The binding of hemoglobin to haptoglobin and its relation to subunit dissociation of hemoglobin.

The binding of liganded hemoglobin to haptoglobin, a plasma cr2-glycoprotein, is an irreversible and stoichiometric reaction that occurs physiologically at the micromolar concentration level. The study of the reaction between deoxyhemoglobin which does not bind haptoglobin and a haptoglobin solution saturated with carbon monoxide indicates that hemoglobin tetramer is incapable of binding haptoglobin and its dissociation to dimers is a prerequisite for the reaction. The reaction between haptoglobin and the liganded dimers proceeds with a rate constant of about 5.5 X lo5 M-I see-l and the results can be fitted with a dissociation constant of 1.5 X 10v6M for the hemoglobin tetramer. The study of the reaction of isolated o(and @hemoglobin chains towards haptoglobin half-saturated by CY chains or Hb A, has permitted a detailed analysis of this reaction. The haptoglobin, on the basis of the data presented here, probably contains four binding sites, two for each hemoglobin dimer (a$). These two pair of sites are independent and noninteracting but within each pair a strong interaction is observed between the a-specific site and the allosterically induced ,L3 site. A detailed model of the reaction under physiological conditions is proposed on the basis of these results.