Electron transfer chain

The electron transfer chain is a series of protein complexes; NADH-Q oxioreductase, Q-cytochrome C oxioreductase, cytochrome C oxidase and ATP synthase. In between these components are electrons carriers which will transfer the electrons from a complex to another. The protein complexes contain certain redox centres, and electrons move to carriers with increasing electron affinity along the chain:

[Q is coenzyme Q, also known as ubiquinone, carries electrons from NADH and FADH₂. It is hydrophobic and diffuses rapidly in the inner mitochondrial membrane.]

• Complex I – NADH-Q oxioreductase
         o Transfer of two high-potential electrons from NADH to FMN
         o Electrons from FMNH₂ transferred to a series of Fe-S clusters
         o Electrons from Fe-S clusters shuttled to Coenzyme Q
         o Net Effect –
                   4H⁺ pumped out of matrix into intermembrane space
                   2 chemical H⁺ removed from matrix

• Complex II – Succinate-Q reductase complex
         o Succinate dehydrogenase (in Cirtic Acid Cycle) part of succinate-Q reductase complex
         o Electrons from FADH₂ transferred to Fe-S clusters and then to Q
         o Does not pump protons - therefore less ATP is formed by oxidation of FADH₂ than NADH

• Complex III – Q-cytochrome C oxioreductase
         o Cytochrome C is in all organisms with mitochondrial respiratory chains
         o Small soluble protein containing C-type heme
         o Carries one electron from Q-cytochrome C oxioreductase to cytochrome C oxidase
         o Electrons transferred from QH₂ to oxidised cytochrome C (Cyt_Cox)

         o Mechanism that couples electron transfer is known as the Q Cycle

• Complex IV – Cytochrome C Oxidase
         o 4 electrons transferred from cytochrome C to O₂
         o 4H⁺ from matrix allow the complete reduction of O₂ to H₂O
         o 4 more H⁺ pumped across membrane

The electron motive force created by the flow of electrons down electron transport chain are used to pump H⁺ into the intermembrane space. This creates a concentration gradient between the inner membrane and the intermembrane space where the concentration of H in the intermembane space is higher than the concetration of H in the inner membrane space. As the inner membrane of the mitochondria is not permeable to H, this forces the H to go through the ATP synthase which will create a proton motive force to convert ADP + Pi into ATP.