14-8-24 BSc MLT Glycogen metabolism.pptx

Glycogen metabolism

1. Glycogen Dr. G. Revathy Associate Professor Dept of Biochemistry, SVMCHRC 14-08-2024 1

2. Glycogen Metabolism 14-08-2024 2

3. Glycogen: Physiological significance Storage form of polysaccharide in human Stored in liver (5%) & skeletal muscle (1%) Liver glycogen - maintenance of blood sugar levels especially between the meals. Muscle glycogen is a storage form of fuel for muscle contraction 14-08-2024 3

4. ↓ blood glucose  liver glycogen is broken down  blood glucose. After taking food, ↑ blood glucose  glycogen deposition in liver. About 5 hours after taking food, ↓ blood glucose  met by glycogen After about 18 hours fasting, liver glycogen is depleted,  fats are hydrolysed (fatty acid oxidation). Glycogen

5. Glycogenesis 14-08-2024 5

6. Glycogenesis Definition : synthesis of glycogen from glucose. STEP 1: Activation of Glucose : Enzyme: Hexokinase (muscle) / Glucokinase (liver). Glucose + ATP → Glucose-6-phosphate + ADP STEP 2: Isomerisation Enzyme: Phosphoglucomutase . Glucose-6-phosphate → Glucose-1-phosphate 14-08-2024 6

7. STEP 3: Glucose-1-phosphate + UTP → UDP-glucose + PPi Enzyme: UDP Glucose pyrophosphorylase UDP glucose is active form of glucose Glucose from this UDPGlc alone can be used for glycogenesis. 14-08-2024 7

8. Glycogen Primer Glycogenin-37kDa protein-carbohydrate complex. dimeric protein acceptor of the glycosyl unit. Oligosaccharide chain of 7 glucose units is added through glycosylation of specific tyrosine residues 14-08-2024 8

9. Glycogenesis.. STEP 4: C-1 of the UDP glucose is added to the C-4 of the already existing glycogen ( Primer ) through α -1, 4 glycosidic linkage, liberating UDP . Enzyme: Glycogen synthase. ‘rate limiting enzyme’ UDP Glucose Primer ( n Glu) Primer ( n+1 Glu ) Glycogen synthase 14-08-2024 9

10. Glycogenesis.. STEP 5: Glucose from UDP glucose is added sequentially to the non reducing (outer) end of the growing glycogen / glycogen primer. till the new chain length is extended to 11 – 12 glucose units. 14-08-2024 10

11. STEP 6: Branching enzyme ( α -1, 4 to α -1, 6 glucan transferase ): transfers 6 - 8 glucose residues from this chain to another site - branching point - α -1, 6 glycosidic linkage. To this newly created branch, further glucose units can be added in alpha-1,4 linkage by glycogen synthase. New 1, 6-glycosidic linkage (branching point) 14-08-2024 11

12. 14-08-2024 12

13. Glycogenolysis 14-08-2024 13

14. Glycogenolysis Definition : Breakdown of glycogen into glucose. It is NOT reversal of glycogenesis & vice versa It is a different pathway. 14-08-2024 14

15. Glycogenolysis STEP 1: Glycogen Phosphorylase : ‘ rate limiting enzyme ’. Removes glucose units sequentially as Glucose-1- phosphate cleaving α -1, 4-glycosidic linkage using phosphate PLP as prosthetic group Glycogen ( n ) Glycogen ( n-1 ) Glucose-1-Phosphate 14-08-2024 15

16. Glycogenolysis.. 14-08-2024 16

17. Glycogenolysis.. Phosphorylase removes glucose units one by one till it reaches 4 glucose residues, left at the branching point. it can’t cleave α -1,6 glycosidic linkage (branching point). If glycogen phosphorylase alone acts on a glycogen molecule, the final product is a highly branched molecule; it is called limit dextrin. 14-08-2024 17

18. Glycogenolysis.. STEP 2: α -1, 4 to α -1, 4 glucan transferase : transfers 3 - glucose (trisaccharide) units from this branch to another branch exposing the branching point ( α -1, 6 glycosidic linkage) free. Branching point is exposed 14-08-2024 18

19. Glycogenolysis.. STEP 3: Debranching ( α -1, 6 glucosidase) enzyme: hydrolytically removes the single glucose unit at the branching point release it as free glucose. Ratio of Glucose-1-phosphate & free glucose is 8:1. Branching point ( α -1, 6 glycosidic linkage) Free glucose + 14-08-2024 19

