title: Ligand binding intermediates of nitrosylated human hemoglobin induced at low temperature by X-ray irradiation.
abstract: Under prolonged X-ray irradiation, the ferrous heme of nitrosylated human adult hemoglobin derivative (HbNO) undergoes a reversible transition generating a 5-coordinate species, due to release of the Fe-NO bond. The overall process can be investigated using X-ray absorption near edge structure (XANES) spectroscopy. In this work, Fe K-edge XANES spectra were measured at T < 15 K, pH 9.2, i.e., on a high-affinity state (R-HbNO) where all the hemes are 6-coordinate, and at pH 6.5 in the presence of inositol hexakis-phosphate (IHP), i.e., on a low-affinity ligated state (T-HbNO) where the iron-hemes of the α-chains are 5-coordinate due to breaking of the Fe-proximal histidine bond. Under X-ray irradiation, 5-coordinate Fe-hemes are populated in both R-HbNO and T-HbNO, the Fe-NO bond lysis induced in T-HbNO involving rebinding of the proximal histidine to the transiently populated 4-coordinate hemes of the α-chains. A detailed analysis of the spectra confirms that different intermediate states in the ligand binding cooperative process of hemoglobin can be populated by X-ray irradiation, and that the part of the energy associated to the R-T quaternary transition, that is transmitted to the heme site, can be monitored by XANES spectroscopy.
authors: Istituto di Biochimica e Biochimica Clinica, Università Cattolica del Sacro Cuore, Largo F. Vito 1,00168, Roma, Italy. alessandro.arcovito@rm.unicatt.it
authors: Alessandro
authors: A
authors: Arcovito
authors: Stefano
authors: S
authors: Della Longa
mesh_terms: D003080:Cold Temperature; D006442:Glycated Hemoglobin / Q000737:chemistry*; D006801:Humans; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D009569:Nitric Oxide / Q000737:chemistry*; D011487:Protein Conformation; D056928:X-Ray Absorption Spectroscopy
chemical_list: D006442:Glycated Hemoglobin; D007501:Iron; D009569:Nitric Oxide; C058377:hemoglobin A, glycosylated-nitric oxide complex