Axial coordination of ferric Aplysia myoglobin.
Resonance Raman spectra of ferric Aplysia myoglobin in the ligand-free and the azide-bound forms have been studied over a wide pH range to determine the coordination states of the heme iron atom. In the hydroxide form at high pH (approximately 9) the iron is six-coordinate and is in a high/low spin equilibrium. As the pH is lowered below the acid/alkaline transition (pKa = 7.5), the heme becomes five-coordinate. When the pH is lowered even further no other changes in the resonance Raman spectrum are detected; thus, the heme remains five-coordinate down to pH 4, the lowest value studied. For ferric azide-bound Aplysia myoglobin, the iron is six-coordinate in a high/low spin equilibrium at all pH values (4.8-9). These data indicate (i) that the unusual reactivity toward azide previously observed at neutral pH is indeed related to the absence of a coordinated water molecule, and (ii) that causes other than the heme coordination are responsible for the spectral differences and the ligand-binding kinetics differences observed below pH 6.
Rousseau
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AT&T Bell Laboratories, Murray Hill, New Jersey 07974.
Ching
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YC
Brunori
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Giacometti
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D000818:Animals; D001048:Aplysia / Q000032:analysis*; D001386:Azides / Q000378:metabolism; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism*; D009211:Myoglobin / Q000378:metabolism*; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman
D001386:Azides; D009211:Myoglobin; D007501:Iron