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II. Energetics, Enzymes and Redox • 3.3 Energy/Carbon Source Classes of Microorganisms • 3.4 Bioenergetics • 3.6 Electron Donors and Electron Acceptors • 3.7 Energy-Rich Compounds © 2015 Pearson Education, Inc. 3.3 Energy Classes of Microorganisms • Metabolism • The sum total of all of the chemical reactions that occur in a cell • Catabolic reactions (catabolism) • Energy-releasing metabolic reactions • Anabolic reactions (anabolism) • Biosynthetic metabolic reactions © 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. 3.3 Energy/Carbon Source Classes of Microorganisms • Microorganisms have a variety of ways to conduct their metabolism • Grouped into carbon source classes • Heterotrophs rely on reduced carbon • Autotrophs fix CO2 or reduce CO2 • Grouped into energy source classes • Chemotrophs use chemical energy • Chemolithotrophs use inorganic compounds • Chemoorganotrophs use carbon compounds • Phototrophs use light energy © 2015 Pearson Education, Inc. 3.3 Energy Classes of Microorganisms • Autotrophs may be chemotrophs or phototrophs with respect to energy source • Phototrophic autotrophs (photoautotrophs) are familiar • Cyanobacteria (Sec.14.3) • Photosynthesize using chlorophyll a • Fix CO2 to carbohydrate using Calvin Cycle • Chemotrophic autotrophs (chemoautotrophs) are less familiar • Methanogens (Sec.13.20) = Archaea; sediments, animals • Reduce CO2 to CH4 with unusual enzymes for energy • Utilize methanol or acetate from CH4 for carbon © 2015 Pearson Education, Inc. 3.3 Energy Classes of Microorganisms • Heterotrophs may be chemotrophs or phototrophs with respect to energy source • Chemotrophic heterotrophs (chemoheterotrophs) are very common • Phototrophic heterotrophs (photoheterotrophs) also exist • Heliobacter (Sec. 14.8) a gram + rod • Carries out photosynthesis using bacteriochlorophyll g • Relies on pyruvate, lactate, butyrate, acetate for carbon © 2015 Pearson Education, Inc. 3.4 Bioenergetics • In any chemical reaction, some energy is lost as heat but some energy (Free energy = G): energy released that is available to do work • The change in free energy during a reaction at standard conditions is referred to as ΔG0′ • ΔG: free energy that occurs under actual conditions in a cell © 2015 Pearson Education, Inc. 3.6 Electron Donors and Electron Acceptors • Energy from oxidation–reduction (redox) reactions is used in synthesis of energy-rich compounds (e.g., ATP) • Redox reactions occur in pairs (two half reactions; Figure 3.8) • Electron donor: the substance oxidized in a redox reaction (loses electrons) • Electron acceptor: the substance reduced in a redox reaction (gains electrons) © 2015 Pearson Education, Inc. 3.6 Electron Donors and Electron Acceptors Half reaction donating e– Half reaction accepting e– Electron donor Formation of water Electron acceptor Net reaction Energy from oxidation–reduction (redox) reactions is used in synthesis of energy-rich compounds (e.g., ATP) © 2015 Pearson Education, Inc. Figure 3.8 3.6 Electron Donors and Electron Acceptors • Reduction potential (E0′): measurement of energy transfer in redox reactions or tendency to donate electrons • Expressed as volts (V) • Half reactions with highly positive reduction potentials occur easily • Reduced substance with a more negative E0′ donates electrons to the oxidized substance with a more positive E0′ © 2015 Pearson Education, Inc. 3.6 Electron Donors and Electron Acceptors • The redox tower represents the range of possible reduction potentials (Figure 3.9) • The reduced substance at the top of the tower donates electrons • The oxidized substance at the bottom of the tower accepts electrons • The farther the electrons "drop," the greater the amount of energy released © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Figure 3.9 3.6 Electron Donors and Electron Acceptors • Redox reactions usually involve reactions between intermediates (carriers) • Electron