GLYCOGEN METABOLISM

Glycogen Metabolism Notes

By: Sarmad Sirag Bashir, M.Sc. Biochemistry and Molecular Biology

Introduction to Glycogen

• Definition: Glycogen is the primary storage form of carbohydrates in animals and is comparable to starch in plants.

• Location: It is predominantly found in the liver and muscle tissues.

• Production: Glycogen is synthesized from glucose in the cytosol of cells.

Structure of Glycogen

• Type of Polysaccharide: Glycogen is a homopolysaccharide formed of branched α-D glucose units.

• Glycosidic Bonds:

  • The primary type of glycosidic bond in glycogen is the α-1,4 linkage.

  • Branching occurs at the α-1,6 linkage at specific points.

• Branching Details: Each branch is composed of approximately 12-14 glucose units.

• Visualization:

  • Nonreducing ends and reducing end present structural variations in how glucose units are linked together.

Functions of Glycogen

1. Liver Glycogen:

   • Its primary function is to release glucose into the bloodstream to maintain normal blood glucose levels, particularly during fasting (12-18 hours after the last meal).

2. Muscle Glycogen:

   • Acts as an energy source for muscle tissues, crucial during muscle contractions; significantly depleted only after extended intense exercise.

Synthesis of Glycogen (Glycogenesis)

• Definition: Glycogenesis refers to the process of forming glycogen from glucose via intracellular synthesis.

• Physiological Condition: Glycogenesis primarily occurs post-meal.

• Location: It occurs in the cytosol of liver and muscle cells, where glycogen constitutes 8-10% of the liver's wet weight and 1-2% in muscles.

Substrates for Glycogen Synthesis

In the Liver:

• Elevated blood glucose

• Other hexoses such as fructose and galactose

• Non-carbohydrate sources including glycerol and lactate

In Muscles:

• Blood glucose is the only source.

• Other requirements include:

  • Insulin (stimulates glucose uptake and activates glycogen synthesis enzymes)

  • Various enzymes

  • UTP (uridine triphosphate)

  • Glycogen primer

Steps of Glycogenesis

1. Activation of Glucose:

   • Involves the transformation of glucose to glucose-1-phosphate through the action of enzymes like glucokinase and phosphoglucomutase, ultimately forming UDP-glucose.

2. Initiation:

   • Glycogenin catalyzes the transfer of the first 8 glucose molecules to its own tyrosine residues (auto-glycosylation), providing a primer for glycogen synthesis.

3. Elongation:

   • Glycogen synthase adds UDP-glucose molecules to the glycogen primer, elongating the α-1-4 branches.

4. Glycogen Branching:

   • A branching enzyme transfers segments (5-8 glucose residues) from the elongated chains to another chain at the C6 position, forming a new α-1-6 linkage.

   • Subsequent elongation of the new branches occurs through glycogen synthase.

Breakdown of Glycogen (Glycogenolysis)

• Definition: Glycogenolysis is the enzymatic breakdown of glycogen into glucose in the liver and into lactic acid in muscles.

• Physiological Conditions:

  • Occurs in the liver predominantly between meals and during fasting (8-12 hours).

  • In muscles, it occurs primarily during exercise.

Key Enzymes Involved

1. Phosphorylase:

   • Breaks down α-1-4 bonds via phosphorolysis, releasing glucose units as glucose-1-phosphate.

2. Debranching Enzyme:

   • Functions to remove trisaccharide units (α-1-4 transferase) and hydrolyze α-1-6 linkages (α-1-6 glucosidase), releasing free glucose.

Steps of Glycogenolysis

• Action of Glycogen Phosphorylase:

  • Acts near branching points to release glucose-1-phosphate units sequentially.

• Transferase Enzyme: Transfers trisaccharide units to other branches.

• Debranching Enzyme: Hydrolyzes α-1-6 linkages, allowing further release of glucose units.

Glucose-6-Phosphate Role

• In the liver, glucose-6-phosphate is converted to glucose by the enzyme glucose-6-phosphatase.

• In muscles, glucose-6-phosphate cannot be converted to glucose due to the absence of glucose-6-phosphatase; instead, it is utilized for ATP production via glycolysis, generating lactate.

Transport Mechanism

• Glucose-6-phosphate transporters assist in moving glucose-6-phosphate through cellular membranes to facilitate metabolic processes.

Regulation of Glycogenesis and Glycogenolysis

• Hormonal Control:

  • Post-meal, insulin inhibits glycogen phosphorylase and activates glycogen synthase, promoting glycogenesis and inhibiting glycogenolysis.

  • During fasting, glucagon and epinephrine inhibit glycogen synthase and stimulate glycogen phosphorylase, resulting in stimulated glycogenolysis and inhibited glycogenesis.

• Energy Requirements:

  • Utilized ATP/UTP: Glycogenesis and glycogenolysis are energetically demanding, requiring specific substrates to facilitate these processes.

Clinical Aspects of Glycogen Metabolism

Glycogen Storage Diseases

• Definition: A group of inherited disorders characterized by abnormal accumulation of glycogen in tissues.

• Causes: Mainly results from deficiencies in enzymes responsible for glycogen metabolism.

  • Defective liver enzymes lead to hypoglycemia.

  • Defective muscle enzymes result in weakness and exercise intolerance.

Types of Glycogen Storage Diseases

Conclusion

• The highly branched structure of glycogen allows for efficient simultaneous action of glycogen metabolic enzymes, facilitating rapid mobilization and storage of glucose as needed.

Thank You!