F0F1 ATPase

**Structure** - **F0 (note: "zero," the oligomycin-sensitive part)**: Integral to the inner mitochondrial membrane. It contains the c-ring and a-subunit, which form the proton channel. Protons flow down their electrochemical gradient from the intermembrane space into the matrix, which rotates the c-ring and the central stalk (γ subunit). - **F1**: Peripheral, matrix-facing head with an α3β3 hexamer plus γ, δ, ε subunits. The catalytic sites are on the β subunits. Rotation of the γ stalk causes conformational changes in the β subunits, which synthesize ATP from ADP + Pi (binding change mechanism: loose → tight → open states).

**Mechanism and chemiosmotic coupling (Mitchell)** - Complexes I, III, and IV pump H+ from the matrix to the intermembrane space. This creates a proton motive force (a pH gradient plus a membrane potential). - Protons re-enter the matrix through F0, and this flow drives ATP synthesis by F1. - Approximate yield: about 2.5 ATP per NADH and about 1.5 ATP per FADH2. Complex II does not pump protons, so FADH2 yields less. - ATP is exported to the cytosol by the adenine nucleotide translocase (ANT), in exchange for ADP.

**Inhibitors and uncouplers (high-yield pharmacology/biochem)** - **ATP synthase inhibitors (Complex V)**: Oligomycin binds F0 and blocks the proton channel. The proton gradient builds up, electron transport stops, and O2 consumption falls. Note that the ETC is *coupled* to ATP synthesis. - **Electron transport inhibitors**: - Rotenone: Complex I - Antimycin A: Complex III - Cyanide, CO, azide (H2S too): Complex IV - These reduce the proton gradient, so ATP synthesis falls. - **Uncouplers** (2,4-dinitrophenol, aspirin overdose, thermogenin/UCP-1 in brown fat): They dissipate the proton gradient, bypassing ATP synthase. Electron transport continues and O2 consumption increases, but ATP synthesis falls. The energy is released as heat, causing hyperthermia. This is why salicylate toxicity causes fever and metabolic acidosis.

**Clinical correlates** - **Mitochondrial diseases** (maternal/mitochondrial inheritance, heteroplasmy): Defects in oxidative phosphorylation, including ATP synthase subunits (e.g., MT-ATP6 mutations cause NARP: neuropathy, ataxia, retinitis pigmentosa; also some Leigh syndrome). Tissues with high energy demand (brain, muscle, retina, heart) are most affected. Look for lactic acidosis, myopathy, and ragged red fibers (MELAS, MERRF). - **Ischemia/hypoxia**: Without O2 as the terminal electron acceptor, the proton gradient collapses and ATP synthesis stops. In severe cases, ATP synthase can run in reverse and hydrolyze ATP, which worsens energy depletion. - **Regulation**: Oxidative phosphorylation is controlled mainly by the ADP/ATP ratio (respiratory control). High ADP increases flux, and high ATP decreases flux.

**Memory aids** - "F0 = Oligomycin" (the "O" in F-zero/oligomycin). - F1 is the catalytic head that faces the matrix. - Inhibitor of the ETC: stops O2 use and ATP production. Uncoupler: increases O2 use but decreases ATP and increases heat.