Fig. 1.

Hydrogenotrophic (a), methylotrophic (b) and aceticlastic (c) methanogenesis pathways. The ferredoxin electron carrier is a two-electron carrier. Some methanogens use a H₄MPT derivative called tetrahydrosarcinopterin (H₄SPT). The Na⁺/H⁺ translocation stoichiometry is not represented in the figure. FwdA-F/FmdA-F: formylmethanofuran dehydrogenase, Ftr: formylmethanofuran-tetrahydromethanopterin formyl-transferase, Mch: methenyl-tetrahydromethanopterin cyclohydrolase, Mtd: methylenetetrahydromethanopterin dehydrogenase, Mer: 5,10-methylenetetrahydromethanopterin reductase, MtrA-H: tetrahydromethanopterin S-methyl-transferase, McrABCDG methyl-coenzyme M reductase, FrhABG: coenzyme F₄₂₀-reducing hydrogenase, HdrABC: soluble heterodisulfide reductase, MvhAGD: F₄₂₀-non-reducing hydrogenase, FdhAB: formate dehydrogenase, FpoA-O: F₄₂₀H₂ dehydrogenase, HdrDE: membrane-bound heterodisulfide reductase, Ech-H₂ase: energy-converting hydrogenase, Rnf: Na⁺-translocating ferredoxin:NAD⁺ oxidoreductase complex, ATPase: ATP synthase, CODH-ACS: Acetyl-CoA decarbonylase/synthase, MTI and MTII: methyltransferase, CoB: coenzyme B, CoM: coenzyme M, H₄MPT: tetrahydromethanopterin, MFR: methanofuran, Fd: ferredoxin, F₄₂₀H₂: reduced coenzyme F₄₂₀, MP: methanophenazine, CO(III): cobalamin binding protein