SEARCH

SEARCH BY CITATION

Keywords:

  • basidiomycetes;
  • cellobiohydrolase;
  • cellulose dehydrogenase;
  • endoglucanase;
  • β-glucosidase;
  • quinone redox cycling

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Cellulose is the main polymeric component of the plant cell wall, the most abundant polysaccharide on Earth, and an important renewable resource. Basidiomycetous fungi belong to its most potent degraders because many species grow on dead wood or litter, in environment rich in cellulose. Fungal cellulolytic systems differ from the complex cellulolytic systems of bacteria. For the degradation of cellulose, basidiomycetes utilize a set of hydrolytic enzymes typically composed of endoglucanase, cellobiohydrolase and β-glucosidase. In some species, the absence of cellobiohydrolase is substituted by the production of processive endoglucanases combining the properties of both of these enzymes. In addition, systems producing hydroxyl radicals based on cellobiose dehydrogenase, quinone redox cycling or glycopeptide-based Fenton reaction are involved in the degradation of several plant cell wall components, including cellulose. The complete cellulolytic complex used by a single fungal species is typically composed of more than one of the above mechanisms that contribute to the utilization of cellulose as a source of carbon or energy or degrade it to ensure fast substrate colonization. The efficiency and regulation of cellulose degradation differs among wood-rotting, litter-decomposing, mycorrhizal or plant pathogenic fungi and yeasts due to the different roles of cellulose degradation in the physiology and ecology of the individual groups.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Cellulose is the main polymeric component of the plant cell wall, the most abundant polysaccharide on Earth, and an important renewable resource. The chemical composition is simple, it consists of d-glucose residues linked by β-1,4-glycosidic bonds to form linear polymeric chains of over 10 000 glucose residues. Cellulose contains both highly crystalline regions where individual chains are linked to each other and less-ordered amorphous regions. Although chemically simple, the intermolecular bonding pattern can result in a very complex morphology (Hon, 1994). Basidiomycetes are the most potent degraders of this polymer because many species grow on dead wood or litter, in environment rich in cellulose. Fungal cellulolytic systems differ from the complexed cellulolytic systems of bacteria while the differences between individual taxonomic groups are less pronounced (Lynd et al., 2002). Although cellulose degradation by basidiomycetes has been studied extensively since the middle of the last century, e.g. (Reese & Levinson, 1952), the view of cellulose degradation changed in the last few years. The main reasons were the formulation of the contribution of oxidative systems to cellulose degradation, including the first attempts to quantify their importance (Suzuki et al., 2006), the detection of processive endoglucanases in brown rot fungi (Cohen et al., 2005; Yoon et al., 2007), as well as the appearance of the first genome sequence of a wood-rotting basidiomycete, Phanerochaete chrysosporium (Martinez et al., 2004), that greatly enhanced the power of proteomic and computational methods for detection of individual components of its model cellulolytic system (van den Wymelenberg et al., 2005, 2006; Kersten & Cullen, 2007; Sato et al., 2007). Last, but not least, basidiomycetes from habitats other than wood, for example, the litter-decomposers and mycorrhizal species, attracted more attention in the past years. All of the above achievements contribute to a better picture of the different processes participating in cellulose degradation by basidiomycetes. Previous reviews on cellulose degradation by fungi were usually limited to the description of the properties of enzymatic systems or focused only on one of the several-redox based systems active upon all plant cell wall components. The aim of this review is to present both the information about the composition and biochemical properties of enzymatic systems utilized by basidiomycetous fungi for cellulose degradation and the redox, radical-generating systems, and to point out the main differences. It should be made clear that the degradation of cellulose is a complex process where several components may be acting at the same time. We hope that it will further promote research in the physiology and ecology of lignocellulose degradation.

Degradation of cellulose using hydrolytic enzymes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

The classical array of fungal cellulose-degrading enzymes is composed of endo-cleaving (endoglucanases) and exo-cleaving (cellobiohydrolases, exocellulases) enzymes acting on cellulose. The resulting cellobiose or cello-oligosaccharides are usually processed by extracellular or intracellular β-glucosidases or subject to dehydrogenation by cellobiose dehydrogenase.

Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Endoglucanases (EGs) were isolated from several wood-rotting basidiomycetes, brown rot and white rot fungi and also from the plant pathogen Sclerotium rolfsii, the basidiomycetous yeast Rhodotorula glutinis and the symbiont of termites Termitomyces sp. (Table 1). Because endoglucanase activity was also documented in cultures of litter-decomposing basidiomycetes (Steffen et al., 2007; Valáškováet al., 2007), ectomycorrhizal fungi (Maijala et al., 1991; Cao & Crawford, 1993) and wood-associated yeasts (Jimenez et al., 1991), it seems that this enzyme is common among basidiomycetes.

Table 1.   Selected properties of isolated endoglucanases
Fungus and enzymeGroup*Molecular mass (kDa)pIKM (g L−1)pH optimumReferences
  • *

    BR, brown rot; LD, litter decomposer; P, phytopathogen; S, symbiotic; WR, white rot; Y, yeast.

  • Substrate, carboxymethyl cellulose.

Coniophora cerebella ABR42  4.7Goksoyr & Eriksen (1980)
Coniophora cerebella BBR39  4.2Goksoyr & Eriksen (1980)
Coniophora puteanaBR    Schmidhalter & Canevascini (1992)
Gloeophyllum sepiariumBR85   Bhattacharjee et al. (1993)
Gloeophyllum sepiarium EGSBR453.87.64.1Mansfield et al. (1998)
Gloeophyllum trabeumBR29 13.14.4Herr et al. (1978a)
Gloeophyllum trabeum Cel12ABR284.8  Cohen et al. (2005)
Gloeophyllum trabeum Cel5ABR424.9  Cohen et al. (2005)
Gloeophyllum trabeum EGTBR413.16.34.2Mansfield et al. (1998)
Fomitopsis palustrisBR40   Ishihara & Shimizu (1984)
Fomitopsis palustris EG47BR47   Yoon et al. (2007)
Fomitopsis palustris EG35BR35   Yoon et al. (2007)
Irpex lacteusWR65  4.0Kanda et al. (1976)
Irpex lacteus E2-AWR    Kubo & Nisizawa (1983)
Irpex lacteus E2-BWR    Kubo & Nisizawa (1983)
Irpex lacteus En-1WR36  4.0Kanda et al. (1980)
Phanerochaete chrysosporium EG 28WR285.2  Henriksson et al. (1999)
Phanerochaete chrysosporium EG 38WR384.9  Uzcategui et al. (1991a)
Phanerochaete chrysosporium EG 44WR444.3  Uzcategui et al. (1991a)
Piptoporus betulinus EG1BR622.6–2.82.22.5Valášková & Baldrian (2006b)
Polyporus arcularius IWR39 0.354.4–4.6Ishihara et al. (2005)
Polyporus arcularius IIWR36 0.264.4–4.6Ishihara et al. (2005)
Polyporus arcularius IIIaWR24 0.264.9Ishihara et al. (2005)
Polyporus schweinitziiWR45  4.0Bailey et al. (1969), Keilich et al. (1969)
Postia placentaBR35–40   Clausen (1995)
Rhodotorula glutinisY408.6114.5Oikawa et al. (1998)
Schizophyllum communeWR41   Willick & Seligy (1985)
Schizophyllum communeWR39   Willick & Seligy (1985)
Sclerotium rolfsii EG AP78 2.54.0Sadana et al. (1984)
Sclerotium rolfsii EG BP52 4.82.8–3.2Sadana et al. (1984)
Sclerotium rolfsii EG CP28 2.24.0Sadana et al. (1984)
Serpula incrassata Cel 25BR25<3.6 2.5–4.0Kleman-Leyer & Kirk (1994)
Serpula incrassata Cel 49BR49<3.6 2.5–4.0Kleman-Leyer & Kirk (1994)
Serpula incrassata Cel 57BR57<3.6 2.5–4.0Kleman-Leyer & Kirk (1994)
Termitomyces sp.S36 7.54.4Rouland et al. (1988)
Trametes versicolorWR30  5.0Pettersson & Porath (1963), Idogaki & Kitamoto (1992)
Volvariella volvacea EG1LD427.747.5Ding et al. (2001)

The enzymes are monomeric, with molecular masses typically between 22 and 45 kDa but enzymes almost double the size were found in Sclerotium rolfsii and Gloeophyllum sepiarium (Sadana et al., 1984; Bhattacharjee et al., 1993). Class 7 endoglucanases produced e.g. by Phanerochaete chrysosporium contain a large catalytic domain and a 4-kDa cellulose-binding domain (CBD) that can be detached by papain cleavage (Uzcategui et al., 1991a), while some smaller endoglucanases lack the CBD (Henriksson et al., 1999). Although some enzymes are not glycosylated, endoglucanases typically contain a relatively low amount of carbohydrate ranging from 1 to 12% (Eriksson & Pettersson, 1975a, b; Kanda et al., 1976, 1980; Henriksson et al., 1999). Isoelectric points are usually acidic, between 2.6 and 4.9, but the enzymes isolated from Volvariella volvacea and Rhodotorula glutinis exhibited pI above 7 (Oikawa et al., 1998; Ding et al., 2001).

Multiple endoglucanases are produced by many fungi. At least three different enzymes were isolated from Gloeophy-llum trabeum, Phanerochaete chrysosporium, Sclerotium rolfsii and Serpula incrassata. The most studied endoglucanase system in Phanerochaete chrysosporium was originally described to contain five different enzymes (Eriksson & Pettersson, 1975a, b), but some of them were later identified as cleavage products of other endoglucanases. There are two typical endoglucanases with a CBD, EG38 and EG44 belonging to class 7 glycosyl hydrolases (Uzcategui et al., 1991a) and another 28 kDa endoglucanase belonging to family 12. The latter enzyme lacks the CBD; it induces the swelling of filter paper and was proposed to catalyze the fast cleavage of amorphous cellulose regions that are inaccessible to larger endoglucanases (Henriksson et al., 1999). All Phanerochaete chrysosporium enzymes exhibit endo–exo synergism with cellobiohydrolases.

Endoglucanases of basidiomycetes show catalytic optima at pH between 4.0 and 5.0, i.e. near to the pH values found in fungus-colonized wood (Suzuki et al., 2006; Valášková & Baldrian, 2006b). Only the Volvariella volvacea enzyme heterologously expressed in Pichia sp. exhibits a neutral pH optimum (Ding et al., 2002). Temperature optima are between 50 and 70 °C (Kanda et al., 1976, 1980; Herr et al., 1978a, b; Lachke & Deshpande, 1988; Valášková & Baldrian, 2006b), i.e. well above the values occurring under natural conditions.

Carboxymethylcellulose and amorphous cellulose (e.g. phosphoric acid swollen cellulose, PASC) are good – although not natural – substrates of most endoglucanases and indicate that the enzyme activity is mainly directed towards amorphous regions in the cellulose molecule. The KM values for carboxymethylcellulose are in the range from 0.26 in Polyporus arcularius to 13 g L−1 in G. trabeum (Herr et al., 1978a; Ishihara et al., 2005). Significant activity towards crystalline cellulose was found only with the Irpex lacteus 65 kDa cellulase, the Cel5A from G. trabeum and EG35 from Fomitopsis palustris (Kanda et al., 1976; Cohen et al., 2005; Yoon et al., 2007). The latter two enzymes belong to a group of processive endoglucanases, originally reported from cellulolytic bacteria. These enzymes cleave cellulose internally but also release soluble oligosaccharides before detaching from the polysaccharide and thus act as a combination of endoglucanase and cellobiohydrolase (Tomme et al., 1996; Gilad et al., 2003). Cel5A from G. trabeum hydrolyze Avicel to cellobiose as the major product while introducing only a small proportion of reducing sugars into the remaining, insoluble substrate. It produced up to 4.5 nmol glucose equivalents from Avicel per minute and milligram protein (Cohen et al., 2005). The other processive endoglucanase EG35 was recently isolated from another brown rot fungus F. palustris, which was previously reported to degrade crystalline cellulose. The partial amino acid sequence of the protein did not show any similarity to known glycosyl hydrolases (Yoon & Kim, 2005, 2007) and it can be considered for sure that there is no corresponding enzyme in the white rot fungus Phanerochaete chrysosporium. In brown rot fungi, processive endoglucanases could potentially substitute for the absence of cellobiohydrolases. However, because most brown rot fungi are reported to be unable to degrade crystalline cellulose the prevalence of these enzymes is questionable. Some processivity was also shown for the EGS and EGT enzymes from G. trabeum and EG1 from Piptoporus betulinus, which liberate cellobiose and glucose from amorphous cellulose, although all of them are inactive with Avicel (Mansfield et al., 1998; Valášková & Baldrian, 2006b).

EG28 from Phanerochaete chrysosporium and endoglucanases from Gloeophyllum spp., Irpex lacteus, Piptoporus betulinus, Sclerotium rolfsii and Trametes versicolor are also active on cello-oligomers, cellotetraose is cleaved to cellobiose; EG1 from Piptoporus betulinus can even produce glucose and cellobiose from cellotriose (Kanda et al., 1980; Lachke & Deshpande, 1988; Idogaki & Kitamoto, 1992; Mansfield et al., 1998; Henriksson et al., 1999; Valášková & Baldrian, 2006b). KM for p-nitrophenyl-β-d-cellobioside (pNPC) is between 7 and 16 mM (Mansfield et al., 1998; Henriksson et al., 1999; Valášková & Baldrian, 2006b).

Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Cellobiohydrolases have so far been isolated from several white rot basidiomycetes, the plant pathogen Sclerotium rolfsii and from Termitomyces sp. (Table 2). They are apparently absent from most brown rot fungi, and also the genomes of the human pathogen Cryptococcus neoformans and the plant pathogen Ustilago maydis lack the corresponding genes (Loftus et al., 2005; Kamper et al., 2006). Cellobiohydrolase activity was also documented in litter-decomposing fungi (Steffen et al., 2007; Valáškováet al., 2007) and some ectomycorrhizal fungi (Cao & Crawford, 1993; Burke & Cairney, 1998).

Table 2.   Selected properties of isolated cellobiohydrolases
Fungus and enzymeGroup*Molecular mass (kDa)pIKM (mM)pH optimumReferences
  • *

    BR, brown rot; P, phytopathogen; S, symbiotic; WR, white rot.

  • Substrate: p-nitrophenyl cellobioside.

  • Substrate: p-nitrophenyl cellobioside.

  • §

    Substrate: p-nitrophenyl lactoside.

  • Substrate: p-nitrophenyl lactoside.

  • Isolated from the termite Macrotermes muelleri, but apparently originating from its fungal symbiont.

Coniophora puteana CBH IBR523.66.85.0Schmidhalter & Canevascini (1993a)
Coniophora puteana CBH IIBR503.64.35.0Schmidhalter & Canevascini (1993a)
Dichomitus squalens Ex-1WR394.6 5.0Rouau & Odier (1986)
Dichomitus squalens Ex-2WR364.5 5.0Rouau & Odier (1986)
Fomitopsis palustrisBR 2.3 4.0Hishida et al. (1997)
Irpex lacteusWR65  5.0Kanda & Nisizawa (1988)
Irpex lacteus Ex-1WR534.5 5.0Hamada et al. (1999)
Irpex lacteus Ex-2WR564.8 5.0Hamada et al. (1999)
Phanerochaete chrysosporium CBH 50WR504.9  Uzcategui et al. (1991b)
Phanerochaete chrysosporium CBH 58WR583.82.1§ Uzcategui et al. (1991b)
Phanerochaete chrysosporium CBH 62WR624.93.4 Uzcategui et al. (1991b)
Schizophyllum communeWR59   Willick & Seligy (1985)
Schizophyllum communeWR58   Willick & Seligy (1985)
Sclerotium rolfsiiP42  4.2–4.5Patil & Sadana (1984)
Termitomyces sp.S52  4.4Rouland et al. (1988)

The enzymes are monomeric with molecular masses typically between 50 and 65 kDa though Dichomitus squalens and Sclerotium rolfsii cellobiohydrolases are smaller (Rouau & Odier, 1986; Sadana & Patil, 1988b). Papain cleavage of family 7 cellobiohydrolases yields a large domain that is catalytically active with low molecular mass substrates and a 4–5 kDa CBD similar to that of Trichoderma reesei (Uzcategui et al., 1991b). A structural model for family 7 CBH58 from Phanerochaete chrysosporium is available (Munoz et al., 2001). Similar to endoglucanases, glycosylation of cellobiohydrolases is none or low, <12% (Eriksson & Pettersson, 1975a, b; Schmidhalter & Canevascini, 1993a; Hamada et al., 1999); and the isoelectric points are acidic, typically between 3.6 and 4.9 (but only 2.3 in F. palustris).

The necessity for at least two cellobiohydrolases in the ascomycete Hypocrea jecorina (anamorph Trichoderma reesei) has been attributed to their particular preference for the reducing (CBHI) and nonreducing (CBHII) ends of cellulose chains. This notion has also been supported by the exo–exo synergy observed between these two enzymes (Lynd et al., 2002). This is also the probable reason for the multiplicity of cellobiohydrolases detected in most basidiomycetes studied so far (Table 2). Phanerochaete chrysosporium produces three cellobiohydrolases, CBH58 (originally designated CBH I), CBH62 and CBH50 (Uzcategui et al., 1991b). CBH58 and CBH62 liberate cellobiose from reducing ends of cellulose and their CBDs are located at the C terminus while CBH50 cleaves from the nonreducing end and its CBD is located at the N terminus. All combinations of Phanerochaete chrysosporium cellobiohydrolases exhibit considerable synergistic action.

Similar to most endoglucanases, catalytic optima of cellobiohydrolases are situated in a narrow pH range between 4.0 and 5.0. Temperature optima are between 37 and 60 °C depending on the enzyme and the substrate (Rouau & Odier, 1986; Sadana & Patil, 1988b; Hamada et al., 1999). Cellobiohydrolases are typically active on crystalline cellulose, e.g. Avicel. Interestingly, CBH58 and CBH50 from Phanerochaete chrysosporium were not active on carboxymethylcellulose and CBH62 as well as both cellobiohydrolases from Pleurotus ostreatus exhibited only weak activity against this substrate (Uzcategui et al., 1991b; Garzillo et al., 1994). On the other hand, CBHI and CBHII from Coniophora puteana are both active on amorphous cellulose (Schmidhalter & Canevascini, 1993a). Only enzymes acting from the reducing ends are able to liberate cellobiose from pNPC or p-nitrophenyl-β-d-lactoside (pNPL), the KM values are between 2 and 7 mM. Cellobiohydrolases are also active on cellotriose, cellotetraose or higher cellodextrins (Kanda et al., 1989; Schmidhalter & Canevascini, 1993a; Hishida et al., 1997).

Not surprisingly, cellobiose acts as a competitive inhibitor of cellobiohydrolases. The Ki is 1.2–2.4 mM in Coniophora puteana but as low as 65 μM in Phanerochaete chrysosporium (Schmidhalter & Canevascini, 1993a; Igarashi et al., 1998). It is proposed that the positive effect of cellobiose dehydrogenase (CDH) on cellulose hydrolysis is due to relieving the product inhibition of cellobiohydrolases by cellobiose oxidation (Igarashi et al., 1998).

β-Glucosidase (EC 3.2.1.21)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Because cellobiose is a largely available substrate, β-glucosidases are produced by the majority of microorganisms (Lynd et al., 2002). Among basidiomycetes, enzymes were isolated from several wood-rotting fungi, both white rot and brown rot ones, the mycorrhizal fungi Pisolithus tinctorius and Tricholoma matsutake, the plant pathogen Sclerotium rolfsii and Termitomyces sp. (Table 3). β-Glycosidases were also isolated from, and their activity detected in, basidiomycetous yeasts, although some of the wood-associated yeasts were unable to use cellobiose as a substrate (Peciarova & Biely, 1982; Onishi & Tanaka, 1996; Middelhoven, 2006). Activity of β-glucosidase was also detected in pure cultures of litter-decomposing basidiomycetes (Steffen et al., 2007; Valáškováet al., 2007) and ectomycorrhizal species (Burke & Cairney, 1998; Mucha et al., 2006).

Table 3.   Selected properties of isolated β-glucosidases
Fungus and enzymeGroup*Molecular mass (kDa)pIKM (μM)pH optimumReferences
  • *

    BR, brown rot; LD, litter decomposer; M, mycorrhizal; P, phytopathogen; S, symbiotic; WR, white rot; Y, yeast.

  • Substrate: p-nitrophenyl glucoside.

  • Intracellular enzyme.

  • §

    Also β-xylosidase activity.

  • Also 1,3-β-glucosidase activity.

  • Recombinant protein expressed in Escherichia coli.

  • ††

    Substrate: o-nitrophenyl glucoside.

Ceriporiopsis subvermisporaWR110 3903.5Magalhaes et al. (2006)
Ceriporiopsis subvermisporaWR53   Magalhaes et al. (2006)
Gloeophyllum trabeumBR320 414.5Herr et al. (1978a, b)
Phanerochaete chrysosporiumWR90 1605.5Smith & Gold (1979)
Phanerochaete chrysosporiumWR114 964.0–5.2Lymar et al. (1995)
Phanerochaete chrysosporiumWR410 1107.0Smith & Gold (1979)
Phanerochaete chrysosporium§WR454.753005.0Copa-Patino & Broda (1994)
Phanerochaete chrysosporiumWR116 3350 Igarashi et al. (2003)
Phanerochaete chrysosporium A1WR1654.81504.0–4.5Deshpande et al. (1978)
Phanerochaete chrysosporium A2WR1724.51504.0–4.5Deshpande et al. (1978)
Phanerochaete chrysosporium BWR165–1824.6–5.22104.0–4.5Deshpande et al. (1978)
Phanerochaete chrysosporium BGL1AWR53 230 Tsukada et al. (2006)
Phanerochaete chrysosporium BGL1BWR60 620 Tsukada et al. (2006)
Piptoporus betulinusM362.618004.0Valášková & Baldrian (2006b)
Pisolithus tinctoriusM4503.88704.0Cao & Crawford (1993)
Pleurotus ostreatusWR357.52300 Morais et al. (2002)
Pleurotus ostreatusWR507.32360 Morais et al. (2002)
Pleurotus ostreatusWR668.52430 Morais et al. (2002)
Poria vailantiiBR   4.2Sison & Schubert (1958)
Rhodotorula minutaY1444.812004.7–5.2Onishi & Tanaka (1996)
Schizophyllum communeWR94–96   Willick & Seligy (1985)
Schizophyllum commune IWR110   Lo et al. (1988)
Schizophyllum commune IIWR96   Lo et al. (1988)
Sclerotium rolfsii BG1P904.165004.2Shewale & Sadana (1981)
Sclerotium rolfsii BG2P904.676004.2Shewale & Sadana (1981)
Sclerotium rolfsii BG3P1075.169004.2Shewale & Sadana (1981)
Sclerotium rolfsii BG4P925.664004.2Shewale & Sadana (1981)
Sporobolomyces singularisY146 19603.5Ishikawa et al. (2005)
Termitomyces sp.S  260†† Osore & Okech (1983)
Termitomyces clypeatusS450  5.0Sengupta et al. (1991)
Trametes gibbosaWR640  3.5Bhattacharjee et al. (1992)
Trametes versicolorWR300 276 Evans (1985)
Tricholoma matsutakeLD160 2205.0Kusuda et al. (2006)
Volvariella volvacea BGL-1LD1585.6907.0Cai et al. (1998)
Volvariella volvacea BGL-2LD2565.0–5.25006.2Cai et al. (1998)

β-Glucosidases isolated so far exhibit high structural variability, partly reflecting the intracellular/extracellular localization of the enzyme (Table 3). The detected molecular masses range from 35 to 640 kDa. While the small enzymes with molecular masses around 100 kDa are monomeric and usually extracellular, homo-oligomeric enzymes have also been isolated. The yeasts Rhodotorula minuta and Sporobolomyces singularis produce dimeric, cell wall-associated β-glycosidases with high affinity for p-nitrophenyl-β-d-glucoside (pNPG) and a transgalactosidase activity (Onishi & Tanaka, 1996; Ishikawa et al., 2005). The enzymes are also able to cleave galactose and fucose units and lactose disaccharides, but the KM for pNPG is the lowest and they can thus be designated as β-glucosidases. The cell wall-associated enzyme from Pisolithus tinctorius is a homotrimer (Cao & Crawford, 1993) and the 450-kDa β-glucosidase from Termitomyces clypeatus is composed of 110-kDa subunits (Sengupta et al., 1991). On the other hand, the large 300-kDa β-glucosidase from Trametes versicolor is a monomeric glycoprotein with 90% glycosylation. The quaternary structure of the large intracellular enzymes from Phanerochaete chrysosporium (410 kDa) and Volvariella volvacea (256 kDa) is not known (Smith & Gold, 1979; Cai et al., 1998). β-Glucosidases are typically glycosylated but the sugar unit content varies (Evans, 1985; Lymar et al., 1995). Isoelectric points of cell wall-associated and extracellular enzymes are acidic, typically between 3.5 and 5.2 while the intracellular enzymes have pIs from 6.2 to 7.0.

As already mentioned, β-glucosidases can be extracellular, cell wall-associated and intracellular. The mycelia-associated fraction in Pleurotus ostreatus, Trametes versicolor and Piptoporus betulinus accounted for 65%, 13%, and 35% of the total activity, respectively (Valášková & Baldrian, 2006a). Only 10% of β-glucosidase in Volvariella volvacea cultures was extracellular while 26% were associated with cell wall fraction and 64% were located intracellularly (Cai et al., 1999).

The intracellular localization of some β-glucosidases requires the transport of cellobiose into the cell. Although not proven, it is probable that a permease similar to the Trichoderma reesei diglucoside permease also operates in basidiomycetes (Kubicek et al., 1993). The presence of extracellular and intracellular enzymes is reflected in the production of enzyme molecules differing in structural and catalytic properties. In Phanerochaete chrysosporium, an array of enzymes with β-glucosidase activity was independently isolated by several groups (Table 3). The smallest 45 kDa protein is extracellular, and in addition to pNPG it can also cleave p-nitrophenyl-β-d-xylopyranoside (pNPX) and laminaribiose (β-1,3 linkages). It is almost inactive with polymeric cellulose and xylan (Copa-Patino & Broda, 1994). Three extracellular β-glucosidases were found in cellulose-grown cultures (Lymar et al., 1995). The 114 kDa enzyme contains a CBD that can be detached by papain treatment. The 96 and 98 kDa proteins that do not bind cellulose are probably fragments of this larger molecule. β-Glucosidase activity is competitively inhibited by glucose and gluconolactone, but also by cellobionolactone, the product of CDH oxidation of cellobiose (Lymar et al., 1995). The 116-kDa extracellular enzyme belongs to family 3 glycosidases (Igarashi et al., 2003). It has high affinity for cellobionolactone (KM 1.29 mM) and laminaribiose (2.03 mM) compared with cellobiose (3.35 mM), but the kcat for cellobionolactone is 76 × lower than that for cellobiose. Cellobionolactone produced in high amounts by CDH thus effectively inhibits its activity (Igarashi et al., 2003). An array of monomeric extracellular enzymes with 165–182 kDa were isolated from cultures growing on cellulose; cellobiose induced only the activity of cell-bound enzyme with lower affinity for pNPG –KM 2.0 mM compared with 0.15–0.21 mM reported for the extracellular enzymes (Deshpande et al., 1978). The 410-kDa intracellular enzyme was strongly induced by cellobiose but only weakly by cellulose (Smith & Gold, 1979). Glucose acted as a repressor of β-glucosidase synthesis in Phanerochaete chrysosporium (Smith & Gold, 1979). Two more intracellular β-glucosidases were recently characterized which belong to glycosyl hydrolase family 1 (Tsukada et al., 2006). Recombinant proteins expressed in Escherichia coli have 53 and 60 kDa; BGL1A is also active as β-xylosidase with KM similar to that for pNPG; its crystal structure was recently determined (Nijikken et al., 2007).

