Chemistry Reference
In-Depth Information
Scheme 4.6 1,3-Diketone 15, used as a hapten to raise antibody
38C2, traps the reactive, active-site amine of Lys
H93
to form a stable,
chromophoric vinylogous amide.
two-step mechanism involving transient acylation of an active site nucleophile [49].
Other mechanism-based inactivators of hydrolytic enzymes have afforded antibodies
with
-lactamase [50] and galactosidase activity [51]. Versatile aldolase catalysts have
been created with immunogenic 1,3-diketones (
15
) [52], which trap reactive lysines via
Schiff base formation, then rearrange to a more stable and readily detected vinylogous
amide (
16
) (Scheme 4.6). In the presence of a
b
-hydroxyketone instead of the 1,3-di-
ketone, analogous enamine chemistry results in a retro-aldol reaction. In the synthetic
direction, a wide range of aldol condensations is catalyzed [53].
Mechanistically, the antibody aldolases resemble natural class I aldolase enzymes
(Scheme 4.7) [52]. In the first step of a condensation reaction, the
b
-amino group of the
catalytic lysine reacts with a ketone to form a Schiff base. Deprotonation of this species
yields a nucleophilic enamine, which condenses with electrophilic aldehydes in a sec-
ond step to form a new carbon-carbon bond. Subsequent hydrolysis of the Schiff base
releases product and regenerates the active catalyst.
Aldolase antibodies obtained by reactive immunization are notable for high activity,
broad substrate specificity, and high selectivities [53]. Rate accelerations are typically in
the range 10
5
to 10
7
-fold over background. Although the k
cat
/K
m
values are 10
2
to 10
4
lower than those of aldolase enzymes, these are among the most efficient antibody
catalysts described to date. Their efficacy is all the more notable in light of the inher-
ently complex, multistep process they catalyze.
e
Scheme 4.7 Antibodies generated
against hapten 15 promote diverse aldol
condensations through enamine chemis-
try.
