title: Structural study of X-ray induced activation of carbonic anhydrase.
abstract: Carbonic anhydrase, a zinc metalloenzyme, catalyzes the reversible hydration of carbon dioxide to bicarbonate. It is involved in processes connected with acid-base homeostasis, respiration, and photosynthesis. More than 100 distinct human carbonic anhydrase II (HCAII) 3D structures have been generated in last 3 decades [Liljas A, et al. (1972) Nat New Biol 235:131-137], but a structure of an HCAII in complex with CO(2) or HCO(3)(-) has remained elusive. Here, we report previously undescribed structures of HCAII:CO(2) and HCAII:HCO(3)(-) complexes, together with a 3D molecular film of the enzymatic reaction observed successively in the same crystal after extended exposure to X-ray. We demonstrate that the unexpected enzyme activation was caused in an X-ray dose-dependent manner. Although X-ray damage to macromolecular samples has long been recognized [Ravelli RB, Garman EF (2006) Curr Opin Struct Biol 16:624-629], the detailed structural analysis reports on X-ray-driven reactions have been very rare in literature to date. Here, we report on enzyme activation and the associated chemical reaction in a crystal at 100 K. We propose mechanisms based on water photoradiolysis and/or electron radiolysis as the main cause of enzyme activation.
authors.affiliation: Department for Structural and Computational Biology, Max F Perutz Laboratories, University of Vienna, Campus Vienna Biocenter 5, A-1030 Vienna, Austria.
authors.forename: Björn
authors.initials: B
authors.lastname: Sjöblom
authors.forename: Maurizio
authors.initials: M
authors.lastname: Polentarutti
authors.forename: Kristina
authors.initials: K
authors.lastname: Djinovic-Carugo
mesh_terms: D001639:Bicarbonates / Q000737:chemistry / Q000378:metabolism; D002245:Carbon Dioxide / Q000737:chemistry / Q000378:metabolism; D024402:Carbonic Anhydrase II / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002384:Catalysis / Q000528:radiation effects; D020134:Catalytic Domain; D003460:Crystallization; D004789:Enzyme Activation / Q000528:radiation effects; D006639:Histidine / Q000737:chemistry; D006801:Humans; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction; D014965:X-Rays; D015032:Zinc / Q000737:chemistry
chemical_list: D001639:Bicarbonates; D011994:Recombinant Proteins; D014867:Water; D002245:Carbon Dioxide; D006639:Histidine; D024402:Carbonic Anhydrase II; D015032:Zinc