Glycolysis - Glucose oxidation

Glucose oxidation
major pathway

1. Glucose Oxidation major Pathway

2. I. Glycolysis Definition: Glycolysis means oxidation of glucose to give: Pyruvate (in the presence of oxygen) or, Lactate (in the absence of oxygen )

3. Site: Cytoplasm of all tissue cells, but it is of physiological importance in: 1. Tissues with no mitochondria: mature RBCs, cornea and lens. 2. Tissues with few mitochondria: Testis, leucocytes, medulla of the kidney , retina, skin and gastrointestinal tract. 3. Tissues undergo frequent oxygen lack: skeletal muscles especially during exercise.

4. Glycolysis What are the possible fates of glucose?

5. 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

6. 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

7. Glycolysis – Stage 1 glucose converted to glucose-6-PO 4 ATP is needed catalyzed by hexokinase or glucokinase

8. Glycolysis – Stage 1 phosphoglucoisomerase aldose is converted to ketose

9. Glycolysis – Stage 1 rate limiting enzyme – phosphofructokinase inhibited by high ATP, citric acid, long-chain fatty acids stimulated by ADP or AMP

10. Glycolysis

11. Glycolysis – Stage 2 six carbon molecule split into 2- 3 carbon molecules aldose and ketose

12. Glycolysis – Stage 3

13. Glycolysis – Stage 3 redox reaction energy from redox used to form acyl phosphate

14. Glycolysis – Stage 3 Consists of two coupled processes

15. Glycolysis – Stage 3 formation of ATP – substrate level phosphorylation

16. Glycolysis – Stage 3 phosphoryl shift – uses 2,3 bisphosphoglycerate

17. Glycolysis

18. In the energy investment phase, ATP provides activation energy by phosphorylating glucose. This requires 2 ATP per glucose. In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD + is reduced to NADH. 2 ATP (net) and 2 NADH are produced per glucose.

19. Energy Investment Phase

20. Energy-Payoff Phase

21. Fate of Pyruvate

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

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

24. Substrate level phosphorylation : This means phosphorylation of ADP to ATP at the reaction itself . In glycolysis , there are 2 examples: - 1.3 Bisphosphoglycerate + ADP 3 Phosphoglycerate + ATP - Phospho-enol pyruvate + ADP Enolpyruvate + ATP .

25. Special features of glycolysis in RBCs 1. Mature RBCs contain no mitochondria, thus: a) They depend only upon glycolysis for energy production (=2 ATP). b) Lactate is always the end product. 2. Glucose uptake by RBCs is independent on insulin hormone. 3. Reduction of met-hemoglobin: Glycolysis produces NADH+H+, which used for reduction of met-hemoglobin in red cells

26. Biological importance (functions) of glycolysis : Energy production: a) anaerobic glycolysis gives 2 ATP. b) aerobic glycolysis gives 8 ATP. Provides important intermediates: a) Dihydroxyacetone phosphate: can give glycerol-3phosphate, which is used for synthesis of triacylglycerols and phospholipids ( lipogenesis ). b) 3 Phosphoglycerate : which can be used for synthesis of amino acid serine. c) Pyruvate : which can be used in synthesis of amino acid alanine .

27. Aerobic glycolysis provides the mitochondria with pyruvate , which gives acetyl CoA Krebs' cycle.

28. Glycolysis How can fructose be used for energy?

29. Glycolysis To use galactose it must be converted to glucose-6-PO 4

30. Glycolysis

31. Glycolysis What causes lactose intolerance?

32. Reversibility of glycolysis (Gluconeoqenesis): 1. Reversible reaction means that the same enzyme can catalyzes the reaction in both directions. 2. all reactions of glycolysis -except 3- are reversible. 3. The 3 irreversible reactions (those catalyzed by kinase enzymes) can be reversed by using other enzymes. Glucose-6-p  Glucose F1, 6 Bisphosphate  Fructose-6-p Pyruvate  Phosphoenol pyruvate 4. During fasting, glycolysis is reversed for synthesis of glucose from non- carbohydrate sources e.g. lactate. This mechanism is called: gluconeogenesis.

33. Importance of lactate production in anerobic glycolysis : 1. In absence of oxygen, lactate is the end product of glycolysis : 2 . In absence of oxygen, NADH + H + is not oxidized by the respiratory chain. 3 . The conversion of pyruvate to lactate is the mechanism for regeneration of NAD + . 4. This helps continuity of glycolysis , as the generated NAD+ will be used once more for oxidation of another glucose molecule. Glucose  Pyruvate  Lactate

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

35. 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

36. 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

37. Hexokinase Hexokinase is inhibited by its product glucose-6-PO 4 glucose remains in blood Glucokinase, an isozyme of hexokinase is not inhibited by glucose-6-PO 4 found in liver has lower affinity for glucose

38. 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

39. Pyruvate Kinase

40. 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

41. 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

42. Gluconeogenesis

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

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

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

46. 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

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

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

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

50. Regulation of Glycolysis and Gluconeogenesis What are some of the factors that ensure the reciprocal regulation of these processes? allosteric regulators of key enzymes energy charge fructose 2,6-bisphosphate hormones

51. Regulation of Glycolysis and Gluconeogenesis

52. Regulation of Glycolysis and Gluconeogenesis fructose 2,6-bisphosphate stimulates PFK and inhibits fructose 1,6-bisphosphase controlled by insulin and glucagon and reflects the nutritional status of the cell

53. Regulation of Glycolysis and Gluconeogenesis How do hormones influence the enzymes associated with these processes? influence gene expression change transcription rate influence degradation of m-RNA insulin PFK, PK glucagon PEPCK, fructose 1,6-bisphosphatase

54. Regulation of Glycolysis and Gluconeogenesis What are substrate cycles and why are they important? can amplify metabolic signals can generate heat

55. Regulation of Glycolysis and Gluconeogenesis What is the Cori cycle and why is it important?

56. Regulation of Glycolysis and Gluconeogenesis