REVIEWER-Chapter-5-Microbial-Metabolism PDF

CHAPTER 5 - MICROBIAL METABOLISM CHEM REACTIONS UNDERLYING METABOLISM - the collection of controlled biochemical reactions that takes place within a microbe. - ultimate function of an organism’s metabolism is to reproduce the organism and that metabolic processes are guided by the following eig...

CHAPTER 5 - MICROBIAL METABOLISM CHEM REACTIONS UNDERLYING METABOLISM - the collection of controlled biochemical reactions that takes place within a microbe. - ultimate function of an organism’s metabolism is to reproduce the organism and that metabolic processes are guided by the following eight elementary statements: Every cell acquires nutrients, which are the chemicals necessary as building blocks and energy sources for metabolism. OXIDATION AND REDUCTION REACTIONS Metabolism requires energy from light or from the catabolism (breakdown) of acquired nutrients. - many metabolic reactions involve the transfer of electron Energy is often stored in the chemical bonds of which carry energy adenosine triphosphate (ATP). Using enzymes, cells catabolize nutrient molecules to Reduction refers to the overall electrical charge on a molecule. form elementary building blocks called precursor Oxidation-reduction reactions or redox reactions - metabolites. electrons are transferred from an electron donor (a molecule Using precursor metabolites, other enzymes, and that donates an electron; oxidized) to an electron acceptor (a energy from ATP, cells construct larger building blocks molecule that accepts an electron; reduced). in anabolic (biosynthetic) reactions. Cells use enzymes and additional energy from ATP to OIL RIG: oxidation involves loss; reduction involves gain. anabolically link building blocks together to form macromolecules in polymerization reactions. o Reduction and oxidation reactions always happen Cells grow by assembling macromolecules into cellular simultaneously because every electron donated by one structures such as ribosomes, membranes, and cell chemical is accepted by another chemical. walls. A chemical may be reduced by: Cells typically divide into two once they have doubled gaining either a simple electron or in size. electron that is part of a hydrogen atom—which Metabolism can be divided into two major classes of reactions: is composed of one proton and one electron. 1. Catabolism - cells have catabolic pathways, which A molecule may be oxidized in one of three ways: break larger molecules into smaller products. by losing a simple electron - release energy, some of which is stored in ATP by losing a hydrogen atom molecules, though most of the energy is lost as heat; by gaining an oxygen atom exergonic ex. breakdown of lipids into glycerol and fatty acids. o Biological oxidations often involve the loss of hydrogen 2. Anabolism - synthesize large molecules from atoms; such reactions are also called dehydrogenation smaller ones. reactions - require energy, typically provided by ATP produced from o Electrons rarely exist freely in cytoplasm; instead, they catabolism; endergonic orbit atomic nuclei. - increasing either the concentrations of reactants or ambient temperatures increases the number of Important electron carrier molecules, derived from vitamins: collisions and produces more chemical reactions 1. Nicotinamide adenine dinucleotide (NAD+) – ex. synthesis of cell membrane lipids from transfer of two electron and a hydrogen ion glycerol and fatty acids. 2. Nicotinamide adenine dinucleotide phosphate (NADP+) - transfer of two electron and a hydrogen ion o The products of catabolism provide many of the building 3. Flavin adenine dinucleotide (FAD) – transfer of two blocks (precursor metabolites) for anabolic reactions. hydrogen These reactions produce macromolecules and cellular structures, leading to cell growth and division. Pathway – a series of reactions  CHAPTER 5 - MICROBIAL METABOLISM Oxidoreductases - remove electrons from (oxidize) or ATP PRODUCTION AND ENERGY STORAGE add electrons to (reduce) various substrates. ex. Lactic acid dehydrogenase - oxidizes Phosphorylation – inorganic phosphate (PO4 3) is added to a lactic acid to form pyruvic acid during substrate. fermentation ex. cells phosphorylate adenosine diphosphate (ADP), Transferases - transfer functional groups, such as an which has two phosphate groups, to form adenosine amino group (NH2), a phosphate group (PO4 3-), or a triphosphate (ATP), which has three phosphate groups two-carbon (acetyl) group, between molecules. ex. Hexokinase - transfers phosphate from Why is ATP well suited to serve as the primary short-term energy ATP to glucose in the first step of glycolysis carrier in metabolic pathways? Makeup of Enzymes 1. It is multifunctional as a ribonucleotide for use in Apoenzymes – proteins; inactive if they are not bound to one or synthesizing RNA. more of the nonprotein substances called cofactors. 