Glycogen is a molecule made up of glucose, the body’s main source of energy. While a large part of the body’s cells allow the synthesis and destruction of glycogen, only the liver and muscles ensure its storage. We are talking about liver and muscle glycogen, respectively.

Le glycogen is a macromolecule (i.e. a very large molecule) of glucose with a molecular weight of up to 10 million daltons (dalton is a unit of measure of mass equivalent to one sixteenth of the mass of an oxygen atom. For comparison, glucose has a molecular weight of 180 daltons).

Glycogen allows the glucose storage in the body: this is called a polysaccharide, that is, a highly branched polymer made up of saccharides (or sugar. There are a large number of saccharides such as galactose, ribose, fructose. But in this case, glycogen consists of glucose: more 50 glucose molecules).

Glucose, absorbed by the body

As a reminder, glucose is a six-carbon molecule whose crude formula is C6H12O6. It is an essential source of energy for the functioning of the organism and more particularly for the brain, being the only fuel it can consume (apart from the ketone bodies produced by the body from the fats stored in the event of fasting ).

Carbohydrates are chains of saccharides (or sugars) that we consume like starch (found in starches such as rice, wheat, pasta, etc.) and sucrose (made up of fructose and glucose). Unlike glucose which is a simple molecule, carbohydrates are complex molecules. It is during the digestion of carbohydrates that the glucose molecules that constitute them will be absorbed by the body.

Glycogen, absorbed by the cell

Glucose is stored in the body’s cells in the form of glycogen. Indeed, glucose molecules cannot be absorbed continuously by cells which cannot ensure their storage. The cells would be compromised by the storage of small molecules in large quantities: a high molecular concentration would make the interior of the cell strongly hypertonic which could lead to a massive entry of water at the risk of the cell bursting.

This is why glucose is stored in an osmotically inactive form: glycogen.

Glycogen is a large molecule found in various organs of the body. Most cells in the body can synthesize and break down glycogen (including red blood cells, nerve cells (or oligodendrocytes), vaginal epithelial cells, etc.). However, the storage capacities of these cells remain limited. There are only two main sites for glycogen storage:

• Liver cells (or hepatocytes) : we are talking about hepatic glycogen. The liver can store 10% of its weight in glycogen, i.e. 15 to 50 g / kg of liver ;
• Muscle fibers (or myocytes) : these cells can store up to 2% of their weight in glycogen, i.e. 1 to 10 g / kg of muscle (we are talking about muscle glycogen). Be careful, as muscles have a greater mass than the liver, the amounts of glycogen present in the muscles are therefore greater than those contained in the liver.

Note that glycogen is stored in the cytosol of cells.

Le hepatic glycogen is stored in liver cells (hepatocytes). Indeed, the synthesis of glycogen (or glycogenogenesis or glycogenesis) occurs when the level of glucose in the blood is high. The pancreas then releases the hormone insulin. This will promote the capture of glucose by the liver (which is one of the only organs able to store it in the form of glycogen with the muscles). However, the capture of glucose by the liver is guaranteed without even the action of insulin, but the latter optimizes this phenomenon.

In addition, when necessary, the liver supplies the body with glucose which it stores in the form of glycogen: this is the glycogénolyse. In the event of a drop in blood sugar, the release of glucagon by the pancreas will activate glycogen destroying enzymes in the liver, resulting in the release of glucose by the latter organ. Adrenaline has a reaction analogous to glucagon, it is secreted by the adrenal glands under stress.

You have understood it: in the event of a drop in blood sugar (as part of a sporting activity or a fast for example) or stress, the liver supplies the body with glucose by destroying glycogen. Glucose is then released into the bloodstream to stabilize blood sugar levels and provide cells with energy.

Muscle cells, like those in the liver, store glucose. Glucose is also stored there in the form of glycogen: we are talking about muscle glycogen.

When blood sugar levels increase, the pancreas releases the hormone insulin which promotes glucose uptake by muscle fibers (myocytes). Glucose is then stored in the muscles in the form of glycogen thanks to synthetic enzymes (this is glycogenogenesis or glycogenesis).

Conversely, in the event of a drop in blood sugar, the release of glucagon by the pancreas will activate the enzymes for destroying glycogen in the muscle fibers. This results in the release of glucose to the muscles. Adrenaline has an action similar to glucagon, it is secreted by the adrenal glands under stress. Glucose will not be released from the muscles into the bloodstream (unlike the release of glucose from the liver) but will remain in the muscles to serve directly as a source of ATP (a molecule that provides cells with energy).

Better understand

How is glycogen formed in cells?

Most cells in the body can make glycogen, although it is stored only by liver and muscle cells. Each glycogen molecule has a primer molecule called glycogeny (this molecule is at the heart of the glycogen molecule and it is from this that its biosynthesis starts). Glucose units will then “cling” to glycogenesis in a linear fashion. The covalent bonds between each unit of glucose are termed 1,4 glycosidic.

Glycogen is therefore made up of glucose molecules placed one after the other. It is therefore a macromolecule whose structure is very branched, forming many branches on a main chain at 1,4. Every 10 glucose units on average found a 1,6 glycosidic bond.

A glycogen storage disorder is diagnosed by:

• a biopsy, which involves examining a sample of muscle or liver tissue under a microscope;
• magnetic resonance imaging (MRI) to detect glycogen in tissue.

The diagnosis can be confirmed by DNA analysis.

Other tests may be performed (liver, skin, muscle and blood tests) in order to more precisely determine the glycogen storage disorder in question.

Finally, genetic analyzes are used to determine whether a couple has an increased risk of having a child with an inherited genetic disease.

Treatment for a glycogen storage disorder varies depending on the type of disorder and generally involves regulating carbohydrate intake.