Allosteric effects of chloride ions at the intradimeric alpha1beta1 and alpha2beta2 interfaces of human hemoglobin.
The structural transition induced by ligand binding in human hemoglobin encompasses quaternary structure changes at the interfaces between the two alphabeta dimers. In contrast, the interfaces between alpha and beta subunits within the same dimer (i.e., alpha1beta1 and alpha2beta2 interfaces) are structurally invariant. Previous work from this laboratory using NMR spectroscopy has identified four sites at the intradimeric alpha1beta1 and alpha2beta2 interfaces that, although structurally invariant, experience significant changes in the rates of proton exchange upon ligand binding. These sites are Hisalpha103(G10) and Hisalpha122(H5) in each alpha subunit of the hemoglobin tetramer. In the present work, we show that the proton exchange at the Hisalpha103(G10) sites is affected by the interactions of hemoglobin with chloride ions. Increasing concentrations of chloride ions at pH 6.45 and at 37 degrees C enhance the exchange rate of the Hisalpha103(G10) N(epsilon 2) proton. The enhancement is greater in deoxygenated than in ligated hemoglobin. In the framework of the local unfolding model for proton exchange, these results suggest that the structural free energy and/or the proton transfer reactions at the Hisalpha103(G10) sites depend on the concentration of chloride ions. Therefore, the ligand-induced changes at the Hisalpha103(G10) sites are modulated by the allosteric effect of chloride ions on hemoglobin.
D000494:Allosteric Regulation; D002712:Chlorides / Q000737:chemistry / Q000494:pharmacology*; D019281:Dimerization; D006441:Hemoglobin A / Q000737:chemistry* / Q000187:drug effects; D006639:Histidine / Q000737:chemistry; D006801:Humans; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D020836:Protein Structure, Quaternary; D021122:Protein Subunits; D011522:Protons
D002712:Chlorides; D021122:Protein Subunits; D011522:Protons; D006639:Histidine; D006441:Hemoglobin A