Determination and restrained least-squares refinement of the structures of ribonuclease Sa and its complex with 3'-guanylic acid at 1.8 A resolution

Acta Crystallogr B. 1991 Apr 1;47 ( Pt 2):240-53.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Binding Sites
  • Chemical Phenomena
  • Chemistry, Physical
  • Crystallization
  • Guanosine Monophosphate / metabolism*
  • Hydrogen Bonding
  • Molecular Sequence Data
  • Molecular Structure
  • Protein Conformation
  • Ribonucleases / chemistry*
  • Ribonucleases / metabolism
  • Streptomyces aureofaciens / enzymology
  • X-Ray Diffraction


  • 3'-guanylic acid
  • Guanosine Monophosphate
  • Ribonucleases