Spectroscopic studies of myoglobin at low pH: heme structure and ligation.
We explore heme structure and ligation subsequent to a low-pH conformational transition in sperm whale myoglobin. Below pH 4.0, the iron-histidine bond breaks in metMb and deoxyMb. In MbCO, the majority of the iron-histidine bonds remain intact down to pH 2.6; however, the observation of a weak Fe-CO mode at 526 cm-1 indicates that a small fraction of the sample has the histidine replaced by a weak ligand, possibly water. The existence of a sterically hindered CO subpopulation in MbCO and the continued association of the four-coordinate heme with the protein in deoxyMb suggest that the heme pocket remains at least partially intact in the acid-induced conformation. The global pH-dependent conformational change described here is clearly distinguished from the local "closed" to "open" transition described previously in MbCO [Morikis et al. (1989) Biochemistry 28, 4791-4800]. Further observations of the four-coordinate heme state yield insights on the mechanism of heme photoreduction and the assignment of the 760-nm band in deoxyMb.
Sage
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JT
Department of Physics, Northeastern University, Boston, Massachusetts 02115.
Morikis
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Champion
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D000818:Animals; D002942:Circular Dichroism; D006418:Heme / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008024:Ligands; D008786:Metmyoglobin / Q000737:chemistry; D009211:Myoglobin / Q000031:analogs & derivatives / Q000737:chemistry / Q000648:ultrastructure*; D011487:Protein Conformation; D013059:Spectrum Analysis, Raman; D014907:Whales
D008024:Ligands; D009211:Myoglobin; C055547:deoxymyoglobin; D008786:Metmyoglobin; D006418:Heme