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 co-enzyme 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 ₄₂₀-nonreducing hydrogenase, and formate dehydrogenase. In addition to establishing a physical basis for the electron-bifurcation model of energy conservation, the composition of the complex also suggests that either H ₂ or formate (two alternative electron donors for methanogenesis) can donate electrons to the heterodisulfide-H₂ via F₄₂₀- nonreducing hydrogenase or formate via formate dehydrogenase. Electron flow from formate to the heterodisulfide rather than the use of H₂ 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 F₄₂₀-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 H₂, is closely integrated into the methanogenic pathway.