Nonequivalence of the α and β Chains in the Reaction of Hybrid-Heme Hemoglobin with Carbon Monoxide

Abstract

Abstract The rates of binding and dissociation of CO and hybrid-heme hemoglobins, in which one chain contains protoheme and the other chain meso- or deuteroheme, were measured at each heme site independently of changes at the partner chain site. The rates were also determined on isolated chains. The binding reaction of CO to hybrid-heme hemoglobins follows an autocatalytic process characteristic of a heme-heme interaction, and the initial and final values of the apparent second order rate constants of this reaction were obtained. The rate constant of the dissociation of the CO molecule from saturated hemoglobins was also estimated by the NO replacement method. All of these rate constants were compared with those determined for isolated chains containing the corresponding heme. For isolated chains, the rate of CO binding to β chains is usually larger than that to α chains containing the same heme, and the rate of CO dissociation from β chains is slower than that from the corresponding α chains. In hybrid-heme hemoglobins, when the rates were compared with those of corresponding isolated chains, CO binding to the β chains was slowed down more than that to the α chains. Moreover, the rate of CO dissociation from β chains was increased, while that from α chains was decreased. It was suggested that a conformational change occurs upon formation of a tetramer molecule (composed of α and β chains) which changes β chains from a more accessible state to the CO molecule than α chains to a less accessible state with respect to both the binding and dissociation processes.