CO2 REDUCTION TO ACETATE IN ANAEROBIC-BACTERIA

The reduction of 2 CO2 to acetate is catalyzed in the energy metabolism of homoacetogenic bacteria, which couple acetate formation to the synthesis of ATP. The carboxyl group of acetate is formed from CO2 via reduction to a bound carbonyl (]CO[), a reaction that requires the input of metabolic energy when hydrogen is used as the electron donor. The methyl group of acetate is formed via formate and tetrahydrofolate bound C1 intermediates including methyl tetrahydrofolate as the intermediates. The methyl group is then 'condensed' with the carbonyl and CoA to acetyl-CoA, which is converted to acetate in the energy metabolism or to cell carbon in the anabolism of the bacteria. The mechanism of ATP synthesis coupled to CO2 reduction to acetate is still unclear. The only reaction sufficiently exergonic is the reduction of methylene tetrahydrofolate to methyl tetrahydrofolate. Indirect evidence was presented that this reaction in homoacetogens might be coupled to the electrogenic transport of sodium across the cytoplasmic membrane. The sodium gradient formed via methylene-THF reduction could be transformed into a proton gradient via a sodium/proton antiporter. ATP would then be synthesized by a proton translocating ATP synthase.