Oligomycin completely inhibits ATP synthase (Complex V) in isolated mitochondria. What will be the immediate effect on the proton motive force (PMF) and on the rates of ETC complexes I-IV?
- A PMF increases (hyperpolarization); electron transport rate decreases ✓
- B PMF collapses; electron transport rate increases
- C PMF is unchanged; only ADP phosphorylation stops
- D PMF increases; electron transport rate increases compensatorily
Explanation
ATP synthase is the exit route for protons accumulated in the intermembrane space by complexes I, III, and IV. Blocking ATP synthase prevents proton re-entry, causing progressive buildup of the proton gradient — the membrane becomes hyperpolarized (increased PMF). This high back-pressure prevents further proton pumping by the respiratory chain complexes, which slows and eventually halts electron transport (state 4 respiration = resting state with no ADP phosphorylation). This demonstrates that oxidative phosphorylation is tightly coupled: electron transport is driven by, and regulated by, ATP synthesis.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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