Biology 12

48 MHR • Unit 1 Metabolic Processes

from this venom, contains enzyme inhibitors that

prevent blood clots from forming. In 1999,

pharmaceutical researchers found that more than

40% of stroke patients who received ANCROD

recovered all of their mental faculties. Other

venom, such as scorpion venom, is being used

to treat autoimmune disorders.

Figure 2.9Venom from this Malayan pit viper can be used

to treat patients who have had a stroke.

There are two kinds of inhibition that can affect

the activity of enzymes. In non-competitive

inhibition, an inhibitor molecule binds to the

enzyme at a site known as the allosteric site. As a

result, the three-dimensional structure of the enzyme

is altered, which prevents the substrate from binding

to the active site (see Figure 2.10). Most metabolic

pathways are regulated by feedback inhibition. This

is a type of non-competitive inhibition in which the

end product of the pathway binds at an allosteric

site on the first enzyme of the pathway. In this way,

non-competitive inhibitors can play a key role in

the normal functioning and regulation of metabolic

pathways. Study Figure 2.11 to learn how a metabolic

pathway is regulated by feedback inhibition.

Figure 2.10Non-competitive inhibition

Figure 2.11This hypothetical metabolic pathway is

regulated by feedback inhibition.

Molecules that promote the action of enzymes

can also bind to the allosteric site. These molecules

are known as activators. The activity of any

enzyme can change, depending on the number of

activators and inhibitors in its environment. The

regulation of enzyme activity by inhibitors and

activators is known as allosteric regulation.

Competitive inhibitioninvolves chemical

compounds that bind to the active site of the

enzyme and inhibit enzymatic reactions. The

compounds compete with the true substrate for

access to the active site. This competition is

possible because competitive inhibitors are very

similar in shape and structure to the enzyme’s

View of active pathway

View of inhibited pathway

stop

first

reactant

A

end

product

F

end

product

F

EDCB

first

reactant

A

A

E 1

E 1

E 1

E 1

Overall view of pathway

EDCB

first

reactant

A

end

product

F

E 1 E 2 E 3 E 4 E 5

E 1

E 2 E 3 E 4 E 5

substrate

active site

enzyme

allosteric site

inhibitor

molecule

Inhibitor binds to the

enzyme at the allosteric site

A

Shape of

the active

site is

altered

B

Substrate

can no

longer fit

into the

active site

C

The substrate binds to the

active site of the enzyme.

The pathway is now active.

A

Overall view of pathwayOnce enough end product

has been produced, it binds to the allosteric site of

the enzyme. The pathway is no longer active and the

end product is no longer produced.

D

View of active pathway

The pathway remains

active as long as there is

insufficient end product.

B

View of inhibited

pathwayOnce there is

sufficient end product,

the product binds with

the enzyme, changing

the shape of the active

site. This prevents the

enzyme from binding

with additional

substrate and inhibits

the pathway.

C