title: The active site of aspartic proteinases.
abstract: The active site of the aspartic proteinase, endothiapepsin, has been defined by X-ray analysis and restrained least-squares refinement at 2.1 A resolution with a crystallographic agreement value of 0.16. The environments of the two catalytically important aspartyl groups are remarkably similar and the contributions of the NH2- and COOH-terminal domains to the catalytic centre are related by a local 2-fold axis. The carboxylates of the aspartyls share a hydrogen bond and have equivalent contacts to a bound water molecule or hydroxonium ion lying on the local diad. The main chains around 32 and 215 are connected by a novel interaction involving diad-related threonines. It is suggested that the two pKa values of the active site aspartyls arise from a structure not unlike that in maleic acid with a hydrogen-bonded intermediate species and a dicarboxylate characterised by electrostatic repulsions between the two negatively charged groups.
mesh_terms: D000595:Amino Acid Sequence; D016282:Aspartic Acid Endopeptidases; D001665:Binding Sites; D010450:Endopeptidases; D006860:Hydrogen Bonding; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D010450:Endopeptidases; D016282:Aspartic Acid Endopeptidases; C042466:Endothia aspartic proteinase