Biological Oxidation & Electron Transport Chain
The Electron Transport Chain (ETC) couples the oxidation of NADH and FADH₂ to the synthesis of ATP. It is located in the inner mitochondrial membrane and accounts for ~90% of cellular ATP production.
ETC Components (4 Complexes)
- Complex I (NADH Dehydrogenase): NADH → NAD+ (electrons enter via FMN → Fe-S clusters → CoQ). Pumps 4 H+ across membrane. Inhibited by: Rotenone, Amytal (barbiturate), MPP+ (Parkinson's toxin).
- Complex II (Succinate Dehydrogenase): FADH₂ → FAD (electrons enter via Fe-S → CoQ). Does NOT pump H+ — that is why FADH₂ yields less ATP than NADH. Inhibited by Malonate (competitive), Carboxin.
- Coenzyme Q (Ubiquinone): Mobile electron carrier in the inner membrane. Accepts electrons from both Complex I and II → transfers to Complex III. Can accept 2e⁻ (fully reduced = QH₂, ubiquinol).
- Complex III (Cytochrome bc1): QH₂ → Cyt c (Q cycle). Pumps 4 H+ per 2e⁻. Inhibited by: Antimycin A, Myxothiazol.
- Cytochrome c: Mobile peripheral protein on outer face of inner membrane; carries electrons one at a time from Complex III → IV. Also released during apoptosis.
- Complex IV (Cytochrome c Oxidase): 4 Cyt c → O₂ → 2H₂O (final electron acceptor). Pumps 4 H+ per 4e⁻. Inhibited by: Cyanide, Azide, CO (bind Fe of Cyt a3), H₂S. CO poisoning: cells cannot use O₂ despite adequate delivery.
- Complex V (ATP Synthase, F₁F₀): Proton flow back through F₀ (driven by ΔΨ + ΔpH = proton-motive force) → rotates γ subunit → synthesizes ATP in F₁. Inhibited by Oligomycin (F₀ channel; increases proton-motive force). DCCD also inhibits.
Chemiosmotic Theory (Peter Mitchell, Nobel 1978)
ETC pumps H+ from matrix to intermembrane space → creates electrochemical gradient (proton-motive force: ΔΨ ~−180 mV + ΔpH). H+ flows back through ATP synthase → drives ATP synthesis. ~2.5 ATP per NADH; ~1.5 ATP per FADH₂ (P/O ratios).
Uncouplers
Dissipate proton gradient WITHOUT making ATP → energy released as heat.
- Thermogenin (UCP-1): Brown adipose tissue; natural uncoupler; generates heat (non-shivering thermogenesis — newborns, cold adaptation)
- DNP (2,4-dinitrophenol): Proton ionophore; was used as weight loss drug (fatal — no antidote if overdose)
- Aspirin overdose: Metabolite salicylate can uncouple ETC → high fever, respiratory alkalosis then metabolic acidosis
ATP Yield — Complete Glucose Oxidation
- Glycolysis: 2 ATP + 2 NADH → 5 ATP (from NADH via malate-aspartate shuttle)
- Pyruvate → Acetyl-CoA: 2 NADH → 5 ATP
- TCA (×2): 6 NADH + 2 FADH₂ + 2 GTP → 20 ATP
- Total: ~30–32 ATP per glucose (modern estimates using actual P/O ratios)
Shuttle Systems
NADH cannot cross inner mitochondrial membrane. Electrons transferred via shuttles:
- Malate-Aspartate Shuttle (Liver, Heart): Transfers cytosolic NADH → mitochondrial NADH → 2.5 ATP
- Glycerol-3-Phosphate Shuttle (Brain, Muscle): Cytosolic NADH → mitochondrial FADH₂ → 1.5 ATP only (less efficient)