Effects of calcium ions on pyruvate kinase from human erythrocytes

Ca2+ ions have a biphasic effect on the allosteric pyruvate kinase (EC 2.7.1.40) from human erythrocytes: Ca2+ is an activator at low phosphoenolpyruvate (PEP) concentrations: at increased PEP concentrations Ca2+ behaves as an inhibitor. In the presence of ATP the same effect was observed and at low PEP concentrations Ca2+ ions can completely abolish the ATP inhibitory effect. At high Ca2+ concentrations there is a loss of the cooperativity towards PEP. The enzyme activated by fructose-1,6-diphosphate (FDP) is inhibited by Ca2+ ions at all concentrations of PEP tested. Mg2+ ions are not able to counteract the activation by Ca2+ ions at low PEP concentrations. The results are interpreted on the basis of the model of Monod.