Role of catalytic residues in the formation of a tetrahedral adduct in the acylation reaction of bovine beta-trypsin. A molecular orbital study.

In the acylation reaction of serine proteases the effect of amino acid residues on the geometrical change of the catalytic site from Michaelis to tetrahedral state was studied by using ab initio molecular orbital calculations. Amino acid residues in the catalytic site and the peptide substrate were calculated as a quantum mechanical region, and all the other amino acid residues and the calcium ion were included in the calculation as the electrostatic effects. The effects of Asp102, Asp194, N-terminus and the oxyanion binding site are large. The oxyanion binding site directly stabilizes the tetrahedral substrate. Asp102 stabilizes the enzyme intermediate, interacting with the protonated His57 residue. In order to elucidate the roles of Asp102 and the oxyanion binding site, energy decomposition analyses were done for the intermolecular interactions. The contribution of Asp102 and the oxyanion binding site to the decrease of energy in the geometrical change is due to the electrostatic effect. The energies of the proton shuttle from Ser195 O gamma to the leaving group of the substrate were calculated for amide and ester substrate models.