As already mentioned for Phanerochaete chrysosporium, also β-glucosidases from brown rot fungi are relatively nonspecific and can also cleave xylose, mannose and galactose units from corresponding oligosaccharides, although with higher KM values (Herr et al., 1978b; Valášková & Baldrian, 2006b). Cello-oligosaccharides are usually good substrates but the enzymes are inactive on crystalline cellulose and exhibit only low activity on amorphous high molecular mass cellulose (Herr et al., 1978b; Sadana et al., 1988; Valášková & Baldrian, 2006b). Activity is competitively inhibited by glucose (Ki 0.2–6 mM), gluconolactone and cellobionolactone. The pH optimum is usually between 3.5 and 5.5, but the intracellular enzyme from Phanerochaete chrysosporium has a neutral pH optimum (Smith & Gold, 1979); the temperature optima are between 45 and 75 °C.

Phosphorolytic degradation of cello-oligosaccharides

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Although phosphorolytic degradation is more typical for polysaccharides with α-1,4 bonds, several bacterial species possess enzymes capable of cleaving cellobiose or cellodextrins by Pi-mediated (ATP-independent) phosphorolytic reactions of cellobiose and cellodextrin phosphorylases.

  • image

Owing to intracellular localization of the enzymes, the reaction is not involved in cellulose hydrolysis but is a part of the intracellular utilization of oligosaccharides (Kitaoka & Hayashi, 2002; Lynd et al., 2002). Cellobiose phosphorylase (EC 2.4.1.20) was not screened for in fungi but it was occasionally detected in the white rot root pathogen Heterobasidion annosum. This fungus exhibited better growth on cellobiose than on glucose due to the energy-saving phosphorolysis (Hüttermann & Volger, 1973).

Oxidative decomposition of cellulose

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

The hypothesis that the decomposition system used by wood-rotting basidiomycetes to degrade plant cell-wall polysaccharides also involves a nonenzymatic component, which was already formulated 40 years ago (Halliwell, 1965; Koenigs, 1974). The detection of hydrogen peroxide (H2O2) production by several fungi led to the proposal of a degradation pathway based on the Fenton reaction

  • image

This reaction is a well-recognized route of •OH production in biological systems. H2O2 is produced by white rot fungi by the action of enzymes such as glyoxal oxidase (Kersten & Kirk, 1987), glucose oxidase (Kelley & Reddy, 1986), and aryl alcohol oxidase (Guillén et al., 1990; Muheim et al., 1990), and also by brown rot fungi (Ritschkoff & Viikari, 1991). In addition to free •OH radicals, certain states of hypervalent iron where the radical remains associated with iron were also considered by some authors as potential oxidizing agents (Wood, 1994; Branchaud, 1999; Welch et al., 2002a, b). Although iron is generally sequestered in redox-inactive complexes in most biological systems to prevent oxidative damage (Halliwell & Gutteridge, 1999), this does not hold for wood, where sufficient iron concentrations make •OH generation feasible (Koenigs, 1974), provided that chelators or reductants are available to solubilize the metal. However, because Fe2+ is usually absent in oxygenated environment, there was a question as to how the necessary reduction of Fe3+ is achieved.

Several systems of polysaccharide decomposition have been proposed where enzymes do not directly react with cellulose undergoing oxidative cleavage. Three oxidative systems operated by wood-rotting basidiomycetes have already received sufficient experimental evidence. These include (1) CDH catalysed reactions (2) redox cycling by small-molecular mass quinones or other redox compounds and (3) •OH production catalysed by small glycopeptides (Hammel et al., 2002; Goodell, 2003; Tanaka et al., 2007).

The CDH-based decomposition differs from the other oxidative systems in two ways: (1) it depends on the presence of cellobiose and its degradation products and the action is more specific due to enzyme binding to cellulose and (2) in addition to oxidative cleavage of polysaccharides it also transforms cellobiose and cello-oligosaccharides, major products of cellulose hydrolysis.

In all three oxidative systems, •OH is responsible for polysaccharide scission. Hydroxyl radicals can abstract hydrogen atoms from the sugar subunits of cellulose or other polysaccharides with high rate constants around 109 M−1 s−1 (Ek et al., 1989). These reactions produce transient carbon-centred radicals that react rapidly with O2 to give peroxyl radical species. If the ROO• carries a hydroxyl group on the same carbon, it eliminates •OOH (Halliwell & Gutteridge, 1999). If there is no α-hydroxyl group present, the molecule undergoes a variety of oxidoreductions, some of which can result in the cleavage of the cellulose chain (Kirk et al., 1991).

Cellobiose dehydrogenase (CDH; EC 1.1.99.18)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

CDH is an extracellular enzyme produced by basidiomycetes and ascomycetes. It efficiently oxidizes cellobiose but also soluble cellodextrins, mannodextrins and lactose to their corresponding lactones using a wide spectrum of electron acceptors including quinones, phenoxyradicals, Fe3+, Cu2+, cytochrome c or triiodide ion (Henriksson et al., 2000; Zamocky et al., 2006).

CDH activity was first discovered as a cellobiose-dependent reduction of quinones in white rot fungi (Westermark & Eriksson, 1974a, b), and the enzyme named cellobiose quinone oxidoreductase (CBQ) carrying a flavin group was isolated from Phanerochaete chrysosporium (Westermark & Eriksson, 1975). Subsequently, a form containing both flavin and heme, that is now named CDH, has been isolated from the same fungus and named cellobiose oxidase (CBO) because it was incorrectly assumed to prefer O2 as an electron acceptor (Ayers et al., 1978). CBQ was later identified as a catalytic active fragment of CBO appearing in the cultures of some fungi probably due to the action of proteases (Wood & Wood, 1992; Henriksson et al., 2000). The FAD and heme prosthetic groups are contained within two separable domains.

Typical CDH produced by basidomycetes is a monomeric protein of 90–110 kDa with glycosylation in the range of 10–20% (Schmidhalter & Canevascini, 1993b; Fang et al., 1998; Baminger et al., 2001); the pI is typically around 4.0 (Table 4). The enzyme has been isolated mainly from white rot basidiomycetes but its activity has also been detected in some other basidiomycetous species (Zamocky et al., 2006). The brown rot fungi do not produce CDH with the only known exception of Coniophora puteana (Schmidhalter & Canevascini, 1993b). It was also isolated from the soil plant pathogen Sclerotium rolfsii and detected in the ectomycorrhizal fungi Pisolithus tinctorius, Suillus variegatus and Cortinarius sp. (Burke & Cairney, 1998).

Table 4.   Selected properties of isolated cellobiose dehydrogenases
FungusGroup*Molecular mass (kDa)pIKM (μM)pH optimumReferences
  • *

    BR, brown rot; P, phytopathogen; WR, white rot.

  • Substrate: cellobiose.

  • Quinones as electron acceptors; other acceptors can exhibit different optima.

Coniophora puteanaBR1113.946–844.0Schmidhalter & Canevascini (1993b), Kajisa et al. (2004)
Irpex lacteusWR97 34 Hai et al. (2000)
Phanerochaete chrysosoriumWR894.216–1105.0Henriksson et al. (1995), Zamocky et al. (2006)
Pycnoporus cinnabarinusWR92 1114.5Sigoillot et al. (2002)
Pycnoporus cinnabarinusWR1013.8 4.5Temp & Eggert (1999)
Schizophyllum communeWR102 304.5Fang et al. (1998)
Sclerotium rolfsiiP1014.2–5.0120 Sadana & Patil (1988a); Baminger et al. (2001)
Trametes hirsutaWR924.2 5.0Nakagame et al. (2006)
Trametes pubescensWR904.22104.5–5.0Ludwig et al. (2004)
Trametes versicolorWR974.21205.0Roy et al. (1996)
Trametes villosaWR984.42104.5–5.0Ludwig et al. (2004)

CDH is a typical oxidoreductase with oxidative and reductive half reactions that occur separately (Fig. 1). The oxidative half reaction represents an oxidation in the C1 position of a saccharide; the hemiacetal at this position is converted to a lactone that hydrolyzes spontaneously to a carboxylic acid. The two electrons taken up by the enzyme are later transferred further to one two-electron acceptor, or to two one-electron acceptors (Morpeth, 1985; Henriksson et al., 1993). All results indicate that the oxidation of an electron donor is carried out by the FAD group, which is converted to FADH2 (Henriksson et al., 1991). The reduction of cytochrome c and other electron acceptors by CDH is carried out by the heme domain following flavin-to-heme intramolecular electron transfer. However, direct reduction of electron acceptors by the FAD domain of the protein is also possible, although sometimes slower (Henriksson et al., 2000; Zamocky et al., 2006).

image

Figure 1.  Reactions of cellobiose dehydrogenase based on Henriksson et al. (2000). ‘Fe’ represents the heme iron, ‘A’ represents the one-electron acceptor.

Download figure to PowerPoint

The substrate specificity has been investigated most carefully with the Phanerochaete chrysosporium enzyme (Zamocky et al., 2006). CDH readily oxidizes cellobiose and higher cellodextrins, as well as lactose, maltose mannobiose and galactosylmannose, although the latter substrates display 10–100 × higher KM values (Ayers et al., 1978; Morpeth, 1985; Bao et al., 1993; Henriksson et al., 1998; Zamocky et al., 2006). These ‘true’ substrates are all di- or oligosaccharides with β-1,4 bonds and a glucose or mannose residue at the reducing end. The monosaccharides glucose and mannose and the 1,4-α-diglucoside maltose have very high KM values. Monosaccharides and maltose also have substantially lower kcat values (Henriksson et al., 2000; Zamocky et al., 2006). The isolated enzymes show KM for cellobiose in the range of 10–200 μM and a pH optimum for quinone reduction between 4 and 5 (Table 4). Temperature optima are typically between 30 and 55 °C (Henriksson et al., 1995; Baminger et al., 2001), but can be as high as 75 °C as in Pycnoporus cinnabarinus (Temp & Eggert, 1999).

CDH was identified as the first nonhydrolytic enzyme binding to cellulose (Renganathan et al., 1990; Henriksson et al., 1991). Unlike many other cellulose-binding proteins, CDH binds specifically to cellulose, i.e. it does not bind to insoluble xylan, mannan, starch and chitin (Henriksson et al., 1997; Temp & Eggert, 1999), and this specificity can be a key to its biological function. In contrast to some CDHs from ascomycetes, basidiomycete enzymes do not possess a typical cellulose binding motif (Hallberg et al., 2002); the binding to cellulose is of a hydrophobic nature (Henriksson et al., 1997).

The biological function of CDH is not fully understood, although Phanerochaete chrysosporium produces relatively high levels of the enzyme, c. 0.5% of the secreted protein. CDH moderately enhances the activity of a crude mixture of cellulases and also of isolated cellobiohydrolase (Bao & Renganathan, 1992; Igarashi et al., 1998). CDH has been shown to degrade not only cellulose, but also xylan and lignin in the presence of H2O2 and chelated Fe ions (Henriksson et al., 1995). It is produced along with cellulases and hemicellulases under cellulolytic conditions, i.e. when cellulose is the major carbon source. It seems likely that CDH is induced by low concentrations of cellobiose, but it is also subject to catabolite repression by excessive concentrations of cellobiose or glucose (Schmidhalter & Canevascini, 1993b; Henriksson et al., 2000). mRNA of CDH was found together with mRNAs for endoglucanase and MnP in Phanerochaete chrysosporium growing on wood (Vallim et al., 1998).

Several hypothetical mechanisms of CDH involvement in the degradation of cellulose (but also hemicelluloses and lignin) have been proposed, e.g. the reduction of substrate inhibition by cellulolysis products, reduction of quinones to be used by ligninolytic enzymes or the support of a Mn-peroxidase reaction (Henriksson et al., 2000). The currently accepted hypothesis is that CDH degrades and modifies cellulose, hemicelluloses and lignin by generating hydroxyl radicals in a Fenton-type reaction (Kremer & Wood, 1992; Henriksson et al., 1995; Mansfield et al., 1997). The enzyme can reduce Fe3+ to Fe2+, or Cu2+ to Cu+ by oxidation of cellobiose. Subsequent reaction between the reduced species and H2O2 generates hydroxyl radicals that may modify and depolymerize plant cell wall polymers. The iron is present in wood and H2O2 is readily produced by CDH itself or by other extracellular fungal redox enzymes (Ander & Marzullo, 1997). Depolymerization with CDH, cellobiose, Fe3+ and H2O2 was demonstrated for carboxymethylated cellulose (CMC), water-soluble xylan, radioactively labelled synthetic lignin (Henriksson et al., 1995) and for insoluble cellulose in the form of kraft pulp (Mansfield et al., 1997). Some depolymerization occurred even without added H2O2 due to its formation by the enzyme itself (Nutt et al., 1997).

