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Mitochondrial Electron Transport

Mitochondrial Electron Transport. The cheetah, whose capacity for aerobic metabolism makes it one of the fastest animals. Citric acid cycle supplies NADH and FADH 2 to the electron transport chain. Fatty Acids. Acetyl Co A. Amino Acids. Pyruvate. Glucose.

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Mitochondrial Electron Transport

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  1. Mitochondrial Electron Transport • The cheetah, whose capacity for aerobic metabolism makes it one of the fastest animals

  2. Citric acid cycle supplies NADH and FADH2 to the electron transport chain Fatty Acids Acetyl Co A Amino Acids Pyruvate Glucose

  3. Reduced coenzymes NADH and FADH2 are formed in matrix from: (1) Oxidative decarboxilation of pyruvate to acetyl CoA (2) Aerobic oxidation of acetyl CoA by the citric acid cycle (3) Oxidation of fatty acids and amino acids The NADH and FADH2 are energy-rich molecules because each contains a pair of electrons having a high transfer potential.

  4. The reduced and oxidized forms of NAD

  5. The reduced and oxidized forms of FAD

  6. Electrons of NADH or FADH2 are used to reduce molecular oxygen to water. A large amount of free energy is liberated. The electrons from NADH and FADH2are not transported directly to O2 but are transferred through series of electron carriers that undergo reversible reduction and oxidation.

  7. The flow of electrons through carriers leads to the pumping of protons out of the mitochondrial matrix. The resulting distribution of protons generates a pH gradient and a transmembrane electrical potential that creates a protonmotive force.

  8. ATP is synthesized when protons flow back to the mitochondrial matrix through an enzyme complex ATP synthase. The oxidation of fuels and the phosphorylation of ADP are coupled by a proton gradient across the inner mitochondrial membrane. Oxidative phosphorylation is the process in which ATP is formed as a result of the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers.

  9. OXIDATIVE PHOSPHORYLATION IN EUKARYOTES TAKES PLACE IN MITOCHONDRIA Two membranes: outer membrane inner membrane (folded into cristae) Two compartments: (1) the intermembrane space (2) the matrix Location of mitochondrial complexes • Inner mitochondrial membrane: Electron transport chainATP synthase • Mitochondrial matrix:Pyruvate dehydrogenase complexCitric acid cycleFatty acid oxidation The outer membrane is permeable to small molecules and ions because it contains pore-forming protein (porin). The inner membrane is impermeable to ions and polar molecules. Contains transporters (translocases).

  10. NADH FMN Fe-S Co-Q Fe-S cyt c1 cyt c cyt a cyt a3 O2 cyt b succinate FAD Fe-S THE ELECTRON TRANSPORT CHAIN Series of enzyme complexes (electron carriers) embedded in the inner mitochondrial membrane, which oxidize NADH2 and FADH2 and transport electrons to oxygen is calledrespiratory electron-transport chain (ETC). The sequence of electron carriers in ETC

  11. High-Energy Electrons: Redox Potentials and Free-Energy Changes In oxidative phosphorylation, the electron transfer potentialof NADH or FADH2 is converted into the phosphoryl transfer potentialof ATP. Phosphoryl transfer potential is G°' (energy released during the hydrolysis of activated phos-phate compound). G°' for ATP = -7.3 kcal mol-1 Electron transfer potential is expressed as E'o, the (also called redox potential, reduction potential, or oxidation-reduction potential).

  12. NADH FMN Fe-S Co-Q Fe-S cyt c1 cyt c cyt a cyt a3 O2 cyt b succinate FAD Fe-S E'o (reduction potential)is a measure of how easily a compound can be reduced (how easily it can accept electron). All compounds are compared to reduction potential of hydrogen wich is 0.0 V. The larger the value of E'o of a carrier in ETC the better it functions as an electron acceptor (oxidizing factor). Electrons flow through the ETC components spontaneouslyin the direction of increasing reduction potentials. E'o of NADH= -0.32 volts (strong reducing agent)E'o of O2 = +0.82 volts (strong oxidizing agent)

  13. Important characteristic of ETC is the amount of energy released upon electron transfer from one carrier to another. This energy can be calculated using the formula: Go’=-nFE’o n – number of electrons transferred from one carrier to another; F – the Faraday constant (23.06 kcal/volt mol); E’o– the difference in reduction potential between two carriers. When two electrons pass from NADH to O2 : Go’=-2*96,5*(+0,82-(-0,32)) = -52.6 kcal/mol

  14. NADH FMN Fe-S Co-Q Fe-S cyt c1 cyt c cyt a cyt a3 O2 cyt b succinate FAD Fe-S THE RESPIRATORY CHAIN CONSISTS OF FOUR COMPLEXES I II Components of electron-transport chain are arranged in the inner membrane of mitochondria in packages called respiratory assemblies (complexes). III IV III IV I II

  15. 26.8 The energy is released not in a single step of electron transfer but in incremental amount at each complex. Energy released at three specific steps in the chain is collected in form of transmembrane proton gradient and used to drive the synthesis of ATP.

