Q.57. Match the following in CORRECT combination between Group I and Group II with reference to
the agents that interfere with oxidative phosphorylation
Group I
P. Cyanide
Q. Antimycin A
R. Aurovertin
S. 2,4-Dinitrophenol
(A) P-iv, Q-i, R-ii, S-iii
(B) P-v, Q-i, R-iii, S-iv
(C) P-iv, Q-iii, R-ii, S-v
(D) P-v, Q-ii, R-iii, S-iv
Group II
i. Blocks electron transfer from cyt 𝑏 to cyt 𝑐1
ii. Inhibits F1
iii. Uncoupling of phosphorylation from electron transfer
iv. Inhibits cytochrome oxidase
v. Inhibits K+ionophore

The correct answer is option (A): P-iv, Q-i, R-ii, S-iii. Cyanide inhibits cytochrome oxidase, Antimycin A blocks electron transfer from cytochrome b to cytochrome c1, Aurovertin inhibits F1-ATP synthase, and 2,4-Dinitrophenol uncouples oxidation from phosphorylation.

Correct matching and final answer

The correct matching between Group I and Group II is:

• P. Cyanide → iv. Inhibits cytochrome oxidase

• Q. Antimycin A → i. Blocks electron transfer from cyt b to cyt c1

• R. Aurovertin → ii. Inhibits F1

• S. 2,4-Dinitrophenol → iii. Uncoupling of phosphorylation from electron transfer

Therefore, the correct option is (A) P-iv, Q-i, R-ii, S-iii.

Explanation of each agent (Group I)

Cyanide (P → iv. Inhibits cytochrome oxidase)

• Cyanide binds to the ferric iron of cytochrome oxidase (Complex IV), the terminal enzyme of the electron transport chain. This prevents transfer of electrons to oxygen and blocks oxidative phosphorylation.

• As a result, electron flow stops, proton pumping ceases, ATP synthesis falls dramatically, and cells undergo histotoxic hypoxia despite adequate oxygen supply.

Antimycin A (Q → i. Blocks electron transfer from cyt b to cyt c1)

• Antimycin A is an antibiotic that specifically inhibits Complex III (cytochrome bc1 complex) of the respiratory chain. It blocks electron transfer from cytochrome b to cytochrome c1.

• This inhibition stops the Q cycle in Complex III, halts further electron flow to cytochrome c and Complex IV, decreases proton gradient formation, and thereby prevents ATP synthesis.

Aurovertin (R → ii. Inhibits F1)

• Aurovertin is a potent inhibitor of the F1 sector (catalytic head) of ATP synthase. It binds to the β-subunits of the F1-ATPase and inhibits ATP synthesis activity.

• Because F1 is responsible for the actual formation of ATP from ADP and inorganic phosphate, inhibition of F1 directly suppresses ATP production even if a proton gradient still exists.

2,4-Dinitrophenol (S → iii. Uncoupling of phosphorylation from electron transfer)

• 2,4-Dinitrophenol (DNP) is a classical uncoupler of oxidative phosphorylation. It is lipid-soluble and carries protons across the inner mitochondrial membrane, dissipating the proton gradient.

• Electron transport and oxygen consumption continue, but ATP synthesis falls sharply because the proton motive force is collapsed; the energy is released as heat instead of being conserved as ATP.

Why the other Group II option (v) is incorrect here

• Statement v: “Inhibits K⁺ ionophore” does not correctly describe any of the given agents (cyanide, antimycin A, aurovertin, 2,4-DNP).

• K⁺ ionophores (like valinomycin) typically facilitate K⁺ transport; classical respiratory inhibitors and uncouplers listed in this question act at cytochromes, ATP synthase, or proton gradient level, not by inhibiting a K⁺ ionophore.

Option-by-option analysis

Option (A): P-iv, Q-i, R-ii, S-iii ✅ (Correct)

• P-iv: Cyanide → inhibits cytochrome oxidase (Complex IV) – correct.

• Q-i: Antimycin A → blocks electron transfer from cyt b to cyt c1 – correct.

• R-ii: Aurovertin → inhibits F1-ATPase – correct.

• S-iii: 2,4-DNP → uncouples phosphorylation from electron transfer – correct.

Option (B): P-v, Q-i, R-iii, S-iv ❌

• P-v: Cyanide does not inhibit a K⁺ ionophore; it inhibits cytochrome oxidase.

• R-iii: Aurovertin is not an uncoupler; it directly inhibits F1.

• S-iv: 2,4-DNP does not inhibit cytochrome oxidase; it uncouples oxidative phosphorylation.

Option (C): P-iv, Q-iii, R-ii, S-v ❌

• Q-iii: Antimycin A is not an uncoupler; it is a Complex III inhibitor.

• S-v: 2,4-DNP does not inhibit a K⁺ ionophore.

Option (D): P-v, Q-ii, R-iii, S-iv ❌

• P-v: Cyanide again wrongly matched to K⁺ ionophore inhibition.

• Q-ii: Antimycin A does not inhibit F1; it acts at Complex III.

• R-iii: Aurovertin is not an uncoupler.

• S-iv: 2,4-DNP does not inhibit cytochrome oxidase.

Only option (A) correctly matches each agent with its specific site or mode of action in oxidative phosphorylation.