Carboxyl-terminal modification influences subunit assembly of sickle hemoglobin beta chains

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.

Fluorescence studies of human semi-beta-hemoglobin assembly

The intrinsic fluorescence properties of human alpha apohemoglobin at protein concentrations from 1 to 5 microM in 0.1 M potassium phosphate buffer, pH 7 or 8 at 5 degrees C were monitored in the absence and presence of a fixed concentration (5 microM) of a fluorescence quenching heme-containing native or Des (146-His, 145-Tyr) beta chain partner. These "reverse quenching" studies revealed that the emission intensity changes observed correlated well with protein concentration and theoretical extent of semi-beta-hemoglobin assembly. Furthermore, the relative quenching efficiencies were calculated to be 0.32, 0.25 and 0.61 for beta (pH 7), beta (pH 8) and Des beta (pH 7) chains, respectively. Thus, heme-mediated quenching was sensitive to the expected pH induced alpha apohemoglobin conformational change and to alteration in beta chain structure. Intramolecular changes induced by carboxylterminal modification (decreased "beta chain self-quenching") appeared to enhance the intermolecular rearrangements (increased "alpha chain partner quenching") seen upon subunit assembly.