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Carbohydrate Metabolism for HO.pptx

  1. By: Oman Ph.
  2. Glycogen Metabolism 2 By: Oman Ph. 3/30/2023
  3. Structure of glycogen Glycogen is homopolysaccaharides formed of branched α-D glucose. The main glycosidic bond is α1-4 linkage. Only at the branching point, the chain is attached by α1-6 linkage. Unlike starch the branching is frequent. Location of Glycogen: is present mainly in cytoplasm of liver and muscles. Although almost 6-10% of the liver by weight is composed of glycogen, its contribution in muscle by weight never exceeds 1%. Still, owing to the huge total muscle mass, the total amount of glycogen stored in muscles is much higher than that in liver. 3 By: Oman Ph. 3/30/2023
  4. Functions of Glycogen Glucose cannot be stored as such within the cells because it is osmotically active and at equivalent amounts to glycogen, it will induce osmotic lysis of the cell due to the uptake of considerable amounts of water. Liver glycogen maintain normal blood glucose concentration especially during the early stage of fasting (between meals). Liver glycogen is depleted after 12-18 hours fasting. Muscle glycogen serves as emergency store for its own usage during exercise. 4 By: Oman Ph. 3/30/2023
  5. Dynamic nature of glycogen stores in liver. Glycogen molecule is regularly build and degraded to store or supply glucose that furnishes body energy. 5 PPi UTP UDP Glycogen (Glucose)n+1 UDP-Glucose Glucose-1-P Pi Glucose-6-P 2 Pi Glycogen (Glucose)n Glycogen (Glucose)n Glycogen Synthase Glycogen Phosphorylase Phosphoglucomutase UDP-Glucose Pyrophosphorylase Pyrophosphatase By: Oman Ph. 3/30/2023
  6. 1. Synthesis of glycogen (glycogenesis) It is the formation of glycogen in liver and muscle Substrates for glycogen synthesis: 1. In liver: a) Blood glucose b) Other hexoses; fructose & galactose c) Non-carbohydrate sources; glycerol and lactate. These compounds (non-carbohydrate sources) first converted in to glucose by gluconeogenesis. 2. In muscle: Blood glucose only. 6 By: Oman Ph. 3/30/2023
  7. Glycogenesis… Glycogen(n residues) + UDP-glucose glycogen(n +1 residues) + UDP A branching enzyme transfers a segment from the end of a glycogen chain to the C6 hydroxyl of a glucose residue of glycogen to yield a branch with an α(1→6) gycosidic linkage. 7 By: Oman Ph. 3/30/2023
  8. Glycogen catabolism (breakdown): glycogenolysis Enzymes of glycogenolysis Glycogen Phosphorylase Glycogen Debranching Enzyme Phosphoglucomutase Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the α (1→4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as reaction product. Glycogen(n residues) + Pi → glycogen (n–1 residues) + glucose-1- phosphate 8 By: Oman Ph. 3/30/2023
  9. Cont… Liver Glucose-6-phosphatase catalyzes the following, essential to the liver's role in maintaining blood glucose: Glucose-6-phosphate + H2O glucose + Pi Muscle tissues lack this enzyme that is why muscles do not contribute glucose for circulation . Glucose -6-phosphate in muscle enters glycolysis. 9 By: Oman Ph. 3/30/2023
  10. 10 By: Oman Ph. 3/30/2023
  11. Fates of glycogen In liver – The synthesis and breakdown of glycogen is regulated to maintain blood glucose levels. In muscle - The synthesis and breakdown of glycogen is regulated to meet the energy requirements of the muscle cell. 11 By: Oman Ph. 3/30/2023
  12. Regulation of glycogenesis and glycogenolysis There is coordinated regulation of glycogenesis and glycogenolysis i.e. conditions leading to stimulation of glycogenolysis inhibitng at the same time glycogenesis and vise versa. During fasting glycogenolysis is stimulated and glycogenesis is inhibited. This provides blood glucose from liver glycogen. After meal part of absorbed glucose (40%) goes to general circulation to be utilized. The remaining (60%) is converted into glycogen in liver. So after meal, glycogenesis is stimulated and glycogenolysis is inhibited. 12 By: Oman Ph. 3/30/2023
