Understanding the molecular basis of the high oxygen affinity variant human hemoglobin Coimbra

Human hemoglobin (Hb) Coimbra (βAsp99Glu) is one of the seven βAsp99 Hb variants described to date. All βAsp99 substitutions result in increased affinity for O₂ and decreased heme-heme cooperativity and their carriers are clinically characterized by erythrocytocis, caused by tissue hypoxia. Since βAsp99 plays an important role in the allosteric α1β2 interface and the mutation in Hb Coimbra only represents the insertion of a CH₂ group in this interface, the present study of Hb Coimbra is important for a better understanding of the global impact of small modifications in this allosteric interface. We carried out functional, kinetic and dynamic characterization of this hemoglobin, focusing on the interpretation of these results in the context of a growth of the position 99 side chain length in the α1β2 interface. Oxygen affinity was evaluated by measuring p50 values in distinct pHs (Bohr effect), and the heme-heme cooperativity was analyzed by determining the Hill coefficient (n), in addition to the effect of the allosteric effectors inositol hexaphosphate (IHP) and 2,3-bisphosphoglyceric acid (2,3-BPG). Computer simulations revealed a stabilization of the R state in the Coimbra variant with respect to the wild type, and consistently, the T-to-R quaternary transition was observed on the nanosecond time scale of classical molecular dynamics simulations.