2. It is highly water soluble and can accumulate to high - Urea as holoenzyme concentrations in cells with no ill effects. 3. It has two different levels of energy donation, ATP to Cofactors - either inorganic ions (such as Fe, Mg, Zn, or Cu ADP and ATP to AMP, depending on what is needed for ions) or certain organic molecules called coenzymes. a reaction. Fourth, ATP also can also serve as a phosphate donor. Coenzymes - All coenzymes are either vitamins or contain vitamins, which are organic molecules that are required for Cells phosphorylate ADP to form ATP in three specific ways: metabolism but cannot be synthesized by certain organisms. Substrate-level phosphorylation - involves the Holoenzyme – active enzyme formed from the binding of transfer of phosphate to ADP from another apoenzyme and its cofactor(s). phosphorylated organic compound Oxidative phosphorylation - energy from redox reactions of respiration (described shortly) is used to attach inorganic phosphate to ADP Photophosphorylation - light energy is used to phosphorylate ADP with inorganic phosphate ROLES OF ENZYMES IN METABOLISM Enzymes – organic catalysts - increase the likelihood of a reaction o The names of enzymes usually end with the suffix -ase, and the name of each enzyme often incorporates the name of Ribozymes – RNA enzymes; not all enzymes are proteins that enzyme’s substrate, which is the molecule the enzyme - process other RNA molecules by removing sections of acts on. RNA and splicing the remaining pieces together - all protein enzymes are made by ribozymes Basic categories of enzymes: Enzyme Activity Hydrolases - catabolize molecules by adding water in a decomposition process known as hydrolysis. Activation energy - amount of energy needed to trigger a Hydrolases are used primarily in depolymerizing chemical reaction catabolism of macromolecules. - enzymes catalyze reactions by lowering the AE ex. Lipase - breaks down lipid molecules Isomerases2 - rearrange atoms within a molecule but do not add or remove anything. ex. Phosphoglucoisomerase - converts glucose 6-phosphate into fructose 6- phosphate during glycolysis Ligases or polymerases - join two molecules together. They often use energy supplied by ATP. ex. Acetyl-CoA synthetase - combines acetate and coenzyme A to form acetyl-CoA for the Krebs cycle Lyases - split large molecules; not using water The activity of enzymes depends on the closeness of fit between the ex. Fructose-1,6-bisphosphate aldolase - functional sites of an enzyme and its substrate. splits fructose 1,6-bisphosphate into G3P and DHAP Active site – the shape of an enzyme’s functional site is complementary to the shape of the enzyme’s substrate  CHAPTER 5 - MICROBIAL METABOLISM o Generally, the shapes and locations of only a few amino Factors influencing the rate of enzymatic reactions: acids in a protein enzyme or nucleotides in a ribozyme determine the shape of that enzyme’s active site. A change Higher temperatures increase the rate of most chemical in a single component—for instance, through mutation— reactions, but cause the enzyme to lose structure & active can render the enzyme less effective or even completely site above optimal temperature. nonfunctional. Denatured enzymes – not functional; lose 3D structure of the Enzyme-substrate specificity - likened to the fit between a lock active site and key - Denaturation can be permanent (can’t regain original Induced-fit model - enhanced enzyme-substrate specificity; structure) or reversible (noncovalent bonds re-form). enzymes’ active sites change shape slightly when they bind to their substrate. Extreme pH can denature enzymes, disrupting an enzyme’s secondary and tertiary structures. - the enzyme’s optimal pH is approximately 7.2. As substrate concentration increases, enzymatic activity increases as more and more enzyme active sites bind more and more substrate molecules. - many enzymes are produced in the amounts and at the times they are needed to maintain metabolic activity Saturation point – at which all activities are utilized Process of the activity of enzymes, which depicts the catabolic lysis of fructose 1,6-bisphosphate: Organisms can influence enzymatic activity to control 1. An enzyme associates with its specific substrate metabolic activity. molecule, which has a shape complementary to that Allosteric – a site located away from the active site enzyme’s active site. the binding of an activator to an allosteric site causes 2. The enzyme and its substrate bind to form a temporary the enzyme’s active site to change shape, which intermediate compound called an enzyme-substrate activates the enzyme com plex. The binding of the substrate induces the enzyme to fit the shape of the substrate even more Inhibitors – block enzyme activity closely—the induced-fit model. 