Food Biochemistry and Food Processing (2 edition)

(Steven Felgate) #1

BLBS102-c08 BLBS102-Simpson March 21, 2012 12:8 Trim: 276mm X 219mm Printer Name: Yet to Come


170 Part 2: Biotechnology and Enzymology

Figure 8.1.A transition state diagram, also called reaction
coordinate diagram, shows the activation energy profile of the
course of an enzyme-catalyzed reaction. One curve depicts the
course of the reaction in the absence of the enzyme, while the other
depicts the course of reaction in the presence of the enzyme that
facilitates the reaction. The difference in the level of free energy
between the beginning state (ground state) and the peak of the
curve (transition state) is the activation energy of the reaction. The
presence of enzyme lowers the activation energy (EA) but does not
change the overall free energy (G).

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 reaction, 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 reaction rate is the speed
at which the reaction proceeds toward equilibrium, the speed of
the reaction does not affect the equilibrium point.

How Does an Enzyme work?

An enzyme carries out the reaction by temporarily combining
with its specific kind of substrate, resulting in a slight change
of their structures to produce a very precise fit between the two
molecules. The introduction of strains into the enzyme and sub-
strate shapes allows more binding energy to be available for the
transition state. Two models of this minor structure modification
are the induced fit, in which the binding energy is used to distort
the shape of the enzyme, and the induced strain, in which bind-
ing energy is used to distort the shape of the substrate. These
two models, in addition to a third model, the nonproductive
binding model, have been proposed to describe conformational
flexibility during the transition state. The chemical bonds of the

substrate break and form new ones, resulting in the formation of
new product. The newly formed product is then released from
the enzyme, and the enzyme combines with another substrate
for the next reaction.

ENZYME KINETICS AND MECHANISM


Regulatory Enzymes

For the efficient and precise control of metabolic pathways in
cells, the enzyme, which is responsible for a specific step in a
serial reaction, must be regulated.

Feedback Inhibition

In living cells, a series of chemical reactions in a metabolic path-
way occurs, and the resulting product of one reaction becomes
the substrate of the next reaction. Different enzymes are usu-
ally responsible for each step of catalysis. The final product of
each metabolic pathway will inhibit the earlier steps in the serial
reactions. This is called feedback inhibition.

Noncompetitive Inhibition

An allosteric site is a specific location on the enzyme where an
allosteric regulator, a regulatory molecule that does not directly
block the active site of the enzyme, can bind. When the allosteric
regulator attaches to the specific site, it causes a change in the
conformation of enzyme active site, and the substrate therefore
will not fit into the active site of enzyme; this results in the inhi-
bition phenomenon. The enzyme cannot catalyze the reaction,
and not only the amount of final product but also the concen-
tration of the allosteric regulator decrease. Since the allosteric
regulator (the inhibitor) does not compete with the substrate for
binding to the active site of enzyme, the inhibition mechanism
is called noncompetitive inhibition.

Competitive Inhibition

A regulatory molecule is not the substrate of the specific enzyme
but shows affinity for attaching to the active site. When it occu-
pies the active site, the substrate cannot bind to the enzyme for
the catalytic reaction, and the metabolic pathway is inhibited.
Since the regulatory molecule will compete with the substrate
for binding to the active site on enzyme, this inhibitory mecha-
nism of the regulatory molecule is called competitive inhibition.

Enzyme Kinetics

During the course of an enzyme-catalyzed reaction, the plot
of product formation over time (product formation profile) re-
veals an initial rapid increase, approximately linear, of prod-
uct, and then the rate of increase decreases to zero as time
passes (Fig. 8.2A). The slope of initial rate (viorv), also called
the steady state rate, appears to be initially linear in a plot
where the product concentration versus reaction time follows the
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