  13. Cont… After meal→ ↑ blood glucose →insulin secretion = stimulates glycogen synthase and inhibit glycogen phosphorylase. Hence, glycogenesis is favored. During fasting→ ↓blood glucose→ Glucagon , Epinephrine,… → Stimulate glycogen phosphorylase and inhibit glycogen synthase. 13 By: Oman Ph. 3/30/2023
  14. 14 By: Oman Ph. 3/30/2023
  15. Infusion of glucose into mouse liver 15 By: Oman Ph. 3/30/2023
  16. Covalent modification of glycogen metabolism enzymes…??? 3/30/2023 By: Oman Ph. 16
  17. Glycogen storage disease “Glycogen storage disease” is a generic term to describe a group of inherited disorders characterized by deposition of an abnormal type or quantity of glycogen in the tissues. It is also called glycogenosis. They are mainly due to deficiency of Enzymes of glycogen metabolism. There are 8 types of glycogen storage diseases (read). 17 By: Oman Ph. 3/30/2023
  18. Glycolysis  Catabolism of glucose to provide ATPs and NADH molecules  Also provides building blocks for anabolic pathways.  Glycolytic cleavage of glucose or other hexoses into pyruvate/lactate forms the backbone of the carbohydrate metabolism.  However, because it can run independent of mitochondria/O2, it is mainly called anaerobic oxidation of glucose. 3/30/2023 18 By: Oman Ph.
  19. Cont… Can catabolize sugars other than glucose: Fructose ----> 2 glyceraldehyde- 3-phosphate Galactose --> glucose 1-phosphate --> glucose - 6- phosphate mannose ---> mannose 6-phosphate --> fructose -6- phosphate The enzymes of glycolysis pathway are also utilized in the opposite direction to synthesize glucose in gluconeogenesis. Therefore, glycolysis is an amphipathic pathway. 3/30/2023 19 By: Oman Ph.
  20. Intracellular site & tissue distribution • Glycolysis occurs in the cell cytoplasm of all tissues of the body. It is especially important in the tissues with unutilizability or short supply of oxygen. • Mammalian erythrocyte (RBCs): The red cells are devoid of mitochondria and depend on glycolysis as the main source of energy. RBCs are unique in that about 90% of its total energy requirement is provided by glycolysis. • Contracting muscles: Due to partial occlusion of blood vessels by the muscular contraction, there is a decrease in oxygen availability that creates temporary anaerobic conditions. Most of the energy of the rapidly contracting muscles comes from the anaerobic oxidation of glucose through glycolysis. 3/30/2023 20 By: Oman Ph.
  21. Cont… Cornea, lens and some parts of retina: They have a limited blood supply and lack of mitochondria. Therefore, needed energy is derived from glycolysis. Stages of glycolysis 1. Energy requiring stage - One molecule of glucose is converted in to 2 molecules of gyceraldehyde-3-phosphate -These steps require 2 molecules of ATP. 2. Energy producing stages -These steps produce ATP molecules 3/30/2023 21 By: Oman Ph.
  22. Activation of Glucose Circulating blood glucose metabolically inert unless it is activated to glucose-6-phosphate (Glu-6-P) inside the cells. Once phosphorylated, glucose is trapped inside the cells because cell membrane is impermeable to it. The activation takes place with the help of tissue-specific isoenzymes. 3/30/2023 22 By: Oman Ph.
  23. Hexokinase and Glucokinase Hexokinase • Found in the cytosol of most tissues • Low specificity: it’s a common for hexoses • Low Km: high affinity for glucose • Inhibited by Glucose-6-phosphate Glucokinase • Found in the Liver and pancreatic β-cells • Also a ‘hexokinase’ • High specificity for glucose • High Km and not inhibited by glucose-6-phosphate 3/30/2023 23 By: Oman Ph.
