chemiosmotic hypothesis

**Core concept:** Oxidative phosphorylation couples two processes: (1) exergonic electron transfer from NADH/FADH2 to O2, and (2) endergonic ATP synthesis. The coupling is not via a high-energy chemical intermediate but via a proton gradient across the inner mitochondrial membrane (IMM).

**Mechanism:** - NADH donates electrons to Complex I (NADH dehydrogenase); FADH2 donates via Complex II (succinate dehydrogenase). Electrons pass to CoQ (ubiquinone), then Complex III (cytochrome bc1), cytochrome c, Complex IV (cytochrome c oxidase), and finally O2, which is reduced to H2O. - Complexes I, III, and IV pump H+ from the matrix into the intermembrane space (Complex II does NOT pump protons, which is why FADH2 yields less ATP). - This generates the proton motive force: a chemical gradient (ΔpH; the intermembrane space is more acidic) plus an electrical gradient (Δψ; the matrix is negative). - Protons flow back down the gradient through the F0 subunit of ATP synthase (Complex V). This rotation drives conformational changes in the F1 subunit, which catalyzes ADP + Pi → ATP (binding change mechanism). - Approximate yield: ~2.5 ATP per NADH and ~1.5 ATP per FADH2 (older texts cite 3 and 2).

**Respiratory control and coupling:** - ATP synthesis is tightly coupled to electron transport. If ADP is low, the gradient is not dissipated, and electron transport slows (respiratory control). High ADP increases flux.

**Clinical correlates (high-yield):** - *ETC inhibitors* (block electron flow, so no gradient and no ATP; O2 consumption falls): Rotenone (Complex I), Antimycin A (Complex III), Cyanide, CO, azide, H2S (Complex IV). - *ATP synthase inhibitors:* Oligomycin (blocks F0 proton channel). The gradient builds up, which slows electron transport. ATP production falls. - *Uncouplers* (dissipate the gradient, so electron transport continues, and O2 consumption rises, but ATP synthesis falls; energy is released as heat): 2,4-dinitrophenol (DNP), aspirin (salicylate) overdose (causes hyperthermia), and thermogenin (UCP-1) in brown fat, which produces heat in neonates and in cold adaptation. - *Mitochondrial diseases:* Defects in oxidative phosphorylation proteins (e.g., Leber hereditary optic neuropathy, MELAS, MERRF) show maternal (mitochondrial) inheritance, heteroplasmy, and lactic acidosis. Muscle biopsy shows ragged red fibers. - *Cyanide poisoning:* Treat with hydroxocobalamin, or nitrites (induce methemoglobin, which binds CN) plus thiosulfate.

**Test tip:** Know which agents decrease O2 consumption (ETC inhibitors, ATP synthase inhibitors) vs. increase it (uncouplers), and the effect on the proton gradient (↓ with uncouplers; ↑ with oligomycin).