The effect of the apolar mutation of the distal histidine (His69 → Val) has been studied in the cooperative homodimeric hemoglobin from the mollusc Scapharca inaequivalvis. Absorption, circular dichroism, and resonance Raman spectroscopy point to a more symmetric heme structure of the deoxy derivative, which is indicative of an R-like conformation of the deoxy heme. Resonance Raman spectroscopy also brings out alterations in the geometry and interactions of the bound CO molecule. The iron-carbon stretching frequency is decreased by about 30 cm-1 with respect to the native protein, while the diatomic ligand stretching frequency is increased by about the same degree. Consistent with the structural changes, the ligand binding properties are significantly altered. In the mutant the overall rate and the affinity for CO binding are increased about 100-fold with respect to the native protein, and cooperativity is abolished. In addition, the amplitude and the rate of the geminate rebinding process increase significantly. This finding may be correlated to the longer average residence time of the photolyzed CO molecule within the heme pocket of the H69V mutant, as indicated by molecular dynamics simulations.
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