Evidence for two oxygen-linked binding sites for polyanions in dromedary hemoglobin

The functional properties of dromedary hemoglobin have been studied as a function of chloride, polyphosphates and pH and compared with those of human hemoglobin. The two proteins have the same amino acid residues at the anion-binding sites as well as at the level of the groups responsible for the alkaline Bohr effect. Analysis of the experimental data reveals that: (a) intrinsic oxygen affinity and the Bohr effect are very similar for the two proteins; (b) the association equilibrium constants of chloride are substantially higher in the dromedary system, both in the unligated and ligated state; (c) two polyanion-binding sites occur in dromedary oxy and deoxyhemoglobin; (d) association constants of polyphosphates for the higher-affinity binding site (probably in the cavity between beta chains) are comparable for the two proteins under physiological conditions; association constants for the second binding site in dromedary hemoglobin are not affected by pH changes; (e) the dependence of oxygen affinity in dromedary hemoglobin upon chloride concentration is complex, this anion at relatively low concentrations reverses the effect of millimolar polyphosphate; (f) both in stopped-flow and flash photolysis experiments the kinetic behaviour of dromedary hemoglobin is consistent with the equilibrium results. The pronounced sensitivity to solvent composition of the functional properties of dromedary hemoglobin even in the oxy state stresses the potential relevance of this conformation for regulating the oxygen transport in vivo.