The 'pH optimum anomaly' of intracellular enzymes of Ferroplasma acidiphilum.
Citrate synthase (EC 126.96.36.199) was purified from the acidophilic bacterium Acetobacter europaeus to electrophoretic homogeneity. The specific activity was 228 units/mg of protein during the exponential ethanol-oxidation growth phase. The enzyme has a molecular mass of 280 kDa and is a hexamer with a subunit size of 46 kDa. The apparent K(m) values were 20 microM for oxaloacetate and 51 microM for acetyl-CoA. Unlike citrate synthase from other Gram-negative bacteria, the activity of the enzyme was inhibited by ATP, slightly enhanced by ADP and not effected by NADH. Acetate caused activation of the enzyme. The pH optimum on the citrate synthase activity in vitro was 8.1. The amino-terminal amino acid sequence of the purified enzyme was ENGKSATISLNGKDVALPVL.