The X-ray crystal structure of inactivated carboxypeptidase A by
(2R,3S)-2-benzyl-3,4-epoxybutanoic
acid,
a pseudomechanism-based inactivator, was determined to show that the
carboxylate of Glu-270 at the active site of
the enzyme is covalently modified: the inhibitor is tethered to the
carboxylate forming an ester linkage which is
brought about by the attack of the carboxylate on the oxirane ring of
the inhibitor. Examination of the crystal
structure in comparison with that inactivated by its enantiomer,
(2S,3R)-2-benzyl-3,4-epoxybutanoic acid,
shows
that the two inhibitors are bound covalently to the enzyme in a
symmetrical fashion. An explanation for the lack of
inactivating activity found previously with
(2R,3R)- as well as
(2S,3S)-2-benzyl-3,4-epoxybutanoic acids was
offered.