Oxidative phosphorylation

High-energy compounds

• ATP hydrolysis releases about 30.5 kJ/mol. Phosphoenolpyruvate has the highest transfer potential, then 1,3-bisphosphoglycerate and creatine phosphate; acetyl-CoA and succinyl-CoA are high-energy thioesters.
• Substrate-level phosphorylation makes ATP without oxygen: phosphoglycerate kinase and pyruvate kinase in glycolysis, succinyl-CoA synthetase in the citric acid cycle.

Electron transport chain

Electrons from NADH and FADH2 pass along complexes in the inner mitochondrial membrane to oxygen. The energy pumps protons into the intermembrane space, and ATP synthase lets them flow back to make ATP (the chemiosmotic mechanism).

• P:O ratio: about 2.5 ATP per NADH and 1.5 per FADH2. One glucose yields about 30–32 ATP aerobically.
• Cytosolic NADH enters by the malate–aspartate shuttle (heart, liver; 2.5 ATP) or glycerol-3-phosphate shuttle (skeletal muscle, brain; 1.5 ATP).
• ATP leaves the matrix through the ATP–ADP translocase (blocked by atractyloside).
• Electron transport and ATP synthesis are normally coupled: if ATP synthase is blocked (oligomycin), protons cannot return, the gradient becomes maximal and electron flow and oxygen use fall.

Uncouplers and heat

• Uncouplers carry protons back into the matrix without passing through ATP synthase. Electron transport runs unchecked, oxygen consumption rises, ATP synthesis falls and the energy is released as heat.

Poisoning of respiration

• Cyanide binds Fe3+ in cytochrome a3, so cells cannot use oxygen (histotoxic hypoxia): lactic acidosis, high venous oxygen saturation, bitter almond smell, collapse. Sources: smoke inhalation, nitroprusside, cassava. Treat with hydroxocobalamin (forms cyanocobalamin), or nitrites (methaemoglobin binds cyanide) plus sodium thiosulphate (rhodanese makes thiocyanate).
• Carbon monoxide binds haemoglobin about 240 times more avidly than oxygen and shifts the dissociation curve left; it also inhibits complex IV. PaO2 and pulse oximetry appear falsely normal. Treat with 100% oxygen (CO half-life falls from about 5 hours to about 1 hour) and hyperbaric oxygen in severe cases.

Mitochondrial disease

• Mitochondrial DNA is inherited only from the mother; heteroplasmy (mixed normal and mutant mitochondria) gives variable severity. Tissues with high energy demand (brain, muscle, retina) suffer most.
• MELAS (encephalopathy, lactic acidosis, stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibres, deafness, dementia), Leber hereditary optic neuropathy (young men, painless central vision loss; complex I subunit mutations).

Muscle energy and dystrophin

• Muscle draws first on stored ATP and creatine phosphate (via creatine kinase), then glycolysis, then oxidative metabolism. Both isotonic and isometric contractions use high-energy phosphate bonds.
• In Duchenne muscular dystrophy (X-linked frameshift deletions) dystrophin is absent, so the sarcolemma tears during contraction, fibres die and creatine kinase leaks out (very high CK). Boys show Gowers sign (climbing up the legs to stand) and calf pseudohypertrophy.

Free radicals

• Free radicals have an unpaired electron: superoxide, hydroxyl radical (the most reactive and damaging), nitric oxide and peroxynitrite. Hydrogen peroxide is not a radical but generates hydroxyl radicals through the Fenton reaction with Fe2+.
• Sources: electron leak from the respiratory chain; the neutrophil respiratory burst (NADPH oxidase); ionising radiation, which splits water into hydroxyl radicals that break DNA; drugs (NAPQI from paracetamol, doxorubicin); and reperfusion injury, where returning oxygen meets xanthine oxidase and recruited neutrophils.
• Damage: lipid peroxidation of membranes, protein cross-linking and oxidation, DNA strand breaks and mutations (radiation carcinogenesis appears years later). Dry heat sterilisation kills bacteria by oxidation; moist heat by protein coagulation.

Antioxidant defences

• Chronic granulomatous disease: NADPH oxidase defect, no respiratory burst; infections with catalase-positive organisms (Staphylococcus aureus, Aspergillus, Serratia, Burkholderia); abnormal nitroblue tetrazolium or dihydrorhodamine test.

Fuel use in fed and fasting states

• Respiratory quotient: carbohydrate 1.0, protein 0.8, fat 0.7. Basal metabolic rate rises with thyroid hormone, fever (about 10–13% per °C) and pregnancy.

Remember Coupled chain: block ATP synthase and oxygen use falls; uncouple it and oxygen use and heat rise while ATP falls.

Exam trap Complex II pumps no protons, so FADH2 yields less ATP than NADH.

Exam trap Cyanide gives high venous oxygen saturation because tissues cannot extract oxygen.

How FCPS asks about this

• Brown fat heat production in newborns: uncoupling by thermogenin (UCP1).
• Fever and sweating on dinitrophenol slimming pills: uncoupling of oxidative phosphorylation.
• Cyanide poisoning affects: complex IV (cytochrome c oxidase).
• Oligomycin: blocks the ATP synthase proton channel.
• Most reactive free radical; radiation damage: hydroxyl radical, damaging DNA.
• Selenium-dependent antioxidant enzyme: glutathione peroxidase; membrane antioxidant vitamin E.
• Boy with Gowers sign, big calves and very high CK: absent dystrophin.