Inhibitors of Electron Transport Chain and Oxidative Phosphorylation and MCQs with answer

Inhibitors of Electron Transport Chain and Oxidative Phosphorylation and MCQs with answer

In a cell, electron flow in the ETC is done in a proper sequence from either NADH or FAD to oxygen to form ATP. The 3 complexes of ETC are associated with generation of ATP. All these complexes do not function independently. So any blockage at any point in the chain prevents the formation of ATP. If the movement is blocked anywhere the carrier on the oxidized site of the block has no source of electron and carrier on the reduced site has no electron to donate.

Inhibitors of respiratory chain have been categorized into 3 types:

  • Inhibitors of electron transfer chain
  • Inhibitors of F0F1ATPase
  • Un-couplers of oxidative phosphorylation

Inhibitors of electron transfer chain

Inhibitors of ETC are the one which interrupts the flow of electron through the respiratory chain and thus block the respiratory chain at 3 sites: complex-1, complex-3 and complex-4. This results in the blockage of proton pumping, ATP synthesis and oxygen uptake.

Inhibitors of complex-1 include:

  • Barbiturates like amobarbital
  • Piericidin-A (antibiotic)
  • Rotenone and myxathiazole (insecticide)

These inhibitors blocks the transfer of reducing equivalents from Fe-S to ubiquinone-Q

Inhibitors of complex-3

  • Dimercaprol
  • Anti-mysin A

These inhibitors prevent the transfer of electron from cytochrome b to cytochrome C1.

Inhibitors of complex-4

  • Cyanide
  • Carbon mono-oxide
  • H2S

These inhibitors prevent the transfer of electrons from cytochrome aa3 to molecular oxygen by inhibiting cytochrome oxidase

Inhibitors of oxidative phosphorylation (F0F1ATPase)

These inhibitors inhibit the phosphorylation by inhibiting F0F1ATPase enzyme. In a coupled system, inhibition of phosphorylation can lead to inhibition of oxidation. For example:

Oligomycin, dicyclohexylcarbodiimide (antibiotics) completely blocks the oxidation and phosphorylation by inhibiting F0F1ATPase.

Un-couplers of oxidative phosphorylation:

These are the chemical substances which allow the ETC to take place but prevent the phosphorylation of ADP to ATP by uncoupling the linkage between ETC and phosphorylation and thus prevent synthesis of ATP.

Un-couplers are lipid soluble compounds which pass through the mitochondrial membrane and binds to H+.

Examples of un-couplers of oxidative phosphorylation include:

  • 2,4-dinitrophenol
  • Dicoumarol (anticoagulant)
  • Salicylate (aspirin metabolite)
  • Bilirubin, free fatty acids and thyroxin also work as un-couplers.

Multiple Choice Questions (MCQs)

1. Which of the following is the main issue caused by the inhibitors?
A. Blocks F1F0ATPase
B. Prevents formation of ATP
C. Prevents ETC to take place
D. All of the above

2. Inhibitors of respiratory chain are classified in how many types?
A. 3
B. 4
C. 5
D. 6

3. Inhibition of which enzyme inhibits the oxidative phosphorylation?
A. Dehydrogenase
B. Oxygenase
C. F1F0ATPase
D. Both A and C

4. Fill in the blanks- ETC is done in a sequence from either —– or —- to oxygen.
A. NAD or FAD
B. GTP or NAD
C. NADH or FAD
D. None of the above

5. What do the inhibitors of ETC does?
A. Blockage of proton pumping
B. Prevent ATP synthesis
C. Blocks oxygen uptake
D. All of the above

6. Do the complexes of ETC function independently?
A. Yes
B. No
C. Depends on the situation
D. None of the above

7. Inhibitors of ETC blocks the respiratory chain at which site?
A. Complex-1
B. Complex-3
C. Complex-4
D. All of the above

8. Match the following examples-
A. Complex-1 1. Dimercaprol
B. Un-couplers 2. CO
C. Complex-4 3. Thyroxin
D. Complex-3 4. Rotenone

9. Amobarbital is an inhibitor of which complex?
A. Complex-1
B. Un-couplers
C. Complex-3
D. Complex-2

10. What do inhibitors of complex 3 do?
A. Blocks the transfer of reducing equivalents from Fe-S to ubiquinone-Q
B. Prevent transfer of electrons from cytochrome b to cytochrome C1
C. Prevents transfer of electrons from cytochrome aa3 to molecular oxygen
D. ETC takes place but inhibits phosphorylation

11. What do un-couplers do?
A. Blocks the transfer of reducing equivalents from Fe-S to ubiquinone-Q
B. Prevent transfer of electrons from cytochrome b to cytochrome C1
C. Prevents transfer of electrons from cytochrome aa3 to molecular oxygen
D. ETC takes place but inhibits phosphorylation

12. Which of the following statement is NOT true?
A. Inhibition of phosphorylation can lead to inhibition of oxidation
B. Piericidin is an insecticide
C. Complexes are linked to each other
D. Un-couplers are lipid-soluble compounds

13. Which type of inhibitors prevents the transfer of electrons from cytochrome aa3 to molecular oxygen?
A. Un-couplers
B. Complex-4
C. Complex-1
D. None of the above

14. Which of the following is an example of un-couplers?
A. Cyanide
B. CO
C. H2S
D. None of the above

15. Antibiotics inhibit which of the following enzyme?
A. Dehydrogenase
B. Oxygenase
C. F1F0ATPase
D. Both A and C

ANSWERS-

1. Prevents formation of ATP
2. 3
3. F1F0ATPase
4. NADH or FAD
5. All of the above
6. No
7. All of the above
8. A – 4 B – 3 C – 2 D – 1
9. Complex-1
10. Prevent transfer of electrons from cytochrome b to cytochrome C1
11. Complex-4
12. Piericidin is an insecticide
13. ETC takes place but inhibits phosphorylation
14. None of the above
15. F0F1ATPase

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REFERENCE:– 1. Pankaja Naik- Biochemistry; 4th edition; page no:- 150-153

 

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