Activation of the low oxygen affinity-inducing potential of the Asn108(beta)-->Lys mutation of Hb-Presbyterian on intramolecular alpha alpha-fumaryl cross-bridging.

Abstract

The Asn108 beta-->Lys mutation in hemoglobin (HbPresbyterian mutation) endows a low O(2) affinity-inducing propensity to the protein. Introduction of a fumaryl cross-bridge between its two alpha 99 lysine residues also induces a low O(2) affinity into HbA. We have now engineered an alpha alpha-fumaryl cross-bridge into Hb-Presbyterian to determine the synergy or additivity, if any, that can be achieved between these two low O(2) affinity-inducing structural perturbations. Despite the presence of the additional epsilon-amino group of Lys108(beta) within the central cavity, the epsilon-amino group of Lys99(alpha alpha) of deoxy Hb-Presbyterian retained high selectivity for alpha alpha-fumaryl cross-bridging, with an overall efficiency comparable to that with HbA. The alpha alpha-fumaryl cross-linking of Hb-Presbyterian reduced its O(2) affinity much more significantly than that observed with HbA, indicating a synergy between the two low O(2) affinity-inducing structural perturbations. Apparently, the alpha alpha-fumaryl cross-bridge in Hb-Presbyterian activates part of the latent low O(2) affinity-inducing potential of Lys108(beta) that is generally activated in the presence of chloride. The synergy between the Asn108(beta)-->Lys mutation and the alpha alpha-fumaryl cross-bridging was conserved in the presence of chloride, but not in the presence of DPG. Furthermore, in the presence of chloride and DPG, alpha alpha-fumaryl Hb-Presbyterian accessed a low O(2) affinity T-state that is accessed by HbA, alpha alpha-HbA and Hb-Presbyterian only in the presence of IHP. Isoelectric focusing analysis suggested that the alpha alpha-fumaryl cross-linking of Hb-Presbyterian induces changes in the ionization behavior of one or more of the functional groups neighboring Lys99(alpha) and Lys108(beta) [presumably His103(alpha) and/or Glu101(beta)] to compensate for the extra positive charge of Lys108(beta). Molecular modeling studies identified two potential chloride binding sites per alpha beta dimer within the middle of the central cavity of alphaalpha-fumaryl HbA involving residues His103(alpha), Arg104(beta) and Asn108(beta). The affinity of these sites is increased in alpha alpha-fumaryl Hb-Presbyterian as a result of the Asn108(beta)-->Lys mutation. Thus, the results of the present study suggest that the enhanced neutralization of the positive charges in the middle of the central cavity of Hb achieved by these two electrostatic modifications, one (the alpha alpha-fumaryl cross-bridge) acting directly and the other (the Presbyterian mutation) acting indirectly through the mediation of chloride ion binding, facilitates the alpha alpha- fumaryl-Hb Presbyterian to access a low O(2) affinity T-state structure much more readily than either Hb-Presbyterian or alpha alpha-fumaryl HbA.