Relative stabilities of the two quaternary conformations of human fetal hemoglobin

The pH dependence of several functional properties of human fetal and adult hemoglobins have been studied to determine the relative stabilities of the high and low affinity (R and T) quaternary conformations of the two proteins under different conditions. Fetal aqumethemoglobin undergoes changes in sulfhydryl reactivity, absorption spectrum, and circular dichroism in the presence of insitol hexaphospahte which are consistent with a transition from the R to T quaternary state, but only at pH values below 6.8. In adult hemoglobin this transition can be induced pH values below 7.2. Even in the absence of phosphates, the ultraviolet (uv) circular dichroism spectrum of fetal aquomethemoglobin at low pH indicates the presence of some T conformation. The initial value for the second-order rate constant for combination of fetal deoxyhemoglobin with carbon monoxide is comparable to that for adult hemoglobin in the absence of organic phosphates and is not reduced by organic phosphates as much as that for the adult protein. The apparent first-order rate constant for dissociation of CO from fully liganded fetal hemoglobin, measured by replacement with NO, increases threefold in the absence of organic phosphates, and fourfold in the presence of organic phosphates, with decreasing pH; the midpoint of the pH dependent transition occurs around 6.8. A similar increase in the apparent first-order rate constant for O2 dissociation as measured by replacement with CO, can also be seen with decreasing pH. NO-hemoglobin F can be converted to the T state even when fully liganded simply by lowering the pH, as judged by uv circular dichroism, visible difference spectrum in the region of the alpha and beta bands, and a dramatic increase in the rate of NO dissociation, measured by replacement with CO in the presence of dithionite. These results are all consistent with a model for fetal hemoglobin in which the organic phosphate site may be functionally weakened by replacement of a residue involved in ionic interactions with the negatively charged phosphate groups, but in which the low affinity T conformation is intrinsically more stable than that of adllt hemoglobin. According to this model,the differences between fetal and adult hemoglobin can be accounted for primarily in terms of the relative stabilities of R and T conformations in each of the proteins with differences in the intrinsic properties of the individual conformations contributing effects of only secondary importance.