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Week 3 LectureOctober 2001 Metabolism Continued
Lecture Review • Metabolism Basics • Aerobic Metabolism of Organics
This Week’s Lecture • Anaerobic Respiration • nitrate (NO3-), CO2, sulfate (SO42-), ferric iron (Fe3+), organics, and others • Fermentation • Syntrophic Association During Conversion of Mixed Acid Products to Methane • Chemolithotrophy • Photosynthesis
Aerobic Respiration Overview • carbon flows to carbon dioxide • electrons flow to external acceptor • energy produced by oxidative phosphorylation through PMF
ADP GDP ATP GTP Respiration of Glucose glucose glycolysis pyruvate ½ O2 Citric Acid Cycle e- Electron Transport System H20 Electrons flow in the form of reduced dinucleotides (NADH and FADH) CO2
Question? • What happens when the environment is anoxic or anaerobic? • What is the difference between anaerobic and anoxic? • What impact does this have on organic carbon biodegradation? • What is the significance of these changes in environmental management and design?
Anaerobic Respiration • Some bacteria are capable of aerobic respiration and anaerobic respiration (aerobic is preferred due to more favorable energy production) • Other bacteria that carry out anaerobic respiration are obligate anaerobes • In either case, the electron acceptor chosen is based on maximizing free energy production for cell growth
Anaerobic (Anoxic) Respiration of Organics • Organic compounds are most often the original electron donor • Most electron acceptors are inorganics • Electron transport systems in anaerobic respiration is similar to that of aerobic metabolism
-0.50 + 0.90 Examples of Anoxic Respiration NAD+/NADH2 So/HS- • terminal electron acceptors other than oxygen used • less energy produced • carbon flow the same as in aerobic respiration CO2/CH4 SO4/S2- increasing energy production Eo’ Fumarate/Succinate NO3- /NO2- Fe3+/Fe2+ ½ O2/H20
Nitrate Reduction(Denitrification) • Conversion of nitrate (NO3-) as an electron acceptor to ammonia (NH4+) or nitrate (NO2-) • Nitrite undergoes further reduction to produce nitric oxide (NO), nitrous oxide (N2O), and nitrogen gas (N2), all of which are lost to the atmosphere • Denitrification results in a loss of nitrogen from ecosystems and is only carried out biologically by bacteria • Nitrogen removal treatment processes incorporate denitrification
Aerobic Respiration and Denitrification • During aerobic respiration, three areas where H+ is pumped out to establish PMF
Denitrification • Only two areas in ETC that pump out H+ as compared to three for aerobic respiration • Less energy generated
Methanogenesis and AcetogenesisCO2 as an electron acceptor CO2 acetogenesis methanogenesis H2 acetate • CH4
Sulfate Reduction • sulfate (SO4)reduction to sulfide (S2-)requires eight electrons • the first intermediate in this process is the production of sulfite (SO32-) and requires two electrons • conversion of sulfite to sulfide requires an additional six electrons
Why does sulfate inhibit methane formation? • Hydrogen is needed for both processes • Sulfate/sulfide (SO4/S2-) redox pair has a more positive reduction potential • How would sulfate presence in an anaerobic digester affect methane formation?
Iron Reduction • Ferric iron (Fe3+) reduction to ferrous iron (Fe2+) • Relatively large positive Eo’ indicates that Fe3+ is an attractive electron acceptor • Ferrous iron is much more soluble and this process has been used in mining iron ore • Because of the high concentrations of iron in some groundwaters, iron reduction is a common reaction in groundwater remediation • Very little Fe3+ in surface waters at neutral pH
Other Metals as Electron Acceptors • Mn+4 to Mn+2 • important in drinking water and groundwater systems • Cr+6 to Cr+3 • Cr+3 much less toxic and soluble and is precipitated out • AsO43- to AsO33- • mining wastes • SeO42- to SeO42- • major problem in agriculture lands in California
If there are no external electron acceptors?? • Suppose there are no electron acceptors like nitrate, various metals, etc. • What happens to the electrons associated with the organic carbon that is oxidized? • How do cells handle this condition? • What is this called?
