Autoxidation of native oxymyoglobin. Thermodynamic analysis of the pH profile.

Abstract

A complete kinetic description has been made on the pH profile for the autoxidation rate in terms of displacement of superoxide anion, O2- from MbO2 by the entering water molecule or hydroxyl ion. Using the equation, the effect of temperature on the autoxidation rate has been studied over the pH range 4.8-12.6 in 0.1 M buffer at 15 degrees, 25 degrees and 35 degrees C. The resulting thermodynamic parameters characterize the dissociation groups involved in the reaction as histidyl and tyrosyl residues. Despite the fact that each elementary process of the reaction is primarily protected against autoxidation by the high energy barrier of approximately 85-150 kJ . mol-1, the catalytic proton participates not only in decreasing the value of delta H degrees not equal to but also in increasing the value of delta S degrees not equal to to facilitate the formation of the activated complex, thereby promoting most of the autoxidation reaction of MbO2. The proton-catalyzed process is therefore of primary importance and a mechanistic detail of the reaction is discussed.