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Transcript
Essential Concept of Metabolism METABOLISM: AN OVERVIEW Metabolism is the sum of all chemical processes in a living organism. It consists of anabolism, reactions that require energy to synthesize complex molecules from simpler ones, and catabolism, reactions that release energy by breaking complex molecules into simpler ones. Autotroph, which use carbon dioxide to synthesize organic molecules, include photoautotrophs (which carry on photosynthesis) and chemoautotrophs. Heterotrophs, which use organic molecules made by other organisms, include chemoheterotrophs, and photohetrotrophs (obtain chemical energy from light). For growth, movement, and other activities, metabolic pathways use energy captured in the catabolic pathways. ENZYMES Properties of Enzymes Enzymes are proteins that catalyze chemical reactions in living organisms by lowering the activation energy needed for a reaction to occur. Enzymes have an active site, the binding site to which the substrate (the substance on which the enzymes act) attaches to form an enzyme-substrate complex. Enzymes typically exhibit a high degree of specificity in the reactions they catalyze. Properties of Coenzyme and Cofactors Some enzymes require coenzymes, nonprotein organic molecules that can combine with the apoenzyme, the protein portion of the enzyme, to form holoenzyme. Some enzymes also require inorganic ions as cofactors. Enzyme Inhibition Enzyme activity can be reduced by competitive inhibitors, molecules that compete with the substance for the enzyme’s active site, or by noncompetitive inhibitors, molecules that bind to an allosteric site, a site other than the active site. Factors That Affect Enzyme Reaction Factors that affect the rate of enzyme reactions include temperature, pH, and concentrations of substrate, product, and enzyme. Organic molecules have energy associated with the electrons that form bonds between their atoms. When this energy is released, it is trapped into the bonds of ATP. Biochemical Reactions 1. Metabolism Chemical reactions that change or transform energy in cells. a.Anabolic (synthesis) reactions Chemical reactions that build large molecules from small ones. require energy (endergonic) - products contain more energy than reactants. b.Catabolic (degredation) reactions Chemical reactions that: break down large molecules into small ones. release energy (exergonic) - products contain less energy than reactants. 2. Oxidation - Reduction Reactions Most energy transformations in organisms involve oxidation & reduction. Oxidation - a molecule loses one or more electrons, lose a hhydrogen ion. Reduction - a molecule gains one or more electrons, gain a hydrogen ion. Oxidation and reduction reactions are always coupled; each time one substance is oxidized, another is simultaneously reduced. Cells take nutrients and degrade them from highly reduced compounds (with many hydrogen atoms) to highly oxidized compounds. Ex. electron transport chain Electron transport chain Fe3+ gains an electron - is reduced to Fe2+ Fe2+ loses an electron - is oxidized to Fe3+ Molecules Involved in Energy Transformations 1. ATP (adenosine triphosphate) Energy currency of cells. Generation of ATP 1). Substarte-level phosphorylation ATP generated when a high energy phosphate is directly transferred from a phosphorylated compound (a substrate) to ADP. 2). Oxidation phosphorylation When electrons are transferred from organic compounds to electron carriers in an electron transport chain – ATP generated from ADP through a process called Chemiosmosis Cells couple ATP formation & breakdown to other reactions. 2. Cofactors Inorganic helpers (usually ions) needed by some enzymes to function. Ex. Mg2+ 3. Coenzymes Organic cofactors (vitamin derived) needed by some enzymes to function, pick up electrons when they are released. Nicotinamide adenine dinucleotide (NAD+) - coenzyme that transfers electrons; derived from niacin. Oxidized form is NAD+, reduced form is NADH2. Flavin adenine dinucleotide (FAD) - coenzyme that transfers electrons; derived from riboflavin. Oxidized form is FAD, reduced form is FADH2. 4. Cytochromes Iron containing molecules that transport electrons. Ex. electron transport chains 5. Enzymes (biological catalysts) Proteins that speed up chemical reactions without being altered in the process. The generation of ATP 1. Energy released during certain metabolic reactions can be trapped to form ATP from ADP and P. 2. Addition of a p to a molecule is called phosphorylation. 