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Transcript
CHAPTER 20 The ElectronTransport Chain The cheetah, whose capacity for aerobic metabolism makes it one of the fastest animals The role of Oxidative Phosphorylation and Electron-transport in Mitochondria - Oxidative phosphorylation is the process by which NADH and FADH2 (QH2) are oxidized and ATP is formed - Glycolysis and citric acid cycle are carried out to produce the reduced forms of NAD+ and FAD (Q) from oxidation of glucose. - The membrane-associated electron transport system is a series of enzyme complexes embedded in the inner mitochondrial membrane, which oxidize NADH and QH2. Oxidation energy is used to transport protons across the inner mitochondrial membrane, creating a proton gradient - The electrons from the oxidation NADH and QH2 are passed to a terminal electron acceptor usually oxygen (O2) to produce water - ATP synthase (ATPase) is a key enzyme that used the proton gradient energy to produce ATP Figure 20.2 Structure of the mitochondrion - Outer membrane has few proteins. Channels are present that allow free diffusion of ions and water soluble metabolites. - Inner membrane is very rich in protein (protein : lipid ratio of 4:1) - Permeable to neutral molecules (O2 and CO2) - Is a barrier to protons and large polar and ionic substances - Polar substances must be actively transported (pyruvate transferase) - Intermembrane space is where the protons are transported during the membrane-associated electron transport process - Matrix contents include the enzymes associated with production of acetyl-CoA and the citric acid cycle (except succinate dehydrogenase complex). Protons are removed from the matrix during electron transport Figure 20.15: The electron transport chain 4 H+ / 2e- 4 H+ / 2e- 2 H+ / 2e- V 3 H+ / ATP - Complexes I, III, and IV pump protons across the inner membrane as electrons are transferred Mobile coenzymes: ubiquinone (Q) and cytochrome c serve as links between complexes Complex IV reduces O2 to water Complex V is ATP synthase, which uses the proton gradient across the membrane to make ATP Figure 20.15: The electron transport chain 4 H+ / 2e- V - NADH donates electrons two at a time to complex I (NADH-Q reductase complex) of the electron transport chain - 4 H+ are pumped across the inner mitochondrial membrane Figure 20.15: The electron transport chain 4 H+ / 2e- X 4 H+ / 2e- V - Complex I donates 2 e- to Ubiquinone (Q), forming QH2 - QH2 donates 2 e- to Complex III (cytochrome c reductase complex) Mobile electron carrier 1 Ubiquinone (Q) Q is a lipid soluble molecule that diffuses within the lipid bilayer of the inner mitochondrial membrane, accepting electrons from Complex I and Complex II and passing them to Complex III Figure 20.15: The electron transport chain 4 H+ / 2e- 4 H+ / 2e- 2 H+ / 2e- V - Complex III donates one e- to cytochrome c cytochrome c transfers one e- to Complex IV (cytochrome c oxidase complex) This one e- transfer is repeated to transfer both electrons Mobile electron carrier 2 Cytochrome c A protein associated with the outer face of the inner mitochondrial membrane. Transports electrons from complex III to complex IV. Figure 20.15: The electron transport chain 4 H+ / 2e- 4 H+ / 2e- 2 H+ / 2e- V A total of 10 H+ are pumped across the inner mitochondrial membrane for every two electrons donated to Complex I and the electrons transferred to oxygen to make H2O. Figure 20.15: The electron transport chain X V - FADH2 is bound to Complex II (succinate dehydrogenase) Complex II. Succinate dehydrogenase - Transfers electrons from succinate to flavin adenine dinucleotide (FAD) as a hydride ion (H:-), to an Fe-S complex (one electron at a time), to ubiquinone (Q), making QH2 - Complex II does not pump protons Structure of E. Coli succinate dehydrogenase complex Figure 20.15: The electron transport chain X V - Complex II donates 2 e- to Ubiquinone (Q), forming QH2 - QH2 donates 2 e- to Complex III (cytochrome c reductase complex) Figure 20.15: The electron transport chain 4 H+ / 2e- X - V Complex III donates one e- to cytochrome c cytochrome c transfers one e- to Complex IV (cytochrome c oxidase complex) This one e- transfer is repeated to transfer both electrons Figure 20.15: The electron transport chain 4 H+ / 2e- X 2 H+ / 2e- V A total of 6 H+ are pumped across the inner mitochondrial membrane for every two electrons of FADH2 and the two electrons transferred to oxygen to make H2O. Electron Transport involving Complexes I-IV Figure 20.6 Iron and Copper in metalloenzymes are important in electron transport Iron can undergo reversible oxidation and reduction: - Enzyme heme groups and cytochromes contain iron and are important in the electron transport process - Nonheme iron exists in iron-sulfur clusters. iron is bound by sulfide ions and S- groups from cysteine (iron-sulfur clusters can accept only one e- in a reaction) - Copper (Cu) assists in the electron transport in Complex IV Cu2+ Cu+ Figure 20.7 Iron – sulfur proteins Iron atoms are complexed with an equal number of sulfide ions (S2-) and with thiolate groups of Cys side chains Each can undergo reduction-oxidation reactions Figure 20.8 Heme Fe(II)-protoporphyrin component of cytochrome c oxidase Heme consists of a tetrapyrrole porphyrin ring system complexed with iron Figure 20.15: The electron transport chain 4 H+ / 2e- 4 H+ / 2e- 2 H+ / 2e- V 3 H+ / ATP Next: Use of proton gradient for synthesis of ATP by ATP synthase (Complex V). Assignment Read Chapter 20 Read Chapter 21 Topics not covered: Standard Reduction Potentials (Fig 20.1) Details of the inner workings of the Complexes