WHY IS THIS IMPORTANT?

Transcription

1 CHAPTER 3 ESSENTIALS OF METABOLISM WHY IS THIS IMPORTANT? It is important to have a basic understanding of metabolism because it governs the survival and growth of microorganisms The growth of microorganisms can have a direct effect on infectious disease Good metabolic function makes pathogens more successful at causing disease OVERVIEW Essentials of Metabolism BASIC CONCEPTS ENZYMES CATABOLISM ANABOLISM OF METABOLISM 1

2 BASIC CONCEPTS OF METABOLISM Metabolism is: A series of chemical processes that go on in living organisms Used to obtain energy Linked to growth BASIC CONCEPTS OF METABOLISM Carbon and energy are required for growth The body has two processes by which it can obtain carbon: Autotrophy carbon is obtained from inorganic substances Heterotrophy carbon is obtained from other organic molecules Nearly all infectious organisms are chemoheterotrophs Chemoheterotrophs obtain energy by breaking down other organic molecules and compounds OXIDATION & REDUCTION REACTIONS Metabolism is broken down into two parts: Catabolism molecules are broken down through metabolic processes to release the energy stored in their chemical bonds Anabolism metabolic processes in which the energy derived from catabolism is used to build large organic molecules from smaller ones Both processes involve electron transfer and oxidation and reduction reactions 2

3 OXIDATION & REDUCTION REACTIONS An oxidation reaction is a chemical reaction in which an atom, ion or molecule loses one or more electrons A reduction reaction is a chemical reaction in which an atom, ion or molecule gains one or more electrons OXIDATION & REDUCTION REACTIONS Oxidation and reduction reactions always occur together The combination of an oxidation reaction and a reduction reaction are jointly referred to as redox reactions When a substance is oxidized, it loses electrons When a substance is reduced, it gains electrons RESPIRATION In metabolism, respiration occurs at the cellular level and is not the same as breathing (respiration at the macroscopic level) Cellular respiration describes catabolic processes and is divided into: Aerobic respiration metabolism that uses oxygen Anaerobic respiration metabolism that does not use oxygen Facultatively anaerobic respiration metabolism that can use oxygen but can also occur without it 3

4 METABOLIC PATHWAYS Metabolic reactions occur in series of chemical reactions called pathways The following is an example of a pathway. A is the initial substrate and E is the final product of the pathway, with B, C, and D being intermediates A B C D E Each step in the pathway is mediated or facilitated by a specific enzyme ENZYMES Enzymes are proteins that act as catalysts for metabolic reactions, making the reaction go faster Each enzyme is specific for a reaction Enzymes are found in all living organisms and most cells contain hundreds of types which are constantly being manufactured and replaced Enzymes work by lowering the energy of activation ENZYMES 4

5 PROPERTIES OF ENZYMES Enzymes have specific three-dimensional shapes: if the shape changes, activity is inhibited The shape of the molecule provides a distinctive site called the active site. It is here that: The substrate fits into the enzyme and the reaction occurs The enzyme and substrate interact to form the enzymesubstrate complex The active site has to have the proper shape for the enzyme to work PROPERTIES OF ENZYMES Enzymes are generally highly specific A given enzyme catalyzes only one type of reaction Most enzymes react with only one particular substrate The shape of an enzyme molecule and the electrical charges found at the active site allow for the reaction to work and are responsible for the enzyme s specificity COENZYMES AND COFACTORS Many enzymes can catalyze a reaction only if other substances are present at the active site Cofactors are helper substances that are inorganic ions such as magnesium, zinc, manganese, or iron Coenzymes are helper substances that are nonprotein organic molecules Cofactors or coenzymes bind to the active site and change the shape of the active site so the substrate now fits 5

6 COENZYMES AND COFACTORS Coenzymes and cofactors can also be used as carrier molecules When a carrier molecule receives either electrons or hydrogen atoms, it becomes reduced When a carrier molecule loses electrons or hydrogen atoms, it becomes oxidized COENZYMES AND COFACTORS Two coenzyme carrier molecules frequently encountered in biological reactions are: NAD + = nicotinamide adenine dinucleotide FAD = flavin adenine dinucleotide ENZYME INHIBITION Enzyme inhibition takes place in three ways: Competitive inhibition Allosteric inhibition Feedback inhibition 6

7 COMPETITIVE INHIBITION The inhibitor molecule is similar in structure to the substrate and competes with the substrate to bind to the active site When the inhibitor has bound to the active site, the substrate cannot bind The binding of the competitor is reversible and dependent upon the relative numbers of inhibitor molecules and substrate molecules present COMPETITIVE INHIBITION ALLOSTERIC INHIBITION This activity also involves inhibitor molecules but they do not block the active site Inhibitor molecules bind to a part of the enzyme away from the active site: the allosteric site This binding changes the shape of the active site in such a way that it can no longer fit properly with the substrate The binding of some allosteric inhibitors is reversible 7

