Tosyl phenylalanyl chloromethyl ketone
) is a protease inhibitor
. Its structural formula
TPCK is the irreversible inhibitor of chymotrypsin
. Also inhibits some cysteine proteases
such as papain
. It does not inhibit trypsin
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 chymotrypsin).