Coupling of ferredoxin and heterodisulfide reduction via electron bifurcation in hydrogenotrophic methanogenic archaea

摘要

In methanogenic archaea growing on H_2 and CO_2 the first step in methanogenesis is the ferredoxin-dependent endergonic reduction of CO_2 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 CO_2 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.