Chymotrypsin:
a Serine Protease

The serine proteases are a well characterized family of enzymes. Trypsin, chymotrypsin, and elastase are all serine proteases. All of these enzymes carry out the same type of reaction - the cleavage of a peptide chain.

Chymotrypsin, shown here, cleaves peptide chains on the carbonyl side of aromatic residues.

It contains a C-terminal alpha helix and two beta sheets (shown in green and orange).

Three important residues are found in the active site and form the catalytic triad:
His57
Ser195
Asp102
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When the peptide substrate binds, it forms hydrogen bonds with numerous residues on the protein. . Upon binding, the peptide is bent so that the bond (which is to be cleaved) is located near His57 and Ser195

His-57 - - acts both as a general acid and general base during the reaction.

Asp-102 - - functions to orient His-57 properly, immobilizing it with a hydrogen bond.

Ser-195 - - forms a covalent bond with the peptide to be cleaved, facilitating the cleavage reaction.

Initially His57 acts as a general base and abstracts a proton from Ser 195 Ser 195 may then attack the carbonyl carbon of the peptide bond (which is to be cleaved).

His57 then donates a proton to the amide nitrogen of the bound peptide leading to the subsequent breaking of the peptide bond and the eventual dissociation of the amine product.

His57 again acts as a general base, abstracting a proton from an attacking water molecule. It then donates this proton to the oxygen of Ser-195 during cleavage of the tetrahedral oxyanion intermediate, facilitating release of the second peptide product of the reaction.

The formation of an ionic tetrahedral intermediates in the reaction sequence of chymotrypsin is facilitated by groups on the protein that stabilize the developing negative charge. The amide nitrogens of Ser-195 and Gly-193 form an 'oxyanion hole' in which the substrate carbonyl oxygen is hydrogen bonded to the amide N-H group.

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