The serine proteases are among the most thoroughly studied enzymes, and numerous crystal structures representing the enzymesubstrate complex and intermediates in the hydrolysis reactions have been reported. Some aspects of the catalytic mechanism remain controversial, however, especially the role of conformational changes in the reaction. We describe here a high-resolution (1.46 Å) crystal structure of a complex formed between a cleaved form of bovine pancreatic trypsin inhibitor (BPTI) and a catalytically inactive trypsin variant with the BPTI cleavage site ideally positioned in the active site for resynthesis of the peptide bond. This structure defines the positions of the newly generated amino and carboxyl groups following the 2 steps in the hydrolytic reaction. Comparison of this structure with those representing other intermediates in the reaction demonstrates that the residues of the catalytic triad are positioned to promote each step of both the forward and reverse reaction with remarkably little motion and with conservation of hydrogen-bonding interactions. The results also provide insights into the mechanism by which inhibitors like BPTI normally resist hydrolysis when bound to their target proteases.trypsin ͉ bovine pancreatic trypsin inhibitor ͉ enzyme mechanisms S erine proteases are found throughout all 3 domains of cellular life and function in a wide range of physiological processes, including digestion, protein maturation and turnover, hemostasis, and immune responses (1). Approximately 0.6% of human protein-encoding genes are predicted to specify serine proteases, and this family is even more prevalent in other organisms, notably the arthropods (2, 3). A large body of biochemical and structural data have established a 2-step mechanism for hydrolysis of peptide bonds by this class of proteases (4), as shown in Scheme 1.The first step of the reaction is a nucleophilic attack by the catalytic serine residue (Ser-195 in trypsin and other members of the chymotrypsin, or S1, family) on the carbonyl carbon atom of the residue labeled P1, generating a covalent acyl-enzyme intermediate and a new peptide amino terminus, on the P1Ј residue. A second nucleophilic attack, by a water molecule, leads to hydrolysis of the acyl-enzyme, releasing the new carboxyl group and restoring the catalytic Ser residue to its initial state.