Pyruvate and Acetate Metabolism in the Photosynthetic Bacterium Rhodobacter capsulatus

Summary: Enzymes involved in pyruvate metabolism were assayed in crude extracts of Rhodobacter capsulatus cells grown photosynthetically with different carbon sources. Pyruvate dehydrogenase, pyruvate carboxylase, pyruvate kinase and phosphoenolpyruvate carboxykinase (ATP) were all present in extracts of cells grown on DL-lactate, whereas phosphoenolpyruvate carboxylase, phosphoenolpyruvate synthase and pyruvate, orthophosphate dikinase were undetectable in extracts of cells grown on either DL-lactate, DL-malate or acetate. Malate-grown cells and acetate-grown cells contained lower levels of pyruvate carboxylase and higher levels of pyruvate kinase than did lactate-grown cells, and malate-grown cells also contained lower levels of phosphoenolpyruvate carboxykinase. Pyruvate dehydrogenase activity was undetectable in extracts of acetate-grown cells. Two mutant strains, which were unable to grow on pyruvate, lactate or any other compound metabolized via pyruvate, were found to be deficient in pyruvate carboxylase activity, indicating that this anaplerotic enzyme is essential for growth on pyruvate or related compounds as sole added carbon source. This requirement for pyruvate carboxylase can be bypassed to some extent, however, since the mutants grew on acetate, albeit more slowly than the wild-type and after a long lag period. The key enzymes of the glyoxylate cycle, isocitrate lyase and malate synthase, were present at high levels in extracts of acetate-grown cells, both of the mutants and of several wild-type strains tested, whereas isocitrate lyase activity was undetectable in three different strains of Rhodobacter sphaeroides. This is consistent with previous suggestions that acetate is assimilated via the glyoxylate cycle in R. capsulatus but not in R. sphaeroides.