Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

264 Chapter 12
CO 2
−
− O 2 C
O
CO 2
−
− O 2 C
O
CO 2
−
OH
O
CO 2
−
OH
OH
− O 2 C
Hapten
O
CO 2
−
O
H
N
NH-protein
O
Figure 12.11 A catalytic antibody for the chorismate mutase reaction.
have been synthesised for the chorismate mutase-catalysed rearrangement of
chorismate to prephenate shown in Figure 12.11. One such analogue has
been used to generate catalytic antibodies capable of accelerating the rearrangement
by 10 4 -fold (as compared to 10 6 -fold for the enzyme-catalysed reaction).
The kinetic parameters for this catalytic antibody are K M 260 mm and k cat
2:7 min 1 .
The ability of catalytic antibodies to catalyse reactions which are diYcult to
achieve chemically has been demonstrated by the examples shown in Figure
12.12. Antibodies raised against a cyclic N-oxide were found to catalyse the
intramolecular ring opening of a structurally related epoxide, via a disfavoured
6-endo-tet transition state, to give only the six-membered ring product. Antibodies
capable of catalysing cationic cyclisation reactions have also been isolated:
using cyclic N-oxide and ammonium salt haptens, antibodies capable of
cyclisation of the corresponding sulphonate were found. Unexpectedly, it was
found that cyclisation of the substituted alkene substrate gave a cyclopropane
product, reminiscent of the monoterpene cyclase-catalysed reactions.
The fact that catalytic antibodies can be generated supports the idea that
enzymes achieve much of their rate acceleration through transition state stabilisation.
However, the observation that most catalytic antibodies are much
slower than their enzyme counterparts raises the question: what other factors
do enzymes utilise to achieve their additional rate acceleration Perhaps the
answer is that enzymes have had millions of years to perfect their catalytic
abilities through such subtle ploys as use of strain, bifunctional catalysis, or
protein dynamics. The other common limitation of catalytic antibodies is that
they suVer from product inhibition, whereas enzymes usually bind their substrates
and products relatively weakly (as discussed in Section 3.4 it is thermodynamically
unfavourable for enzymes to bind their substrates strongly).