Three-megadalton complex of methanogenic electron-bifurcating and CO₂-fixing enzymes

The first reaction of the methanogenic pathway from carbon dioxide (CO₂) is the reduction and condensation of CO₂ to formyl-methanofuran, catalyzed by formyl-methanofuran dehydrogenase (Fmd). Strongly reducing electrons for this reaction are generated by heterodisulfide reductase (Hdr) in complex with hydrogenase or formate dehydrogenase (Fdh) using a flavin-based electron-bifurcation mechanism. Here, we report enzymological and structural characterizations of Fdh-Hdr-Fmd complexes from Methanospirillum hungatei. The complexes catalyze this reaction using electrons from formate and the reduced form of the electron carrier F₄₂₀. Conformational changes in HdrA mediate electron bifurcation, and polyferredoxin FmdF directly transfers electrons to the CO₂ reduction site, as evidenced by methanofuran-dependent flavin-based electron bifurcation even without free ferredoxin, a diffusible electron carrier between Hdr and Fmd. Conservation of Hdr and Fmd structures suggests that this complex is common among hydrogenotrophic methanogens.