Chapter 6 Microbial Metabolism

6.1 Enzymes and Energy Drive Cellular Metabolism Metabolism is all the biochemical reactions taking place in an organism Enzymes catalyze all chemical reactions in cells Enzymes are reusable. Enzyme activity is highly specific. Enzymes have an active site. Enzymes are used in very small amounts.

Figure 06.02: The mechanism of enzyme action. Enzymes act through enzyme-substrate complexes. Enzymes increase the probability of a chemical reaction. Enzymes bind to the substrate at the active site, which is specific to the substrate. Figure 06.02: The mechanism of enzyme action.

Figure 06.03: Enzymes and activation energy. Enzymes lower the activation energy so a reaction is more likely to occur. Enzymes weaken chemical bonds in the substrate. Enzymes can: be made entirely of protein or contain a - metal ion (cofactor). - or an organic molecule (coenzyme). Figure 06.03: Enzymes and activation energy.

Figure 06.04: Metabolic pathways and enzyme inhibition. Enzymes often team up in metabolic pathways. A metabolic pathway is a sequence of chemical reactions. Each reaction is catalyzed by a different enzyme. The product of one reaction serves as the substrate for the next. Figure 06.04: Metabolic pathways and enzyme inhibition.

Figure 06.04: Metabolic pathways and enzyme inhibition. Enzyme Activity Can Be Inhibited. Feedback inhibition hinders metabolic pathways. It inhibits an enzyme in the pathway so no product is available to feed the next reaction. Other types of inhibition include: changing the shape of an active site (noncompetitive inhibition). blocking an active site (competitive inhibition). Figure 06.04: Metabolic pathways and enzyme inhibition.

Figure 06.05A: Adenosine Triphosphate. Energy in the form of ATP is required for metabolism. ATP (adenosine triphosphate) is the cellular “energy currency,” providing energy for: movement. cell division. protein synthesis, etc. Figure 06.05A: Adenosine Triphosphate.

Figure 06.05B: ATP/ADP Cycle. Energy is released from ATP when the bond holding the last phosphate group on the molecule is broken, producing: adenosine diphosphate (ADP). a free phosphate group. Adding a phosphate group to a molecule is called phosphorylation. ATP cannot be stored because it is relatively unstable. Energy must be stored in more stable forms like glycogen or lipids in microbes. Figure 06.05B: ATP/ADP Cycle.

6.2 Glucose Catabolism Generates Cellular Energy Glucose contains stored energy that can be extracted. Energy in glucose is released slowly by converting to ATP through metabolic pathways. Cellular respiration is a series of catabolic pathways for the production of ATP. If oxygen is consumed while making ATP, it is aerobic respiration. If oxygen is not used, it is anaerobic respiration. Figure 06.06: A Metabolic Pathway Coupled to the ATP/ADP Cycle.

Glycolysis is the first stage of energy extraction. Glycolysis is the splitting of 1-6 C glucose molecule into 2-3C pyruvate molecules. Figure 06.07: A Metabolic Map of Aerobic and Anaerobic Pathways for ATP Production.

Figure 06.08: The Reactions of Glycolysis. This requires 2 ATP molecules to start glycolysis. This releases 4 ATP with a net gain of 2 ATP and 2 NADH molecules Figure 06.08: The Reactions of Glycolysis.

Figure 06.10: Summary of Glycolysis and the Citric Acid Cycle. The citric acid cycle (Krebs Cycle) extracts more energy from pyruvate. Before entering the Krebs cycle, enzymes: remove a carbon from each pyruvate molecule. combine the carbon with coenzyme A (CoA) to form acetyl-CoA. This releases 2 NADH and 2 CO2. Figure 06.10: Summary of Glycolysis and the Citric Acid Cycle.

Figure 06.09: The Reactions of the Citric Acid Cycle. The Krebs cycle is like a constantly turning wheel: picking up pyruvate molecules from glycolysis. spitting out carbon dioxide, ATP, NADH, and FADH2. Figure 06.09: The Reactions of the Citric Acid Cycle.

Figure 6.10: Summary of Glycolysis and the Citric Acid Cycle. For each two pyruvate molecules that enter the cycle, the following molecules are formed 4 CO2 2 ATP 6 NADH 2 FADH2 Figure 6.10: Summary of Glycolysis and the Citric Acid Cycle.