Fermentation • organic compounds serve as both e donor and acceptor • no externally supplied e donor • oxidized and reduced products formed • carbohydrates are primary fermentable substrates • ATP production occurs via substrate level phosphorylation
Fermentation • Fermentation reactions are important in: • wastewater treatment processes • phosphorus removal • sludge digestion • BOD removal • wetland systems, especially in bottom • sediments (PCB dechlorination) • agricultural management plans for manure • landfill leachate management
Fermentation Carbon and Energy Flow Organic Compound e donor intermediate P intermediate ~P substrate level phosphorylation ADP electron carrier ATP Oxidized Organic intermediate e acceptor Reduced Organic fermentation product
NADH NAD+ NAD+ NADH NADH NAD+ Pyruvic Acid Fermentation organics ADP ATP pyruvic acid acetaldehyde + CO2 lactic acid mixed acids ethanol
pyruvic acid butyric acid propionic acid formic acid acetic acid CO2 H2 Mixed Acid Fermentation Complex Organics
Mixed Acid Fermentation • important in the breakdown of organic compounds in anaerobic environments • primary products are organic acids, carbon dioxide, and hydrogen
Conversion of Mixed Acid Fermentation Products to Methane • acetic acid and carbon dioxide are converted to methane in anaerobic environments • hydrogen is consumed in the process • butyric and propionic acid are not converted directly to methane
CH4 CO2 Methane Formation pyruvic acid butyric acid propionic acid formic acid acetic acid H2
CH4 CO2 Methane Formation pyruvic acid butyric acid propionic acid DGo’ + DGo’ + formic acid acetic acid CH4 CO2 H2
Mixed Acid Conversion to Acetic Acid • Breakdown of acids such as butyric and propionic to acetic is required prior to methane formation • This breakdown is energetically non-favorable at standard conditions • How do organisms alter the environment to achieve this reaction?
Non-Standard Conditions DG =DGo’ + RT ln ([C][D]/[A][B]) • Conversion of butyric and propionic acids results in acetic acid and H2 • H2 is consumed by methanogens in the conversion of both acetic acid and CO2 to methane • The reduction in H2 makes these reactions possible by lowering the product concentrations in the above equation
Syntrophic Association • Where a H2 producing organism can only grow in the presence of a H2 consuming organism • The coupling of H2 formation and use is called interspecies hydrogen transfer • If H2 builds up in a process it is indicative of an unbalanced consortium • A H2 build-up will result in a build up of acids resulting in pH decreases and process failure
Fermentation Summary • Little free energy available for growth • for example in glucose fermentation to ethanol, 2 moles of ATP produced/mole of glucose • Most energy is tied up as products (alcohols, acids, methane, H2) • These products produced as intermediate electron acceptors are reduced • A key intermediate is pyruvate
Can other substances besides organic carbon serve as electron donors?
