DPT - Biochemistry & Genetics - II (Metabolism of Carbohydrates) Metabolism of Glycogen

DPT - Biochemistry & Genetics - II (4th Semester)

1. METABOLISM OF CARBOHYDRATES METABOLISM OF GLYCOGEN by , Miss Areesha Ahmad Lecturer

2. CONTENTS Harper's Biochemistry, 28th Ed. Glycogen Chemistry of Glycogen Functions of Glycogen Glycogen Metabolism Steps of Glycogen Metabolism Glycogenesis Steps of Glycogenesis Glycogenolysis Steps of Glycogenolysis Summary References

3. GLYCOGEN Glycogen is the storage form of glucose It is Hompolysachharide Also known as animal starch It is a very large, branched polymer of glucose residues that can be broken down to yield glucose molecules when energy is needed. Harper's Biochemistry, 28th Ed. 3

4. GLYCOGEN Harper's Biochemistry, 28th Ed. 4

5. GLYCOGEN Glucose is stored in the form of Glycogen in Liver & Muscles . In liver (6- 8%) and in muscles (1- 2%) stored . Most of the glucose residues in glycogen are linked by α- 1,4- glycosidic bonds . Branches at about every tenth residue are created by α- 1,6- glycosidic bonds. Harper's Biochemistry, 28th Ed.

6. FUNCTIONS OF GLYCOGEN Harper's Biochemistry, 28th Ed. 6 Maintain Blood glucose levels. Liver glycogen stores increase metabolic condition of the body after consuming a meal , which are used up d uring fasting. Glycogen stored in muscles is used as an energy source to produce ATP (adenosine triphosphate), which is the primary energy carrier in cells.

7. CHEMISTRY OF GLYCOGEN Harper's Biochemistry, 28th Ed. 7 Glycogen is present in the cytosol in the form of granules ranging in diameter from 10 to 40 nm . It has a molecular mass of 107 Daltons ( Da ) and consists of polysaccharide chains, each containing about 13 glucose residues .

8. The chains are either branched or unbranched and are arranged in 12 concentric layers. CHEMISTRY OF GLYCOGEN

9. The branched chains (each has two branches) are found in the inner layers and T he unbranched chains in the outer layer . CHEMISTRY OF GLYCOGEN Glycogenin : It is  an enzyme that helps convert glucose into glycogen. It is the primer molecule for glycogen synthesis . It is a branched polymer of glucose that stores energy in tissues. 

10. Glycogenin is a protein that serves as the starting point, or "primer," for the synthesis of glycogen. Glycogen is a large, branched polysaccharide that the body uses to store glucose, particularly in the liver and muscle tissues. CHEMISTRY OF GLYCOGEN

11. The body stores glucose as glycogen instead of as glucose itself for several reasons: Osmotic Pressure : Glucose is a small molecule that, if stored in high concentrations, would draw water into cells through osmosis, potentially causing cellular damage or swelling. Glycogen, being a large, branched polysaccharide, does not exert the same osmotic pressure, making it more suitable for storage. Efficient Storage : Glycogen is a more compact way to store glucose. Its branched structure allows for the storage of a large amount of glucose in a relatively small space. This efficient packing helps keep the storage sites, like the liver and muscles, from becoming excessively large or burdensome. Regulation : Glycogen storage and breakdown are tightly regulated processes. Glycogen can be quickly mobilized when glucose is needed, such as between meals or during exercise. This regulation helps maintain stable blood glucose levels and ensures a readily available supply of energy. Energy Release : Glycogen can be broken down into glucose molecules rapidly when needed. This breakdown is controlled and more efficient than if glucose were stored directly, allowing the body to respond quickly to changes in energy demands. Preventing Toxicity : High levels of free glucose in the bloodstream can be harmful. By storing glucose as glycogen, the body minimizes the risk of toxic effects from elevated glucose concentrations. WHY BODY STORES GLUCOSE AS GLYCOGEN AND NOT A GLUCOSE ITSELF?

12. ROLE OF LIVER GLYCOGEN Harper's Biochemistry, 28th Ed. Only immediately available reserve store of blood glucose. High liver glycogen level protects the liver cells against harmful effects many poisons and chemicals Certain forms of detoxication (removal of toxic substances) e.g. conjugation ( type of chemical reaction where a molecule is attached to another molecule to make it more water-soluble) with glucouronic acid ( molecule derived from glucose) . Like when a substance is conjugated with glucuronic acid, it becomes more water-soluble. This increased solubility allows the substance to be more easily excreted from the body through urine or bile . Rate of deamination ( the process of removing an amino group) of amino acid in the liver is depressed as the glycogen level rises . High level of liver glycogen depresses the rate of ketone bodies.

13. GLYCOGEN METABOLISM Harper's Biochemistry, 28th Ed. 13 There are Two Pathways of Glycogen Metabolism. Glycogenesis: Is the Pathway of Synthesis of Glycogen. Glycogenolysis: Is the Pathway of Breakdown of Glycogen. They are different from each other. Breakdown pathway is not reversal of synthetic pathways. When glycogenesis occurs glycogenolysis does not take place.

