1. Enzyme 4 - the Inhibition Lecture NO: 1st MBBS
Dr Muhammad Ramzan

2. Enzyme inhibition (EI) – the definition
It is an interaction B/W a molecule and enzyme that blocks the catalytic action of the enzyme in a normal way
Enzyme inhibitor is a substance that slows down /stops the enzyme catalyzed reaction

3. Enzyme Inhibition - the background
Enzyme inhibition is the major mechanism for the physiological regulation of enzymes
Some body processes are controlled by enzyme inhibition like blood coagulation and clot dissolution/Fibrinolysis
This process has led to the development of drugs that act by inhibiting enzyme metabolic pathway

4. Types of enzyme inhibition - 2
There are 2 types of enzyme inhibition, depending upon the type/nature of inhibitor :
Reversible inhibition
Irreversible Inhibition

5. Enzyme inhibitors - types

6. Reversible inhibition Inhibitors forms non covalent bonds with E
Inhibitor, binds reversibly to an enzyme, establishing non covalent interactions with the Enzyme,
Once inhibitor binds with the enzyme, active site is blocked and there is no further catalysis
The inhibitor can be removed from E by:
↑ing the substrate or diluting the inhibitor

7. Irreversible inhibition
Inhibitor, binds irreversibly to an enzyme through Covalent bonds
These bonds are permanent and cannot be removed
There is loss of folding, 3D tertiary structure and function
Are also called as suicide substrates
Inhibitors can also bind at the allosteric site

8. Difference B/W types of inhibitions
Serial NO:
Parameter
Reversible
Irreversible 1
Type of bonds
Temporary – non covalent
Permanent- Covalent bond 2
Chemical change
No change in active site/Enzyme
Permanent change in active site / E
loss of folding 3D 3
Loss of function
Reversible by removal of inhibitor or addition of substrate

9. Types of reversible inhibition - 3
There are 4 types of reversible inhibition
Competitive
Non competitive
Uncompetitive and
Feed back inhibition

10. Competitive inhibition (Blockers) the imposter and substrate
Competitive inhibitor binds to the enzyme’s active site and competes with the normal substrate molecule.
These imposters often have structures that are similar to those of the normal substrate
The inhibitor thus decreases the enzyme activity
Competitive inhibition can be reversed by ↑ing the Substrate or decreasing the conc. Of the Inhibitor

11. Competitive inhibition cont.
In competitive inhibition, inhibitor is bound to:
Active site or the product or neither.
It cannot bind to both at the same time
Shape of the enzyme or active site is unchanged
Statins and ACE inhibitors are the commonest inhibitors

12. Competitive inhibition

13. Competitive inhibition – the example the Statins
Statins are the lipid lowering agents that compete with the HMG COA – A substrate for the HMG CoA Reductase( HMGCR)
HMGCR is a rate limiting Enzyme for the CH synthesis
HMGCR : 3 - hydroxy-3- methyl- Glutaryl CoA Reductase
Statins thus prevent the De novo synthesis of CH
HMG-Co A is a precursor for CH synthesis

14. Example of Competitive inhibition the Statins

15. Competitive inhibition – Example 2 ACE Inhibitors –Lipitor/Captopril
ACE inhibitors are the antihypertensive drugs that prevent the conversion of Angiotensin 1 to Angiotensin 11
These drugs compete with Angio1 for the Enzyme ACE in the pulmonary circulation
ACE – I is the Angiotensin converting Enzyme Inhibitors
ACE inhibitors are Captopril (Capoten) and Atrovastatin (Lipotar)

16. Completive inhibition - ACE inhibitors (Captopril)

17. Non competitive inhibition (Allosteric) inhibitors bind to Allosteric sites
Noncompetitive inhibitor binds reversibly with the enzyme at a site other than the active site :
Changes the 3D structure of the active site, So that substrate no longer fits correctly to :
give a catalytic reaction
It makes the enzyme less active

18. Non competitive inhibition (Allosteric)

19. Competitive VS. non competitive inhibition
Serial number
Parameter
Competitive inhibition
Non competitive inhibition 1
Structure of inhibitor
similar to substrate
Different from substrate 2
Site of binding
Active site
Allosteric site 3
Changes in active site /enzymes
No change
Changed 4
Dilution
Can be diluted
Cannot be diluted

20. Uncompetitive inhibition inhibitor binds to ESC
The inhibitor binds to the Enzyme substrate complex - ESC
This decreases the maximum velocity of the enzyme due to delayed removal of the activated complex
Substrate cannot bind to the active site during this period and activity of the enzyme is slowed down
Mixed inhibition is a term in which : inhibitor can bind to active site in competitive and to ESC in uncompetitive inhibition

21. End product or Negative Feed back inhibition inhibition by end product
It is a negative feed back to regulate the production of a given mole like AA. Via enzymatic metabolic pathway
End product of this pathway; then binds with the allosteric site of the 1st enzyme of the pathway to inhibit its activity
This process prevents the excessive production of end products like AA. Isoleucine from AA. Threonine
Threonine is an initial substrate and Isoleucine is the end product

22. End product or negative feed back inhibition

23. Negative feed back inhibition Threonine to Isoleusine

24. Irreversible inhibition – Poisoning of E Inhibitor forms covalent bond with E
1. Irreversible inhibitor modifies the E chemically and makes
covalent bonds with Aminoacyl groups at active site
These groups are essential for S binding ,catalysis at the active site and enzyme conformation
E loses its folding, 3D tertiary structure and function - Denaturation
These include Cyanides, heavy metals and antibiotics

25. Cyanide - An irreversible Inhibitor
Cyanide is a poison which prevents ATP production via aerobic respiration, leading to eventual death
It binds to an allosteric site on Cytochrome Oxidase
A carrier molecule that forms part of the ECT
It changes shape of the active site and enzyme can no longer pass electrons to the final acceptor (oxygen)
Consequently, the ETC cannot continue to function and ATP is not produced via aerobic respiration

26. Cyanide poisoning

27. Irreversible inhibitors - Heavy metal poisoning and antibiotics
Heavy metals like Mercury and Lead ions produce protein Denaturation and neurological damage by:
binding with SH group of the enzymes
Antibiotics are enzyme inhibitors that act on the life processes that are essential to certain strains of bacteria
These include the sulfa drugs and penicillin
Penicillin prevents synthesis of bacterial cell wall and growth

28. Diagnostic importance – enzymes localization in specific tissues
Certain enzymes are found exclusively in specific tissues .
Their presence in the blood indicates damage to the tissues
↑ in the serum level of the enzymes is propornal to the damage of the affected organ
Measurement of the level of Enzymes is the diagnostic tool for a no: of diseases especially of :
Heart; Liver, Pancrease, Prostate and bone

29. Enzymes – Diagnostic importance in MI
Elevated levels of the following Enzymes is important in Diagnosis of Myocardial Infarction (MI)
Creatine Kinase (CK) CK MB
Lactic Dehydrogenase (LDH2)
Cardiac Troponin I and Cardiac Troponin T tests

30. Enzymes in the diagnosis of liver diseases
Elevated levels of enzymes in Liver diseases are :
Alanine amino Transferase ALT
Lactic Dehydrogenase LDH and
AST and alkaline Phosphatase

31. Enzymes in the diagnosis of bones and prostate
Elevated levels of enzymes in bone diseases are:
Alkaline Phosphatase
Enzymes in Prostate cancer with
Acid Phosphatase