8 ALLOSTERIC INHIBITION FEEDBACK INHIBITION Feedback inhibition is used in many of the metabolic pathways found in the cell The final product in a pathway accumulates and begins to bind to and inactivate the enzyme that catalyzes the first reaction of the pathway It is reversible and, when the level of end product decreases, the inhibition stops and the pathway begins to function again FEEDBACK INHIBITION 8

9 FACTORS THAT AFFECT ENZYME REACTIONS Three major factors affect enzyme activity: Temperature can break hydrogen bonds and change shape ph can alter electrical charges in the enzyme Concentration of substrate, product & enzyme lower numbers of substrate, product, and enzyme molecules means a lower level of activity CATABOLIC PROCESSES IN METABOLISM Catabolic processes in metabolism cause the breakdown of large organic molecules into smaller ones These reactions cause a release of energy CATABOLIC PROCESSES IN METABOLISM There are three important pathways by which most organisms release energy from nutrient molecules: Glycolysis Krebs cycle Electron transport chain 9

10 GLYCOLYSIS The catabolic pathway is used by most organisms The best example of this pathway is glucose breakdown The process itself is a series of chemical reactions GLYCOLYSIS The reactions occur in the cytoplasm and do not require oxygen Four ATP molecules are produced in glycolysis The first steps of the pathway consume two ATP molecules The net gain is two ATP molecules GLYCOLYSIS During glycolysis: Phosphates are transferred to substrates by phosphorylation Phosphorylation makes the substrates more energetic After a series of steps, the 6-carbon glucose molecule is broken into two 3-carbon pyruvate molecules NAD + carries electrons to the electron transport chain 10

11 GLYCOLYSIS GLYCOLYSIS Glycolysis can lead to further pathways Krebs cycle and cellular respiration (aerobic) Fermentation (anaerobic) GLYCOLYSIS 11

12 THE KREBS CYCLE The Krebs cycle is an aerobic catabolic pathway seen in aerobic cellular respiration Pyruvate is further metabolized in this process THE KREBS CYCLE Pyruvate is first modified with coenzyme A This produces the acetyl-coa complex The Krebs cycle is a series of reactions in which chemical changes occur Within these reactions, hydrogen atoms are removed and their electrons are transferred to coenzyme carrier molecules The hydrogen atoms are carried by NAD + and FAD to the electron transport system THE KREBS CYCLE Three important things happen in the Krebs cycle: Carbon is oxidized as CO 2 Electrons are transferred to coenzyme carrier molecules that take the electrons to the electron transport chain Energy is captured and stored when ADP is converted to ATP 12

13 THE KREBS CYCLE ELECTRON TRANSPORT CHAIN The electron transport chain is a sequence of molecules In eukaryotes, they are found in the inner mitochondrial membrane In prokaryotes, they are organized in the plasma membrane ELECTRON TRANSPORT CHAIN Electrons are transferred to a final electron acceptor In aerobic respiration, the final acceptor is oxygen In anaerobic respiration, the final acceptor is an inorganic oxygen-containing molecule 13

14 ELECTRON TRANSPORT CHAIN ELECTRON TRANSPORT CHAIN Electron transport differs from organism to organism and some organisms use more than one type As electrons move from one molecule to another in the chain, energy is released via a process called chemiosmosis CHEMIOSMOSIS As electrons are transferred along the electron transport chain, protons are pumped out of the cell This causes the proton concentration outside the cell to be higher than inside the cell, causing a concentration gradient to form 14

15 CHEMIOSMOSIS Specialized membrane proteins allow protons to re-enter the cell Energy is released as protons re-enter the cell This energy is used to bind phosphate to ADP, making the high-energy molecule ATP The difference in proton concentration in this process is called the proton motive force CHEMIOSMOSIS Cells using anaerobic respiration generate 2 molecules of ATP from one glucose molecule, in glycoloysis Cells using aerobic respiration generate 2 more molecules of ATP from one glucose molecule, in the Krebs cycle Cell using aerobic respiration generate 38 total molecules of ATP from one glucose molecule FERMENTATION Fermentation is the enzymatic breakdown of carbohydrates in which the final electron acceptor is an organic molecule ATP is synthesized by substrate-level phosphorylation but is not linked to electron transport 15

16 FERMENTATION No oxygen is required for ATP to be synthesized Fermentation by itself yields no ATP per glucose molecule, but glycolysis before fermentation yields 2 ATP Different microorganisms use different fermentation pathways ALCOHOLIC FERMENTATION During alcoholic fermentation, pyruvate is first converted into acetaldehyde CO 2 is released and NADH is oxidized Alcohol (ethanol) is the final product HOMOLACTIC FERMENTATION During alcoholic fermentation, pyruvate is converted into lactate CO 2 is not produced, but NADH is oxidized Lactate is the final product 16

17 FERMENTATION ANABOLISM Anabolic reactions are biosynthetic reactions because they are used to synthesize all the biological molecules needed by the cells of living organisms Biosynthetic reactions form the network of pathways that produce the components required by the cell for growth and survival These reactions are fueled by the energy stored in high-energy bonds in ATP 17