Question

Explain the reactions taking place in mitochondrial inner membrane.

Answer 1

Electron and hydrogen (proton) transport takes place across four multiprotein complexes (I-IV). They are.

1. Complex-I (NADH dehydrogenase):
   It contains a flavoprotein (FMN) and associated with non-heme iron Sulphur protein (Fe-S). This complex is responsible for passing electrons and protons from mitochondrial NADFI (Internal) to Ubiquinone (UQ)
   \[\ce{NADH + H+ + UQ ⇌ NAD+ + UQH2}\]
2. In plants, an additional NADH dehydrogenase (External) complex is present on the outer surface of inner membrane of mitochondria which can oxidise cytosolic NADH + H⁺.
   Ubiquinone (UQ) or Coenzyme Quinone (CoQ) is a small, lipid-soluble electron, proton carrier located within the inner membrane of mitochondria).
3. Complex-II (succinic dehydrogenase):
   It contains FAD flavoprotein is associated with non-heme iron Sulphur (Fe-S) protein. This complex receives electrons and protons from succinate in Kerbs cycle and is converted into fumarate and passes to ubiquinone.
   \[\ce{Succinate + UQ -> Fumaraic + UQH2}\]
4. Complex-III (Cytochrome bc₁ complex): This complex oxidises reduced ubiquinone (ubiquinol) and transfers the electrons through Cytochrome bc₁ Complex (Iron Sulphur centci bcl complex) to cytochrome c.
   
5. Complex IV (Cytochrome c oxidase): Complex IV is the terminal oxidase and brings about the reduction of 1/2 O₂ to H₂O. TWO protons are needed to form a molecule of H₂O (terminal oxidation).