title: Structure of relaxed-state human hemoglobin: insight into ligand uptake, transport and release.
abstract: Hemoglobin was one of the first protein structures to be determined by X-ray crystallography and served as a basis for the two-state MWC model for the mechanism of allosteric proteins. Since then, there has been an ongoing debate about whether Hb allostery involves the unliganded tense T state and the liganded relaxed R state or whether it involves the T state and an ensemble of liganded relaxed states. In fact, the former model is inconsistent with many functional observations, as well as the recent discoveries of several relaxed-state Hb structures such as RR2, R3 and R2. One school of thought has suggested the R2 state to be the physiologically relevant relaxed end state, with the R state mediating the T-->R2 transition. X-ray studies have been performed on human carbonmonoxy Hb at a resolution of 2.8 A. The ensuing liganded quaternary structure is different from previously reported liganded Hb structures. The distal beta-heme pocket is the largest when compared with other liganded Hb structures, partly owing to rotation of betaHis63(E7) out of the distal pocket, creating a ligand channel to the solvent. The structure also shows unusually smaller alpha- and beta-clefts. Results from this study taken in conjunction with previous findings suggest that multiple liganded Hb states with different quaternary structures may be involved in ligand uptake, stabilization, transport and release.
authors.affiliation: Department of Medicinal Chemistry, School of Pharmacy and Institute for Structural Biology and Drug Discovery, Virginia Commonwealth University, Richmond, VA 23219, USA.
authors.forename: Joy D
authors.initials: JD
authors.lastname: Jenkins
authors.forename: Faik N
authors.initials: FN
authors.lastname: Musayev
authors.forename: Richmond
authors.initials: R
authors.lastname: Danso-Danquah
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Abraham
authors.forename: Martin K
authors.initials: MK
authors.lastname: Safo
mesh_terms: D000494:Allosteric Regulation; D002248:Carbon Monoxide / Q000737:chemistry / Q000378:metabolism*; D003460:Crystallization; D018360:Crystallography, X-Ray; D006454:Hemoglobins / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D008024:Ligands; D011485:Protein Binding; D011487:Protein Conformation; D021381:Protein Transport; D040681:Structural Homology, Protein
chemical_list: D006454:Hemoglobins; D008024:Ligands; D002248:Carbon Monoxide