Allosteric Properties of Skeletal Muscle Pyruvate Kinase

Abstract

Abstract The phenylalanine inhibition of skeletal muscle pyruvate kinase has been further studied. Even at high levels of the inhibitor the plots of reaction rates as a function of phosphoenolpyruvate concentration give hyperbolic curves; the inhibition by phenylalanine is of the mixed type. The enzyme exhibits a homotropic cooperative effect with respect to this inhibitor. This property is strongly dependent on pH. Serine, cysteine, and alanine reversed this inhibition. The reactivating amino acids greatly increase the I0.5 but do not alter the cooperativity of the inhibitor. These effects were not found with 2-phosphoglyceric acid, an isosteric inhibitor. In fact, the inhibition is independent of pH and it is not reversed by the amino acids. Furthermore the enzyme follows Michaelis-Menten kinetics with respect to this inhibitor. The reassociation of the enzyme from its subunits markedly decreases the homotropic cooperative effect of phenylalanine. It is concluded that this amino acid is an allosteric inhibitor.