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Non-Enzymatic Biological Catalysis 263
parameters K M and k cat can be measured as 0.56 mm and 0:08 min 1 , respectively.
These catalytic properties are slow by the standards of enzyme-catalysed
reactions, but they validate the idea that catalytic antibodies can, in principle,
be designed and generated.
How are hydrolysis reactions catalysed at the antigen combining sites of
these antibodies The amide and ester hydrolysis reactions catalysed by antibody
43C9 have been analysed in some detail, and evidence has been obtained
for the existence of a covalent intermediate in the reaction formed by nucleophilic
attack of a histidine side chain. Determination of the amino acid sequence
of the antibody, together with molecular modelling studies, suggested
that His-L91 and Arg-L96 might be involved in antibody catalysis. Mutation of
either residue to glutamine gave catalytically inactive antibodies, conWrming
this suggestion. A mechanism proposed for the ester hydrolysis reaction is
shown in Figure 12.10. In this mechanism His-L91 acts as a nucleophile and
Arg-L96 stabilises the tetrahedral transition state in the reaction. Not surprisingly,
this antibody is strongly inhibited by the original phosphonamidate
analogue.
Catalytic antibodies have now been generated for many types of reactions,
and the interested reader is referred to several reviews at the end of the chapter.
The Wnal example was mentioned in Section 10.5. Transition state analogues
His L91
H
N
His L91
N
R
O
N
H
N
O
O
H 2 N NH +
2
NH
Arg L96
NO 2
R
O
N
H
N
O
O–
H H
H
N N +
NH
Arg L96
H
NO 2
R
O
N
H
His L91
O
H
N
N
OH
H 2 O
R
O
N
H
His L91
N
N
– O
O
Figure 12.10 Proposed mechanism for the antibody 43C9-catalysed ester hydrolysis reaction.
NO 2