Carbon Monoxide Oxidation by Methanogenic Bacteria

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Different species of methanogenic bacteria growing on CO2 and H2 were shown to remove CO added to the gas phase. Rates up to 0.2 μmol of CO depleted/min per 10 ml of culture containing approximately 7 mg of cells (wet weight) were observed. Methanobacterium thermoautotrophicum was selected for further study based on its ability to grow rapidly on a completely mineral medium. This species used CO as the sole energy source by disproportionating CO to CO2 and CH4 according to the following equation: 4CO + 2H2O → 1CH4 + 3CO2. However, growth was slight, and the growth rate on CO was only 1% of that observed on H2/CO2. Growth only occurred with CO concentrations in the gas phase of lower than 50%. Growth on CO agrees with the finding that cell-free extracts of M. thermoautotrophicum contained both an active factor 420 (F420)-dependent hydrogenase (7.7 μmol/min per mg of protein at 35°C) and a CO-dehydrogenating enzyme (0.2 μmol/min per mg of protein at 35°C) that catalyzed the reduction of F420 with CO. The properties of the CO-dehydrogenating enzyme are described. In addition to F420, viologen dyes were effective electron acceptors for the enzyme. The apparent Km for CO was higher than 1 mM. The reaction rate increased with increasing pH and displayed an inflection point at pH 6.7. The temperature dependence of the reaction rate followed the Arrhenius equation with an activation energy (ΔH‡) of 14.1 kcal/mol (59.0 kJ/mol). The CO dehydrogenase activity was reversibly inactivated by low concentrations of cyanide (2 μM) and was very sensitive to inactivation by oxygen. Carbon monoxide dehydrogenase of M. thermoautotrophicum exhibited several characteristic properties found for the enzyme of Clostridium pasteurianum but differed mainly in that the clostridial enzyme did not utilize F420 as the electron acceptor.

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