A nickel hydride complex in the active site of methyl-coenzyme M reductase: Implications for the catalytic cycle

Methanogenic archaea utilize a specific pathway in their metabolism, converting C, substrates (i.e., CO2) or acetate to methane and thereby providing energy for the cell. Methyl-coenzyme M reductase (MCR) catalyzes the key step in the process, namely methyl-coenzyme M (CH3-S-COM) plus coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. The active site of MCR contains the nickel porphinoid F-430. We report here on the coordinated ligands of the two paramagnetic MCRred2 states, induced when HS-CoM (a reversible competitive inhibitor) and the second substrate HS-CoB or its analogue CH3-S-CoB are added to the enzyme in the active MCRred1 state ((NiF430)-F-I). Continuous wave and pulse EPR spectroscopy are used to show that the MCRred2a state exhibits a very large proton hyperfine interaction with principal values A(H-1) = [-43,-42,-5] MHz and thus represents formally a (NiF430)-F-III hydride complex formed by oxidative addition to Nil. In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence for the involvement of MCR in reverse methanogenesis (anaerobic oxidation of methane), we believe that the nickel hydride complex reported here could play a key role in helping to understand both the mechanism of reverse and forward methanogenesis.