Glycogen Metabolism

Glycogen is the storage from of glucose. The metabolism of glycogen both as glycogenolysis, breakdown of glycogen, and glycogenesis, formation of glycogen along with their regulation is briefed in the slides.

1. GLYCOGEN METABOLISM

2. Outline: General view & biomedical importance Synthesis of glycogen Degradation of glycogen Regulation.

3. Glycogen- bank account. “a friend in need is a friend indeed” Glucose- essential for energy Glycogen - stored - when glucose is abundant - degraded - when glucose is below normal glycogen glucose

4. Claude Bernard , 1857, isolated glycogen Carl Cori & Gerty Cori, NP, 1947, glycogen degradation Luis Leloir, Argentina, NP, 1970, glycogen synthesis Earl Sutherland, NP 1971, role of cAMP General overview Polymer of α-D glucose, 10 8 Da Liver and muscle, liver content more than muscle Muscle has more glycogen than liver. M uscle- synthesis of ATP, Liver- blood glucose %age of weight Tissue weight Body content Liver glycogen 5.0 1.8 90 gm. Muscle glycogen 0.7 35 245 gm. Extracellular glucose 0.1 10 L 10 gm.

5. General view & biomedical importance Liver glycogen after a fast of 12-18 hours falls by 125% Muscle  pyruvate  transamination  Gluconeogenesis  liver  muscle 8% muscle glycogen release glucose Exercise triggers mobilization to form ATP

6. In muscles- Red muscle White muscle Rich blood flow Poor blood supply Large no of mitochondria & Oxygen Lesser Pyruvate Lactate End product- CO2 & H2O Substrate for glycolysis Can work for longer period Short period

7. Glycogen Composed entirely of glucosyl residues Linked together by α- 1, 4 glycosidic linkages 8-10: branch, α- 1 , 6 linkage Branch- large sites for glycogenolysis : glucose 1-(P) Stored as granules: cytoplasm Formation of branch: slower Liver glycogen store increase in well fed state Depleted during fast Muscle glycogen not affected by fasting

8. Synthesis of glycogen: Synthesis of UDP-glucose Glucose-6-℗ Glucose-1- ℗ phosphoglucomutase UTP PPi UDP- glucose UDP- glucosepyrophosphorylase 2 Pi H 2 Pyrophosphatase hydrolysis

9. Inter-conversion of glucose-6-phosphate to glucose-1-phosphate Glucose-6-℗ Glucose - 1,6 bisphosphate Glucose -1- ℗ phosphoglucomutase phosphoglucomutase

10. B. Synthesis of a primer to initiate glycogen synthesis UDP- glucose UDP - UDP Glycogen synthase Tyr-OH Tyr-O- Tyr-O- Glycogen synthase Glycogenin primer

11. Glycogenin Protein An enzyme 37 kDa Constitutes of 332 amino acids Glycosylation occurs at tyrosine residue The –OH group of Tyr serves as the site Reaction catalyzed by Glycogenin itself

12. Tyr-O- α- 1, 4 glycosidic linkages C. Elongation of chain by glycogen synthase Non-reducing end Glycogen synthase O O O CH 2 OH CH 2 OH UDP + ATP UTP + ADP UDP Nucleoside diphosphate kinase

13. Summary Glucose-6-℗ Glucose-1- ℗ phosphoglucomutase UTP PPi UDP- glucose UDP- glucosepyrophosphorylase 2 Pi H 2 hydrolysis UDP Glycogen synthase Tyr-OH Tyr-O- Tyr-O- Glycogen synthase

14. D. Formation of branches Tyr-O- Action of enzyme α- (1 , 4), α- (1 , 6) transglucosidase α- 1, 6 glycosidic linkage 4:6 transferase Non-reducing ends

15. E. Synthesis of additional branches After elongation of the two ends The new formed 6- 8 glucosyl residues are removed Added & the additional branches made α - (1, 4), α- (1, 6) transglucosidase and 4:6 transferase are together called the “branching enzyme” O O CH 2 OH CH 2 OH O α- (1, 6) glycosidic linkage

16. ll. Degradation of Glycogen Not a reversal of synthetic pathway A separate set of cytosolic enzyme is required Primary product is G lucose-1- phosphate Glucose