A mechanism of CDH participation in the Fenton reaction has been proposed for the brown rot fungus Coniophora puteana (Hyde & Wood, 1997) in wood containing oxalic acid which strongly chelates Fe3+ and Fe2+ (Espejo & Agosin, 1991). The redox properties of Fe-oxalate complexes can have a large influence on Fenton chemistry (Hyde & Wood, 1997; Park et al., 1997, 1999). CDH can reduce Fe3+-oxalate effectively only at pH values below c. 2.5, where it is present as Fe3+-dioxalate, because the reduction potential of the Fe3+-trioxalate complex that predominates above this pH is too negative. The reduction of the Fe3+-dioxalate by CDH results in uncomplexed Fe2+ or in the Fe2+-mono-oxalate complex. These Fe2+ species are relatively stable at pH 2.5, but as they diffuse away from the hyphae, they will encounter a region with lower oxalate concentration and higher pH, which will result in the formation of the Fe2+-dioxalate complex. Around pH 4, Fe2+-dioxalate complexes autoxidize rapidly and as a result peroxyl radical is produced. This •OOH is reduced by Fe2+ or dismutates, thus generating H2O2, the second substrate for Fenton reaction (Hyde & Wood, 1997; Park et al., 1997).

This model proposes that a complete Fenton system is formed only after Fe2+ has diffused some distance from the fungal hyphae. In this way, Fenton reagent might be produced by wood-rotting fungi within the secondary wood cell wall, where it is needed to initiate the degradation of lignin or polysaccharides. This mechanism would also protect the fungus from the oxidative damage that can occur if hydroxyl radicals are produced near the hyphae. The model assumes that H2O2 is produced only via Fe2+ autoxidation. However, if CDH also uses O2 as an electron acceptor, which seems possible given that Fe3+-oxalate complexes are relatively difficult to reduce, then H2O2 will be produced at the same site as Fe2+. In addition, in white rot fungi that secrete H2O2-producing enzymes it is unlikely that H2O2 production could depend only upon Fe2+ autoxidation. Another potential problem with this model is the pH below c. 2.5 required near the hyphae. Although some wood-rotting fungi have been shown to reduce pH as much as to 1.6–2.5 (Green et al., 1991), it is not clear whether these conditions generally occur.

Oxalic acid, the compound essential for the functioning of the proposed mechanism is secreted by many brown rot and white rot fungi (Takao, 1965; Dutton et al., 1993). White rot fungi are usually reported to produce less oxalate than brown rot fungi due to the fact that excess oxalate inhibits the activity of ligninolytic peroxidases (Akamatsu et al., 1994; Shimada et al., 1997). Because a stable concentration of oxalate is necessary for the functioning of the ligninolytic system of white rot fungi, its concentration is regulated by the production of oxalate decarboxylase (Shimada et al., 1997; Kurek & Gaudard, 2000). In addition to the co-operation with CDH, oxalate was also proposed to directly participate in cellulose hydrolysis in brown rot fungi by a Fenton-type mechanism while H2O2 is produced during the synthesis of oxalate (Shimada et al., 1997). Oxalate in a certain range of concentrations greatly enhanced cellulose degradation by Fenton oxidation in vitro (Tanaka et al., 1994) and the wild-type brown rotter Postia placenta caused higher wood mass loss than its less oxalate-producing mutant (Micales & Highley, 1991). However, because it is still unclear how the strict regulation of oxalate concentration can be achieved in vivo it seems more probable that oxalic acid is just a part of one of the more complex systems of cellulose hydrolysis discussed here.

Interestingly, the analysis of Phanerochaete chrysosporium genome has identified, in addition to a CDH gene, a separate gene encoding a heme domain similar to that of CDH fused to a highly conserved family 1 CBD (Kersten & Cullen, 2007). The predicted cytochrome b562 protein is 46% identical to the corresponding region of CDH, and the isolated recombinant enzyme has the expected electron transfer activity (Yoshida et al., 2005). The structure and regulation of this CDH-like protein are compatible with a role in Fenton chemistry similar to CDH (Kersten & Cullen, 2007).

Quinone redox cycling

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Low-molecular-weight chelators of catecholate origin were isolated from white rot and brown rot fungi in the late 1980s and early 1990s (Fekete et al., 1989; Jellison et al., 1991; Chandhoke et al., 1992). The research was focused on the chelators produced by G. trabeum (‘Gt chelators’) that were of small molecular size (<1000 Da) and, unlike enzymes, could penetrate through the wood cell wall matrix (Jellison et al., 1991; Goodell et al., 1997; Filley et al., 2002; Goodell et al., 2002). Chelators with iron-reducing capacity have been also documented in rotted wood, e.g. palo podrido (Ferraz et al., 2001). It was discovered later that some of the participating compounds are probably quinones (Goodell et al., 1997; Goodell, 2003).

The principle of the quinone redox cycling mechanism is in the fugal reduction of quinones to the corresponding hydroquinones, which then react with Fe3+ to give Fe2+ and semiquinone radicals. The semiquinones can reduce O2 to give •OOH and the original quinones. Because •OOH is a source of H2O2, this cycle will generate a complete Fenton system (Kerem et al., 1999).

This mechanism requires the reduction of extracellular quinones to their hydroquinone forms by the fungus. Enzymes potentially capable of this reaction include intracellular benzoquinone reductases (Brock et al., 1995) and extracellular sugar dehydrogenases such as CDH, which have been shown to use quinones as alternate electron acceptors (Henriksson et al., 2000). This model also assumes an adequate source of quinones. It has been suggested that quinones can be found in wood extractives or generated during lignin transformation by white rot fungi (Guillén et al., 2000). It was also proposed that demethylated lignin, produced during brown-rot degradation of wood, may also function as a redox active compound, serving as an electron source for Fe3+ to Fe2+ reduction (Xu & Goodell, 2001; Filley et al., 2002). It has also been shown that iron reduction capacity in soils is dependent on the amount of soil organic matter (Keppler et al., 2000). Because the humic portion of soils contains breakdown products of lignin, it seems likely that compounds formed during wood degradation might also promote redox reactions in soils (Goodell et al., 2006).

Because the redox-active quinones and hydroquinones produced in decayed wood are also a subject of •OH damage, it seems likely that the fungus has to provide its own source of extracellular quinones. Their production was originally identified in the brown rot fungi of the genus Gloeophyllum that secrete two hydroquinones, 2,5-dimethoxyhydroquinone (2,5-DMHQ) and 4,5-dimethoxycatechol (4,5-DMC). These compounds are able to reduce Fe3+ to give Fe2+ and semiquinone radicals (Kerem et al., 1999; Paszczynski et al., 1999; Jensen et al., 2001; Newcombe et al., 2002). The semiquinones reduce both O2 and Fe3+, giving peroxyl radicals, additional Fe2+ and the two quinones 2,5-dimethoxy-1,4-benzoquinone (2,5-DMBQ) and 4,5-dimethoxy-1,2-benzoquinone (4,5-DMBQ). In subsequent oxidoreductions, Fe2+/Fe3+ couple equilibrates with the •OOH/O2 couple, while Fe2+ reduces •OOH to give H2O2 (Buettner, 1993; Halliwell & Gutteridge, 1999) (Fig. 2). Production of 2,5-DMHQ was also found during the growth of G. trabeum and Postia placenta on wood (Cohen et al., 2002) and during the degradation of cellulose by Serpula lacrymans, another brown rot fungus (Shimokawa et al., 2004). In G. trabeum that produced both 2,5-DMHQ and 4,5-DMC, 2,5-DMHQ/2,5-DMBQ was the more efficient hydroquinone/quinone couple. It was always present in higher concentrations, 2,5-DMHQ also reduced O2 more rapidly and 2,5-DMBQ was more rapidly reduced by the fungus (Jensen et al., 2001; Cohen et al., 2002; Suzuki et al., 2006). In addition to 2,5-DMHQ and 4,5-DMC, other unidentified compounds of phenolic origin were found in wood extracts that can potentially also participate in the redox chemistry (Goodell et al., 1997; Suzuki et al., 2006).

image

Figure 2.  Reactions involved in the quinone redox cycling in the brown rot fungus Gloeophyllum trabeum (shown for 2,5-dimethoxyhydroquinone). Based on Jensen et al. (2002) and Suzuki et al. (2006).

Download figure to PowerPoint

Due to high production of oxalate, pH in G. trabeum cultures can be near 4.1 and even lower values were found near the hyphae in wood (Jensen et al., 2001; Suzuki et al., 2006). 2,5-DMHQ is very stable in the absence of iron at pH 2–4 and it readily reduces Fe3+ with a rate constant of 4.5 × 103 M−1 s−1 at pH 4.0. Fe2+ is also very stable at low pH. H2O2 generation results from the autoxidation of the semiquinone radical and was observed only when 2,5-DMHQ was incubated with Fe3+. At low concentrations of oxalate, around 50 μM, ferric ion reduction and production of •OH is enhanced. The enhancement of both Fe3+ reduction and •OH production may be due to the promotion of the ferric ion solubility by oxalate. On increasing the oxalate concentration the oxalate/ferric ion ratio favours formation of the 2 : 1 and 3 : 1 complexes and results in slower Fe3+ reduction and •OH formation (Varela & Tien, 2003).

Recently, (Suzuki et al., 2006) tried to quantify the relative importance of quinone redox cycling in the decay of spruce wood by G. trabeum. They found the highest decrease of holocellulose polymerization degree in 1-week-old cultures. Highest concentrations of 2,5-DMHQ and 4,5-DMC (in total 40 μM) were found in the same time interval, but the concentrations decreased rapidly (7 μM in week 5) along with decreasing pH and increasing oxalate concentrations. The rate constants for the reactions of 2,5-DMHQ and 4,5-DMC with the Fe3+-oxalate complexes were determined as 43 and 65 L mol−1 s−1, respectively. Calculations showed that quinone cycling is responsible for a significant fraction of cellulose scissions (in average more than 25%). However, the results also showed that there are also hydroquinone-independent mechanisms for holocellulose cleavage during early decay (Suzuki et al., 2006).

Quinone redox cycling can only be efficient provided the quinones are rapidly reduced to hydroquinones. An intracellular NADH:quinone oxidoreductase capable of 2,5-DMBQ and 4,5-DMBQ reduction was originally isolated from G. trabeum (Jensen et al., 2002). The isolated enzyme has a pI of 3.3 and it is a homodimer of 22 kDa subunits, each with an FMN. The KM and kcat for 2,5-DMBQ and 4,5-DMBQ are 5–7 μM and 1100–1600 s−1, respectively. Because the sugar oxidases that can also use quinones as alternative electron acceptors (Leitner et al., 2001) were present in mycelial extracts, the enzyme is probably responsible for intracellular quinone reduction (Jensen et al., 2002). Later work showed that there are two quinone reductases of which QRD1 is produced during growth on wood when 2,5-DMBQ is present while QRD2 is probably involved in intracellular quinone detoxification (Cohen et al., 2004).

The main obstacle for the functioning of the proposed quinone cycling process can be the intracellular localization of quinone reductases (Fig. 2). For it to be efficient, rapid transfer from and into the fungal hyphae must be possible and the fungus has to deal with the toxic quinones in the cell. Because no quinone reduction was found in cell wall fraction of G. trabeum, one of the alternatives is that the redox potential of native cytoplasmic membrane can be responsible for quinone reduction. However, so far there are no clear indications that such a process really works (Qi & Jellison, 2004).

Extracts from wood colonized by several brown rot fungi showed high iron-reducing capability in a low-molecular-mass fraction (<5000 Da) that was significantly greater than in extracts from wood colonized by white rot or nondecay fungi (Goodell et al., 2006). Although the nature of these compounds is unclear, hydroquinones are among the potential candidates for the detected activity.

Although the relationship to cellulose degradation is not clear, the ability to reduce quinones was also detected in the white rot species. Phanerochaete chrysosporium produces intracellular benzoquinone reductases (Brock et al., 1995) and reduction of quinones by a plasma membrane redox system was also demonstrated in this species (Stahl et al., 1995). Furthermore, quinone redox cycling was demonstrated for Pleurotus eryngii in the context of superoxide anion radical production (Guillén et al., 1997). Potentially, this could lead to redox cycling-driven Fenton reactions in white-rot fungi.

Glycopeptide-catalysed Fenton reaction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Low molecular mass compounds termed ‘glycopeptides’ were first detected in brown rot fungi in 1980s (Enoki et al., 1989; Tanaka et al., 1991) and identified as compounds capable of catalyzing redox reactions between O2 and electron donors to produce •OH, reduce Fe3+ to Fe2+ and to strongly bind Fe2+ (Enoki et al., 1992; Tanaka et al., 1993, 1996; Hirano et al., 1995, 1997, 2000). These compounds were initially described as iron containing, with c. 22% neutral carbohydrate, 12% protein and a molecular mass of 1.5–5 kDa in F. palustris (Hirano et al., 1995), G. trabeum (Enoki et al., 1992) and Irpex lacteus (Tanaka et al., 1993). Later, larger glycopeptides have been described in white rot fungi Trametes versicolor and Phanerochaete chrysosporium where they were claimed to be the major source of •OH in wood degrading cultures (Tanaka et al., 1999a, b, c) and to act synergistically with phenol oxidases in lignin degradation (Tanaka et al., 1999b; Yamakawa et al., 2005).

