1) Enzyme inhibition occurs when a compound decreases the rate of an enzyme-catalyzed reaction by binding to the enzyme and altering its catalytic activity. 2) There are several types of enzyme inhibition including competitive, non-competitive, irreversible, and allosteric inhibition. Competitive inhibitors bind to the enzyme's active site, non-competitive inhibitors bind elsewhere, and irreversible inhibitors bind permanently through covalent bonds. 3) Allosteric inhibition involves binding at allosteric sites which induces a conformational change that decreases catalytic activity, and feedback inhibition occurs when a product of a pathway inhibits an earlier enzyme in the pathway.

1. ENZYME INHIBITIION
By
Dr.Nivethitha.K, MDS
Periodontist

2. CONTENTS
 Introduction
 Key terms
 Enzyme inhibition
 Types of enzyme inhibition
1. Reversible & irreversible inhibition
2. Competitive, uncompetitive & non-competitive inhibition
3. Suicidal inhibition
4. Allosteric inhibition
 Summary
 References

3. WHAT ARE ENZYMES ??
 In Greek Enzyme – “enzymos” – “ leavened”
 Enzymes are biocatalysts
A substance produced by a living organism which
acts as a catalyst to bring about a specific
biochemical reactions

4. KEY TERMS

5. Active site is the region where
substrate molecules bind and
undergo a chemical reaction
Catalytic site:
The residues that
catalyse a reaction
of the substrate
Binding site:
The residues that
form temporary
bonds with the
substrate
KEY TERMS

6. Michaelis theory:
E + S E-S E+P
MICHAELIS CONSTANT & Vmax
K1 K3
K2
Michaelis constant (Km):
Km = K2 + K3
K1
* V max = maximum velocity

7.  A compound that decreases or diminishes the rate or velocity of an enzyme –
catalysed reaction by influencing the binding of the substrate / its turnover
number.
 The inhibitor may be organic / inorganic in nature
 Inhibitors – drugs, antibiotics, toxins & antimetabolite or natural products of
enzyme reaction
ENZYME INHIBITOR

9. REVERSIBLE INHIBITION
Inhibitor binds non – covalently with the enzyme
Reversible - If the inhibitor is removed , the action of the E is fully restored
An equilibrium is established between the free inhibitor & EI complex & is defined by
an equilibrium constant (Ki)
The activity of the enzyme is fully restored on removing the inhibitor by dialysis
E I I
E

10. COMPETITIVE INHIBITION
 Inhibitor binds reversibly to the same site that the substrate binds – competes with
the S for binding
 Substrate analogues – I closely resembles to the S
 I can be reversed by increasing the conc of S – reversible
 Degree of inhibition – depends on the conc. of S & I & on the relative affinities of
the enzyme for S & I

12. Velocity is decreased – effective concentration of enzyme is reduced
Km is increased affinity of the enzyme towards substrate is apparently decreased in
presence of the inhibitor
Vmax is not changed

13. Succinate Dehydrogenase:
Oxidation of succinate to fumarate.
This is the only citric acid cycle enzyme
that is tightly bound to the inner
mitochondrial membrane.
It is an FAD dependent enzyme
Malonate:
It has similar structure to Succinate
& it competitively inhibits SDH
COMPETITIVE INHIBITION

14. CLINICALLY USEFUL COMPETITIVE
INHIBITORS
DRUGS TARGET ENZYME THERAPEUTIC USE
STATINS – Atorvastin,
Simvastatin
HMG CoA reductase Decrease plasma
cholesterol level –
Antihyperlipidemic
agents
Allopurinol Xanthine oxidase Gout
Methotrexate Dihydrofolate reactions Cancer
Captopril & Enalapril Angiotensin converting
enzyme
High blood pressure
Dicoumarol Vit – K epoxide -
reductase
Anti - coagulant

15. NON - COMPETITIVE INHIBITION
 Inhibitor binds at a site other than the active site of the enzyme
 I has no structural resemblance to the S – No competition for binding
 Increase in the S conc. doesn’t relieve this I
 I & S binds at different site – formation of both EI & EIS complexes is possible
 EIS – forms product at a slower rate than ES
 Reaction is slowed down but not halted

17. Km value is unchanged – I do not interfere with the binding of S to E
Vmax decreases – I cannot be overcome by increasing the conc. of S

18. INHIBITOR ENZYME INHIBITED
Heavy metals – Ag2+, Hg2+,Pb2+ Binding with cysteinyl SH group of
enzyme
Pepstatin Pepsin
Soyabean trypsin inhibitor Trypsin
Ethanol or narcotic drugs Acid phosphatase
NON - COMPETITIVE INHIBITION

19. UNCOMPETITIVE INHIBITION
 I binds only to the ES complex, not to free E
 I – causes structural distortion of the active site – E catalytically inactive
 I can’t be reversed by increasing the (S) since I doesn’t compete with S for the
same binding site

22. IRREVERSIBLE INHIBITION
 Inhibitor binds covalently with the enzyme irreversibly
 Hence it can’t dissociate from the enzyme
 Inhibitor cause conformation change at active site of the E – destroying their
capacity to function as catalysts
 Enzyme activity is not regained on dialysis / by increasing the conc. of S
 A variety of poisons such as iodoacetate, OP poisoning & oxidising agents acts
as irreversible inhibition.