  24. Cont… 3/30/2023 24 By: Oman Ph.
  25. Cont…. 3/30/2023 25 By: Oman Ph.
  26. Energy from Glycolysis Under anaerobic conditions: ATP invested in the activation phase • One ATP in the activation of glucose to Glu-6-P. • One ATP in the activation of Fru-6-P to Fru-1,6-DiP. Total ATP invested = 2 ATP ATP Gained: • 2 ATP by substrate level phosphorylation from 1,3- diphosphoglycerate. • 2 ATP from substrate level phosphorylation from phosphoenol pyruvate. Net ATP gained = 4 ATP gained - 2 ATP lost = 2 ATP 3/30/2023 26 By: Oman Ph.
  27. Cont… During rapid contraction (exertion), muscle cells require a very fast supply of energy in the form of ATP. The extra demand of ATP is met by glycolysis because substrate level phosphorylation is 100-times faster than oxidative phosphorylation and oxygen is low because of vessels occlusion by exertion. 3/30/2023 27 By: Oman Ph.
  28. Under aerobic conditions: Total ATP invested in the activation phase = 2 ATP, as under anaerobic conditions. Total ATP gained = 8 or 10 ATP: a) 4 ATP (obtained by substrate level phosphorylation) b) 2NADH, H+ (produced from oxidation of glyceraldehyde-3- phosphate); One NADH can give rise to either two or three ATP through respiratory chain depending upon which shuttle system is used for its transfer across the mitochondrial membrane ,therefore, 2 X 2 or 3 ATP = 4 or 6 ATP. Net ATP gained : 8 ATP – 2 ATP = 6 ATP, Or, 10 ATP – 2 ATP = 8 ATP 3/30/2023 28 By: Oman Ph.
  29. Special features of Glycolysis in RBCs Mature RBCs contains no mitochondria, thus they depend up on only glycolysis for energy production. Lactate is always the end product. The RBCs have ability to form 2,3-bisphosphogycerate (2,3-BPG) through what is called Rapoport-Luebering cycle. 2,3-BPG decrease affinity of hemoglobin to oxygen, thus good tissue oxygenation. 3/30/2023 29 By: Oman Ph.
  30. Fig. Rapoport-Luebering cycle 3/30/2023 30 By: Oman Ph.
  31. Significance of 2,3-BPG It decreases the affinity of hemoglobin (Hb) to O2 to help dissociation and unloading of O2 in tissue capillaries. Therefore, the level of 2,3-BPG increases markedly under hypoxic conditions (high altitudes, stagnant hypoxia and anemic hypoxia). Red blood cells have a high concentration of 2,3-BPG (4-5 mM, equimolar to Hb) in contrast to presence in trace amounts in other cells. 3/30/2023 31 By: Oman Ph.
  32. Biological functions of Glycolysis Energy production Oxygenation of tissue: through 2,3-BPG formation that decreases the affinity of hemoglobin to oxygen Provides important intermediates for synthesis of molecules 3/30/2023 32 By: Oman Ph.
  33. Fig. Function of Glycolysis 3/30/2023 33 By: Oman Ph.
  34. Fate of pyruvate 1. lactate production in the absence of oxygen/mitochondria This reaction helps for regeneration of NAD+. 3/30/2023 34 By: Oman Ph.
  35. Conversion of lactate back to glucose The Cori cycle involves the utilization of lactate, produced by glycolysis in non-hepatic tissues, (such as muscle and erythrocytes) as a carbon source for hepatic gluconeogenesis. In this way the liver can convert the anaerobic byproduct of glycolysis, lactate, back into more glucose for reuse by non-hepatic tissues. 3/30/2023 35 By: Oman Ph.
  36. 3/30/2023 36 By: Oman Ph.
  37. 2. Conversion to ethanol It is fermentation by yeast enzymes Fig. general fate of pyruvate 3/30/2023 37 By: Oman Ph.
  38. 3) entry into citric acid cycle in the mitochondria 3/30/2023 38 By: Oman Ph.
