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Temperature Jump Studies of Sperm Whale Metmyoglobin

III. EFFECT OF HEME-LINKED GROUPS ON LIGAND BINDING

From the ¹ From the Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706

The binding kinetics of H₂S and imidazole by sperm whale metmyoglobin has been studied in the pH range from pH 5.0 to 8.0 by the temperature jump technique.

The pH dependence of the bimolecular binding rate constant has been interpreted in terms of mechanisms involving zero, one, and two heme-linked ionizing groups on the protein. Only a mechanism involving two ionizing groups on the protein with pK values of 4.7 and 5.7 can fit all the data. The rate constants obtained for the individual steps of this mechanism are consistent with an electrostatic model for the binding process.

The E-7 histidine group and the carboxyl group of a propionic acid residue attached to the heme group are postulated to be the ionizing groups with pK values of 5.7 and 4.7, respectively.

A systematic method for analyzing pH-dependent bimolecular rate data is outlined for those reactions which have reactants with several ionizable groups affecting the binding kinetics.

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