Ligand binding kinetic studies on the hybrid hemoglobin alpha (human):beta (carp): a hemoglobin with mixed...

Oxygen and CO ligand binding kinetics have been studied for the hybrid hemoglobin (Hb) alpha (human):beta (carp), hybrid II. Valency and half-saturated hybrids were used to aid in the assignment of the conformations of both chains. In hybrid II, an intermediate S state occurs, in which one chain has R- and the other T-state properties. In HbCO at pH 6 (plus 1 mM inositol hexaphosphate), the human alpha-chain is R state and the carp beta-chain is T state. We have no evidence at this pH that the carp beta-chain ever assumes the R conformation. At pH 6, the human alpha-chain shows human Hb R-state kinetics at low fractional photolysis and T-state rates for CO ligation by stopped flow. At pH 7, the human-chain R-state rate slows toward a carp hemoglobin rate. The carp beta-chains, on the other hand, react 50% more rapidly in the liganded conformation than in carp hemoglobin, and while the human alpha-chains are in the R state, the two beta-chains appear to function as a cooperative dimer. In this hemoglobin, the chains appear to be somewhat decoupled near pH 7, allowing a sequential conformational change from the R state in which the beta-chains first assume T-state properties, followed by the alpha-chains. The rate of the R-T conformational change for the carp beta-chains is at least 300 times greater than that for the human alpha-chains. At pH 9, the R----T conformational transition rate is at least 200 times slower than that for human hemoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)