Crystal structure of 6-guanidinohexanoyl trypsin near the optimum pH reveals the acyl-enzyme intermediate to be deacylated.

The force driving the conversion from the acyl intermediate to the tetrahedral intermediate in the deacylation reaction of serine proteases remains unclear. The crystal structure of 6-guanidinohexanoyl trypsin was determined at pH 7.0, near the optimum reaction pH, at 1.94 Å resolution. In this structure, three water molecules are observed around the catalytic site. One acts as a nucleophile to attack the acyl carbonyl carbon while the other two waters fix the position of the catalytic water through a hydrogen bond. When the acyl carbonyl oxygen oscillates thermally, the water assumes an appropriate angle to catalyze the deacylation.