Electron transfer chain

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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: Electron transfer chain.jpg

[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 INADH-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 IISuccinate-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 IIIQ-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 (CytCox)

         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.

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