Glycogen Metabolism By Dr. Amr S. Moustafa, MD, PhD

Objectives: By the end of these 2 lectures, you should be able to: Understand the structure-function relationship of glycogen Identify glycogen as a storage of carbohydrates in liver & muscle Understand glycogen synthesis (glycogenesis) Understand glycogen breakdown (glycogenolysis) Understand the regulation of both glycogenesis and glycogenolysis Identify examples of glycogen storage diseases

Structure of Glycogen Glycogen is a branched-chain homopolysaccharide made exclusively from a- D-glucose Glucose residues are bound by a(1 - 4) glucosidic linkage Branches (every 8-10 residue) are linked by a(1-6) glucosidic linkage Glycogen is present in the cytoplasm in the form of granules which contain most of the enzymes necessary for glycogen synthesis & degradation

Structure of Glycogen: a- D-glucose

Structure of Glycogen a(1 - 4) glucosidic linkage a(1 - 6) glucosidic linkage at the branch point (every 8-10) Many nonreducing ends

Why is glycogen highly branched? More-soluble Increases the number of non-reducing ends to which new glucosyl residues can be added (glycogenesis) or removed (glycogenolysis) i.e., glycogen can readily accept or degrade more glucose units at a time

Nonreducing Vs Reducing ends The nonreducing end of a carbohydrate chain is one in which the anomeric carbon of the terminal sugar is linked by a glycosidic bond to another compound, making the terminal sugar nonreducing If the O on the anomeric C of a sugar is not attached to any other structure (Free), that end of the sugar is reducing

Location & Functions of Glycogen Location of glycogen in the body: Mainly in skeletal muscle & liver 400 g in muscles (1-2% of resting muscles weight) 100 g in liver (~ 10% of well-fed liver) Functions of glycogen: Function of muscle glycogen: fuel reserve (ATP) (during muscular exercise) Function of liver glycogen: a source for blood glucose (especially during early stages of fasting)

Metabolism of Glycogen Glycogenesis: Synthesis of glycogen from glucose Glycogenolysis: Breakdown of glycogen into glucose-6-phosphate (in muscle) or glucose in liver

GLYCOGENESIS 1- Building blocks: UDP-GLUCOSE 2- Initiation of synthesis: Elongation of pre-existing glycogen fragment OR The use of glycogen primer (glycogenin) 3- ELONGATION: Glycogen synthase (for a1-4 linkages) The rate-limiting enzyme of glycogenesis 4- BRANCHING: Branching enzyme (for a1-6 linkages) Glycogen synthase cannot initiate synthesis but only elongates pre-existing glycogen fragment or glycogen primer (glycogenin)

Synthesis of Glycogen

Synthesis of Glycogen: Branching Enzyme Amylo-α(1→4)→α(1→6)-transglucosidase removes a set of 6-8 glucosyl residues from the nonreducing end of the glycogen chain and thus creating a new nonreducing end (breaking an α(1→4) bond, and attaches the removed set to a nonterminal glucosyl residue by an α(1→6) linkage, thus functioning as a 4:6 transferase)

Glycogenolysis 1- Shortening of glycogen chain: by glycogen phosphorylase Cleaving of a(1-4) bonds of the glycogen chain producing glucose 1-phosphate Glucose 1-phosphate is converted to glucose 6-phosphate (by mutase enzyme) 2- Removal of branches : by debranching enzymes Cleaving of a(1-6) bonds of the glycogen chain producing free glucose (few)

Glycogenolysis in Liver Vs Muscle 3- Fate of glucose 6-phosphate (G-6-P): In the liver: G-6-P is converted into free glucose by glucose-6-phosphatase In the muscle: G-6-P is not converted to free glucose due to lack of glucose-6-phosphatase in the muscle G-6-P is used as a source of energy for skeletal muscles during muscular exercise (by anaerobic glycolysis starting from G-6-P step)

Glycogenolysis Glycogen phosphorylase: Rate-limiting enzyme Requires pyridoxal phosphate as a co-enzyme Products: Glucose-1-phosphate Remaining glycogen chain (n-1) (Pyridoxal phosphate)

Glycogenolysis Debranching enzyme: A bifunctional enzyme: A) Oligo-α(1→4)→α(1→4)- glucantransferase activity (4:4 transferase): Breaks 1:4 bond near branch point and makes 1:4 bond at another part of the chain. B) Amylo-α(1→6)-glucosidase activity: Breaks 1:6 bond, releasing free glucose

Glycogenolysis Fate of G-6-P in muscle and liver In the muscle: G-6-P in muscle continue into anaerobic glycolysis for ATP production during exercise (absence of G-6-Pase in muscle) In the liver: G-6-P is converted into glucose by G-6-Pase, to maintain blood glucose level during early fasting.

