Glycogen Metabolism in Muscle and Liver

What is Glycogen?

• Polysaccharide: storage form of glucose in the body

• Stored in granules predominantly in liver and muscle as an energy reserve

• Glycogen is formed from dietary glucose by the process of glycogenesis

What is Glycogen used for in Muscles and Liver?

Liver glycogen is utilised to maintain plasma glucose levels between meals, whereas muscle glycogen is required to sustain muscle contraction

How is Glycogen Broken Down?

• Glycogen is degraded between meals in the liver by the glycogenolysis pathway to produce glucose-1-phosphate which can be converted to free glucose and exported into the bloodstream to maintain plasma glucose levels. 

• It can also be broken down in muscle to provide the energy to support muscle contraction.

Where is Glycogen Most Stores?

What is the Structure of Glycogen?

What is Glycogenolysis?

Glycogen Breakdown in Muscles

Glycogen Breakdown in Liver

What is the Relationship between Blood Glucose and Liver Glycogen Stores throughout the day?

Mechanism of Glycogen Breakdown

How is Glycogen Degraded?

Cleavage of α-1,6-linkages at branch points

Summary of Glycogenolysis

What is Glycogenesis?

Formation of Glycogen from UDP-glucose

How does Glycogen Synthesis Start?

Introduction of Branches

Why is Glycogen a Good Energy Store?

Why is Glycogen a Bad Energy Store?

BUT it is a BAD store because glucose is hydrophilic and associates with water increasing the overall weight and bulk

Summary of Glycogen Metabolism

Regulation of glycogen metabolism- Glycogen Mobilisation

Glycogen mobilisation (glycogenolysis) is greatly accelerated:

• In liver in between meals or during extended fasting, when glucose is required for glycolysis by the brain and red blood cells

• In muscle to fuel glycolysis during vigorous exercise

Regulation of glycogen metabolism- Glycogen Synthesis

Glycogen synthesis (glycogenesis) is activated:

• To replenish liver glycogen stores after feeding or muscle stores when exercise ceases. 

• The pathway for glycogen synthesis is not a simple reversal of breakdown and requires energy input

What is Allosteric Regulation of Phosphorylase?

What is Allosteric Regulation of Glycogen Synthase?

Summary of Allosteric Regulation

Regulation of Glycogen Metabolism by Covalent Modification

• Mediated by the addition (and removal) of a phosphate group

• Addition of a phosphate group is known as phosphorylation and is catalysed by protein kinases

• This is a reversible modification; removal of phosphate groups (dephosphorylation) is catalysed by protein phosphatases

How is Phosphorylase Activated by cAMP-dependent Phosphorylation?

How does Induction of cAMP Cascade have Opposite Effect on Glycogen Synthase?

Reciprocal Regulation of Phosphorylase and Glycogen Synthase