17. A. Shortening of chains: Tyr-O- Glycogen phosphorylase 6 Pi PLP

18. B. Removal of branches: Tyr-O- Oligo α - (1, 4), α- (1, 4) glucantransferase Formation of α- (1, 4 ) linkage by 4:4 transferase Action of amylo α - (1, 6) glucosidase H 2 Tyr-O- Glucose-1- ℗ Glycogen phosphorylase

19. C . Fate of glucose-1- phosphate in liver and muscled Glucose-1-℗ Glucose-6- ℗ glucose H 2 O Pi Glucose-6- phosphatase R eleased into blood to maintain blood glucose level Glycolysis Energy for muscle contraction

20. D. Lysosomal degradation of glycogen Small amount: glycogen, 1-3% degraded continuously Purpose: unknown The enzyme: alpha (1, 4) glucosidase , alias acid maltase Deficiency: accumulation of glycogen Pompe’s disease type II: only lysososmal storage disease

21. SPECIAL FEATURES OF GLYCOGEN DEGRADATION AND SYNTHESIS WHY STORE GLUCOSE AS GLYCOGEN? WHY NOT JUST PUMP GLUCOSE INTO CELLS? WHY GLYCOGEN IS A BRANCHED MOLECULE WITH ONLY ONE BEGINNING AND MANY BRANCHES TERMINATING WITH NON REDUCING GLUCOSYL END? WHY IS PRIMER NEEDED FOR GLYCOGEN SYNTHESIS? WHY DOES GLYCOGEN LIMIT ITS OWN SYNTHESIS?

22. WHY STORE GLUCOSE AS GLYCOGEN? Why not store it as fat? Why waste so many ATP to synthesize glycogen? The answer is Fat stored, not mobilized rapidly as glycogen. Cannot be used as source of energy: absent O 2 Fat: cannot be converted to glucose to maintain its level

23. WHY NOT JUST PUMP GLUCOSE INTO CELLS ? Glucose: osmotically active Costs ATP to pump glucose Concn . of 400 mM to match the “glucose reserve” Balanced by outward movement Uptake of water: lysis High MW; 400 mM glucose stored; intracellular glycogen; concentration of 0.01 mM No osmotic pressure problem

24. WHY GLYCOGEN IS A BRANCHED MOLECULE WITH ONLY ONE BEGINNING AND MANY BRANCHES TERMINATING WITH NON REDUCING GLUCOSYL END? Numerous sites: glycogen phosphorylase & glycogen synthase α amylose: polymer: only one non reducing end Slower glycogen phosphorylase & glycogen synthase: tight association with glycogen Ready access to multitude of non- reducing sugars

25. WHY IS PRIMER NEEDED FOR GLYCOGEN SYNTHESIS? glycogen synthase : low Km- large glycogen Km glycogen Glucose alone: can’t act as primer Glycogen: immortal Glycogenin : a primer A las! Glycogen: mortal

26. WHY DOES GLYCOGEN LIMIT ITS OWN SYNTHESIS? glycogen synthase efficient with larger glycogen How does glycogenesis stop? glycogen synthase ‘a’: decreases with accumulation of glycogen Glycogen inhibits the dephosphorylation of glycogen synthase ‘b’ by phosphoprotein phosphatase

27. III. REGULATION OF GLYCOGEGESIS & GLYCOGENOLYSIS LIVER: glycogenolysis accelerates in fasting MUSCLE: glycogenolysis in active exercise Glycogenesis when muscle is at rest 2 levels: Hormonal regulation Allosterically controlled

28. A. Activation of glycogen degradation by cAMP mediated pathway I. Activation of protein kinase A glucagon epinephrine GPCR ATP cAMP Active Adenyl cyclase PKA ‘b’ PKA Protein kinase A Large enzyme complex 1.3x 10 6 Da Composed of 4 subunits α, β, γ & δ α, β & δ are regulatory γ is catalytic PKA ‘a’ Inactive enzymes Active enzyme

29. II. Activation of phosphorylase kinase cAMP dependent PKA ‘a’ Glycogen phosphorylase kinase ‘b’ Glycogen phosphorylase kinase ‘a’ ATP ADP H 2 O Pi Protein phosphatase-1 INSULIN

