The Functional Properties of Hemoglobin Bethesda

Hemoglobin Bethesda is a /3 chain variant of human hemoglobin in which histidine is substituted for tyrosine at position 145. From chemical and crystallographic studies, it has been postulated that the tyrosine normally found at this position is critical for the maintaince of a normal deoxy structure. Kinetic studies of the carbon monoxide and Rbutyl isocyanide binding reactions show that deoxyhemoglobin Bethesda in low ionic strength buffers at pH 7.0 exhibits noncooperative, chain-like behavior. Under the same conditions, the Soret absorption spectrum of deoxyhemoglobin Bethesda is more like that of the isolated a~ and fl chains than that of the normal deoxyhemoglobin tetramer. In addition, reversible dissociation of deoxyhemoglobin Bethesda can be demonstrated by ultracentrifugation experiments. All of these results suggest that at neutral pH deoxyhemoglobin Bethesda exists in a conformation analogous to that postulated for liganded hemoglobin A. However, in the presence of inositol hexaphosphate, more normal, cooperative kinetic behavior is observed, subunit dissociation is reduced, and the absorption spectrum in the Soret region is found to be virtually identical with that of deoxyhemoglobin A. Thus, the substitution of histidine for tyrosine 145 /3 drastically alters the ability of hemoglobin to form a normal deoxy structure. Studies of the dissociation of oxygen and n-butyl isocyanide from saturated hemoglobin Bethesda indicate, however, that this mutation does not alter the intrinsic reactivities of the (Y and (3 heme sites within the liganded conformation. Rather, this amino acid substitution appears to inhibit the quatemary change to the deoxy conformation in such a manner that only on the addition of organic phosphates is behavior observed which resembles that of normal deoxyhemoglobin A.