title: Dual nature of the distal histidine residue in the autoxidation reaction of myoglobin and hemoglobin comparison of the H64 mutants.
abstract: 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 degrees 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.
authors.affiliation: Biological Institute, Graduate School of Science, Tohoku University, Sendai, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Suzuki
authors.forename: Y H
authors.initials: YH
authors.lastname: Watanabe
authors.forename: M
authors.initials: M
authors.lastname: Nagasawa
authors.forename: A
authors.initials: A
authors.lastname: Matsuoka
authors.forename: K
authors.initials: K
authors.lastname: Shikama
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D004175:Diptera; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007814:Larva; D016297:Mutagenesis, Site-Directed; D009211:Myoglobin / Q000737:chemistry* / Q000378:metabolism*; D010084:Oxidation-Reduction; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013053:Spectrophotometry; D014907:Whales
chemical_list: D006454:Hemoglobins; D009211:Myoglobin; D011994:Recombinant Proteins; D006639:Histidine