Electron Transport Chains (ETC)

Overview

• ETC is a series of protein complexes in and around a membrane.
• Couples exergonic redox reactions to endergonic proton pumping.
• Generates an electrochemical gradient (proton motive force).
• Proton motive force is used in various cellular processes.

Steps of ETC

1. Electron Entry:
   • Electrons enter from electron donors like NADH or FADH2.
   • Entry points vary based on reduction potentials.
2. Redox Reaction:
   • Initial electron donor oxidized; acceptor reduced.
   • ΔG (free energy change) = -nFE (Faraday’s constant).
   • Larger positive E means more exergonic reaction.
3. Energy Transfer:
   • Energy from redox reactions can pump protons across the membrane.
4. Terminal Electron Acceptor:
   • Final acceptor in humans is oxygen; varies in nature.

Electron Donors and Acceptors

• Not limited to NADH and FADH2.
• Examples of electron donors: Hydrogen (H2), Iron (Fe).
• Various bacteria use different donors and acceptors.

Complexes of the ETC

• Membrane-associated red/ox protein complexes.
• Move electrons from donors to terminal acceptors.
• Oxidoreductases: Enzyme complexes that facilitate redox reactions.
• Oxidase: Terminal complex for aerobic respiration (oxygen as acceptor).
• Prosthetic Groups: Directly involved in catalyzing redox reactions.
  • Types include electron/proton carriers and electron-only carriers.

Mobile Energy Carriers

• Molecules that transfer energy within cells.
• Exist in pools, analogous to delivery vehicles.
• Include NAD+/NADH, NADP+/NADPH, FAD/FADH2.

Redox Chemistry

• Oxidation is electron removal, reduction is electron gain.
• Redox reactions dictate spontaneity and energy transfer.
• Electron carriers facilitate these reactions, including NAD+/NADH and others.

Aerobic vs Anaerobic Respiration

• Aerobic Respiration:
  • Oxygen as terminal electron acceptor.
  • Common in humans and many organisms.
• Anaerobic Respiration:
  • Other compounds (e.g., nitrate, nitrite) as acceptors.
  • Provides metabolic flexibility for survival.

Example: Two-Complex ETC

• Involves two integral membrane complexes (Complex I & II).
• Electron transfer leads to proton translocation, creating membrane potential.
• Example includes aerobic and anaerobic ETC processes.

Detailed Look at Aerobic Respiration

• Eukaryotic mitochondria have four complexes (I-IV).
• Complex I: NADH donates electrons, includes FMN and Fe-S proteins.
• Complex II: FADH2 transfers electrons directly to ubiquinone (Q).
• Complex III: Contains cytochromes, transfers electrons to cytochrome c.
• Complex IV: Reduces oxygen to form water, pumps protons.

Chemiosmosis

• Protons pumped across membrane create an electrochemical gradient.
• Protons move through ATP synthase, synthesizing ATP from ADP + Pi.
• Cyanide Effect: Inhibits cytochrome c oxidase, affecting ATP synthesis.
• Chemiosmosis is critical in aerobic glucose catabolism and photosynthesis.

Conclusion

• ETC is essential for cellular respiration and energy production.
• Understanding electron donors, acceptors, and complex interactions is crucial for grasping metabolic processes.