title: Ibuprofen induces an allosteric conformational transition in the heme complex of human serum albumin with significant effects on heme ligation.
abstract: Human serum albumin (HSA), the most prominent protein in blood plasma, is able to bind a wide range of endogenous and exogenous compounds. Among the endogenous ligands, HSA is a significant transporter of heme, the heme-HSA complex being present in blood plasma. Drug binding to heme-HSA affects allosterically the heme affinity for HSA and vice versa. Heme-HSA, heme, and their complexes with ibuprofen have been characterized by electronic absorption, resonance Raman, and electron paramagnetic resonance (EPR) spectroscopy. Comparison of the results for the heme and heme-HSA systems has provided insight into the structural consequences on the heme pocket of ibuprofen binding. The pentacoordinate tyrosine-bound heme coordination of heme-HSA, observed in the absence of ibuprofen, becomes hexacoordinate low spin upon ibuprofen binding, and heme dissociates at increasing drug levels. The electronic absorption spectrum and nu(Fe-CO)/nu(CO) vibrational frequencies of the CO-heme-HSA-ibuprofen complex, together with the observation of a Fe-His Raman mode at 218 cm(-1) upon photolysis of the CO complex and the low spin EPR g values indicate that a His residue is one of the low spin axial ligands, the sixth ligand probably being Tyr161. The only His residue in the vicinity of the heme Fe atom is His146, 9 A distant in the absence of the drug. This indicates that drug binding to heme-HSA results in a significant rearrangement of the heme pocket, implying that the conformational adaptability of HSA involves more than the immediate vicinity of the drug binding site. As a whole, the present spectroscopic investigation supports the notion that HSA could be considered as the prototype of monomeric allosteric proteins.
authors.affiliation: Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino (FI), Italy.
authors.forename: Francesco P
authors.initials: FP
authors.lastname: Nicoletti
authors.forename: Barry D
authors.initials: BD
authors.lastname: Howes
authors.forename: Maria
authors.initials: M
authors.lastname: Fittipaldi
authors.forename: Gabriella
authors.initials: G
authors.lastname: Fanali
authors.forename: Mauro
authors.initials: M
authors.lastname: Fasano
authors.forename: Paolo
authors.initials: P
authors.lastname: Ascenzi
authors.forename: Giulietta
authors.initials: G
authors.lastname: Smulevich
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry; D006418:Heme / Q000737:chemistry*; D006801:Humans; D007052:Ibuprofen / Q000737:chemistry*; D008958:Models, Molecular; D011487:Protein Conformation / Q000187:drug effects; D017434:Protein Structure, Tertiary; D012709:Serum Albumin / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; D012709:Serum Albumin; D006418:Heme; D007052:Ibuprofen