Enzyme: laccases (EC 126.96.36.199).
First evidence of catalytic mediation by phenolic compounds in the laccase-induced oxidation of lignin models
Article first published online: 1 AUG 2003
European Journal of Biochemistry
Volume 270, Issue 17, pages 3634–3640, September 2003
How to Cite
d'Acunzo, F. and Galli, C. (2003), First evidence of catalytic mediation by phenolic compounds in the laccase-induced oxidation of lignin models. European Journal of Biochemistry, 270: 3634–3640. doi: 10.1046/j.1432-1033.2003.03752.x
- Issue published online: 1 AUG 2003
- Article first published online: 1 AUG 2003
- (Received 19 May 2003, revised 10 July 2003, accepted 14 July 2003)
- phenolic mediators;
- lignin models;
The sulfonephthalein indicator, phenol red, exhibits an unusually slow rate of oxidation by laccase from Poliporus pinsitus, in spite of the fact that it is a phenol and therefore a natural substrate for this phenoloxidase enzyme. Nevertheless, after prolonged exposure to laccase (24 h) phenol red is oxidized by more than 90%. We found that phenol red, which can be oxidatively converted into a resonance-stabilized phenoxy radical, performs as a mediator in the laccase-catalyzed oxidation of a nonphenolic substrate (4-methoxybenzyl alcohol) and also of a hindered phenol (2,4,6-tri-tert-butylphenol). In particular, phenol red was found to be at least 10 times more efficient than 3-hydroxyanthranilate (a reported natural phenolic mediator of laccase) in the oxidation of 4-methoxybenzyl alcohol. Other phenols, which do not bear structural analogies to phenol red, underwent rapid degradation and did not perform as laccase mediators. On the other hand, several variously substituted sulfonephthaleins, of different pK2 values, mediated the laccase catalysis, the most efficient being dichlorophenol red, which has the lowest pK2 of the series. The mediating efficiency of phenol red and dichlorophenol red was found to be pH dependent, as was their oxidation Ep value (determined by cyclic voltammetry). We argue that the relative abundance of the phenoxy anion, which is easier to oxidize than the protonated phenol, may be one of the factors determining the efficiency of a phenolic mediator, together with its ability to form relatively stable oxidized intermediates that react with the desired substrate before being depleted in undesired routes.