title: The regulatory properties of rabbit muscle pyruvate kinase. The effect of pH.
abstract: 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.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D008954:Models, Biological; D009132:Muscles / Q000201:enzymology*; D010728:Phosphoenolpyruvate / Q000378:metabolism; D011770:Pyruvate Kinase / Q000378:metabolism*; D011817:Rabbits
chemical_list: D000244:Adenosine Diphosphate; D010728:Phosphoenolpyruvate; D011770:Pyruvate Kinase; D008274:Magnesium