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Parts of Chapter 2, and 24 Figure 2.11 Patterns of chemical reactions. (a) Synthesis reactions Smaller particles are bonded together to form larger, more complex molecules. (b) Decomposition reactions Bonds are broken in larger molecules, resulting in smaller, less complex molecules. (c) Exchange reactions Bonds are both made and broken (also called displacement reactions). Example Example Example Amino acids are joined together to form a protein molecule. Glycogen is broken down to release glucose units. ATP transfers its terminal phosphate group to glucose to form glucose-phosphate. + Amino acid molecules Glycogen Protein molecule Glucose molecules Glucose Adenosine triphosphate (ATP) + Glucose phosphate Adenosine diphosphate (ADP) (a) Synthesis reactions Smaller particles are bonded together to form larger, more complex molecules. Example Amino acids are joined together to form a protein molecule. Amino acid molecules Protein molecule . (b) Decomposition reactions Bonds are broken in larger molecules, resulting in smaller, less complex molecules. Example Glycogen is broken down to release glucose units. Glycogen Glucose molecules . (c) Exchange reactions Bonds are both made and broken (also called displacement reactions). Example ATP transfers its terminal phosphate group to glucose to form glucose-phosphate. + Glucose Adenosine triphosphate (ATP) + Glucose phosphate Adenosine diphosphate (ADP) . Amino acid Amino acid Amino acid Amino acid Amino acid (a) Primary structure: The sequence of amino acids forms the polypeptide chain. (b) Secondary structure: The primary chain forms spirals (-helices) and sheets (-sheets). -Helix: The primary chain is coiled to form a spiral structure, which is stabilized by hydrogen bonds. -Sheet: The primary chain “zig-zags” back and forth forming a “pleated” sheet. Adjacent strands are held together by hydrogen bonds. (c) Tertiary structure: Superimposed on secondary structure. -Helices and/or -sheets are folded up to form a compact globular molecule held together by intramolecular bonds. (d) Quaternary structure: Two or more polypeptide chains, each with its own tertiary structure, combine to form a functional protein. Tertiary structure of prealbumin (transthyretin), a protein that transports the thyroid hormone thyroxine in serum and cerebrospinal fluid. Quaternary structure of a functional prealbumin molecule. Two identical prealbumin subunits join head to tail to form the dimer. Figure 2.19a Levels of protein structure. Amino acid Amino acid Amino acid Amino acid Amino acid (a) Primary structure: The sequence of amino acids forms the polypeptide chain. Figure 2.19b Levels of protein structure. -Helix: The primary chain is coiled to form a spiral structure, which is stabilized by hydrogen bonds. -Sheet: The primary chain “zig-zags” back and forth forming a “pleated” sheet. Adjacent strands are held together by hydrogen bonds. (b) Secondary structure: The primary chain forms spirals (-helices) and sheets (-sheets). . Tertiary structure of prealbumin (transthyretin), a protein that transports the thyroid hormone thyroxine in serum and cerebrospinal fluid. (c) Tertiary structure: Superimposed on secondary structure. -Helices and/or -sheets are folded up to form a compact globular molecule held together by intramolecular bonds. . Quaternary structure of a functional prealbumin molecule. Two identical prealbumin subunits join head to tail to form the dimer. (d) Quaternary structure: Two or more polypeptide chains, each with its own tertiary structure, combine to form a functional protein. Figure 2.20 Enzymes lower the activation energy required for a reaction to proceed rapidly . WITHOUT ENZYME WITH ENZYME Activation energy required Less activation energy required Reactants Reactants Product Product Figure 2.21 Mechanism of enzyme action. Substrates (S) e.g., amino acids + Product (P) e.g., dipeptide Energy is absorbed; bond is formed. Water is released. Peptide bond Active site Enzyme (E) Enzyme-substrate complex (E-S) 1 Substrates bind 2 Internal at active site. rearrangements Enzyme changes leading to shape to hold catalysis substrates in occur. proper position. Enzyme (E) 3 Product is released. Enzyme returns to original shape and is available to catalyze another reaction. . Stage 1 Digestion in GI tract lumen to