Editing Active site (section)

===Chymotrypsin===
[[File:Mechanism of peptide bond cleavage in a-chymotrypsin.svg|thumb|Mechanism of peptide bond cleavage by chymotrypsin.|400px]]
[[Chymotrypsin]] is a [[serine protease|serine endopeptidase]] that is present in [[pancreatic juice]] and helps the [[hydrolysis]] of [[proteins]] and [[polypeptides|peptide]].<ref name=":0" />{{Rp|84–6}} It  catalyzes the hydrolysis of peptide bonds in [[Stereoisomerism|L-isomers]] of [[tyrosine]], [[phenylalanine]], and [[tryptophan]]. In the active site of this enzyme, three amino acid residues work together to form a [[catalytic triad]] which makes up the catalytic site. In chymotrypsin, these residues are Ser-195, His-57 and Asp-102.

The mechanism of chymotrypsin can be divided into two phases. First, Ser-195 nucleophilically attacks the [[peptide bond]] carbon in the substrate to form a tetrahedral intermediate. The nucleophilicity of Ser-195 is enhanced by His-57, which abstracts a proton from Ser-195 and is in turn stabilised by the negatively charged [[carboxylate]] group (RCOO<sup>−</sup>) in Asp-102. Furthermore, the tetrahedral [[oxyanion]] intermediate generated in this step is stabilised by [[hydrogen bonds]] from Ser-195 and Gly-193.

In the second stage, the R'NH group is protonated by His-57 to form [[Amine|R'NH<sub>2</sub>]] and leaves the intermediate, leaving behind the [[acyl group|acylated]] Ser-195. His-57 then acts as a base again to abstract one proton from a water molecule. The resulting [[hydroxide]] anion nucleophilically attacks the acyl-enzyme complex to form a second tetrahedral oxyanion intermediate, which is once again stabilised by H bonds. In the end, Ser-195 leaves the tetrahedral intermediate, breaking the CO bond that connected the enzyme to the peptide substrate. A proton is transferred to Ser-195 through His-57, so that all three amino acid return to their initial state.