title: Direct observation of photolysis-induced tertiary structural changes in hemoglobin.
abstract: Human Hb, an alpha2beta2 tetrameric oxygen transport protein that switches from a T (tense) to an R (relaxed) quaternary structure during oxygenation, has long served as a model for studying protein allostery in general. Time-resolved spectroscopic measurements after photodissociation of CO-liganded Hb have played a central role in exploring both protein dynamical responses and molecular cooperativity, but the direct visualization and the structural consequences of photodeligation have not yet been reported. Here we present an x-ray study of structural changes induced by photodissociation of half-liganded T-state and fully liganded R-state human Hb at cryogenic temperatures (25-35 K). On photodissociation of CO, structural changes involving the heme and the F-helix are more marked in the alpha subunit than in the beta subunit, and more subtle in the R state than in the T state. Photodeligation causes a significant sliding motion of the T-state beta heme. Our results establish that the structural basis of the low affinity of the T state is radically different between the subunits, because of differences in the packing and chemical tension at the hemes.
authors.affiliation: RIKEN Harima Institute/SPring-8, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo 679-5148, Japan. sadachi@spring8.or.jp
authors.forename: Shin-Ichi
authors.initials: S
authors.lastname: Adachi
authors.forename: Sam-Yong
authors.initials: SY
authors.lastname: Park
authors.forename: Jeremy R H
authors.initials: JR
authors.lastname: Tame
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Shiro
authors.forename: Naoya
authors.initials: N
authors.lastname: Shibayama
mesh_terms: D002263:Carboxyhemoglobin / Q000737:chemistry / Q000528:radiation effects; D003080:Cold Temperature; D018360:Crystallography, X-Ray; D006454:Hemoglobins / Q000737:chemistry* / Q000528:radiation effects; D006801:Humans; D066298:In Vitro Techniques; D008958:Models, Molecular; D010777:Photochemistry; D010782:Photolysis; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D055672:Static Electricity
chemical_list: D006454:Hemoglobins; D021122:Protein Subunits; D002263:Carboxyhemoglobin