Multiple heme pocket subconformations of methemoglobin associated with distal histidine interactions.
Electron paramagnetic resonance spectra of methemoglobin reveal that, in addition to the major tetragonal high-spin aqueous complex and the low-spin hydroxide complex, three other complexes associated with the interaction of the distal histidine are resolved. These are a rhombic high-spin and two classes of low-spin bis-histidine complexes. By freeze-quenching experiments it is shown that the rhombic high-spin and one of the low-spin bis-histidine complexes (B) are at equilibrium with the dominant species. Incubation in the 210-260 K temperature range shifts the total equilibrium toward a low-energy state with the distal histidine coordinated to the iron (complex C).
Levy
A
A
Laboratory of Cellular and Molecular Biology, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224.
Kuppusamy
P
P
Rifkind
J M
JM
D000818:Animals; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006736:Horses; D006801:Humans; D008706:Methemoglobin / Q000737:chemistry*; D011487:Protein Conformation; D013696:Temperature; D013816:Thermodynamics
D006418:Heme; D006639:Histidine; D008706:Methemoglobin