ANALYSIS OF THE GLOBAL ARCHITECTURE OF HEMOGLOBIN A(2) BY HEME-BINDING STUDIES AND MOLECULAR MODELING

The kinetics of CNProto- and CNDeutero-hemin binding to apohemoglobin A(2) was investigated in a stopped-flow device in 0.05 M potassium pho sphate buffer, pH 7, at 10 degrees C. The overall kinetic profile exhi bited multiple phases: Phases I-TV corresponding with heme insertion ( 8.5-13 x 10(7) M-1 s(-1)), local structural rearrangement (0.21-0.23 s (-1)), global alpha delta structural event (0.071-0.098 s(-1)), and fo rmation of the Fe-His bond (0.009-0.012 s(-1)), respectively. Kinetic differences observed between apohemoglobin A(2) and apohemoglobin A (p reviously studied) prompted an analysis of the structures of beta and delta chains through molecular modeling. This revealed a structural re positioning of the residues not: only at, but also distant from the si te of the amino acid substitutions, specifically those involved in the heme contact and subunit interface. A significant global change was o bserved in the structure of the exon-coded 3 region and provided addit ional evidence for the designation of this as the subunit assembly dom ain.