carriers are divided into two classes • Prosthetic groups (attached to enzymes) • Example: heme • Coenzymes (diffusible) • Examples: NAD+, NADP © 2015 Pearson Education, Inc. 3.7 Energy-Rich Compounds • Chemical energy released in redox reactions is primarily stored in certain phosphorylated compounds (Figure 3.12) • ATP; the prime energy currency • Phosphoenolpyruvate • Glucose 6-phosphate • Chemical energy also stored in coenzyme A, a high energy sulfur compound © 2015 Pearson Education, Inc. Anhydride bonds Ester bond Ester bond Anhydride bond Phosphoenolpyruvate Adenosine triphosphate (ATP) Glucose 6-phosphate Compound Thioester bond Acetyl Anhydride bond Coenzyme A Acetyl-CoA © 2015 Pearson Education, Inc. Acetyl phosphate G0′kJ/mol ΔG0′< 30kJ Phosphoenolpyruvate 1,3-Bisphosphoglycerate Acetyl phosphate ATP ADP Acetyl-CoA –51.6 –52.0 –44.8 –31.8 –31.8 –35.7 ΔG0′< 30kJ AMP Glucose 6-phosphate –14.2 –13.8 Figure 3.12 3.7 Energy-Rich Compounds • Long-term energy storage involves insoluble polymers that can be oxidized to generate ATP • Examples in prokaryotes • Glycogen • Poly-β-hydroxybutyrate and other polyhydroxyalkanoates • Elemental sulfur • Examples in eukaryotes • Starch • Lipids (simple fats) © 2015 Pearson Education, Inc. 3.7 Energy-Rich Compounds • An energy rich compound that also serves as an electron carrier • Reduced NAD carries high energy electrons • Provides “reducing power” for biosynthesis © 2015 Pearson Education, Inc. Modes of ATP Synthesis • Substrate level phosphorylation • Transmembrane gradient (eg proton gradient or pmf) • Respiration • Aerobic • Anaerobic • Photosynthesis • Anoxygenic • Oxygenic © 2015 Pearson Education, Inc. • Direct transfer of high energy phosphate © 2015 Pearson Education, Inc. • Example: pyruvate kinase reaction • Part of catabolic pathway called glycolysis or EMP pathway • PEP is a product of another reaction Substrate level phosphorylations are common in all organisms © 2015 Pearson Education, Inc. Metabolic pathways that use a gradient to make ATP • Respiration • Aerobic-use oxygen as final electron acceptor • Anaerobic-use something other than oxygen as final acceptor • Photosynthesis • Anoxygenic-use something other than water as original electron donor • Oxygenic-use water as original electron donor © 2015 Pearson Education, Inc. Transmembrane gradient © 2015 Pearson Education, Inc. Figure 3.20 • Transmembrane gradient • Fig. 3.20 shows NADH as the electron donor • And oxygen as the electron acceptor • But…………. • MICROBES CAN USE MANY OTHER SUBSTANCES AS DONORS OR ACCEPTORS!!!!!! © 2015 Pearson Education, Inc. • ATP Synthase aka F1F0 Synthase or ATPase • Controlled entry of protons drives ATP synthesis • 3-4 protons/ATP • Can work in reverse © 2015 Pearson Education, Inc. • Anaerobic respiration • Nitrate (NO3–), ferric iron (Fe3+), sulfate (SO42–), carbonate (CO32–), fumarate, DMSO are examples of acceptors • Less energy released compared to aerobic respiration • “Primitive” form of respiration? © 2015 Pearson Education, Inc. Figure 13.39 © 2015 Pearson Education, Inc. Figure 13.39 Nitrate Reduction is a Good Example of Anaerobic Respiration (Ch.13.16 and 13.17) • Nitrate Reduction = use of nitrate as electron acceptor in anaerobic respiration (nitrate to nitrite) • A dissimilative process • Conversion of nitrate to more reduced substances such as N2O or N2 = denitrification © 2015 Pearson Education, Inc. Nitrate reductase Nitrate reduction (Escherichia coli) Nitrite reductase Nitric oxide reductase Denitrification (Pseudomonas stutzeri) Gases Nitrous oxide reductase © 2015 Pearson Education, Inc. Figure 13.40 © 2015 Pearson Education, Inc. Figure 13.41 • Lithotroph: type of chemotroph that uses inorganic substance (e.g. a mineral) as a source of energy: in other words as an electron donor. • Anaerobic respiration: process in which electrons are transferred to a final acceptor that is not oxygen-can be organic or inorganic © 2015 Pearson