1. Competitive inhibitors are shaped such that they fit 3. Bonds within the substrate are broken, forming two or into an enzyme’s active site and thus prevent the more products in catabolic reactions. normal substrate from binding. 4. The enzyme dissociates from the newly formed do not undergo a chemical reaction to form products; products, which diffuse away from the site of the can bind permanently or reversibly to an active site reaction. a. Inhibitory moleculesncompete for and block 5. The enzyme resumes its original configuration and is active sites. ready to associate with another substrate molecule. b. Reversible inhibition can be overcome by an increase in substrate concentration. 2. Noncompetitive inhibitors do not attach to the active site but instead bind to an allosteric site on the enzyme. This binding alters the shape of the active site so that enzymatic activity is reduced or blocked completely. 3. Cells often control the action of enzymes through feedback inhibition (also called negative feedback or end-product inhibition) Allosteric feedback inhibition - functions in much the way a thermostat controls a heater  CHAPTER 5 - MICROBIAL METABOLISM CARBOHYDRATE CATABOLISM Glucose is catabolized via one of two processes: Cellular respiration - a process that results in the complete breakdown of glucose to carbon dioxide and water - via the Krebs cycle and an electron transport chain Fermentation which results in organic waste products. - involves the conversion of pyruvic acid into other organic compounds - lacks a Krebs cycle, resulting in less ATP production GLYCOLYSIS - first step in the catabolism of glucose, catabolizing a single molecule of glucose to two molecules of pyruvic acid (also called pyruvate) and results in ATP production. - follows the Embden-Meyerhof-Parnas (EMP) pathway Glucose catabolism begins with glycolysis, which forms pyruvic acid and two molecules of both ATP and NADH. Two pathways branch from pyruvic acid: respiration and fermentation. In aerobic respiration, the Krebs cycle and the electron transport chain completely oxidize pyruvic acid to CO2 and H2O, in the process synthesizing many molecules of ATP. Fermentation results in the incomplete oxidation of pyruvic acid to form organic fermentation products. The EMP pathway of glycolysis occurs in the cytosol and can be divided into three stages involving a total of 10 steps: 1. Energy-investment stage (stage 1-3) – the energy in two molecules of ATP is invested to phosphorylate a six-carbon glucose molecule and rearrange its atoms CELLULAR RESPIRATION to form fructose 1,6-bisphosphate 2. Lysis stage (steps 4-5) - Fructose 1,6-bisphosphate is - a metabolic process that involves the complete oxidation of cleaved into glyceraldehyde 3-phosphate (G3P)5 and substrate molecules and then production of ATP by a series dihydroxyacetone phosphate (DHAP). Each of these of redox reactions com pounds contains three carbon atoms and is freely Three stages of cellular respiration: convertible into the other. DHAP is converted to G3P before the next stage. 1. Synthesis of Acetyl-CoA – pyruvic acid must first be 3. Energy-conserving-stage (steps 6-10) - Each G3P is converted to acetyl-coenzyme A, or acetyl-CoA oxidized to pyruvic acid, yielding two ATP molecules - Decarboxylation - removing one carbon from each, for a total of four ATP; it is also oxidized to pyruvic pyruvic acid as CO2 acid, yielding another two ATP molecules, for a total of - An enzyme then joins the remaining two-carbon four ATP molecules. molecule (called acetate) to coenzyme A with a high-energy bond to form acetyl-CoA Substrate-level phosphorylation - direct transfer of the - This reaction produces one molecule of NADH phosphate between the two substrates 2. Krebs cycle - a “circular” series of eight enzymatically - PEP (one substrate) is transferred to an ADP catalyzed reactions that transfer much of this stored molecule (a second substrate) to form ATP energy via electrons to the coenzymes NAD+ and FAD - aka tricarboxylic acid or citric acid cycle Glucose is cleaved and ultimately transformed into two molecules of pyruvic acid in this process. Four ATPs are 1. Acetyl-CoA enters the Krebs cycle by joining with formed and two ATPs are used, so the result is a net gain of oxaloacetic acid to form citric acid, releasing two ATPs. Two molecules of NAD+ are reduced to NADH. coenzyme A. 2. Two oxidations and two decarboxylation and the addition of coenzyme A yield succinyl-CoA. 3. Substrate-level phosphorylation produces ATP and again releases coenzyme A.  