Glycogen Metabolism and Enzymatic Control

Introduction

• Lecture focused on biochemistry: specifically glycogen metabolism.
• Format: Live lecture on Tuesday and recorded lecture on Thursday.
  • Homework due this week related to prior week's lecture.
  • Short quiz on lecture two material (introduction to metabolism).

Overview of Metabolism

• Importance of regulating metabolic pathways: catabolic (breakdown) vs anabolic (synthesis).
• Emphasis on reciprocal control of pathways like glycogen breakdown and synthesis; they should not happen simultaneously to avoid unnecessary energy expenditure.

Hormonal Control of Metabolism

• Major hormones involved in carbohydrate metabolism:
  • Insulin
  • Glucagon
  • Epinephrine
• Hormones regulate metabolic activities, including glycogen metabolism.

Polysaccharides

• Definition: Large carbohydrates made of monosaccharide units (e.g., glucose, fructose).

• **Functions:

  1. Structural:** e.g. cellulose in plant cell walls.
  2. Energy Storage:** e.g. starches (plants) and glycogen (animals).

• Other roles: Can be part of glycoproteins and glycolipids on cell surfaces, contribute to the extracellular matrix (ECM).

Structural Features of Glycogen

• Glycogen consists of linear and branched structures:
  • Linear chains formed by alpha 1-4 glycosidic bonds.
  • Branch points formed by alpha 1-6 glycosidic bonds.
• Glycogen synthesized from glucose units activated by UDP (uridine diphosphate).

Energy Storage: Glycogen Characteristics

• Major storage sites:
  • Liver (largest amount)
  • Muscle
• Glycogen is stored as granules, making it less soluble, which helps maintain osmotic balance in cells.
• Avoids hypertonicity issues that could cause cells to burst due to high glucose concentrations.

Enzymatic Control in Glycogen Metabolism

• Glycogen Breakdown (Glycogenolysis):
  • Enzymes involved:
  1. Glycogen Phosphorylase: Removes glucose units, phosphorylating them to glucose-1-phosphate.
  2. Phosphoglucomutase: Converts glucose-1-phosphate to glucose-6-phosphate.
  3. Glucose-6-Phosphatase: In liver, dephosphorylates glucose-6-phosphate to release glucose into the bloodstream.
  4. Debranching Enzyme: Acts to remove branches from glycogen; has two activities:
     • Debranching activity: Transfers three glucose units from a branch to a linear chain.
     • Alpha 1-6 Glucosidase activity: Removes the last glucose at a branch point.

Glycogen Synthesis (Glycogenesis)

• Initiated by glycogenin:
  • Catalyzes the synthesis of a primer (8 glucose units linked).
  • Subsequent extension of this primer by glycogen synthase and branching enzyme.

Reducing and Non-Reducing Ends in Glycogen

• Reducing end: Can undergo oxidation (active in reactions), usually linked to non-glycosidic bonded glucose.
• Non-reducing end: Where enzymes act during glycogen metabolism, where glucose residues are added or removed.

Summary of Enzyme Functions

1. Glycogen Phosphorylase: Breaks down glycogen into glucose-1-phosphate.
2. Phosphoglucomutase: Converts glucose-1-phosphate to glucose-6-phosphate.
3. Glucose-6-phosphatase: Converts glucose-6-phosphate to free glucose.
4. Debranching Enzyme: Handles the removal of branches during glycogenolysis.

Conclusion

• Key focus on regulation of metabolism as it relates to hormonal control and the functionality of enzymes in glycogen metabolism.
• Understanding these processes critical for insight into energy storage and regulation in biological systems.