Interaction between the carboxyl groups of Asp127 and Asp129 in the active site of Escherichia coli phosphofructokinase

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Abstract
The pH dependence of the enzymic properties of the phosphofructokinase from Escherichia coli was compared to those of two mutants in which one carboxyl group of the active site has been removed from either Asp127 or Asp129. All measurements of activity were made in the presence of allosteric activator ADP or GDP to eliminate any cooperative process. Asp129 is a crucial residue for the activity of phosphofructokinase since its conversion to Ser decreases the catalytic activity by 2-3 orders of magnitude in both the forward and reverse reactions, but the ionization of Asp129 is not directly related the pH dependence of phosphofructokinase activity. This pH dependence is however modified by the Asp129----Ser mutation, which decreases the pK of another residue, Asp127, by as much as pH of 1.5. The side chain of Asp127 has the catalytic role proposed earlier: its deprotonated form acts as a base in the forward reaction, and its protonated form acts as an acid in the reverse reaction. The protonated form of Asp127 is also required for the binding of fructose 1,6-bisphosphate. The electrostatic interaction between the carboxyl groups of Asp127 and Asp129 seems different in free phosphofructokinase to that in enzyme/substrate complexes, suggesting that a conformational change occurs upon substrate binding. The pH dependence of phosphofructokinase activity involves one other ionizable group with a pK of approximately 6 which does not belong to the side chains of Asp127 or Asp129.