A Kinetic study of the pH dependence of the reaction between metmyoglobin and hydrogen peroxide
Watson, Lenetta Gleeson
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The kinetics of the bimolecular reaction between metmyoglobin and hydrogen peroxide were studied through the use of stopped-flow techniques. The reactions were performed within the pH range of 5.0 to 11.0 in buffer solutions having ionic strengths of 0.10 M. The reactions were studied under pseudo-first order conditions with hydrogen peroxide present in excess, in one case, and with metmyoglobin present in excess, in another case. Second order rate constants were calculated frotn the slope of the lines constructed from plots of observed pseudo-first order rate constants versus concentrations of hydrogen peroxide. The rate constants of the reaction were independent of the pH of the reaction within the pH range of 5.0 to 8.5, but within the pH range of 8.5 to 11.0, the rate constants of the reaction decreased with increasing pH. The pH dependence of the rate constants for the reaction between metmyoglobin and hydrogen peroxide was fit to a mechanism involving a single pK, in the protein. From the spectra of metmyoglobin, taken in buffers prepared at pH values within the entire range of the protein’s stability, there appeared to be two different forms of metmyoglobin. One form existed at pH below 8.5 and the other form existed above pH of 8.5. The acidic form of metmyoglobin reacts with hydrogen peroxide with a rate of 315 ± 15 M'V1, while the alkaline form of metmyoglobin was unreactive. The transition from the acidic form to the basic form occurred at approximately pH 9.5.