Kinetics of ligation reactions of rabbit hemoglobin in quaternary R and T states.

Rabbit hemoglobin shows significantly lower affinity for CO than does human hemoglobin (Hb A). The overall ligand combination and dissociation rate constants reveal, however, only small differences between Hb A rabbit Hb; this is mainly due to the fact that beta chains in rabbit hemoglobin determine the kinetics of ligand dissociation and combination. The heme environment in these chains is probably not very different in rabbit Hb and human Hb A. Rabbit hemoglobin alpha chains, on the other hand, exhibit greatly reduced CO and O2 combination rates in the R state and are primarily responsible for the overall low CO affinity of rabbit Hb. We postulate that the low ligand affinity of alpha chains in rabbit Hb is due to the substitution of larger residues at positions B10(Leu leads to Val), Cd6(Leu leads to Phe), and CD7(Ser leads to Thr). The possible implication of such substitutions for the kinetic and equilibrium properties of rabbit Hb are discussed.