  39. Reversibility of glycolysis  Reversible reactions means that the same enzymes catalyze the reaction in both direction.  All reactions of glycolysis except three are reversible.  Irreversible reactions are sites of regulation of glycolysis. Pasteur effect –is the inhibition of glycolysis in the presence of oxygen. Aerobic condition of glucose produce increase amount of ATP and citrate-those inhibit PFK- 1=inhibition of glycolysis. 3/30/2023 39 By: Oman Ph.
  40. Regulation of glycolysis • The rate of glycolysis is regulated by controlling the three irreversible enzymes. • Glucokinase (hexokinase), Phosphofructokinase-1, and pyruvate kinase. • These enzymes catalyze what is called committed reactions of the pathway. 1. Hormonal regulation a. Insulin: stimulates synthesis of all key enzymes of glycolysis b. Glucagon: inhibits the activity of all key enzyme of glycolysis. 3/30/2023 40 By: Oman Ph.
  41. Cont.. 2. Energy regulation a) High level of ATP inhibits PFK-1 and pyruvate kinase b) High level of ADP and AMP stimulate PFK-1. 3. Substrate and product regulation a) Glucose-6-phosphate inhibits hexokinase (and not glucokinase) b) Citrate inhibits PFK-1 c) Fructose 1,6-bisphosphate stimulates pyruvate kinase d) Fructose 2,6-bisphosphate stimulate PFK-1. 3/30/2023 41 By: Oman Ph.
  42. Fig. Regulation of Glycolysis 3/30/2023 42 By: Oman Ph.
  43. In vitro inhibition of glycolysis Arsenate and iodoacetate: inhibit glycolysis by inhibiting glyceraldeyde-3-phosphate dehydrogenase enzyme. Fluoride-inhibits enolase enzyme-clinical laboratories use fluoride to inhibit glycolysis by adding it to the blood before measuring blood glucose. 3/30/2023 43 By: Oman Ph.
  44. Clinical aspects of glycolysis • There are many diseases associated with impaired glycolysis. They include 1. Pyruvate kinase (PK) deficiency • This leads of excessive hemolysis of RBCs leading to hemolytic anemia. •Genetic deficiency of PK enzyme causes decrease the rate of glycolysis and decrease production of ATP. 2. Hexokinase deficiency It leads to hemolytic anemia due to decrease ATP production. 3/30/2023 44 By: Oman Ph.
  45. Lactic acidosis • It is the lowered blood PH and bicarbonate levels due to increase blood lactate above normal level. • Causes- increase formation as in severe muscular exercise ( increased glycolysis) or decrease utilization in case of lack of oxygen. 3/30/2023 45 By: Oman Ph.
  46. The TCA cycle
  47. Pyruvate dehydrogenase reaction & control mechanism • Glycolysis occurs in the cytosol of cells and ends up with pyruvate in aerobic conditions. • Pyruvate should enter the mitochondrion to be metabolized further. • The inner membrane is the major permeability barrier of the mitochondria. • How does the pyruvate get into the mitochondrion from the cytosol? • To get from intermembrane space to matrix involves pyruvate translocase (symporter that also moves H+ into matrix). • The matrix contains Pyruvate Dehydrogenase, enzymes of Krebs Cycle, and other pathways, e.g., fatty acid oxidation & amino acid metabolism. 3/30/2023 By: Oman Ph. 47
  48. Cont… Pyruvate Dehydrogenase catalyzes oxidative decarboxylation of pyruvate, to form acetyl-CoA. Pyruvate Dehydrogenase: a large complex containing multienzyme . Oxidative decarboxylation of pyruvate is a preparatory stage for Krebs' cycle and involves removal of CO2 and hydrogen from pyruvate to be oxidized into acetyl-CoA 3/30/2023 By: Oman Ph. 48