The glycopeptide from F. palustris was reported to have a molecular mass between 7.2 and 12 kDa, and contain 54–61% protein (Enoki et al., 2003; Kaneko et al., 2004). Recently, the glycopeptide from Phanerochaete chrysosporium was characterized in more detail (Tanaka et al., 2007). The preparation had a molecular mass of about 14 kDa and contained 25% neutral carbohydrate and 0.04% Fe. Moreover, cDNAs and two putative genes encoding glycoproteins have been sequenced – the 875-bp glp1 and 864-bp glp2. The glycoprotein contained an 1-amino-1-deoxy-2-ketose (ketoamine) produced by the condensation of an amino acid side chain and a carbohydrate. By tautomerization, this structure can yield a 2,3-enediol (Fig. 3) that can reduce Fe3+ to Fe2+ and produce H2O2 from O2 (Oak et al., 2000).

image

Figure 3.  Glycopeptide-catalysed Fenton reaction. Modified from Enoki et al. (2003) and Yamakawa et al. (2005).

Download figure to PowerPoint

The size of glycopeptides does not allow them to penetrate the intact wood cell wall (Flournoy et al., 1991), and the reduction of their substrates thus probably occurs close to fungal hyphae although some diffusion into the cell wall was demonstrated (Hirano et al., 2000). To perform the complete catalytic cycle, oxidized saccharidic moieties of the glycopeptides have to be reduced again. This can potentially be performed by a cell wall-associated reductase that is most probably NADH-dependent (Enoki et al., 2003). However, the identity of the electron donor is not yet known.

Degradation of other plant cell wall material by cellulolytic enzymes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Nonspecificity of oxidation by hydroxyl radicals that can lead to modification or cleavage of both polysaccharides and lignin was discussed above. Also, some of the enzymes of the cellulolytic system exhibit broader specificity and can thus act in the degradation of hemicelluloses. Several basidiomycete endoglucanases can also act on hemicelluloses, galactomannan (Keilich et al., 1969), galactoglucomannan (Mansfield et al., 1998), or mannan (Henriksson et al., 1999). Xylan can be cleaved by endoglucanases from G. trabeum, Piptoporus betulinus and Sclerotium rolfsii (Lachke & Deshpande, 1988; Mansfield et al., 1998; Henriksson et al., 1999; Valášková & Baldrian, 2006b). Endoglucanases EG28 and EG38 from Phanerochaete chrysosporium are active on xylan, the latter even with higher kcat than for carboxymethylcellulose. Both of them are inactive on mannan. EG44 is inactive on xylan but degrades mannan with kcat comparable to carboxymethylcellulose (Lawoko et al., 2000). The enzyme Xyn10A firstly isolated as endoglucanase from G. trabeum is actually more active on xylan (Cohen et al., 2005). Among cellobiohydrolases, the enzymes from Dichomitus squalens showed some activity with xylan and o-nitrophenyl-β-d-xylobioside (Rouau & Odier, 1986) while CBH50 and CBH58 from Phanerochaete chrysosporium are inactive on xylan or mannan (Lawoko et al., 2000). The participation of Cellobiohydrolases (and cellulose-binding endoglucanases) in hemicellulose hydrolysis is, however, limited because they specifically bind to cellulose. β-Glucosidases generally exhibit low specificity and are able to cleave mannose, xylose or galactose units from oligosaccharides with varying affinity and the extracellular enzymes can thus participate in hemicellulose degradation.

Saprotrophic basidiomycetes, in particular, those growing on wood or litter, produce a rich array of hemicellulose-degrading enzymes (Baldrian, 2008). It can be anticipated that hemicellulolytic enzymes can also exhibit some activity against cellulose and in fact only a few have been explicitly reported not to act on cellulose or cellobiose. Endoxylanase from Termitomyces sp. is also active on carboxymethylcellulose (Faulet et al., 2006) and the endoxylanases XynA and XynC from Phanerochaete chrysosporium slowly cleave pNPC, although they are not active on carboxymethylcellulose (Decelle et al., 2004). Some endoglucanase activity was also demonstrated for G. trabeum endoxylanase (Ritschkoff et al., 1994). Similar to β-glucosidases, a broader substrate range can be also anticipated in β-glycosidases of other basidiomycetes. β-Galactosidase of Cryptococcus laurentii can be an example of enzyme active on pNPG and cellobiose (Ohtsuka et al., 1990).

Enzymatic vs. oxidative degradation of cellulose

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

Current results indicate that wood-rotting fungi possess two independent types of systems capable of internal scission of cellulose molecules – enzymatic and radical-based ones. Probably all wood-rotting fungi produce endoglucanases and a radical-based system. The radical-based systems share several features: they are based on redox chemistry and the produced hydroxyl radicals nonspecifically cleave cellulose and hemicelluloses and also modify (or perhaps also cleave) lignin. The CDH system is different in that redox reactions with cellobiose can occur extracellularly and are not dependent on NADH or other energy equivalents. This system is, however, dependent on the production of cellobiose and thus dependent on cellobiohydrolase. This is the main reason why brown rot fungi except Coniophora sp. cannot use it. Owing to the diffusible nature of oxidants produced, cleavage by radical-based systems can occur at larger distances from hyphae. If the pH near hyphal surfaces is low, generation of hydroxyl radicals occurs only at some distance with a higher pH value. The endocleavage of cellulose by endoglucanases is specific for cellulose, energy independent and, due to the enzyme size, it probably occurs immediately near hyphae. Moreover, a significant part of cellobiohydrolase which continues cellulose hydrolysis is localized in association with fungal hyphae (Valášková & Baldrian, 2006a).

In the case of the brown rot fungus Postia placenta, endoglucanases were only active on cellulose after oxidative pretreatment (Ratto et al., 1997), which suggests that the radical generation precedes the action of endoglucanases. This is in agreement with the fact that quinone redox cycling that occurs in this species is most active during initial wood degradation (Suzuki et al., 2006). On the other hand, CDH and endoglucanase are produced concomitantly (Henriksson et al., 2000). There is also as yet no explanation as to why some fungi produce more than one oxidative system; Phanerochaete chrysosporium apparently produces both CDH and glycopeptides and G. trabeum produces redox-active quinones and glycopeptides.

The answer to the question of why white rot fungi produce endoglucanases and do not use the combination of radical-based cellulose scission and cellobiose production by cellobiohydrolase lies probably in the differences in localization of degradation processes. We propose that enzymatic hydrolysis of cellulose has mainly or exclusively a nutritive role because it is localized near the hyphae. Endoglucanases probably generate more cuts in the cellulose chains accessible to cellobiohydrolases than do the nonspecific oxidative systems. The main role of the radical-based mechanisms can thus be in the structural degradation of wood to promote fungal colonization and resource capture. This is in agreement with the fact that •OH production during the growth of brown rot fungi on wood or cellulose is directly proportional to substrate mass loss (Kaneko et al., 2005). Where present, processive endoglucanases are involved in oligosaccharide liberation by brown rot fungi.

Future perspectives in the research on cellulose degradation by basidiomycetes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References

The research on cellulose degradation by basidiomycetes that has continued for several decades has reached considerable achievements in the description of the degradation system of the white rot fungus Phanerochaete chrysosporium as well as in the significant contribution to the mechanisms of brown rot decay. There are, however, still many fields where more efforts are needed in order to increase the understanding of the composition of cellulolytic systems, their regulation and ecological significance. One of the most important questions is how the brown rot fungi cope with the degradation of crystalline cellulose. It is not clear as to how abundant is the production of processive endoglucanases and which mechanisms of carbon acquisition from cellulose are used by the species unable to produce these enzymes. The finalization of the Postia placenta genome project in the near future will hopefully bring the possibility of highly sensitive proteomic analysis of the cellulolytic system of brown rot fungi and help us to answer some of the above questions.

The understanding of the mechanisms of radical-based cellulose degradation should be extended by the study of the relative importance of enzymatic/oxidative degradation and their physiological and ecological significance. Such quantitative studies would, however, require a very complex experimental setup.

