complex V

**Structure:** Two domains. F0 is membrane-embedded and forms the proton channel (c-ring + a subunit). F1 projects into the mitochondrial matrix and contains the catalytic sites (α3β3γδε). Proton flow through F0 rotates the c-ring and γ-stalk, causing conformational changes in the β subunits (binding change mechanism: loose, tight, open states) that drive ATP synthesis.

**Function and chemiosmotic coupling (Mitchell):** The electron transport chain (complexes I, III, IV) pumps H+ from the matrix to the intermembrane space. This creates an electrochemical gradient (proton motive force). H+ flows back down its gradient through ATP synthase, and the energy released drives ADP + Pi → ATP. About 4 H+ are needed per ATP, including the cost of Pi/ATP/ADP transport. Per NADH, this gives about 2.5 ATP. Per FADH2, it gives about 1.5 ATP. Older textbooks use 3 and 2.

**Complex V does not transport electrons.** Complex II (succinate dehydrogenase) is the only ETC complex that does not pump protons, and it is also a TCA cycle enzyme.

**Inhibitors:** - **Oligomycin** (and venturicidin) blocks the F0 proton channel. This stops ATP synthesis, causes the proton gradient to build up, and secondarily inhibits electron transport (reduced O2 consumption). - Complexes I-IV inhibitors: rotenone and amytal (I), antimycin A (III), CN-, CO, azide, H2S (IV).

**Uncouplers:** 2,4-dinitrophenol, aspirin overdose (salicylates), thermogenin (UCP-1) in brown fat, and FCCP. They dissipate the proton gradient, so electron transport continues and O2 consumption rises, but ATP is not made. Energy is released as heat. Clinical features include hyperthermia, sweating, and tachycardia. Uncoupling is seen in aspirin toxicity, which also causes anion gap metabolic acidosis with respiratory alkalosis.

**Net effects of inhibitors vs. uncouplers:** - ETC inhibitors: ↓ O2 consumption, ↓ ATP, ↓ proton gradient. - ATP synthase inhibitors: ↓ O2 consumption, ↓ ATP, ↑ proton gradient. - Uncouplers: ↑ O2 consumption, ↓ ATP, ↓ proton gradient, ↑ heat.

**Clinical associations:** - Mitochondrial diseases (maternal/matrilineal inheritance, heteroplasmy). Mutations in the mtDNA gene MT-ATP6 (F0 subunit a) cause NARP (neuropathy, ataxia, retinitis pigmentosa) and maternally inherited Leigh syndrome. These are classically high-yield mitochondrial disorders. - Mitochondrial disease often affects high-energy tissues (brain, muscle, retina, heart) and may present with lactic acidosis, ragged red fibers on Gomori trichrome stain (seen in MELAS/MERRF), and myopathy. - Bedaquiline treats multidrug-resistant TB by inhibiting mycobacterial ATP synthase (high-yield pharmacology). It is associated with QT prolongation.

**Mnemonic:** "Oligomycin = Obstructs the proton channel of ATP synthase." The names ATP synthase, F0F1 ATPase, and complex V all refer to the same enzyme. F0 has the "o" for oligomycin sensitivity.