Citrate Synthase

Citrate synthase (citrate oxaloacetate-lyase (CoA-acetylating), EC 4.1.3.7) is very active in Rhodopseudomonas capulata, a nonsulfur purple bacterium. This activity varies greatly from carbon source to carbon source. The highest specific activity for this enzyme was obtained in acetate and acetate-bicarbonate-grown cells (1.7 to 1.8 µmoles per mg of protein-min). Citrate synthase activity was 2-fold greater in cells grown aerobically in the dark than in cells grown semiananaerobically in the light, when either malate or pyruvate was the carbon source. The specific activities of this enzyme from glucose dark-grown and light-grown cells, and from pyruvate dark-grown cells were similar, and they were about one-half the specific activity of the enzyme from acetate-grown cells. Citrate synthase in this organism was found to be a regulatory enzyme. DPNH was a strong inhibitor of the enzyme and AMP was found to relieve this inhibition. DPNH, at 0.1 mm, increased the S0.5 value and the n value (Hill constant) for acetyl-CoA. It also increased the S0.5 value but did not change the n value for oxalacetate. Increasing amounts of DPNH increased the n values for AMP, the deinhibitor. AMP, 0.46 mm, in the presence of DPNH, restored the S0.5 and the n values for acetyl-CoA to those obtained in the absence of effector molecules. AMP (0.46 mm) alone did not affect these values. Thus, AMP acts strictly as a deinhibitor of the DPNH effect with respect to acetyl-CoA. AMP, in the presence or absence of DPNH, decreased the S0.5 value and the n value for oxalacetate acting here both as a deinhibitor and as an activator. These results suggest that citrate synthase activity in R. capsulata is modulated both by the energy charge and the reducing state of the cell.