Oxidative Phosphorylation PDF

Oxidative Phosphorylation Learning objectives - Explain the significances of oxidative phosphorylation - Understand the overall electron flow and proton pumping in the electron transport chain (ETC) complexes - Recognize the initial electron donor and final electron acceptor in...

Oxidative Phosphorylation Learning objectives - Explain the significances of oxidative phosphorylation - Understand the overall electron flow and proton pumping in the electron transport chain (ETC) complexes - Recognize the initial electron donor and final electron acceptor in each ETC complex - Explain the process of ATP formation by ATP synthase - Recognize the two different shuttle systems for cytosolic NADH to enter the ETC  Oxidative Phosphorylation - Glycolysis + TCA cycle produce very little ATP directly (substrate level phosphorylation). - For each glucose - Glycolysis: 2 ATPs and TCA cycle : 2 ATPs - The rest of the energy is stored in the reduced cofactors: 10 NADH + 2 FADH2 per glucose - Consider that: C6H12O6 + 6 O2 6 CO2 + 6 H2O (G' = -2870 kcal/mol) - Oxidation of the reduced co-factors releases energy: NADH + H+ + 1/2 O2 NAD+ + H2O (G' = -220 kcal/mol) FADH2 + 1/2 O2 FAD + H2O (G' = -182 kcal/mol) - Thus, the reduced co-factors from oxidation of 1 mole of glucose store 10 X 220 + 2 X 182 = 2564 kJ of energy, accounting for about 90% of the total realizable energy (under standard conditions) NAD+/NADH and NADP+/NADPH - Diffusible 2-electron carriers - Accept or donate 1 hydride (H-) ion (1 proton and 2 electrons) FAD/FADH2 and FMN/FMNH2 - Bound in enzymes as prosthetic groups + - - Accept or donates 1H + 1e at a time Oxidation of fatty acids and some amino acids also generates acetyl-CoA as well as NADH and FADH2 The mitochondrion - Aerobic oxidation of biomolecules Alligator jaw muscle - white muscle Flight muscle - red muscle Electron transport chain (ETC) in mitochondria Electron transfer reactions - The driving force is expressed as a difference in the standard redox potentials of the components, and this is equivalent to the change in free energy: n = no. of e- transferred G’ = -nF E' F = 96.4 kJ mol-1 V-1 E‘ = EA' - ED' - Electron flow is favorable from donors of lower reduction potential (i.e. more negative E values) to acceptors of higher reduction potential. Four integral membrane protein complexes: Summary of electron flow in inner membrane: Complex I Complex IV Complex II Complex III NADH dehydrogenase (Complex I) Cofactors (electron carriers): riboflavin coenzyme FMN, iron-sulfur cluster, transfers electrons from NADH to unbound ubiquinone (Q) the mobile ubiquinol (QH2) carries electrons and diffuses through inner membrane to cytochrome bc1 complex (Complex III) Overall: NADH + H+ + Q NAD+ + QH2 (intermembrane space) Proton pumping (matrix) FMN - Protein-bound co-factor + - - Accepts or donates 1H + 1e at a time Iron-sulfur (Fe-S) clusters Single electron carriers: Fe2+  Fe3+ + e- Protein Protein - each Fe is always coordinated by 4 S - 4 cysteine residues from the protein also contribute 4S to the cluster - it is also possible to have a single Fe coordinated by 4 cysteine residues Q (or CoQ)/QH2 - Diffusible through inner membrane - Accepts/donates 1 electron + 1 proton at a time Succinate dehydrogenase (Complex II) The only membrane-bound enzyme in TCA cycle Contains an internal chain of electron transfer cofactors No proton pumping in Complex II QH2 carries electrons and diffuses through inner membrane to Complex III (intermembrane space) (matrix) Cytochrome bc1 complex (Complex III) Electron carriers: Fe-S clusters, cytochrome b and cytochrome c1 Transfers electrons from ubiquinol (QH2) to cytochrome c (cyt c) QH2 + 2 cyt c (oxidized) + 2 H+ Q + 2 cyt c (reduced) + 4H+ Cytochrome c is a water-soluble protein coenzyme in the intermembrane space. Complex III pumps 4 protons per 2 electrons transferred (Iron-sulfur protein) (intermembrane space) (matrix) Cytochromes - Proteins containing a heme group as a tightly bound prosthetic group - Heme is a tetrapyrrole coordinating a single atom of Fe(II/III) - Single electron-carrier (Fe2+  Fe3+ + e-) Cytochrome c - Contains heme C - Membrane-bound or Cytochrome a diffusible - Contains heme A - Membrane-bound Cytochrome b - Contains heme B - Membrane-bound The Q cycle in Complex III Cytochrome oxidase (Complex IV) Electron carriers: 2 Cu ions and 2 heme A groups (cytochrome a proteins) Transfers electrons from cytochrome c to oxygen 4 cytochrome c (reduced) + O2 4 cytochrome c (oxidized) + 2 H2O The electron carriers in this complex only transfer electron one at a time Incompletely reduced intermediates (e.g. hydrogen peroxide and hydroxyl free radicals) remain tightly bound until complete reduction to water. (x2) (intermembrane space) (matrix)  Electron transport and proton pumps Intermembrane space inner membrane Matrix Proton gradient (intermembrane (matrix) space) A protein gradient is then established across the inner membrane The energy stored in the gradient, proton-motive force, has two components: chemical potential energy and electrical potential energy The electrochemical energy released when protons flow spontaneously down the gradient can be used to drive the synthesis of ATP from ADP and Pi ATP synthesis by ATP synthase (Oxidative Phosphorylation) ATP synthase: F0F1 complex in the inner membrane Protons flow through the F0 unit down the gradient ATP is synthesized by the F1 unit (ATPase) from ADP and Pi For every 2 electrons donated by NADH, 2.5 ATPs are synthesized. For every 2 electrons donated by FADH2, 1.5 ATPs are synthesized. NADH + H+ NAD+ Shuttle systems for NADH generated in cytoplasm Inner mitochondrial membrane is impermeable to NADH Cytosolic NADH generated from glycolysis is shuttled indirectly into the mitochondria as reducing equivalents Reducing equivalents are molecules that can be transported into the mitochondria (1) Glycerol 3-P shuttle - Skeletal muscle and brain (DHAP) Complex III Complex IV (2) Malate-aspartate shuttle - Liver, kidney, and heart (Complexes I, III, IV) Net profit of aerobic metabolism If we start from glucose: - Glycolysis to 2 pyruvate yields: Cytosol 2 ATP 2 NADH 2 X 2.5 (malate-aspartate shuttle) = 5 ATP or 2 X 1.5 (glycerol 3-P shuttle) = 3 ATP - Conversion of 2 pyruvate to 2 acetyl-CoA: Mitochondria 2 NADH = 2 X 2.5 = 5 ATP - Oxidation of 2 acetyl-CoA in the TCA cycle: 20 ATP 32 or 30 ATP for complete oxidation of glucose to CO Net yield = __________ 2