Bacterial Membrane Bioenergetics: Chemiosmotic Theory and Electron Transport - Prof. Ranja, Study notes of Health sciences

Download Bacterial Membrane Bioenergetics: Chemiosmotic Theory and Electron Transport - Prof. Ranja and more Study notes Health sciences in PDF only on Docsity! Bacterial Physiology HSCI5607 Chapter 3-4 Membrane Bioenergetics and Electron Transport Chemiosmotic Theory: Introduction of chemiosmotic theory by Peter Mitchell has revolutionized the field of membrane bioenergetics. According to the chemiosmotic theory membranes pump protons across the membrane generating electrochemical gradient spanning outside and inside the membrane. The gradient drives the flow of protons from outside to inside generating proton motive force which is used for many functions besides ATP generation. This mechanism operates not only in bacterial membranes but also mitochondrial as well as chloroplast membranes. In order to generate energy protons must return through specialized conductors which may be transport membrane proteins or ATP synthesizing unit or motor that drives flagellar rotation. Chemiosmotic mechanism is central to bacterial physiology and bacteria commonly use ion gradients to couple energy yielding (exergonic) and energy requiring (endergonic) reactions. Electron Transport: The energy required for the growth related biosynthetic as well as nutrient transport activities is obtained by coupling these activities to the flow of electrons in membrane and creation of electrochemical proton gradient. The electrons flow from primary electron donors to terminal electron acceptors through a series of electron carriers proteins and lipids called quinones. These flow of electron through the carrier is referred to as respiration. If the final electron acceptor is oxygen, it is termed as aerobic respiration while if it is not oxygen, it is known as anaerobic respiration (nitrate or sulfate or organic compound as Fumerate). Proton translocation takes place during respiration and electrochemical potential is created at the coupling site. The proton potential is then used to drive solute transport, ATP synthesis, flagella rotation, and other membrane activities. Electron transport pathways are almost same in bacteria and in mitochondria. Within prokaryotes the types of primary donor and terminal acceptor may vary. The Electron Carriers: There are different types of electron carrier through which electrons flow: 1. Flavoproteins (Hydrogen and electron carriers) 2. Quinones (Hydrogen and electron carrier) 3. Iron-Sulfur proteins (electron carrier) 4. Cytochromes (Electron carrier) Amongst these carriers, quinones are lipids while others are proteins and exist as multienzyme complexes known as ‘Oxidoreductases’. The electrons are carried by nonproteinic part known as ‘Prosthetic group’ such as FeS in iron-sulfur proteins and flavin in flavoprotein. Flavoproteins: Flavoproteins have organic molecule ‘flavin’ as a prosthetic group. Its yellow in color and is synthesized by cells from riboflavin, vitamin B2. Two types of flavins found are, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). When they are reduced, they carry two hydrogens/electrons. They are involved in many cytoplasmic oxidation-reduction reactions besides the electron transport. Quinones: Quinones are lipid electron carriers. Due to their hydrophobicity they are mobile in lipid bilayer and carry hydrogens and electrons to and from the immobile protein carriers. Isoprenoid side chains contribute to their hydrophobicity. Bacteria make two types of quinones, a ‘Ubiquinone’(UQ), and a ‘Menaquinone’(MK). Menaquinones are derived from Vitamin K, have lower electrode potential than UQ and are used predominantly during anaerobic respiration. A third type of quinone, known as ‘Plastoquinone’ is used in photosynthetic electron transport. Bacterial ETC…. The quinones accept electrons from dehydrogenases and transfer these to oxidase complexes that reduce the terminal electron acceptor. In bacteria terminal electron acceptor may be molecular oxygen or inorganic compound like nitrate or organic compound like fumerate under anaerobic respiration. Similarly bacteria also contain variety of terminal oxidases, e.g. quinol oxidases, cytochrome c oxidases. Sometimes 2 or 3 different oxidases may be found in the same bacteria. The oxidases differ in their affinities for oxygen as well as the type of metal and heme they contain. Some of them are proton pumps while some are not. Two major differences between the bacterial and mitochondrial ETC is that (1). The routes to oxygen in the bacteria are branched (specially under aerobic conditions), the branched point being at the quinone or cytochrome, (2). Many bacteria can alter their ETC depending upon their growth conditions. Branching gives flexibility under different growth conditions. Coupling sites: The sites in electron transport chain where redox reactions are coupled to proton extrusion creating a electrochemical potential p are called ‘Coupling sites’. Each of these sites is also a site for ATP synthesis, since the protons extruded reenter via ATP synthase to make ATP. The number of protons extruded per two electrons vary depending upon the complex. 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