Chapter 23 Metabolic Pathways for Carbohydrates 23.4 Digestion of Carbohydrates 23.5 Glycolysis: Oxidation of Glucose 23.6 Pathways for Pyruvate

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1 Chapter 23 Metabolic Pathways for Carbohydrates 23.4 Digestion of Carbohydrates 23.5 Glycolysis: Oxidation of Glucose 23.6 Pathways for Pyruvate 1

2 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. 2

3 Digestion of Carbohydrates 3

4 Glycolysis In Stage 2, the metabolic pathway called glycolysis degrades glucose (6C) to pyruvate (3C). 4

5 5

6 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. 6

7 Glycolysis: Energy Investment

8 Glycolysis: Energy-Production In reactions 7 and 10, the hydrolysis of phosphates in the triose phosphates generates four ATP molecules. 8

9 Glycolysis: Reactions

10 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-6- phosphate. Four ATP are formed in energy-generation by direct transfers of phosphate groups to four ADP. Glucose + 2ADP + 2P i + 2NAD + 2Pyruvate + 2ATP + 2NADH + 4H + 10

11 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. 11

12 Learning Check In glycolysis, what compounds provide phosphate groups for the production of ATP? 12

13 Solution In glycolysis, what compounds provide phosphate groups for the production of ATP? In reaction 7, phosphate groups from two 1,3- bisphosphoglycerate molecules are used to form two ATP. In reaction 10, phosphate groups from two phosphoenolpyruvate molecules are used to form two more ATP. 13

14 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 CO 2. O CH 3 C COO - + HS CoA + NAD + pyruvate O CH 3 C S CoA + CO 2 + NADH + H + acetyl CoA 14

15 Lactate Formation When oxygen is not available (anaerobic conditions), pyruvate is reduced to lactate, which replenishes NAD + to continue glycolysis. O lactate dehydrogenase CH 3 C COO - + NADH + H + pyruvate OH CH 3 CH COO - + NAD + lactate 15

16 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. 16

17 Pathways for Pyruvate 17

18 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. 18

19 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. 19

20 23.7 Regulation of Glucose Metabolism and energy Production High glucose levels and insulin promote glycolysis. Low glucose levels and glucagon promote gluconeogenesis. 20

21 23.10 Glycogen Metabolism: Glycogenesis and Glycogenolysis 21

22 Glycogenesis Glycogenesis: Stores glucose by converting glucose to glycogen. Operates when high levels of glucose-6- phosphate 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. 22

23 Diagram of Glycogenesis 23

24 Formation of Glucose-6-Phosphate Glucose is converted to glucose-6-phosphate using ATP. P O OH CH 2 OH O OH OH Glucose-6-phosphate 24

25 Formation of Glucose-1-Phosphate Glucose-6-phosphate is converted to glucose-1-phosphate. P O CH 2 H O CH 2 O O OH OH OH OH OH O P OH OH Glucose-6-phosphate Glucose-1-phosphate 25

26 UDP-Glucose UTP converts glucose-1-phosphate to UDP-glucose and pyrophosphate (PP i ). OH CH 2 OH H O OH O O O O P O P O CH 2 OH O - O - O UDP-glucose OH O N N OH 26

27 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 27

28 Glycogenolysis Glycogenolysis is the break down of glycogen to glucose. 28

29 Glycogenolysis Glycogenolysis: Is activated by glucagon (low blood glucose). Bonds glucose to phosphate to form glucose-1-phosphate. Glycogen-glucose + P i Glycogen + glucose-1-phosphate 29

30 Isomerization of Glucose-1- phosphate The glucose-1-phosphate isomerizes to glucose- 6-phosphate, which enters glycolysis for energy production. 30

31 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. 31

32 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-6- phosphate. C. Activated by high levels of glucose-6-phosphate. D. Glucose + UTP UDP-glucose + PP i 32

33 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-6- phosphate. C. 1 Activated by high levels of glucose-6-phosphate. D. 1 Glucose + UTP UDP-glucose + PP i 33

34 Glucose: Utilization of Glucose Is the primary energy source for the brain, skeletal muscle, and red blood cells. Deficiency can impair the brain and nervous system. 34

35 23.11 Gluconeogenesis: Gluconeogenesis is: The synthesis of glucose from carbon atoms of noncarbohydrate compounds. Required when glycogen stores are depleted. Glucose Synthesis 35

36 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 36

37 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. 37

38 Phosphoenolpyruvate to Fructose- 1,6-bisphosphate Phosphoenolpyruvate is converted to fructose- 1,6-bisphosphate using the same enzymes in glycolysis. 38

39 Glucose Formation A loss of a phosphate from fructose-1,6- bisphosphate forms fructose-6-phosphate and P i. A reversible reaction converts fructose-6- phosphate to glucose-6-phosphate. The removal of phosphate from glucose-6- phosphate forms glucose. 39

40 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. 40

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42 Pathways for Glucose 42

43 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. 43

44 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. 44

45 Tutorials: Glycolysis URLs