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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 9 Metabolism: Energy Release and Conservation 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sources of energy •most microorganisms use one of three energy sources •the sun •reduced organic compounds •reduced inorganic compounds •the chemical energy obtained can be used to do work Figure 9.1 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemoorganotrophic fueling processess Figure 9.2 3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemoorganic fueling processes-respiration • most respiration involves use of an electron transport chain • aerobic respiration • final electron acceptor is oxygen • anaerobic respiration – final electron acceptor is different exogenous acceptor such as NO3-, SO42-, CO2, Fe3+ or SeO42-. – organic acceptors may also be used • as electrons pass through the electron transport chain to the final electron acceptor, a proton motive force (PMF) is generated and used to synthesize ATP 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemoorganic fueling processes - fermentation • uses an endogenous electron acceptor – usually an intermediate of the pathway used to oxidize the organic energy source e.g., pyruvate • does not involve the use of an electron transport chain nor the generation of a proton motive force • ATP synthesized only by substrate-level phosphorylation 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Overview of aerobic catabolism • three-stage process – large molecules (polymers) small molecules (monomers) – initial oxidation and degradation to pyruvate – oxidation and degradation of pyruvate by the tricarboxylic acid cycle (TCA cycle) 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. many different energy sources are funneled into common degradative pathways Figure 9.3 7 ATP made primarily by oxidative phosphorylation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Amphibolic Pathways • function both as catabolic and anabolic pathways • important ones – Embden-Meyerhof pathway – pentose phosphate pathway – tricarboxylic acid (TCA) cycle Figure 9.4 8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Breakdown of Glucose to Pyruvate • Three common routes – Embden-Meyerhof pathway – pentose phosphate pathway – Entner-Doudoroff pathway 9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Embden-Meyerhof Pathway • occurs in cytoplasmic matrix of both procaryotes and eucaryotes • the most common pathway for glucose degradation to pyruvate in stage two of aerobic respiration 10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. addition of phosphates “primes the pump” oxidation step – generates NADH high-energy molecules – used to synthesize ATP by substrate-level phosphorylation Figure 9.5 11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Summary of glycolysis glucose + 2ADP + 2Pi + 2NAD+ 2 pyruvate + 2ATP + 2NADH + 2H+ 12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Pentose Phosphate Pathway • also called hexose monophosphate pathway • can operate at same time as glycolytic pathway or Entner-Doudoroff pathway • can operate aerobically or anaerobically • an amphibolic pathway 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. oxidation steps produce NADPH, which is needed for biosynthesis Figure 9.6 14 sugar transformation reactions produce sugars needed for biosynthesis sugars can also be further degraded Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 9.7 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Summary of pentose phosphate pathway glucose-6-P + 12NADP+ + 7H2O 6CO2 + 12NADPH + 12H+ Pi 16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Entner-Doudoroff Pathway reactions of pentose phosphate pathway • yield per glucose molecule: – 1 ATP – 1 NADPH – 1 NADH Figure 9.8 17 reactions of glycolytic pathway Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Tricarboxylic Acid Cycle • also called citric acid cycle and Kreb’s cycle • common in aerobic bacteria, freeliving protozoa, most algae, and fungi • major role is as a source of carbon skeletons for use in biosynthesis 18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 9.9 19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Summary • for each acetyl-CoA molecule oxidized, TCA cycle generates: – 2 molecules of CO2 – 3 molecules of NADH – one FADH2 – one GTP 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Electron Transport and Oxidative Phosphorylation • only 4 ATP molecules synthesized directly from oxidation of glucose to CO2 • most ATP made when NADH and FADH2 (formed as glucose degraded) are oxidized in electron transport chain (ETC) 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Electron Transport Chain • series of electron carriers that operate together to transfer electrons from NADH and FADH2 to a terminal electron acceptor • electrons flow from carriers with more negative E0 to carriers with more positive E0 22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Electron transport chain… • as electrons transferred, energy released • in eucaryotes the electron transport chain carriers are within the inner mitochrondrial membrane 23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mitochondrial ETC Figure 9.11 24 electron transfer accompanied by proton movement across inner mitochondrial membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Procaryotic ETCs • located in plasma membrane • some resemble mitochondrial ETC, but many are different – different electron carriers – may be branched – may be shorter – may have lower P/O ratio 25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Electron Transport Chain of E. coli branched pathway upper branch – stationary phase and low aeration lower branch – log phase and high aeration Figure 9.12 26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oxidative Phosphorylation • Process by which ATP is synthesized as the result of electron transport driven by the oxidation of a chemical energy source 27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemiosmotic hypothesis • is the most widely accepted hypothesis to explain oxidative phosphorylation – electron transport chain organized so protons move outward from the mitochondrial matrix as electrons are transported down the chain – proton expulsion during electron transport results in the formation of a concentration gradient of protons and a charge gradient – The combined chemical and electrical potential difference make up the proton motive force (PMF) 28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PMF drives ATP synthesis • diffusion of protons back across membrane (down gradient) drives formation of ATP • ATP synthase – enzyme that uses PMF down gradient to catalyze ATP synthesis 29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inhibitors of ATP synthesis • blockers – inhibit flow of electrons through ETC • uncouplers – allow electron flow, but disconnect it from oxidative phosphorylation – many allow movement of ions, including protons, across membrane without activating ATP synthase • destroys pH and ion gradients – some may bind ATP synthase and inhibit its activity directly 30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ATP Yield During Aerobic Respiration • maximum ATP yield can be calculated – includes P/O ratios of NADH and FADH2 – ATP produced by substrate level phosphorylation • the theoretical maximum total yield