Role of $ 37 Gln in the Inhibition of Hemoglobin S Polymerization by Hemoglobin

Previous studies suggested that y87 Gln in hemoglobin (Hb) F is an important site for promoting inhibition of Hb S (cy#: polymerization by H b F. We engineered and isolated the double mutant ) using a yeast expression system and characterized polymerization properties of this modified tetramer in an effort to clarify the role of Gln at position 87 in inhibiting H b S polymerization. Electrophoretic mobility and absorption spectra of this double mutant were the same as that of Hb S, while oxygen affinity was higher, and effects of organic phosphates on oxygen affinity were reduced. The deoxy form of the double mutant showed a characteristic delay time prior to polymerization in uitro. The critical concentration for polymerization of the double mutant was about 1.5 times higher than Hb S, and delay and polymerization times were much longer than Hb S at the same hemoglobin concentrations. The logarithmic plot of delay time uersus hemoglobin concentration for the double mutant showed a straight line that was intermediate between lines for AS and FS mixtures. These results and those of kinetics of polymerization of H b Wdouble mutant mixtures indicate that substitution of Gln for Thr at p87 in H b S prolongs delay time and inhibits polymerization, although the double mutant forms polymers like Hb s. (Hb cyzpzS Glu f Val, 87 Thr f Gln