Subunit interacting surfaces of human hemoglobin in solution: localization of the alpha-beta subunit interacting surfaces on the alpha-chain by a comprehensive synthetic strategy.

By using synthetic overlapping peptides encompassing the entire alpha-chain of adult human hemoglobin (HbA), we have mapped on the alpha-chain the regions responsible for its binding to the beta-chain in solution. These binding surfaces were, in general, in good agreement with those expected from the crystal structure (peptides alpha81-95, alpha101-115, alpha111-125, and alpha131-141). However, we observed some significant differences in the levels of binding found here in solution and those expected from the crystal structure. Peptide alpha31-45, which in the crystal had the highest number of contact residues of all the alpha-chain peptides, did not bind the beta-chain in solution. Similarly, peptide alpha91-105, with seven contact residues in the crystal, showed low binding with the beta-chain in solution. On the other hand, peptides alpha41-55 and alpha121-135 possessed much higher binding activity in solution than would be expected from their contribution to subunit association in the crystal. In fact, peptide alpha121-135 had the highest binding activity of the alpha-chain peptides. These studies and our previous findings, which localized on the beta-chain the regions that bind to the alpha-chain in solution, have shown that the regions of subunit association in solution are close to, but not identical with, those in the crystal. The approach should be quite useful for mapping subunit association in oligomeric proteins and could even be applied to proteins that are isolated only in traces or whose three-dimensional structure is not yet known.