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METABOLISM CATABOLISM AND ANABOLISM Functions of food source of energy essential nutrients stored for future use Metabolism is all the chemical reactions of the body some reactions produce the energy which is stored in ATP that other reactions consume all molecules will eventually be broken down and recycled or excreted from the body 26-1 Catabolic reactions breakdown complex organic compounds providing energy (exergonic) glycolysis, Krebs cycle and electron transport A b li reactions Anabolic ti synthesize th i complex l molecules from small molecules requiring energy (endergonic) Exchange of energy requires use of ATP (adenosine triphosphate) molecule. 26-2 ENERGY TRANSFER ATP MOLECULE & ENERGY Energy is found in the bonds between atoms Oxidation is a decrease in the energy content of a molecule Reduction is the increase in the energy content of a molecule Oxidation-reduction reactions are always coupled within the body Each cell has about 1 billion ATP molecules that last for less than one minute Over half of the energy released from ATP is converted to heat whenever a substance is oxidized, another is almost simultaneously reduced. 26-4 26-3 OXIDATION AND REDUCTION Biological oxidation involves the loss of electron and a proton (hydrogen atom) dehydrogenation reactions require coenzymes to transfer hydrogen atoms to another compound li i cells ll th common coenzymes off living thatt carry H H+ NAD (nicotinamide adenine dinucleotide ) NADP (nicotinamide adenine dinucleotide phosphate ) FAD (flavin adenine dinucleotide ) NAD+ + 2 H NADH + H+ Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). 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Biological reduction is the addition of electron and a proton (hydrogen atom) to a molecule increase in potential energy of the molecule 26-5 1 CARBOHYDRATE METABOLISM GLUCOSE CATABOLISM Dietary carbohydrate burned as fuel within hours of absorption All oxidative carbohydrate consumption is essentially a matter of glucose catabolism Glucose catabolism – a series of small steps, each controlled by a separate enzyme, in which energy is released in small manageable amounts, and as much as possible, is transferred to ATP and the rest is released as heat Three major pathways of glucose catabolism glycolysis C6H12O6 + 6O2 6CO2 + 6H2O+ energy glucose (6C) split into 2 pyruvic acid molecules (3C) anaerobic fermentation occurs in the absence of oxygen reduces pyruvic acid to lactic acid Function of this reaction is to transfers energy from glucose to ATP not aerobic respiration occurs in the presence of oxygen completely oxidizes pyruvic acid to CO2 and H2O to produce carbon dioxide and water 26-7 26-8 OVERVIEW OF ATP PRODUCTION MECHANISMS OF ATP GENERATION Copyright © The McGraw-Hill Companies, Inc. 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Key Glucose Carbon atoms ATP Phosphate groups 1 Phosphorylation ADP Phosphorylation is bond attaching 3rd phosphate group contains stored energy Mechanisms of phosphorylation within animals Glucose 6-phosphate Glycogen Fat Fructose 6-phosphate ATP 2 Priming ADP Fructose 1,6-diphosphate 3 Cleavage 2 PGAL 2 Pi 2 NAD+ substrate-level phosphorylation in cytosol oxidative phosphorylation in mitochondria 2 NADH + 2 H+ 4 Oxidation 2 2 ADP 2 H2O in chlorophyll-containing plants or bacteria 2 ATP 5 Dephosphorylation 2 2 ADP 2 ATP 2 2 pyruvic acid photophosphorylation. 2 NADH + 2 H+ 2 NAD+ 2 2 lactic acid 26-9 Anaerobic fermentation STEPS OF GLYCOLYSIS (1) glucose enters cell has phosphate added - ATP used maintains favorable concentration gradient, prevents glucose from leaving cell Oxidation isomerization occurs phosphorylation further activates molecule - ATP used Cleavage molecule split into 2 threecarbon molecules 26-11 removes H+ NAD+ + H NADH Dephosphorylation Priming 26-10 STEPS OF GLYCOLYSIS (2) Phosphorylation Aerobic respiration transfers phosphate groups to ADP to form ATP 4 ATPs produced (2 ATP used) for a net gain of 2 ATP produces 2 pyruvic acid Animation 26-12 2 ANAEROBIC FERMENTATION STEPS OF GLYCOLYSIS 4 ATP are produced but 2 ATP were consumed to initiate glycolysis, so net gain is 2 ATP per glucose molecule Fate of pyruvic acid depends on oxygen availability In an exercising muscle, demand for ATP > oxygen supply; ATP produced by glycolysis Some energy originally in the glucose is contained in the ATP, some in the NADH, some is lost as heat, but most of the energy remains in the pyruvic acid Lactic acid travels to liver to be oxidized back