An Introduction to Metabolism

Page
7

Substrate Specificity of Enzymes

The reactant that an enzyme acts on is called the enzyme’s substrate

The enzyme binds to its substrate, forming an enzyme-substrate complex

The active site is the region on the enzyme where the substrate binds

Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction

Slide 54

Fig. 8-16

Substrate

Active site

Enzyme

Enzyme-substrate

complex

(b)

(a)

Slide 55

Catalysis in the Enzyme’s Active Site

In an enzymatic reaction, the substrate binds to the active site of the enzyme

The active site can lower an EA barrier by

Orienting substrates correctly

Straining substrate bonds

Providing a favorable microenvironment

Covalently bonding to the substrate

Slide 56

Fig. 8-17

Substrates

Enzyme

Products are

released.

Products

Substrates are

converted to

products.

Active site can lower EA

and speed up a reaction.

Substrates held in

active site by weak

interactions, such as

hydrogen bonds and

ionic bonds.

Substrates enter active site; enzyme

changes shape such that its active site

enfolds the substrates (induced fit).

Active

site is

available

for two new

substrate

molecules.

Enzyme-substrate

complex

5

3

2

1

6

4

Slide 57

Effects of Local Conditions on Enzyme Activity

An enzyme’s activity can be affected by

General environmental factors, such as temperature and pH

Chemicals that specifically influence the enzyme

Slide 58

Effects of Temperature and pH

Each enzyme has an optimal temperature in which it can function

Each enzyme has an optimal pH in which it can function

Slide 59

Fig. 8-18

Rate of reaction

Optimal temperature for

enzyme of thermophilic

(heat-tolerant)

bacteria

Optimal temperature for

typical human enzyme

(a) Optimal temperature for two enzymes

(b) Optimal pH for two enzymes

Rate of reaction

Optimal pH for pepsin

(stomach enzyme)

Optimal pH

for trypsin

(intestinal

enzyme)

Temperature (ºC)

pH

5

4

3

2

1

0

6

7

8

9

10

0

20

40