Electron transport chain Biochem 4511 Chapter 15 RedOx reactions Biological role Chemiosmotic theory Thermodynamics Organization Proton Transport 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 TCA as a source of proton gradient for oxidative phosphorylation in mitochondria 3NADH and 1FADH2 Oxidative phosphorylation in mitochondria Review of Oxidation Reduction reactions One reactant is in its oxidized state while the other is in its reduced state A helpful mnemonic LEO the lion says GER Loss of Electrons is Oxidation Gain of Electrons is Reduction 2014 John Wiley Sons Inc All rights reserved How much energy can be released in this redox reaction This significant amount of energy is released in a number of steps that are coupled to proton transport across the inner mitochondrial membrane Home assignment How much energy is released upon electron transfer from FADH2 to 1 2O2 under standard conditions use the provided table Any RedOx reaction can be written as a sum of two halfreactions Ability of a compound to accept electrons is measured by its reduction potential E0 The higher the E0 value the more likely a compound to serve as an electron acceptor Electrode potential E0 for the standard half reaction 2H 2e H2 is declared to be zero All other half reactions are compared to the standard one Compounds with lower than H affinity to electrons have negative standard potentials Compounds with higher than H affinity for electrons have positive standard potentials Electron transport chain components TCA cycle half reactions 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 source donor of electrons In aerobic oxidation of organic molecules food molecular oxygen is the ultimate acceptor of electrons Two ways of writing half reactions The most common way table reactions Electron acceptor is on the left Electron donor is on the right E0 is E0 a E0 d An alternative way Both electron acceptor and electron donor are on the left E0 is E0 a E0 d The Nernst Equation The free energy change inversely proportional to the reduction potential The actual reduction potential depends on the concentrations of oxidized and reduced species R Gas Constant 8 3145 J mol 1 K 1 n of electrons T temperature in Kelvin F Faraday s constant 96 485 J V 1 K 1 Oxidative phosphorylation in evolution Oxygen is the final acceptor of electrons in the mitochondrial electron chain Oxygen is poisonous to all forms of life in the absence of enzymes that can reduce the highly reactive byproducts of oxidation and oxidative metabolism Photosynthesis 6CO2 6H2O light CH2O 6 6O2 Multicellular life Photosynthetic bacteria prokaryotes Recall what compounds and enzymes are involved in deactivation of ROS O2 is the ultimate acceptor of e in the mitochondrial e chain electron sink Where is the place for Mitochondrial electron transport Chain NADH and FADH2 do not donate electrons directly to O2 Instead the electrons are sent through the Electron Transport Chain Why Oxidative phosphorylation Chemiosmotic theory by Peter Mitchell in 60s Nobel Prize in 1978 A Mitochondrial enzyme complexes generate a proton gradient across the inner mit membrane B This gradient provides the energy for ADP phosphorylation This energy is often called proton motive force Therefore the REDOX reactions in the electron chain are indirectly coupled to ATP production Proton gradient powers the synthesis of ATP High energy electrons generated in TCA and glycolysis are transported to O2 by the mitochondrial electron transport chain The energy of this chain of redox reactions is accumulated as a proton gradient proton motive force across the inner membrane The energy of proton gradient is converted to the chemical energy of ATP by the mitochondrial enzyme ATP synthase Electron chain in the inner membrane of mitochondria Proton pump of the ATP synthase Localization of oxidative phosphorylation Mitochondria retain some similarity to bacteria Outer mitochondrial membrane contains porins and therefore permeable for most small molecules In contrast inner mitochondrial membrane impermeable for most molecules ideal for maintaining gradients Human body contains 14 000m2 of mt inner membrane Roughly equal to 3 football fields Mitochondrial content Images of the Mitochondria Electron Micrograph of a Fibroblast Electron Micrograph 3D Reconstruction by Electron Tomography Mitochondria in Green Electron transport 3 types of electron transport 1 Direct transport of e2 Proton electron 3 Hydride ion with two electrons Fe3 Fe2 2 5 electron transport molecules 1 2 3 4 5 NADH FADH2 Ubiquinol Fe proteins cytochromes FeS protein complexes 2 2x 2x ee NAD NADH H 2e 1H FMN FMNH2 FAD FADH2 2 H e Ubiquinone Ubiquinol Q QH2 Ubiquinone Q 2 H Ubiquinol QH2 e Q QH2 is a lipid soluble molecule that is not tightly bound to any particular complex Therefore it can transport e from one reaction to another within the electron transport chain Fe containing electron transporters Cytochrome heme iron prosthetic groups 1e Cytochromes are proteins with heme prosthetic groups Unlike the heme groups in myoglobin and hemoglobin the heme in cytochrome c undergoes reversible oneelectron transfers The central iron atom can be oxidized Fe3 or reduced Fe2 Fe containing electron transporters FeS Iron sulfur clusters nonheme iron comlexes 1e 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 How can iron have many reduction potentials How can Fe proteins participate in the electron chain Redox reactions at different levels Answer The actual reduction potentials of ironcontaining compounds are altered by the enzymes of the chain Electron flow in the respiratory chain Electron chain complexes I II 4H III QH2 IV 4H 2H 10 H are moved across the membrane for each pair of electrons transferred from NADH to O2 How many protons are moved from each pair of e from FADH2 Electron Transport Generates a Proton Gradient FADH2 Two entry points for high energy electrons from NADH Complex I and FADH2 Complex II 10 H are transported per NADH molecule oxidized 6 H are transported per FADH2 molecule oxidized The P O ratio P O Phosphate Oxygen