Compared with wood degradation, the research on cellulose degradation by other groups of basidiomycetes, e.g. the soil saprotrophic species is far less developed. There are as yet no isolated enzymes, not to speak about the functioning of a complete cellulolytic system in the model species. The potential role of radical-generating mechanisms in soil saprotrophs remains to be addressed as well as the existence of nonligninolytic ‘brown rot type’ basidiomycetes in soils. To understand the contribution of basidiomycetes to the carbon cycling in the environment, the contribution of different fungal/microbial groups to the utilization of cellulose in different habitats should be quantified. This is impossible without linking specific producers to measured enzyme activities in natural substrates, e.g. wood, litter or soils, perhaps by transcriptome and proteome analyses. The development of methodology in the last few years gives us hope that the study of cellulose degradation by basidiomycetes in the future will offer a challenging work that will greatly increase our knowledge in the fields ranging from fungal physiology to ecosystem processes.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Degradation of cellulose using hydrolytic enzymes
  5. Endo-1,4-β-glucanase (EC 3.2.1.4, endocellulase)
  6. Cellobiohydrolase (CBH, EC 3.2.1.91; exocellulase)
  7. β-Glucosidase (EC 3.2.1.21)
  8. Phosphorolytic degradation of cello-oligosaccharides
  9. Oxidative decomposition of cellulose
  10. Cellobiose dehydrogenase (CDH; EC 1.1.99.18)
  11. Quinone redox cycling
  12. Glycopeptide-catalysed Fenton reaction
  13. Degradation of other plant cell wall material by cellulolytic enzymes
  14. Enzymatic vs. oxidative degradation of cellulose
  15. Future perspectives in the research on cellulose degradation by basidiomycetes
  16. Acknowledgements
  17. References
  • Akamatsu Y, Takahashi M & Shimada M (1994) Production of oxalic acid by wood-rotting basidiomycetes grown on low and high-nitrogen culture media. Mater Organism 28: 251264.
  • Ander P & Marzullo L (1997) Sugar oxidoreductases and veratryl alcohol oxidase as related to lignin degradation. J Biotechnol 53: 115131.
  • Ayers AR, Ayers SB & Eriksson KE (1978) Cellobiose oxidase, purification and partial characterization of a hemoprotein from Sporotrichum pulverulentum. Eur J Biochem 90: 171181.
  • Bailey PJ, Liese W, Roesch R, Keilich G & Afting EG (1969) Cellulase (beta-1,4-glucan 4-glucanohydrolase) from wood-degrading fungus Polyporus schweinitzii Fr. I. Purification. Biochim Biophys Acta 185: 381391.
  • Baldrian P (2008) Enzymes of Saprotrophic Basidiomycetes. Ecology of Saprotrophic Basidiomycetes (BoddyL, FranklandJ & VanWestP, eds), pp. 1941. Academic Press, New York.
  • Baminger U, Subramaniam SS, Renganathan V & Haltrich D (2001) Purification and characterization of cellobiose dehydrogenase from the plant pathogen Sclerotium (Athelia) rolfsii. Appl Environ Microbiol 67: 17661774.
  • Bao WJ & Renganathan V (1992) Cellobiose oxidase of Phanerochaete chrysosporium enhances crystalline cellulose degradation by cellulases. FEBS Lett 302: 7780.
  • Bao WJ, Usha SN & Renganathan V (1993) Purification and characterization of cellobiose dehydrogenase, a novel extracellular hemoflavoenzyme from the white-rot fungus Phanerochaete chrysosporium. Archiv Biochem Biophys 300: 705713.
  • Bhattacharjee B, Roy A & Majumder AL (1992) Beta-glucosidase of a white-rot fungus Trametes gibbosa. Biochem Intl 28: 783793.
  • Bhattacharjee B, Roy A & Majumder AL (1993) Carboxymethylcellulase from Lenzites saepiaria, a brown-rotter. Biochem Mol Biol Intl 30: 11431152.
  • Branchaud BP (1999) Free radicals as a result of dioxygen metabolism. Metal Ions in Biological Systems, Vol. 36 (SigelA & SigelH, eds), pp. 79102. Marcel Dekker, New York.
  • Brock BJ, Rieble S & Gold MH (1995) Purification and characterization of a 1,4-benzoquinone reductase from the basidiomycete Phanerochaete chrysosporium. Appl Environ Microbiol 61: 30763081.
  • Buettner GR (1993) The pecking order of free radicals and antioxidants – lipid peroxidation, alpha tocopherol, and ascorbate. Archiv Biochem Biophys 300: 535543.
  • Burke RM & Cairney JWG (1998) Carbohydrate oxidases in ericoid and ectomycorrhizal fungi: a possible source of Fenton radicals during the degradation of lignocellulose. New Phytol 139: 637645.
  • Cai YJ, Buswell JA & Chang ST (1998) beta-Glucosidase components of the cellulolytic system of the edible straw mushroom, Volvariella volvacea. Enzyme Microb Technol 22: 122129.
  • Cai YJ, Chapman SJ, Buswell JA & Chang ST (1999) Production and distribution of endoglucanase, cellobiohydrolase, and beta-glucosidase components of the cellulolytic system of Volvariella volvacea, the edible straw mushroom. Appl Environ Microbiol 65: 553559.
  • Cao WG & Crawford DL (1993) Purification and some properties of beta-glucosidase from the ectomycorrhizal fungus Pisolithus tinctorius Strain Smf. Can J Microbiol 39: 125129.
  • Chandhoke V, Goodell B, Jellison J & Fekete FA (1992) Oxidation of 2-keto-4-thiomethylbutyric acid (KTBA) by iron-binding compounds produced by the wood-decaying fungus Gloeophyllum trabeum. FEMS Microbiol Lett 90: 263266.
  • Clausen CA (1995) Dissociation of the multienzyme complex of the brown-rot fungus Postia placenta. FEMS Microbiol Lett 127: 7378.
  • Cohen R, Jensen KA, Houtman CJ & Hammel KE (2002) Significant levels of extracellular reactive oxygen species produced by brown rot basidiomycetes on cellulose. FEBS Lett 531: 483488.
  • Cohen R, Suzuki MR & Hammel KE (2004) Differential stress-induced regulation of two quinone reductases in the brown rot basidiomycete Gloeophyllum trabeum. Appl Environ Microbiol 70: 324331.
  • Cohen R, Suzuki MR & Hammel KE (2005) Processive endoglucanase active in crystalline cellulose hydrolysis by the brown rot basidiomycete Gloeophyllum trabeum. Appl Environ Microbiol 71: 24122417.
  • Copa-Patino JL & Broda P (1994) A Phanerochaete chrysosporium beta-d-glucosidase/beta-d-xylosidase with specificity for (1[RIGHTWARDS ARROW]3)-beta-d-glucan linkages. Carbohydr Res 253: 265275.
  • Decelle B, Tsang A & Storms RK (2004) Cloning, functional expression and characterization of three Phanerochaete chrysosporium endo-1,4-beta-xylanases. Curr Genet 46: 166175.
  • Deshpande V, Eriksson KE & Pettersson B (1978) Production, purification and partial characterization of 1,4-beta-glucosidase enzymes from Sporotrichum pulverulentum. Eur J Biochem 90: 191198.
  • Ding SJ, Ge W & Buswell JA (2001) Endoglucanase I from the edible straw mushroom, Volvariella volvacea– Purification, characterization, cloning and expression. Eur J Biochem 268: 56875695.
  • Ding SJ, Ge W & Buswell JA (2002) Secretion, purification and characterisation of a recombinant Volvariella volvacea endoglucanase expressed in the yeast Pichia pastoris. Enzyme Microb Technol 31: 621626.
  • Dutton MV, Evans CS, Atkey PT & Wood DA (1993) Oxalate production of basidiomycetes including the white rot species Coriolus versicolor and Phanerochaete chrysosporium. Appl Microbiol Biotechnol 39: 510.
  • Ek M, Gierer J & Jansbo K (1989) Study on the selectivity of bleaching with oxygen-containing species. Holzforschung 43: 391396.
  • Enoki A, Tanaka H & Fuse G (1989) Relationship between degradation of wood and production of H2O2-producing or one-electron oxidases by brown-rot fungi. Wood Sci Technol 23: 112.
  • Enoki A, Hirano T & Tanaka H (1992) Extracellular substance from the brown rot basidiomycete Gloeophyllum trabeum that produces and reduces hydrogen peroxide. Mater Organism 27: 247261.
  • Enoki A, Tanaka H & Itakura S (2003) Physical and chemical characteristics of glycopeptide from wood decay fungi. Wood Deterioration and Preservation (GoodellB, NicholasDD & SchultzTP, eds), pp. 140153. Oxford University Press, Washington, DC.
  • Eriksson KE & Pettersson B (1975a) Extracellular enzyme system utilized by fungus Sporotrichum pulverulentum (Chrysosporium lignorum) for breakdown of cellulose. 1. Separation, purification and physicochemical characterization of 5 endo-1,4-beta-glucanases. Eur J Biochem 51: 193206.
  • Eriksson KE & Pettersson B (1975b) Extracellular enzyme system utilized by fungus Sporotrichum pulverulentum (Chrysosporium lignorum) for breakdown of cellulose. 3. Purification and physico-chemical characterization of an exo-1,4-beta-glucanase. Eur J Biochem 51: 213218.
  • Espejo E & Agosin E (1991) Production and degradation of oxalic acid by brown rot fungi. Appl Environ Microbiol 57: 19801986.
  • Evans CS (1985) Properties of the beta-d-glucosidase (cellobiase) from the wood-rotting fungus, Coriolus versicolor. Appl Microbiol Biotechnol 22: 128131.
  • Fang J, Liu W & Gao PJ (1998) Cellobiose dehydrogenase from Schizophyllum commune: purification and study of some catalytic, inactivation, and cellulose-binding properties. Archiv Biochem Biophy 353: 3746.
  • Faulet BM, Niamke S, Gonnety JT & Kouame LP (2006) Purification and biochemical properties of a new thermostable xylanase from symbiotic fungus, Termitomyces sp. Afric J Biotechnol 5: 273282.
  • Fekete FA, Chandhoke V & Jellison J (1989) Iron binding compounds produced by wood decaying basidiomycetes. Appl Environ Microbiol 55: 27202722.
  • Ferraz A, Parra C, Freer J, Baeza J & Rodriguez J (2001) Occurrence of iron-reducing compounds in biodelignified “palo podrido” wood samples. Intl Biodeter Biodegrad 47: 203208.
  • Filley TR, Cody GD, Goodell B, Jellison J, Noser C & Ostrofsky A (2002) Lignin demethylation and polysaccharide decomposition in spruce sapwood degraded by brown rot fungi. Org Geochem 33: 111124.
  • Flournoy DS, Kirk TK & Highley TL (1991) Wood decay by brown rot fungi – changes in pore structure and cell wall volume. Holzforschung 45: 383388.
  • Garzillo AMV, Dipaolo S, Ruzzi M & Buonocore V (1994) Hydrolytic properties of extracellular cellulases from Pleurotus ostreatus. Appl Microbiol Biotechnol 42: 476481.
  • Gilad R, Rabinovich L, Yaron S, Bayer EA, Lamed R, Gilbert HJ & Shoham Y (2003) Ce1I, a noncellulosomal family 9 enzyme from Clostridium thermocellum, is a processive endoglucanase that degrades crystalline cellulose. J Bacteriol 185: 391398.
  • Goksoyr J & Eriksen J (1980) Cellulases. Microbial Enzymes and Bioconversions (RoseAH, ed), pp. 283330. Academic Press, London.
  • Goodell B (2003) Brown-rot fungal degradation of wood: our evolving view. Wood Deterior Preserv 845: 97118.
  • Goodell B, Jellison J, Liu J et al. (1997) Low molecular weight chelators and phenolic compounds isolated from wood decay fungi and their role in the fungal biodegradation of wood. J Biotechnol 53: 133162.
  • Goodell B, Qian Y, Jellison J, Richard M & Qi W (2002) Lignocellulose oxidation by low molecular weight metal-binding compounds isolated from wood degrading fungi: a comparison of brown rot and white rot systems and the potential application of chelator-mediated Fenton reactions. Progress in Biotechnology 21, Biology in the Pulp and Paper Industry (ViikariL & LanttoR, eds), pp. 3747. Elsevier, New York.
  • Goodell B, Daniel G, Jellison J & Qian YH (2006) Iron-reducing capacity of low-molecular-weight compounds produced in wood by fungi. Holzforschung 60: 630636.
  • Green F, Larsen MJ, Winandy JE & Highley TL (1991) Role of oxalic acid in incipient brown rot decay. Mater Organism 26: 191213.
  • Guillén F, Martínez AT & Martínez MJ (1990) Production of hydrogen peroxide by aryl alcohol oxidase from the ligninolytic fungus Pleurotus eryngii. Appl Microbiol Biotechnol 32: 465469.
  • Guillén F, Martínez MJ, Munoz C & Martínez AT (1997) Quinone redox cycling in the ligninolytic fungus Pleurotus eryngii leading to extracellular production of superoxide anion radical. Archiv Biochem Biophys 339: 190199.
  • Guillén F, Munoz C, Gomez-Toribio V, Martínez AT & Martínez MJ (2000) Oxygen activation during oxidation of methoxyhydroquinones by laccase from Pleurotus eryngii. Appl Environ Microbiol 66: 170175.
  • Hai PQ, Nozaki K, Amano Y & Kanda T (2000) Purification and characterization of cellobiose dehydrogenase from Irpex lacteus and its adsorption on cellulose. J Appl Glycosci 47: 311318.
  • Hallberg BM, Henriksson G, Pettersson G & Divne C (2002) Crystal structure of the flavoprotein domain of the extracellular flavocytochrome cellobiose dehydrogenase. J Mol Biol 315: 421434.
  • Halliwell B & Gutteridge JMC (1999) Free Radicals in Biology and Medicine. Oxford University Press, Oxford.
  • Halliwell G (1965) Catalytic decomposition of cellulose under biological conditions. Biochem J 95: 3541.
  • Hamada N, Ishikawa K, Fuse N et al. (1999) Purification, characterization and gene analysis of exo-cellulase II (Ex-2) from the white rot basidiomycete Irpex lacteus. J Biosci Bioeng 87: 442451.
  • Hammel KE, Kapich AN, Jensen KA & Ryan ZC (2002) Reactive oxygen species as agents of wood decay by fungi. Enzyme Microb Technol 30: 445453.
  • Henriksson G, Pettersson G, Johansson G, Ruiz A & Uzcategui E (1991) Cellobiose oxidase from Phanerochaete chrysosporium can be cleaved by papain into 2 domains. Eur J Biochem 196: 101106.
  • Henriksson G, Johansson G & Pettersson G (1993) Is cellobiose oxidase from Phanerochaete chrysosporium a one-electron reductase. Biochim Biophys Acta 1144: 184190.
  • Henriksson G, Ander P, Pettersson B & Pettersson G (1995) Cellobiose dehydrogenase (cellobiose oxidase) from Phanerochaete chrysosporium as a wood degrading enzyme – studies on cellulose, xylan and synthetic lignin. Appl Microbiol Biotechnol 42: 790796.
  • Henriksson G, Salumets A, Divne C & Pettersson G (1997) Studies of cellulose binding by cellobiose dehydrogenase and a comparison with cellobiohydrolase 1. Biochem J 324: 833838.