absorbable forms. Transport via blood to tissue cells. PROTEINS CARBOHYDRATES Amino acids Glucose and other sugars Stage 2 Anabolism Proteins (incorporation into molecules) and catabolism of nutrients NH3 to form intermediates within tissue cells. FATS Glycogen Glucose Fatty acids Glycerol Fats Pyruvic acid Acetyl CoA Stage 3 Oxidative breakdown of products of stage 2 in Infrequent mitochondria of tissue cells. CO2 is liberated, and H atoms removed are ultimately delivered to molecular oxygen, forming water. Some energy released is used to form ATP. Catabolic reactions Anabolic reactions Krebs cycle H CO2 Oxidative phosphorylation (in electron transport chain) O2 H2O Chemical energy (high-energy electrons) Chemical energy Glycolysis Glucose Cytosol Krebs cycle Pyruvic acid Mitochondrial cristae Via substrate-level phosphorylation 1 During glycolysis, each glucose molecule is broken down into two molecules of pyruvic acid in the cytosol. Electron transport chain and oxidative phosphorylation Mitochondrion 2 The pyruvic acid then enters the mitochondrial matrix, where the Krebs cycle decomposes it to CO2. During glycolysis and the Krebs cycle, small amounts of ATP are formed by substratelevel phosphorylation. Via oxidative phosphorylation 3 Energy-rich electrons picked up by coenzymes are transferred to the electron transport chain, built into the cristae membrane. The electron transport chain carries out oxidative phosphorylation, which accounts for most of the ATP generated by cellular respiration. Glucose Glycolysis Krebs cycle Electron transport chain and oxidative phosphorylation Carbon atom Phosphate Phase 1 Sugar Activation Glucose is activated by phosphorylation 2 ADP and converted to fructose-1, Fructose-1,66-bisphosphate bisphosphate Phase 2 Sugar Cleavage Fructose-1, 6-bisphosphate is cleaved into two 3-carbon Dihydroxyacetone fragments phosphate Glyceraldehyde 3-phosphate Phase 3 Sugar oxidation and formation 2 NAD+ of ATP 4 ADP The 3-carbon frag2 NADH+H+ ments are oxidized (by removal of hydrogen) and 4 ATP 2 Pyruvic acid molecules are formed 2 NADH+H+ 2 NAD+ 2 Lactic acid To Krebs cycle (aerobic pathway) Glycolysis Krebs cycle Electron transport chain and oxidative phosphorylation Carbon atom Inorganic phosphate Coenzyme A Cytosol Pyruvic acid from glycolysis Transitional phase Mitochondrion (matrix) NAD+ CO2 NADH+H+ Acetyl CoA Oxaloacetic acid NADH+H+ (pickup molecule) Citric acid (initial reactant) NAD+ Malic acid Isocitric acid NAD+ Krebs cycle CO2 NADH+H+ -Ketoglutaric acid Fumaric acid CO2 FADH2 Succinic acid FAD GTP ADP Succinyl-CoA GDP + NAD+ NADH+H+ Glycolysis Krebs cycle Electron transport chain and oxidative phosphorylation Intermembrane space Inner mitochondrial membrane Mitochondrial matrix 2 H+ + FADH2 NADH + (carrying from food) 1 2 ATP synthase FAD H+ NAD+ Electron Transport Chain Electrons are transferred from complex to complex and some of their energy is used to pump protons (H+) into the intermembrane space, creating a proton gradient. ADP + Chemiosmosis ATP synthesis is powered by the flow of H+ back across the inner mitochondrial membrane through ATP synthase. Krebs cycle NADH+H+ Electron transport chain and oxidative phosphorylation FADH2 Free energy relative to O2 (kcal/mol) Glycolysis Enzyme Complex I Enzyme Complex II Enzyme Complex III Enzyme Complex IV Figure 24.12 Energy yield during cellular respiration. Cytosol Mitochondrion 2 NADH + H+ Electron shuttle across mitochondrial membrane Glycolysis Glucose Pyruvic acid 2 NADH + H+ 2 Acetyl CoA 6 NADH + H+ Krebs cycle (4 ATP–2 ATP used for activation energy) Net +2 ATP by substrate-level phosphorylation 2 FADH2 Electron transport chain and oxidative phosphorylation 10 NADH + H+ x 2.5 ATP 2 FADH2 x 1.5 ATP +2 ATP by substrate-level phosphorylation About 32 ATP Maximum ATP yield per glucose + about 28 ATP by oxidative phosphorylation Figure 24.15 Metabolism of triglycerides. Glycolysis Glucose Stored fats in adipose tissue Dietary fats Glycerol Triglycerides (neutral fats) Lipogenesis Fatty acids Ketone bodies Ketogenesis (in liver) Glyceraldehyde phosphate Pyruvic acid Certain amino acids Acetyl CoA CO2 + H2O + Steroids Bile salts Catabolic reactions Anabolic reactions Cholesterol Krebs cycle Electron transport Figure 24.16 Transamination, oxidative deamination, and keto acid are utilized for Transamination modification: