title: Structures of R- and T-state hemoglobin Bassett: elucidating the structural basis for the low oxygen affinity of a mutant hemoglobin.
abstract: The crystal structures of R- and T-state hemoglobin (Hb) Bassett have been determined to 2.15 and 1.80 A resolution, respectively. Physiologically, Hb Bassett (alphaAsp94-->Ala) is characterized by a low affinity for oxygen, a reduced Bohr effect and low cooperativity, as well as being slightly unstable (compared with normal adult hemoglobin; HbA). Comparisons between the Hb Bassett structures and previously determined R- and T-state HbA structures revealed that this mutant shares similar tertiary and quaternary structures with other Hbs. However, this analysis did identify localized structural differences between R-state Hb Bassett and R-state HbA at the alpha1beta2 (alpha2beta1) dimer interface and at the beta-cleft. Specifically, the beta-FG corner has shifted closer to the alpha-C helix in the mutant R structure. In addition, four intersubunit hydrogen bonds found at the alpha1beta2 interfaces of native R-state Hb structures are abolished or weakened and subsequently replaced by two new intersubunit hydrogen bonds in R-state Hb Bassett. Remarkably, the newly formed hydrogen bonds in the R-state mutant structure are also observed in T-state Hb structures. At the beta-cleft, betaHis46, which is known to contribute to the Bohr effect in Hb, makes a unique hydrogen-bonding interaction with betaAsn139 in the R-state Hb Bassett. Unlike the R-state mutant, the T-state Hb Bassett structure does not display any significant structural changes at both the alpha1beta2 (alpha2beta1) dimer interface and the beta-cleft. Quite significantly, the mutation has led to removal of an interdimer repulsion involving alpha1Asp94 and beta2Asp99. The R- and T-state structures of Hb Bassett suggest a stereochemical basis for the observed functional properties of this mutant.
authors.affiliation: Department of Medicinal Chemistry, School of Pharmacy and Institute for Structural Biology and Drug Discovery, Virginia Commonwealth University, Richmond, VA 23298, USA.
authors.forename: Martin K
authors.initials: MK
authors.lastname: Safo
authors.forename: Osheiza
authors.initials: O
authors.lastname: Abdulmalik
authors.forename: Hsiang Ru
authors.initials: HR
authors.lastname: Lin
authors.forename: Toshio
authors.initials: T
authors.lastname: Asakura
authors.forename: Donald J
authors.initials: DJ
authors.lastname: Abraham
mesh_terms: D018360:Crystallography, X-Ray; D006454:Hemoglobins / Q000737:chemistry* / Q000235:genetics; D009154:Mutation; D011487:Protein Conformation
chemical_list: D006454:Hemoglobins