Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymes are Wonderful Catalysts 31
the reaction catalysed by the enzyme, being the production of a single regio- and
stereo-isomer of the product. Both are properties which are highly prized in
synthetic reactions used in organic chemistry: enzymes are able to do both.
3.2 A thermodynamic model of catalysis
A catalyst may be deWned as a species which accelerates the rate of a chemical
reaction whilst itself remaining unchanged at the end of the catalytic reaction.
In thermodynamic terms, catalysis of a chemical reaction is achieved by reducing
the activation energy for that reaction, the activation energy being the
diVerence in free energy between the reagent(s) and the transition state for the
reaction. This reduction in activation energy can be achieved either by stabilisation
(and hence reduction in free energy) of the transition state by the catalyst,
or by the catalyst Wnding some other lower energy pathway for the reaction.
Figure 3.4 illustrates the free energy proWle of a typical acid-catalysed
chemical reaction which converts a substrate, S, to a product, P. In this case
an intermediate chemical species SH þ is formed upon protonation of S. If
the conversion of SH þ to PH þ is ‘easier’ than the conversion of S to P, then
the activation energy for the reaction will be reduced and hence the reaction will
go faster. It is important at this point to deWne the diVerence between an
intermediate and a transition state: an intermediate is a stable (or semi-stable)
chemical species formed during the reaction and is therefore a local energy
minimum, whereas a transition state is by deWnition a local energy maximum.
The rate of a chemical reaction is related to the activation energy of the
reaction by the following equation:
k ¼ A:e (
E act=RT)
Therefore, the rate acceleration provided by the catalysis can simply be
calculated:
k cat =k uncat ¼ e (E uncat
E cat =RT)
If, for example, a catalyst can provide 10 kJ mol 1 of transition stabilisation
energy for a reaction at 258C a 55-fold rate acceleration will result, whereas a
20 kJ mol
1 1
stabilisation will give a 3000-fold acceleration and a 40 kJ mol
stabilisation a 10 7 -fold acceleration! A consequence of the exponential relationship
between activation energy and reaction rate is that a little extra transition
state stabilisation goes a long way!
An enzyme-catalysed reaction can be analysed thermodynamically in the
same way as the acid-catalysed example, but is slightly more complicated. As
explained in Chapter 2, enzymes function by binding their substrate reversibly
at their active site, and then proceeding to catalyse the biochemical reaction