The oxidation of myoglobin to metmyoglobin by oxygen. 2. The relation between the first order rate constant and the partial pressure of oxygen

Abstract

Oxidation of myoglobin to metmyoglobin by O2 at 30[degree] in 0.6 [image] phosphate buffer, pH 5.69, was shown to be 1st order in unoxidized myoglobin over a range of O2 pressures from 0.3 to 760 mm. The observed 1st order rate constant at first increases with increasing O2 pressures, shows a well defined max. value at 1-1.4 mm. partial pressure of O2 and then decreases to a constant value above 30 mm. The detn. of the equilibrium constant for the myoglobin-O2 reaction under the conditions of the oxidation expts. at 30[degree] gave Ke= 0.88 [plus or minus] 0.12 mm.1. Hence the partial pressure for half saturation is 1-1.31 mm. and, thus kobs. has its max. value at half saturation. A free radical mechanism for the autoxidation is discussed which involves the participation of an auxiliary electron-accepting group on the protein molecule acting as a catalyst in a reaction regenerating the unoxidized hemoprotein and thus serving to "protect" the heme from oxidation. However, this mechanism is not complete for it does not account for the additional consumption of O2 above that required for oxidizing the heme group.