BIOC 2300-Lecture 18-Glucagon I-Fall2024 (1)

Glycogen Overview

  • Glycogen is a polymer of glucose, specifically a highly branched structure utilized by animals for glucose storage.

  • It is formed mainly in liver and muscle tissue.

Location and Function of Glycogen

Liver

  • Can make up to 10% of liver weight.

  • Sources blood glucose after meals; when depleted, triacylglycerols are utilized as an energy source.

Muscle

  • Constitutes about 2% of muscle weight.

  • Serves as a source of glucose exclusively for muscle activity, particularly during short-term exercise.

Glycogen Structure

  • Contains one reducing end (where the anomeric carbon can react) and multiple non-reducing ends (allowing glucose release).

  • Displayed as granules visible under a microscope.

Glycogenolysis: Breakdown of Glycogen

Overview

  • Glycogenolysis involves several enzymatic steps:

    1. Glycogen phosphorylase releases glucose-1-phosphate from the non-reducing ends.

    2. The debranching enzyme facilitates the removal of branched structures near branch points.

    3. Conversion from glucose-1-phosphate to glucose-6-phosphate by phosphoglucomutase for glycolysis.

    4. In the liver, glucose-6-phosphate is further processed for release into the bloodstream.

Key Enzymes in Glycogenolysis

  • Glycogen Phosphorylase: Cleaves terminal glucose units, releasing glucose-1-phosphate by breaking the α(1-4) bonds.

  • Debranching Enzyme: Rearranges the structure of glycogen and removes branched chains by transferring glucose residues and cleaving α(1-6) bonds.

  • Phosphoglucomutase: Converts glucose-1-phosphate to glucose-6-phosphate, allowing entry into glycolysis.

  • Glucose-6-Phosphatase: Only found in the liver; releases glucose from glucose-6-phosphate for blood glucose regulation.

Glycogenesis: Formation of Glycogen

Steps of Glycogenesis

  1. Activation of Glucose: Glucose is converted to UDP-glucose by UTP.

  2. Primer Formation: Glycogenin generates an 8-glucose primer.

  3. Chain Elongation: Glycogen synthase adds glucose units to the growing chain.

  4. Branch Formation: Glycogen-branching enzyme creates branches in the glycogen structure.

Precursor for Glycogen Synthesis

  • UDP-Glucose: The activated form of glucose that facilitates glycogen synthesis through the formation of glycosidic bonds.

Role of Glycogenin

  • Glycogenin serves as a primer for glycogen synthesis by covalently linking glucose units to itself.

  • Produces an initial chain of 8 glucose units which allows glycogen synthase to begin elongation.

Glycogen Synthase and Branching Enzyme Actions

  • Glycogen Synthase: Responsible for the polymerization of glucose.

  • Glycogen-Branching Enzyme: Forms branches in glycogen by cleaving α(1-4) bonds and creating α(1-6) glycosidic bonds.

Conclusion

  • Understanding glycogen storage and metabolism is fundamental in biochemistry, linking nutrition and energy regulation within the body.

Learning Goals

  • Define, understand, and apply key concepts and procedures related to glycogen metabolism introduced in this lecture.