CHAPTER 5 - MICROBIAL METABOLISM 4. Further oxidations and rearrangements - In mitochondria, the ubiquinone is called regenerate oxaloacetic acid, and the cycle can coenzyme Q. begin anew. Two molecules of acetyl-CoA enter the cycle for each molecule of glucose undergoing glycolysis. Metal-containing proteins are a mixed group of integral proteins with a wide-ranging number of iron, sulfur, or 6 types of reactions: copper atoms that can alternate between reduced and oxidized states. Anabolism (step 1) - splitting of the high-energy bond between acetate and coenzyme A releases enough Iron-sulfur proteins occur in various places in electron energy to enable the binding of the freed two-carbon transport chains of many organisms. Copper proteins are acetate to a four-carbon compound called oxaloacetic found only in electron transport chains involved in acid, forming the six-carbon compound citric acid photosynthesis Isomerization (step 2) Cytochromes are integral proteins associated with heme. Redox Reactions (step 3, 4, 6, 8) - reduce FAD to Iron can alternate between a reduced (Fe2+) state and an FADH2 (6) and NAD+ to NADH (3, 4, 8); so, for the two oxidized (Fe3+) state. molecules of acetyl-CoA derived from the original glucose molecule, six molecules of NADH and two of Electrons carried by NADH enter a transport chain earlier than FADH2 are formed. electrons carried by FADH2, which are passed to a ubiquinone. Decarboxylations (steps 3, 4) - after isomerization, Thus, more energy can be harvested from electrons carried by decarboxylations release two molecules of CO2 for NADH than from those carried by FADH2 and more molecules each acetyl-CoA that enters of ATP can be generated from NADH than from FADH2. Substrate-level phosphorylation (step 5) - For every two molecules of acetyl-CoA that pass through the Anaerobic respiration – aerobes use oxygen as the final Krebs cycle, two molecules of ATP are generated electron acceptor Hydration (step 7) Anaerobes - perform anaerobic respiration by using inorganic The Krebs cycle occurs in the cytosol of prokaryotes and in chemicals other than oxygen as the final electron acceptor. the matrix of mitochondria in eukaryotes. 3. Final series of redox reactions Chemiosmosis - general term for the use of ion gradients to generate ATP; Electron Transport that is, ATP is synthesized utilizing energy released by the flow of ions down their electrochemical gradient across a - stepwise release of energy from a series of redox reactions membrane. - consists of a series of membrane-bound carrier molecules - uses the potential energy of electrochemical gradients to that pass electrons from one to another and ultimately to a phosphorylate ADP into ATP final electron acceptor. - relates to oxidative phosphorylation - NAD+ is an “empty-handed firefighter”; NADH is a “firefighter with a bucket.” Electrochemical gradient - differences in concentration and - located in the cytoplasmic membranes of prokaryotes charge (and in the inner mitochondrial membranes of eukaryotes) Proton gradient – electrochemical gradient created from the Lithotrophs - acquire electrons from inorganic sources such as transport of protons to one side of the membrane H2, NO2 -, or Fe2+ ATP synthases (ATPases) – protein channels where protons, o Electrons pass sequentially from one membrane- propelled by the proton motive force flow down their bound carrier molecule to another, each time losing electrochemical gradient some energy. Eventually, they pass to a final acceptor Oxidative phosphorylation - proton gradient is created by the molecule. oxidation of carriers in an electron transport chain o The electrons’ energy is used to pump protons across the membrane. METABOLIC DIVERSITY Categories of carriers in the transport chains: Pentose phosphate pathway - alternative to glycolysis for the breakdown of glucose and is named for the phosphorylated Flavoproteins are integral membrane proteins, many of pentose sugars (ribulose, xylulose, and ribose) that it produces which contain flavin, a coenzyme derived from riboflavin (vitamin (vitamin B2). yields molecules of NADPH that carries electrons used - flavin mono nucleotide (FMN), which is the initial in anabolic reactions such as photosynthesis carrier molecule breakdown of glucose nets a single molecule of ATP - flavoproteins alternate between reduced and from each molecule of glucose, so the pathway is half oxidized states. as energy efficient as glycolysis Ubiquinones are lipid-soluble, nonprotein carriers that are present universally in cells, derived from vitamin K  CHAPTER 5 - MICROBIAL METABOLISM FERMENTATION PHOTOSYNTHESIS - partial oxidation of sugar (or other metabolites) to release - a process of capturing light energy from the sun and use it energy using an organic molecule from within the cell as the to drive the syn thesis of carbohydrates from CO2 and H2O final electron acceptor - metabolic reactions that oxidize