A Signature of the T → R Transition in Human Hemoglobin

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 98; no. 7; pp. 3773 - 3777
• Main Authors: Mihailescu, Mihaela-Rita, Russu, Irina M.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 27.03.2001; National Acad Sciences; The National Academy of Sciences
• Subjects: Adult; Allosteric Regulation; Biochemistry; Biological Sciences; Catalysis; Chemical bases; Dimers; Energy Metabolism; Exchange rates; Free energy; Hemoglobins; Hemoglobins - chemistry; Histidine - chemistry; Humans; Ligands; Magnetic Resonance Spectroscopy; Molecules; Protons
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.071493598

Allosteric effects in hemoglobin arise from the equilibrium between at least two energetic states of the molecule: a tense state, T, and a relaxed state, R. The two states differ from each other in the number and energy of the interactions between hemoglobin subunits. In the T state, constraints between subunits oppose the structural changes resulting from ligand binding. In the R state, these constraints are released, thus enhancing ligand-binding affinity. In the present work, we report the presence of four sites in hemoglobin that are structurally stabilized in the R relative to the T state. These sites are Hisα103(G10) and Hisα122(H5) in each α subunit of hemoglobin. They are located at the α1β1and α2β2interfaces of the hemoglobin tetramer, where the histidine side chains form hydrogen bonds with specific residues from the β chains. We have measured the solvent exchange rates of side chain protons of Hisα103(G10) and Hisα122(H5) in both deoxygenated and ligated hemoglobin by NMR spectroscopy. The exchange rates were found to be higher in the deoxygenated-T than in ligated-R state. Analysis of exchange rates in terms of the local unfolding model revealed that the structural stabilization free energy at each of these two histidines is larger by ≈1.5 kcal/(mol tetramer) in the R relative to the T state. The location of these histidines at the intradimeric α1β1and α2β2interfaces also suggests a role for these interfaces in the allosteric equilibrium of hemoglobin.