Interaction of zinc with hemoglobin: binding of zinc and the oxygen affinity

Abstract

Stripped human Hb has a high apparent Zn association constant of 1.3 .times. 107 M-1 with a stoichiometry of 1 Zn for every 2 hemes. The saturation of this site produces a dramatic 3.7-fold increase in the O2 affinity. The effect of Zn on the O2 affinity is interrelated with the interaction of 2,3-diphosphoglyceric acid (2,3-DPG) and Hb. A smaller Zn effect is observed in the presence of added 2,3-DPG. Information about the location of the Zn-binding site responsible for the increased O2 affinity was obtained by comparing the binding of Zn to various Hb. Blocking the .beta.93 sulfhydryl group decreases the apparent Zn association constant by an order of magnitude. The substitution of histidine-.beta.143 in Hb Abruzzo [.beta.143 (H21) His .fwdarw. Arg] and Hb Little Rock [.beta.143 (H21) His .fwdarw. Gln] decreases the apparent Zn association constant by 2 orders of magnitude. The substitution of histidine-.beta.143 by other amino acids and the reaction of the .beta.93 sulfhydryl group produce dramatic increases in the O2 affinity. The binding of Zn to 1 or both of these amino acids can explain the Zn-induced increase in the O2 affinity.