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.
