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Chapter 17

http://WWW.YAHOOLE.WEEBLY.COM. Chapter 17. Metabolic Diversity: Phototrophy Chemolithotrophy Anaerobes. Phototrophy. Using light as energy source. CO 2 (inorganic) is usually sole C source = autotrophy .

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Chapter 17

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  1. http://WWW.YAHOOLE.WEEBLY.COM Chapter 17 Metabolic Diversity: Phototrophy Chemolithotrophy Anaerobes

  2. Phototrophy Using light as energy source. CO2 (inorganic) is usually sole C source = autotrophy. Some organisms get energy from light, but C from organic C sources = photoheterotrophy. Photosynthetic pigments, ex. chlorophylls, capture light energy, allowing its conversion to chemical energy.

  3. Light vs. Dark Reactions Light reactions: light energy is converted to chemical energy (ex. ATP). Dark reactions: chemical energy is used to reduce CO2 to organic C compounds. Photosynthesis may be oxygenic (O2 is produced) or anoxygenic (no O2 is produced).

  4. Chlorophyll/Bacteriochlorophyll Photosynthesis occurs only in organisms that possess some type of chlorophyll. Why is chlorophyll a green? At what wavelength does chlorophyll a maximally absorb ? Chlorophyll b? Anoxygenic phototrophs have bacteriochlorophylls, ex. bacteriochlorophyll a , which absorbs maximally at 800-925 nm. Why do organisms have several kinds of chlorophylls absorbing light at different wavelengths?

  5. Photosynthetic Membranes and Chloroplasts Chlorophyll pigments and all the other components of the light-gathering apparatus are found in the photosynthetic membranes of cells. In euk. photosynthesis occurs in membrane-bound organelles called chloroplasts. The chlorophyll pigments are attached to lamellar membrane systems called thylakoids. Stacks of thylakoids are called grana. In prok. photosynthesis occurs in internal membrane systems that arise from invaginated cytoplasmic membrane.

  6. Chloroplasts

  7. Reaction Centers and Antenna Pigments Most of the photosynthetic pigments in a cell are called light-harvesting or antenna chlorophylls. Only a very small number are involved in converting light energy into ATP and are called reaction centers. Chlorosomes (specialized membrane-enclosed structures containing photosynthetic pigments in green sulfur bacteria and Chloroflexus) absorb low light intensities, enabling these organisms to grow at the lowest light intensities of any know phototrophs.

  8. Carotenoids Photoprotective accessory pigment. Are always found in phototrophic organisms. Are yellow, brown, red, or green.

  9. Phycobilins Serve as the main light-harvesting pigments of red algal chloroplasts: red pigment (phycoerythrin) - absorbs most strongly at 550nm, blue pigment (phycocyanin) - absorbs most strongly at 620 nm. Phycobiliproteins occur as aggregates called phycobilisomes attached to photosynthetic membranes. Accessory pigments allow the organism to capture more of the available light than chlorophylls could alone.

  10. Anoxygenic Photosynthesis Purple bacterial photosynthetic apparatus: reaction center, light-harvesting I and light-harvesting II components, and the cytochrome bc1 complex. The cytochrome bc1 complex is common to both respiratory and photosynthetic e- flow. The reaction center contains 3 polypeptides: L, M, and H subunits, which traverse the membrane several times, bacteriochlorophyll a, bacteriopheophytin, quinone, and a carotenoid pigment  e- transfer reactions  ATP production.

  11. Cyclic Photophosphorylation It is similar to respiration in that e- flow through the membrane establishes a proton motive force. It is unlike respiration in that there is no net input or consumption of e-  e- travel a closed route.

  12. Photosynthetic Gene Cluster These genes encode proteins involved in: (1) bacteriochlorophyll biosynthesis (2) carotenoid biosynthesis (3) polypeptides that bind pigment molecules in the reaction center and light-harvesting complexes. Synthesis of these gene products must be highly coordinated so that the correct proportions of each are available.

  13. Photosynthesis Superoperons Allow for transcription of many functionally related genes whose products interact and form the photosynthetic complexes that integrate into the membrane. O2 = master regulatory signal governing the transcription of the photosynthetic gene cluster  represses pigment synthesis in anoxygenic phototrophs so that photosynthesis occurs only under anoxic conditions.

  14. Purple Anoxygenic Phototrophic Photosynthetic Membrane

  15. Autotrophy in Purple Anoxygenic Phototrophs (e- donors and reverse e- flow) Reducing power (NADH or NADPH) must be made so that CO2 can be reduced. Reducing power = usually H2S or thiosulfate (S2O32-) = reduced substrates oxidized by cytochromes c and e- from them end up in “quinone pool” of photosynthetic membrane. e- from quinone pool must be forced backward against the thermodynamic gradient to reduce NAD+ to NADH = reverse e- flow driven by energy from the proton motive force.

  16. Autotrophy in Purple Anoxygenic Phototrophs (e- donors and reverse e- flow)

  17. Electron Flow in Purple Bacteria vs. Green Sulfur Bacteria vs. Heliobacterium

  18. Oxygenic Photosynthesis Involves 2 distinct interconnected photochemical reactions: photosystem I and photosystem II. Use light to generate ATP and NADPH. e- flow = “Z” scheme ATP is generated by noncyclic photophosphorylation.

  19. The Calvin Cycle (Autotrophic CO2 Fixation) Requires NAD(P)H and ATP Requires 2 key enzymes: ribulose bisphosphate carboxylase (RubisCO) and phosphoribulokinase. Ribulose bisphosphate + CO2  2 3-phosphoglyceric acid (PGA) 2 3-PGA  glucose (reverse glycolysis) 12 NADPH and 18 ATP are required to synthesize 1 hexose (ex. glucose) molecule from 6 molecules CO2 by the Calvin Cycle  storage polymers, ex. glycogen, starch, etc.

