Activation of the low oxygen affinity-inducing potential of the Asn108(β)→Lys mutation of Hb-Presbyterian on intramolecular αα-fumaryl cross-bridging

The Asn108β→Lys mutation in hemoglobin (Hb-Presbyterian mutation) endows a low O₂ affinity-inducing propensity to the protein. Introduction of a fumaryl cross-bridge between its two α99 lysine residues also induces a low O₂ affinity into HbA. We have now engineered an αα-fumaryl cross-bridge into Hb-Presbyterian to determine the synergy or additivity, if any, that can be achieved between these two low O₂ affinity-inducing structural perturbations. Despite the presence of the additional ε-amino group of Lys108(β) within the central cavity, the ε-amino group of Lys99(αα) of deoxy Hb-Presbyterian retained high selectivity for αα-fumaryl cross-bridging, with an overall efficiency comparable to that with HbA. The αα-fumaryl cross-linking of Hb-Presbyterian reduced its O₂ affinity much more significantly than that observed with HbA, indicating a synergy between the two low O₂ affinity-inducing structural perturbations. Apparently, the αα-fumaryl cross-bridge in Hb-Presbyterian activates part of the latent low O₂ affinity-inducing potential of Lys108(β) that is generally activated in the presence of chloride. The synergy between the Asn108(β)→Lys mutation and the αα-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, αα-fumaryl Hb-Presbyterian accessed a low O₂ affinity T-state that is accessed by HbA, αα-HbA and Hb-Presbyterian only in the presence of IHP. Isoelectric focusing analysis suggested that the αα-fumaryl cross-linking of Hb-Presbyterian induces changes in the ionization behavior of one or more of the functional groups neighboring Lys99(α) and Lys108(β) [presumably His103(α) and/or Glu101(β)] to compensate for the extra positive charge of Lys108(β). Molecular modeling studies identified two potential chloride binding sites per αβ dimer within the middle of the central cavity of αα-fumaryl HbA involving residues His103(α), Arg104(β) and Asn108(β). The affinity of these sites is increased in αα-fumaryl Hb-Presbyterian as a result of the Asn108(β)→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 αα-fumaryl cross-bridge) acting directly and the other (the Presbyterian mutation) acting indirectly through the mediation of chloride ion binding, facilitates the αα-fumaryl-Hb Presbyterian to access a low O₂ affinity T-state structure much more readily than either Hb-Presbyterian or αα-fumaryl HbA.