Probing the diphosphoglycerate binding pocket of HbA and HbPresbyterian (β108Asn → Lys)

HbPresbyterian (β108Asn → Lys, HbP) contains an additional positive charge (per αβ dimer) in the middle of the central cavity and exhibits a lower oxygen affinity than wild-type HbA in the presence of chloride. However, very little is known about the molecular origins of its altered functional properties. In this study, we have focused on the ββ cleft of the Hb tetramer. Recently, we developed an approach for quantifying the ligand binding affinity to the β-end of the Hb central cavity using fluorescent analogues of the natural allosteric effector 2,3- diphosphoglycerate (DPG) [Gottfried, D. S., et al. (1997) J. Biol. Chem. 272, 1571-1578]. Time-correlated single-photon counting fluorescence lifetime studies were used to assess the binding of pyrenetetrasulfonate to both HbA and HbP in the deoxy and CO ligation states under acidic and neutral pH conditions. Both the native and mutant proteins bind the probe at a weak binding site and a strong binding site; in all cases, the binding to HbP was stronger than to HbA. The most striking finding was that for HbA the binding affinity varies as follows: deoxy (pH 6.35) > deoxy (pH 7.20) > CO (pH 6.35); however, the binding to HbP is independent of ligation or pH. The mutant oxy protein also hydrolyzes p-nitrophenyl acetate, through a reversible acyl- imidazole pathway linked to the His residues of the ββ cleft, at a considerably higher rate than does HbA. This implies a perturbation of the microenvironment of these residues at the DPG binding pocket. Structural consequences due to the presence of the new positive charge in the middle of the central cavity have been transmitted to the ββ cleft of the protein, even in its liganded conformation. This is consistent with a newly described quaternary state (B) for liganded HbPresbyterian and an associated change in the allosteric control mechanism.