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. t p c k is an amino acid analog of phenylalanine where the carboxyl group is replaced by c o c h 2 c l and the amine is attached to s o 2 bonded to benzene with a methyl group in the para position. why does chymotrypsin bind tpck? the phenyl group of tpck is structurally similar to regular chymotrypsin substrates. tpck is a polar molecule and easily forms several noncovalent interactions with the active site of chymotrypsin, reversibly outcompeting the usual substrate. the chlorine of tpck is a strong nucleophile and attacks the his 157 residue. the serine residue at position 195 in chymotrypsin is a strong nucleophile. trypsin, another serine protease, does not bind tpck. how could tpck be modified to enable its recognition by trypsin? choose the most likely substrate.