Electron Transport Chain

ETC Overview - Electron Expressway

• The "Electron Expressway": Final common pathway for oxidizing fuel molecules (e.g., glucose, fatty acids).
• Location: Inner Mitochondrial Membrane (IMM); cristae increase surface area for efficiency.
• Purpose: Harvests energy from $NADH$ and $FADH_2$ by transferring their electrons sequentially to $O_2$.
• Process: Electrons flow through multi-protein complexes (I-IV) and mobile carriers (Coenzyme Q, Cytochrome c).
• Energy released pumps protons (H⁺), creating an electrochemical gradient (proton-motive force) that drives ATP synthesis.

⭐ Cyanide and Carbon Monoxide (CO) are potent inhibitors of Complex IV (Cytochrome c oxidase), halting cellular respiration and ATP production. This is a frequently tested toxicological point!

ETC Components - The Electron Team

• Complex I (NADH Dehydrogenase): $NADH \rightarrow NAD^+$ (FMN, Fe-S). Pumps 4H⁺; e⁻ to CoQ.
  • Inhibitors: Rotenone, Amytal, Piericidin A (📌 RAP-1).
• Complex II (Succinate Dehydrogenase): $FADH_2 \rightarrow FAD$ (FAD, Fe-S). e⁻ to CoQ; No H⁺ pump.
  • Inhibitor: Malonate. ⭐ > Complex II is unique: part of TCA cycle & ETC; does NOT pump H⁺.
• Coenzyme Q (Ubiquinone): Mobile lipid carrier; e⁻ from C-I & C-II to C-III.
• Complex III (Cytochrome bc₁): $QH_2 \rightarrow Q$ (Hemes, Fe-S). Pumps 4H⁺; e⁻ to Cyt c; Q-cycle.
  • Inhibitor: Antimycin A (📌 Anti-3).
• Cytochrome c: Mobile protein carrier (intermembrane space).
• Complex IV (Cytochrome c Oxidase): Uses Cyt c e⁻ (Heme a, a₃, CuA, CuB). Pumps 2H⁺; $\frac{1}{2}O_2 + 2H^{+} \rightarrow H_2O$.
  • Inhibitors: CN⁻, CO, N₃⁻ (Azide) (📌 C.O.N. IV).

Mechanism & PMF - Proton Power Play

• NADH & FADH₂ donate e⁻; flow via CoQ, Cyt c to O₂ (final acceptor).
  • NADH path: Cmplx I → CoQ → Cmplx III → Cyt c → Cmplx IV → O₂
  • FADH₂ path: Cmplx II → CoQ → Cmplx III → Cyt c → Cmplx IV → O₂
• Proton Pumping: H⁺ actively moved from matrix to IMS by Complexes I, III, IV.
  • Complex I: 4H⁺
  • Complex III: 4H⁺
  • Complex IV: 2H⁺ (Note: Complex II pumps 0H⁺)
• PMF: Electrochemical H⁺ gradient (ΔpH chemical, ΔΨ electrical) across inner membrane.
  • Drives ATP synthesis via ATP Synthase (Complex V).

⭐ Peter Mitchell's Chemiosmotic theory: PMF powers ATP synthesis.

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Oxidative Phosphorylation - ATP Factory Firing

• ATP Synthase (Complex V): F₀ ($H^+$ channel) & F₁ (catalytic site, makes ATP).
• Chemiosmosis: $H^+$ pumped to intermembrane space (IMS) creates proton-motive force (PMF).
• PMF drives $H^+$ via F₀, rotating F₁ ($ ext{γ}$ subunit), making ATP from ADP + $P_i$.
• Oligomycin: Inhibits F₀, blocks $H^+$ flow, stops ATP synthesis.
• Uncouplers (DNP, thermogenin): Disrupt PMF; energy lost as heat. 📌 UCPs = UnCouple Protons.

⭐ ATP synthase's F₁ uses a "binding change mechanism" (Open, Loose, Tight states) for ATP synthesis.

Inhibitors & Uncouplers - Chain Blockers Breakers

• ETC Inhibitors (Blockers): Halt electron (e⁻) flow, ↓ATP synthesis, ↓O₂ consumption.
  • C-I: Rotenone, Amytal, Piericidin A. 📌 R.A.P. at One.
  • C-III: Antimycin A.
  • C-IV: Cyanide (CN⁻), CO, H₂S, Azide (N₃⁻).
  • ATP Synthase (Complex V): Oligomycin.
• Uncouplers (Breakers): Disrupt proton (H⁺) gradient; ↓ATP synthesis, BUT ↑O₂ consumption, ↑Heat production.
  • E.g., 2,4-Dinitrophenol (DNP), Aspirin (high doses), Thermogenin (UCP1).

⭐ Cyanide inhibits Complex IV (Cytochrome c oxidase), causing histotoxic hypoxia; characterized by almond odor on breath (not always present).

High‑Yield Points - ⚡ Biggest Takeaways

• ETC in inner mitochondrial membrane: major ATP production via oxidative phosphorylation.
• Electrons from NADH/FADH2 flow via complexes I-IV to Oxygen (final acceptor).
• Proton pumping creates electrochemical gradient (proton-motive force).
• ATP synthase (Complex V) uses this gradient for ATP synthesis (chemiosmosis).
• Inhibitors (e.g., Cyanide, Rotenone) block ETC; Uncouplers (e.g., DNP) disrupt gradient, make heat.
• P:O Ratios: ~2.5 ATP/NADH, ~1.5 ATP/FADH2.