Acetate oxidation to CO2 via a citric acid cycle involving an ATP-citrate lyase: a mechanism for the synthesis of ATP via substrate level phosphorylation in Desulfobacter postgatei growing on acetate and sulfate

Desulfobacter postgatei is an acetate-oxidizing, sulfate-reducing bacterium that metabolizes acetate via the citric acid cycle. The organism has been reported to contain a si-citrate synthase (EC 4.1.3.7) which is activated by AMP and inorganic phosphate. It is show now, that the enzyme mediating citrate formation is an ATP-citrate lyase (EC 4.1.3.8) rather than a citrate synthase. Cell extracts (160,000xg supernatant) catalyzed the conversion of oxaloacetate (apparent Km=0.2 mM), acetyl-CoA (app. Km=0.1 mM), ADP (app. Km=0.06 mM) and phosphate (app. Km=0.7 mM) to citrate, CoA and ATP with a specific activity of 0.3 μmol·min-1·mg-1 protein. Per mol citrate formed 1 mol of ATP was generated. Cleavage of citrate (app. Km=0.05 mM; Vmax=1.2 μmol · min-1 · mg-1 protein) was dependent on ATP (app. Km=0.4 mM) and CoA (app. Km=0.05 mM) and yielded oxaloacetate, acetyl-CoA, ADP, and phosphate as products in a stoichiometry of citrate:CoA:oxaloacetate:ADP=1:1:1:1. The use of an ATP-citrate lyase in the citric acid cycle enables D. postgatei to couple the oxidation of acetate to 2 CO2 with the net synthesis of ATP via substrate level phosphorylation.