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 COCH2Cl and the amine is attached to SO2 bonded to benzene with a methyl group in the para position. Why does chymotrypsin bind TPCK?

a. TPCK is a polar molecule and easily forms several non-covalent interactions with the active site Of chymotrypsin, reversibly outcompeting the usual substrate.
b. The chlorine of TPCK is a strong nucleophile, and attacks the His 157 residue.
c. The serine residue at position 195 in chymotrypsin is a strong nucleophile.
d. The phenyl group Of TPCK is structurally similar to regular chymotrypsin substrates.