processes that occur when amino acids Amino acid + Keto acid Keto acid + Amino acid energy. (-keto(glutamic acid) Liver 3 During keto acid modification the keto acids formed during transamination are altered so they can easily enter the Krebs cycle pathways. glutaric acid) Oxidative deamination NH3 (ammonia) Keto acid modification Urea CO2 Modified keto acid Blood Enter Krebs cycle in body cells Krebs cycle Urea Kidney Excreted in urine 1 During transamination an amine group is switched from an amino acid to a keto acid. 2 In oxidative deamination, the amine group of glutamic acid is removed as ammonia and combined with CO2 to form urea. Figure 24.18 Interconversion of carbohydrates, fats, and proteins. Proteins Carbohydrates Fats Proteins Glycogen Triglycerides (neutral fats) Glucose Amino acids Glucose-6-phosphate Keto acids Glycerol and fatty acids Glyceraldehyde phosphate Pyruvic acid Lactic acid NH3 Acetyl CoA Ketone bodies Urea Excreted in urine Krebs cycle Figure 24.17 Carbohydrate-fat and amino acid pools. Food intake Dietary proteins and amino acids Pool of free amino acids Components of structural and functional proteins Nitrogen-containing Urea derivatives (e.g., hormones, neurotransmitters) Some lost via cell sloughing, hair loss Excreted in urine Dietary carbohydrates and lipids NH3 Structural components of cells (membranes, etc.) Pool of carbohydrates and fats (carbohydrates fats) Specialized derivatives (e.g., steroids, acetylcholine); bile salts Some lost via surface secretion, cell sloughing Catabolized Storage for energy forms CO2 Excreted via lungs Figure 24.19a Major events and principal metabolic pathways of the absorptive state. Major metabolic thrust: anabolism and energy storage Amino acids Glucose Major energy fuel: glucose (dietary) Glycerol and fatty acids Glucose Liver metabolism: amino acids deaminated and used for energy or stored as fat Amino acids CO2 + H2O Keto acids + Proteins Glycogen Triglycerides (a) Major events of the absorptive state Fats CO2 + H2O + In all tissues: In muscle: Glycogen Glucose Gastrointestinal tract CO2 + H2O Glucose + Protein Amino acids In liver: Glucose Fats Glucose Glycogen Keto acids Fatty acids Glyceraldehydephosphate Glycerol Protein CO2 + H2O In adipose tissue: Fats + (b) Principal pathways of the absorptive state Fatty acids Glycerol Fats Fatty acids Initial stimulus Blood glucose Physiological response Stimulates Result Beta cells of pancreatic islets Blood insulin Targets tissue cells Active transport of amino acids into tissue cells Facilitated diffusion of glucose into tissue cells Protein synthesis Enhances glucose conversion to: Cellular respiration CO2 + H2O + Fatty acids + glycerol Glycogen Figure 24.21a Major events and principal metabolic pathways of the postabsorptive state. Major metabolic thrust: catabolism and replacement of fuels in blood Proteins Glycogen Major energy fuels: glucose provided by glycogenolysis and gluconeogenesis, fatty acids, and ketones Triglycerides Glucose Liver metabolism: amino acids converted to glucose Amino acids Fatty acids and ketones Keto acids CO2 + H2O Amino acids Glucose Glycerol and fatty acids (a) Major events of the postabsorptive state + Glucose Glycogen 2 In muscle: In adipose tissue: CO2 + H2O + Fat Protein Pyruvic and lactic acids 4 3 Amino acids In most tissues: 4 2 Fat 3 In liver: Amino acids Pyruvic and lactic acids 4 Keto acids Fatty acids Glycerol CO2 + H2O 2 3 Fatty acids + glycerol + Glucose CO2 + H2O + Ketone bodies Keto acids Blood glucose 1 Stored glycogen (b) Principal pathways of the postabsorptive state In nervous tissue: CO2 + H2O + Increases, stimulates Reduces, inhibits Initial stimulus Plasma glucose (and rising amino acid levels) Physiological response Result Stimulates Alpha cells of pancreatic islets Negative feedback: rising glucose levels shut off initial stimulus Plasma glucagon Stimulates glycogenolysis and gluconeogenesis Liver Stimulates fat breakdown Adipose tissue Plasma fatty acids Plasma glucose (and insulin) Fat used by tissue cells = glucose sparing Table 24.4 Thumbnail Summary of Metabolic Reactions Table 24.5 Profiles of the Major Body Organs in Fuel Metabolism Table 24.6 Summary of Normal Hormonal Influences on Metabolism Table 24.7 Summary of Metabolic Functions of the Liver (1 of 2) Table 24.7 Summary of Metabolic Functions of the Liver (2 of 2)