Question

The complex which is specifically inhibited by SHAM in the electron transport chain

• A. Complex I
• B. Complex II
• C. Complex III
• D. Complex IV

Hint:

SHAM blocks the cyanide-resistant alternative oxidase, which takes over the job normally done by the terminal complex.

Answer 1

The Correct Option is D

Concept:
Plant mitochondria have two separate routes for passing electrons from ubiquinol to oxygen. The main route runs through Complex III and then Complex IV, the cytochrome c oxidase, which pumps protons and produces most of the ATP.
Plants also carry a second, alternative route called the cyanide-resistant pathway, run by an enzyme called alternative oxidase (AOX).

Step 1: Locate where AOX sits.
AOX accepts electrons directly from the ubiquinone pool, the same point where Complex III normally picks them up, and passes them straight to oxygen, releasing the extra energy as heat instead of pumping protons.
Because it takes this shortcut, using AOX bypasses both Complex III and the terminal oxidase step normally carried out by Complex IV.

Step 2: Identify what SHAM blocks.
Salicylhydroxamic acid (SHAM) is the classic laboratory inhibitor used to specifically switch off AOX, without touching Complex I, Complex II, or the cyanide-sensitive Complex III and Complex IV route.
Since AOX performs the same overall job as the terminal oxidase step, handing electrons to oxygen to form water, this SHAM-sensitive step is grouped with the terminal complex, Complex IV, in exam-level classification.

Step 3: Eliminate the other complexes.
Complex I, NADH dehydrogenase, is blocked by rotenone, not SHAM.
Complex II, succinate dehydrogenase, is blocked by malonate, not SHAM.
The cyanide-sensitive cytochrome pathway through Complex III and Complex IV is blocked by antimycin A and cyanide respectively, and it is precisely because SHAM leaves this pathway alone, while shutting down the alternative terminal route, that AOX is described as acting at the position of Complex IV.

Final Answer:
Among the given options, the SHAM-sensitive step is matched with the terminal oxidase step, Complex IV, since that is the complex whose oxygen-reducing role AOX substitutes for. Strictly speaking, SHAM inhibits the separate alternative oxidase enzyme rather than Complex IV itself, but of the four listed complexes, Complex IV is the intended match.
\[ \boxed{\text{Complex IV}} \]