title: Structure of cyanase reveals that a novel dimeric and decameric arrangement of subunits is required for formation of the enzyme active site.
abstract: BACKGROUND
  Cyanase is an enzyme found in bacteria and plants that catalyzes the reaction of cyanate with bicarbonate to produce ammonia and carbon dioxide. In Escherichia coli, cyanase is induced from the cyn operon in response to extracellular cyanate. The enzyme is functionally active as a homodecamer of 17 kDa subunits, and displays half-site binding of substrates or substrate analogs. The enzyme shows no significant amino acid sequence homology with other proteins.
  
  
  RESULTS
  We have determined the crystal structure of cyanase at 1.65 A resolution using the multiwavelength anomalous diffraction (MAD) method. Cyanase crystals are triclinic and contain one homodecamer in the asymmetric unit. Selenomethionine-labeled protein offers 40 selenium atoms for use in phasing. Structures of cyanase with bound chloride or oxalate anions, inhibitors of the enzyme, allowed identification of the active site.
  
  
  CONCLUSIONS
  The cyanase monomer is composed of two domains. The N-terminal domain shows structural similarity to the DNA-binding alpha-helix bundle motif. The C-terminal domain has an 'open fold' with no structural homology to other proteins. The subunits of cyanase are arranged in a novel manner both at the dimer and decamer level. The dimer structure reveals the C-terminal domains to be intertwined, and the decamer is formed by a pentamer of these dimers. The active site of the enzyme is located between dimers and is comprised of residues from four adjacent subunits of the homodecamer. The structural data allow a conceivable reaction mechanism to be proposed.
authors.affiliation: Biosciences Division/Structural Biology Center, Argonne National Laboratory, Argonne, IL 60439, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: Walsh
authors.forename: Z
authors.initials: Z
authors.lastname: Otwinowski
authors.forename: A
authors.initials: A
authors.lastname: Perrakis
authors.forename: P M
authors.initials: PM
authors.lastname: Anderson
authors.forename: A
authors.initials: A
authors.lastname: Joachimiak
mesh_terms: D001665:Binding Sites; D019759:Carbon-Nitrogen Lyases / Q000737:chemistry*; D020134:Catalytic Domain; D018360:Crystallography, X-Ray / Q000379:methods; D019281:Dimerization; D004926:Escherichia coli / Q000201:enzymology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D017356:Synchrotrons
chemical_list: C019953:cyanate hydrolase; D019759:Carbon-Nitrogen Lyases