CO2 reduction to the level of formylmethanofuran in Methanosarcina barkeri is non-energy driven when CO is the electron donor

A Methanosarcina barkeri mutant lacking Ech hydrogenase does not catalyze CH(4) formation from H(2)/CO(2) since, as was shown previously, the energy-driven reduction of CO(2) to formylmethanofuran by H(2) is blocked. CH(4) formation by this mutant could be restored in the presence of CO or pyruvate. Furthermore, CH(4) formation from H(2)/CO(2) plus CO by the Deltaech mutant was not inhibited by the protonophore TCS. These data show that in vivo the reduction of CO(2) to formylmethanofuran can be coupled to the oxidation of CO or pyruvate via a common electron carrier and that the reduction of this electron carrier by H(2), catalyzed by Ech hydrogenase, is the energy-driven step in formylmethanofuran-synthesis from CO(2), H(2) and methanofuran.