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M Oxidative Phosphorylation.pdf

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Boris Zhorov

Oxidative Phosphorylation Learning Objectives •Enzyme complexes in the ETC Chemiosmotic theory • •Structure of ATP synthase •Oxidation of 1 NADH = 10 H+ = 2.5 ATP/NADH molecule. - not strictly stoichiometric •4H : 1 ATP Complex I: NADH-CoQ Oxidoreductase Catalyzes oxidation of NADH coupled to the reduction of CoQ •This is the largest protein complex in the mitochondrial membrane, •Contains 46 subunits with 10 redox centers: •1 FMN and 9 Fe-S clusters. •FMN becomes FMNH upon N2DH reduction •Electrons pass between centers in a stepwise fashion. Oxidative Phosphorylation •NADH donates two electrons. •Cytochrome c can accept only one electron. •CoQ can transfer one or two electrons. Electron transfer between the 10 redox centers is coupled to 4 proton pumps • which pump H+ out of the matrix. The detailed mechanism remains unknown. Complex II: Succinate-CoQ Oxidoreductase This complex contains succinate dehydrogenase, the only membrane-bound enzyme of the citric acid cycle, along with 3 other hydrophobic subunits. •The complex contains five redox centers: •1 FMN - Flavin Mononucleotide •3 Fe-S clusters •Cytochrome b. •Electrons move from succinate to FADH and 2hen, through Fe-S clusters and cytochrome b, to CoQ. •CoQ moves freely within the lipid bilayer. It collects electrons from complexes I and II and transfers them to complex III. Complex III:CoQ-Cytochromec Oxidoreductase This complex passes electrons from the reduced CoQ to cytochrome c. •The contex contains four redox centers: •2 cytochrome b •1 cytochrome c •1 Fe-S cluster •Cytochrome c is a peripheral membrane protein. It carries electrons in the intermembrane space from Complex III to complex IV. Oxidative Phosphorylation Complex IV: Cytochrome c Oxidase Cytochrome c oxidase catalyzes the overall reaction •Contains four redox centers •Coupled to the pumping of 2 protons from the matrix to the intermembrane space.\ •Oxidation of NADH in the ETC results in the expelling of 10 protons from the matrix to the intermembrane space. The obtained proton gradient is used as a driving force in the endergonic reaction of ATP synthesis. Oxidative Phosphorylation Chemiosmotic Theory: The free energy of electron transport is conserved by pumping H+ from the matrix to the intermembrane space. The obtained electrochemical potential drives ATP synthesis. •Proposed in 1961 by Peter Mitchel to explain the following observations: ! Chemiosmotic Theory: ions do not permeate via an intact inner mitochondrial membrane. ATP synthesis is blocked by increasing permeability of the inner mitochondrial membrane for ions and protons. Oxidation of NADH creates an electrochemical gradient across the inner membrane. ATP synthesis is thus stimulated by increasing cytosolic [H+]. Oxidative Phosphorylation ATP Synthase: Free energy in expelling one proton from the matrix (21.5kJ/mol) is less than that required to synthesize one ATP molecule from ADP and Pi (>40kJ/mol). •ATP synthase catalyzes the endergonic synthesis of ATP by coupling it to the exergonic flow of protons to the matrix. Downhill movement of 3 protons to the matrix releases energy necessary to synthesize 1 ATP molecule. •3 H+ in = 1 ATP out. •Multisubunit protein composed of 2 functional parts •ransmembrane proton channel: •ater-soluble peripheral membrane protein: •Compounds that block proton permeation through the channel (oligomycin B) inhibit ATP synthesis. CELLULARRMEETABOLISM FOR 1MOOLECULEEOF GLUCOOSE STEP ELECTRON CARRIERS PRODUCTS Glycolysis 2 NADH produced 2 ATP (4total-2consumed) Oxidative Phosphorylation Commplexes Complexes Reactions
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