Tungsten Enzymes and Models

Abstract

AbstractThe chemical properties of the metals tungsten (W) and molybdenum (Mo) are very similar. Mo plays a key role in biological systems and molybdoenzymes are ubiquitous; however, it is only recently that a biological role for W has been established, and so far only in a few microorganisms. Three different types of tungstoenzyme have been purified and these are abbreviated as AOR, F(M)DH, and AH. Members of the AOR group catalyze the reversible oxidation of aldehydes and include three ferredoxin‐dependent oxidoreductases that oxidize various aldehydes (AOR), formaldehyde (FOR), and glyceraldehyde‐3‐phosphate (GAPOR). The F(M)DH group utilizes carbon dioxide as a substrate and is represented by formate dehydrogenase (FDH) and formyl methanofuran dehydrogenase (FMDH), while acetylene hydratase (AH) is the sole member of the AH class. These tungstoenzymes catalyze reduction–oxidation reactions of very low potential (less than ‐420 mV) except one (AH), which catalyzes a hydration reaction. All of these enzymes contain a single W atom in the +IV, +V, or +VI redox state that is coordinated to the protein via an organic moiety known as a pterin. The pterin is similar to that found in molybdoenzymes. The crystal structure of the first W‐containing (or pterin‐containing) enzyme, that of AOR fromPyrococcus furiosus, revealed a catalytic site with one W atom coordinated of four sulfur atoms from two pterin molecules. The two pterins are also bridged by a magnesium ion. the catalytic and spectroscopic propeties of tungstoenzymes and synthetic analogues of their active sites are discussed, together with the factors that might contribute to the biochemical utilization of W rather than Mo.