NADH to NAD+ while CHEMICAL AND STRUCTURES reducing cellular organic molecules Chlorophylls – pigment molecules - Fermentation reactions can be used to identify microbes - composed of a hydrocarbon tail attached to a light- ex. NADH reduces pyruvic acid to form lactic acid absorbing active site centered around a Mg2+ vary slightly in the lengths and structures of their hydrocarbon tails and in the atoms that extend from their active sites: chlorophyll a – green plants, algae, photosynthetic protozoa, and cyanobacteria chlorophyl b chlorophyll c Photosystems - light-harvesting matrices embedded in thylakoids, cellular membranes Thylakoids - invaginations of photosynthetic prokaryotes’ cytoplasmic membranes - thylakoids of eukaryotes appear to be formed from infoldings of the inner membranes of chloroplasts Grana – stacks, arrangement of thylakoids Stroma - space between the outer membrane and the thylakoid membrane LIPID CATABOLISM Thylakoid space - a narrow, convoluted cavity - The most common lipids involved in ATP and metabolite production are fats, which typically consist of glycerol and 2 types of photosystems: fatty acids - Photosystem I (PS I) – Lipases – enzymes that hydrolyze the bonds attaching glycerol - Photosystem II (PS II) - to the fatty acid chains in the first step of triglyceride catabolism Photosystems absorb light energy and use redox reactions to Beta-oxidation – a catabolic process of degrading fatty acids store this energy in molecules of ATP and (NADPH) enzymes repeatedly split off pairs of the hydrogenated carbon atoms that make up a fatty acid and join each Light-dependent reactions – depend on light energy pair to coenzyme A to form acetyl-CoA which can then Light independent reactions - synthesize glucose from carbon feed into the Krebs cycle dioxide and water generates NADH and FADH2, resulting in ATP production LIGHT-DEPENDENT REACTIONS PROTEIN CATABOLISM Reaction center chlorophyll - the pigments of photosystem I absorb light energy and transfer it to a neighboring molecule Proteases – secreted during the first step in the process of within the photosystem until the energy eventually arrives at a protein catabolism outside the cell special chlorophyll molecule Deamination – a reaction where special enzymes split off amino Light energy excites electrons passes its excited electrons groups when amino acids are transported into the cell to the reaction center’s electron acceptor energy is used Secreted proteases hydrolyze proteins, releasing amino acids, to pump protons across the membrane, creating a proton motive which are deaminated after uptake to produce molecules used force as substrates in the Krebs cycle. R indicates the side group, In prokaryotes, protons are pumped out of the cell; in which varies among amino acids. eukaryotes, they are pumped from the stroma into the interior of the thylakoids—the thylakoid space Proton motive force - protons flow down their electrochemical gradient through ATPases, which generate ATP  CHAPTER 5 - MICROBIAL METABOLISM o Oxygenic photosynthesis: uses water and carbon Cyclic Photophosphorylation dioxide, oxygen is released as a waste product o Aerobic respiration: oxygen serves as the final electron - the final electron acceptor is the original reaction center acceptor to produce carbon dioxide and water. chlorophyll that donated the electrons - when light energy excites electrons in PS I, they pass down OTHER ANABOLIC PATHWAYS an electron transport chain and return to PS I - Anabolic reactions are synthesis reactions Noncyclic Phosphorylation Amphibolic reactions - reactions that can proceed in either - requires both PS I and PS II, generates molecules of ATP direction—toward catabolism or toward anabolism and also reduces molecules of coenzyme NADP+ to NADPH CARBOHYDRATE BIOSYNTHESIS 1. Light energy excites electrons of PS II Gluconeogenesis – pathways that enable some cells to 2. They are passed to PS I through an electron transport synthesize sugars from noncarbohydrate precursors, such as chain. (photosystem II occurs first in the pathway and amino acids, glycerol, and fatty acids photosystem I second) - amphibolic; using enzymes of glycolysis in reverse, but 3. PS I further energizes the electrons with additional light four of the reactions require unique enzymes energy and transfers them through an electron transport - highly endergonic chain to NADP+, which is thereby reduced to NADPH 4. Hydrogen ions added to NADPH LIPID BIOSYNTHESIS 5. NADPH participates in the synthesis of glucose in the Lipids - function as energy storage compounds and as light- independent reactions components of membranes Oxygenic - derive electrons from the dissociation of H2O Triglycerides - most energy-efficient energy storage form - two molecules of water give up their