to pyruvic when O2 is available (oxygen debt) Fermentation is inefficient, not favored by brain or heart End-products of glycolysis are: 2 pyruvic acid + 2 NADH + 2 ATP glycolysis can not continue without supply of NAD+ NADH reduces pyruvic acid to lactic acid acid, restoring NAD+ then stored as glycogen or released as glucose 26-14 26-13 ANAEROBIC FERMENTATION Most ATP generated in mitochondria, which requires oxygen as final electron acceptor In the presence of oxygen, pyruvic acid enters the mitochondria and is oxidized by aerobic respiration p y polymerize it to form gglycogen y g for storage g remove phosphate group and release free glucose into the blood Occurs in two principal steps: matrix reactions – their controlling enzymes are in the fluid of the mitochondrial matrix membrane reactions - whose controlling enzymes are bound to the membranes of the mitochondrial cristae Drawbacks of anaerobic fermentation enter bloodstream and transported to the liver when oxygen becomes available the liver oxidized it back to pyruvic acid oxygen is part of the oxygen debt created by exercising muscle Liver can also convert lactic acid back to G6P and can: AEROBIC RESPIRATION Lactic acid leaves the cells that generate it wasteful, because most of the energy of glucose is still in the lactic acid and has contributed no useful work lactic acid is toxic and contributes to muscle fatigue Skeletal muscle is relatively tolerant of anaerobic fermentation, cardiac muscle less so the brain employs no anaerobic fermentation 26-15 26-16 MITOCHONDRIAL MATRIX REACTIONS MITOCHONDRIAL MATRIX REACTIONS Copyright © The McGraw-Hill Companies, Inc. 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Pyruvic acid (C3) 6 CO2 Pyruvic acid oxidation NAD+ Three steps prepare pyruvic acid to enter citric acid cycle 7 NADH + H+ Acetyl group (C2) decarboxylation so that a 3-carbon compound becomes a 2-carbon compound convert that to an acetyl group (acetic acid) 8 Acetyl-Co A Coenzyme A H2O 9 Citric acid (C6) Oxaloacetic acid (C4) H2O 10 NADH + H+ NAD+ (C6) Citric acid cycle 18 H2O NAD+ 11 Citric acid (Krebs) Cycle NADH + H+ (C4) 12 CO2 17 (C5) H2O 13 Occurs in mitochondrial matrix (C4) NADH + H+ (C4) CO2 FAD (C4) Pi 15 GTP NAD+ removes hydrogen atoms from the C2 compound acetyl group binds to coenzyme A 14 16 FADH2 NAD+ CO2 removed from pyruvic acid results in acetyl-coenzyme A (acetylCoA) GDP 26-17 ADP ATP 26-18 3 MITOCHONDRIAL MATRIX REACTIONS Citric Acid Cycle acetyl-Co A (a C2 compound) combines with a C4 to form a C6 compound (citric acid)-- start of cycle hydrogen atoms are removed and accepted by NAD+ another CO2 is removed and the substrate five-carbon becomes a five carbon chain previous step repeated removing another free CO2 leaving a four-carbon chain ATP two hydrogen atoms are removed and accepted by the coenzyme FAD two final hydrogen atoms are removed and transferred to NAD+ reaction generates oxaloacetic acid, which starts the cycle again Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). 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Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 26-19 SUMMARY OF MATRIX REACTIONS MEMBRANE REACTIONS 2 pyruvate + 6H2O 6CO2 2 ADP + 2 Pi 2 ATP 8 NAD+ + 8 H2 8 NADH + 8 H+ 2 FAD + 2 H2 2 FADH2 Carbon atoms of glucose have all been carried away as CO2 and exhaled Energy lost as heat, stored in 2 ATP, 8 reduced NADH, 2 FADH2 molecules of the matrix reactions and 2 NADH from glycolysis Citric acid cycle is a source of substances for synthesis of fats and nonessential amino acids Membrane reactions have two purposes: to further oxidize NADH and FADH2 and transfer their energy to ATP to regenerate NAD+ and FAD and make them available again to earlier reaction steps Mi h d i l electron-transport l h i Mitochondrial chain – series of compounds that carry out this series of membrane reactions most bound to the inner mitochondrial membrane arranged in a precise order that enables each one to receive a pair of electrons from the member on the left side of it. pass electrons to member on the other side 26-21 26-22 ELECTRON TRANSPORT CHAIN Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 50 NADH + H+ NAD+ FMN Relative free energy ((kcal/mole) Fe-S Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 40 FADH2 Enzyme complex 1 FAD CoQ 30 Figure 26 26.5 5 Cyt b Fe-S Cyt c1 20 Enzyme complex 2 Cyt c Cu 10 Cyt a Cyt a3 Enzyme complex 3 ½ O2 + 2 H+ H2O 0 Reaction progress 26-24 4 CHEMIOSMOTIC MECHANISM Electron transport chain energy fuels respiratory enzyme complexes pump protons from matrix into space between inner and outer mitochondrial membranes creates steep electrochemical gradient for H+ across inner mitochondrial membrane Inner membrane is permeable to H+ at channel proteins called ATP synthase Chemiosmotic mechanism - H+ current rushing back through these ATP synthase channels drives ATP synthesis (ANIMATION) Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. 