23. Irreversible is similar to non – competitive inhibition
Vmax – decreased
Km – No change

24. INHIBITORS ENZYME INHIBITED THERAPEUTIC USES
Disulfiram Aldehyde
dehydrogenase
Treatment of Alcoholism
Cyanide Cytochrome oxidase Inhibits respiratory chain
Fluoride Enolase Inhibits Glycolysis
Melathion Acetylcholine esterase Organophosphorus
insecticide
Di – isopropropyl
fluorophosphate
Serine proteases,
Acetylcholine esterase
Nerve gas
BAL – British Anti
Lewisite
Reacts with the SH
group of enzyme
Antidote for heavy metal
poisoning

25. SUICIDE INHIBITION
 Special type of irreversible inhibition of enzyme activity
 Also known as mechanism based inactivation
 The structural analogue ( inhibitor) is converted to a more effective
inhibitor with the help of the enzyme to be inhibited
 It makes use of the enzyme’s own reaction mechanism to inactivate it
 The Enzyme literally commits suicide

26. DRUGS PRODUCT TARGET ENZYME THERAPEUTIC
USE
Allopurinol Alloxanthin Xanthine oxidase Gout
5 - fluorouracil Fluorodeoxy
uridylate
Thymidylate
synthase
Cancer
Aspirin Acetylates serine
residue in the
active center of
cyclo - oxygenase
Cyclo - oxygenase NSAIDS
Difluro methyl
ornithine (DFMO)
Irreversible
covalent complex
with the co
enzyme
Ornithine
Decarboxylase
Trypanosomiasis
( Sleeping
sickness)

27. ALLOSTERIC INHIBITION
 Some Enzymes posses additional site other than the active site called as
allosteric site & the enzymes – Allosteric enzymes
 Allosteric enzymes follows a sigmoidal curve
1. Positive Allosteric effectors:
The binding of the substrate to one of the subunits of the enzyme may
enhance (increase) substrate binding by other subunits
2. Negative Allosteric effectors:
If the binding of the substrate to one of the subunits decreases the avidity of
substrate binding by other sites

28. Inhibitor is not a substrate analog
It is partially reversible, when excess
substrate is added
Km is usually increased
Vmax is reduced
When an inhibitor binds to the
allosteric site, the configuration of
catalytic site is modified such that
substrate cannot bind properly
ALLOSTERIC INHIBITION

29. ALLOSTERIC INHIBITION
 Phosphofructokinase is the key enzyme
in the control of glycolysis
 High levels of ATP allosterically inhibits the
enzyme in the liver, thus lowering its affinity
for fructose – 6- phosphate.
 A high concentration of ATP converts the
hyperbolic binding curve of fructose – 6 –
phosphate into a sigmoidal curve
 AMP reverses the inhibitory action of ATP &
so the activity of the enzyme increases,
when ATP/AMP ratio is lowered

30. FEEDBACK INHIBITION
 Specialised type of allosteric inhibition
 It means that the activity of the
enzyme is inhibited by the final
product of the biosynthetic pathway
Note:
In this pathway if D inhibits E1, it is called
feedback inhibition

31. Product Enzyme inhibited Pathway
Heme ALA synthase Heme synthesis
Cholesterol HMG CoA reductase Cholesterol synthesis
Glucose – 6 – Phosphate Hexokinase Glycolysis
Acyl coA Acetyl CoA carboxylase Fatty acid synthesis
FEEDBACK INHIBITION

32. IMPORTANCE OF ENZYME INHIBITION
 For understanding the regulation of enzyme activity within the living cells
 Useful in elucidating the cellular metabolic pathways by causing accumulation of
intermediates
 Identification of the catalytic / functional groups at the active site
 Provides information about substrate specificity of the enzymes
 Useful to study the mechanism of catalytic activity

33. COMPETITIVE
INHIBITION
NON
COMPETITIVE
IRREVERSIBLE
INHIBITION
ALLOSTERIC
INHIBITION
SUICIDE
INHIBITION
FEEDBACK
INHIBITION
Structural
similarity to S
No structural
similarity
Binds tightly to
E by covalent
bonds
I binds to
Allosteric site
of the E
I makes use of
the Enzymes
own reaction
mechanism to
inactivate them
End products
inhibits earlier
enzymatic
activity of the
metabolic
pathway
Compete with S
for active site
Do not
compete
Binds to the
site different
from active site
Causes
conformational
change in
active site of E
Km – Increased
Vmax – No
change
Km – No
change
Vmax -
decreased
Km – no
change
Vmax -
decreased
Km – increased
Vmax –
decreased
Drugs –
Sulfonamides,S
tatins,
Methotrexate
Heavy metal
poisons
Cyanide, OP
poisoning,
Iodoacetate
PFK – 1 – ATP
& citrate
Allopurinol,
Fluorouracil
Heme – ALA
synthesis
Cholesterol –
HMG – CoA
reductase
SUMMARY

34. REFERENCES
 Textbook of biochemistry for medical students – 3rd edition – D.M. Vasudevan
 Essentials of biochemistry – Sathyanarayanan
 Video – http://shomusbiology.weebly.com
 www.google.com