The Electron-Transport Chain and Oxidative Phosphorylation

CHAPTER 20 The Electron-Transport Chain

• The electron-transport chain (ETC) is analogous to a bicycle chain, converting energy from biological processes into usable energy (ATP) through a series of redox reactions.

20.1 Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria

• Location: Oxidative phosphorylation occurs in the inner mitochondrial membrane while the citric acid cycle operates in the mitochondrial matrix.
• Structure: Mitochondria consist of a double membrane:
  • Outer membrane: Permeable to small molecules (due to mitochondrial porin).
  • Inner membrane: Highly folded into cristae, providing a large surface area for oxidative phosphorylation. It contains proton pumps vital for ATP synthesis.
• Proton Gradient: Protons are pumped into the intermembrane space, creating a gradient that powers ATP synthesis through ATP synthase.

20.2 Oxidative Phosphorylation Depends on Electron Transfer

• Mechanism: Oxidation-reduction reactions transfer electrons from NADH and other electron donors to oxygen, with four protein complexes involved:
  1. NADH-Q oxidoreductase (Complex I)
  2. Succinate-Q reductase (Complex II)
  3. Q-cytochrome c oxidoreductase (Complex III)
  4. Cytochrome c oxidase (Complex IV)
• Flow of Electrons: Electrons flow down the chain, releasing energy that is harnessed to pump protons across the inner mitochondrial membrane, creating a proton gradient (the proton motive force).

Key Components of Electron Transport

• NADH and: High-electron carriers involved in electron transfer to oxygen.
• Reduction Potentials: Electrons transfer potential is quantified using redox potential.
• Coenzyme Q (Ubiquinone): A hydrophobic electron carrier that aids in the mobility of electrons.

Key Reactions and Interactions

• NADH-Q oxidoreductase (Complex I):

  • Accepts electrons from NADH, transfers them through FMN and iron-sulfur clusters to ubiquinone.
  • Pumps protons out of the mitochondrial matrix (4 protons).

• Succinate-Q reductase (Complex II):

  • Does not pump protons. It accepts electrons from succinate (produced during the citric acid cycle) and transfers them to ubiquinone.

• Q-cytochrome c oxidoreductase (Complex III):

  • Passes electrons to cytochrome c and pumps protons (2 protons per pair of electrons). This complex operates via the Q cycle.

• Cytochrome c oxidase (Complex IV):

  • Catalyzes the reduction of oxygen to water and pumps protons across the membrane (also contributes significant protons to the gradient).

20.3 The Respiratory Chain Consists of Proton Pumps and a Physical Link to the Citric Acid Cycle

• Enzyme Coupling: Complexes are organized efficiently to ensure effective electron transport and proton pumping, resulting in high ATP yield through adherence to redox potential gradients.
• Respirasome Formation: The ETC components may exist as a supercomplex (respirasome), enhancing electron transfer efficiencies.
• Clinical Insight: Mutations affecting electron transport components can lead to diseases such as Friedreich's Ataxia.

Impacts of the Electron Transport Chain

• Reactive Oxygen Species (ROS): Byproducts of oxygen reduction can be harmful, necessitating cellular defense mechanisms like superoxide dismutase and catalase.
• Oxygen as Final Electron Acceptor: Required for the aerobic respiration process, generating the maximum ATP yield compared to anaerobic methods.

Conclusion

• The electron transport chain and oxidative phosphorylation are crucial for ATP generation in aerobic organisms and highlight the intricate relationship between metabolic pathways and cellular respiration.
• Understanding these mechanisms is vital for deciphering energy metabolism and its implications for health and disease.