30. INSULIN SIGNAL CASCADE INSULIN Insulin receptor tyrosine kinase ‘b’ Insulin receptor tyrosine kinase ‘a’ Insulin receptor substrate ‘b’ (IRS-Tyr) Insulin receptor substrate ‘a’ (IRS-Tyr) Protein phosphatase ‘b’ Protein phosphatase ‘a’ Biological effect

31. III. Activation of glycogen phosphorylase Glycogen phosphorylase kinase ’a’ Glycogen phosphorylase ‘b’ Glycogen phosphorylase ‘a’ ATP ADP H 2 O Pi Protein phosphatase-1 INSULIN GLYCOGENOLYSIS

32. Summary glucagon epinephrine GPCR ATP cAMP Active Adenyl cyclase PKA ‘b’ PKA ‘a’ Glycogen phosphorylase kinase ‘b’ Glycogen phosphorylase kinase ‘a’ ATP ADP H 2 O Pi Protein phosphatase-1 INSULIN Glycogen phosphorylase ‘b’ Glycogen phosphorylase ‘a’ ATP ADP H 2 O Pi Protein phosphatase-1 GLYCOGENOLYSIS

33. B. Inhibition of glycogen synthesis by cAMP directed pathway glucagon epinephrine GPCR ATP cAMP Active Adenyl cyclase PKA ‘b’ PKA ‘a’ Glycogen synthase ‘a’ Glycogen synthase ‘b’ ATP ADP H 2 O Pi Protein phosphatase-1 INSULIN INHIBITION OF GLYCOGEN SYNTHESIS

34. C. Allosteric regulation of glycogen synthesis and degradation Glycogen synthase & glycogen phosphorylase respond to the energy needs of the cell Glycogenesis: glucose is high Glycogenolysis : glucose; energy level low Allosteric regulation: rapid response Can override the effects of hormone mediated regulation

35. I. Regulation of glycogen synthesis and degradation in well-fed state GLYCOGEN GLUCOSE-1-℗ Glycogen synthase Glycogen phosphorylase GLYCOGEN GLUCOSE-1-℗ Glycogen synthase Glycogen phosphorylase GLUCOSE GLUCOSE-6-℗ ATP AMP

36. II. Activation of glycogen degradation by calcium a. Calcium activation of muscle phosphorylase kinase Nerve impulse Membrane depolarisation Ca Ca Ca Ca Calmodulin Ca Ca Ca Ca Muscle phosphorylase ‘b’ Muscle phosphorylase ‘a’ Glycogen phosphorylase ‘b’ Glycogen phosphorylase ‘a’ Pi H 2 GLYCOGENOLYSIS

37. b. Calcium activation of liver phosphorylase kinase ER Membrane depolarisation Ca Ca Ca Ca Calmodulin Ca Ca Ca Ca Liver phosphorylase kinase ‘b’ Liver phosphorylase kinase ‘a’ Glycogen phosphorylase ‘b’ Glycogen phosphorylase ‘a’ GLYCOGEN SYNTHESIS- INHIBITION

38. ER Membrane depolarisation Ca Ca Ca Ca Calmodulin Ca Ca Ca Ca Protein kinase ‘b’ Protein kinase ‘a’ Glycogen synthase ‘b’ Glycogen synthase ‘a’ GLYCOGENOLYSIS

39. Bibliography Lipincott’s Illustrated Reviews, Biochemistry 5 th edition, Richard Harvey, Denise Terrier, Unit II, Chapter 11, Page no: 125- 136 Jaypee’s Texbook of Biochemistry for medical students, 6 th edition, D M Vasudevan , Sreekumari S, Kannan Vaidyanath , Section B, Unit 9, Chapter 9, glycogenolysis , glycogen synthesis, page no 106-112 McGraw Hills LANGE’s Harper’s Illustrated Biochemistry, R K Murray, D A Bender, P A Weil, 28 th edition, Section 11, chapter 19, page no: 157-164 Wiley- Liss’s Textbook of BIOCHEMISTRY with Clinical Correlations, Thomas M Devlin, 4 th edition Chapter 7, carbohydrate metabolism I, major metabolic pathways and their control, page no: 311-334 Central’s Fundamentals of Biochemistry, A C Deb, 8 th edition, Chapter 17, glycogenolysis , clinical orientationof glycogen. Page no: 240-242

40. God not only plays dice, he throws them in the corner you can’t see them. - Stephen Hawking