The catalytic mechanism of aspartic proteinases.

The highly symmetric active site of an aspartic proteinase, endothiapepsin, binds a water molecule ideally situated for nucleophilic attack on a substrate peptide bond whose distortion from planarity is stabilised by interactions of the substrate with the extended binding cleft. The apparent electrophilicity of the catalysis results from this distortion. The scissile peptide bond is orientated with the carbonyl oxygen hydrogen bonding to the tip of the beta-hairpin 'flap' which lies over the cleft. Nucleophilic attack by the bound water leads to a tetrahedral intermediate similar to observed complexes with hydroxyl inhibitors and stabilised by hydrogen bonds with the flap.