Ligand binding in hemoglobin J Capetown

Abstract

Hemoglobin J Capetown (α 92Gln 2β 2) exhibits a two- to threefold increase of oxygen affinity and, in preliminary evaluation, a normal Bohr effect. The combination velocity constant for CO is less than twice as great as that for Hb A, while the dithionite reaction with oxy Hb J Capetown shows a decreased rate as compared with Hb A. These data suggest that the kinetic basis of the high affinity of this hemoglobin variant is a combination of an increased combination velocity and a decreased dissociation velocity for ligands. The most striking finding is the presence of considerable co-operativity ( n in Hill's equation = 2.2) in the reaction of the hemes with ligands. The difference between the rate of the replacement reaction of O 2 by CO and the rate observed with the dissociation of O 2 by dithionite was comparable to Hb A, confirming the existance of co-operativity. Finally, the binding of haptoglobin occurs at a somewhat faster rate than normal Hb A. These findings in Hb J Capetown are in distinct contrast with the ligand binding properties of another mutation of the same site, Hb Chesapeake (α 92Leuβ 2). In this Hb, a much higher oxygen affinity is observed, in addition to a drastically reduced heme-heme interaction ( n = 1.37). Additionally, Hb Chesapeake exhibits a slower binding rate with haptoglobin as compared with Hb A. The differences in the type of perturbation introduced by the polar glutamine as compared with the hydrophobic leucine residue in the α 1 β 2 area of contact must explain the different array of properties in these two hemoglobins.