Pyruvate kinase from Lactobacillus bulgaricus: possible regulation by competition between strong and weak effectors

The pyruvate kinase from Lactobacillus bulgaricus has been purified to homogeneity. The native enzyme is composed of four probably identical subunits of relative molecular mass M(r) 72,000 +/- 4,000. The unique N-terminal amino acid sequence is homologous to those of other pyruvate kinases, especially of type I and II enzymes from Escherichia coli. The saturation of the pyruvate kinase from Lactobacillus bulgaricus is hyperbolic for ADP and cooperative for the other substrate phospho-enol-pyruvate. The enzyme is strongly activated by glucose-6-phosphate, ribose-5-phosphate, and fructose-6-phosphate, which increase the affinity for phospho-enol-pyruvate. These activators seem to stabilize the same state of the enzyme, since their maximum activations are not additive, but their partial activations can be cumulated. Pyruvate kinase is also weakly activated by AMP and inhibited by fructose-1,6-bisphosphate. However, both AMP and fructose-1,6-bisphosphate act as strong inhibitors in the presence of a strong activator, because these weak effectors suppress the activation by glucose-6-phosphate, ribose-5-phosphate, or fructose-6-phosphate. This mutual exclusion of strong and weak effectors, which appears as an original regulatory mechanism, could reflect either the binding of different effectors to different interacting sites or their competition for a unique polyvalent regulatory site in the pyruvate kinase from Lactobacillus bulgaricus.