Question

Match List-I with List-II

List-I (Inhibitor/Redox carrier in ETC) & List-II (Action)
(A) Amytal & (I) Inhibits FAD-linked oxidation
(B) Antimycin & (II) Inhibits $\text{NAD}^+$-linked oxidation
(C) Tetramethyl-p-phenylenediamine & (III) Completely inhibits oxidation of all
(TMPD) & electron donors
(D) Sodium azide & (IV) Transfers electrons directly to
& Cytochrome C

• (A) - (I), (B) - (II), (C) - (III), (D) - (IV)
• (A) - (II), (B) - (I), (C) - (III), (D) - (IV)
• (A) - (II), (B) - (I), (C) - (IV), (D) - (III)
• (A) - (I), (B) - (II), (C) - (IV), (D) - (III)

Hint:

Inhibitors of Complex IV (like cyanide, azide, and carbon monoxide) are highly toxic because they shut down the entire respiratory chain, preventing cells from synthesizing ATP via oxidative phosphorylation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The mitochondrial Electron Transport Chain (ETC) is composed of four multi-protein complexes that transfer electrons from electron donors ($\text{NADH}$ and $\text{FADH}_2$) to oxygen, driving ATP synthesis.
Specific chemical inhibitors and artificial electron donors target distinct steps of this electron flow, altering respiration rates.

Step 2: Detailed Explanation:
Let us analyze the specific action of each molecule in List-I:
1. Amytal (a barbiturate): This is a classic inhibitor of Complex I (NADH-coenzyme Q oxidoreductase).
By blocking the transfer of electrons from Fe-S clusters to ubiquinone, it directly prevents the oxidation of NADH-linked substrates like malate or glutamate.
Thus, it inhibits $\text{NAD}^+$-linked oxidation. (A) matches with (II).
2. Antimycin: This is an antibiotic that binds to the $\text{Q}_i$ site of Cytochrome $b$ in Complex III, blocking the Q-cycle.
Because it blocks the downstream pathway, it prevents electron flow from both Complex I and Complex II (succinate dehydrogenase, which is FAD-linked).
As a result, it inhibits FAD-linked oxidation. (B) matches with (I).
3. Tetramethyl-p-phenylenediamine (TMPD): This is an artificial electron donor.
It is chemically oxidized by transferring its electrons directly to cytochrome $c$, bypassing Complexes I, II, and III.
Thus, it transfers electrons directly to Cytochrome C. (C) matches with (IV).
4. Sodium azide: This is a potent inhibitor of Complex IV (Cytochrome $c$ oxidase).
It binds tightly to the ferric ($\text{Fe}^{3+}$) iron of the heme $a_3$ site, blocking the transfer of electrons to oxygen, the terminal electron acceptor.
Because this blocks the entire ETC, it completely inhibits the oxidation of all electron donors. (D) matches with (III).
Combining these matches, we get: (A) - (II), (B) - (I), (C) - (IV), (D) - (III).

Step 3: Final Answer:
The correct matching sequence is (A) - (II), (B) - (I), (C) - (IV), (D) - (III), which corresponds to option (C).