A proton nuclear magnetic resonance investigation of human hemoglobin A 2: Implications on the intermolecular contacts in sickle hemoglobin fibers and on the bohr effect of human normal adult hemoglobin

Abstract

High-resolution proton nuclear magnetic resonance spectroscopy at 300 and 600 MHz has been used to investigate the conformation of a minor hemoglobin component of human blood, hemoglobin A 2 ( α 2 β 2), in solution. We have found that (i) the replacement of the β chains by the δ chains in hemoglobin A 2 conserves the α 1 δ 2 interface but slightly perturbs the α 1 δ 2 interface, and (ii) one surface histidine residue in the deoxy form and one in the carbonmonoxy form of hemoglobin A 2 have local conformations and / or electrostatic environments which are different from the corresponding ones in human normal adult hemoglobin. By comparing the proton nuclear magnetic resonance titration of individual histidine residues in hemoglobin A 2 and in human normal adult hemoglobin, we can conclude that in human normal adult hemoglobin, both β116 and β117 histidine residues are titratable in both the deoxy and the carbonmonoxy forms. Thus, these two histidine residues can contribute to the Bohr effect of human normal adult hemoglobin. The present nuclear magnetic resonance data on hemoglobin A 2 and those previously obtained in our laboratory in sickle hemoglobin suggest that the antisickling property of hemoglobin A 2 does not originate from an alteration of the intermolecular contact site at the β6 position, but involves additional amino-acid residues which are different in the β and δ chains. We have found that the replacement of the β116 and β117 histidine residues in the δ chains does not play a significant role in the antisickling effect of hemoglobin A 2 and, thus, these amino-acid residues do not participate in the intermolecular interactions responsible for the polymerization of sickle hemoglobin.