Food Biochemistry and Food Processing (2 edition)

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