Oxidative Phosphorylation Biochem 4511 Figures Essential Biochemistry 3rd Ed Pratt and Cornely Principles of Biochemistry 5th Ed Moran et al Fundamentals of Biochemistry 2nd Ed Voet Voet and Pratt Oxidative phosphorylation contains several steps of exergonic electron transfer It represents the final phase of the catabolism It is the major source of the cellular ATP Oxidation Reduction Reactions Oxidation reduction reactions involve the transfer of electron s from one molecule to another The molecule which loses electrons is oxidized The molecule which gains electrons is reduced Reduction Potential Reduction Potential indicates how likely a molecule is to gain an electron Electrons travel from molecules with the lowest reduction potential to molecules with the highest In the electron transport chain NADH is the reduced molecule which is the initial source of electrons Reduction Potential Molecule reduction potentials are analogous to Gibb s free energy in thermodynamics The standard reduction potential indicates a substance s tendency to become reduced The actual reduction potential depends on the concentrations of reactants Electrons are transferred from a substance with a lower reduction potential to a substance with a higher reduction potential The free energy change for an oxidation reduction reaction depends on the change in reduction potential The Nernst Equation The actual reduction potential depends on the concentrations of oxidized and reduced species R Gas Constant 8 3145 J mol 1 K 1 T temperature in Kelvin n of electrons F Faraday s constant 96 485 J V 1 K 1 The free energy change can be calculated from the change in reduction potential More equations for electrochemical reactions Anode Cathode Overall Nernst Equation At equilibrium e111OxdRe 222dReOx e 12212112OxdReOxdRe QnFnFdOdOlnRT cccclnRT 21Re12x12Re21x eqreactionKRTnFGln nFGreaction 0 0 reactionGeqKnFQnFlnRTlnRT Overall V 0 105 aq NADH 2e aq H aq NAD V 0 20 aq H2e aq H O2 COOgCV 0 095 0 20 105 0 NADH CO aq NAD aq HCOO2 reactiong eqKnFQnFlnRTlnRT 1640 40 715 298314 8095 0964852RT nFln eqeqKxxxK Overview of Mitochondrial Electron Transport There are four protein complexes associated with the electron transport chain Complex I transfers electrons from NADH to ubiquinone Q The citric acid cycle fatty acid oxidation and other processes also generate mitochondrial ubiquinol QH2 Complex II is succinate dehydrogenase which produces QH2 Complex III transfers electrons from ubiquinol to reduce cytochrome c Complex IV transfers electrons from cytochrome c to reduce O2 to H2O Electron Transport Takes Place in Mitochondria The inner mitochondrial membrane encloses the matrix and includes specific transport proteins As electrons are being passed through the electron transport chain protons are pumped from the matrix into the intermembrane space The generated proton concentration gradient provides the energy for ATP synthesis Mitochondria are Compartmentalized The outer mitochondrial membrane contains porins and therefore is permeable to most small molecules The inner mitochondrial membrane is impermeable to most molecules The inner membrane is ideal for maintaining concentration gradients The human body contains 14 000 m2 of mitochondrial inner membrane Roughly equal to 3 football fields Net ATP yield of glucose catabolism 30 or 32 depends on how electrons from cytoplasmic NADH are transported into the mitochondrial matrix 3 Phosphoglycerol Shuttle Muscle Malate Aspartate Shuttle Heart and Liver NADH FADH2 QH2 1 5 ATP NADH cyt NADH mit 2 5 ATP ATP and ADP Transport in Mitochondria is Tightly Regulated ATP translocase protein imports ADP and exports ATP A symport permits simultaneous movement of Pi and H Electron Transport Generates a Proton Gradient The number of protons transported by each complex is unclear but it is widely accepted that 10 H are transported per NADH molecule oxidized Naming the Electron Transport Chain Complexes 1 Name the initial electron donor e g NADH 2 Name the terminal acceptor of electrons e g ubiquinone 3 Add oxidoreductase for complexes I III Complex I NADH Ubiquinone Oxidoreductase Complex II Succinate Ubiquinone Oxidoreductase Succinate Dehydrogenase Complex III Ubiquinol Cytochrome C Oxidoreductase Complex IV Cytochrome C Oxidase Final e acceptor is O2 If you remember the name you know the function and vice versa Complex I Transfers Electrons from NADH to Ubiquinone Q to Generate Ubiquinol QH2 In complex I electrons transfer from NADH to FMN then from FMN to Q There are several co factors required for electron transfer including multiple iron sulfur clusters FeS NADH FMN FeS1 FeSx Q Ubiquinone is reduced to ubiquinol QH2 which can diffuse through the membrane to Complex III As electrons are transferred from NADH to ubiquinone Complex I transfers four protons from the matrix to the intermembrane space Architecture of Complex I Structure of a bacterial Complex I has been solved FMN purple and Fe S clusters orange Q binding site is highlighted The protein has many transmembrane TM helices Complex I Electron Transport 1 NADH initially transfers two electrons to FMN 2 Electrons are next transferred to a series of FeS clusters Electrons transferred one at a time Iron Sulfur Clusters There are several different classes of iron sulfur clusters Comprised of iron ions coordinated with Cys thiols as well as free sulfur anions Transfer one electron at a time Complex I Electron Transport 3 Electrons from the FeS clusters are transferred one at a time to ubiquinone Q 4 Ubiquinone is fully reduced to ubiquinol QH2 and diffuses through the membrane to complex III Oxidation Reduction Reactions Contribute to the Ubiquinol Pool Succinate dehydrogenase Complex II produces QH2 during the citric acid cycle Ubiquinol QH2 is also produced during fatty acid oxidation Electrons from cytosolic NADH may enter the mitochondrial ubiquinol pool through the 3 phosphoglycerol pathway Complex III Electron Transfer Complex III transfers electrons from ubiquinol QH2 to a series of cytochromes one electron at a time Cytochromes are proteins with heme prosthetic groups Unlike the heme groups in myoglobin and hemoglobin the heme in cytochrome c undergoes reversible one electron transfers The central iron atom can be oxidized Fe3 or reduced Fe2 The Q Cycle In complex III electrons are transferred one at a time from ubiquinol to cytochrome c Reduced cytochrome c transfers electrons to complex IV Transfer of two electrons from one