Oxidative phosphorylation & its mechanism

• Oxidative phosphorylation represents the culmination of energy-producing metabolic pathways in eukaryotic cells, such as glycolysis, the citric acid cycle, and fatty acid oxidation.
• It occurs in the inner membrane of mitochondria, where it harnesses the energy released from electron transfer to synthesize adenosine triphosphate (ATP), the primary energy currency of the cell.
• This process involves two main components: the electron transport chain (ETC) and chemiosmosis.

Mechanism of Oxidative Phosphorylation

Electron Transport Chain (ETC)

• Composed of four protein complexes (I-IV) and two electron carriers (ubiquinone and cytochrome c).
• Electrons from NADH and FADH2 pass through the ETC and ultimately reduce oxygen to water, while the movement of electrons powers proton pumping, creating a proton gradient.

  • Key Steps:

    1. Complex I (NADH oxidoreductase): Receives electrons from NADH, transfers them to ubiquinone (Q), and pumps protons.
    2. Complex II (succinate oxidoreductase): Transfers electrons from FADH2 to ubiquinone, without proton pumping.
    3. Ubiquinone (Q): Transfers electrons to Complex III.
    4. Complex III (cytochrome bc1 complex): Passes electrons to cytochrome c while pumping protons.
    5. Cytochrome c: Transfers electrons to Complex IV.
    6. Complex IV (cytochrome c oxidase): Accepts electrons, reduces oxygen to water, and pumps protons.

Chemiosmosis

• The proton gradient generated by the ETC creates a proton motive force.
• ATP Synthase (Complex V): Protons flow back into the mitochondrial matrix through ATP synthase, driving the conversion of ADP and inorganic phosphate (Pi) into ATP.

Final Electron Acceptor

• Oxygen: Acts as the final electron acceptor, combining with protons and electrons to form water. Oxygen is essential for keeping the ETC running and sustaining aerobic respiration.

Importance and Regulation

• Oxidative phosphorylation is crucial for efficient ATP production, providing energy for cellular processes.
• The system is tightly regulated to match energy demands and minimize the production of reactive oxygen species (ROS), which can cause cellular damage.

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