Hydrogenases from Methanogenic Archaea, Nickel, a Novel Cofactor, and H-2 Storage

Most methanogenic archaea reduce CO2 with H-2 to CH4. For the activation of H-2, they use different [NiFe]-hydrogenases, namely energy-converting [NiFe]-hydrogenases, heterodisulfide reductase-associated [NiFe]-hydrogenase or methanophenazine-reducing [NiFe]-hydrogenase, and F-420-reducing [NiFe]-hydrogenase. The energy-converting [NiFe]-hydrogenases are phylogenetically related to complex T of the respiratory chain. Under conditions of nickel limitation, some methanogens synthesize a nickel-independent [Fe]hydrogenase (instead of F420-reducing [NiFe]-hydrogenase) and by that reduce their nickel requirement. The [Fe]-hydrogenase harbors a unique iron-guanylylpyridinol cofactor (FeGP cofactor), in which a low-spin iron is ligated by two CO, one C(O)CH2-, one S-CH2-, and a sp(2)-hybridized pyridinol nitrogen. Ligation of the iron is thus similar to that of the low-spin iron in the binuclear active-site metal center of [NiFe]- and [FeFe]-hydrogenases. Putative genes for the synthesis of the FeGP cofactor have been identified. The formation of methane from 4 H-2 and CO2 catalyzed by methanogenic archaea is being discussed as an efficient means to store H-2.