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Tungsten in biological systems

Authors

  • Arnulf Kletzin,

    1. Center for Metalloenzyme Studies, Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA 30602-7229, USA
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    • 1

      Institut für Mikrobiologie, Technische Hochschule Darmstadt, Schnittspahnstr. 10, 64287 Darmstadt, Germany.

  • Michael W.W. Adams

    Corresponding author
    1. Center for Metalloenzyme Studies, Department of Biochemistry and Molecular Biology, University of Georgia, Athens, GA 30602-7229, USA
      *Corresponding author. Tel.: + 1 (706) 542 2060; Fax: + 1 (706) 542 0229; E-mail: adamsm@bscr.uga.edu
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*Corresponding author. Tel.: + 1 (706) 542 2060; Fax: + 1 (706) 542 0229; E-mail: adamsm@bscr.uga.edu

Abstract

Abstract: Tungsten (atomic number 74) and the chemically analogous and very similar metal molybdenum (atomic number 42) are minor yet equally abundant elements on this planet. The essential role of molybdenum in biology has been known for decades and molybdoenzymes are ubiquitous. Yet, it is only recently that a biological role for tungsten has been established in prokaryotes, although not as yet in eukaryotes. The best characterized organisms with regard to their metabolism of tungsten are certain species of hyperthermophilic archaea (Pyrococcus furiosus and Thermococcus litoralis), methanogens (Methanobacterium thermoautotrophicum and Mb. wolfei), Gram-positive bacteria (Clostridium thermoaceticum, C. formicoaceticum and Eubacterium acidaminophilum), Gram-negative anaerobes (Desulfovibrio gigas and Pelobacter acetylenicus) and Gram-negative aerobes (Methylobacterium sp. RXM). Of these, only the hyperthermophilic archaea appear to be obligately tungsten-dependent. Four different types of tungstoenzyme have been purified: formate dehydrogenase, formyl methanofuran dehydrogenase, acetylene hydratase, and a class of phylogenetically related oxidoreductases that catalyze the reversible oxidation of aldehydes. These are carboxylic reductase, and three ferredoxin-dependent oxidoreductases which oxidize various aldehydes, formaldehyde and glyceraldehyde 3-phosphate. All tungstoenzymes catalyze redox reactions of very low potential (≤−420 mV) except one (acetylene hydratase) which catalyzes a hydration reaction. The tungsten in these enzymes is bound by a pterin moiety similar to that found in molybdoenzymes. The first crystal structure of a tungsten- or pterin-containing enzyme, that of aldehyde ferredoxin oxidoreductase from P. furiosus, has revealed a catalytic site with one W atom coordinated to two pterin molecules which are themselves bridged by a magnesium ion. The geochemical, ecological, biochemical and phylogenetic basis for W- vs. Mo-dependent organisms is discussed.

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