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Abstract:
Purified formylmethanofuran dehydrogenase from Methanobacterium thermoautotrophicum, which is a thermophilic methanogenic Archaeon growing on H2 and CO2, was shown to catalyze the reversible reduction of CO2 to N-formylmethanofuran with 1,1?,2,2?-tetramethylviologen (E?o=? 550 mV) as electron donor. The rate of CO2 reduction was approximately 25 times higher than the rate of N-formylmethanofuran dehydrogenation. From determinations of equilibrium concentrations at 60°C and pH 7.0 a midpoint potential (E?o) for the CO2+ methanofuran/formylmethanofuran couple of approximately ? 530 mV was estimated. The initial step of methanogenesis from CO2 thus has a midpoint potential considerably more negative than that of the H+/H2 couple (E?o=?460 mV at 60°C). Evidence is described indicating that the as-yet unidentified physiological electron donor of the formylmethanofuran dehydrogenase is present in the soluble cell fraction.
