Oxidative Phosphorylation P1

1. ATP: The Cell's Energy Currency

• ATP → ADP + Pi releases -7.3 kcal/mol

• Used as the immediate source of energy (not long-term storage).

• Adult body contains ~100g ATP but recycles its entire body weight in ATP daily.

• Hence, constant regeneration of ATP is essential.

2. Where the Energy Comes From

• ATP is generated by coupling the transfer of high-energy electrons (from food) to phosphorylation of ADP.

• One molecule of glucose produces ~30 ATP:

  • 26 via oxidative phosphorylation

  • 2 from glycolysis

  • 2 from the TCA cycle

3. The Role of Oxygen

• Oxygen is the terminal electron acceptor in the chain.

• The challenge: safely and gradually transfer electrons to O₂ (a reactive species) without causing cellular damage.

4. The Electron Transport Chain (ETC)

• Located on the inner mitochondrial membrane.

• Electrons pass through a series of carriers with increasing redox potential.

• Energy is released stepwise → used to pump protons into the intermembrane space.

5. Redox Potential and Free Energy

• Electrons flow from low to high redox potential:

  • e.g., NADH (E°′ = –0.32 V) → O₂ (E°′ = +0.82 V)

  • ΔE = +1.14 V → ΔG = –220 kJ/mol

• This energy is used to establish a proton gradient (not directly to make ATP).

6. Chemiosmotic Theory

• Proposed by Peter Mitchell.

• The proton motive force (PMF) generated by proton pumping drives ATP synthesis.

• ATP synthase uses this PMF to convert ADP + Pi → ATP.

7. Structure of the Mitochondrion

• Double membrane: outer (permeable), inner (impermeable, with cristae).

• Cristae increase surface area for ETC.

• Compartments:

  • Matrix: enzymes of TCA, fatty acid oxidation

  • Intermembrane space: site of proton accumulation

8. Electron Transport Chain Complexes

• Complex I (NADH:Q oxidoreductase)

  • NADH → FMN → Fe-S clusters → Q → QH₂

  • Pumps 4 H⁺

• Complex II (Succinate-Q reductase)

  • Succinate → FADH₂ → Fe-S clusters → Q → QH₂

  • No proton pumping

• Q Pool

  • Mobile pool of ubiquinone (Q) / ubiquinol (QH₂)

• Complex III (Q-cytochrome c oxidoreductase)

  • QH₂ donates e⁻ via Q cycle to cytochrome c

  • Pumps 4 H⁺

• Cytochrome c

  • Carries electrons between Complex III → Complex IV

  • Contains heme group; mobile and conserved

• Complex IV (Cytochrome c oxidase)

  • Accepts electrons → O₂ → H₂O

  • Uses heme a, a₃ and copper centers (CuA, CuB)

  • Pumps 2 H⁺ per 2 e⁻

9. Cofactors in the ETC

• FMN: accepts 2 e⁻ from NADH

• Fe-S clusters: 1 e⁻ at a time

• Hemes (a, b, c): single e⁻ transfers

• Copper centers: involved in O₂ reduction in Complex IV

• Ubiquinone (CoQ): lipid-soluble, shuttles 1 or 2 e⁻

10. Mitochondrial Membrane Lipids

• Cardiolipin: unique to inner mitochondrial membrane; stabilizes ETC complexes

• PE and PC: structural support and membrane curvature

11. Endosymbiotic Origin of Mitochondria

• All mitochondria evolved from an engulfed bacterial ancestor (e.g., R. prowazekii).

• Retain bacterial-type genome and ribosomes.

✅ Summary of Key Steps (Biological Order)

1. Food → NADH/FADH₂ (TCA Cycle)

2. NADH/FADH₂ → ETC Complexes (I–IV)

3. Electron flow → proton gradient

4. Proton flow through ATP synthase → ATP

5. O₂ accepts electrons → H₂O formation