GLYCOGEN METABOLISM

1. Biochemical Importance of Glycogen

  • Storage Form of Glucose:

    • Found primarily in the liver and muscles.

    • Linked by α-1,4 glycosidic bonds with α-1,6 branches.

  • Functions:

    1. Immediate Energy Source (Muscles): Provides glucose-6-phosphate for glycolysis during contraction.

    2. Blood Glucose Regulation (Liver): Prevents hypoglycemia during fasting by glycogenolysis.

    3. Anaerobic Fuel: Supplies energy during oxygen-limited activities.

2. Glycogenesis (Glycogen Synthesis)

  • Energy-requiring process converting glucose to glycogen.

  • Key Steps:

    1. Activation of Glucose:

      • Glucose → Glucose-6-phosphate → Glucose-1-phosphate.

      • Catalyzed by UDP-glucose pyrophosphorylase.

    2. Glycogen Synthase: Adds glucose from UDP-glucose to the glycogen primer.

    3. Branch Formation:

      • Branching enzyme (α-1,6 glycosidic linkages).

      • Branching increases solubility and enzymatic access.

3. Glycogenolysis (Glycogen Degradation)

  • Not the reverse of glycogenesis.

  • Key Enzymes:

    1. Glycogen Phosphorylase: Removes glucosyl units (α-1,4 bonds) to form glucose-1-phosphate.

    2. Debranching Enzyme: Resolves branch points.

      • Glucan Transferase: Transfers outer branches to another chain.

      • α-1,6 Glucosidase: Releases branch-point glucose.

    3. Phosphoglucomutase: Converts glucose-1-phosphate → glucose-6-phosphate.

    4. Glucose-6-Phosphatase: Converts glucose-6-phosphate to free glucose (liver-specific).

4. Regulation of Glycogen Metabolism

  • Liver:

    • Maintains blood glucose homeostasis.

    • Regulated by insulin (synthesis), glucagon, and epinephrine (degradation).

  • Muscle:

    • Provides ATP for contraction.

    • Stimulated by AMP, Ca²⁺, and epinephrine.

  • Key Enzymes:

    • Glycogen synthase (glycogenesis).

    • Glycogen phosphorylase (glycogenolysis).

5. Clinical Relevance: Glycogen Storage Diseases (GSDs)

  • Von Gierke’s Disease (GSD Type I):

    • Deficiency in glucose-6-phosphatase.

    • Leads to hypoglycemia, hepatomegaly, lactic acidosis, and hyperlipidemia.

    • Dental implications: Periodontitis, delayed dentition, and recurrent infections.