Propionate metabolism: V. The physiological significance of isocitrate lyase during growth of E. coli on propionate

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The role of isocitrate lyase during growth on propionate was investigated using three strains (E-26, K-12, and W) of Escherichia coli. The activity of this enzyme during growth on propionate varied with respect to the strain employed, the concentration of propionate added to the mineral salts culture medium, and the phase of growth at which cells were harvested. The growth behavior of E-26 and of K-12 mutants indicates that in these strains of E. coli, the formation of C 4 acids from propionate can be accomplished either by the phosphoenolpyruvate (PEP) carboxylase pathway or by the glyoxylate bypass. In the early phase of growth, pyruvate formed from the oxidation of propionate is metabolized primarily via the PEP carboxylase pathway. In such cultures, the activity of isocitrate lyase and malate synthase is low. As growth proceeds, there occurs a shift in metabolism, such that the carboxylase pathway is inhibited and the decarboxylation of pyruvate to form acetyl-CoA is stimulated. In such cultures, the formation of the enzymes of glyoxylate bypass is derepressed. In contrast to E-26 and K-12, the formation of isocitrate lyase is not derepressed in E. coli W cultured on propionate.