Hb Bruxelles, deletion of Phebeta42, shows a low oxygen affinity and low cooperativity of ligand binding

Hemoglobin variants: biochemical properties and clinical correlates

Diseases affecting hemoglobin synthesis and function are extremely common worldwide. More than 1000 naturally occurring human hemoglobin variants with single amino acid substitutions throughout the molecule have been discovered, mainly through their clinical and/or laboratory manifestations. These variants alter hemoglobin structure and biochemical properties with physiological effects ranging from insignificant to severe. Studies of these mutations in patients and in the laboratory have produced a wealth of information on hemoglobin biochemistry and biology with significant implications for hematology practice. More generally, landmark studies of hemoglobin performed over the past 60 years have established important paradigms for the disciplines of structural biology, genetics, biochemistry, and medicine. Here we review the major classes of hemoglobin variants, emphasizing general concepts and illustrative examples.

Additive effects of beta chain mutations in low oxygen affinity hemoglobin betaF41Y,K66T

In order to decrease significantly the oxygen affinity of human hemoglobin, we have associated the mutation betaF41Y with another point mutation also known to decrease the oxygen affinity of Hb. We have synthesized a recombinant Hb (rHb) with two mutations in the beta chains: rHb betaF41Y,K66T. In the absence of 2, 3-diphosphoglycerate, additive effects of the mutations are evident, since the doubly mutated Hb exhibits a larger decrease in oxygen 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 binding still indicate combined effects on the allosteric equilibrium, as evidenced 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 betaF41Y, K66T at pH 7.5 showed a K(4,2) value of 0.26 microM. Despite the lower oxygen affinity, the single mutant betaF41Y and double mutant betaF41Y,K66T show only a moderate increase of 20% in the autoxidation rate. These mutations are thus of interest in developing a Hb-based blood substitute.

Tetramer-dimer equilibrium of oxyhemoglobin mutants determined from auto-oxidation rates

One of the main difficulties with blood substitutes based on hemoglobin (Hb) solutions is the auto-oxidation of the hemes, a problem aggravated by the dimerization of Hb tetramers. We have employed a method to study the oxyHb tetramer-dimer equilibrium based on the rate of auto-oxidation as a function of protein concentration. The 16-fold difference in dimer and tetramer auto-oxidation rates (in 20 mM phosphate buffer at pH 7.0, 37 degrees C) was exploited to determine the fraction dimer. The results show a transition of the auto-oxidation rate from low to high protein concentrations, allowing the determination of the tetramer-dimer dissociation coefficient K4,2 = [Dimer] 2/[Tetramer]. A 14-fold increase in K4,2 was observed for addition of 10 mM of the allosteric effector inositol hexaphosphate (IHP). Recombinant hemoglobins (rHb) were genetically engineered to obtain Hb with a lower oxygen affinity than native Hb (Hb A). The rHb alpha2beta2 [(C7) F41Y/(G4) N102Y] shows a fivefold increase in K4,2 at pH 7.0, 37 degrees C. An atmosphere of pure oxygen is necessary in this case to insure fully oxygenated Hb. When this condition is satisfied, this method provides an efficient technique to characterize both the tetramer-dimer equilibrium and the auto-oxidation rates of various oxyHb. For low oxygen affinity Hb equilibrated under air, the presence of deoxy subunits accelerates the auto-oxidation. Although a full analysis is complicated, the auto-oxidation studies for air equilibrated samples are more relevant to the development of a blood substitute based on Hb solutions. The double mutants, rHb alpha2beta2 [(C7) F41Y/(G4) N102A] and rHb alpha2beta2 [(C7) F41Y/(E10) K66T], show a lower oxygen affinity and a higher rate of oxidation than Hb A. Simulations of the auto-oxidation rate versus Hb concentration indicate that very high protein concentrations are required to observe the tetramer auto-oxidation rate. Because the dimers oxidize much more rapidly, even a small fraction dimer will influence the observed oxidation rate.