Kinetic analysis of metsulphmyoglobin and metmyoglobin reduction by Fe(EDTA)2-

Metsulphmyoglobin prepared from horse heart myoglobin was purified by ion-exchange chromatography to yield a product that on reduction with Fe(EDTA)2- has an A617/A561 ratio greater than 3.5:1. The kinetics of reduction of this purified metsulphmyoglobin and of native metmyoglobin by Fe(EDTA)2- were studied under various conditions of pH, ionic strength and temperature to compare the relative electron-transfer reactivities of a metallochlorin and a metalloporphyrin in identical protein environments. Although the rate of metsulphmyoglobin reduction is 2-7 times that of metmyoglobin under a variety of conditions, this difference can be more than compensated for by the reported difference in mid-point reduction potential between the two forms of the protein. The electrostatic and activation parameters observed for native metmyoglobin and metsulphmyoglobin are essentially identical, and small differences are found in the pH-dependence of the reduction reaction. These findings lead us to conclude that conversion of the porphyrin prosthetic group into a chlorin has relatively little effect on the electron-transfer reactivity of the central metal atom.