1402 Glycogen Metabolism

• Glycogen

  Storage form of glucose in animals

  
• Glycogen


  • Stored in liver (6-8%) and muscle (1-2%)
  • Helps to maintain the blood glucose levels between meals
  • Glycogen stores increase in a well-fed state, depleted during fasting
  • Muscle glycogen serves as a fuel reserve for the supply of ATP during muscle contraction

  
• Glycogenesis


  Synthesis of glycogen from glucose which occurs in liver and muscle

  
• Glycogenesis


  1. Glucose converted to glucose-6-phosphate

  2. Glucose-1-phosphate formed

  3. UDPG (uridine diphosphate glucose) formed

  4. Glucose from UDPG attached to glycogen primer

  5. Branching enzyme forms 1,6 linkages

  
• Glycogenin


  Protein that can accept glucose from UDPG to initiate glycogen synthesis

  
• Glycogenolysis


  Breakdown of glycogen to glucose

  
• Glycogenolysis


  1. Phosphorylase splits 1,4 glucosidic bonds

  2. 1,4 glucan transferase transfers glucose residues

  3. Amylo 1,6 glucosidase splits 1,6 linkages

  4. Acid maltase degrades small quantity of glycogen

  
• Muscle glycogenolysis


  Glycogen -> Glucose-1-phosphate -> Glucose-6-phosphate -> Glycolysis -> Lactate

  
• Glycogen synthase


  Exists in active and inactive forms, dephosphorylated form is active
• Phosphorylase


  Exists in active and inactive forms, phosphorylated form is active
• High cAMP level


  Activates phosphorylase, inactivates glycogen synthase
• Glucose-6-phosphate, ATP


  Allosterically inhibit phosphorylase

  
• Glucose


  Acts as inhibitor to phosphorylase

  
• Adrenaline, Glucagon


  Activate glycogenolysis
• Insulin


  Activates glycogenesis
• Glycogen storage diseases


  Genetic diseases with abnormal type or increased quantity of glycogen deposition in tissues
• Types of glycogen storage diseases

  • Type I (Glucose-6-phosphatase deficiency, Von Gierke's disease)
  • Type II (Lysosomal α-1,4-glucosidase deficiency, Pompe's disease)
  • Type III (Debranching enzyme deficiency, Limit dextrinosis/Cori's disease)
  • Type IV (Branching enzyme deficiency, Andersons disease)
  • Type V (Muscle glycogen phosphorylase deficiency, McArdle's disease)
  • Type VI (Liver glycogen phosphorylase deficiency, Hers disease)

  
• Von Gierke's disease features:
• Treatment for glycogen storage diseases is to provide small frequent meals to maintain blood glucose levels