Competitive inhibitors bind to the active site of an enzyme, preventing substrate binding. Non-competitive inhibitors bind elsewhere on the enzyme, altering its shape. Uncompetitive inhibitors only bind after substrate binding, blocking the reaction.

1. Inhibition of enzyme activity
Some substances reduce or even stop the catalytic activity of
enzymes in biochemical reactions. They block or distort the active
site. These chemicals are called inhibitors, because they inhibit
reaction.
Inhibitors that occupy the active site and prevent a substrate
molecule from binding to the enzyme are said to be active site-
directed (or competitive, as they 'compete' with the substrate for the
active site).
Inhibitors that attach to other parts of the enzyme molecule, perhaps
distorting its shape, are said to be non-active site-directed (or non
competitive).
1

2. i) inhibitor competes with substrate for the enzyme active site
ii) non-covalent interactions
iii) structures similar to substrate, products or transition state
Reaction scheme for a reversible enzyme inhibitor and the definition of the
dissociation/inhibition constant.
Competitive inhibitors (Michaelis-Menton kinetics)
2

3. P + E
S
+
E
S
E
I S
E
I
S S
S
I
S
E
X
I
I
S
Competitive Inhibition
P + E
Formation of the E-I complex prevents
binding of substrate, therefore the reaction
cannot proceed to the normal physiological
product, P.
High [S] overcome
inhibition
3

4. Formation of the E-I complex prevents binding of substrate,
therefore the reaction cannot proceed to the normal physiological
product, P.
Larger [I] leads to larger [E-I], leaving less free enzyme to which
the substrate can bind.
Limitation: must maintain high [I] to maintain inhibition, requiring
multiple administrations or extended release formulations.
Note that high [S] can overcome inhibition by favoring equilibrium
to the E-S complex.
Competitive inhibitors
4

5. A Competitive Inhibitor has a chemical similarity to the substrate and competes with
the substrate for binding to the active site of the enzyme. A good example to
describe competitive inhibition is the mitochondrial enzyme, succinate
dehydrogenase:
(A) The reaction catalyzed by succinate dehydrogenase is the oxidation of succinate
to fumarate. (B) Malonate and oxaloacetate are competitive inhibitors of succinate
dehydrogenase.
Both these competitive inhibitors, malonate and oxaloacetate, look like succinate in
their chemical character. Both inhibitors can bind in the same places in the active
site of succinate dehydrogenase as the substrate. However, neither inhibitor has the
capacity to undergo the reaction and so the enzyme is inhibited. The hallmark of
competitive inhibition is that it can be overcome by a sufficiently high concentration
of substrate.
C
CH2
CH2
O-
O
O-
O
C
2H
-
Succinate
dehydrogenase
C
CH
C
O-
O
O-
O
C
H
C
C O
CH2
O-
O
O-
O
C
C
CH2
O-
O
O-
O
C
A B
5

6. 6

7. 7

8. Competitive inhibition occurs when both substrate and
inhibitor will fit the active site, and compete with each
other to occpy it.
y = mx + c
1
[S]
1
V
Vmax unaltered
Km increased
E
I
E-I P + E
X
E-S P + E
s
8

9. 9

10. Non-competitive inhibition
 In this case of the non-competitive inhibition, the inhibitor
reacts with the enzyme at a site other than the active site,
causing conformational change in enzyme which decreases or
stops catalytic activity.
 Both the free enzyme (E) and the enzyme-substrate complex
(E-S) react with inhibitor.
Non-competitive inhibition is irreversible and the substrate can
not over come the inhibitors impact on the enzyme
E
S
E
S
P
E
+
E
S
E
S
E
+
I
EI
ES
ESI
10

11. 11

12. Non-competitive inhibition occurs when the inhibitor binds
to a site on the enzyme other than the active site or binds
irreversibly to the active site.
y = mx + c
1
[S]
1
V
Vmax reduced
Km unaltered
1
Km
12

13. Non-competitive inhibition occurs when the inhibitor binds
to a site on the enzyme other than the active site or binds
irreversibly to the active site.
E ES
EI ESI
E + P
S
S
I I
I I
ki k’i
13
Two Ki's have the same value, i.e. the
inhibitor binds equally well to E and ES,
then a mixed inhibitor is usually called a
noncompetitive inhibitor.

14. 14

15. Mixed Inhibitor.
y = mx + c
1
[S]
1
V + I
1
Km
1
Km
app
1
Vmax
1
Vmax app
E ES
EI ESI
E + P
S
S
I I
I I
ki k’i
A mixed inhibitor binds to both E and ES,
not at the substrate binding site:
Note that there are 2 Ki's. If they have the same
value, i.e. the inhibitor binds equaly well to E and ES,
then a mixed inhibitor is usually called a
noncompetitive inhibitor.
In the presence of a mixed inhibitor, the
following Lineweaver-Burk plot is obtained:
Both the apparent Km and the apparent Vmax
are changed by the inhibitor
ki k’i
15

16. Uncompetitive inhibition occurs when the inhibitor binds after
the substrate has bound to the enzyme, and then stops the
reaction occurring.
Uncompetitive inhibition
occurs when the inhibitor
binds only to the ES
complex:
S
+
E E
S
P +
E
E
S
I
16