The regulatory behavior of rabbit pyruvate kinase has been studied as a function of pH. The initial velocity of the enzyme-catalysed reaction as a function of ADP concentration was analysed with the exponential model for a regulatory enzyme. The analysis of the exponential model parameters as functions of pH provided pK values of 6.6 and 8.08 for the free enzyme in its fully ADP-bound conformation. By contrast, the binding of ADP to the ADP-free conformation of the free enzyme did not involve groups that ionize within the pH range (6.2-8.5) of these experiments. The results suggest that homotropic allosteric interactions actually alter the mode of ADP binding. The pK values of 6.63 and 9.00 determined from the analysis of V as a function of pH are readily interpreted in terms of a direct phosphoryl-transfer mechanism in which the beta-phosphoryl group of ADP (pK 6.63) acts as the nucleophile and a lysine epsilon-amino group (pK 9.0) acts as the proton donor in the pyruvate kinase reaction.

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