Analogous Effect of Protons and Inositol Hexaphosphate on the Alteration of Structure of Nitrosyl Fetal Human Hemoglobin

We have determined the low temperature EPR spectra and room temperature ligand dissociation rate constants of human NO-hemoglobins F and A as a function of pH and inositol hexaphosphate levels in order to assess the contribution of a quaternary structural equilibrium in the two proteins to their spectral and functional properties. Our results are consistent with an increased stability of a ligated low affinity structure in the fetal protein; the functional properties of this structure appear to be essentially the same in the two hemoglobins, even though its stability relative to a high affinity conformation is different. The pH dependence of the NO dissociation constant in both adult and fetal hemoglobin can be assigned primarily to the pH-dependent equilibria of high and low affinity forms as monitored by EPR.