  • Henriksson G, Sild V, Szabo IJ, Pettersson G & Johansson G (1998) Substrate specificity of cellobiose dehydrogenase from Phanerochaete chrysosporium. Biochim Biophys Acta – Protein Struct Mol Enzymol 1383: 4854.
  • Henriksson G, Nutt A, Henriksson H, Pettersson B, Stahlberg J, Johansson G & Pettersson G (1999) Endoglucanase 28 (cel12A), a new Phanerochaete chrysosporium cellulase. Eur J Biochem 259: 8895.
  • Henriksson G, Johansson G & Pettersson G (2000) A critical review of cellobiose dehydrogenases. J Biotechnol 78: 93113.
  • Herr D, Baumer F & Dellweg H (1978a) Purification and properties of an extracellular endo-1,4-beta-glucanase from Lenzites trabea. Archiv Microbiol 117: 287292.
  • Herr D, Baumer F & Dellweg H (1978b) Purification and properties of an extracellular beta-glucosidase from Lenzites trabea. Eur J Appl Microbiol Biotechnol 5: 2936.
  • Hirano T, Tanaka H & Enoki A (1995) Extracellular substance from the brown rot basidiomycete Tyromyces palustris that reduces molecular oxygen to hydroxyl radicals and ferric iron to ferrous iron. Mokuzai Gakkaishi 41: 334341.
  • Hirano T, Tanaka H & Enoki A (1997) Relationship between production of hydroxyl radicals and degradation of wood by the brown-rot fungus, Tyromyces palustris. Holzforschung 51: 389395.
  • Hirano T, Enoki A & Tanaka H (2000) Immunogold labeling of an extracellular substance producing hydroxyl radicals in wood degraded by brown-rot fungus Tyromyces palustris. J Wood Sci 46: 4551.
  • Hishida A, Suzuki T, Iijima T & Higaki M (1997) An extracellular cellulase of the brown-rot fungus, Tyromyces palustris. Mokuzai Gakkaishi 43: 686691.
  • Hon DNS (1994) Cellulose: a random walk along its historical path. Cellulose 1: 125.
  • Hüttermann A & Volger C (1973) Cellobiose phosphorylase in Fomes annosus. Nature – New Biol 245: 64.
  • Hyde SM & Wood PM (1997) A mechanism for production of hydroxyl radicals by the brown-rot fungus Coniophora puteana: Fe(III) reduction by cellobiose dehydrogenase and Fe(II) oxidation at a distance from the hyphae. Microbiology-UK 143: 259266.
  • Idogaki H & Kitamoto Y (1992) Purification and some properties of a carboxymethyl cellulase from Coriolus versicolor. Biosci Biotechnol Biochem 56: 970971.
  • Igarashi K, Samejima M & Eriksson KEL (1998) Cellobiose dehydrogenase enhances Phanerochaete chrysosporium cellobiohydrolase I activity by relieving product inhibition. Eur J Biochem 253: 101106.
  • Igarashi K, Tani T, Kawai R & Samejima M (2003) Family 3 beta-glucosidase from cellulose-degrading culture of the white-rot fungus Phanerochaete chrysosporium is a glucan 1,3-beta-glucosidase. J Biosci Bioeng 95: 572576.
  • Ishihara H, Imamura K, Kita M, Aimi T & Kitamoto Y (2005) Enhancement of the viscometric endocellulase activity of Polyporus arcularius CMCase IIIa by cellobiose and cellooligosaccharides. Mycoscience 46: 148153.
  • Ishihara M & Shimizu K (1984) Purification and properties of two extracellular endo-cellulases from the brown-rotting fungus Tyromyces palustris. Mokuzai Gakkaishi 30: 7987.
  • Ishikawa E, Sakai T, Ikemura H, Matsumoto K & Abe H (2005) Identification, cloning, and characterization of a Sporobolomyces singularis beta-galactosidase-like enzyme involved in galacto-oligosaccharide production. J Biosci Bioeng 99: 331339.
  • Jellison J, Chandhoke V, Goodell B & Fekete FA (1991) The isolation and immunolocalization of iron-binding compounds produced by Gloeophyllum trabeum. Appl Microbiol Biotechnol 35: 805809.
  • Jensen KA, Houtman CJ, Ryan ZC & Hammel KE (2001) Pathways for extracellular Fenton chemistry in the brown rot basidiomycete Gloeophyllum trabeum. Appl Environ Microbiol 67: 27052711.
  • Jensen KA, Ryan ZC, Wymelenberg AV, Cullen D & Hammel KE (2002) An NADH: quinone oxidoreductase active during biodegradation by the brown-rot basidiomycete Gloeophyllum trabeum. Appl Environ Microbiol 68: 26992703.
  • Jimenez M, Gonzalez AE, Martinez MJ, Martinez AT & Dale BE (1991) Screening of yeasts isolated from decayed wood for lignocellulose-degrading enzyme activities. Mycolog Res 95: 12991302.
  • Kajisa T, Yoshida M, Igarashi K, Katayama A, Nishino T & Samejima M (2004) Characterization and molecular cloning of cellobiose dehydrogenase from the brown-rot fungus Coniophora puteana. J Biosci Bioeng 98: 5763.
  • Kamper J, Kahmann R, Bolker M et al. (2006) Insights from the genome of the biotrophic fungal plant pathogen Ustilago maydis. Nature 444: 97101.
  • Kanda T & Nisizawa K (1988) Exocellulase of Irpex lacteus (Polyporus tulipiferae). Methods Enzymol 160: 403408.
  • Kanda T, Wakabayashi K & Nisizawa K (1976) Purification and properties of an endocellulase of avicelase type from Irpex lacteus (Polyporus tulipiferae). J Biochem 79: 977988.
  • Kanda T, Wakabayashi K & Nisizawa K (1980) Purification and properties of a lower molecular weight endo-cellulase from Irpex lacteus (Polyporus tulipiferae). J Biochem 87: 16251634.
  • Kanda T, Yatomi H, Makishima S, Amano Y & Nisizawa K (1989) Substrate specificities of exo-type and endo-type cellulases in the hydrolysis of beta-(1[RIGHTWARDS ARROW]3)-mixed and beta-([RIGHTWARDS ARROW]4)-mixed d-glucans. J Biochem 105: 127132.
  • Kaneko S, Hirano T, Tanaka H, Itakura S & Enoki A (2004) Physical and chemical properties of an extracellular low-molecular-weight substance from the brown-rot basidiomycete Fomitopsis palustris. Biocontr Sci 9: 1115.
  • Kaneko S, Yoshitake K, Itakura S, Tanaka H & Enoki A (2005) Relationship between production of hydroxyl radicals and degradation of wood, crystalline cellulose, and a lignin-related compound or accumulation of oxalic acid in cultures of brown-rot fungi. J Wood Sci 51: 262269.
  • Keilich G, Bailey PJ, Afting EG & Liese W (1969) Cellulase (beta-I,4-glucan 4-glucanohydrolase) from wood degrading fungus Polyporus schweinitzii Fr. 2. Characterization. Biochim Biophys Acta 185: 392401.
  • Kelley RL & Reddy CA (1986) Identification of glucose oxidase activity as the primary source of hydrogen peroxide production in ligninolytic cultures of Phanerochaete chrysosporium. Archiv Microbiol 144: 248253.
  • Keppler F, Eiden R, Niedan V, Pracht J & Scholer HF (2000) Halocarbons produced by natural oxidation processes during degradation of organic matter. Nature 403: 298301.
  • Kerem Z, Jensen KA & Hammel KE (1999) Biodegradative mechanism of the brown rot basidiomycete Gloeophyllum trabeum: evidence for an extracellular hydroquinone-driven Fenton reaction. FEBS Lett 446: 4954.
  • Kersten P & Cullen D (2007) Extracellular oxidative systems of the lignin-degrading basidiomycete Phanerochaete chrysosporium. Fungal Genet Biol 44: 7787.
  • Kersten PJ & Kirk TK (1987) Involvement of a new enzyme, glyoxal oxidase, in extracellular H2O2 production by Phanerochaete chrysosporium. J Bacteriol 169: 21952201.
  • Kirk TK, Ibach R, Mozuch MD, Conner AH & Highley TL (1991) Characteristics of cotton cellulose depolymerized by a brown-rot fungus, by acid, or by chemical oxidants. Holzforschung 45: 239244.
  • Kitaoka M & Hayashi K (2002) Carbohydrate-processing phosphorolytic enzymes. Trends Glycosci Glycotechnol 14: 3550.
  • Kleman-Leyer K & Kirk TK (1994) Three native cellulose-depolymerizing endoglucanases from solid-dubstrate cultures of the brown-rot fungus Meruliporia (Serpula) incrassata. Appl Environ Microbiol 60: 28392845.
  • Koenigs JW (1974) Hydrogen peroxide and iron: a proposed system for decomposition of wood by brown-rot basidiomycetes. Wood Fiber 6: 6679.
  • Kremer SM & Wood PM (1992) Production of fenton reagent by cellobiose oxidase from cellulolytic cultures of Phanerochaete chrysosporium. Eur J Biochem 208: 807814.
  • Kubicek CP, Messner R, Gruber F, Mandels M & Kubicekpranz EM (1993) Triggering of cellulase biosynthesis by cellulose in Trichoderma reesei– Involvement of a constitutive, sophorose-inducible, glucose-inhibited beta-diglucoside permease. J Biol Chem 268: 1936419368.
  • Kubo K & Nisizawa K (1983) Purification and properties of 2 endo-type cellulases from Irpex lacteus (Polyporus tulipiferae). J Ferment Technol 61: 383389.
  • Kurek B & Gaudard F (2000) Oxidation of spruce wood sawdust by MnO2 plus oxalate: a biochemical investigation. J Agri Food Chem 48: 30583062.
  • Kusuda M, Ueda M, Konishi Y et al. (2006) Detection of β-glucosidase as saprotrophic ability from an ectomycorrhizal mushroom, Tricholoma matsutake. Mycoscience 47: 184189.
  • Lachke AH & Deshpande MV (1988) Sclerotium rolfsii– status in cellulase research. FEMS Microbiol Rev 54: 177194.
  • Lawoko M, Nutt A, Henriksson H, Gellerstedt G & Henriksson G (2000) Hemicellulase activity of aerobic fungal cellulases. Holzforschung 54: 497500.
  • Leitner C, Volc J & Haltrich D (2001) Purification and characterization of pyranose oxidase from the white rot fungus Trametes multicolor. Appl Environ Microbiol 67: 36363644.
  • Lo AC, Willick G, Bernier R & Desrochers M (1988) Purification and assay of beta-glucosidase from Schizophyllum commune. Methods Enzymol 160: 432437.
  • Loftus BJ, Fung E, Roncaglia P et al. (2005) The genome of the basidiomycetous yeast and human pathogen Cryptococcus neoformans. Science 307: 13211324.
  • Ludwig R, Salamon A, Varga J, Zamocky M, Peterbauer CK, Kulbe KD & Haltrich D (2004) Characterisation of cellobiose dehydrogenases from the white-rot fungi Trametes pubescens and Trametes villosa. Appl Microbiol Biotechnol 64: 213222.
  • Lymar ES, Li B & Renganathan V (1995) Purification and characterization of a cellulose-binding beta-glucosidase from cellulose-degrading cultures of Phanerochaete chrysosporium. Appl Environ Microbiol 61: 29762980.
  • Lynd LR, Weimer PJ, Van Zyl WH & Pretorius IS (2002) Microbial cellulose utilization: fundamentals and biotechnology. Microbiol Mol Biol Rev 66: 506577.
  • Magalhaes PO, Ferraz A & Milagres AFM (2006) Enzymatic properties of two beta-glucosidases from Ceriporiopsis subvermispora produced in biopulping conditions. J Appl Microbiol 101: 480486.
  • Maijala P, Fagerstedt KV & Raudaskoski M (1991) Detection of extracellular cellulolytic and proteolytic activity in ectomycorrhizal fungi and Heterobasidion annosum (Fr) Bref. New Phytol 117: 643648.
  • Mansfield SD, DeJong E & Saddler JN (1997) Cellobiose dehydrogenase, an active agent in cellulose depolymerization. Appl Environ Microbiol 63: 38043809.
  • Mansfield SD, Saddler JN & Gübitz GM (1998) Characterization of endoglucanases from the brown rot fungi Gloeophyllum sepiarium and Gloeophyllum trabeum. Enzyme Microb Technol 23: 133140.
  • Martinez D, Larrondo LF, Putnam N et al. (2004) Genome sequence of the lignocellulose degrading fungus Phanerochaete chrysosporium strain RP78. Nature Biotechnol 22: 695700.
  • Micales JA & Highley TL (1991) Factors associated with decay capacity of the brown-rot fungus Postia placenta. Biodeterioration Research III (LlewllynGG & ORearCE, eds), pp. 285302. Plenum, New York.
  • Middelhoven WJ (2006) Polysaccharides and phenolic compounds as substrate for yeasts isolated from rotten wood and description of Cryptococcus fagi sp. nov. Antonie Van Leeuwenhoek Intl J Gen Mol Microbiol 90: 5767.
  • Morais H, Ramos C, Matos N et al. (2002) Liquid chromatographic and electrophoretic characterisation of extracellular beta-glucosidase of Pleurotus ostreatus grown in organic waste. J Chromatogr B – Analyt Technol Biomed Life Sci 770: 111119.
  • Morpeth FF (1985) Some properties of cellobiose oxidase from the white rot fungus Sporotrichum pulverulentum. Biochem J 228: 557564.
  • Mucha J, Dahm H, Strzelczyk E & Werner A (2006) Synthesis of enzymes connected with mycoparasitism by ectomycorrhizal fungi. Archiv Microbiol 185: 6977.
  • Muheim A, Waldner R, Leisola MSA & Fiechter A (1990) An extracellular aryl alcohol oxidase from the white rot fungus Bjerkandera adusta. Enzyme and Microbial Technology 12: 204209.
  • Munoz IG, Ubhayasekera W, Henriksson H et al. (2001) Family 7 cellobiohydrolases from Phanerochaete chrysosporium: CRYSTAL structure of the catalytic module of Cel7D (CBH58) at 1.32 angstrom resolution and homology models of the isozymes. J Mol Biol 314: 10971111.
  • Nakagame S, Furujyo A & Sugiura J (2006) Purification and characterization of cellobiose dehydrogenase from white-rot basidiomycete Trametes hirsuta. Biosci Biotechnol Biochem 70: 16291635.
  • Newcombe D, Paszczynski A, Gajewska W, Kroger M, Feis G & Crawford R (2002) Production of small molecular weight catalysts and the mechanism of trinitrotoluene degradation by several Gloeophyllum species. Enzyme Microb Technol 30: 506517.
  • Nijikken Y, Tsukada T, Igarashi K, Samejima M, Wakagi T, Shoun H & Fushinobu S (2007) Crystal structure of intracellular family 1 [beta]-glucosidase BGL1A from the basidiomycete Phanerochaete chrysosporium. FEBS Lett 581: 15141520.
  • Nutt A, Salumets A, Henriksson G, Sild V & Johansson G (1997) Conversion of O2 species by cellobiose dehydrogenase (cellobiose oxidase) and glucose oxidase – a comparison. Biotechnol Lett 19: 379383.
  • Oak J-H, Nakagawa K & Miyazawa T (2000) Synthetically prepared Amadori-glycated phosphatidylethanolamine can trigger lipid peroxidation via free radical reactions. FEBS Lett 481: 2630.
  • Ohtsuka K, Tanoh A, Ozawa O, Kanematsu T, Uchida T & Shinke R (1990) Purification and properties of a beta-galactosidase with high galactosyl transfer activity from Cryptococcus laurentii Okn-4. J Ferment Bioeng 70: 301307.
  • Oikawa T, Tsukagawa Y & Soda K (1998) Endo-beta-glucanase secreted by a psychrotrophic yeast: purification and characterization. Biosci Biotechnol Biochem 62: 17511756.
