Effects of Substitutions of Lysine and Aspartic Acid for Asparagine at β108 and of Tryptophan for Valine at α96 on the Structural and Functional Properties of Human Normal Adult Hemoglobin: Roles of α1β1 and α1β2 Subunit Interfaces in the Cooperative Oxygenation Process

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Abstract
Using our Escherichia coli expression system, we have produced five mutant recombinant (r) hemoglobins (Hbs): r Hb (αV96W), r Hb Presbyterian (βN108K), r Hb Yoshizuka (βN108D), r Hb (αV96W, βN108K), and r Hb (αV96W, βN108D). These r Hbs allow us to investigate the effect on the structure−function relationship of Hb of replacing β108Asn by either a positively charged Lys or a negatively charged Asp as well as the effect of replacing α96Val by a bulky, nonpolar Trp. We have conducted oxygen-binding studies to investigate the effect of several allosteric effectors on the oxygenation properties and the Bohr effects of these r Hbs. The oxygen affinity of these mutants is lower than that of human normal adult hemoglobin (Hb A) under various experimental conditions. The oxygen affinity of r Hb Yoshizuka is insensitive to changes in chloride concentration, whereas the oxygen affinity of r Hb Presbyterian exhibits a pronounced chloride effect. r Hb Presbyterian has the largest Bohr effect, followed by Hb A, r Hb (αV96W), and r Hb Yoshizuka. Thus, the amino acid substitution in the central cavity that increases the net positive charge enhances the Bohr effect. Proton nuclear magnetic resonance studies demonstrate that these r Hbs can switch from the R quaternary structure to the T quaternary structure without changing their ligation states upon the addition of an allosteric effector, inositol hexaphosphate, and/or by reducing the temperature. r Hb (αV96W, βN108K), which has the lowest oxygen affinity among the hemoglobins studied, has the greatest tendency to switch to the T quaternary structure. The following conclusions can be derived from our results: First, if we can stabilize the deoxy (T) quaternary structure of a hemoglobin molecule without perturbing its oxy (R) quaternary structure, we will have a hemoglobin with low oxygen affinity and high cooperativity. Second, an alteration of the charge distribution by amino acid substitutions in the α1β1 subunit interface and in the central cavity of the hemoglobin molecule can influence the Bohr effect. Third, an amino acid substitution in the α1β1 subunit interface can affect both the oxygen affinity and cooperativity of the oxygenation process. There is communication between the α1β1 and α1β2 subunit interfaces during the oxygenation process. Fourth, there is considerable cooperativity in the oxygenation process in the T-state of the hemoglobin molecule.