Education, Inc. Photosynthesis • Uses light energy to remove an electron from an electron donor and boost it to a high energy level • Photons captured by pigment molecules • Chlorophylls, bacteriochlorophylls, accessory pigments such as carotenoids • Absorb different wavelengths © 2015 Pearson Education, Inc. Photosynthesis-Anoxygenic • Anoxygenic photosynthesis uses an electron donor other than water • H2S is a common donor • Green sulfur bacteria, purple bacteria, Heliobacter, some Archaea • Use a transmembrane gradient to generate ATP via cyclic photophosphorylation © 2015 Pearson Education, Inc. © 2015 Pearson Education, Inc. Photosynthesis-Oxygenic • Oxygenic photosynthesis uses water as electron donor • Cyanobacteria • 2 Photosystems linked by electron carriers • Use a transmembrane gradient to generate ATP via either cyclic photophosphorylation or noncyclic photophosphorylation © 2015 Pearson Education, Inc. Makes reducing power in the form of NADPH When reducing power is not needed-cyclic photophosphorylation © 2015 Pearson Education, Inc. III. Fermentation and Respiration Overview • 3.8 Glycolysis • 3.9 Fermentative Diversity and the Respiratory Option • 3.10 Respiration: Electron Carriers • 3.11 Respiration: The Proton Motive Force • 3.12 Respiration: Citric Acid and Glyoxylate Cycle • 3.13 Catabolic Diversity © 2015 Pearson Education, Inc. III. Fermentation and Respiration Overview • Two key metabolic pathways • Complementary • Overlapping • Definition depends on context-industrial, medical, biochemical © 2015 Pearson Education, Inc. Fermentation • In food science fermentation can refer to the production of foods such as yogurt • In chemical engineering it can refer to the production of ethanol as an additive for gasoline • In microbiology it refers to the breakdown of carbon compounds (eg glucose) to smaller compounds with a limited harvest of energy through substrate level phosphorylation and no oxygen used © 2015 Pearson Education, Inc. Respiration • In medicine or exercise science respiration refers to breathing • In microbiology respiration refers to the removal of electrons from a substance and their transfer to a terminal acceptor with a significant harvest of energy through oxidative phosphorylation (redox reactions). Oxygen may be used as the terminal acceptor (aerobic respiration) or not (anaerobic respiration). © 2015 Pearson Education, Inc. • Fermentation: substrate-level phosphorylation; ATP is directly synthesized from an energy-rich intermediate • Respiration: oxidative phosphorylation; ATP is produced from proton motive force formed by transport of electrons © 2015 Pearson Education, Inc. Fermentation • A basic and important process for microorganisms • A sugar is the starting material and the end product depends on the species • Three Stages (I) Preparation, (II) Energy Harvesting, (III) Reboot • Reboot stage is very diverse! © 2015 Pearson Education, Inc. 3.8 Glycolysis • Stage I and Stage II (Figure 3.14) called Glycolysis (Embden–Meyerhof-Parnas or EMP pathway): a common pathway for catabolism of glucose • End product of glycolysis is pyruvate (pyruvic acid) • In fermentation pyruvate is processed through Stage III • It accepts electrons so is reduced © 2015 Pearson Education, Inc. Fermentation with lactic acid produced © 2015 Pearson Education, Inc. Figure 3.14 3.9 Fermentative Diversity and the Respiratory Option • Fermentations may be classified by products formed (See Sec.13.12) in Stage III • Ethanol • Lactic acid (homolactic vs heterolactic) • Propionic acid • “Mixed acids” • Butyric acid (extra ATP generated) • Butanol © 2015 Pearson Education, Inc. 3.9 Fermentative Diversity and the Respiratory Option • Fermentations may be classified by substrate fermented (See Sec.13.12) • Usually NOT glucose • Amino acids • Purines and pyrimidines • Aromatic compounds © 2015 Pearson Education, Inc. 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) of NAD and NADH. © 2015 Pearson Education, Inc.