Wk 7&8 Lecture 5: The Electron Transport Chain

Mitochondrial Electron Transport

Overview of Oxidative Phosphorylation

• Oxidative phosphorylation begins the process of synthesizing ATP using electrons from mitochondrial oxidation.

• Crucial for activities requiring sustained ATP production, such as running.

• Key players: NADH and FADH2 donate electrons to the electron transport chain (ETC) located in the inner mitochondrial membrane.

Electron Flow and Proton Gradient

• Electron Transport Chain (ETC):

  • Electrons pass through a series of carriers, releasing energy used to pump protons (H+) across the inner membrane.

  • Establishes a proton gradient, essential for ATP synthesis.

• ATP Synthase:

  • Protons flow back into the matrix through ATP synthase, driving ATP production from ADP and inorganic phosphate (Pi).

Structure of Mitochondria

• Mitochondria have two membranes:

  • Outer Membrane: Contains porins that allow the transit of small molecules.

  • Inner Membrane: Highly folded (cristae) to increase surface area for ETC and ATP synthase.

  • Matrix: Contains enzymes for the citric acid cycle (CAC) and fatty acid oxidation.

• The inner membrane lacks pores and selectively transports metabolites.

Origin of Mitochondria

• Mitochondria likely evolved from bacterial endosymbionts, forming a symbiotic relationship with larger host cells.

• They contain their own DNA, a remnant from their bacterial ancestors, coding for some mitochondrial proteins.

Oxidative Phosphorylation Components

• Two main components:

  1. Electron Transport: Electrons from NADH and FADH2 drive the reduction of oxygen to water.

  2. ATP Synthesis: Proton gradient created by electron transport powers ATP synthesis.

• Electron transfer potential is characterized by standard reduction potentials (E°'):

  • NADH E°': -0.32 volts (good electron donor)

  • Oxygen E°': +0.82 volts (strong oxidizing agent)

• Change in standard reduction potential (ΔE°') for electron flow results in a significant free energy change, enabling ATP synthesis.

Electron Carriers in the ETC

• Electrons flow through complexes

  • Complex I (NADH-Q Oxidoreductase): accepts electrons from NADH.

  • Complex II (Succinate-Q Reductase): accepts electrons from FADH2.

  • Complex III (Q-Cytochrome c Oxidoreductase): transfers electrons to cytochrome c.

  • Complex IV (Cytochrome c Oxidase): reduces oxygen to water and pumps protons across the membrane.

• Ubiquinone (Coenzyme Q) and cytochrome c function as mobile electron carriers between the complexes.

Complexes of the Electron Transport Chain

• Complex I:

  • Contains FMN and iron-sulfur clusters; accepts electrons from NADH and reduces ubiquinone (Q) to QH2.

  • Pumps 4 protons across the membrane.

• Complex II:

  • Receives electrons from FADH2, does not pump protons.

• Complex III:

  • Uses QH2 to transfer electrons to cytochrome c; pumps 4 protons across the membrane.

• Complex IV:

  • Accepts electrons from cytochrome c, reduces oxygen to water; pumps 2 protons across the membrane.

Energy Efficiency and ATP Yield

• NADH generates approximately 2.5 ATP; FADH2 generates approximately 1.5 ATP.

• Total ATP yield from glucose breakdown varies:

  • Glycolysis: 2 ATP (net gain) and 2 NADH

  • CAC: Generates 6 NADH and 2 FADH2 from glucose through 2 acetyl CoA, producing ATP through substrate-level phosphorylation.

• Total ATP synthesis from complete breakdown of glucose (glycolysis + PDH + CAC + ETC) can reach up to 30-38 ATP, depending on reducing agents used and cellular conditions.

Inhibitors of Electron Transport

• Certain poisons can inhibit electron transport:

  • Rotenone: Blocks electron flow from iron-sulfur clusters to ubiquinone in Complex I.

  • Cyanide: Blocks Complex IV, preventing oxygen reduction.

  • Carbon monoxide: Inhibits electron transport; may be lethal in high concentrations.

• Importance of monitoring environmental conditions to prevent poisoning.

Conclusion

• The mitochondria play a critical role in cellular respiration and energy production through electron transport and ATP synthesis.

• Understanding the function and mechanism of the ETC and how energy is harvested from electrons is key for comprehending cellular metabolism.