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Dual nature of the distal histidine residue in the autoxidation reaction of myoglobin and hemoglobin: Comparison of the H64 mutants

Suzuki, Tatsuya, Watanabe, Yo‐hei, Nagasawa, Masashi, Matsuoka, Ariki, Shikama, Keiji
European journal of biochemistry 2000 v.267 no.20 pp. 6166-6174
Physeter macrocephalus, aqueous solutions, autoxidation, heme, hemoglobin, histidine, mammals, mutants, mutation, myoglobin, pH, solvents, superoxide anion
The oxygenated form of myoglobin or hemoglobin is oxidized easily to the ferric met‐form with generation of the superoxide anion. To make clear the possible role(s) of the distal histidine (H64) residue in the reaction, we have carried out detailed pH‐dependence studies of the autoxidation rate, using some typical H64 mutants of sperm whale myoglobin, over the wide range of pH 5–12 in 0.1 m buffer at 25 °C. Each mutation caused a dramatic increase in the autoxidation rate with the trend H64V ≥ H64G ≥ H64L ≫ H64Q > H64 (wild‐type) at pH 7.0, whereas each mutant protein showed a characteristic pH‐profile which is essentially different from that of the wild‐type or native sperm whale MbO2. In particular, all the mutants have lost the acid‐catalyzed process that can play a dominant role in the autoxidation reaction of most mammalian myoglobins or hemoglobins. Kinetic analyses of various types of pH‐profiles lead us to conclude that the distal histidine residue can play a dual role in the nucleophilic displacement of O2 − from MbO2 or HbO2 in protic, aqueous solution. One is in a proton‐relay mechanism via its imidazole ring, and the other is in the maximum protection of the FeO2 center against a water molecule or an hydroxyl ion that can enter the heme pocket from the surrounding solvent.