Published Online: 15 DEC 2011
Copyright © 2011 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Inorganic and Bioinorganic Chemistry
How to Cite
Adams, M. W. W. and Roy, R. 2011. Tungsten Proteins. Encyclopedia of Inorganic and Bioinorganic Chemistry. .
- Published Online: 15 DEC 2011
Tungsten (W, atomic number 74) and molybdenum (Mo, atomic number 42) are very similar metals chemically, and are equally abundant elements on this planet. Molybdenum has been known for decades to play an essential role in biology, and molybdenum-containing enzymes are ubiquitous in both prokaryotes and eukaryotes. In contrast, tungsten appears to play a much more limited role in biological systems, and one that was established relatively recently. So far, tungsten-containing enzymes have been found only in prokaryotes. The best-characterized organisms with regard to their metabolism of tungsten are certain species of anaerobic archaea that grow at extreme temperatures (the hyperthermophiles) or produce methane gas (the methanogens), and anaerobic bacteria that produce acetate (the acetogens) or utilize acetylene. Of these, only the hyperthermophilic archaea appear to be obligately tungsten-dependent. That is, they do not synthesize molybdenum-containing homologs of key tungstoenzymes when grown on molybdenum rather than tungsten. Three fundamentally different types of tungstoenzyme have been characterized so far. The best characterized is the Aldehyde ferredoxin oxidoreductase (AOR) family, represented by aldehyde ferredoxin oxidoreductase. Hyperthermophilic archaea such as Pyrococcus furiosus contain at least four members of the AOR family of tungstoenzymes. These appear to play key roles in the metabolism of sugar and peptides. The second family, termed F(M)DH, contains two types of tungstoenzyme, formate dehydrogenase and formyl methanofuran dehydrogenase. These are involved in CO2 metabolism in acetogens and methanogens, respectively. The F(M)DH members are much more complex enzymes than the AOR family and show no amino acid sequence similarity to them. The third family is represented by the enzyme acetylene hydratase (AH). It catalyzes the hydration of acetylene and is found in acetylene-utilizing bacteria. Members of the AOR and F(M)DH family catalyze redox reactions of very low potential (≤−420 mV), in which tungsten undergoes a two-electron redox reaction [W(IV) to W(VI)]. Crystal structures are available for two members of the AOR family and reveal that each subunit of the proteins contains a single W atom coordinated to two organic pterin molecules, which are bridged by a magnesium ion. An iron–sulfur cluster is situated near the W atom and is thought to be involved in electron transfer to it. The geochemical, ecological, and biochemical basis for tungsten- rather than molybdenum-dependent organisms is discussed.