Chapter 6 Metabolism of Microorganisms

6.1 Enzymes and Energy in Metabolism Enzymes catalyze all cellular reactions. Enzymes are not changed by the reactions and can be reused. Enzyme activity is highly specific. Enzymes act on specific substrates. Enzymes are used in very small amounts.

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 2: The mechanism of enzyme action

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 6.3: Enzymes and activation energy

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 4: Metabolic pathways and enzyme inhibition

Metabolism is regulated and can be inhibited by enzymes. 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).

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 5A Adenosine Triphosphate

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 prokaryotes).

FIGURE 5 B: The ATP/ADP cycle

6.2 The Catabolism of Glucose 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. Cells make ATP by harvesting energy through cellular respiration. If oxygen is consumed while making ATP, it is aerobic respiration. If not, it is anaerobic respiration.

Figure 06: A Metabolic Pathway Coupled to the ATP/ADP Cycle.

Glycolysis is the first stage of energy extraction. Figure 07: A Metabolic Map of Aerobic and Anaerobic Pathways for ATP Production.

Glycolysis is the splitting of 1-6C glucose molecule into 2-3C pyruvate molecules. This requires 2 ATP molecules to start glycolysis. This releases 4 ATP with a net gain of 2 ATP and 2 NADH molecules.

Figure 08: The Reactions of Glycolysis

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 CO 2. The Krebs cycle is like a constantly turning wheel: picking up pyruvate molecules from glycolysis. spitting out carbon dioxide, ATP, NADH, and FADH 2.

Figure 10: Summary of Glycolysis and the Citric Acid Cycle.

For each two pyruvate molecules that enter the cycle, the following molecules are formed 4 CO 2 2 ATP 6 NADH 2 FADH 2 Figure 09: The Reactions of the Citric Acid Cycle.

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 FADH 2 provide the electrons for oxidative phosphorylation.

Figure 12: The ATP Yield from Aerobic Respiration.

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. 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.

Figure 11: Oxidative Phosphorylation in Bacterial Cells.

6.3 Other Aspects of Catabolism 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 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 carbonyl group in protein breakdown. Fatty acids are broken down through beta oxidation.

Anaerobic respiration produces ATP using other final electron acceptors. In anaerobic respiration, anaerobes use molecules other than oxygen as the final electron receptor in the electron transport chain. Anaerobic respiration produces less ATP than aerobic respiration.

Figure 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 reform 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.

6.4 The Anabolism of Carbohydrates 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 15: Photosynthetic Microbes. © Dr. Dennis Kunkel/Visuals Unlimited

Photosynthesis is divided into two sets of reactions: energy-fixing reactions. carbon-fixing reactions. Figure 16: Photosynthesis in Cyanobacteria and Algae.

6.5 Patterns of Metabolism 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.

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 17: Microbial Metabolic Diversity.