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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, Yoâhei, 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 metâform 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 pHâdependence studies of the autoxidation rate, using some typical H64 mutants of sperm whale myoglobin, over the wide range of pHâ5â12 in 0.1âm buffer at 25âÂ°C. Each mutation caused a dramatic increase in the autoxidation rate with the trend H64Vââ¥âH64Gââ¥âH64Lââ«âH64Qâ>âH64 (wildâtype) at pHâ7.0, whereas each mutant protein showed a characteristic pHâprofile which is essentially different from that of the wildâtype or native sperm whale MbO2. In particular, all the mutants have lost the acidâcatalyzed process that can play a dominant role in the autoxidation reaction of most mammalian myoglobins or hemoglobins. Kinetic analyses of various types of pHâprofiles lead us to conclude that the distal histidine residue can play a dual role in the nucleophilic displacement of O2 â from MbO2 or HbO2 in protic, aqueous solution. One is in a protonârelay 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.