TPCK is an irreversible inhibitor of chymotrypsin. Also inhibits some cysteine proteases such as caspase, papain, bromelain or ficin.  It does not inhibit trypsin or zymogens.
TPCK is observed covalently bound in the active site of Caspase 3 in the crystal structure of the complex solved in 2010. The chloromethyl group reacts with the active site cysteine to form a covalent bond with the loss of the chlorine.
TPCK is chosen for the chemical labelling of active histidine in enzyme analysis.  The phenylalanine moiety is bound to the enzyme because of specificity for aromatic amino acid residues at the active site (as in chymotrypsin, in which it binds to the Histidine-57 residue in the active site).