title: Determination and restrained least-squares refinement of the structures of ribonuclease Sa and its complex with 3'-guanylic acid at 1.8 A resolution.
abstract: The crystal structures of ribonuclease from Streptomyces aureofaciens (RNase Sa) and its complex with 3'-guanylic acid (guanosine 3'-monophosphate, 3'-GMP) have been determined by the method of isomorphous replacement. The atomic parameters have been refined by restrained least-squares minimization using data in the resolution range 10.0-1.8 A. All protein atoms and more than 230 water atoms in the two crystal structures have been refined to crystallographic R factors of 0.172 and 0.175 respectively. The estimated r.m.s. error in the atomic positions ranges from 0.2 A for well-defined atoms to about 0.5 A for more poorly defined atoms. There are two enzyme molecules in the asymmetric unit, built independently, and referred to as molecules A and B. The value of the average B factor for protein atoms in both structures is about 19 A2 and for water molecules about 35 A2. Electron density for the substrate analogue 3'-GMP was found only at the active site of molecule A. The density was very clear and the positions of all 3'-GMP atoms were refined with precision comparable to that of the protein.
authors.affiliation: Institute of Molecular Biology, Slovak Academy of Sciences, Bratislava, Czechoslovakia.
authors.forename: J
authors.initials: J
authors.lastname: Sevcik
authors.forename: E J
authors.initials: EJ
authors.lastname: Dodson
authors.forename: G G
authors.initials: GG
authors.lastname: Dodson
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003460:Crystallization; D006157:Guanosine Monophosphate / Q000378:metabolism*; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D015394:Molecular Structure; D011487:Protein Conformation; D012260:Ribonucleases / Q000737:chemistry* / Q000378:metabolism; D013304:Streptomyces aureofaciens / Q000201:enzymology; D014961:X-Ray Diffraction
chemical_list: C068152:3'-guanylic acid; D006157:Guanosine Monophosphate; D012260:Ribonucleases