Isolated mitochondria are incubated with succinate as respiratory substrate. Addition of antimycin A halts oxygen consumption almost completely, even though succinate donates electrons through complex II rather than complex I. Antimycin A acts at which site?

• A Complex III (cytochrome bc1 complex) ✓
• B Complex I (NADH dehydrogenase)
• C Complex IV (cytochrome c oxidase)
• D ATP synthase Fo channel

Explanation

Antimycin B blocks electron flow within complex III by binding the Qi site between cytochrome b and ubiquinone. Because electrons from succinate must pass through complex II, ubiquinone, and then complex III before reaching cytochrome c and oxygen, blocking complex III stops respiration regardless of whether the entry point is complex I or II. Rotenone acts at complex I and would not fully block succinate-driven respiration, which is why succinate is used experimentally to localize such inhibitors.

Reference: Lehninger Principles of Biochemistry, 8th ed.

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