1. Enzymes can be inhibited through various mechanisms including competitive, non-competitive, uncompetitive, and allosteric inhibition. Competitive inhibitors compete with the substrate for the active site, increasing Km and leaving Vmax unchanged. Non-competitive inhibitors bind elsewhere, decreasing Vmax but not affecting Km. 2. Enzyme inhibition has clinical significance in drug action and toxicology. Competitive inhibitors like sulfonamides and methotrexate are used as drugs, while non-competitive inhibitors like cyanide and heavy metals can be toxic. 3. Allosteric inhibition involves effectors binding at sites other than the active site to induce a conformational change, decreasing Vmax

1. Enzyme Inhibition
Prof. (Dr.) V.P. Acharya

2. All things are difficult before they
are easy.
Thomas Fuller

4. Many different kinds of molecules inhibit
enzymes and act in a variety of ways
Enzyme inhibition
Competitive
Non-competitive
Reversible Irreversible
Uncompetitive
Suicide
Allosteric
Feedback

5. Competitive inhibition
• Inhibitor competes with the substrate for the active site
• Inhibitor is substrate analogue
• Usually reversible
• ↑ [S] abolishes inhibition
• ↓ velocity of reaction
• ↑ Km
• Vmax unchanged

6. COMPETITIVE
INHIBITION

7. Clinical significance
• Sulfonamide– structural analogue of PABA
(PABA with pteroyl glutamic acid synthesises
Folic acid)
• Methotrexate -- structural analogue of folic
acid
↓
Inhibits folate reductase
↓
↓DNA synthesis & cell division
↓
Death of cancer cells

8. • INH– Similar structure to pyridoxal
↓
Pyridoxal deficiency
↓
Peripheral neuropathy
• Methanol↔ Ethanol
• Allopurinol↔ Xanthine oxidase
• Dicoumarol ↔ Vit. K epoxide reductase
↓
Act as anticoagulant

10. Non-competitive inhibition
• No competition between substrate and
inhibitor
• Different binding sites
• No structural similarities
• ↑ [S] doesn’t resolve the inhibition
• Usually irreversible
• May be reversible when inhibitor is
removed
• Km value unchanged
• Vmax reduces

12. Noncompetitive inhibition

13. Clinical significance
• Cyanide inhibits cytochrome oxidase
• F inhibits enolase- removes Mn & Mg
• Heavy metals react with –SH gr. Of BAL-
hence BAL is used in heavy metal poisoning
Toxicological importance
• Most of the poisons- Irreversible NC
inhibitors- iodoacetate, heavy metal poisons

14. Reversible non-competitive inhibition
Inhibition is reversible if inhibitor is removed
Ex:
Ascaris and soybean are trypsin inhibitors
Ab prepared against any enzyme
precipitates it

15. Competitive Vs Non-competitive
inhibition
Competitive
inhibition
Non-competitive
inhibition
Act on Active site May/may not be
Str of inhibitor Substrate analogue Not an analogue
Reversibility Reversible Mostly irreversible
↑ Substrate Inhibition relieved No effect
Km ↑ No change
Vmax Unchanged ↓
Significance Drug action Toxicological

16. Uncompetitive inhibition
• Inhibitor binds to the ES complex, not to the free
enzyme
• ↓ Vmax, ↓ Km
Ex: Inhibition of
placental ALP by
Phenylalanine

17. Suicide inhibition
• A substrate analogue binds to the enzyme
↓
First few steps of the pathway are catalyzed
↓
New product irreversibly binds to the enzyme
↓
Enzyme inhibited
• Irreversible reaction

18. Ex:
Allopurinol Xanthine oxidase Alloxanthine
• Difluoromethyl ornithine (DFMO) inhibits ornithine
decarboxylase in Trypanosoma
• Purine and Pyrimidine analogues in cancer
chemotherapy
• Aspirin inhibits COX enzyme
(-)

19. not failed. I've just found 10,000 ways that won't w
Thomas A. Edison

20. Allosteric inhibition
• Allos= Different; Steric = site
• Different sites for substrate and modifier binding
• Effectors or modifiers bind covalently
• Most of the hormone action
• Mostly multimeric enzymes; hence co-operative
binding
• Binding of modifier– changes configuration of
active site– release the substrate
• Catalytic and allosteric sites may not lie closely
• Inhibitor is not a substrate analogue

21. • Partially reversible; excess substrate
may reverse the reaction
• ↑ Km
• ↓Vmax
• Effect of modifier is maximum when
[s]≈ Km

22. Allosteric inhibition
Allosteric activation

24. Homotropic effector: If substrate acts as allosteric
effector- presence of the substrate molecule will
enhance activity of other substrate-binding sites-
sigmoid curve
Heterotropic effector: If substrate and effectors are
different- commonly encountered
Key Enzyme:
Usually allosteric enzymes are regulatory enzymes/
key enzymes / Rate limiting enzyme
Succinyl CoA+ Glycine → Delta ALA
Heme (Allosterically inhibits ALA
synthase)
(-)
ALA Synthase

25. • Fine control : Enzyme activity is regulated
by altering existing enzyme activity
• Coarse control: Enzyme activity is
regulated by altering concentration of
enzymes

26. Feedback Inhibition
• End-product inhibits the enzyme
• Usually allosteric inhibition

27. Covalent modification
By addition or removal of a group to the enzyme
protein by covalent bond
Ex: Zymogen activation
Phosphorylation & dephosphorylation
Methylation
Uridylation
Adenylation & deadenylation

28. Induction
Inducer
↓
Relieve repression at the operator site
↓
Biosynthesis of enzyme starts
Constitutive enzyme– Independent of
inducer
Ex: Lac operon
Glucokinase induced by glucose

29. Repression
• Acts at molecular level, on genes
• Effect is noticed after a lag period of hours
and days
• ↓ no. of enzyme molecules
Ex: ALA synthase is regulated by heme by
repression

30. Genetic Engineering & Modified Enzyme
• Hybrid enzymes/ Chimeric enzymes/Fusion
proteins: Fusion of 2 or more enzymes having
desired properties
• Abenzymes / Catmab: Enzymes+ monoclonal
antibodies
Treatment for autoimmune diseases, HIV

31. Non-protein enzymes
• Ribonuclease P (1983)
 Found to have true enzyme activity
 Obeyed M-M equation
Name the other ribozymes??

32. Walking with a friend in the dark is better than
walking alone in the light.
Helen Keller