Bioenergetics and Oxidative Phosphorylation PDF

Bioenergetics and oxidative phosphorylation Amr Yehia, PhD Lecturer of Biochemistry and Molecular Biology Faculty of pharmacy, Al-Azhar University Outlines of Bioenergetics and oxidative phosphorylation Bioenergetics and types of energy ATP acts as an energy...

Bioenergetics and oxidative phosphorylation Amr Yehia, PhD Lecturer of Biochemistry and Molecular Biology Faculty of pharmacy, Al-Azhar University Outlines of Bioenergetics and oxidative phosphorylation Bioenergetics and types of energy ATP acts as an energy carrier Biological oxidation Respiratory chain (Electron Transport Chain ETC) Inhibitors of respiratory chain Oxidative phosphorylation ( chemiosmotic theory) Uncouplers Oxidation of extramitochondrial NADH + H+ Course Learning outcomes of the Bioenergetics and oxidative phosphorylation Explains the sources of energy supply of the processes running in the living organisms and molecular mechanisms of the energy transformation in the cells. Describes the thermodynamics of the biological (living) systems: exergonic and endergonic reactions, free energy, spontaneous and non spontaneous reactions, free energy changes of coupled reactions. Describes the biological oxidation-reduction reactions and mechanisms of electron transfer by transporters of respiratory chains. Describes the mechanism and explain the significance of substrate-level phosphorylation in the intracellular energy storage; structure and roles of high-energy compounds, the ways of ATP synthesis in the cell and processes of cellular activities in which ATP energy is used. Course Learning outcomes of the Bioenergetics and oxidative phosphorylation Describes the molecular principles of composition and structure of biological membranes. Explains the chemiosmotic theory of energy transformation and coupling, chemiosmotic cycle of protons, the mechanisms of oxidative phosphorylation, the generation of proton electrochemical gradient as well as pathways of electron transport and H+ translocation by the components of biological membranes. Describes the different electron transport chains, to deepen knowledge on mitochondrial respiratory chain, its components and complexes; inhibitors of electron transport systems and the mechanisms of action of uncouplers of oxidative phosphorylation.  Bioenergetics Bioenergetics: It is study of energy changes that accompanies biochemical reactions. Bioenergetics describes the transfer and utilization of energy in biological systems. Types of Energy: 1. Heat Energy: is used to maintain body temperature. 2. Free Energy: is used for body activities or to do useful work.  Forms of Free Energy (Free Energy Change of a Reaction) ▪ The change in free energy (Δ G) can be used to predict the direction of a reaction. ▪ - Δ G means loss of energy and reaction goes spontaneously. The reaction is exergonic. ▪ + Δ G means gain of energy and reaction is not spontaneously going. The reaction is endergonic. ▪ If Δ G is zero, reaction is in equilibrium. Exergonic reaction Endergonic reaction  Source of Energy: Both catabolism and anabolism constitute metabolism ▪ ATP as an energy carrier: ▪ The free energy produced through the catabolism of fuels (carbohydrates, lipids, and amino acids) is not transmitted directly to the reactions requiring energy. ▪ Instead it is used to synthesize a compound that acts as a carrier of free energy, which is adenosine triphosphate (ATP). ▪ ATP molecules are generated by exergonic reactions (catabolism) and utilized by endergonic reactions (anabolism) for different body works.  ATP : ATP is called high energy phosphate compound which is known as energy currency of the living cells. Breakdown of one high energy bond of ATP gives -7.3 Kcal/mol. (ΔG=-7300 calorie/mol). ATP→ADP→AMP Any bond whose breakdown produce a large decrease in free energy (~ 5 Kcal/mol) is termed a high energy bond (~)  Sources of ATP: 1. Substrate level phosphorylation: Formation of ATP from ADP and a phosphorylated substrate e.g. Glycolysis & Krebs cycle. 2. Respiratory chain (Oxidative phosphorylation): From NADH or FADH2 along the respiratory chain (electron transport chain, ETC) where electrons are transferred over various carriers to oxygen finally to form water with production of ATP.  Biological Oxidation Energy is required to maintain the structure and function of the living Cells. This energy is derived from oxidation of carbohydrates, lipids and proteins of diet. Oxidation and reduction reactions are always coupled and they are called (Redox reactions). LEO: Loss Electron Oxidation & GER: Gain Electron Reduction  Stages of foodstuffs oxidation  First stage: Digestion converts the macromolecules into small units.  Second stage: The products of digestion are absorbed, catabolized to smaller components, and oxidized to CO2.  The reducing equivalents (NADH and FADH2) are generated by the common oxidative pathways.  Third stage: These reduced equivalents (NADH and FADH2) enter into the electron transport chain (ETC), or Respiratory chain, where energy is released.  Redox potential E0 (Electron affinity)  It is the tendency of the reactants in an oxidation-reduction reactions to donate or accept electrons.  Oxygen has the highest electron affinity. While Hydrogen has the lowest electron affinity. Redox Chain:  Chain of different compounds of increasing redox potential from H and O2.  Redox chain contains compounds that have redox potentials higher than H and lower than O2.  Therefore, electrons flow from the pair with the low E0 to that with the more E0.  Increasing redox potential from H and O2.  Electron Transport Chain (ETC) Respiratory Chain Respiratory chain and ATP synthesis occur in all tissues that contain mitochondria. Location of ETC: Inner mitochondrial membrane.  