1.6-ANGSTROM STRUCTURE OF A SEMISYNTHETIC RIBONUCLEASE CRYSTALLIZED FROM AQUEOUS-ETHANOL - COMPARISON WITH CRYSTALS FROM SALT-SOLUTIONS ANDWITH RIBONUCLEASE-A FROM AQUEOUS ALCOHOL-SOLUTIONS

The non-covalent combination of residues 1-118 of RNase A with a synth etic 14-residue peptide containing residues 111-124 of the molecule fo rms a highly active semisynthetic enzyme, RNase 1-118:111-124. With th is enzyme, the roles played by the six C-terminal residues in generati ng the catalytic efficiency and substrate specificity of RNase can be studied using chemically synthesized analogs. The structure of RNase 1 -118:111-124 from 43% aqueous ethanol has been determined using molecu lar-replacement methods and refined to a crystallographic R factor of 0.166 for all observed reflections in the range 7.0-1.6 Angstrom (Prot ein Data Bank file 1SSC). The structure is compared with the 2.0 Angst rom, structure of RNase A from 43% aqueous 2-methyl-2-propanol and wit h the 1.8 Angstrom structure of the semisynthetic enzyme obtained from crystals grown in concentrated salt solution. The structure of RNase 1-118:111-124 from aqueous ethanol is virtually identical to that of R Nase A from aqueous 2-methyl-2-propanol, Half of the crystallographica lly bound water molecules are not coincident, however. The structure i s somewhat less similar to that of RNase 1-118:111-124 from salt solut ions, with a major difference being the positioning of active-site res idue His 119.