Additive effects of beta chain mutations in low oxygen affinity hemoglobinbeta F41Y,K66T

In order to decrease significantly the oxygen affinity of human hemoglobin, we have associated the mutation beta F41Y with another point mutation also known to decrease the oxygen affinity of Hb. We have synthesized a recombi nant Hb (rHb) with two mutations in the beta chains: rHb beta F41Y,K66T. In the absence of 2,3-diphosphoglycerate, additive effects of the mutations a re evident, since the doubly mutated Hb exhibits a larger decrease in oxyge n affinity than for the individual single mutations. In the presence of 2,3 -diphosphoglycerate, the second mutation did not significantly increase the P-50 value relative to the single mutations. However, the kinetics of CO b inding still indicate combined effects on the allosteric equilibrium, as ev idenced by more of the slow bimolecular phase characteristic of binding to the deoxy conformation. Dimer-tetramer equilibrium studies indicate an increase in stability of the mutants relative to rHb A; the double mutant rHb beta F41Y,K66T at pH 7.5 showed a K-4,K-2 value of 0.26 mu M. Despite the lower oxygen affinity, the single mutant beta F41Y and double mutant beta F41Y,K66T show only a moder ate increase of 20% in the autoxidation rate. These mutations are thus of i nterest in developing a Hb-based blood substitute.