electrons, - cells polymerize glycerol and three fatty acids producing O2 as a waste product Phospholipids - most common form of fat synthesized in cells Anoxygenic - get electrons from inorganic compounds such as H2S, resulting in a nonoxygen waste Carotenoids – lipids; reddish pigments found in many bacterial and plant photosystems LIGHT-INDEPENDENT REACTIONS - do not require light directly; instead, they use large quantities of ATP and NADPH Carbon fixation by the Calvin-Benson cycle - involves the attachment of molecules of CO2 to molecules of ribulose 1,5- bisphosphate (RuBP). - occurs in the cytoplasm of photosynthetic bacteria or the interior stroma of chloroplasts in eukaryotes Endergonic - requires a great deal of energy AMINO ACID BIOSYNTHESIS Three steps of the Calvin-Benson cycle: Other organisms, including humans, cannot synthesize certain amino acids, called essential amino acids 1. Fixation of CO2 - An enzyme attaches 3 ex. Lactobacillus cannot synthesize any amino acids; molecules of carbon dioxide (3 carbon atoms) to 3 it acquires all of them by catabolizing proteins in its molecules of RuBP (15 carbon atoms), which are environment. then split to form 6 molecules of 3-phosphoglyceric acid (18 carbon atoms). Amination – process of converting precursor metabolites to 2. Reduction - molecules of NADPH reduce the 6 amino acids by the addition of an amine group (NH3) molecules of 3-phosphoglyceric acid to form 6 - reverse of the catabolic deamination molecules of G3P (18 carbon atoms). These ex. Formation of aspartic acid from amination of the reactions require 6 molecules of ATP and 6 Krebs cycle intermediate oxaloacetic acid molecules of NADPH 3. Regeneration of RuBP - The cell regenerates 3 molecules of RuBP (15 carbon atoms) from 5 Transamination - amine group is moved from one amino acid molecules of G3P (15 carbon atoms). It uses the to a metabolite, producing a different amino acid remaining molecule of G3P to synthesize glucose by - use a coenzyme, pyridoxal phosphate, which is reversing the reactions of glycolysis. derived from vitamin B6  CHAPTER 5 - MICROBIAL METABOLISM Cells use inhibitory and excitatory allosteric sites on enzymes to control the activity of enzymes. Feedback inhibition slows or stops anabolic pathways when the product is in abundance. Cells regulate catabolic and anabolic pathways that use the same substrate molecules by requiring different coenzymes for each. ex. NADH is used almost exclusively with catabolic enzymes, whereas NADPH is typically used for NUCLEOTIDE BIOSYNTHESIS anabolism Nucleotides - building blocks of nucleic acids, each of which Regulatory mechanisms are generally of two types: consists of a five-carbon sugar, a phosphate group, and a purine or pyrimidine base 1. Control of gene expression - cells control the amount - produced from precursor metabolites of glycolysis and timing of protein (enzyme) production and the Krebs cycle 2. Control of metabolic expression - cells control the pentose sugars—ribose in RNA and activity of proteins (enzymes) once they have been deoxyribose in DNA—are derived from ribose produced 5-phosphate from the pentose phosphate pathway phosphate group is derived ultimately from ATP Purines and pyrimidines are synthesized in a series of ATP- requiring reactions from the amino acids glutamine and aspartic acid derived from Krebs cycle intermediates, ribose 5- phosphate, and folic acid. INTEGRATION AND REGULATION OF METABOLIC FUNCTIONS Mechanisms of how cells regulate metabolism: Cells synthesize or degrade channel and transport proteins to increase or decrease the concentration of chemicals in the cytosol or organelles. ex. enzymes of beta-oxidation are not produced when there are no fatty acids to catabolize Cells often synthesize the enzymes needed to catabolize a particular substrate only when that substrate is available. ex. the enzymes of beta-oxidation are not produced when there are no fatty acids to catabolize. If two energy sources are available, cells catabolize the more energy efficient of the two. ex. a bacterium growing in the presence of both glucose and lactose will produce enzymes only for the transport and catabolism of glucose. Once the supply of glucose is depleted, lactose-utilizing proteins are produced Cells synthesize the metabolites they need, but they typically cease synthesis if a metabolite is available as a nutrient. ex. bacteria grown in an environment containing an excess of aspartic acid will cease the amination of oxaloacetic acid Eukaryotic cells keep metabolic processes from interfering with each other by isolating particular enzymes within membrane-bound organelles. ex. proteases sequestered within lysosomes digest phagocytized proteins without destroying vital proteins in the cytosol CHAPTER 5 - MICROBIAL METABOLISM