The allosteric effect of inositol hexasulfate on oxygen binding by hemoglobin

Myoinositol hexasulfate (IHS) is a powerful allosteric effector of oxygen binding by hemoglobin. It binds to deoxyhemoglobin at the same site as 2,3-diphosphoglycerate (DPG) and inositol hexaphosphate (IHP) with an affinity which is intermediate between that of the two phosphate esters. The binding constant calculated from the displacement of the oxygenation curve in the presence of low concentrations of IHS is 0.9X10(6) M. The value obtained directly from the number of protons bound as a function of IHS concentration is 1.0X10(6) M. The agreement between these two independent measurements provides an experimental verification of the empirical equation, relating the oxygen affinity to the binding constants, proposed previously (Benesch, R.E., Benesch, R., Renthal, R and Gratzer, W.B. (1971), Nature (London), New Biol. 2348 174).