Kinetic properties of pyruvate kinase of rabbit brain

Summary

The kinetic properties of rabbit brain pyruvate kinase have been studied to determine its role in the regulation of glycolysis. One of the substrates of the enzyme, phosphoenolpyruvate, exhibits homotropic cooperativity (Hill coeff. of 1.45); thus, it is a moderate activator of the enzyme. The other substrate, ADP, shows normal Michaelis-Menton kinetics. Fructose-6-phosphate and glucose-6-phosphate activate the enzyme only slightly at the 1mm level and inhibit slightly at higher levels, and hence have no metabolic influence on the enzyme activity. Fructose-1, 6-diphosphate also has a slight activation up to 0.5 mm but no inhibition at higher level; therefore, it has no influence either. ATP, 2-phosphoglycerate, and phenylalanine are inhibitors of the enzyme. ATP, being the energy reservoir derived from glycolysis as well as a product of the reaction catalyzed by the enzyme, is a significant feedback inhibitor of the enzyme. These kinetic properties suggest a key role for pyruvate kinase in the regulation of glycolysis. Phenylalanine inhibition of the enzyme has been reported to be a possible mechanism of damage to the developing brain in phenylketonuria. The inhibition by phenylalanine at 10 mm in the assay mixture is reversed by alanine, cysteine, or serine at 0.2 mm level. Furthermore, the effect of these amino acids in reversing the phenylalanine inhibition are mutually enhancing. Consequently phenylalanine cannot have a significant inhibition on the activity of pyruvate kinase in brain.