Microbial Metabolism

Microbial Metabolism Cellular Respiration and Fermentation What happens after glycolysis?  After glucose is broken down to pyruvic acid, pyruvic acid can be channeled into either Fermentation OR  Cellular Respiration   Aerobic respiration    Requires oxygen Final electron acceptor is O2 Anaerobic respiration   No oxygen Final electron acceptor is an inorganic molecule other than O2 Aerobic Respiration  Tricarboxylic acid (TCA) cycle Kreb’s cycle or citric acid cycle  A large amount of potential energy stored in acetyl CoA is released by a series of redox reactions that transfer electrons to the electron carrier coenzymes (NAD+ and FAD)  Acetyl CoA     Where does it come from? Pyruvic acid, from glycolysis, is converted to a 2-carbon (acetyl group) compound (decarboxylation) The acetyl group then combines with Coenzyme A through a high energy bond NAD+ is reduced to NADH TCA cycle Pyruvate NAD+ CoA  For every molecule of glucose (2 acetyl CoA) the TCA cycle generates     CO2 NADH FADH2 ATP NADH CO2 CoA Acetyl-CoA CoA NADH Oxaloacetate Citrate NAD+ Isocitrate NAD+ Malate NADH CO2 H2O Fumarate α-ketoglutarate NAD+ FADH2 CoA P FAD Succinate CoA ATP CoA Succinyl-CoA ADP CO2 NADH Where to now?  All the reduced coenzyme electron carriers make their way to the electron transport chain NADH from glycolysis  NADH from pyruvic acid to acetyl CoA conversion  NADH and FADH2 from the TCA cycle   The electron transport chain indirectly transfers the energy from these coenzymes to ATP The electron transport chain     Sequence of carrier molecules capable of oxidation and reduction Electrons are passed down the chain in a sequential and orderly fashion Energy is released from the flow of electrons down the chain This release of energy is coupled to the generation ATP by oxidative phosphorylation Membrane location of the ETC  The electron transport chain is located in the inner membrane of the mitochondria of eukaryotes  the plasma membrane of prokaryotes  The ETC players   Three classes of ETC carrier molecules Flavoproteins Contain a coenzyme derived from riboflavin  Capable of alternating oxidations/reductions  Flavin mononucleotide (FMN)   Cytochromes   Have an iron-containing group (heme) which can exist in alternating reduced (Fe2+) and oxidized (Fe3+) forms Coenzyme Q (Ubiquinone)  Small non protein carrier molecule Are all ETCs the same?  Bacterial electron transport chains are diverse     Particular carriers and their order Some bacteria may have several types of electron transport chains Eukaryotic electron transport chain is more unified and better described All have the same goal to capture energy into ATP The ETC sets up a proton gradient   As energetic electrons are passed down the ETC some carriers (proton pumps) actively pump H+ across the membrane Proton motive force results from an excess of protons on one side of the membrane Generation of ATP by chemiosmosis   Protons can only diffuse back along the gradient through special protein channels that contain the enzyme ATP synthase ATP synthase uses the energy released by the diffusion of H+ across the membrane to synthesize ATP from ADP ETC drives chemiosmosis   NADH FADH2 ATP ATP Why aren’t electrons transferred directly to O2?    Electrons from the breakdown of nutrients are transferred through various carriers to O2 This allows the release of small packets of energy that can be efficiently trapped and stored in ATP If these electrons were transferred directly from NADH to O2, the release of energy would be too large and most would be lost from the cell Aerobic respiration     Complete oxidation of glucose molecule generates 38 ATP in prokaryotes from each of glycolysis and the TCA cycle by substrate level phosphorylation 34 from oxidative phosphorylation as a result of 10 NADH and FADH2 from glycolysis and the TCA cycle Eukaryotes only produce 36 ATP from aerobic respiration Anaerobic Respiration   Like aerobic respiration, it involves glycolysis, the TCA cycle and an electron transport chain But  The final electron acceptor is an inorganic molecule other than O2     Some bacteria use NO3- and produce either NO2-, N2O or N2 (Pseudomonas and Bacillus) Desulfovibrio use SO42- to form H2S Methanogens use carbonate to form methane The amount of ATP generated varies with the pathway    Only part of the TCA cycle operates under anaerobic conditions Not all ETC carriers participate in anaerobic respiration ATP yield never as high as aerobic respiration Fermentation      Releases energy from sugars or other organic molecules Does not require O2 (can occur in the presence of O2) Does not use the TCA cycle or ETC, but does use glycolysis Uses an organic molecule as the final electron acceptor Produces only small amounts of ATP (from glycolysis) Most of the energy remains in the end product Recycling the NAD   In fermentation, pyruvic acid or its derivatives are reduced by NADH to fermentation end products Ensures recycling of NAD+ for glycolysis Why bother with fermentation?   Fermenting bacteria can grow as fast as those using aerobic respiration by markedly increasing the rate of glycolysis Fermentation permits independence from molecular oxygen and allows colonization of anaerobic environments Types of fermentation  Alcohol fermentation  Acid fermentation Homolactic  Only lactic acid  Streptococcus and Lactobacillus  Heterolactic  Mixture of lactic acid, acetic acid and CO2   Can result in food spoilage  Can produce Yogurt from milk  Sauerkraut from fresh cabbage  Pickles from cucumber  Bring on the good stuff  Alcohol fermentation by the yeast Saccharomyces is responsible for some of the better things in life  CO2 produced causes bread to rise  Ethanol is used in alcoholic beverages End products of fermentation Metabolic pathways of Energy Use    The complete oxidation of glucose to CO2 and H2O is considered an efficient process But, 45% of the energy from glucose is lost as heat (would be much higher without the ETC) Cells use the remaining energy (in ATP) in a variety of ways E.g., active transport of molecules across membrane or flagella motion  Most is used for the production of new cellular components  Integration of metabolic pathways     Carbohydrate catabolic pathways are central to the supply of cellular energy in most microorganisms However, rather than being dead end pathways, several intermediates in these pathways can be diverted into anabolic pathways This allows the cell to derive maximum benefit from all nutrients and the metabolites in the cell pool Amphibolism-integration of catabolic and anabolic pathways to improve cell efficiency Amphibolic view of metabolism    Principle sites of amphibolic interaction Glycolysis  glyceraldehyde-3phosphate  pyruvate TCA cycle  acetyl-CoA  oxaloacetic acid  α-ketoglutaric acid