26-25 OVERVIEW OF ATP PRODUCTION NADH releases an electron pair to electron transport system and H+ to prime pumps enough FADH2 releases its electron pairs further along electron-transport system enough energy to synthesize 3 ATP energy to synthesize 2 ATP Complete aerobic oxidation of glucose to CO2 and H2O produces 36-38 ATP efficiency rating of 40% - 60% is lost as heat 26-28 26-27 ATP GENERATED BY OXIDATION OF GLUCOSE GLYCOGEN METABOLISM Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glucose ATP is quickly used after it is formed 2 ATP Glycolysis (net) 2 NADH + 2 H+ 2 pyruvate Mitochondria CO2 6 NADH + 6 H+ 2 ATP 2 FADH2 O2 H2 O 4 ATP 28–30 ATP Total 36–38 ATP releases glucose between meals stimulated by glucagon and epinephrine only liver cells can release glucose back into blood Gluconeogenesis - synthesis of glucose from noncarbohydrates, such as glycerol and amino acids 26-29 stimulated by insulin chains glucose monomers together Glycogenolysis – hydrolysis of glycogen Citric acid cycle Electron-transport chain Glycogenesis - synthesis of glycogen Cytosol 2 NADH + 2 H+ it is an energy transfer molecule, not an energy storage molecule converts the extra glucose to other compounds better suited for energy storage (glycogen and fat) occurs chiefly in the liver and later, kidneys if necessary 26-30 5 LIPIDS GLUCOSE STORAGE AND USE Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood glucose Triglycerides are stored in body’s adipocytes constant Extracellular Intracellular Glucose 6-phosphatase (in liver, kidney, and intestinal cells) released into blood, transported and either oxidized or redeposited in other fat cells Hexokinase (in all cells) Glucose 6-phosphate Glycogen synthase Key Pi Glycogen phosphorylase Lipogenesis - synthesis of fat from other types of molecules amino Glucose 1-phosphate Glycogenesis Glycogenolysis turnover of lipid molecules every 2 - 3 weeks acids and sugars used to make fatty acids and glycerol PGAL can be converted to glycerol Glycogen Pi Glycolysis Figure 26.8 LIPIDS LIPOGENESIS AND LIPOLYSIS PATHWAYS Lipolysis – breaking down fat for fuel begins with the hydrolysis of a triglyceride to glycerol and fatty acids stimulated by epinephrine, norepinephrine, glucocorticoids, thyroid hormone, and growth hormone glycerol easily converted to PGAL and enters the pathway of glycolysis Copyright © The McGraw-Hill Companies, Inc. 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Glucose Glucose 6-phosphate Stored triglycerides Glycerol generates only half as much ATP as glucose Citric acid cycle Lipogenesis Lipolysis 26-33 26-34 PROTEINS citric acid cycle as acetyl-CoA ketogenesis g Amino acids in the pool can be converted to others Free amino acids also can be converted to glucose and fat or directly used as fuel Conversions involve three processes: undergo g metabolized New triglycerides Key Fatty acids catabolized into acetyl groups (by beta-oxidation in mitochondrial matrix) may: enter Fatty acids Acetyl-Co A Ketone bodies β-hydroxybutyric acid Acetoacetic acid Acetone KETOGENESIS Pyruvic acid Acetyl groups removes two carbon atoms at a time which bonds to coenzyme A forms acetyl-CoA, the entry point for the citric acid cycle richer source of energy than the glucose molecule PGAL Fatty acids a fatty acid with 16 carbons can yield 129 molecules of ATP Glycerol Beta oxidation beta oxidation in the mitochondrial matrix catabolizes the fatty acid components 26-32 26-31 by liver to produce ketone bodies acetoacetic acid -hydroxybutyric acid acetone As fuel - first must be deaminated (removal of -NH2) rapid or incomplete oxidization of fats raises blood ketone levels (ketosis) and may lead to a pH imbalance (ketoacidosis) 26-35 deamination – removal of an amino group (-NH2) amination – addition of -NH2 transamination – transfer of -NH2 from one molecule to another what remains is keto acid and may be converted to pyruvic acid, acetyl-CoA, or one of the acids of the citric acid cycle during shortage of amino acids, citric acid cycle intermediates can be aminated and converted to amino acids in gluconeogenesis, keto acids are used to synthesis glucose 26-36 6