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Chapter 14 Lecture Outline Respiration, Lithotrophy, and Photolysis Common Principles of Respiration, Lithotrophy and Photolysis Electron transport system (electron transport chain) Electron transfer reactions (oxidation-reduction reactions) A is oxidized, B is reduced Energy of electron flow powers the cell Storage of energy from electron transfer in form of an electrochemical potential (voltage) across the membrane Voltage potential includes a concentration gradient of ions (H+, Na+) plus charge difference Voltage potential drives ATP synthesis and other processes Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 2 Electricity from Iron-Reducing Bacterium Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. Soil bacteria commonly oxidize organic nutrients Geobacter metallireducens transfers electrons to iron ions (F3+) via their pili Pili act as nanowires Can power an electrical clock 3 Distinction of Respiration, Lithotrophy and Photolysis Respiration Organic molecules are electron donors (oxidation of organic electron donors) Final electron acceptor is oxygen (aerobic respiration) or inorganic molecules (anaerobic respiration) Lithotrophy Inorganic molecules are electron donors (oxidation of inorganic electron donors) Sugars, lipids, amino acids Fe2+ , H2 Final electron acceptor is oxygen or inorganic molecule Photolysis Light capture coupled to splitting of H2S or H2O Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 4 How does Fermentation fit in? Electrons passed to electron acceptors Respiration Electrons passed to through electron transport system to inorganic acceptors Aerobic respiration: O2 Anaerobic respiration: nitrogen, sulfur compounds Fermentation Electrons passed to organic receptors without electron transport system Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 5 Refresh Once More the Sources of Energy, Electrons, and Carbon Energy photo- Electrons litho- (light) vs. chemo- (inorganic) vs. organo- Carbon auto- (CO2) vs. hetero- (all else) Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 6 Electron Transport Systems Electron transport occurs on membranes Electron acceptor usually present outside cell (exogenous) Needed in large quantities for respiration Electron passage energy must be captured by cell cytoplasm Inner (cell) membrane of bacteria, archaea Inner membrane of mitochondria, chloroplasts Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. Selenium granule deposited at inner membrane 7 Members of an Electron Transport System (ETS) NADH or other electron donor Electrons Oxidoreductase protein complexes Cytochromes Colored proteins Absorbance spectrum shifts with change in redox state Cytochrome C Oxidase detected in clinical diagnostic kits Cofactors like FMN Quinones Terminal oxidase Terminal electron acceptor Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 8 The Respiratory ETS Electrons from NADH O2 release energy Too much energy to capture in one step Requires intermediates Multiple steps Common features in many ETS pathways NADH Oxidase Quinones Cytochromes Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 9 Electron Transport is Coupled to Proton Transport Sequential electron transfer yields energy to pump ions across the membrane Most often H+ Proton concentration gradient is established Concentration gradient plus charge (chemiosmosis) difference creates proton motive force Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 10 The Proton Motive Force Energy of electron transport is captured As a gradient across a membrane Gradient of protons Charge and concentration of electrons Drives protons out of cell Gradient of protons has charge, concentration Both tend to drive protons back into cell Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 11 The Chemiosmotic Hypothesis Electron transport system pumps protons out of the cell Resulting electrochemical gradient of protons drives conversion of ADP to ATP through ATP synthase Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 12 Processes Driven by Proton Motive Force Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. ATP Synthase Uptake of nutrients Drug efflux pumps Flagellar rotation 13 ATP Synthase (ATPase) Flux protons is coupled to converting ADP + Pi to ATP F0 Protons enter c subunits of the F0 complex The F0 subunits rotate relative to the F1 complex ADP + Pi F1 ATP Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 14 E. coli Respiratory ETS Animation: A Bacterial Electron Transport System Click box to launch animation Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 15 Proton Potential Creates ATP Animation: ATP Synthase Mechanism Click box to launch animation Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 16 Anaerobic Respiration Many environments lack oxygen Gut, deep soil, deep ocean Less energy producing than aerobic respiration Use other terminal electron acceptors Nitrogen compounds NO3- + 2e- + 2H+ NO2- + H2O 2 NO2- + 2e- + 4H+ 2 NO + 2H2O 2 NO + 2e- + 2H+ N2O + H2O N2O + 2e- + 2H+ N2 + H2O NO2- + 6e- + 8H+ NH4+ + 2H2O Funnels into nitrogen oxidation Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 17 Lithotrophy Reduced minerals serve as electron donors for an electron transport system Only prokaryotes can grow by metabolizing inorganic compounds without photosynthesis Fill many key niches in ecosystems Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 18 Examples for Lithotrophy Nitrogen oxidation Anaerobic ammonium oxidation plays major role in waste water treatment Sulfur oxidation production of sulfuric acid Supplemental to commercial mining Metal oxidation Hydrogenothrophy Oxidation of H2 by sulfur leads to H2S Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 19 Methanogenesis Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 20 Methanogenesis Reduction of CO2 and other single carbon compounds to methane Metabolized by methanotrophs Only observed in a special group of archaea Methanogens Found in Landfills Natural methane gas can be harvested Intestine of cows and humans Deep oceans Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 21 Do not Confuse Methanogens Generate Methanotrophs Oxidize methane (metabolize) methane Methylotrophs Oxidize single C molecules other than methane such as methanol or methylamine Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 22 Phototrophy and Photolysis Phototrophy: all forms of energy yielding metabolism that involve absorption of light energy Photolysis: light absorption coupled to splitting an electron from a molecule Photosynthesis: photolysis with CO2 fixation and biosynthesis Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 23 Photolysis Photoexcitation of a light absorbing pigment leads to electron transfer through an ETS Light-driven separation of electrons from a molecule Electron passes to quinols From quinols to cytochromes Energy of passage pumps H+ outside membrane Photolytic ETS generates a proton potential and the reduced cofactor NADH Photolytic proton potential drives ATP synthase Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 24 Lightabsorbing Pigments Used in Prokaryots Chlorophyll (in cyanobacteria) Bacteriochlorophyll In Conduct photolysis green and purple bacteria Caretenoid Accessory pigment used by purple bacteria Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 25 Common Principle of Photolysis Membrane embedded chain of oxidoreductases and quinones Common design Antenna system Reaction center complex ETS Energy carriers Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 26 Photolytic Electron Transport Chain Three systems Photosystem I Photosystem II Oxygenic Z pathway Includes PSI and II components Molecular oxygen is generated Only in cyanobacteria (and green plants) Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc. 27