TPCK inactivates chymotrypsin but not trypsin because:
a. It looks like the substrate for chymotrypsin (but not trypsin) and thus can bind in its active site and modify His-57.
b. Trypsin does not have a His residue present in the active site to react with TPCK.
c. TPCK is a transition state analog for chymotrypsin but not trypsin.