A complete citric acid cycle in assimilatory metabolism of Pelobacter acidigallici, a strictly anaerobic, fermenting bacterium

Abstract

Pelobacter acidigallici is a strictly anaerobic bacterium that ferments trihydroxybenzenes to 3 mol acetate/mol substrate. The key intermediate linking the catabolic sequences to the formation of cell matter is acetyl-CoA. Since P. acidigallici is independent of further external electron donors, it must oxidize part of the acetyl-CoA to provide reducing equivalents for anabolism. In this study we demonstrate the presence of all enzymes necessary to operate a modified citric acid cycle, with activities sufficient to support growth. Unusual enzymes in the cycle are 2-oxoglutarate synthase and succinyl- CoA:acetoacetate CoA transferase. Anaplerotic reactions are catalyzed by pyruvate synthase, PEP synthetase and PEP carboxylase. No CO dehydrogenase, hydrogenase, or formate dehydrogenase activity could be detected. The phylogenetic implications of these findings with respect to the relatedness of P. acidigallici to gramnegative, sulfur-reducing bacteria by 16S rRNA cataloguing are discussed.