  20. The Calvin Cycle (Autotrophic CO2 Fixation)

  21. Carboxysomes Inclusions containing stores of RubisCO without affecting the osmolarity of the cytoplasm.

  22. Reverse Citric Acid Cycle and Hydroxypropionate Cycle (Autotrophic CO2 fixation) Alternative mechanisms for autotrophic CO2 fixation Special enzyme catalyze the reductive fixation of CO2 into intermediates of the Calvin Cycle. Hydroxyproprionate cycle = another mechanism for autotrophic CO2 fixation without using the Calvin Cycle. Hydroxyproprionate = key intermediate.

  23. Chemolithotrophy

  24. Inorganic e- Donors and Energetics Most chemolithotrophs are autotrophs. Need: (1) energy (ATP) (2) reducing power - obtained from the inorganic compound or from reverse e- transport reactions. ATP synthesis is coupled to ox. of e- donor. Sources of inorganic e- donors: H2S, H2, NH3 Must conduct energy calculations to predict the kinds of chemolithotrophs found in nature - refer to Table 17.1 of the text.

  25. Hydrogen Oxidation H2 = e- donor e- acceptors = nitrate, sulfate, ferric iron, etc. Hydrogenases catalyze reactions to obtain energy from H2 ox. and to reduce NAD+ to NADH. Most H bacteria can grow as chemoorganotrophs, but when growing as chemolithotrophs, they get C from CO2 via the Calvin Cycle.

  26. Oxidation of Reduced Sulfur Compounds e- donors = H2S, S0 (elemental sulfur), S2O32- (thiosulfate)  final product is usually SO42- + H+  H2SO4  lowers pH of medium e- for autotrophic CO2 fixation come from reverse e- flow  NADH CO2 is actually fixed via the Calvin Cycle. Most sulfur chemolithotrophs are aerobic, but some are anaerobic.

  27. Iron Oxidation Aerobic oxidation of Fe2+ (ferrous iron) to Fe3+ (ferric iron) yields a small amount of energy, so iron bacteria must ox. large amounts of iron in order to grow. These bacteria are common in coal mining dumps. Ferrous iron is stable under oxic conditions, therefore, iron-ox. bacteria are obligately acidophilic. ATP is generated via the proton motive force. CO2 is fixed via the Calvin Cycle. These organisms generate a lot of ferric iron in the environment. Ferrous iron can be ox. under anoxic cond. by anoxygenic phototrophs, but in this case, ferrous iron does not serve as e- donor for energy metabolism, but as e- donor for CO2 reduction (autotrophy).

  28. Nitrification and Anammox Most common inorganic N compounds used as e- donors are ammonia and nitrite by nitrifying bacteria under aerobic cond. Proton motive force is generated to drive ATP production. CO2 is fixed via the Calvin Cycle. NADH for the Calvin Cycle is generated via reverse electron flow. Anammox = anoxic ammonia ox. to yield nitrogen gas. These organisms are autotrophic, but lack Calvin Cycle enzymes, so mechanism of CO2 fixation is unclear.

  29. Anaerobic Respiration Ferric Iron, Mn, Chlorate, and Organic Electron Acceptors These molecules can serve as electron acceptors in anaerobic respiration.

  30. O2 as a Reactant in Biochemical Processes O2 can be incorporated into organic compounds by oxygenases, ex. for sterol biosynthesis. Monooxygenase catalyzes the oxidation of aliphatic (linear, open-chained) hydrocarbons (for energy) by the addition of O2. Aromatic (ring) hydrocarbons can be used as electron donors aerobically by microbes to generate compounds that can enter the citric acid cycle. Oxygenases are involved. Aromatic hydrocarbons can be degraded anaerobically if the hydrocarbon already contains an oxygen atom.

  31. Methanotrophy The utilization of methane as both electron donor and C source. Involves ox. of methane using methane monooxygenase.

  32. Polysaccharides  Hexoses Polymeric substances, such as polysaccharides, must first be hydrolyzed to monomeric units before energy-generating mechanisms can be employed. Cellulose and starch are 2 important polysaccharides and disaccharides, such as lactose or sucrose. These disaccharides and polysaccharides must be broken down into monomeric hexoses (6C sugars) in order to be used as electron donors for energy as well as serve as important structural components of microbial cell walls, capsules, slimes, etc.

  33. Organic Acid Metabolism A variety of organic acids can be used by microbes as C sources and electron donors. The acids of the citric acid cycle are common products formed by plants and are ferm. products of microbes. (Remember that the citric acid cycle is biosynthetic as well as bioenergetic). Oxaloacetate is regenerated for the citric acid cycle through the glyoxylate cycle. Pyruvate and other 3C compounds can be converted into oxaloacetate with specific enzymes in order to allow the citric acid cycle to continue to function.

  34. Lipids as Microbial Nutrients Extracellular enzymes called lipases break the ester bonds of glycerol and fatty acids forming fats in order to use these subunits for C and energy. Fatty acids are oxidized by a process called beta oxidation in which 2C of fatty acid are split off at a time (remember how fats are synthesized?).

  35. Nitrogen Fixation N2  nitrogenase  ammonia  organic form Nitrogenase = actually 2 enzymes: dinitrogenase and dinitrogenase reductase The triple bond between the 2 N’s make this a hard bond to break and requires a lot of energy. This reaction takes place in the absence of O2 because O2 irreversibly inactivates dinitrogenase reductase. The enzyme is often protected from oxygen in special structures called heterocysts. Nitrogen fixation is a very highly genetically regulated process.

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