Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymes are Wonderful Catalysts 35
(3)
O
O
(6)
O
OPh
OPh
OPh
O
O
unreactive
conformation
O
O
reactive
conformation
O
O
held in reactive
conformation
Figure 3.7 Intramolecular hydrolysis of phenyl succinate (3) versus phenyl maleate (6).
the observed rates of hydrolysis are even faster. Phenyl phthalate (see Figure 3.5
(5) ) has an eVective concentration of acetate ions of 2 10 5 m, whilst phenyl
maleate (see Figure 3.5 (6) ) has an astonishing eVective concentration of 10 10 m!
Yet the same molecule containing a trans- double bond has no rate acceleration
at all. So it is clear that by holding the reactive groups rigidly in close proximity
to one another remarkable rate acceleration can be achieved. Why is the
hydrolysis of phenyl maleate, in which a Wve-membered anhydride is formed,
so much faster than the hydrolysis of phenyl succinate, in which an apparently
similar Wve-membered anhydride is formed The answer is that in phenyl
maleate the reactive groups are held in the right orientation to react, as
shown in Figure 3.7, so the probability of the desired reaction is increased.
In thermodynamic terms, the restriction of the double bond in the case of
phenyl maleate has removed rotational degrees of freedom, so that in going to
the transition state for the intramolecular reaction fewer degrees of freedom are
lost, which means that the reaction is entropically more favourable. If you think
of entropy as a measure of order in the system, then in the case of phenyl
maleate the molecule is already ordered in the right way with respect to the
reacting groups. Thus, there is a large kinetic advantage in intramolecular
chemical reactions due to the ordering of reactive groups.
The same eVect operates in enzyme active sites, and is a major factor in
enzyme catalysis. The binding of substrates and cofactors at an enzyme active
site of deWned three-dimensional structure brings the reagents into close proximity
to one another and to the enzyme active site functional groups. This
increases the probability of correct positioning for reaction to take place, so it
speeds up the reaction. An important factor in this analysis is that the enzyme
structure is already held rigidly (or fairly rigidly at least) in the correct conformation
for binding and catalysis. Recent studies have argued that the
thermodynamic origin of this kinetic advantage in enzyme catalysis is primarily
enthalpic, rather than entropic. Nevertheless, the catalytic power of proximity
eVects, or ‘pre-organisation’, has been demonstrated by the synthesis of host–
guest systems which mimic enzymes by binding substrates non-covalently (see
Chapter 12).