Food Biochemistry and Food Processing

7 Enzyme Activities 159

HOWDOES ANENZYMEWORK?

An enzyme carries out the reaction by temporarily
combining with its specific kind of substrate, result-
ing 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 substrate
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 binding
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 com-
bines with another substrate for the next reaction.

ENZYME KINETICS AND

MECHANISM

REGULATORYENZYMES

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 regu-

lated.

Feedback Inhibition

In living cells, a series of chemical reactions in a

metabolic pathway occurs, and the resulting product

of one reaction becomes the substrate of the next

reaction. Different enzymes are usually 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 inhibi-

tion.

Figure 7.1.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).