Review of the Comparative Biochemistry of Pyruvate Kinase

Pyruvate kinase (EC 2.7.1.40 ATP: pyruvate phosphotransferase) from mammalian sources functions predominantly in an aerobic environment. Basically two forms of the enzyme are found, one in gluconeogenie tissues which has allosteric properties; the other in non-gluconeogenic tissues which exhibits classical Michaelis-Menten kinetics. By contrast, pyruvate kinase from many invertebrate sources functions in an oxygen-depleted or oxygen-free environment, while some invertebrates undergo alternate periods of aerobiosis and anaerobiosis, e.g. the intertidal bivalve molluscs. Adaptations to environments such as these have meant that under anaerobic conditions dramatic changes in the overall metabolism of the organism have taken place. Often little of the glycolytic end product, lactate, accumulates in the organism. This is associated with an alternative route for phosphoenolpyruvate metabolism such that additional reactions occur that produce ATP by substrate level phosphorylation of ADP. Obviously under conditions such as these the activity of pyruvate kinase must be diminished below that which is observed under aerobic conditions. Concomittantly alternative compound(s) must be produced that will act as an electron sink for the oxidation of NADH since the formation of pyruvate will no longer be stoichiometric with that of the reduced pyridine nucleotide.