Protein—Heme Interactions in Hemoglobin from the Mollusc Scapharca inaequivalvis: Evidence from Resonance Raman Scattering

Resonance Raman spectra of the Scapharca inaequivalvis homodimeric hemoglobin (HbI) have been measured for the ligand-bound and ligand-free ferrous forms of the protein. In the deoxy derivative, the iron-histidine (Fe-His) stretching mode, proposed as a marker of the oxygen affinity and a conduit linking the hemes to the subunit interface, gives rise to a Raman peak centered at 203 cm⁻¹, an unusually low frequency compared to that reported for other hemoglobins and myoglobins. In the CO-bound derivative, three isotope-sensitive lines at 517, 583, and 1945 cm⁻¹ have been assigned to the Fe-CO stretching, Fe-C-O bending, and C-O stretching modes, respectively. From the frequencies of these modes and from their relative intensities, the Fe-C-O geometry appears to be tilted from axial coordination and shows a bending angle which has been estimated to be about 171 ± 5°. For the oxygen derivative, only one isotope–sensitive peak has been detected at 570 cm⁻¹, in line with the values found for myoglobin and other hemoglobins. Resonance Raman spectra of HbI modified with p-–(chloromercuri)benzoate (PMB) at Cys⁹² have been measured in parallel with those of the native protein. Despite the large increase in oxygen affinity produced by the PMB modification, the frequency of the Fe-His stretching mode is unshifted in the deoxy derivative. Thus, in HbI, the frequency of the Fe-His stretching mode does not correlate with the dissociation constant of the last oxygen molecule as found in human hemoglobins. In turn, this finding indicates that the iron-histidine bond does not play the same role in heme-heme communication in HbI as it does in the vertebrate hemoglobins. The low-frequency modes, which are sensitive to the conformation of the peripheral substituents on the heme, differ substantially from those in mammalian hemoglobins. The results show that in the dimeric hemoglobin the interactions between the heme and the protein pocket are substantially different and are consistent with a mechanism of cooperativity involving direct communication between the two hemes.