Interaction of inositol hexakisphosphate with liganded ferrous human hemoglobin. Direct evidence for two functionally operative binding sites.

Inositol hexakisphosphate (InsP6) binding to the oxygenated, carbonylated and nitrosylated derivatives of ferrous human hemoglobin (HbO2, HbCO and HbNO, respectively) has been measured at pH 7.0 (0.1 M Bis-Tris buffer, 0.1 M NaCl) and 20 degrees C. The observations indicate the presence of two InsP6 binding sites per tetramer in all the heme liganded hemoglobin derivatives, with different affinities for the polyphosphate. For each binding site, InsP6 interacts with similar affinity constants to HbO2, HbCO and HbNO. Such a finding indicates that different heme ligands do not alter significantly the stereochemistry of the polyphosphate binding cleft. This behaviour seems to indicate that, even though different heme ligands are likely to affect the tertiary conformation of the subunit in a different fashion, the perturbation does not seem to be transmitted to the quaternary arrangement of the whole macromolecule, and, thus, to the InsP6 binding site.