Scheme of hydrogenotrophic methanogenesis involving Sec-containing proteins (orange); the Cys-containing isoforms are in white. CoM-S-S-CoB, heterodisulfide of coenzyme M and coenzyme B; Fd, ferredoxin; F, oxidized Fd; F, reduced Fd; FDH, formate dehydrogenase; FMD, formyl-methanofuran dehydrogenase; Fru, F₄₂₀-reducing hydrogenase; F₄₂₀, (oxidized) 8-hydroxy-5-deazariboflavin derivative; F₄₂₀H₂, reduced coenzyme F₄₂₀; H₄MPT, tetrahydromethanopterin, HDR, heterodisulfide reductase; HS-CoB, coenzyme B (N-7-mercaptoheptanoyl-O-phospho-L-threonine); HS-CoM, coenzyme M (2-mercaptoethanesulfonic acid); MF, methanofuran; Vhu, F₄₂₀-nonreducing hydrogenase.