Figure 06.12: The ATP Yield from Aerobic Respiration. Oxidative phosphorylation makes the most ATP molecules. Pairs of electrons are passed from one chemical to another (electron transport), releasing energy. The energy released is used to combine phosphate with ADP to form ATP. The electron transport chain is composed of electron carriers called cytochromes. Coenzyme carriers NADH and FADH2 provide the electrons for oxidative phosphorylation. Figure 06.12: The ATP Yield from Aerobic Respiration.

Figure 06.11: Oxidative Phosphorylation in Bacterial Cells. As electrons move down the electron transport chain they pump protons out of the cell (chemiosmosis). The protons outside the membrane build up a concentration gradient. Figure 06.11: Oxidative Phosphorylation in Bacterial Cells.

Microinquiry 6 Figure A: The Machine That Makes ATP. A channel opens and the protons flow in through a channel called ATP synthase. ATP synthase harnesses the energy from the flowing protons to phosphorylate ADP into ATP. Oxygen accepts the electron pair at the end of the chain, acquires 2 protons, and becomes water. Microinquiry 6 Figure A: The Machine That Makes ATP.

6.3 There Are Other Pathways to ATP Production Other nutrients represent potential energy sources. Many mono-, di-, and polysaccharides can be energy sources for prokaryotes. They must all be prepared before being processed by: Glycolysis. the Krebs cycle. oxidative phosphorylation. Figure 06.13: Carbohydrate, protein, and fat metabolism.

Figure 06.13: Carbohydrate, protein, and fat metabolism. Chemical bonds in fats store large amounts of energy, making fats good energy sources. Cells use proteins for energy when fats and carbohydrates are lacking. Deamination is the replacement of the amino group in a protein with a carboxyl group in protein breakdown. Fatty acids are broken down through beta oxidation. Figure 06.13: Carbohydrate, protein, and fat metabolism.

Figure 06.14A: Microbial Fermentation. Anaerobic respiration produces ATP using other final electron acceptors in the electron transport chain Anaerobic respiration produces less ATP than aerobic respiration. Figure 06.14A: Microbial Fermentation.

Figure 06.14A: Microbial Fermentation. Fermentation produces ATP using an organic final electron receptor. Fermentation is used when oxygen and other alternative electron acceptors are unavailable. Pyruvate can be converted to lactic acid to form NAD+ coenzymes so glycolysis can produce ATP from glucose. Eukaryotes also perform fermentation, such as the yeast used in alcoholic fermentation to create alcoholic beverages. Figure 06.14A: Microbial Fermentation.

6.4 Photosynthesis Converts Light Energy to Chemical Energy Photosynthesis is a process to acquire chemical energy. In photosynthesis, light energy is converted to chemical energy, which is stored as an organic compound. In prokaryotes, it is carried out in the cell membrane, in eukaryotes in organelles called chloroplasts. The green pigment chlorophyll a absorbs light energy. Some bacteria use other pigments, such as bacteriochlorophylls. Some archaea use bacteriorhodopsin. Figure 06.15A: Photosynthetic Microbes. © Dr. Dennis Kunkel/Visuals Unlimited

Figure 06.16: Photosynthesis in Cyanobacteria and Algae. Photosynthesis is divided into two sets of reactions: energy-fixing reactions. carbon-fixing reactions. Figure 06.16: Photosynthesis in Cyanobacteria and Algae.

6.5 Microbes Exhibit Metabolic Diversity Autotrophs and heterotrophs get their energy and carbon in different ways. Autotrophs synthesize their own foods from simple carbon sources like carbon dioxide. Photoautotrophs use light as their energy source. Chemoautotrophs use inorganic compounds as their energy source. Figure 06.17: Microbial Metabolic Diversity.

Figure 06.17: Microbial Metabolic Diversity. Heterotrophs gain energy and carbon from outside sources. Photoheterotrophs use light as their energy source and organic compounds as their source of carbon. Chemoheterotrophs use organic compounds both for energy and carbon sources. Saprobes feed exclusively on dead organic matter. Parasites feed on living organic matter. Figure 06.17: Microbial Metabolic Diversity.