3. During substrate-level phosphorylation, a high-energy p from an intermediate in catabolism is added to ADP. 4. During oxidative phosphorylation, energy is released as electrons are passed to a series of electron acceptors (an electron transport chain) and finally to O2 or another inorganic compound. 5. During photophosphorylation, energy from light is trapped by chlorophyll, and electrons transfer release energy used for the synthesis of ATP. Most cells break down glucose to make ATP by: Cellular respiration (aerobic process) – converts the storage form to the direct form of energy Fermentation and anaerobic respiration – anaerobic process CELLULAR RESPIRATION – AEROBIC AND ANAEROBIC METABOLISM: AND FERMENTATION Most of a cell’s energy is produced from the oxidation of carbohydrates. Glucose is the most commonly used carbohydrates. In aerobic,glucose is completely degrades through a). glycolysis; b). Kreb’s cycle (also known as tricarboxylic acid cycle c). Electron transport chain. Molecular oxygen is final acceptor for electrons and hydrogen:produces relatively large amount of ATP; ex. many bacteria, fungi, protozoa and animals. In anaeroboic , the metabolic reactions involve the same three steps as for aerobic respiration, but does not use molecular oxygen as the final electron acceptor [rather oxygen containing salts (nitrates, nitrite, carbonate, sulfate are final electron acceptors; because only part of Kreb’s cycle operates under anaerobic conditions, ATP yield is never as high as in aerobic respiration] ex. anaerobic microorganisms Both processes start with Glycolysis Glycolysis Glycolysis is a metabolic pathway by which glucose is oxidized to pyruvic acid. Under anaerobic conditions, glycolysis yields a net of two ATP’s per molecule of glucose. 1. Glycolysis Occurs in cells of most eukaryotes & some prokaryotes. General equation for cellular respiration of glucose: C6H12O6 + 6O2 6CO2 + 6H2O + 30 ATP Cellular respiration occurs in 3 stages: Eukaryotic cells Cytoplasm - Glycolysis Mitochondria - Krebs Cycle inner membrane of Mitochondria - Electron Transport Chain Glucose (6C) is split into two pyruvate (3C) molecules. Glucose + NAD+ ADP + Pi 2 pyruvic acid (3Carbon) + 2 NADH + 2 ATP + 2 H+ does not require oxygen, in the cytosol. energy harvested/glucose: 2 ATP (via substrate-level phosphorylation) 2 NADH (actively transported into mitochondria of eukaryotic cells) First half of glycolysis activates glucose. Second half of glycolysis extracts energy. Total energy yield during the 2nd half of glycolysis is 4 ATPs and 2 NADHs. Since 2 ATPs were used to activate glucose, there is a net gain of only 2 ATPs. 2 ATP are used to activate glucose to unstable fructose1,6-diphosphate then form 2 stable glyceraldehydes phosphate (P-GAL) - important intermediate compound. Note: ATP is synthesized in glycolysis by substrate-level phosphorylation. This means that an enzyme transfers a phosphate group from an organic molecule (substrate) to ADP, forming ATP. Pyruvic acid must be converted to Acetyl CoA before it can enter Krebs cycle. Pyruvic acid breaks down to acetyl CoA produce 2 NADH Pyruvic acid decarboxylation add coenzyme A CO2 releases Acetyl - CoA Alternative to Glycolysis 1. Pentose-phosphate pathway is used to metabolize five-carbon sugars. 2. Entner-Doudoroff pathway yields one ATP and two NADH molecules. Anaerobic respiration - Fermantation Fermantation refers to the reactions of metabolic pathways by which NADH is oxidized to NAD. An organic molecules is the final electron acceptor. Six pathwayts of fermentation are summarized in figure 5.12. Alcohol fermentation and Lactic acid fermentation (muscle cramp) most important and commonly occurring fermentation pathways. Oxygen is not required. Organic compounds are the final electron acceptors in fermentation. AEROBIC METABOLISM: RESPIRATION Anaerobes do not use oxygen: aerobes use oxygen and obtain energy chiefly via aerobic respiration. The Krebs cycle The Krebs cycle metabolizes two-carbon compounds to CO2 and H2O, produces one ATP directly from each acetyl group, and transfers hydrogen atoms to the electron transport chain. In energy production the Krebs cycle processes acetyl-CoA, so that (in the electron transport chain) hydrogen atoms can be oxidized for energy. Fig. 7.7 Key to remember the Kreb cycle chemical compound: CIA SS FMO, (CIA wants social security number for mails only) Electron Transport and Oxidative Phosphorylation Electron transport is the transfer of eklectrons to oxygen (the final electron acceptor). Oxidative phosphorylation involves the electron transport chain for ATP synthesis and is a membrane-regulated process not directly related to the metabolism of specific substrates Fate of electrons: 1. Electron transport chains splits the hydrogen atoms from NADH and FADH2 into H+ and electrons. 