Net synthesis of acetate from CO2byEubacterium acidaminophilumthrough the glycine reductase pathway

Eubacterium acidaminophilum combines the oxidation of amino acids such as alanine or valine with the reduction of glycine to acetate in a two-substrate fermentation (Stickland reaction). In the absence of glycine, dense cell suspensions oxidized alanine or valine only to a small extent, with limited production of hydrogen and acetate. Experiments with 14C-labeled carbonate revealed that acetate was formed under these conditions by net reduction of CO2/HCO3−; 14C-labeled formate was formed as an intermediate. E. acidaminophilum did not grow with hydrogen plus CO2; dense cell suspensions under H2/CO2 produced only very small amounts (<0.5 mM) of acetate. There was no activity of carbon monoxide dehydrogenase, indicating that the glycine pathway was used for acetate synthesis. The results are explained on the basis of biochemical and energetic considerations.