Regulation at the phosphoenolpyruvate branchpoint in Azotobacter vinelandii: pyruvate kinase.

Pyruvate kinase (EC 2.7.1.40) from Azotobacter vinelandii responds sharply to the adenylate energy charge, with a decrease in activity at high values of charge, as expected for an enzyme of an adenosine triphosphate-regenerating sequence. Glycolytic intermediates, especially glucose 6-phosphate, fructose 6-phosphate, and fructose-1,6-diphosphate, strongly stimulate the reaction and overcome the inhibition caused by high values of energy charge. Thus, the properties of this enzyme depend on interaction between energy charge and the concentrations of hexose phosphates. The properties of pyruvate kinase, together with those of phosphoenolpyruvate carboxylase, aspartokinase, and citrate synthase, seem adapted to provide appropriate partitioning of phosphoenolpyruvate between competing pathways in response to metabolic need.