A theoretical study of torsional flexibility in the active site of aspartic proteinases: implications for catalysis.

We have performed ab initio Hartree-Fock self-consistent field calculations on the active site of endothiapepsin. The active site was modeled as a formic acid/formate anion moiety (representing the catalytic aspartates, Asp-32 and -215) and a bound water molecule. Residues Gly-34, Ser-35, Gly-217, and Thr-218, which all form hydrogen bonds to the active site, were modeled using formamide and methanol molecules. The water molecule, which is generally believed to function as the attacking nucleophile in catalysis, was allowed to bind to the active site in four distinct configurations. The geometry of each configuration was optimized using two basis sets (4-31G and 4-31G*). The results indicate that in the native enzyme the nucleophilic water is bound in a catalytically inert configuration. However, by rotating the carboxyl group of Asp-32 by about 90 degrees the water molecule can be reorientated to attack the scissile bond of the substrate. A model of the bound enzyme-substrate complex was constructed from the crystal structure of a difluorostatone inhibitor complexed with endothiapepsin. This model suggests that the substrate itself initiates the reorientation of the nucleophilic water immediately prior to catalysis by forcing the carboxyl group of Asp-32 to rotate. The theoretical results predict that the active site of endothiapepsin undergoes a large distortion during substrate binding and this observation has been used to explain some of the kinetics results which have been reported for mutant aspartic proteinases.