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Abstract

In methanogenic archaea growing on H-2 and CO2 the first step in methanogenesis is the ferredoxin-dependent endergonic reduction of CO2 with H-2 to formylmethanofuran and the last step is the exergonic reduction of the heterodisulfide CoM-S-S-CoB with H-2 to coenzyme M (CoM-SH) and coenzyme B (CoB-SH). We recently proposed that in hydrogenotrophic methanogens the two reactions are energetically coupled via the cytoplasmic MvhADG/HdrABC complex. It is reported here that the purified complex from Methanothermobacter marburgensis catalyzes the CoM-S-S-CoB-dependent reduction of ferredoxin with H-2. Per mole CoM-S-S-CoB added, 1 mol of ferredoxin (Fd) was reduced, indicating an electron bifurcation coupling mechanism: 2H(2) + Fd(ox) + CoM-S-S-CoB -> Fd(red)(2-) + CoM-SH + CoB-SH + 2H(+): This stoichiometry of coupling is consistent with an ATP gain per mole methane from 4 H-2 and CO2 of near 0.5 deduced from an H-2-threshold concentration of 8 Pa and a growth yield of up to 3 g/mol methane.