Chemolithotrophy • theoxidation of inorganics for production of cellular energy • terminal electron acceptor is typically oxygen • most lithotrophs are also autotrophs • accordingly, during lithotrophy there is a need to not only produce energy in the from of ATP but also reduced electron carriers to reduce CO2 to cell carbon
Electron Flow in Lithotrophs e- donor oxid e- donor • energy gained from e- flow through ETC is used to drive reverse electron transport against an unfavorable reduction potential to form NADPH and then reduce CO2 e- e- e- CH2O O2 NADP+ Electron Transport Chain CO2 NADPH H20 ADP ATP
Electron Donors for Chemolithotrophy -0.50 • the greater the reduction potential differences between the donor and oxygen, the greater the energy available for growth 2H+/H2 S0/HS- SO42-/HS- 0.0 NH2OH/NH4+ Eo’ NO3- /NO2- Fe3+/Fe2+ ½ O2/H20 primary electron acceptor for lithotrophy + 1.00
Hydrogen Oxidation • chemolithotrophs use hydrogen as an energy source for growth • those bacteria that use hydrogen as an electron donor and oxygen as a terminal acceptor are referred to as hydrogen bacteria (versus methanogens) • Typically these bacteria are autotrophs that convert carbon dioxide to cell carbon via the Calvin cycle. The energy for this comes from oxidation of hydrogen using oxygen as an electron acceptor 6H2 + 2O2 + CO2 CH20 + H20
Sulfur Oxidation • Oxidation of hydrogen sulfide (H2S), elemental sulfur (So) and thiosulfate (S2O32-) • Final Product is sulfate (SO42-) • Very important in acid mine drainage, biological corrosion
So + H20 + 1/2O2 SO42- + 2H+ S2O32- + H20 + 2O2 2SO42- + 2H+ HS- + 1/2O2 + H+ So + H20 Sulfur Oxidation Reactions H2S + 2O2 SO42- + H+ sulfur storage as granules
4Fe2+ + O2 + 4 H+ 4 Fe3+ + 2 H20 Iron Oxidizing Bacteria • At neutral pH and ambient conditions, ferrous iron (Fe2+) oxidizes quickly to ferric iron (Fe3+) • Under acid conditions this reaction does not occur spontaneously • Lithotrophs (Iron bacteria )biologically convert Fe2+ to Fe3+ under these conditions • Oxidation of iron results in little energy production because reduction potential of to Fe3+ /Fe2+ is so close to that of oxygen/water
Nitrification • Conversion of ammonium (NH4+) to nitrate (NO3-) • Nitrite (NO2-) is an intermediate • Nitrification is a very important process agriculturally as it leads to the oxidation of ammonia to nitrate and potential nitrogen loss through denitrificiation • In wastewater treatment, nitrification is often needed to reduce the oxygen demand the effluent
NH4+ + 3/2O2NO2-+ H20 + 2H+ NO2-+ 1/2O2NO3- Nitrification Reactions Nitrosomonas • Oxidation of ammonia results in production of acidic conditions • Very little energy available to nitrifiers because reduction potential relatively close to that of oxygen Nitrobacter
Energy Production During Phototrophy • the ability to photosynthesize is based on light sensitive pigments called chlorophylls • all cells have chlorophyll A and typically some others • photosynthesis converts light energy to chemical energy • chemical energy produced is used for cell growth in phototrophs which typically are autotrophs (energy is required to reduce CO2 to cellular carbon) • photosynthesis occurs in both anaerobic and aerobic environments
Anoxygenic Photosynthesis • energy production in anoxygenic photosynthesis occurs as a result of electron flow through an electron transport chain • electron flow is cyclic • membrane mediated • process is similar to that in aerobic respiration and some electron transport components are common to both systems
P870 Center Electron Flow and Energy Production in Anoxygenic Photosynthesis -0.60 e- P870 Center act. ADP PMF Eo’ Electron Transport Chain e- ATP Antenna pigment complex + 0.40 e-
reducing power Photoautotrophs • Phototrophs tend to be autotrophs • As such, there is a need to reduce inorganic carbon (CO2) to organic carbon CO2 CH2O • To reduce CO2 to organic carbon takes reducing power (NADPH) • Autotrophs use reverse electron transport to produce NADPH
P870 Center Use of External Electron Acceptors in Anoxygenic Photosynthesis NADP+ e- P870 Center act. e- NADPH ADP PMF Electron Transport Chain e- ATP Antenna pigment complex e- H2S e-
Oxygenic Photosynthesis • involves two distinct but interconnected photoreaction centers • electron flow is non-cyclic • water is the primary electron donor • as water is oxidized, oxygen is liberated • in Eukaryotic organisms oxygenic photosynthesis occurs in chloroplast membranes • in Prokaryotic organism, oxygenic photosynthesis occurs in cytoplasmic membrane
Electron Transport e- Photosystem I e- Photosystem II Oxygenic Photosynthesis NonCyclic Electron Flow -1.0 NADP+ NADPH e- Eo’ ADP + 0.8 H20 PMF 1/2O2 and 2H+ ATP