Studing of biosynthesis and catabolism of glycogen . Regulation of glycogen metabolism.

1. Studing of biosynthesis and catabolism of glycogen. Regulation of glycogen metabolism.

2. GLYCOGEN SYNTHESIS AND DEGRADATION In the well-fed state the glucose after absorption is taken by liver and deposited as a glycogen Glycogen is a very large, branched polymer of glucose residues that can be broken down to yields glucose molecules when energy is needed

3. Most glucose residues in glycogen are linked by a-1,4-glyco-sidic bonds, branches are created by a-1,6-glycosidic bonds

4. Glycogen serves as a buffer to maintain blood-glucose level. Stable blood glucose level is especially important forbrain where it is the only fuel. The glucose from glycogen is readily mobilized and is therefore a good source of energy for sudden, strenuous activity. Liver (10 % of weight) and skeletal muscles (2 %) – two major sites of glycogen storage Glycogen is stored in cytosolic granules in muscle and liver cells of vertebrates

5. Glucose-6-phosphate is the central metabolite in the synthesis and decomposition of glycogen. In the well-fed state glucose is converted to glucose-6-phosphate, which is the precursor for the glycogen synthesis. The glucose-6-phosphate derived from the breakdown of glycogen has three fates: (1) glycolysis; (2) pentose-phosphate pathway; (3) convertion to free glucose for transport to another organs.

6. DEGRADATION OF GLYCOGEN Glycogenolysis- degradation of glycogen The reaction to release glucose from polysaccharide is not simple hydrolysis as with dietary polysaccharides but cleavage by inorganic phosphate – phosphorolytic cleavage Phosphorolytic cleavage or phosphorolysisis catalyzed by enzyme glycogen phosphorylase There are two ends on the molecules of starch or glycogen: a nonreducing end (the end glucose has free hydroxyl group on C4) and a reducing end (the end glucose has an anomeric carbon center (free hydroxyl group on C1)

7. Glycogen phosphorylase removes glucose residues from the nonreducing ends of glycogen Acts only on a-1-4 linkages of glycogen polymer Product is a-D-glucose 1-phosphate (G1P) Cleavage of a glucose residue from the nonreducing end of glycogen

8. Structure of glycogen phosphorylase (GP) • GP is a dimer of identical subunits (97kD each) • Catalyticsites are in clefts between the two domains of each subunit • Bindingsites for glycogen, allosteric effectors and a phosphorylation site • Two forms of GP • Phosphorylase a (phospho- rylated) active form • Phosphorylase b (dephospho- rylated) less active

9. GP catalyzes the sequentialremoval of glucose residues from the nonreducing ends of glycogen • GP stops 4 residues from an a 1-6 branch point • Tranferase shifts a block of three residues from one outer branch to the other • A glycogen-debranchingenzyme or 1,6-glucosidase hydrolyzes the 1-6-glycosidic bond • The products are a free glucose-1-phosphate molecule and an elongated unbranched chain

10. Metabolism of Glucose 1-Phosphate (G1P) • Phosphoglucomutasecatalyzes the conversion of G1P to glucose 6-phosphate (G6P)

11. Glycogen Synthesis • Synthesis and degradation of glycogen require separate enzymatic steps • Cellular glucose converted to G6P by hexokinase • Three separate enzymatic steps are required to incorporate one G6P into glycogen • Glycogen synthase is the major regulatory step

12. Glucose 1-Phosphate formation • Phosphoglucomutasecatalyzes the conversion of glucose 6-phosphate (G6P) to glucose 1-phosphate (G1P).

13. UDP-glucose is activated form of glucose. UDP-glucose is synthesized from glucose-1-phosphate and uridine triphosphate (UTP) in a reaction catalized by UDP-glucose pyrophosphorylase

14. Glycogen synthase adds glucose to the nonreducing end of glycogen

15. A branching enzyme forms -1,6-linkages Glycogen synthase catalyzes only -1,4-linkages. The branching enzyme is required to form -1,6-linkages. Branching is important because it increases the solubility of glycogen. Branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase.

16. Regulation of Glycogen Metabolism • Muscleglycogen is fuel for muscle contraction • Liverglycogen is mostly converted to glucose for bloodstream transport to other tissues • Both mobilization and synthesis of glycogen are regulated by hormones • Insulin, glucagon and epinephrine regulate mammalian glycogen metabolism

17. Hormones Regulate Glycogen Metabolism Insulin • Insulin isproduced by b-cells of the pancreas (high levels are associated with the fedstate) • Insulin increases rate of glucose transport into muscle, adipose tissue via GluT4 transporter • Insulin stimulates glycogensynthesis in the liver via the second messenger phosphatidylinositol 3,4,5-triphosphate (PIP3)

18. Glucagon • Secreted by the a cells of the pancreas in response to lowbloodglucose (elevated glucagon is associated with the fastedstate) • Stimulates glycogendegradation to restore blood glucose to steady-state levels • Only liver cells are rich in glucagon receptors and therefore respond to this hormone

19. Epinephrine (Adrenalin) • Released from the adrenal glands in response to sudden energy requirement (“fight or flight”) • Stimulates the breakdownofglycogen to G1P (which is converted to G6P) • Increased G6P levels increase both the rate of glycolysis in muscle and glucoserelease to the bloodstream from the liver and muscles • Both liver and muscle cells have receptors to epinephrine

20. Effects of hormones on glycogen metabolism

21. Reciprocal Regulation of GlycogenPhosphorylase and Glycogen Synthase • Glycogen phosphorylase (GP) and glycogen synthase (GS) control glycogen metabolism in liver and muscle cells • GP and GS are reciprocallyregulated both covalently and allosterically (when one is active the other is inactive) • Covalent regulation by phosphorylation (-P) and dephosphorylation (-OH) • Allosteric regulation by glucose-6-phosphate (G6P)

22. Reciprocal Regulation of GP and GS COVALENTREGULATION Activeform “a” Inactiveform “b” Glycogen phosphorylase -P -OH Glycogen synthase -OH -P ALLOSTERICREGULATION by G6P GP a (active form) - inhibited by G6P GS b (inactive form) - activated by G6P

23. Activation of GP and inactivation of GS by Epinephrine and Glucagone

24. Activation of GS and inactivation of GP by Insulin