of ATP during aerobic respiration is 38 – the actual number closer to 30 than 38 31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Maximum Theoretic ATP Yield from Aerobic Respiration Figure 9.15 32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Theoretical vs. Actual Yield of ATP • amount of ATP produced during aerobic respiration varies depending on growth conditions and nature of ETC • under anaerobic conditions, glycolysis only yields 2 ATP molecules 33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Anaerobic Respiration • uses electron carriers other than O2 • generally yields less energy because E0 of electron acceptor is less positive than E0 of O2 Table 9.1 34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Microbial Fermentations • oxidation of NADH produced by glycolysis • pyruvate or derivative used as endogenous electron acceptor • substrate only partially oxidized • oxygen not needed • oxidative phosphorylation does not occur – ATP formed by substrate-level phosphorylation 35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fermentations Figure 9.17 36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. homolactic fermenters heterolactic fermenters food spoilage yogurt, sauerkraut, pickles, etc. Figure 9.18 37 alcoholic fermentation alcoholic beverages, bread, etc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 9.2 38 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fermentations of amino acids • Stickland reaction – oxidation of one amino acid with use of second amino acid as electron acceptor – carried out by some Clostridium spp. Figure 9.19 39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Catabolism of Carbohydrates and Intracellular Reserves • many different carbohydrates can serve as energy source • carbohydrates can be supplied externally or internally (from internal reserves) 40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Carbohydrates • monosaccharides – converted to other sugars that enter glycolytic pathway • disaccharides and polysaccharides – cleaved by hydrolases or phosphorylases Figure 9.20 41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Reserve Polymers • used as energy sources in absence of external nutrients – e.g., glycogen and starch • cleaved by phosphorylases (glucose)n + Pi (glucose)n-1 + glucose-1-P • glucose-1-P enters glycolytic pathway – e.g., PHB PHB acetyl-CoA • acetyl-CoA enters TCA cycle 42 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lipid Catabolism • triglycerides – common energy sources – hydrolyzed to glycerol and fatty acids by lipases • glycerol degraded via glycolytic pathway • fatty acids often oxidized via β-oxidation pathway Figure 9.21 43 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. β-oxidation pathway Figure 9.22 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Protein and Amino Acid Catabolism • protease – hydrolyzes protein to amino acids • deamination – removal of amino group from amino acid – resulting organic acids converted to pyruvate, acetyl-CoA, or TCA cycle intermediate • can be oxidized via TCA cycle • can be used for biosynthesis 45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. deamination often occurs by transamination Figure 9.23 46 transfer of amino group from one amino acid to α-keto acid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemolithotrophy • carried out by chemolithotrophs • electrons released from energy source which is an inorganic molecule – transferred to terminal electron acceptor (usually O2 ) by ETC • ATP synthesized by oxidative phosphorylation 47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 9.24 48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 9.3 49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 9.4 50 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nitrifying bacteria oxidize ammonia to nitrate Figure 9.25 51 NH3 NO2 NO3 requires 2 different genera Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sulfur-oxidizing bacteria ATP can be synthesized by both oxidative phosphorylation and substratelevel phosphorylation Figure 9.26 52 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Autotrophic growth by chemolithotrophs • Calvin cycle requires NADH as electron source for fixing CO2 – many energy sources used by chemolithotrophs have E0 more positive than NAD/NADH • use reverse electron flow to generate NADH 53 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolic flexibility of chemolithotrophs • many switch from chemolithotrophic metabolism to chemoorganotrophic metabolism • many switch from autotrophic metabolism (via Calvin cycle) to heterotrophic metabolism 54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phototrophy • photosynthesis – energy from light trapped and converted to chemical energy – a two part process • light reactions in which light energy is trapped and converted to chemical energy • dark reactions in which the energy produced in the light reactions is used to reduce CO2 and synthesize cell constituents 55 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 9.5 oxygenic photosynthesis – eucaryotes and cyanobacteria anoxygenic photosynthesis – all other bacteria 56 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 9.27 57 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Light Reaction in Oxygenic Photosynthesis • chlorophylls – major light-absorbing pigments • accessory pigments – transfer light energy to chlorophylls – e.g., carotenoids and phycobiliproteins 58 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. different chlorophylls have different absorption peaks Figure 9.28 59 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Accessory pigments absorb different wavelengths of light than chlorophylls Figure 9.29 60 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Organization of pigments • antennas – highly organized arrays of chlorophylls and accessory pigments – captured light transferred to special reaction-center chlorophyll • directly involved in photosynthetic electron transport • photosystems – antenna and its associated reaction-center chlorophyll 61 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Green plant photosynthesis noncyclic electron flow – ATP + NADPH made (noncyclic photophosphorylation) cyclic electron flow – ATP made (cyclic photophosphorylation) Figure 9.30 62 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. electron flow PMF ATP Figure 9.31 63 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Light Reaction in Anoxygenic Photosynthesis • H2O not used as an electron source; therefore O2 is not produced • only one photosystem involved • uses different pigments and mechanisms to generate reducing power • carried out by phototrophic green bacteria, phototrophic purple bacteria and heliobacteria 64 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A Photosynthetic Reaction Chain Figure 9.32 65 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Purple nonsulfur bacteria electron source for generation of NADH by reverse electron flow Figure 9.33 66 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Green sulfur bacteria Figure 9.34 67 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bacteriorhodopsin-Based Phototrophy • Some archaea use a type of phototrophy that involves bacteriorhodopsin, a membrane protein which functions as a light-driven proton pump • a proton motive force is generated • an electron transport chain is not involved 68