Heterotropic interactions in Monomeric (IS” Chains from Human Hemoglobin

The O2 affinity of fiSH chains is lowered by H+, inositol hexaphosphate (IHP), and COz. As the oxygen affinity of gSH monomers (flISH) is lower than that of fiSH tetramers (@dSH), it is possible that IHP and CO2 exert their influence on the Oz affinity of PSH chains by increasing the dissociation constant of +3dsH rather than by a direct effect on the molecule. In order to test for this hypothesis we have measured the 02 affinity of fiSH chains as a function of protein concentration at various concentrations of IHP and inorganic phosphates in the absence and presence of COz. From these data association constants for the binding of IHP to /31SH and j34SH as well as for the equilibrium 4j31SH = j3dSH were calculated. We found that IHP and CO2 influence the oxygen affinity of flISH. It was furthermore established that inorganic phosphate enhances the stability of PdSH while IHP favors its dissociation in monomers.