Biomedical Engineering Reference
In-Depth Information
CHAPTER 5
AREAS RELATED TO ENZYME CATALYSIS
5.1. Antibody catalysis
Recent developments of catalysis by antibodies are based on two fundamental ideas
advanced many years ago. Pauling (1946) proposed that the lowering of the activation
energy in enzyme catalysis results from the enzymes affinity for the transition state
exceeding its affinity for the substrate. Developing this idea, Jencks (1969, 1981)
suggested that stable molecules, analogs of the transition state of a reaction could be
used as haptens for the production of antibodies, which catalyze these reactions. These
ideas were first realized in the 1980s by groups of Lerner and Schultz (Tramontano et.
al., 1986; Pollack et al. 1989; Schultz, 1989; Lerner et al. 1991). Mechanistic and
structural studies of antibodies provide insights into the molecular mechanism of
enzymatic catalysis and the evolution of catalytic function. The ability to organize the
immune response to generate selective catalysts for natural enzymatic reactions and for
newly invented chemical processes underscores the chemical potential of large
combinatorial libraries (Schultz and Lerner, 1995).
At present more than 100 reactions have been successfully performed with the use of
catalytic antibodies (Schultz and Lerner, 1995; Hilvert, 2000; Rader and List, 2000;
Blackburn and Garcon, 2000; DeSilva et al., 2000; Kurihara et al. 2000; Vayron et al.,
2000; and references therein). Among them are reactions such as sigmatropic
rearrangements, triterpen synthesis, hydrolysis, deprotonation, complexation of bivalent
metals by protoporphyrin IX, acyl transfer and retroaldol reactions, the regio- and
stereochemical of reactions, cleavage of acetals and glycosides, phosphate ester
cleavage, amides and organophosphorus compounds hydrolysis, removal of the p-
nitrobenzyl ester protecting group, the derivatization of primary amines with
naphthalene-2,3-dicarboxaldehyde, etc.
Success in the synthesis of new catalytic antibodies (CAs) depends on the efficiency
of each of the following steps: 1) hapten design, 2) immunogen synthesis, 3) preparation
of the enzymatic tracer; 4) generation and purification of antibodies; and 5) kinetic
assays.
At the most important step of designing a hapten, an analog of potential transition
state, attention is focused on suggested distinctions between ground and transition states.
These distinctions might be differences in conformation of substrates, changes in
hybridization or in distribution of charges, dipoles and nucleophilic and electrophilic
groups. For reactions involving several substrates, multisubstrate analogues can be used.
Crystallographic and NMR (nuclear Overhauser effects) data on hapten-antibody
complexes can confirm the complementarity of designed haptens and induced binding
pockets.
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