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Chapter 23 Metabolic Pathways for Carbohydrates 23.4 Digestion of Carbohydrates 23.5 Glycolysis: Oxidation of Glucose 23.6 Pathways for Pyruvate Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 1 Stage 1: Digestion of Carbohydrates In the mouth, salivary amylase hydrolyzes α-glycosidic bonds in polysaccharides to give smaller polysaccharides (dextrins), maltose, and some glucose. In the small intestine, pancreatic amylase hydrolyzes dextrins to maltose and glucose. The disaccharides maltose, lactose, and sucrose are hydrolyzed to monosaccharides. The monosaccharides enter the bloodstream for transport to the cells. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 2 Digestion of Carbohydrates Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 3 Glycolysis In Stage 2, the metabolic pathway called glycolysis degrades glucose (6C) to pyruvate (3C). Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 4 Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 5 Glycolysis: Energy-Investment In reactions 1-5 of glycolysis: Energy is used to add phosphate groups to glucose and fructose. Glucose is converted to two three-carbon molecules. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 6 Glycolysis: Energy Investment 4 1 3 5 5 2 Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 7 Glycolysis: Energy-Production In reactions 7 and 10, the hydrolysis of phosphates in the triose phosphates generates four ATP molecules. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 8 Glycolysis: Reactions 6-10 9 6 8 10 7 Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 9 Glycolysis: Overall Reaction Glycolysis generates 2 ATP and 2 NADH. Two ATP are used in energy-investment to add phosphate groups to glucose and fructose-6phosphate. Four ATP are formed in energy-generation by direct transfers of phosphate groups to four ADP. Glucose + 2ADP + 2Pi + 2NAD+ 2Pyruvate + 2ATP + 2NADH + 4H+ Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 10 Regulation of Glycolysis Reaction 1 Hexokinase is inhibited by high levels of glucose-6-phosphate, which prevents the phosphorylation of glucose. Reaction 3 Phosphofructokinase, an allosteric enzyme, is inhibited by high levels of ATP and activated by high levels of ADP and AMP. Reaction 10 Pyruvate kinase, another allosteric enzyme is inhibited by high levels of ATP or acetyl CoA. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 11 Learning Check In glycolysis, what compounds provide phosphate groups for the production of ATP? Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 12 Solution In glycolysis, what compounds provide phosphate groups for the production of ATP? In reaction 7, phosphate groups from two 1,3bisphosphoglycerate molecules are used to form two ATP. In reaction 10, phosphate groups from two phosphoenolpyruvate molecules are used to form two more ATP. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 13 23.5 The Fate of Pyruvate When oxygen is present in the cell, (aerobic conditions), pyruvate from glycolysis is decarboxylated to produce acetyl CoA and CO2. O || CH3—C—COO- + HS—CoA + NAD+ pyruvate O || CH3—C—S—CoA + CO2 + NADH + H+ acetyl CoA Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 14 Lactate Formation When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD+ to continue glycolysis. O lactate || dehydrogenase CH3—C—COO- + NADH + H+ pyruvate OH | CH3—CH—COO- + NAD+ lactate Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 15 Lactate in Muscles Under anaerobic conditions (strenuous exercise): Oxygen in the muscles is depleted. Lactate accumulates in the muscles. Muscles tire and become painful. Rest is needed to repay the oxygen debt and to reform pyruvate in the liver. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 16 Pathways for Pyruvate Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 17 Learning Check Match the following terms with the descriptions: 1) Catabolic reactions 2) Coenzymes 3) Glycolysis 4) Lactate A. Produced during anaerobic conditions. B. Reaction series that converts glucose to pyruvate. C. Metabolic reactions that break down large molecules to smaller molecules + energy. D. Substances that remove or add H atoms in oxidation and reduction reactions. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 18 Solution Match the following terms with the descriptions: 1) Catabolic reactions 2) Coenzymes 3) Glycolysis 4) Lactate A. 4 Produced during anaerobic conditions. B. 3 Reaction series that converts glucose to pyruvate. C. 1 Metabolic reactions that break down large molecules to smaller molecules + energy. D. 2 Substances that remove or add H atoms in oxidation and reduction reactions. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 19 23.7 Regulation of Glucose Metabolism and energy Production High glucose levels and insulin promote glycolysis. Low glucose levels and glucagon promote gluconeogenesis. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 20 23.10 Glycogen Metabolism: Glycogenesis and Glycogenolysis Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 21 Glycogenesis Glycogenesis: Stores glucose by converting glucose to glycogen. Operates when high levels of glucose-6phosphate are formed in the first reaction of glycolysis. Does not operate when energy stores (glycogen) are full, which means that additional glucose is converted to body fat. