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proton motive force

BiochemistryCellular/MitochondrialMetabolismEndocrine/Thermoregulation (brown fat)Nervous system (cyanide/CO toxicity)Musculoskeletal

Summary

Proton motive force (PMF) is the electrochemical gradient of H+ across the inner mitochondrial membrane (or bacterial plasma membrane), created by the electron transport chain pumping protons out of the matrix. It has two components: a chemical gradient (ΔpH) and an electrical gradient (membrane potential, Δψ). Protons flowing back through ATP synthase (Complex V) down this gradient drive ATP synthesis (oxidative phosphorylation).

Detail

**Definition/Formula:** PMF = Δψ − (2.3RT/F)·ΔpH, i.e., electrical potential (inside negative in the matrix) plus the chemical gradient (intermembrane space is more acidic, matrix is more alkaline). Peter Mitchell's chemiosmotic theory (Nobel 1978) describes how this stored energy couples electron transport to ATP synthesis.

**Generation:** NADH and FADH2 donate electrons to the ETC. Complexes I (NADH dehydrogenase), III (cytochrome bc1), and IV (cytochrome c oxidase) pump H+ from the matrix into the intermembrane space. Complex II (succinate dehydrogenase) does NOT pump protons, so FADH2 yields less ATP (~1.5) than NADH (~2.5). O2 is the final electron acceptor at Complex IV and forms H2O.

**Utilization:** H+ re-enters the matrix through the Fo subunit of ATP synthase. This rotation drives conformational changes in the F1 subunit, which phosphorylates ADP to ATP. About 4 H+ are needed per ATP (including phosphate/ATP transport).

**Other uses of PMF:** Transport of pyruvate and Pi into the matrix, the adenine nucleotide translocase, and in bacteria, flagellar rotation and active transport.

**Clinical correlates and high-yield disruptions:** - *ETC inhibitors* stop proton pumping, so the PMF collapses and ATP falls: - Complex I: rotenone, amytal (barbiturates), metformin (mild) - Complex III: antimycin A - Complex IV: cyanide, carbon monoxide, azide, H2S - *ATP synthase inhibitors* (oligomycin) block H+ flow through Fo. The PMF builds up, which inhibits further electron transport, so O2 consumption falls. - *Uncoupling agents* (2,4-dinitrophenol, aspirin overdose/salicylates, CCCP/FCCP) carry H+ across the membrane and dissipate the PMF. Electron transport continues and O2 consumption rises, but ATP synthesis falls. The energy is released as heat, causing hyperthermia, sweating, and tachycardia. - *Uncoupling protein-1 (UCP-1, thermogenin)* in brown fat physiologically dissipates the PMF to produce heat (nonshivering thermogenesis), especially in neonates. - *Mitochondrial diseases* (e.g., Leigh syndrome, MELAS) can impair ETC function and the PMF. They show maternal inheritance, lactic acidosis, and heteroplasmy. - *Antibiotics:* Aminoglycosides need an oxygen-dependent, PMF-driven uptake into bacteria. This is why they are ineffective against anaerobes. - *Bedaquiline* inhibits the mycobacterial ATP synthase (used in MDR-TB).

**Board pearls:** Oxidative phosphorylation is coupled, so substrate oxidation (ETC) depends on ADP availability (respiratory control). Inhibitors cause decreased O2 consumption, while uncouplers cause increased O2 consumption and heat.

Sources

  • First Aid for the USMLE Step 1 (Biochemistry: Oxidative phosphorylation)
  • Lippincott Illustrated Reviews: Biochemistry (Electron Transport Chain and Oxidative Phosphorylation)
  • Lehninger Principles of Biochemistry (Oxidative Phosphorylation and Photophosphorylation)
  • Harper's Illustrated Biochemistry (Respiratory chain and oxidative phosphorylation)
  • Pathoma / Sketchy Medical (Biochemistry: ETC and uncouplers)

Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.