Proton NMR studies of human hemoglobin variants modified in the proximal side of beta heme pocket. Implications for the affinity control and cooperative mechanism

Using high resolution proton NMR spectroscopy, we have investigated 10 human hemoglobin variants modified in the proximal side of the heme pocket in beta subunits. Comparative observation of several resonances in the spectra of liganded and unliganded hemoglobins allowed us to characterize the localization and nature of the structural perturbations induced by amino acid substitutions or chemical modification. The present data indicate that the structural perturbations are localized in the beta subunits, mainly in the tertiary domain surrounding the modification site. Analysis of the aromatic region of the liganded hemoglobin spectra gives substantial information for the assignment of the His-beta 97 C-2H resonance. Correlation of the spectroscopic observations with the functional characteristics of the studied hemoglobins demonstrates that structural factors localized in the proximal side of the heme pocket can control the ligand-iron interaction taking place on the other heme side. The structural perturbations induced by the modifications in the F or FG segments of the beta subunits do not extend to the distal side but rather to the alpha 1 beta 2 interface. This argues the existence of a gradient of tertiary structural stability, indicating a possible structural pattern of heme-heme interaction in the cooperativity control.