One of the ways we know about the mechanism of enzymes is through the use of xray crystallography structures of trapped intermediates or inhibitors bound to enzymes. In a paper by Radisky and Koshland[1], an acyl intermediate of trypsin (PDB code 2AGG was characterized.
Serine proteases use a covalent mechanism to catalyze the hydrolysis of a peptide bond. A covalent bond is formed between and a substrate .
Specificity of the proteases is determined by a binding pocket.