title: Aspartic acid-121 functions at the active site of bovine pancreatic ribonuclease.
abstract: The fully active semisynthetic enzyme formed by the non-covalent interaction of residues 1-118 of bovine pancreatic ribonuclease and a synthetic tetradecapeptide containing residues 111-124 of the enzyme has allowed a direct test of the role of aspartic acid-121 in the functioning of the molecule. Replacement of this residue by asparagine results in a derivative that is 4.5% active against cytidine 2',3'-cyclic phosphate at pH 7.0 under standard assay conditions. Further studies with the same substrate at pH 5.8 reveal that the reduced activity results entirely from a diminished catalytic efficiency and not from a decreased affinity for substrate.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000818:Animals; D001224:Aspartic Acid / Q000502:physiology*; D001665:Binding Sites; D002268:Carboxypeptidases / Q000378:metabolism; D043422:Carboxypeptidases A; D002417:Cattle; D002850:Chromatography, Gel; D006863:Hydrogen-Ion Concentration; D010179:Pancreas / Q000201:enzymology*; D010434:Pepsin A / Q000378:metabolism; D010446:Peptide Fragments / Q000378:metabolism; D012260:Ribonucleases / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D010446:Peptide Fragments; D001224:Aspartic Acid; D012260:Ribonucleases; D002268:Carboxypeptidases; D043422:Carboxypeptidases A; D010434:Pepsin A