Carboxyl-terminal modification alters the subunit interactions and assembly pathways of normal and sickle hemoglobins.

Parallel isofocusing studies established that carboxypeptidase A removal of the His-146 (HC3) and Tyr-145 (HC2) residues of beta heme subunits affected the assembly properties of both Des beta(A) and Des beta(S) with alpha heme chains, albeit to differing degrees. Indeed, the rate of Des beta(A) oligomer dissociation (k1), as determined by visible spectroscopy, was 4.3-fold faster than that of its native beta(A) counterpart. Furthermore, Soret spectral studies have affirmed distinct rates of normal (HbA), sickle (HbS), and Des HbA hemoglobin assembly (k'2) from their alpha and beta [Des beta(A)] heme-containing monomers. Matching kinetic analysis of Des beta(A) and Des beta(S) chain assembly (with an identical a chain) revealed 4.6- and 7.8-fold faster combination rates than those seen for beta(A) and beta(S) chains, respectively. This 3-fold disparity in rates strongly supports the critical role of the beta-6 (A3) residue, and its amino-terminal region, in a chain partner recognition and subsequent human hemoglobin assembly.