The active site histidines of creatine kinase. A critical role of His 61 situated on a flexible loop

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Abstract
A histidine residue with a PKα of 7 has been inferred to act as a general acid‐base catalyst for the reaction of creatine kinase (CK), catalyzing the reversible phosphorylation of creatine by ATP. The chicken sarcomeric muscle mitochon‐drial isoenzyme Mib‐CK contains several histidine residues that are conserved throughout the family of creatine kinases. By X‐ray crystal structure analysis, three of them (His 61, His 92, and His 186) were recently shown to be located close to the active site of the enzyme. These residues were exchanged against alanine or aspartate by in vitro mutagenesis, and the six mutant proteins were expressed in E. coli and purified. Structural integrity of the mutant proteins was checked by small‐angle X‐ray scattering. Kinetic analysis showed the mutant His 61 Asp to be completely inactive in the direction of ATP consumption while exhibiting a residual activity of 1.7% of the wild‐type (wt) activity in the reverse direction. The respective His to Ala mutant of residue 61 showed approximately 1% wt activity in the forward and 10% wt activity in the reverse reaction. All other mutants showed near wt activities. Changes in the kinetic parameters Km or Kmax, as well as a significant loss of synergism in substrate binding, could be observed with all active mutants. These effects were most pronounced for the binding of creatine and phosphocreatine, whereas ATP or ADP binding were less severely affected. Based on our results, we assume that His 92 and His 186 are involved in the binding of creatine and ATP in the active site, whereas His 61 is of importance for the catalytic reaction but does not serve as an acid‐base catalyst in the transphosphorylation of creatine and ATP. In addition, our data support the idea that the flexible loop bearing His 61 is able to move towards the active site and to participate in catalysis.