ROLE OF GLOBIN MOIETY IN THE AUTOXIDATION REACTION OF OXYMYOGLOBIN - EFFECT OF 8 M UREA

It is in the ferrous form that myoglobin or hemoglobin can bind molecu lar oxygen reversibly and carry out its function. To understand the po ssible role of the globin moiety in stabilizing the FeO2 bond in these proteins, we examined the autoxidation rate of bovine heart oxymyoglo bin (MbO(2)) to its ferric met-form (metMb) in the presence of 8 M ure a at 25 degrees C and found that the rate was markedly enhanced above the normal autoxidation in buffer alone over the whole range of pH 5-1 3. Taking into account the concomitant process of unfolding of the pro tein in 8 M urea, we then formulated a kinetic procedure to estimate t he autoxidation rate of the unfolded form of MbO(2) that might appear transiently in the possible pathway of denaturation. As a result, the fully denatured MbO(2) was disclosed to be extremely susceptible to au toxidation with an almost constant rate over a wide range of pH 5-11. At pH 8.5, for instance, its rate was nearly 1000 times higher than th e corresponding value of native MbO(2). These findings lead us to conc lude that the unfolding of the globin moiety allows much easier attack of the solvent water molecule or hydroxyl ion on the FeO2 center and causes a very rapid formation of the ferric met-species by the nucleop hilic displacement mechanism. In the molecular evolution from simple f errous complexes to myoglobin and hemoglobin molecules, therefore, the protein matrix can be depicted as a breakwater of the FeO2 bonding ag ainst protic, aqueous solvents.