The Mechanism of Metmyoglobin Oxidation

From the Division of Physical Chemistry, Commonwealth ScientiJic and Industrial Research Organization, Melbourne, Australia (Received for publication, September 27, 1962) The chemical behavior of metmyoglobin has received con- siderable attention, notably by Keilin and Hartree (1) and by George (2) and George and Irvine (3-5), on the grounds that it has the same prosthetic group as peroxidase and catalase and reacts with II&&, but at a rate more convenient for kinetic investigation. Although the mechanism of HzOz decomposition has proved t.o be different, this in itself is of interest if we can correlate the observed chemical behavior with the nature of the amino acid side chains which surround the iron-porphyrin- histidine-water complex. For such studies the myoglobin molecule is particularly appropriate, since its structure is known with more certainty than that of any other metalloprotein (6). By a combination of electron spin resonance, manometric, and moderately rapid spectrophotometric techniques, applied at low temperatures to slow the reactions, we have found that the complexity of metmyoglobin oxidation is considerably greater than generally believed. The present paper is concerned mainly with the formation and subsequent reaction of the com- plex described by Keilin and Hartree (1) which is characterized by an absorption peak at 545 mp, and with the extent of osida- tion of the protein in the vicinity of the iron atom. In a later paper we shall discuss the nature of the free radical observed by Gibson, Ingram, and Nicholls (7). As an essential part of the argument of the present paper, we show that the free radical is formed extensively at pH 6 and provide evidence for the mech- anism of its appearance and subsequent loss. We wish to point out that the conclusions reached do not necessarily apply below pH 6 or above pH 8.