Fig. 1.
Wolfe cycle of CO2 reduction to methane with 4 H2 in hydrogenotrophic methanogenic archaea. Orange arrows indicate biosynthetic reactions that remove intermediates (thickness of the arrows reflects the quantitative importance), the green arrow illustrates the anaplerotic reaction catalyzed by EhaA-T, and yellow highlighting represents the electron-bifurcating reaction. Reactions f and j are catalyzed by membrane-associated enzyme complexes. All other reactions are catalyzed by cytoplasmic enzymes. F420, coenzyme F420; *Fd, specific ferredoxin; H4MPT, tetrahydromethanopterin; MFR, methanofuran; ΔμNa+, electrochemical sodium ion potential. Enzymes: a, formylmethanofuran dehydrogenase; b, formylmethanofuran/H4MPT formyltransferase; c, methenyl-H4MPT cyclohydrolase; d, methylene-H4MPT dehydrogenase; e, methylene-H4MPT reductase; f, methyl-H4MPT/coenzyme M methyltransferase; g, methyl-coenzyme M reductase; h, electron-bifurcating hydrogenase–heterodisulfide reductase complex; i, F420-reducing hydrogenase; j, energy-converting hydrogenase catalyzing the sodium motive force-driven reduction of ferredoxin with H2. With Methanothermobacter marburgensis, it has been shown that one of the two C1 carbons in purines is derived from formate (C-2) and the other is derived from formyl-H4MPT/methenyl-H4MPT (C-8) (25).