Question

N-Tosylamido-L-phenylethyl, also called tosyl phenylalanyl chloromethyl ketone, (TPCK) irreversibly inhibits chymotrypsin, and is used as to label the active site histidine residue. The COCH2Cl group is the reactive group that binds to the His residue. Why does chymotrypsin bind TPCK? Trypsin, another serine protease, does not bind TPCK. How could TPCK be modified to enable its recognition by trypsin? Choose the most likely substrate, below.

Answer 1

(a) TPCK is specific for chymotrypsin because the phenyl ring of the Phe residue interacts effectively with the binding pocket of the chymotrypsin active site. This positions the chloromethyl group to react with His57. This reaction and binding occurs in the same way as peptide binding and hydrolysis. Thus, the correct option is the phenyl group of TPCK is structurally similar to chymotrypsin substrates.

(b) Replacement of the Phe residue of TPCK with arginine or lysine produces reagents that are specific for trypsin.So, looking at the given structures, the best fitted structure is B.