14. STEPS OF GLYCOGEN METABOLISM Harper's Biochemistry, 28th Ed. 14 GLYCOGENESIS GLYCOGENOLYSIS 1. Activation of Glucose 1. The release of glucose 1-phosphate from glycogen 2. Initiation 2. The remodeling of the glycogen substrate to permit further degradation 3. Elongation 3. The conversion of glucose 1- phosphate into glucose 6-phosphate for further metabolism. 4. Glycogen branching

15. GLYCOGENESIS Synthesis of glycogen from glucose is glycogenesis. It takes place in cytosol and requires ATP and UTP ( uridine triphosphate) . It is a mechanism by which excess glucose is stored in the tissues It occurs in almost all the tissues but the predominant sites are liver and muscles Glycogenesis begins with a small glycogen molecule known as glycogen primer The core of the primer is made up of a protein, glycogenin A glucose molecule is attached to glycogenin via a tyrosine residue ( specific amino acid, tyrosine, that is part of a protein) . Harper's Biochemistry, 28th Ed.

16. (pyrophosphate)

17. 1 ST STEP Harper's Biochemistry, 28th Ed. 17 ACTIVATION OF GLUCOSE Synthesis of glycogen from glucose is carried out by the enzyme glycogen synthase . This enzyme utilizes UDP- glucose as one substrate and the non- reducing end of glycogen as another . UDP- Glucose, t he glucose donor in the biosynthesis of glycogen, is an activated form of glucose.

18. ACTIVATION OF GLUCOSE As in glycolysis, glucose is phosphorylated to glucose 6- phosphate, catalyzed by hexokinase in muscle and glucokinase in liver . Glucose 6-phosphate is isomerized to glucose-1-phosphate by Phosphoglucomutase . Glucose- 1- phosphate reacts with uridine triphosphate (UTP) to form the active nucleotide uridine diphosphate glucose ( UDP) and pyrophosphate. Harper's Biochemistry, 28th Ed. 18

19. 2 ND S T E P Harper's Biochemistry, 28th Ed. 19 I N I T I A T I O N A small fragment of pre – existing glycogen must act as a ‘primer ’ to initiate glycogen synthesis In the absence of glycogen primer, a specific protein – namely Glycogenin can accept glucose from UDPG . The hydroxyl group of the amino acid tyrosine of glycogenin is the site at which the initial glucose unit is attached.

20. 2 ND S T E P Harper's Biochemistry, 28th Ed. 20 I N I T I A T I O N A protein composed of two identical 37- kd ( kilodalton ) subunits, each bearing an oligosaccharide of alpha-1,4-glucose units . A glycosidic bond is formed between the anomeric C1 of the glucose moiety derived from UDP- glucose and the hydroxyl oxygen of a tyrosine sidechain of Glycogenin . UDP is released as a product.

21. 2 ND S T E P I N I T I A T I O N Harper's Biochemistry, 28th Ed. 21

22. 3 RD STEP Harper's Biochemistry, 28th Ed. 22 ELONGATION Third Elongation Step : The actual elongation of the glycogen chain occurs primarily through the action of the enzyme glycogen synthase . Here’s how it works : Glycogen Synthase : This enzyme adds glucose units from UDP-glucose ( uridine diphosphate glucose) to the growing glycogen chain. UDP-glucose is the activated form of glucose, where glucose is linked to UDP, providing the energy needed for the reaction . Formation of Glycosidic Bond : Glycogen synthase catalyzes the formation of an α-1,4- glycosidic bond between the terminal glucose residue of the existing glycogen chain and the glucose unit from UDP-glucose . Glycogen n​ + UDP-glucose→ Glycogen n+1​ + UDP

23. 4 TH STEP Harper's Biochemistry, 28th Ed. 23 GLYCOGEN BRANCHING Glycogen synthase can catalyse the synthesis of a linear unbranched molecule with 1,4 alpha – glycosidic linkages . Glycogen, however, is a branched tree like structure . The formation of branches is brought about by the action of a branching enzyme, namely glucosyl alpha – 4 - 6 transferase.

24. 4 TH STEP Harper's Biochemistry, 28th Ed. 24 GLYCOGEN BRANCHING This enzyme transfers a small fragment of five to eight glucose residues from the non – reducing end of glycogen chain to another glucose residue where it is linked by alpha - 1,6 bond . This leads to the formation of a new non– reducing end, besides the existing one . Glycogen is further elongated and branched, respectively, by the enzymes glycogen synthase and glucosyl 4 – 6 transferase

25. 4 TH STEP GLYCOGEN BRANCHING Harper's Biochemistry, 28th Ed. 25

26. GLYCOGENOLYSIS DEGRADATION OF GLYCOGEN Harper's Biochemistry, 28th Ed. 27 The breakdown of stored glycogen in liver and muscle is called as glycogenolysis. The pathway for the synthesis and degradation of glycogen are irreversible. An independent set of enzymes present in the cytosol carry out glycogenolysis. Degradation of glycogen occurs by breakdown of alpha- 1,4- and alpha- 1,6 glycosidic bonds.

