title: The crystal structure of a tetrameric hemoglobin in a partial hemichrome state.
abstract: Tetrameric hemoglobins are the most widely used systems in studying protein cooperativity. Allosteric effects in hemoglobins arise from the switch between a relaxed (R) state and a tense (T) state occurring upon oxygen release. Here we report the 2.0-A crystal structure of the main hemoglobin component of the Antarctic fish Trematomus newnesi, in a partial hemichrome form. The two alpha-subunit iron atoms are bound to a CO molecule, whereas in the beta subunits the distal histidine residue is the sixth ligand of the heme iron. This structure, a tetrameric hemoglobin in the hemichrome state, demonstrates that the iron coordination by the distal histidine, usually associated with denaturing states, may be tolerated in a native-like hemoglobin structure. In addition, several features of the tertiary and quaternary organization of this structure are intermediate between the R and T states and agree well with the R --> T transition state properties obtained by spectroscopic and kinetic techniques. The analysis of this structure provides a detailed pathway of heme-heme communication and it indicates that the plasticity of the beta heme pocket plays a role in the R --> T transition of tetrameric hemoglobins.
authors.affiliation: Istituto di Biochimica delle Proteine ed Enzimologia, Consiglio Nazionale delle Ricerche, I-80125 Naples, Italy.
authors.forename: Antonio
authors.initials: A
authors.lastname: Riccio
authors.forename: Luigi
authors.initials: L
authors.lastname: Vitagliano
authors.forename: Guido
authors.initials: G
authors.lastname: di Prisco
authors.forename: Adriana
authors.initials: A
authors.lastname: Zagari
authors.forename: Lelio
authors.initials: L
authors.lastname: Mazzarella
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000864:Antarctic Regions; D018360:Crystallography, X-Ray; D005399:Fishes; D006454:Hemoglobins / Q000737:chemistry* / Q000302:isolation & purification; D008958:Models, Molecular; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D021122:Protein Subunits
chemical_list: D006454:Hemoglobins; D021122:Protein Subunits