Ion Binding by Adenosine Triphosphate-Creatine Phosphotransferase

From the ¹ From the Department of Biochemistry, Howard University Medical School, Washington, D. C. 20001

The binding of K⁺, Na⁺, Cl^-, Ca⁺⁺, and Mg⁺⁺ by deionized creatine kinase (ATP-creatine phosphotransferase, EC 2.7.3.2) was investigated as a function of pH at 25° with the use of permselective membrane electrodes. Whereas significant binding of K⁺ to creatine kinase occurs, no binding of Na⁺ can be detected. Cl^- is also bound. Graphs of v/c with respect to v give a linear relationship for K⁺ and Cl^-; association constants and number of maximal binding sites are deduced from such plots for these ions. The number of binding sites on the protein for Mg⁺⁺ is essentially the same as that for Ca⁺⁺, but the affinity of the protein for Mg⁺⁺ is only about 0.6 of that for Ca⁺⁺. The combining sites for each of these 2 divalent ions cannot be considered as belonging to a single class with identical association constants.

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