  49. Fig. Oxidative decarboxylation of pyruvate 3/30/2023 By: Oman Ph. 49
  50. Cont… Acetyl CoA, a product of the Pyruvate Dehydrogenase reaction, is a central compound in metabolism. The "high energy" thioester linkage makes it an excellent donor of the acetate moiety. Oxidative decarboxylation of pyruvate to acetyl COA produces one molecule of NADH. This produces 3 ATP molecule through respiratory chain Phosphorylation. 3/30/2023 By: Oman Ph. 50
  51. Regulation of Pyruvate dehydrogenase • Pyruvate Dehydrogenase exist in two form. Phosphorylated (inactive) and dephosphorylated (active) form. • Factors stimulating Pyruvate Dehydrogenase pyruvate, COASH, NAD+, insulin hormone • Factors inhibiting Pyruvate Dehydrogenase NADH, ATP, acetyl CoA • In vitro inhibition of Pyruvate Dehydrogenase a) Arsenic b) Thiamin deficiency 3/30/2023 By: Oman Ph. 51
  52. Origin of the acetyl group that converted in to acetyl-CoA 3/30/2023 By: Oman Ph. 52
  53. Metabolic fates of acetyl COA glucose-6-P Glycolysis pyruvate fatty acids acetyl CoA ketone bodies cholesterol oxaloacetate citrate Krebs Cycle 3/30/2023 By: Oman Ph. 53
  54. Location & purposes of TCA cycle • TCA cycle also known Krebs’ cycle or citric acid cycle(CAC) or catabolism of acetyl CoA. • It is a serious of reactions in which acetyl CoA is oxidized into Co2, H2O and energy. • Location: matrix of mitochondria. Purpose: Yields reduced coenzymes (NADH and FADH2) and some ATP (2). 3/30/2023 By: Oman Ph. 54
  55. Reactions of the TCA cycle • Composed of 8 reactions • 4 carbon intermediates are regenerated • 2 molecules of CO2 released. • Most of energy stored as NADH and FADH2. 3/30/2023 By: Oman Ph. 55
  56. 3/30/2023 By: Oman Ph. 56
  57. Cont… 3/30/2023 By: Oman Ph. 57
  58. Cont… Net reaction for citric acid cycle: acetyl CoA + 3NAD+ + FAD+ + GDP+ Pi ---> HS-CoA + 3NADH + FADH2 + GTP + 2CO2 Energy Budget so far from 1 molecule of glucose is given below in the table Glycolysis 2ATP 2NADH Prep. step 2NADH TCA cycle 2GTP/2ATP 6NADH &2 FADH2 Total 4ATP 10NADH &2 FADH2 3/30/2023 By: Oman Ph. 58
  59. ATP production from a mole of glucose Substrate level phosphorylation Oxidative phosphorylation Glycolysis 2 ATP 6/4 ATP equivalents Prep. step _ 6 ATP equivalents TCA cycle 2 ATP 18 ATP equivalents + 4 ATP equivalents Total 4 ATP 34 ATP equivalents Total ATP = 38/ 36 3/30/2023 By: Oman Ph. 59
  60. 3/30/2023 By: Oman Ph. 60
  61. Anaplerotic reactions  Removal of any of the intermediates from the TCA cycle removes the 4 carbons that are used to regenerate oxaloacetate during each turn of the cycle. With depletion of oxaloacetate, it is impossible to continue oxidizing acetyl CoA.  Reactions that serves to maintain the operation of the central pathways of metabolism and which allows the operation and coordination of other metabolic pathways that enter and leave the central pathways. 3/30/2023 By: Oman Ph. 61
  62. Fig: Efflux of intermediates from the TCA cycle 3/30/2023 By: Oman Ph. 62
  63. Regulation of TCA cycle TCA is regulated through the key enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase) and availability of oxygen. 1) Citrate synthase Activated by ADP Inhibited by high NADH, succinyl-CoA, citrate, ATP 2) Isocitrate dehydrogenase Activated by high [Ca2+] and high [ADP] Inhibited by high [NADH] 3) α-ketoglutarate dehydrogenase Activated by high [Ca2+] Inhibited by high [NADH] and high [succinyl CoA] 3/30/2023 By: Oman Ph. 63