Question

The following is a list of various inhibitors of respiratory chain (Column I) and their possible mode of action (Column II).
{ll} Column I (Inhibitor) & Column II (Mode of action)
A. Rotenone & I. Completely blocks oxidation and phosphorylation in intact mitochondria
B. Dimercaprol & II. Inhibits cytochrome oxidase
C. Carbon monoxide & III. Prevents the oxidation of substrates that communicate directly via NAD-linked dehydrogenases
D. Oligomycin & IV. Inhibits the respiratory chain between cytochrome b and cytochrome c

• A. A-I, B-II, C-III, D-IV
• B. A-II, B-III, C-IV, D-I
• C. A-III, B-IV, C-II, D-I
• D. A-I, B-III, C-II, D-IV

Hint:

Remember the target complexes of common ETC inhibitors:
Complex I: Rotenone, Amytal
Complex III: Antimycin A, Dimercaprol
Complex IV: Cyanide, CO, Azide
ATP Synthase: Oligomycin

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The mitochondrial electron transport chain (ETC) consists of four multi-protein complexes that transfer electrons to generate a proton gradient.
Specific chemical inhibitors target different complexes or coupling steps, disrupting oxidative phosphorylation.

Step 2: Detailed Explanation:
Let us analyze each inhibitor and match it to its mode of action:
Rotenone: This compound is a classic inhibitor of Complex I (NADH-coenzyme Q oxidoreductase).
By blocking the transfer of electrons from iron-sulfur clusters to ubiquinone, it prevents the oxidation of NADH-linked substrates. This corresponds to III.
Dimercaprol: Also known as British Anti-Lewisite, this agent acts as an inhibitor of Complex III (Coenzyme Q-cytochrome c oxidoreductase), blocking electron flow between cytochrome b and cytochrome c. This corresponds to IV.
Carbon monoxide (CO): CO binds competitively to the reduced iron of heme $a_{3}$ in Complex IV (cytochrome c oxidase), blocking the final transfer of electrons to molecular oxygen. This corresponds to II.
Oligomycin: This antibiotic binds directly to the $F_{o}$ subunit of ATP synthase, blocking proton translocation.
This halts both ATP synthesis and electron transport in intact mitochondria, coupling oxidation and phosphorylation. This corresponds to I.
Matching these pairs results in: A-III, B-IV, C-II, D-I.

Step 3: Final Answer:
The correct matches are A-III, B-IV, C-II, and D-I, which corresponds to Option (C).