The Binding of Hemoglobin to Haptoglobin and Its Relation to Subunit Dissociation of Hemoglobin

The binding of liganded hemoglobin to haptoglobin, a plasma α2-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 105 m-1 sec-1 and the results can be fitted with a dissociation constant of 1.5 x 10-6 m for the hemoglobin tetramer. The study of the reaction of isolated α- and β-hemoglobin chains towards haptoglobin half-saturated by α 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 (αβ). These two pair of sites are independent and noninteracting but within each pair a strong interaction is observed between the α-specific site and the allosterically induced β site. A detailed model of the reaction under physiological conditions is proposed on the basis of these results.