Thermodynamics of the Formylmethanofuran Dehydrogenase Reaction in Methanobacterium Thermoautotrophicum

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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.

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