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Glycolysis and Gluconeogenesis

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  1. Glycolysis and Gluconeogenesis

  2. Glycolysis • What is glycolysis? • sequence of reactions that converts one molecule of glucose to two molecules of pyruvate with the formation of two ATP molecules • anaerobic

  3. Glycolysis • Why is glucose such a commonly used fuel? • tends to exist in ring form, very stable, doesn’t generally glycosylate proteins • formed from formaldehyde under prebiotic conditions

  4. Glycolysis • What are the possible fates of glucose?

  5. Glycolysis • What’s the difference between a facultative anaerobe and an obligate anaerobe? • Can you give an example of habitat-dependent anaerobiosis? • What about activity-dependent anaerobiosis?

  6. Glycolysis • All the intermediates in glycolysis have either 3 or 6 carbon atoms • All of the reactions fall into one of 5 categories • phosphoryl transfer • phosphoryl shift • isomerization • dehydration • aldol cleavage

  7. Glycolysis • Entire reaction sequence may be divided into three stages • glucose is trapped and destabilized • six carbon molecule is split into two three carbon molecules • ATP is generated

  8. Glycolysis – Stage 1 • glucose converted to glucose-6-PO4 • ATP is needed • catalyzed by hexokinase or glucokinase • induced fit • G01= -4.0 kcal/mole

  9. Glycolysis – Stage 1 • phosphoglucoisomerase • aldose is converted to ketose • G01=+0.4 kcal/mole

  10. Glycolysis – Stage 1 • rate limiting enzyme – allosteric • inhibited by high ATP, citric acid, long-chain fatty acids • stimulated by ADP or AMP • G01= - 3.4 kcal/mole

  11. Glycolysis

  12. Glycolysis – Stage 2 • six carbon molecule split into 2- 3 carbon molecules • aldose and ketose • G01=+ 5.73 kcal/mole

  13. Glycolysis – Stage 3 • At equilibrium most mixture exists as dihydroxyacetone phosphate • G01=+ 1.83 kcal/mole

  14. Triose Phosphate Isomerase

  15. Glycolysis – Stage 3 • redox reaction • energy from redox used to form acyl phosphate • G01= +1.5 kcal/mole

  16. Glycolysis – Stage 3 • Consists of two coupled processes

  17. Glycolysis – Stage 3 • formation of ATP – substrate level phosphorylation

  18. Glycolysis – Stage 3 • phosphoryl shift – uses 2,3 bisphosphoglycerate G01= +1.1 kcal/mole • dehydration G01 = +.44 kcal/mole • phosphoryl transfer G01 = -7.5 kcal/mole

  19. Glycolysis

  20. Fate of Pyruvate

  21. Alcoholic Fermentation • Which organisms carry out this process? • yeast • other microorganisms • PDC requires thiamine pyrophosphate as coenzyme • NAD+ is regenerated

  22. Lactic Acid Fermentation • Occurs in muscle cells, microorganisms • Regenerates NAD+

  23. NAD+ and Dehydrogenases • Various dehydrogenases have a similar binding domain for NAD+ showing their common origin • Rossman fold

  24. Glycolysis • How can fructose be used for energy?

  25. Glycolysis • To use galactose it must be converted to glucose-6-PO4

  26. Glycolysis

  27. Glycolysis • What causes lactose intolerance?

  28. Glycolysis • What is galactosemia? • inability to metabolize galactose • missing galactose 1-phosphate uridyl transferase • liver disease • development of cataracts • CNS malfunction

  29. Control of Glycolysis • Of what value is glycolysis for cells? • provides energy in form of ATP • provides building blocks for synthetic reactions • Where are most control points found? • enzymes that catalyze irreversible reactions • hexokinase • phosphofructokinase • pyruvate kinase

  30. Phosphofructokinase • Most important control point in mammalian glycolytic pathway • allosteric enzyme • activated by AMP and fructose 2,6 bisphosphate • inhibited by high levels of ATP, citrate, fatty acids

  31. Phosphofructokinase

  32. Hexokinase • Hexokinase is inhibited by its product glucose-6-PO4 • glucose remains in blood • Glucokinase, an isozyme of hexokinase is not inhibited by glucose-6-PO4 • found in liver • has lower affinity for glucose

  33. Pyruvate Kinase • Pyruvate kinase exists as isozymes • L form – predominates in liver • M form – mostly in muscle and brain • PK is an allosteric enzyme • activated by fructose 1,6 bisphosphate • inhibited by ATP, alanine • L form of PK influenced by covalent modification • inhibited by phosphorylation

  34. Pyruvate Kinase

  35. Glucose Transport • What is the role of glucose transporters in animal cells? • facilitate movement of glucose across cell membrane • What kind of molecule is a transporter and where is it located? • small protein embedded in plasma membrane

  36. Glucose Transport • mammalian glucose transporter

  37. Glucose Transport

  38. Glycolysis and Cancer • Why are rapidly growing tumor cells dependent upon glycolysis? • insufficient oxygen supply • What is the function of HIF-1? • hypoxia-inducible transcription factor stimulates synthesis of many glycolytic enzymes and GLUT-1 and 3 • also stimulates vascular endothelial growth factor

  39. Gluconeogenesis • What is gluconeogenesis? • synthesis of glucose from non-carbohydrate precursors • Why is this an important pathway? • What are some of the major precursors? • lactate, amino acids, glycerol • Where does this process occur? • liver, kidney

  40. Gluconeogenesis • If gluconeogenesis involves the conversion of pyruvate to glucose why is it not simply the reverse of glycolysis? • glycolysis contains several irreversible reactions • Which reactions in glycolysis are irreversible? • phosphoenolpyruvate to pyruvate • fructose 6-phosphate to fructose 1,6-bisphosphate • glucose to glucose 6-phosphate

  41. Gluconeogenesis • What is the first reaction?

  42. Gluconeogenesis • Why is pyruvate carboxylase of special interest? • structural properties • contains ATP-grasp domain at N-terminal end • contains biotin-binding domain at C-terminal end

  43. Gluconeogenesis • What is the role of biotin in this reaction? • prosthetic group lined to -amino group of lysine residue • carrier of activated carbon dioxide

  44. Gluconeogenesis • Pyruvate carboxylase is an allosteric enzyme • activated by acetyl CoA • needed to form carboxybiotin

  45. Gluconeogenesis • Carboxylation of pyruvate occurs in the mitocondria but next step in reaction sequence occurs in cytosol

  46. Gluconeogenesis Decarboxylation of oxaloacetate is coupled with phosphorylation by GTP enzyme is phosphoenolpyruvate carboxykinase

  47. Gluconeogenesis • Which other steps in glycolysis are irreversible? • conversion of fructose 1,6-bisphosphate to fructose 6-phosphate • conversion of glucose 6-phosphate to glucose

  48. Gluconeogenesis • G° = -16.7 kJ mol-1 • fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and activated by ATP

  49. Gluconeogenesis • Enzyme that catalyzes last reaction not found in all tissues • liver and kidney cortex

  50. Gluconeogenesis • Is gluconeogenesis an energetically favorable reaction in the cell? • What drives this reaction? • Are glycolysis and gluconeogenesis active at the same time?