MECHANISM OF FORMATION OF OXALOACETATE AND PHOSPHOENOL PYRUVATE FROM PYRUVATE

Abstract

The pathways by which pyruvate, oxaloa cetate and P-enolpyruvate are interlinked have been reviewed. The enzymes involved (see Table 1) and pathways differ markedly in different cells. Animals and yeast possessing pyruvate carboxylase are able to convert pyruvate to oxaloacetate directly by CO₂ fixation, but the propionic acid bacteria and E. coli use an indirect mechanism. These bacteria lack pyruvate carboxylase but possess an enzyme, P-enolpyruvate synthetase, which phosphorylates pyruvate directly with ATP forming P-enolpyruvate, AMP and Pi. They then convert the P-enolpyruvate to oxaloacetate; E. coli by the enzyme, P-enolpyruvate carboxylase, and the propionic acid bacteria by P-enolpyruvate carboxytransphos-phorylase. Animals lack P-enolpyruvate synthetase and are not able with pyruvate kinase to catalyze the direct synthesis of P-enolpyruvate effectively from pyruvate. They make P-enolpyruvate from oxaloacetate by catalysis with P-enolpyruvate carboxykinase. Not only do the animal cells have different enzymes and use different mechanisms than bacteria, the distribution of the enzymes in the cytosol and mitochondria of cells differ in the different species of animals. This variation in distribution presents a possibility for control of metabolism which varies from species to species. The mechanisms of these controls are still subject to much speculation, but have been considered briefly.