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Glycogenolysis: Glycogen Breakdown Explained

Glycogenolysis is the biochemical process by which stored glycogen in the liver and muscles is broken down into glucose to supply the body with energy.

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Things worth knowing about "Glycogenolysis"

Glycogenolysis is the biochemical process by which stored glycogen in the liver and muscles is broken down into glucose to supply the body with energy.

What is Glycogenolysis?

Glycogenolysis refers to the biochemical pathway by which glycogen – the storage form of carbohydrates in the human body – is broken down into its building blocks. The primary product is glucose-1-phosphate, which is subsequently converted into glucose and used for energy production. This process is essential for maintaining stable blood glucose levels, particularly during fasting or physical exertion.

Where Does Glycogenolysis Take Place?

Glycogenolysis occurs predominantly in two tissues:

Liver: In the liver, glycogen breakdown primarily serves to maintain blood glucose homeostasis. The liver releases the resulting glucose directly into the bloodstream.
Skeletal muscle: In muscle tissue, the glucose derived from glycogen breakdown is used exclusively for local energy production within muscle cells. Because muscle cells lack the enzyme glucose-6-phosphatase, they cannot release free glucose into the blood.

Mechanism of Action

The breakdown of glycogen is carried out by specific enzymes in a series of steps:

Glycogen phosphorylase: This key enzyme cleaves individual glucose units from the glycogen molecule via phosphorolysis, producing glucose-1-phosphate. The reaction continues until four glucose units remain before a branch point.
Glycogen debranching enzyme: This bifunctional enzyme handles branch points in two steps: first, three of the remaining glucose units are transferred to another chain segment (transferase activity); then, the last glucose unit at the branch point is released as free glucose (glucosidase activity).
Phosphoglucomutase: The glucose-1-phosphate produced is converted by this enzyme into glucose-6-phosphate, which can then either enter glycolysis or – in the liver – be hydrolysed to free glucose by glucose-6-phosphatase.

Hormonal Regulation

Glycogenolysis is subject to strict hormonal control, ensuring that the glucose needs of the organism are consistently met:

Glucagon: When blood glucose falls, glucagon – produced by the pancreas – activates a phosphorylation cascade via the second messenger cAMP, ultimately activating glycogen phosphorylase.
Adrenaline (Epinephrine): In stress situations or during physical exercise, adrenaline also stimulates glycogenolysis via cAMP, both in the liver and in muscle tissue.
Insulin: Insulin acts antagonistically, inhibiting glycogenolysis while simultaneously promoting glycogen synthesis (glycogenesis).
Calcium ions: In muscle tissue, glycogenolysis can additionally be activated directly by calcium ions released from the sarcoplasmic reticulum during muscle contraction.

Clinical Relevance

Disorders of glycogenolysis can lead to serious diseases. The so-called glycogen storage diseases (glycogenoses) are rare, mostly hereditary metabolic disorders in which individual enzymes of glycogen metabolism are defective:

Type I (von Gierke disease): Deficiency of glucose-6-phosphatase; leads to severe hypoglycaemia and glycogen accumulation in the liver.
Type V (McArdle disease): Deficiency of muscle glycogen phosphorylase; presents with muscle pain and rapid fatigue during physical activity.
Type VI (Hers disease): Deficiency of hepatic glycogen phosphorylase; causes hepatomegaly and mild hypoglycaemia.

In diabetes mellitus, the regulation of glycogenolysis is also impaired: a relative or absolute lack of insulin leads to uncontrolled activation of glycogenolysis and thereby to persistently elevated glucose release into the blood.

Distinction from Related Metabolic Pathways

Glycogenolysis should be distinguished from closely related metabolic pathways:

Glycogenesis: The reverse process, the synthesis of glycogen from glucose.
Gluconeogenesis: The formation of new glucose from non-carbohydrate precursors (e.g., amino acids, lactate, glycerol) in the liver – a separate pathway that becomes especially important during prolonged fasting.
Glycolysis: The breakdown of glucose to pyruvate for energy production within the cell.

References

Berg, J. M., Tymoczko, J. L., Stryer, L.: Biochemistry. 9th edition, W. H. Freeman and Company, New York, 2019.
Glycogen Storage Diseases. In: OMIM – Online Mendelian Inheritance in Man. Johns Hopkins University. Available at: https://www.omim.org (accessed 2024).
Rhoades, R. A., Bell, D. R.: Medical Physiology: Principles for Clinical Medicine. 5th edition, Wolters Kluwer, Philadelphia, 2018.

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