pH Dependence of the Reaction Catalyzed by Avian Mitochondrial Phosphoenolpyruvate Carboxykinase

The pH dependence of the reaction catalyzed by phosphoenolpyruvate carboxykinase (PEPCK) provides significant insight into the chemical mechanism. The pH dependence of k(cat) shows the importance of two acidic ionizations with pK(a) values of 6.5 and 7.0 assigned to the active site metal ligands H249 and K228. A single basic ionization is observed with an apparent pK(a) value of 8.4 that is assigned to K275 that is located in the P-loop motif and is essential for phosphoryl transfer. The pH dependence of k(cat)/K(M,PEP) demonstrates the importance of the same two acidic ionizations in the interaction of phosphoenolpyruvate with PEPCK and a single basic ionization with a pK(a) value of 8.1 that is assigned to Y220. The interaction of Mg-IDP with PEPCK is dependent upon a single acidic ionization attributed to K228 and two basic ionizations, both having an average pK(a) value of 8.1. One of the basic ionizations is attributed to the P-loop lysine (K275) and the other to C273.