This question is about the plant mitochondrial electron transport chain and its two separate routes for sending electrons to oxygen.
- Complex I: this is NADH dehydrogenase, and its known specific inhibitor is rotenone, not SHAM, so it does not fit.
- Complex II: this is succinate dehydrogenase, blocked specifically by malonate, again unrelated to SHAM.
- Complex III: this is the cytochrome bc1 complex, blocked by antimycin A, and SHAM leaves this complex working normally.
- Complex IV: this is cytochrome c oxidase, the normal terminal step that reduces oxygen to water. Plants have a second, parallel terminal step for this same job, run by the enzyme alternative oxidase (AOX), which SHAM specifically switches off.
Because AOX carries out the same overall function as Complex IV, handing electrons to oxygen, and SHAM is the standard inhibitor used to shut down this AOX route while leaving Complex I through Complex IV of the main chain untouched, this SHAM-sensitive step is treated as corresponding to Complex IV.
Let's summarize:
- Rotenone stops Complex I, malonate stops Complex II, antimycin A stops Complex III, cyanide stops Complex IV.
- SHAM stops the separate alternative oxidase enzyme, which does the job of a terminal oxidase in place of Complex IV.
So among the four listed complexes, the intended answer is Complex IV, keeping in mind that SHAM technically targets the alternative oxidase enzyme rather than Complex IV itself.