Mechanism of peptide bond cleavage in α-chymotrypsin. Chymotrypsin preferentially cleaves C-terminally of large aromatic residues such as phenylalanine (left). The peptide backbone is colored dark red, the sidechains are shown in bright red. In the first step, the catalytic triade enhances the nucleophilicity of Ser₁₉₅ which attacks the carbon atom of the peptide bond, leading to formation of a tetrahedral intermediate. In the next step, the amine leaves the tetrahedral intermediate, its leaving group quality enhanced by His₅₇. During this step, the actual cleavage of the peptide bond takes place (shown by the green arrow). In the following step, a water molecule (blue), the nucleophilicity of which is again enhanced by His₅₇ takes the amine's place. In the last step, serine leaves the tetrahedral intermediate and thereby the catalytic triad is regenerated.