27. GLYCOGENOLYSIS Harper's Biochemistry, 28th Ed. 29 The efficient breakdown of glycogen to provide glucose 6- phosphate for further metabolism requires four enzyme activities: one to degrade glycogen two to remodel glycogen so that it remains a substrate for degradation one to convert the product of glycogen breakdown into a form suitable for further metabolism .

28. STEPS OF GLYCOGENOLYSIS Harper's Biochemistry, 28th Ed. 30 Glycogen degradation consists of three steps: The release of glucose 1- phosphate from glycogen The remodeling of the glycogen substrate to permit further degradation The conversion of glucose 1- phosphate into glucose 6- phosphate for further metabolism.

29. PHOSPHORYLASE ENZYME Glycogen phosphorylase is the key enzyme in glycogen breakdown, cleaves its substrate by the addition of orthophosphate (Pi) to yield glucose 1- phosphate. The cleavage of a bond by the addition of orthophosphate is referred to as phosphorolysis. Harper's Biochemistry, 28th Ed. 31 ENZYMES REQUIRED FOR GLYCOGENOLYSIS

30. TRANSFERASE AND DEBRANCHING ENZYME Harper's Biochemistry, 28th Ed. 32 The Transferase enzyme transfer a single glucose residue joined by an α - 1,6- glycosidic linkage . α- 1,6- Glucosidase, also known as the debranching enzyme, hydrolyzes the α - 1, 6- glycosidic bond, resulting in the release of a free glucose molecule.

31. PHOSPHOGLUCOMUTASE ENZYME Harper's Biochemistry, 28th Ed. 33 Glucose 1- phosphate formed in the phosphoroylytic cleavage of glycogen must be converted into glucose 6- phosphate to enter the metabolic mainstream . This shift of a phosphoryl group is catalyzed by Phosphoglucomutase .

32. STEPS OF GLYCOGENOLYSIS RELEASE OF GLUCOSE 1- PHOSPHATE: Phosphorylase catalyzes the sequential removal of glucosyl residues from the Non- reducing ends of the glycogen molecule (the ends with a free 4- OH group). Orthophosphate splits the glycosidic linkage between C- 1 of the terminal residue and C- 4 of the adjacent one. Harper's Biochemistry, 28th Ed. 34

33. STEPS OF GLYCOGENOLYSIS Harper's Biochemistry, 28th Ed. 35 REMODELLING OF THE GLYCOGEN SUBSTRATE: The α- 1,6- glycosidic bonds at the branch points are not susceptible to cleavage by phosphorylase. Glycogen phosphorylase stops cleaving α - 1,4 linkages when it reaches a terminal residue four residues away from a branch point. Because about 1 in 10 residues is branched, glycogen degradation by the phosphorylase alone would come to a halt after the release of six glucose molécules per branch.

34. REMODELLING OF THE GLYCOGEN SUBSTRATE Harper's Biochemistry, 28th Ed. 36 Transferase and α- 1,6- glucosidase, remodel the glycogen for continued degradation by the phosphorylase . Thus , the Transferase and α-1,6- glucosidase convert the branched structure into a linear one, which paves the way for further cleavage by phosphorylase .

35. REMODELLING OF THE GLYCOGEN SUBSTRATE Debranching enzyme, hydrolyzes the α - 1, 6- glycosidic bond, resulting in the release of a free glucose molecule. Harper's Biochemistry, 28th Ed. 37

36. CO NVERSI O N O F G LUCO SE 1 - PHO PHATE TO G LUCO SE 6 PHO SPHATE Phosphoglucomutase converts glucose 1- phosphate into 6- phosphate in a reversible reaction. Glucose 6-phosphate derived Can be used as a fuel for muscle Can be converted into free glucose in the liver and subsequently released into the blood Can be processed by the pentose phosphate pathway . Harper's Biochemistry, 28th Ed. 38

37. CO NVERSI O N O F G LUCO SE 1 - PHO PHATE TO G LUCO SE 6 - PHO SPHATE The liver contains a hydrolytic enzyme, glucose 6-phosphatase, which cleaves the phosphoryl group to form free glucose and orthophosphate. Glucose 6- phosphatase is absent from most other tissues. Consequently, glucose 6- phosphate is retained for the generation of ATP. The liver releases glucose into the blood during muscular activity and between meals to be taken up primarily by the brain and skeletal muscle. Harper's Biochemistry, 28th Ed. 39

38. SUMMARY Glycogen is a polymer of glucose residues linked by α- (1,4)- and α- ( 1,6)- glycosidic bonds. Stores of readily available glucose, to supply the tissues of the body with an oxidizable energy source, are found as glycogen, solely in the liver . Harper's Biochemistry, 28th Ed. 40

39. REFERENCES Textbook of Biochemistry, Sattaya Narayan, 5 th Edition. Lippincott’s Illustrated Reviews Biochemistry Fifth Edition Textbook of Medical Biochemistry, MN Chatterjea , 8 th Edition. Harper's Biochemistry, 28th Ed. :

40. Harper's Biochemistry, 28th Ed. 42