Glycogen Metabolism Dr. Tarek A Salem Biochemistry Qassim University College of Medicine

Objectives List the steps of glycogen biosynthesis (glycogenesis) Mention site of occurrence, regulation and importance of glycogenesis. Differentiate between liver and muscle glycogen List the steps of glycogenolysis Mention site of occurrence, regulation and importance of glycogenolysis. Explain the biochemical basis of glycogen storage disorders

Liver Cell

Glycogen Glycogen is the main storage form of carbohydrates in animals. It is present mainly in liver and muscle. In the liver, glycogen can compose up to 8% of the fresh weight (100–120 g in an adult) soon after a meal. Only the glycogen stored in the liver can be made accessible to other organs. In the muscles, glycogen is found in a much lower concentration (1% to 2% of the muscle mass), but the total amount exceeds that in the liver.

Glycogen is a polymer of glucose residues linked by  alpha(1  4) glycosidic bonds, mainly  alpha(1  6) glycosidic bonds, at branch points. - Glycogen branches contain about glucose residues.

Glycogenesis It is the formation of glycogen in muscle and liver Its site in the cytoplasm of every cells mainly liver and muscle

Glycogen synthesis

Steps Glucokinase Phosphoglucomutase 1- Glucose G6P G1P Mg ++ ATP ADP UDP-Glucose pyrophosphorylase 2- G-1-P UDP-glucose UTP PPi H 2 O Pyrophosphatase 2Pi

Glycogen synthesis 1-Glycogen synthase enzyme in presence of pre- existing glycogen primer (glycogenin). 2- Chain is lenthened, branching enzyme transfers a part of the α-1:6 glycosidic link. 3- The branches grow up by further addition of 1:4 glucosyl units. 4- The key regulatory enzyme is glycogen synthase which present in 2 forms: - Active (dephosphorylated form) - Inactive (phosphorylated form)

Glycogen synthase It catalyzes transfer of the glucose moiety of UDP- glucose to the hydroxyl at C4 of the terminal residue of a glycogen chain to form an  (1  4) glycosidic linkage: Glycogen (n) + UDP-glucose  Glycogen (n +1) + UDP

Glycogenolysis It is the breakdown of glycogen into glucose in liver and lactic acid in muscles. Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the α(1  4) glycosidic linkages of glycogen, releasing glucose-1- phosphate as reaction product. Glycogen (n) + P i  Glycogen (n–1) + glucose-1-phosphate

Glycogenolysis G-1-P is converted into G-6-P by the action of phosphoglucomutase. In liver, G-6-P is hydrolysed by the action of G-6-phosphatase free glucose In muscle, G-6-P by glycolysis lactic acid because absence of G-6-phosphatase

Pyridoxal phosphate (PLP), a derivative of vitamin B 6, serves as prosthetic group for Glycogen Phosphorylase.

Glycogenolysis The key regulatory enzyme is glycogen breakdown which present in 2 forms: - Active (Phosphorylated form) - Inactive (Dephosphorylated form) = Debranching enzyme is hydrolytic enzyme acts on α1 6 glycosidic link giving free glucose. = Muscle glycogen is to provide muscle with glucose. = Liver glycogen is to maintain blood glucose between meals. = After hr fasting, liver glycogen whereas muscle glycogen is after prolonged exercise.

Difference between muscle and liver glycogen Muscle glycogenLiver glycogen More in amountMore in concentrationAmount Glucose onlyGlucose and other precursoreSource Give lactic acidGive blood glucoseHydrolysis Not affectedConverted into blood glucoseStarvation Depleted Muscular exercise Insulin Adrenalin Thyroxine Glucagon Insulin Adrenaline Thyroxine Glucagon Effect of Hormons

Regulation of glycogen metabolism In Fasting: Glucagon cAMP activates Protein Kinase A which phosphorylates and inactivates glycogen synthase Little glycogen synthesis during fasting

Regulation of glycogen metabolism In Fasting: Glucagon cAMP activates Protein Kinase A which phosphorylates and activates glycogen phosphorylase Fasting results in increased glycogenolysis

Regulation of glycogen metabolism In Feeding: Insulin cAMP is reduceded stimulates phosphodiesterase induces and activates protein phosphatase-1 Activates Glycogen Synthase (Glycogenesis is activated) While Glycogen Phosphorylase is inactivated (Glycogenolysis is inactivated) Feeding results in glycogen synthesis

Coordinated regulation of glycogen metabolism

Glycogen storage diseases (GSD) They are the result of defects in the processing of glycogen synthesis or breakdown within muscles, liver, and other cell types. GSD has two classes of cause: genetic and acquired. Genetic GSD is caused by any inborn error of metabolism. In livestock, acquired GSD is caused by intoxication with the some alkaloids.

Types of GSD There are eleven distinct diseases that are commonly considered to be glycogen storage diseases. Although glycogen synthase deficiency does not result in storage of extra glycogen in the liver, it is often classified with the GSDs as type 0 because it is another defect of glycogen storage and can cause similar problems.)

Glycogen Storage Diseases are genetic enzyme deficiencies associated with excessive glycogen accumulation within cells. Some enzymes whose deficiency leads to glycogen accumulation are part of the inter-connected pathways shown here.

Symptoms in addition to excess glycogen storage:  When a genetic defect affects mainly an isoform of an enzyme expressed in liver, a common symptom is hypoglycemia, relating to impaired mobilization of glucose for release to the blood during fasting.  When the defect is in muscle tissue, weakness & difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.  Additional symptoms depend on the particular enzyme that is deficient.