Blocking any step of the ETC shuts down aerobic ATP production because electrons cannot flow and the proton gradient cannot be maintained. Four classic inhibitors - one per complex plus one for ATP synthase.
Inhibitor
Target
Effect
Rotenone
Complex I
Blocks NADH’s entry point. FADH2 can still deliver to Complex II.
Antimycin A
Complex III
Blocks electron transfer from CoQ to cytochrome c
Cyanide (CN-), Carbon monoxide (CO)
Complex IV
Blocks electron transfer to O2
Oligomycin
ATP synthase (F0)
Blocks proton flow through ATP synthase, stopping ATP production directly
What Happens Downstream
Block any complex and everything upstream of it backs up. Block Complex IV and electrons cannot reach O2; all previous complexes become reduced; NADH cannot be oxidized; TCA cycle stops. Block ATP synthase and protons accumulate outside, increasing PMF, eventually stopping the ETC because no more protons can be pumped against the gradient.
Cyanide Poisoning
Cyanide binds Fe3+ in cytochrome c oxidase (Complex IV), preventing electron transfer to O2. Cellular respiration halts within seconds. All tissues suffer, but the brain and heart (high aerobic demand) are fastest to fail. Paradoxically, blood O2 stays high because cells cannot use it - “histotoxic hypoxia.” Antidote: sodium nitrite + sodium thiosulfate converts cyanide to non-toxic thiocyanate, or hydroxocobalamin (vitamin B12 precursor) binds cyanide.
How does cyanide cause cell death?
Click to reveal answer
Cyanide binds Complex IV (cytochrome c oxidase), preventing electron transfer to O2. The electron transport chain halts. Protons cannot be pumped, ATP synthase stops, NADH accumulates, the TCA cycle halts. Cells die within minutes from ATP depletion. Oxygen is available but unusable - histotoxic hypoxia.
An investigator adds rotenone to isolated mitochondria. O2 consumption drops. When succinate is added, O2 consumption recovers. Explain.
Click to reveal answer
Rotenone blocks Complex I, so NADH cannot deliver electrons. Succinate is oxidized by Complex II (succinate dehydrogenase) to produce FADH2, whose electrons enter the ETC at CoQ (downstream of Complex I). Succinate thus bypasses the rotenone block, and electron flow resumes through Complex III, IV, and O2 consumption continues.
Why does blocking ATP synthase eventually stop the ETC too?
Click to reveal answer
Without ATP synthase to consume protons, protons accumulate in the intermembrane space and the proton motive force grows. Eventually the ETC cannot pump additional protons against the steep gradient - it is thermodynamically blocked. Electrons back up, reduced carriers pile up, and electron flow stops. This is why coupled ETC and ATP synthase are both essential for aerobic respiration.
