The functional changes, associated with the sequential transformation of L′4 into L4 pyruvate kinase (ATP:pyruvate 2-O-phosphotransferase, EC 188.8.131.52) were studied. L′4 enzyme from human erythrocytes shows strong hysteretic behaviour: the initial rate of the enzyme preincubated with an unsaturating concentration of phosphoenolpyruvate is much higher than of the enzyme preincubated with ADP, at the same phosphoenolpyruvate concentration, although the ‘final activity’ (the activity of the linear part of the reaction progress curve) was the same in both cases. This phenomenon was observed both in the presence and absence of fructose 1,6-diphosphate. High concentrations of both Mg2+free and MgATP2- diminish the difference in initial rate, between the ADP and phosphoenolpyruvate preincubated enzymes: Mg2+free by stabilizing the phosphoenolpyruvate-induced form; ATPMg2- by stabilizing the ADP-induced form. The magnitude of the difference in initial rates of the ADP- or phosphoenolpyruvate-preincubated enzyme is a function of both substrates. L4 pyruvate kinase(either from human liver or trypsin treated L′4 enzyme) does not, or to a very slight extent, show such behaviour. L′2L2 pyruvate kinase shows behaviour intermediate between L′4 and L4 enzymes.\ud \ud A model is proposed to describe the kinetic behaviour of L′4 and L4 enzymes
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