158 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
the catalytic activity of the phosphofructokinase,
pyruvate kinase, and pyruvate dehydrogenase com-
plex are enhanced; as [ATP] becomes high, they will
be inhibited. These modulators bind at allosteric
sites on enzymes (Hammes 2002). The structure and
mechanism of allosteric regulatory enzymes such as
aspartate transcarbamylase (Cherfils et al. 1990) and
ribonucleoside diphosphate reductase (Scott et al.
2001) have been well studied. However, allosteric
regulation is not the only way that the enzymatic
activity is influenced. Covalent modification by pro-
tein kinases (Langfort et al. 1998) and phosphatases
(Luan 2003) on enzymes will cause large fluctua-
tions in total enzymatic activity in the metabolism
pool. In addition, sophisticated tuning phenomena
on glycogen phosphorylase and glycogen synthase
through phosphorylation and dephosphorylation
have been observed after hormone signaling (Nuttall
et al. 1988, Preiss and Romeo 1994).
ENZYMESAREPOWERFULCATALYSTS
The compound glucose will remain in a bottle for
years without any detectable changes. However,
when glucose is applied in a minimal medium as the
only carbon source for the growth of Salmonella
typhimurium(or Escherichia coli), phosphorylation
of this molecule to glucose-6-phosphate is the first
chemical reaction that occurs as it enters the cells.
From then on, the activated glucose not only serves
as a fuel compound to be oxidized to produce chem-
ical energy, but also goes through numerous reac-
tions to become the carbon skeleton of various
micro- and macrobiomolecules. To obtain each end
product, multiple steps have to be carried out. It may
take less than 30 minutes for a generation to go
through all the reactions. Only the existence of en-
zymes guarantees this quick utilization and disap-
pearance of glucose.
ENZYMES AND ACTIVATION
ENERGY
ENZYMESLOWER THEACTIVATIONENERGY
Enzymes are mostly protein catalysts; except for the
presence of a group of ribonucleic acid–mediated
reactions, they are responsible for the chemical reac-
tions of metabolism in cells. For the catalysis of a
reaction, the reactants involved in this reaction all
require sufficient energy to cross the potential ener-
gy barrier, the activation energy (EA), for the break-
age of the chemical bonds and the start of the reac-
tion. Few have enough energy to cross the reaction
energy barrier until the reaction catalyst (the en-
zyme) forms a transition state with the reactants to
lower the activation energy (Fig. 7.1). Thus, the en-
zyme lowers the barrier that usually prevents the
chemical reaction from occurring and facilitates the
reaction proceeding at a faster rate to approach equi-
librium. The substrates (the reactants), which are
specific for the enzyme involved in the reaction,
combine with enzyme to lower the activation energy
of the reaction. One enzyme type will combine with
one specific type of substrate, that is, the active site
of one particular enzyme will only fit one specific
type of substrate, and this leads to the formation of
and enzyme-substrate (ES) complex. Once the ener-
gy barrier is overcome, the substrate is then changed
to another chemical, the product. It should be noted
that the enzyme itself is not consumed or altered by
the reaction, and that the overall free-energy change
(G) or the related equilibrium also remain con-
stant. Only the activation energy (EAu) of the uncat-
alyzed reaction decreases to that (EAc) of the
enzyme-catalyzed reaction.
One of the most important mechanisms of the
enzyme function that decreases the activation ener-
gy involves the initial binding of the enzyme to the
substrate, reacting in the correct direction, and clos-
ing of the catalytic groups of the ES complex. The
binding energy, part of the activation energy, is
required for the enzyme to bind to the substrate and
is determined primarily by the structure comple-
mentarities between the enzyme and the substrate.
The binding energy is used to reduce the free energy
of the transition-state ES complex but not to form a
stable, not easily separated, complex. For a reaction
to occur, the enzyme acts as a catalyst by efficiently
binding to its substrate, lowering the energy barrier,
and allowing the formation of product. The enzyme
stabilizes the transition state of the catalyzed reac-
tion, and the transition state is the rate-limiting state
in a single-step reaction. In a two-step reaction, the
step with the highest transition-state free energy is
said to be the rate-limiting state. Though the reac-
tion rate is the speed at which the reaction proceeds
toward equilibrium, the speed of the reaction does
not affect the equilibrium point.