Question

N-Tosylamido-L-phenylethyl, also called tosyl phenylalanyl chloromethyl ketone (TPCK), irreversibly inhibits chymotrypsin, and is used to label the active site histidine residue. The COCH2Cl group is the reactive group that binds to the His residue.

Answer 1

Chymotrypsin is permanently inhibited by TPCK. Additionally inhibits certain cysteine proteases, including ficin, papain, caspase, and bromelain.  For the chemical labeling of active histidine in enzyme analysis, TPCK is chosen. Because the enzyme has a preference for aromatic amino acid residues at the active site, the phenylalanine moiety is attached to the enzyme (as in chymotrypsin, in which it binds to the Histidine-57 residue in the active site).

[1] Neither zymogens nor trypsin are inhibited by it.

In the crystal structure of the complex solved in 2010, TPCK is seen covalently bonded in the active site of Caspase 3.

[2] With the chlorine removed, the chloromethyl group interacts with the cysteine in the active site to form a covalent connection.

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