Perturbation of the intermolecular contact regions (molecular surface) of hemoglobin S by intramolecular low-O2-affinity-inducing central cavity cross-bridges

The general assumption among researchers on hemoglobin is that the intramolecular central cavity cross-bridging of Hb does not result in any generalized perturbations at the protein surface. A corollary of this is that central cavity cross-bridges are unlikely to influence the polymerization of deoxy HbS, since polymerization is a protein surface phenomenon involving the participation of multiple protein surface amino acid residues. In an attempt to evaluate this experimentally, we have introduced two low-O₂-affinity-inducing central cavity cross-bridges into HbS, -sebacyl between the two Lys-82() residues] and -fumaryl between the two Lys-99() residues], and investigated their influence on the polymerization of the deoxy protein. The O₂ affinities of the cross-bridged HbS exhibited sensitivity toward the buffer ions and pH in a cross-link-specific fashion. The modulation of the O₂ affinity of these cross-bridged HbS in the presence of allosteric effectors, DPG and L-35, is also very distinct, reflecting the differences in the conformational features these two cross-bridges induce within the central cavity at the respective effector-binding domains. In addition, the -fumaryl cross bridge inhibited the polymerization, reflecting the perturbation of the microenvironment of one or more intermolecular contact residues, protein surface residues, as a consequence of the central cavity cross-bridge. On the other hand, the -sebacyl cross-bridge exerted a slight potentiating effect on the polymerization of HbS. This reflects the fact that the perturbations at the protein surface are limited and favor polymerization. The results presented demonstrate that the structural changes induced by the central cavity cross-bridges are very specific and not simply restricted to the sites of modification, but are propagated to distant sites/domains, both within and outside the central cavity. It is conceivable that other surface regions that are not involved in the polymerization could also experience similar structural/conformational consequences. These results should be taken into consideration in designing intramolecularly cross-bridged asymmetric hybrid HbS for mapping the contribution of the intermolecular contact residues in the cis and trans dimers of deoxy HbS during polymerization.