Conformational Differences in CO Derivatives of HbA, HbC (Eβ6K) and HbS (Eβ6V) in the Presence and Absence of Inositol Hexaphosphate Detected Using Ultraviolet Resonance Raman Spectroscopy

The 229 nm excited ultraviolet resonance Raman spectra of the carbon monoxide saturated derivatives of adult human hemoglobin (HbA), HbC (Eβ6K) and HbS (Eβ6V) in the presence and absence of inositol hexaphosphate (IHP) at pH 6.35 reveal a distinct pattern of local and global conformational differences. In the absence of IHP, the protein specific spectral differences are all in the form of intensity changes. The intensity differences in the W3 band indicate that, compared with HbA, the mutant Hbs have the β-chain A helix more tightly packed against the E helix, with HbS having the largest change. Intensity differences in other Raman bands indicate that the local perturbation associated with the β6 substitution propagates to other regions of the globin. The IHP-induced changes are consistent with two types of effects, a further tightening of the packing of the A helix against the E helix and the appearance of T state features indicative of a strained or weakened R quaternary conformation. The protein specificity of these effects can be accounted for by a model in which the linkage of the A and H helices of the β-subunits through the Gluβ7-Lysβ132 (H10) salt bridge is modulated by the β6-sensitive packing of the A helix against the E helix. This mechanism also accounts for the protein specificity of the IHP effects since Lysβ132 is linked to two of the residues that form the central cavity IHP binding site,