The three-dimensional structure of Asn¹⁰² mutant of trypsin: Role of Asp¹⁰² in serine protease catalysis

The structure of the Asn¹⁰² mutant of trypsin was determined in order to distinguish whether the reduced activity of the mutant at neutral pH results from an altered active site conformation or from an inability to stabilize a positive charge on the active site histidine. The active site structure of the Asn¹⁰² mutant of trypsin is identical to the native enzyme with respect to the specificity pocket, the oxyanion hole, and the orientation of the nucleophilic serine. The observed decrease in rate results from the loss of nucleophilicity of the active site serine. This decreased nucleophilicity may result from stabilization of a His⁵⁷ tautomer that is unable to accept the serine hydroxyl proton.