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3.9 Fermentative Diversity and the Respiratory Option • Fermentation • Helps detoxify and eliminate waste products • Provides metabolites for other microbes in the environment • May help to recover additional ATP • Maintains redox balance (page 87 and Fig. 3.14) • AND……. • Helps to generate precursor metabolites for anabolism © 2015 Pearson Education, Inc. Broad Overview of Metabolism Prokaryotes will not make something if they can import it There are only a few key precursor molecules (but lots of ways to make them) Energy sources vary © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. 3.9 Fermentative Diversity and the Respiratory Option • Pentose Phosphate Pathway (Shunt) • “Alternate” pathway • Runs “parallel” to glycolysis • Different reactions thus different intermediates • Generates different precursor metabolites • Generates reducing power © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. 3.12 Respiration: Citric Acid and Glyoxylate Cycle • Citric acid cycle (CAC): pathway through which pyruvate is completely oxidized to CO2 (Figure 3.22a) (aka Krebs or TCA cycle) • Initial steps (glucose to pyruvate) same as glycolysis • Subsequently 6 CO2 molecules released and NADH and FADH generated • Plays a key role in both catabolism AND anabolism…why? © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Figure 3.22a 3.12 Respiration: Citric Acid and Glyoxylate Cycle • The citric acid cycle generates many compounds available for biosynthetic purposes • - α-Ketoglutarate and oxaloacetate (OAA): precursors of several amino acids; OAA also converted to phosphoenolpyruvate, a precursor of glucose • Succinyl-CoA: required for synthesis of cytochromes, chlorophyll, and other tetrapyrrole compounds • Acetyl-CoA: necessary for fatty acid biosynthesis © 2015 Pearson Education, Inc. In other words-the citric acid cycle generates key precursor metabolites As well as harvesting energy © 2015 Pearson Education, Inc. 3.12 Respiration: Citric Acid and Glyoxylate Cycle The Citric Acid Cycle is also a key collection point and entry point for metabolites • C4-C6 citric acid cycle intermediates (e.g., citrate, malate, fumarate, and succinate) are common natural plant and fermentation products and can be readily catabolized through the citric acid cycle alone • Fatty acids metabolized via Acetyl-CoA © 2015 Pearson Education, Inc. 3.12 Respiration: Citric Acid and Glyoxylate Cycle • Glyoxylate cycle • A variation of the citric acid cycle with glyoxylate as a key intermediate • Shares enzymes with citric acid cycle • Allows utilization of C2-C3 organic acids if larger molecules not available (Figure 3.23) • Isocitrate to glyoxylate and succinate • For anabolism or catabolism © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Figure 3.23 3.10 Respiration: Electron Carriers Important electron carriers embedded in membranes include: • NADH dehydrogenases • Flavoproteins • Cytochromes (heme) • Iron-sulfur proteins • Quinones © 2015 Pearson Education, Inc. • Respiration is much more productive than fermentation • Aerobic respiration is the most productive of all Figure 3.22b © 2015 Pearson Education, Inc. 13.5 Autotrophic Pathways (pp 390-391) • Carbon fixation is reduction of CO2 to carbohydrate-a key feature of autotrophy • At least 6 pathways exist in various Archaea and Bacteria • But the Calvin cycle (Figure 13.16, 13.17) is the most important © 2015 Pearson Education, Inc. 13.5 Autotrophic Pathways (pp 390-391) • Named for its discoverer, Melvin Calvin • Fixes CO2 into cellular material for autotrophic growth • Requires NADPH, ATP, CO2 and special enzymes e.g. ribulose bisphophate carboxylase (RubisCO), • 6 molecules of CO2 are required to make 1 molecule of glucose (Figure 13.17) © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Figure 13.17 13.5 Autotrophic Pathways: RUBISCO • Responsible for most carbon fixation on planet • Unusual enzyme-”weak”- when CO2 is low • easily inhibited by oxygen • Often found sequestered in carboxysomes to increase CO2 and lower O2 © 2015 Pearson Education, Inc. 3.13 Catabolic Diversity • Chemolithotrophy • Uses inorganic chemicals as electron donors • Examples include hydrogen sulfide (H2S), hydrogen gas (H2), ferrous iron (Fe2+), ammonia (NH3) • Begins with oxidation of inorganic electron donor • Uses an electron transport chain and transmembrane ion gradient © 2015 Pearson Education, Inc. • Dissimilative Iron Oxidizers are chemolithotrophs (13.9 and 14.15) • Oxidize Fe2+ to Fe3+ • Very widely distributed in many environments where Fe2+ is available • Autotrophic or heterotrophic • Aerobic or anaerobic • Archaea or Bacteria © 2015 Pearson Education, Inc. • Acidithiobacillus ferrooxidans is a representative iron oxidizer • Acidophile at pH 2-3 • Acid environments with Fe2+ • Fe2+ -> Fe3+ -> FeOH3 © 2015 Pearson Education, Inc. Out (pH 2) Outer membrane cyt c Electron transport generates proton motive force. Rusticyanin e– Periplasm Reverse e– flow NAD+ Q cyt bc1 e– cyt c cyt aa3 In (pH 6) + ATP NADH © 2015 Pearson Education, Inc. Cell material ADP ATP Figure 13.24 © 2015 Pearson Education, Inc. Figure 13.23 3.17 Nitrogen Fixation (Sec 3.17 also pp 438439) • Living systems require nitrogen in the form of NH3 or R-NH2 • “Fixed” or “reduced” nitrogen, not N2 • Only some prokaryotes can fix atmospheric nitrogen: diazotrophs © 2015 Pearson Education, Inc. 3.17 Nitrogen Fixation • Some nitrogen fixers are free-living, and others are symbiotic • Cyanobacteria are free-living nitrogen fixers • Soybean root nodules contain endosymbiotic Bradyrhizobium japonicum © 2015 Pearson Education, Inc. 3.17 Nitrogen Fixation • Energetically expensive (8 ATP per N atom) • Requires electron donor, often pyruvate • Reaction is catalyzed by nitrogenase • Sensitive to the presence of oxygen • Fe plus various metal cofactors • Can catalyze a variety of reactions © 2015 Pearson Education, Inc.