Electron Transport Chain (ETC) Respiratory Chain It is the final common pathway in aerobic cells by which electrons derived from various substances are transferred to oxygen to form water. A variety of substances (carbohydrates, fatty acids and amino acids) use the respiratory chain as a final pathway where they give electrons to the oxidized coenzymes, nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD+) to form the energy rich reduced coenzymes (NADH+H+ and FADH2).  Electron Transport Chain (ETC) Respiratory Chain The NADH and FADH2 give hydrogen and a pair of electrons to electron carriers collectively called electron transport chain components. Electrons flow through the ETC in steps from the more electronegative component to the more electropositive component. Finally, electrons move to respiratory oxygen to form water. Thus, the ETC is called respiratory chain. Oxygen is the most electropositive component i.e. has the highest electron affinity. So, oxygen is the final acceptor of electrons and protons in the respiratory chain.  Components of the respiratory chain:- They are five protein complexes (I – V) that present in the inner mitochondrial membrane. The components of respiratory chain are arranged according to their redox potential. Hydrogen or electron flow through the chain from a compound of low redox potential to many carriers and finally to oxygen which has the highest redox potential. A lipid-soluble coenzyme Q (Co Q) and a water-soluble protein Cytochrome C (Cyt C) are mobile electron carriers' shuttle between these protein complexes.  Components of the respiratory chain:- ❖ Coenzyme Q (Co Q) is a mobile lipid soluble electron carrier that accepts electrons from complex I and II. ❖ Cytochrome C (Cyt C) is a mobile water-soluble electron carrier.  1) Complex I: NADH Dehydrogenase 2) Complex II: Succinate Dehydrogenase Note: Coenzyme Q works as a collecting point for hydrogen coming from complex I and II in their way to complex III. Coenzyme Q acts as a mobile component of the respiratory chain (lipid soluble compound that can diffuse within the inner membrane). ❖ Respiratory inhibitors: ▪ They are compounds that bind to components of Electron transport chain and prevent electron flow. ▪ Thus, they inhibit both oxidation and subsequent phosphorylation. ▪ These compounds inhibit at specific sites of ETC. ▪ Examples: 1) Rotenone (Insecticides) kills by inhibiting cellular respiration in mitochondria at complex I. 2) Antimycin A is an antibiotic that acts as inhibitor of cellular respiration at complex III. 3) Cyanide and Carbon monoxide toxicities are due to inhibition of cellular respiration at complex IV.  Oxidative Phosphorylation As electrons are passed down the respiratory chain, they lose much of their free energy (ΔG). Part of this energy can be captured and stored by the production of ATP from ADP and inorganic phosphate (Pi). The remainder of ΔG is released as heat. The coupling of electron transport and ATP synthesis is called oxidative phosphorylation.  Importance of Oxidative Phosphorylation:- Coupling mechanism: If the energy liberated from oxidation by respiratory chain is not captured , it will be lost in the form of heat. Thus, the energy liberated is used in phosphorylation of ADP by inorganic phosphate to form ATP which is the stored form of energy.  Mechanism of oxidative phosphorylation Chemiosmotic theory (Mitchell hypothesis( It explains how the energy generated by the transport of electrons through the electron transport chain is used to produce ATP from ADP + Pi. I) Proton Pump: Electron transport at complexes I, III, and IV generates energy that pump protons across the inner mitochondrial membrane from the matrix to the intermembrane space N.B: complexes I, III and IV act as a proton pump BUT complex II does not pump protons as no energy is lost in this process. This process creates: * Electrical gradient (with more positive charges on the outside of the membrane than on the inside). * pH gradient (the outside of the membrane is at a lower pH than the inside). The energy generated by this proton gradient is sufficient to drive ATP synthesis. Thus, the proton gradient serves as the common intermediate that couple's oxidation to phosphorylation. II) ATP synthesis: Protons can only re-enter to the matrix through proton channel in complex V (ATP synthase) decreasing the pH and electrical gradients and at the same time allow the binding of ADP + Pi (i.e. Phosphorylate ADP to ATP, and release ATP). Intermembrane space Mitochondrial Matrix ❖ Amount of ATP produced: 1) NADH + H+ The transport of a pair of electrons from NADH+H+ to O2 through ETC releases sufficient energy available to produce 3 ATP. 2) FADH2 The transport of a pair of electrons from FADH2 to O2 through the ETC releases sufficient energy available to produce 2 ATP. ❖ Uncouplers (UCP): ▪ Uncouplers are molecules disrupt the coupling between oxidation and phosphorylation by causing “proton leak” through allowing protons to re-enter the mitochondrial matrix without energy being captured as ATP. ▪ Electron transport proceeds but ATP synthesis is inhibited as protons do not pass-through complex V. ▪ Energy is released as heat. ❖ Types of uncouplers: ▪ Thermogenin (UCP1) is a natural uncoupling protein found in the inner mitochondrial membrane and responsible for the heat production in the brown adipocytes of mammals through uncoupling of oxidation to phosphorylation. ▪ It is present in large amounts in neonates and hibernating animals. ▪ Synthetic uncouplers: e.g. Oligomycin, 2,4-dinitrophenol & toxic overdoses of aspirin and other salicylates. N.B. this explains the fever that accompanies toxic overdoses of these drugs. Oxidation of Extra Mitochondrial NADH+H+ Thank You