20. Glycogenolysis.. 14-08-2024 20

21. STEP 4: Phosphoglucomutase enzyme: Glucose-1-phosphate ↔ Glucose-6-phosphate 14-08-2024 21

22. STEP 5: Glucose-6-phosphatase Liver: It hydrolyses and remove phosphate from C-6 free glucose is produced. Glucose-6-phosphate → Glucose + Pi Skeletal Muscle : Glucose-6-phosphatase is absent in skeletal muscle. glucose-6-phophate enters glycolysis. Produce ATP for muscle contraction Skeletal muscle does not produce free glucose from glycogen 14-08-2024 22

23. ENERGETICS: In muscle, the energy yield from one glucose residue derived from glycogen is 3 ATP molecules, because no ATP is required for initial phosphorylation of glucose (step 1 of glycolysis). If glycolysis starts from free glucose only 2 ATPs are produced. 14-08-2024 23

24. Regulation of Glycogen metabolism Glycogenesis & Glycogenolysis are reciprocally regulated. Prevent futile cycles 14-08-2024 24 Phosphorylated form Dephosphorylated form Glycogen Phosphorylase ACTIVE INACTIVE Glycogen Synthase INACTIVE ACTIVE

25. Cyclic AMP Mediated Activation Cascade 14-08-2024 25

26. 14-08-2024 26 Reciprocal Regulation of Glycogenolysis and Glycogen Synthesis by Cyclic AMP

27. Effects of Hormones on Glycogen

28. Enzyme Stimulated by Inhibited by Glycogen phosphorylase glucagon, epinephrine , Insulin, ATP cAMP, Ca 2+ , AMP glucose phosphorylation Glycogen synthase Insulin, glucagon, glucose-6-phosphate epinephrine cAMP, Ca 2+ , AMP phosphorylation

29. Muscle Glycogen Phosphorylase: Activation by Ca ++ Muscle Phosphorylase kinase is a tetramer – α , β , γ , and δ . α , β – subunits contain serine residues which can be phosphorylated by protein kinase (cAMP dependent). δ - subunit resembles Calcium binding protein ( calmodulin ) and binds with four Ca ++ δ - subunit with Ca ++ activates catalytic site of γ subunit . Thus, even inactive muscle Phosphorylase can be activated several fold by the calcium ions. 14-08-2024 29

30. Regulators of glycogen metabolism Effectors Glycogen phosphorylase Liver Glycogen synthase Liver Glycogen Phosphorylase Muscle Glycogen synthase Muscle   ATP Inhibition Inhibition Inhibition Inhibition AMP Activation   Activation Inhibition Glucose-6-Phosphate Inhibition Activation Inhibition Activation Ca++ Activation   Activation  

31. Glycogen storage disorders 14-08-2024 31

32. Glycogen Storage Disease Type-I It is also called Von Gierke's Disease. Most common type of glycogen storage disease. Incidence is 1 in 100,000 live births. Glucose-6-phosphatase is deficient. Fasting hypoglycemia that does not respond to stimulation by adrenaline. Glycogen Storage Diseases

33. The glucose cannot be released from liver during overnight fasting. Hyperlipidemia , lactic acidosis and ketosis. Glucose-6-phosphate is accumulated  channeled to HMP shunt pathway  ↑ ribose & nucleotides. Purines  uric acid  hyperuricemia. Glycogen gets deposited in liver  Massive liver enlargement  cirrhosis. Children usually die in early childhood. Treatment is to give small quantity of food at frequent intervals.

34. Glycogen storage diseases Type Disease Biochemical defect / enzyme deficient Biochemical changes / symptoms I Von Gierke’s Disease Glucose-6-phophatase Glycogen is deposited in liver & renal tubules. Hypoglycemia, hyperlipidemia, ketosis, lactic acidosis & hyperuricemia. Death in early childhood. II Pompe’s Disease Lysosomal α -1,4 & 1,6 glucodisase (acid maltase) Glycogen is accumulated in lysosome. Death due to cardiac failure in childhood. III Cori’s Disease / limit dextrinosis Debranching enzyme (liver & muscle) Fasting hypoglycemia, hepatomegaly, muscle weakness. Accumulation of limit dextrin (abnormal glycogen ). IV Anderson Disease / amylopectinosis Branching enzyme Hepatosplenomegaly. Accumulation of glycogen with only few branches. Death due to cardiac / liver failure in childhood. V McArdle’s Disease Muscle phosphorylase Glycogen content in muscle is very high (3 - 4.0 %). Poor tolerance to exercise. No lactate in blood after strenuous exercise. VI Her’s Disease Liver phosphorylase High glycogen content in liver. Hepatomegaly. Tendency to develop Hypoglycemia. VII Tarui’s Disease Muscle PFK Exercise Intolerance. Hemolytic anemia 14-08-2024 34

35. Glycogen Storage Disease Type-VIII, IX, X Deficiencies of phosphorylase kinase of liver, muscle and protein kinase A deficiency respectively. Mild hypoglycemia, exercise intolerance and hepatomegaly They have better prognosis.