Many metalloenzymes have active sites composed of metals coordinated to S-donor ligands. Among the examples are Ni, Fe hydrogenases. These enzymes catalyze the two-electron redox chemistry of H2 and are believed to contain a heterodinuclear active site composed of a Ni center bridged to an Fe center by cysteinate ligands. The possible roles of the thiolate ligands in constructing the active site, in the redox chemistry of the active site, and in the binding of H + are discussed in the context of an overview of the results of physical studies of the enzyme and dinuclear model compounds.A large number of redox metalloproteins that employ thiolate (cysteinate) and/or sulfide as metal ligands is known (see Chapter 1). These proteins play key roles in biological redox processes including respiration and photosynthesis (e.g., ferredoxins (7,2), blue Cu proteins (3), CUA in cytochrome oxidase(4-6)) and in the oxidation or reduction of substrates (e.g., nitrogenase (7), hydrogenase (8-11), sulfite reductase (72)). Among these metalloproteins are a group of enzymes that contain Ni (13). Nickel containing redox metalloenzymes include methylcoenzyme M reductase (74), carbon monoxide dehydrogenase (COdH) (75) and most hydrogenases (leases) (8,16).Methylcoenzyme M reductase is found in methanogenic bacteria where it catalyzes the last step in methanogenesis: CH3SCH2CH2SO3-+ HS-HTP• CH4 + O3SCH2CH2S-S-HTP Me-coenzyme M It contains a unique Ni-containing tetrahydrocorphin cofactor, F430, which is involved in the reduction of the methyl group from methylcoenzyme M (a methylthioether) coupled with the formation of a disulfide involving coenzyme M and N-7-mercaptoheptonyl-0-phospho-L-threonine (thiols), a two-electron redox process. The exact role of the Ni in this enzyme is not well-known (17-19), although it is clear that the net redox chemistry involves thiolate oxidation.Carbon monoxide dehydrogenase catalyzes nature' s version of water-gas shift chemistry-the two-electron redox chemistry of CO: 0097-6156/96A)653-0074$16. 75A)