2. the protons are forced into the space between inner and outer mitochondrial membranes, where they accumulate high levels and lower the pH. 3. The ETC transfer the electrons from one to another, until they are finally accepted by the O2 molecules, which become reduced to Oions. 4. The protons are then allowed to diffuse through special openings in the cristae membranes, this flow of protons is called chemiosmosis. This flow of protons through ATP synthase to combine ADP + Pi to form ATP – (Chemiosmotic phosphorylation). Phosphorylation:adding a phosphate group to chemical The theory of chemiosmosis explains how energy is used to synthesis ATP. energy harvested/NADH: 3 ATPs (via chemiosmotic phosphorylation) energy harvested/FADH2: 2 ATPs (via chemiosmotic phosphorylation) 2 NADH generated from glycolysis 2 NADH generated before entering Kreb’s cycle 6 NADH and 2 FADH produced from ETC 2 ATP was generated in glycolysis 2 ATP was generated in Kreb’s cycle 2 ATP was used in transporting NADH from glycolysis into mitochondria 10 x 3 = 30, 2 x 2 = 4, 30 + 4 -2 = 32, 32 + 2 + 2 = 36 One glucose molecule generates 36 ATP. In eukaryotes, ETC are located in the inner mitochondrial membrane, in prokaryotes, ETC are in the plasma membrane. In prokaryotes , 38 ATP is produced, in Eukaryotes 36 ATP is produced. The Significance of energy capture. The prokaryotes, aerobic (oxidative) metabolism capture 19 times as much energy as does anaerobic metabolisms. THE METABOLISM OF FATS AND PROTEINS Most organisms get energy mainly from glucose. But for almost any organic substance, there is some microorganism that can metabolize it. Fat metabolism Fat metabolism involves hydrolysis and the enzymatic formation of glycerol and free fatty acids. Fatty acids are in turn oxidized by beta oxidation to 2 carbon compound, which results in the release of acetyl-CoA. Acetyl-CoA then enters the Krebs cycle. Protein Metabolism The metabolism of proteins involves the breakdown of proteins to amino acids, the deamination of the amino acids, and their subsequent metabolism in glycolysis, fermentation, or the Krebs cycle. OTHER METABOLIC PROCESSES Photoautotrophy Photosynthesis is the use of light energy to synthesize carbohydrates: (1) The light reactions can include cyclic photophsophorylation or photolysis accompanied by noncyclic photoreduction of NADP; (2) the dark reactions involve the reduction of CO2 to carbohydrate. Photosynthesis in cyanobacteria and algae provides a means of making nutrients, as it does in green plants; however, photosynthetic bacteria generally use some substances besides water to reduce carbon dioxide. Photoheterotrophy Photoheterophy is the use of light as a source of energy. It requires organic compounds as sources of carbon. Chemoautotrophy Chemoautotrophs, or chemolithotrophs, oxidize inorganic substances to obtain energy. Chemolithotrophs require only carbon dioxide as carbon source. THE USE OF ENERGY Biosynthetic Activities An amphibolic pathways is a metabolic pathway that can capture energy or synthesize substances needed by the cell. Figure 5.27 summarized the intermediate products of energy yielding metabolism and some of the building blocks for synthetic reactions that can be made from them. Bacteria synthesize a variety of cell wall polymers. Membrane Transport and Movement Membrane transport uses energy derived from the ATP-producing electron transport system in the membrane to concentrate substances against a gradient. It occurs by active transport and by the phosphotransferase system. Movement in bacteria can be by flagella, by gliding or creeping, or by axial filaments. Bioluminescence The ability of an organism to emit light, may have evolved as a way to remove oxygen from the surroundings of primitive anaerobic microbes early in the Earth’s history. Today it often functions in symbiotic relationships with larger organisms. Dr. Mohammad A.R. Ismaiel