β7E-β132K salt bridge and sickle haemoglobin stability and conformation
The liganded (R-state) form of sickle cell haemoglobin (HbS) is of particular relevance at non-polymerizing concentrations as oxy HbS exhibits unusual properties compared with oxy HbA: mechanical precipitability (resulting from surface denaturation), greater unfolding at an air-water interface and a tendency to oxidize more readily. In human haemoglobins, the beta7 (A4) Glu residue forms an intrachain salt bridge with beta132 (H10) Lys in both liganded and deoxy structures. In the present study, recombinant haemoglobins with substitutions in the beta7 and beta132 sites were studied in order to determine the role of the beta7-beta132 salt bridge on Hb conformational integrity and stability. The elimination of this interhelix bridge correlates with enhanced surface denaturation and conformational alterations in the central cavity 2,3-diphosphoglycerate (DPG) cleft and alpha1beta2 interface. The A-helix beta7 Ala substitution generates a class of conformational change at the DPG pocket and alpha1beta2 interface that is distinct from that dictated by the H-helix beta132 Ala substitution. These results are significant with regard to the communication pathway between the alpha1beta1 and alpha1beta2 interfaces, and the new understanding of Hb allostery dependent upon tertiary structural constraints caused by effector binding to the R-state.