THERMODYNAMICS OF INOSITOL HEXAKISPHOSPHATE INTERACTION WITH HUMAN OXYHEMOGLOBIN

The interaction of inositol hexakisphosphate (IHP) with oxygenated hum an adult hemoglobin (Hb) was investigated at 25 degrees C. The affinit y of IHP for oxygenated Hb is strongly pH-dependent, and potentiometri c measurements of proton uptake and release upon IHP addition have sho wn that over the range between pH 8.0 and pH 6.0 in oxygenated Hb ther e are three groups of residues that change their pK(a) values after IH P addition, likely because of their interaction with negative charges of the heterotropic effector. On the basis of previous calculations on the electrostatic properties of human Hb (Matthew, J. B., Hanania, G, I. H., and Curd, F. R. N. (1979) Biochemistry 18, 1919-1928; Lee,.A W .-m., Karplus, M., Poyart, C., and Bursaux, E, (1988) Biochemistry 27, 1285-1301), two of these groups might be Val(1 beta) and His(143 beta ), which are located in the beta(1)beta(2)dyad axis, where they have b een also proposed to interact with 2,3-diphosphoglycerate, whereas the third group does not appear easily identifiable. Calorimetric measure ments of the heat associated with IHP binding at different pH values o ver the same range indicate that IHP binding is mostly enthalpy-driven at pH < 7 and mostly entropy-driven at pH > 7.