Regulation of Glycogen Metabolism Synthesis & degradation of glycogen are tightly regulated and reciprocally controlled In Skeletal Muscles: Glycogen degradation occurs during active exercise Glycogen synthesis begins when the muscle is at rest In Liver: Glycogen degradation occurs during early fasting Glycogen synthesis occurs in the well-fed state Regulation occurs by 2 mechanisms: 1- Allosteric regulation 2- Hormonal regulation (Covalent modification)

Regulation of Glycogen Metabolism 1. Allosteric Regulation Note that: Glucose is allosteric inhibitor to glycogen phosphorylase in the liver only AMP is allosteric activator to glycogen phosphorylase in the muscle Ca2+ is allosteric activator to glycogen phosphorylase in both muscle and liver Ca2+ +

Role of Calcium for Regulation of Glycogen Phosphorylase Increase of cytosolic calcium during muscle contraction by neural mechanism, and in liver by epinephrine binding to α1-adrenergic receptors. Formation of Ca2+ -calmodulin complex Activation of Ca2+ -dependent enzymes, e.g., glycogen phosphorylase kinase Glycogen phosphorylase kinase phosphorylates and activates glycogen phosphorylase Epinephrine at β-adrenergic receptors activates glycogen degradation via a rise in cAMP, not Ca2+

Hormonal Regulation of Glycogen Phosphorylase by Covalent Modifications: Glucagon & Epinephrine Glucagon & epinephrine (in liver) and epinephrine (in muscle) Increase cAMP Activate protein kinase A (PKA) Activate glycogen phosphorylase kinase (by phosphorylation) Activate glycogen phosphorylase (by phosphorylation): Converting glycogen phosphorylase b (dephospho & inactive) into glycogen phosphorylase a (phospho and active)

Hormonal Regulation of Glycogen Phosphorylase by Covalent Modifications

Hormonal Regulation of Glycogen Synthase by Covalent Modifications: Glucagon & Epinephrine Glucagon & epinephrine (in liver) and epinephrine (in muscle) Increase cAMP Activate protein kinase A (PKA) Inhibit glycogen synthase (by phosphorylation): Converting glycogen synthase a (dephospho & active) into glycogen synthase b (phospho and inactive)

Regulation of Glycogen Metabolism: Hormonal Regulation by Glucagon & Epinephrine Muscle contraction (epinephrine) or hypoglycemia (glucagon/epinephrine) Skeletal muscle: Epinephrine/receptor binding Liver: Glucagon/receptor binding & Epinephrine/receptor binding Second messenger: cAMP Response: Enzyme phosphorylation Glycogen synthase (Inactive form) Inhibition of glycogenesis Glycogen phosphorylase (Active form) Stimulation of glycogenolysis P

Hormonal Regulation of Glycogen Metabolism Glucagon and Epinephrine Effects

Hormonal Regulation of Glycogen Synthase by Covalent Modifications: Insulin Decreases cAMP by activation of phosphodiesterase Deactivates protein kinase A (PKA) Activates glycogen synthase (by dephosphorylation): Converting glycogen synthase b (phospho & inactive) into glycogen synthase a (dephospho and active) Inhibits both glycogen phosphorylase kinase and glycogen phosphorylase (by dephosphorylation) Dephosphorylation effects of insulin are due to decrease of cAMP and also due to stimulation of protein phosphatase enzyme

Regulation of Glycogen Metabolism: Allosteric and Covalent Mechanisms

Glycogen Storage Diseases (GSDs) A group of genetic diseases that result from a defect in an enzyme required for glycogen synthesis or degradation They result in: Formation of abnormal glycogen structure OR Excessive accumulation of normal glycogen in a specific tissue

GSDs: Type Ia von Gierke Disease

GSDs: Pompe, Cori & McArdle GSD Type II: Pompe disease GSD Type III: Cori disease GSD Type V: McArdle syndrome