  • Onishi N & Tanaka T (1996) Purification and properties of a galacto- and gluco-oligosaccharide-producing beta-glycosidase from Rhodotorula minuta IFO879. J Ferment Bioeng 82: 439443.
  • Osore H & Okech MA (1983) The partial purification and some properties of cellulase and beta-glucosidase of Termitomyces conidiophores and fruit bodies. J Appl Biochem 5: 172179.
  • Park JSB, Wood PM, Davies MJ, Gilbert BC & Whitwood AC (1997) A kinetic and ESR investigation of iron(II) oxalate oxidation by hydrogen peroxide and dioxygen as a source of hydroxyl radicals. Free Radical Res 27: 447458.
  • Park JSB, Wood PM, Gilbert BC & Whitwood AC (1999) EPR Evidence for hydroxyl- and substrate-derived radicals in Fe(II)-oxalate/hydrogen peroxide reactions. The importance of the reduction of Fe(III)-oxalate by oxygen-conjugated radicals to regenerate Fe(II) in reactions of carbohydrates and model compounds. J Chem Soc – Perkin Trans 2: 923931.
  • Paszczynski A, Crawford R, Funk D & Goodell B (1999) De novo synthesis of 4,5-dimethoxycatechol and 2,5-dimethoxyhydroquinone by the brown rot fungus Gloeophyllum trabeum. Appl Environ Microbiol 65: 674679.
  • Patil RV & Sadana JC (1984) The purification and properties of (14)-beta-d-glucan cellobiohydrolase from Sclerotium rolfsii– substrate specificity and mode of action. Can J Biochem Cell Biol 62: 920926.
  • Peciarova A & Biely P (1982) Beta-xylosidases and a nonspecific wall-bound beta-glucosidase of the yeast Cryptococcus albidus. Biochim Biophy Acta 716: 391399.
  • Pettersson G & Porath J (1963) Studies on cellulolytic enzymes. 2. Multiplicity of cellulolytic enzymes of Polyporus versicolor. Biochim Biophys Acta 67: 915.
  • Qi WH & Jellison J (2004) Characterization of a transplasma membrane redox system of the brown rot fungus Gloeophyllum trabeum. Intl Biodeter Biodegrad 53: 3742.
  • Ratto M, Ritschkoff AC & Viikari L (1997) The effect of oxidative pretreatment on cellulose degradation by Poria placenta and Trichoderma reesei cellulases. Appl Microbiol Biotechnol 48: 5357.
  • Reese ET & Levinson HS (1952) A comparative study of the breakdown of cellulose by microorganisms. Physiolog Plantar 5: 345366.
  • Renganathan V, Usha SN & Lindenburg F (1990) Cellobiose-oxidizing enzymes from the lignocellulose-degrading basidiomycete Phanerochaete chrysosporium– interaction with microcrystalline cellulose. Appl Microbiol Biotechnol 32: 609613.
  • Ritschkoff AC & Viikari L (1991) The production of extracellular hydrogen peroxide by brown rot fungi. Mater Organism 26: 157167.
  • Ritschkoff AC, Buchert J & Viikari L (1994) Purification and characterization of a thermophilic xylanase from the brown rot fungus Gloeophyllum trabeum. J Biotechnol 32: 6774.
  • Rouau X & Odier E (1986) Purification and properties of 2 enzymes from Dichomitus squalens which exhibit both cellobiohydrolase and xylanase activity. Carbohydr Res 145: 279292.
  • Rouland C, Civas A, Renoux J & Petek F (1988) Purification and properties of cellulases from the termite Macrotermes mulleri (Termitidae, Macrotermitinae) and its symbiotic fungus Termitomyces sp. Compar Biochem Physiol B – Biochem Mol Biol 91: 449458.
  • Roy BP, Dumonceaux T, Koukoulas AA & Archibald FS (1996) Purification and characterization of cellobiose dehydrogenases from the white rot fungus Trametes versicolor. Appl Environ Microbiol 62: 44174427.
  • Sadana JC & Patil RV (1988a) Cellobiose dehydrogenase from Sclerotium rolfsii. Methods Enzymol 160: 448454.
  • Sadana JC & Patil RV (1988b) 1,4-beta-d-glucan cellobiohydrolase from Sclerotium rolfsii. Methods Enzymol 160: 307314.
  • Sadana JC, Lachke AH & Patil RV (1984) Endo-(1-4)-beta-d-glucanases from Sclerotium rolfsii– purification, substrate specificity, and mode of action. Carbohydr Res 133: 297312.
  • Sadana JC, Patil RV & Shewale JG (1988) Beta-d-glucosidases from Sclerotium rolfsii. Methods Enzymol 160: 424431.
  • Sato S, Liu FHK & Tien M (2007) Expression analysis of extracellular proteins from Phanerochaete chrysosporium grown on different liquid and solid substrates. Microbiology 153: 30233033.
  • Schmidhalter DR & Canevascini G (1992) Characterization of the cellulolytic enzyme system from the brown rot fungus Coniophora puteana. Appl Microbiol Biotechnol 37: 431436.
  • Schmidhalter DR & Canevascini G (1993a) Purification and characterization of 2 exocellobiohydrolases from the brown rot fungus Coniophora puteana (Schum Ex-Fr) Karst. Archiv Biochem Biophys 300: 551558.
  • Schmidhalter DR & Canevascini G (1993b) Isolation and characterization of the cellobiose dehydrogenase from the brown rot fungus Coniophora puteana (Schum Ex-Fr) Karst. Archiv Biochem Biophys 300: 559563.
  • Sengupta S, Ghosh AK & Sengupta S (1991) Purification and characterization of a beta-glucosidase (cellobiase) from a mushroom Termitomyces clypeatus. Biochim Biophys Acta 1076: 215220.
  • Shewale JG & Sadana J (1981) Purification, characterization, and properties of beta-glucosidase enzymes from Sclerotium rolfsii. Archiv Biochem Biophys 207: 185196.
  • Shimada M, Akamtsu Y, Tokimatsu T, Mii K & Hattori T (1997) Possible biochemical roles of oxalic acid as a low molecular weight compound involved in brown-rot and white-rot wood decays. J Biotechnol 53: 103113.
  • Shimokawa T, Nakamura M, Hayashi N & Ishihara M (2004) Production of 2,5-dimethoxyhydroquinone by the brown-rot fungus Serpula lacrymans to drive extracellular Fenton reaction. Holzforschung 58: 305310.
  • Sigoillot C, Lomascolo A, Record E, Robert JL, Asther M & Sigoillot JC (2002) Lignocellulolytic and hemicellulolytic system of Pycnoporus cinnabarinus: isolation and characterization of a cellobiose dehydrogenase and a new xylanase. Enzyme Microb Technol 31: 876883.
  • Sison Jr & Schubert WJ (1958) On the mechanism of enzyme action. LXVIII. The cellobiase component of the cellulolytic enzyme system of Poria vaillantii. Archiv Biochem Biophys 78: 563572.
  • Smith MH & Gold MH (1979) Phanerochaete chrysosporiumβ-glucosidases: induction, cellular localization, and physical characterization. Appl Environ Microbiol 37: 938942.
  • Stahl JD, Rasmussen SJ & Aust SD (1995) Reduction of quinones and radicals by a plasma membrane redox system of Phanerochaete chrysosporium. Archiv Biochem Biophys 322: 221227.
  • Steffen KT, Cajthaml T, Šnajdr J & Baldrian P (2007) Differential degradation of oak (Quercus petraea) leaf litter by litter-decomposing basidiomycetes. Rese Microbiol 158: 447455.
  • Suzuki MR, Hunt CG, Houtman CJ, Dalebroux ZD & Hammel KE (2006) Fungal hydroquinones contribute to brown rot of wood. Environ Microbiol 8: 22142223.
  • Takao S (1965) Organic acid production by basidiomycetes. I. Screening of acid-producing strains. Appl Microbiol 13: 732.
  • Tanaka H, Fuse G & Enoki A (1991) An extracellular H2O2-producing and H2O2-reducing glycopeptide preparation from the lignin-degrading white rot fungus, Irpex lacteus. Mokuzai Gakkaishi 37: 986988.
  • Tanaka H, Hirano T & Enoki A (1993) Extracellular substance from the white rot basidiomycete Irpex lacteus for production and reduction of H2O2 during wood degradation. Mokuzai Gakkaishi 39: 493499.
  • Tanaka H, Itakura S, Hirano T & Enoki A (1996) An extracellular substance from the white-rot basidiomycete Phanerochaete chrysosporium for reducing molecular oxygen and ferric iron. Holzforschung 50: 541548.
  • Tanaka H, Itakura S & Enoki A (1999a) Hydroxyl radical generation by an extracellular low-molecular-weight substance and phenol oxidase activity during wood degradation by the white-rot basidiomycete Trametes versicolor. J Biotechnol 75: 5770.
  • Tanaka H, Itakura S & Enoki A (1999b) Hydroxyl radical generation by an extracellular low-molecular-weight substance and phenol oxidase activity during wood degradation by the white-rot basidiomycete Phanerochaete chrysosporium. Holzforschung 53: 2128.
  • Tanaka H, Itakura S & Enoki A (1999c) Hydroxyl radical generation and phenol oxidase activity in wood degradation by the white-rot basidiomycete Irpex lacteus. Mater Organism 33: 91105.
  • Tanaka H, Yoshida G, Baba Y et al. (2007) Characterization of a hydroxyl-radical-producing glycoprotein and its presumptive genes from the white-rot basidiomycete Phanerochaete chrysosporium. J Biotechnol 128: 500511.
  • Tanaka N, Akamtsu Y, Hattori T & Shimada M (1994) Effect of oxalic acid on the oxidative breakdown of cellulose by Fenton reaction. Wood Res 81: 810.
  • Temp U & Eggert C (1999) Novel interaction between laccase and cellobiose dehydrogenase during pigment synthesis in the white rot fungus Pycnoporus cinnabarinus. Appl Environ Microbiol 65: 389395.
  • Tomme P, Kwan E, Gilkes NR, Kilburn DG & Warren RAJ (1996) Characterization of CenC, an enzyme from Cellulomonas fimi with both endo- and exoglucanase activities. J Bacteriol 178: 42164223.
  • Tsukada T, Igarashi K, Yoshida M & Samejima M (2006) Molecular cloning and characterization of two intracellular β-glucosidases belonging to glycoside hydrolase family 1 from the basidiomycete Phanerochaete chrysosporium. Appl Microbiol Biotechnol 73: 807814.
  • Uzcategui E, Johansson G, Ek B & Pettersson G (1991a) The 1,4-beta-d-glucan glucanohydrolases from Phanerochaete chrysosporium– Re-assessment of their significance in cellulose degradation mechanisms. J Biotechnol 21: 143160.
  • Uzcategui E, Ruiz A, Montesino R, Johansson G & Pettersson G (1991b) The 1,4-beta-d-glucan cellobiohydrolases from Phanerochaete chrysosporium. 1. A system of synergistically acting enzymes homologous to Trichoderma reesei. J Biotechnol 19: 271285.
  • Valášková V & Baldrian P (2006a) Estimation of bound and free fractions of lignocellulose-degrading enzymes of wood-rotting fungi Pleurotus ostreatus, Trametes versicolor and Piptoporus betulinus. Res Microbiol 157: 119124.
  • Valášková V & Baldrian P (2006b) Degradation of cellulose and hemicelluloses by the brown rot fungus Piptoporus betulinus– production of extracellular enzymes and characterization of the major cellulases. Microbiology-Sgm 152: 36133622.
  • Valášková V, Šnajdr J, Bittner B, Cajthaml T, Merhautová V, Hofrichter M & Baldrian P (2007) Production of lignocellulose-degrading enzymes and degradation of leaf litter by saprotrophic basidiomycetes isolated from a Quercus petraea forest. Soil Biology and Biochemistry 39: 26512660.
  • Vallim MA, Janse BJH, Gaskell J, Pizzirani-Kleiner AA & Cullen D (1998) Phanerochaete chrysosporium cellobiohydrolase and cellobiose dehydrogenase transcripts in wood. Appl Environ Microbiol 64: 19241928.
  • Van Den Wymelenberg A, Sabat G, Martinez D et al. (2005) The Phanerochaete chrysosporium secretome: database predictions and initial mass spectrometry peptide identifications in cellulose-grown medium. J Biotechnol 118: 1734.
  • Van Den Wymelenberg A, Minges P, Sabat G et al. (2006) Computational analysis of the Phanerochaete chrysosporium v 2.0 genome database and mass spectrometry identification of peptides in ligninolytic cultures reveal complex mixtures of secreted proteins. Fungal Genet Biol 43: 343356.
  • Varela E & Tien M (2003) Effect of pH and oxalate on hydroquinone-derived hydroxyl radical formation during brown rot wood degradation. Appl Environ Microbiol 69: 60256031.
  • Welch KD, Davis TZ & Aust SD (2002a) Iron autoxidation and free radical generation: effects of buffers, ligands, and chelators. Archiv Biochem Biophys 397: 360369.
  • Welch KD, Davis TZ, Van Eden ME & Aust SD (2002b) Deleterious iron-mediated oxidation of biomolecules. Free Radical Biol Med 32: 577583.
  • Westermark U & Eriksson KE (1974a) Cellobiose-quinone oxidoreductase, a new wood-degrading enzyme from white rot fungi. Acta Chem Scand Series B – Org Chem Biochem B 28: 209214.
  • Westermark U & Eriksson KE (1974b) Carbohydrate-dependent enzymic quinone reduction during lignin degradation. Acta Chem Scand Series B – Org Chem Biochem B 28: 204208.
  • Westermark U & Eriksson KE (1975) Purification and properties of cellobiose – quinone oxidoreductase from Sporotrichum pulverulentum. Acta Chem Scand Series B – Org Chem Biochem 29: 419424.
  • Willick GE & Seligy VL (1985) Multiplicity in cellulases of Schizophyllum commune– derivation partly from heterogeneity in transcription and glycosylation. Eur J Biochem 151: 8996.
  • Wood JD & Wood PM (1992) Evidence that cellobiose – quinone oxidoreductase from Phanerochaete chrysosporium is a breakdown product of cellobiose oxidase. Biochim Biophys Acta 1119: 9096.
  • Wood PM (1994) Pathways of production of Fenton reagent by wood-rotting fungi. FEMS Microbiol Rev 13: 313320.
  • Xu G & Goodell B (2001) Mechanisms of wood degradation by brown-rot fungi: chelator-mediated cellulose degradation and binding of iron by cellulose. J Biotechnol 87: 4357.
  • Yamakawa M, Ozaki K, Fujime T, Kakimoto N, Itakura S, Enoki A & Tanaka H (2005) Relationship of phenol oxidase activity and hydroxyl radical generation to wood degradation by white rot basidiomycetes. Biocontr Sci 10: 8590.
  • Yoon JJ & Kim YK (2005) Degradation of crystalline cellulose by the brown-rot basidiomycete Fomitopsis palustris. J Microbiol 43: 487492.
  • Yoon JJ, Cha CJ, Kim YS, Son DW & Kim YK (2007) The brown-rot basidiomycete Fomitopsis palustris has the endo-glucanases capable of degrading microcrystalline cellulose. J Microbiol Biotechnol 17: 800805.
  • Yoshida M, Igarashi K, Wada M et al. (2005) Characterization of carbohydrate-binding cytochrome b562 from the white-rot fungus Phanerochaete chrysosporium. Appl Environ Microbiol 71: 45484555.
  • Zamocky M, Ludwig R, Peterbauer C, Hallberg BM, Divne C, Nicholls P & Haltrich D (2006) Cellobiose dehydrogenase – a flavocytochrome from wood-degrading, phytopathogenic and saprotropic fungi. Curr Prot Peptide Sci 7: 255280.