2009_Book_FoodChemistry

112 2 Enzymes
Fig. 2.13. Example of a transition state analog inhibitor a reaction of triosephosphate isomerase, TT: postulated
transition state; b inhibitor
2.4.2.2 Structural Complementarity
to Transition State
It is assumed that the active conformation of the
enzyme matches the transition state of the reaction.
This is supported by affinity studies which
show that a compound with a structure analogous
to the transition state of the reaction (“transition
state analogs”) is bound better than the substrate.
Hydroxamic acid, for example, is such a transition
state analog which inhibits the reaction of
triosephosphate isomerase (Fig. 2.13). Comparisons
between the Michaelis constant and the
inhibitor constant show that the inhibitor has
a 30 times higher affinity to the active site than
the substrate.
The active site is complementary to the transition
state of the reaction to be catalyzed. This assumption
is supported by a reversion of the concept.
It has been possible to produce catalytically active
monoclonal antibodies directed against transition
state analogs. The antibodies accelerate the
reaction approximating the transition state of the
analog. However, their catalytic activity is weaker
compared to enzymes because only the environment
of the antibody which is complementary to
the transistion state causes the acceleration of the
reaction.
Transition state analog inhibitors were used to
show that in the binding the enzyme displaces
the hydrate shell of the substrates. The reaction
rate can be significantly increased by removing
the hydrate shell between the participants.
Other important factors in catalytic reactions are
the distortion of bonds and shifting of charges.
The substrate’s bonds will be strongly polarized
by the enzyme, and thus highly reactive, through
the precise positioning of an acid or base group
or a metall ion (Lewis acid, cf. 2.3.3.1) (example
see Formula 2.15). These hypotheses are supported
by investigations using suitable transition
state analog inhibitors.
2.4.2.3 Entropy Effect
An interpretation in thermodynamic terms takes
into account that a loss of entropy occurs during
catalysis due to the loss of freedom of rotation
and translation of the reactants. This entropy
effect is probably quite large in the case
of the formation of an enzyme-substrate complex
since the reactants are fairly rigidly positioned before
the transition state is reached. Consequently,
the conversion of the enzyme-substrate complex
to the transition state is accompanied by little or
no change of entropy. As an example, a reaction
running at 27 ◦ C with a decrease in entropy
of 140 JK −1 mol −1 is considered. Calculations
indicate that this decrease leads to a reduction in
free activation energy by about 43 kJ. This value
falls in the range of the amount by which the activation
energy of a reaction is lowered by an enzyme
(cf. Table 2.1) and which can have the effect
of increasing the reaction rate by a factor of 10 8
The catalysis by chymotrypsin, for example,
shows how powerful the entropy effects can be.
In section 2.4.2.5 we will see that this catalysis is
a two-step event proceeding through an acylated
enzyme intermediate. Here we will consider
only the second step, deacylation, thereby
distinguishing the following intermediates:
a) N-acetyl-L-tyrosyl-chymotrypsin
b) Acetyl-chymotrypsin.