Electron Trasport System

CHAPTER: 11(D)

Electron Trasport System

Electron Transport System:

•      In the process of oxidation taking place in kreb’s cycle, dehydrogenase enzymes release pairs of hydrogen and electrons from different substrates .

•      Hydrogen and electron pass through different electrons carriers and ultimately react with oxygen and form water molecules.

•      During transfer of hydrogen atoms from one enzyme carrier to another in ETS , large amount of energy is released, which is stored in the pyrophosphate bond of ATP produced from ADP and inorganic phosphate.

•      The system of transport of electrons from NADH to oxygen releasing water via different electron carriers in mitochondrial matm.rix is known as electron transport system

•      Electron transport & oxidative phosphorylation occur in the inner membrane of mitochondria.

•      These process reoxidize the NADH and FADH₂   that arises from the citric acid cycle, glycolysis and fatty acid oxidation and trap the energy released as ATP.

•      Oxidative phosphorylation is major source of ATP in the cell.

Electron Transport from NADH:

•      Electrons are transferred from NADH to oxygen along the electron transport chain (also called the respiratory chain).

•      NADH passes electrons to NADH dehydrogenase, which contains Flavin mononucleotide  (FMN) and two types of iron-sulfur (FeS) clustres.

•      The electrons are accepted by the FMN to produce FMNH₂ and then passed to the iron atoms of the FeS clusters.

•      Electrons are then passed to ubiquinone (coenzyme Q , CoQ), converting it to ubiquinol (or CoQH₂), and then to cytochrome bc₁   complex.

•      The cytochrome bc₁  complex passes the electron to cytochrome c which in turn passes them to cytochrome oxidase, a complex that contains two cytochromes (cytochrome –a & a₃ ) paired with copper atom.

•      Finally, cytochrome oxidase passes four electrons to molecular oxygen to form two molecules of water.

Electron transport from FADH₂  :

•      FADH₂  is reoxidised to FAD by donating two electron to succinate –CoQ reductase (Complex II), which consist FeS clusters.

•      It passes the electrons onto ubiquinone in the main electron transport chain where their further transport leads to the formation of an H⁺   gradient and ATP synthesis.

Oxidative Phosphorylation:

•      The final step of respiration is the oxidation of the reduced CoA (NADH and NADPH) by molecular oxygen. During this oxidation energy is released and ATP is synthesized.

•      The production of high energy phosphate bonds of ATP from ADP and inorganic phosphate (Pi) is called oxidative phosphorylation.

•      For the production of ATP molecule at least 10-12 K cal of energy is required as shown below:

            ADP + Pi + 10/12 K Cal = ATP

•      The production of ATP in respiration is closely linked with electron transport chain because some energy is released during the transfer of electrons from donor to acceptor molecule.

•      Oxidative phosphorylation is the name given to the synthesis of ATP that occurs when NADH & FADH₂    are oxidized by electron transport through the respiratory chain.

•      The inner membrane of mitochondria (Cristae) has a key role in the synthesis of ATP.

•      Mechanism of oxidative phosphorylation was originally proposed as “Chemiosmotic Hypothesis” by Peter Mitchell in 1961.

•      This hypothesis is now assumed to be theory and state as, “ Energy liberated by a electron transport is used to create a proton gradient across the mitochondrial  inner membrane and that is used to drive ATP synthesis.

•      Approximately 3 ATP are synthesized per NADH oxidised and 2 ATP are synthesized per FADH₂   oxidised.