  16. Complexes I-IV • Mobile coenzymes: ubiquinone (Q) and cytochrome c serve as links between ETC complexes • Complex IV reduces O2 to water

  17. Complex I (NADH-ubiquinone oxidoreductase) Transfers electrons from NADH to Co Q (ubiquinone)Consist of: - enzyme NADH dehydrogenase(FMN - prosthetic group) - iron-sulfur clusters. NADH reduces FMN to FMNH2. Electrons from FMNH2 pass to a Fe-S clusters. Fe-S proteins convey electrons to ubiquinone. QH2 is formed. The flow of two electrons from NADH to coenzym Q leads to the pumping of four hydrogen ions out of the matrix.

  18. NADH-Q oxidoreductase - an enormous enzyme consisting of 34 polypeptide chains. L-shaped (horizontal arm lying in the membrane and a vertical arm that projects into the matrix). NADH FMN matrix Fe-S Iron-sulfur clusters contains two or four iron ions and two or four inorganic sulfides. Clusters are coordinated by four cysteine residues. Iron ions in Fe-S complexes cycle between Fe2+ or Fe3+ states.

  19. Complex II (succinate-ubiquinon oxidoreductase) Transfers electrons from succinate to Co Q.Form 1 consist of: - enzyme succinate dehydrogenase (FAD – prosthetic group) - iron-sulfur clusters. Succinate reduces FAD to FADH2. Then electrons pass to Fe-S proteins which reduce Q to QH2 Form 2 and 3 contains enzymes acyl-CoA dehydrogenase (oxidation of fatty acids) and glycerol phosphate dehydrogenase (oxidation of glycerol) which direct the transfer of electrons from acyl CoA to Fe-S proteins. Complex II does not contribute to proton gradient.

  20. All electrons must pass through the ubiquinone (Q)-ubiquinole (QH2) pair. Ubiquinone Q:- lipid soluble molecule, - smallest and most hydrophobic of all the carriers - diffuses within the lipid bilayer - accepts electrons from I and II complexes and passes them to complex III.

  21. Complex III (ubiquinol-cytochrome c oxidoreductase) Transfers electrons from ubiquinol to cytochrome c.Consist of:cytochrome b, Fe-S clusters and cytochrome c1.Cytochromes – electron transferring proteins containing a heme prosthetic group (Fe2+ Fe3+). Oxidation of one QH2 is accompanied by the translocation of 4 H+ across the inner mitochondrial membrane. Two H+ are from the matrix, two from QH2

  22. Q-cytochrome c oxidoreductase is a dimer. Each monomer contains 11 subunits. Q-cytochrome c oxidoreductase contains three hemes: two b-type hemes within cytochrome b, and one c-type heme within cytochrome c1. Enzyme also contains an iron-sulfur protein with an 2Fe-2S center.

  23. Q cycle • two molecules of QH2 are oxidized to form two molecules of Q, • one molecule of Q is reduced to QH2, • two molecules of cytochrome c are reduced, • four protons are released on the cytoplasmic side, • two protons are removed from the mitochondrial matrix

  24. Complex IV (cytochrome c oxidase) Transfers electrons from cytochrome c to O2.Composed of:cytochromes a and a3. Catalyzes a four-electron reduction of molecular oxygen (O2) to water (H2O): O2 + 4e- + 4H+ 2H2O Translocates 2H+ into the intermembrane space

  25. Cytochrome c oxidase consists of 13 subunits and contains two hemes (two iron atom) and three copper ions, arranged as two copper centers.

  26. The Catalytic Cycle of Cytochrome c Oxidise

  27. The four protons used for the production of two molecules of water come from the matrix. The consumption of these four protons contributes to the proton gradient. Cytochrome c oxidase pumps four additional protons from the matrix to the cytoplasmic side of the membrane in the course of each reaction cycle (mechanism under study). Totally eight protons are removed from the matrix in one reaction cycle (4 electrons)

  28. Cellular Defense Against Reactive Oxygen Species If oxygen accepts four electrons - two molecules of H2O are produced single electron - superoxide anion (O2.-) two electrons – peroxide (O22-). O2.-, O22- and, particularly, their reaction products are harmful to cell components - reactive oxygen species or ROS. DEFENSE superoxide dismutase (manganese-containing version in mitochondria and a copper-zinc-dependent in cytosol) O2.- + O2.- + 2H+ = H2O2 + O2 catalase H2O2 + H2O2 = O2 + 2 H2O antioxidant vitamins: vitamins E and C reduced glutathione

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