Oxidative Phosphorylation
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Direct answer
Chemiosmosis couples electron transport to ATP synthesis through a proton gradient, not a chemical intermediate: electrons descend complex I (NADH-coenzyme Q reductase), coenzyme Q, complex III, cytochrome c and complex IV (cytochrome oxidase, where oxygen is reduced to water), while complexes I, III and IV pump protons from the matrix into the intermembrane space. ATP synthase (complex V, an F0F1 motor) lets protons flow back, rotating its gamma subunit to make ATP — Boyer's binding-change mechanism. P/O ratios give roughly 2.5 ATP per NADH and 1.5 per FADH2 (which enters at coenzyme Q, bypassing complex I), netting about 30-32 ATP per glucose. Inhibitors block the chain at specific complexes; uncouplers let protons leak back without ATP, converting fuel to heat.
What you must remember
- Inhibitor map: rotenone and amobarbital at complex I; malonate at complex II (a succinate analogue); antimycin A at complex III; cyanide, carbon monoxide, azide and hydrogen sulphide at complex IV; oligomycin at ATP synthase; atractyloside at the ATP-ADP translocase.
- Uncouplers: 2,4-dinitrophenol (a protonophore — fatal weight-loss fad of the 1930s), high-dose aspirin, and physiological thermogenin (UCP1) of neonatal brown adipose tissue — electron transport races, ATP falls, heat rises, oxygen consumption climbs.
- Yield bookkeeping: glycolysis nets 2; mitochondrial NADH yields 2.5 and succinate FADH2 1.5 — about 30-32 ATP per glucose; cytosolic NADH feeds the malate-aspartate shuttle (2.5) or the glycerol phosphate shuttle (1.5).
- Cytochrome c release: when the permeability transition pore opens, cytochrome c exits and triggers apoptosome assembly — this machinery's structural link to apoptosis.
- Mitochondrial diseases: MELAS, MERRF, Leber hereditary optic neuropathy and Leigh disease arise from mutations in exactly this machinery, governed by heteroplasmy and maternal inheritance.
- Cyanide antidote package: nitrites (amyl or sodium) create methaemoglobin that scavenges cyanide; sodium thiosulphate with rhodanese converts it to excretable thiocyanate; hydroxocobalamin binds cyanide as cyanocobalamin.
- Oxygen as terminal acceptor: complex IV's cytochrome a3 passes electrons to oxygen — the reason the chain, and all fatty acid oxidation and most ATP production, halts within minutes of anoxia.
- Leakage makes radicals: one to two per cent of consumed oxygen leaves as superoxide, most when the chain is highly reduced (rotenone blockade) — free radicals' mitochondrial source.
A worked case: the smell of bitter almonds
A factory worker collapses minutes after a plating-tank exposure; colleagues note a bitter almond odour; he is deeply acidotic with a high lactate, and strikingly, his measured venous oxygen saturation is high. The chain runs backwards from the end: cyanide binds cytochrome a3 of complex IV, so electrons cannot reach oxygen; every upstream carrier stays reduced; oxygen sits unused in venous blood (the peripheral shunt explaining the high venous saturation); and the cell, unable to phosphorylate, floods with lactate. Treatment follows the chemistry — nitrites create methaemoglobin that outbids cytochrome a3 for cyanide; sodium thiosulphate feeds rhodanese's conversion to thiocyanate; hydroxocobalamin binds cyanide directly; oxygen and acidosis correction support. The companion poison, carbon monoxide, works chiefly on haemoglobin instead — one denies oxygen carriage, the other denies oxygen use.
Where students slip
The yield question is answered with the old integers 3 and 2 ATP — modern bioenergetics gives 2.5 and 1.5, and stating "approximately" with the new figures is the safe answer. Second, coenzyme Q is called a cytochrome; it is a lipid-soluble quinone without haem, the mobile carrier both complexes I and II feed. Third, uncouplers and inhibitors are treated as one category: an inhibitor stops the chain (oxygen consumption falls) whereas an uncoupler accelerates it while ATP synthesis collapses (oxygen consumption rises) — the DNP question turns on that sign. Fourth, only complexes I, III and IV pump protons; complex II (succinate dehydrogenase) contributes no proton gradient, which is exactly why FADH2 is worth less. Fifth, ATP synthase is reversible: under anoxia it runs as an ATPase, hydrolysing ATP to maintain the gradient — a favourite of deeper vivas.
Frequently asked questions
How many ATP are yielded per NADH and per FADH2, and why the difference?
About 2.5 per NADH and 1.5 per FADH2, because FADH2 enters at coenzyme Q and bypasses complex I's proton pumping.
Where does cyanide act and what is its antidote rationale?
At cytochrome a3 of complex IV; nitrites form methaemoglobin that binds cyanide, and thiosulphate with rhodanese converts it to thiocyanate for excretion.
What distinguishes an uncoupler from an electron transport inhibitor?
Uncouplers dissipate the proton gradient so oxygen consumption rises while ATP synthesis falls; inhibitors halt electron flow and oxygen consumption falls with it.
Which complexes pump protons, and which does not?
Complexes I, III and IV pump protons into the intermembrane space; complex II does not, which is why succinate-derived FADH2 yields less ATP.
Why is oxidative phosphorylation central to mitochondrial disease presentations?
The 13 mtDNA-encoded polypeptides of this chain fail in MELAS, MERRF and Leber disease, explaining their energy-hungry tissue patterns — brain, muscle, retina and heart.