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 22 Diagram of Glycogenesis Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 23 Formation of Glucose-6-Phosphate Glucose is converted to glucose-6-phosphate using ATP. P O CH2 O OH OH OH OH Glucose-6-phosphate Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 24 Formation of Glucose-1-Phosphate Glucose-6-phosphate is converted to glucose-1-phosphate. P O CH2 H O CH2 O O OH OH OH OH OH Glucose-6-phosphate Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. O P OH OH Glucose-1-phosphate 25 UDP-Glucose UTP converts glucose-1-phosphate to UDP-glucose and pyrophosphate (PPi). O CH2OH H O OH O O P O OH OH - O O O P O CH2 - O UDP-glucose Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. OH N N O OH 26 Glycogenesis: Glycogen The glucose in UDP-glucose adds to glycogen. UDP-Glucose + glycogen glycogen-glucose + UDP The UDP reacts with ATP to regenerate UTP. UDP + ATP UTP + ADP Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 27 Glycogenolysis Glycogenolysis is the break down of glycogen to glucose. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 28 Glycogenolysis Glycogenolysis: Is activated by glucagon (low blood glucose). Bonds glucose to phosphate to form glucose-1-phosphate. Glycogen-glucose + Pi Glycogen + glucose-1-phosphate Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 29 Isomerization of Glucose-1phosphate The glucose-1-phosphate isomerizes to glucose6-phosphate, which enters glycolysis for energy production. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 30 Glucose-6-phosphate Glucose-6-phosphate: Is not utilized by brain and skeletal muscle because they lack glucose-6-phosphatase. Hydrolyzes to glucose in the liver and kidney, where glucose-6-phosphatase is available providing free glucose for the brain and skeletal muscle. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 31 Learning Check Match each description with: 1) Glycogenesis 2) Glycogenolysis A. Activated by low levels of blood glucose. B. Converts glucose-1-phosphate to glucose-6phosphate. C. Activated by high levels of glucose-6-phosphate. D. Glucose + UTP UDP-glucose + PPi Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 32 Solution Match each description with: 1) Glycogenesis 2) Glycogenolysis A. 2 Activated by low levels of blood glucose. B. 2 Converts glucose-1-phosphate to glucose-6phosphate. C. 1 Activated by high levels of glucose-6-phosphate. D. 1 Glucose + UTP UDP-glucose + PPi Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 33 Utilization of Glucose Glucose: Is the primary energy source for the brain, skeletal muscle, and red blood cells. Deficiency can impair the brain and nervous system. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 34 23.11 Gluconeogenesis: Glucose Synthesis Gluconeogenesis is: The synthesis of glucose from carbon atoms of noncarbohydrate compounds. Required when glycogen stores are depleted. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 35 Gluconeogenesis: Glucose Synthesis Carbon atoms for gluconeogenesis from lactate, some amino acids, and glycerol are converted to pyruvate or other intermediates. Seven reactions are the reverse of glycolysis and use the same enzymes. Three reactions are not reversible. Reaction 1 Hexokinase Reaction 3 Phosphofructokinase Reaction 10 Pyruvate kinase Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 36 Gluconeogenesis: Pyruvate to Phosphoenolpyruvate Pyruvate adds a carbon to form oxaloacetate by two reactions that replace the reverse of reaction 10 of glycolysis. Then a carbon is removed and a phosphate added to form phosphoenolpyruvate. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 37 Phosphoenolpyruvate to Fructose1,6-bisphosphate Phosphoenolpyruvate is converted to fructose1,6-bisphosphate using the same enzymes in glycolysis. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 38 Glucose Formation A loss of a phosphate from fructose-1,6bisphosphate forms fructose-6-phosphate and Pi. A reversible reaction converts fructose-6phosphate to glucose-6-phosphate. The removal of phosphate from glucose-6phosphate forms glucose. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 39 Cori Cycle When anaerobic conditions occur in active muscle, glycolysis produces lactate. The lactate moves through the blood stream to the liver, where it is oxidized back to pyruvate. Gluconeogenesis converts pyruvate to glucose, which is carried back to the muscles. The Cori cycle is the flow of lactate and glucose between the muscles and the liver. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 40 Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 41 Pathways for Glucose Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 42 Learning Check Identify each process as: 1) glycolysis 2) glycogenesis 3) glycogenolysis 4) gluconeogenesis A. The synthesis of glucose from noncarbohydrates. B. The breakdown of glycogen into glucose. C. The oxidation of glucose to two pyruvate. D. The synthesis of glycogen from glucose. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 43 Solution Identify each process as: 1) glycolysis 2) glycogenesis 3) glycogenolysis 4) gluconeogenesis A. 4 The synthesis of glucose from noncarbohydrates. B. 3 The breakdown of glycogen into glucose. C. 1 The oxidation of glucose to two pyruvate. D. 2 The synthesis of glycogen from glucose. Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 44 Tutorials: Glycolysis URLs http://instruct1.cit.cornell.edu/courses/biomi290/ASM/glycolysis.dcr http://programs.northlandcollege.edu/biology/Biology1111/animations/glycolysis.html http://www.tcd.ie/Biochemistry/IUBMB-Nicholson/swf/glycolysis.swf Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings. 45