Biology (Holt)

BIO

graphic

A substrate attaches to an

enzyme's active site.

Enzymes assist biochemical reactions by bringing key molecules together.

Enzyme Action

1 2 The enzyme reduces the activa-tion energy of the reaction. 3 The enzyme is not changed by the reaction.

Active

site

Substrate

Enzyme

Products

Figure 15

Enzyme Specificity
A substance on which an enzyme acts during a chemical reaction is
called a (SUHB strayt).Enzymes act only on specific sub-
strates. For example, the enzyme amylase (AM uh lays)assists in the
breakdown of starch to glucose in the following chemical reaction.
In this reaction, starch is amylase’s substrate.

amylase
starch ←→glucose
The enzyme catalase (KAT uh lays) assists in the breakdown of
hydrogen peroxide, H 2 O 2 , a toxin formed in cells. In this case,
hydrogen peroxide is broken down to water, H 2 O, and oxygen gas,
O 2. In this reaction, hydrogen peroxide is catalase’s substrate.

catalase
2H 2 O 2 →← 2H 2 O O 2
An enzyme’s shape determines its activity. Typically, an enzyme is
a large protein with one or more deep folds on its surface. These
folds form pockets called .As shown in Figure 15,an
enzyme’s substrate fits into the active site. An enzyme acts only on a
specific substrate because only that substrate fits into its active site.

Step When an enzyme first attaches to a substrate during a

chemical reaction, the enzyme’s shape changes slightly so

that the substrate fits more tightly in the enzyme’s active site.

Step At an active site, an enzyme and a substrate interact in a

way that reduces the activation energy of the reaction, mak-

ing the substrate more likely to react.

Step The reaction is complete when products have formed. The

enzyme is now free to catalyze further reactions.

active sites

substrate

SECTION 4Energy and Chemical Reactions 41

Real Life

Some laundry detergents

contain enzymes.

Enzymes break down

carbohydrates or proteins

in common stains such as

food and blood, making

the stains easier to

remove. The enzymes

can tolerate the high

temperatures and alkaline

conditions required for

cleaning fabrics.

Finding Information

Find out the source for

enzymes used in laundry

detergents. 9C