Myoglobin autoxidation

Shikama K. 1998. The molecular mechanism of autoxidation for myoglobin and hemoglobin A venerable puzzle. Chem Rev 98 1357. 

The reactivity of NO with O2 is dramatically affected upon coordination of one of the diatomic components to a metal center. For example, the second-order reactions of NO with oxyhemoglobin, Hb(02) and oxymyoglobin, Mb(02) (e.g. Eq. (47)) are quite fast and have been used as colorimetric tests for NO (105). The nitrogen product is NO3 rather than N02 that is the product of aqueous autoxidation (106). While the reaction of 02 with nitrosyl myoglobin Mb(NO) (Eq. (48)) might superficially appear similar it is much slower and follows a different rate law (107). Possible mechanisms will be discussed below. 

The autoxidation of bovine heart myoglobin and of shark hemoglobin were followed respectively by the decrease of the absorbance at 581 and at 430 nm. BESOD accelerated definitely the autoxidation of the myoglobin but only very... 

In chemistry, it is well known that O2 can be strongly bound to a ferrons iron porphyrin in solvents without any protein matrix however, the oxygenated states of most simple iron porphyrins are irreversibly converted into /r-oxodimers (eqnation3), PFe(III)-0-PFe(III), via peroxo and ferryl intermediates (eqnation 2). The /x-oxodimer is usually very stable in solvents, so it is sometimes called a thermodynamic sink. In addition, autoxidation of PFe(II)-02 to an inert ferric porphyrin easily occurs under aerobic conditions (equation 4). Thus, it is clear that the heme pockets of myoglobin and hemoglobin play an important role in protecting the 02-bound heme from dimerization and autoxidation. 

Table 1 O2 Binding and autoxidation for a series of myoglobins, hemoglobins, and synthetic models (20 °C, pH 7.0)...

The picket fence hinders the face-to-face approach of the two hemes, but less effectively than the polypeptide chains of the heme protein. The autoxidation pathway for the heme in myoglobin (equations 102 and 103) is sufficiently slow to give the protein a useful life of several weeks, but is analogous to that for all reduced-iron systems. 

The most common dioxygen carriers of nature, the heme proteins hemoglobin and myoglobin, bind Oj in a characteristic structure. The Oj molecule binds to the metal atom through one of its atoms, and the M—O—O unit is angular in shape. Studies on small molecules that bind O2 in this way have concentrated on two classes of ligands, porphyrins and Schiff bases.In the first case, bulky superstructures are usually appended to the porphyrin ligand this superstructure inhibits autoxidation of the metal atom in the... 

Kinetic studies of the autoxidation of hemoglobin and myoglobin do not favor intramolecular mechanisms involving reaction between separated groups on the protein molecule, but the form of the rate equations resembles that found when two valency states of a metal ion compete for the same radical intermediate. A free radical mechanism based on competition for the HO2 radical can be developed to account for the observed results. 

Winfield, M.E. (1965). Mechanisms of oxygen uptake The autoxidation of myoglobin and of reduced cyanocobaltates and their significance to oxidase reactions. In T.E. King, H.S. Mason, and... 

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