Page 207 - The Manga Guide to Biochemistry

4. Enzymes and Inhibitors

Why in the world do we have to use equations and graphs—which are no fun at all—to get
the values of Vmax and Km?

One reason is to demonstrate that enzyme reactions are extremely precise and predict-
able. They proceed in strict accordance with chemical and mathematical laws.

More importantly, for scholars doing enzyme research or research concerning the world
surrounding enzymes, the values of Vmax and Km are very useful.

One of these areas of research deals with the relationship between enzymes and
inhibitors. An inhibitor is a substance that affects the binding of an enzyme and substrate,
or affects the enzyme itself. As a result, it inhibits the enzyme’s activity.

Many inhibitors are created artificially and used for enzyme research. Since they inhibit
enzymes, they are often toxic to living organisms. However, they can also be used in positive
ways—to kill cancer cells, for example.

Inhibitors also exist in the natural world. In that case, they’re known as “cellular”
enzyme inhibitors rather than “medicinal” inhibitors. For example, some inhibitors are cre-
ated directly in an organism’s cells and play important roles in regulating enzyme reactions.
Plants can create inhibitors as well. The seeds of certain legumes, for example, contain cel-
lular enzyme inhibitors such as α-amylase inhibitors or trypsin inhibitors. These are known
as anti-nutritional factors. Scientists believe that these inhibitors might be part of the plant’s
natural defense system, which helps protect them against predation.

If the structure of an inhibitor is similar to that of the substrate, it can bond quite eas-
ily with the enzyme. However, since the shape of an inhibitor is subtly different from that of
the substrate, no reaction will occur, and once the enzyme has already bonded with another
substance, it isn’t able to bond with its substrate. Actually, there are several different types
of enzyme inhibition, and we can tell which is occuring by using the Michaelis-Menten
­equation.

Two of the types of enzyme inhibition are competitive inhibition and non-competitive
inhibition.

Competitive inhibition occurs when a substance that is very similar to the substrate
bonds with the enzyme, inhibiting the enzyme reaction.

Inhibitor

Substrate Substrate can no
longer bind with

enzyme!

Active site
of enzyme*

* The active site is the part where the substrate bonds
to the enzyme in order to undergo a catalytic action.

Enzymes Are the Keys to Chemical Reactions 193