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742 Chapter 22
Copyright © 2017 Pearson Education, Inc.
Not all of the salicylic acid would contain
18
O, because the anhydride intermediate has two different
carbonyl groups. If water adds to the benzyl carbonyl group (above), salicylic acid will contain
18
O. If,
however, water adds to the acetyl carbonyl group (below), acetic acid will contain
18
O.
−
O
O CH
3
C
O
O
H
O H
18
CH
3
COH
18
HO
−
O O
C
C
O
Here water adds to the carbonyl group
attached to the methyl group.
14.
Solved in the text.
15.
2
,
3
, and
4
are bases, so they can help remove a proton.
16.
Ser-Ala-Leu is more readily cleaved by carboxypeptidase A, because the nonpolar isobutyl substituent of
phenylalanine is more attracted to the hydrophobic pocket of the enzyme than is the negatively charged
substituent of aspartate.
17.
Glu 270 adds to the carbonyl group of the ester, forming a tetrahedral intermediate. Collapse of the tetra-
hedral intermediate is most likely catalyzed by an acidic group of the enzyme, which increases the leaving
ability of the RO group. (Perhaps the HO substituent of tyrosine is close enough in the esterase to be the
catalyst.) The group that donates the proton can then act as a base catalyst to remove a proton from water
as it hydrolyzes the anhydride.
Glu 270 O
Glu 270 O
−
−
O
+
B H
Glu 270 O O
C
R
B
Glu 270
O
−
O
Glu 270 O
−
C
R
O
HO
+
C
R
O
RO
C
O
C
O
C
O
C
O
C
O
HO R
C
H H
O
RO R
C