FINE CONTROL OF PHOSPHOPYRUVATE CARBOXYLASE ACTIVITY IN ESCHERICHIA COLI

Carbon dioxide fixation and phosphoenolpyruvate carboxylase in Ferrobacillus ferrooxidans

Carbon dioxide fixation was studied in intact cells and cell-free extracts of Ferrobacillus ferrooxidans. The major pathways of fixation were found to be the carboxydismutase system and the phosphoenolpyruvate (PEP) carboxylase system. PEP carboxylase activity was shown to be under metabolic regulation, similar to the regulation established in heterotrophic microorganisms.Acetyl-CoA stimulated PEP carboxylase activity, while aspartate was inhibitory. The F. ferrooxidans enzyme appeared to have a neutral or acidic pH optimum, in contrast to the same enzyme isolated from heterotrophs.

Glucose catabolism in Micrococcus sodonensis

The inability of Micrococcus sodonensis to grow on glucose as the sole source of carbon and energy was investigated. Estimation of pathways of glucose catabolism indicated that both the glycolytic and hexose monophosphate pathways are present in this organism. Comparative studies with Escherichia coli demonstrated that key enzymes for glucose catabolism were present in M. sodonensis in quantities equivalent to those of E. coli. The glucose-6-phosphate and 6-phosphogluconate dehydrogenases of M. sodonensis were nicotinamide adenine dinucleotide phosphate (NADP) specific, and glyceraldehyde-3-phosphate dehydrogenase was nicotinamide adenine dinucleotide specific. Transhydrogenase and reduced NADP oxidase were absent. Growth of the organism in the presence of glucose did not result in a repressed ability to oxidize tricarboxylic acid cycle intermediates, but these cells did have a decreased capacity for glucose degradation. The addition of substrates rich in growth-promoting substances, e.g., yeast extract, did not provide requisite nutrients for growth on glucose. Studies with (32)P suggest that M. sodonensis is incapable of synthesizing energy-rich phosphate compounds during the catabolism of glucose.

Properties and regulation of phosphopyruvate carboxylase activity in Escherichia coli

A procedure is described for the purification of phosphopyruvate ( PEP ) carboxylase (EC 4.1.1.31) from extracts of Escherichia coli , strain W. The enzyme, purified 160-fold, catalysed the stoicheiometric formation of oxaloacetate from phosphopyruvate and potassium bicarbonate. This reaction occurred most readily at pH 8·5, and required the presence of divalent metal ions (Mg 2+ ≥ Mn 2+ > Co 2+ ); the K m for Mg 2+ was 9·8 x 10 -4 M. Addition of acetylcoenzyme A to the reaction system greatly stimulated the rate of oxaloacetate formation. At saturating concentrations of acetyl-coenzyme A (about 1 mM), the reaction rate was thirty times more rapid than that observed if the thiol-ester was omitted; other acyl-coenzyme A derivatives were less effective in stimulating PEP -carboxylase activity (acetyl- > propionyl- > butyryl- ≫ acrylyl- > crotonyl-coenzyme A ). The K m for acetyl-coenzyme A was 1·4 x 10 -4 M. The effect of acetyl-coenzyme A was catalytic, and increased the apparent affinity of the enzyme for PEP from K m = 5.5 x 10 -3 M to K m = 6·4 x 10 -4 M. Since 1 M-urea inhibited the acetyl-coenzyme A -stimulated carboxylation of PEP but did not affect the enzymic activity in the absence of acetyl-coenzyme A , it is suggested that acetyl-coenzyme A exerts its stimulatory effect through interaction with an allosteric site on the enzyme. The results obtained also suggest a mechanism for the physiological regulation of PEP -carboxylase activity in E. coli .

Nature of the effector of catabolite repression of beta-galactosidase in Escherichia coli

Loomis, William F., Jr. (Massachusetts Institute of Technology, Cambridge, Mass.), and Boris Magasanik. Nature of the effector of catabolite repression of beta-galactosidase in Escherichia coli. J. Bacteriol. 92:170-177. 1966.-Many carbon sources were found to give rise to catabolite repression of beta-galactosidase in a mutant strain of Escherichia coli lacking hexose phosphate isomerase activity. Compounds containing glucose or galactose cannot be formed from several of these carbon sources in this mutant strain, and, therefore, appear not to be required for catabolite repression of beta-galactosidase. Glucose was observed to elicit catabolite repression of beta-galactosidase in another mutant strain under conditions in which the formation of compounds of the citric acid cycle is inhibited. If catabolite repression of the lac operon is mediated by a single compound, it appears that the compound is related to the pentoses and trioses of intermediary metabolism. The repression of beta-galactosidase by galactose in galactokinase negative strains was shown to be independent of the gene, CR, which determines catabolite sensitivity of the lac operon, and to be dependent on a functional i gene.