Protein complexing in a methanogen suggests electron bifurcation and electron delivery from formate to heterodisulfide reductase

In methanogenic Archaea, the final step of methanogenesis generates methane and a heterodisulfide of coenzyme M and coenzyme B (CoM-S-S-CoB). Reduction of this heterodisulfide by heterodisulfide reductase to regenerate HS-CoM and HS-CoB is an exergonic process. Thauer et al. [Thauer, et al. 2008 Nat Rev Microbiol 6:579-591] recently suggested that in hydrogenotrophic methanogens the energy of heterodisulfide reduction powers the most endergonic reaction in the pathway, catalyzed by the formylmethanofuran dehydrogenase, via flavin-based electron bifurcation. Here we present evidence that these two steps in methanogenesis are physically linked. We identify a protein complex from the hydrogenotrophic methanogen, Methanococcus maripaludis, that contains heterodisulfide reductase, formylmethanofuran dehydrogenase, [F.sub.420]-nonreducing hydrogenase, and formate dehydrogenase. In addition to establishing a physical basis for the electronbifurcation model of energy conservation, the composition of the complex also suggests that either [H.sub.2] or formate (two alternative electron donors for methanogenesis) can donate electrons to the heterodisulfide-[H.sub.2] via [F.sub.420]-nonreducing hydrogenase or formate via formate dehydrogenase. Electron flow from formate to the heteroclisulfide rather than the use of [H.sub.2] as an intermediate represents a previously unknown path of electron flow in methanogenesis. We further tested whether this path occurs by constructing a mutant lacking [F.sub.420]-nonreducing hydrogenase. The mutant displayed growth equal to wild-type with formate but markedly slower growth with hydrogen. The results support the model of electron bifurcation and suggest that formate, like [H.sub.2], is closely integrated into the methanogenic pathway. energy conservation | Archaea | formate dehydrogenase | forrnylrnethanofuran dehydrogenase | [F.sub.420]-nonreducing hydrogenase doi/ 10.1073/pnas.1003653107