Glycogen consists of glucose units and is a polysaccharide. In the human organism, it serves to provide and store glucose. The buildup of glycogen is called glycogen synthesis, and the breakdown is called glycogenolysis.
What is glycogen?
Glycogen is made up of something called glycogenin, a central protein to which thousands of glucose molecules are then attached. Additional branching also occurs on the glucose molecules, allowing the glycogen molecule to be synthesized or stored in various tissues. These include the muscles, the liver, the vagina, the uterus and the brain, respectively. However, the stored carbohydrates can only be provided or mobilized in the liver because the enzyme glucose-6-phosphatase is found in the liver. Glycogen can be seen as an energy store that is used during physical exertion or hunger. If the glycogen supply runs out, depot fat is subsequently broken down. The breakdown of glycogen to glucose is known as glycolysis, which can be considered a central process of energy metabolism.
Function, effect, and tasks
The glycogen supply is used exclusively by the muscles themselves; the liver stores the glycogen and then makes it available to other cells. This process plays an important role especially in the sleeping state, as the cells of the adrenal medulla or the erythrocytes can be supplied with energy in this way. In addition, blood glucose levels are regulated by various [[hormones]] with the help of the buildup and breakdown of glycogen. Glucagon and adrenaline stimulate the breakdown, whereas the build-up is promoted by insulin. The formation of glucagon and insulin takes place in parts of the pancreas. Glycogen, which is stored in the liver, is used to replenish blood glucose and supply the brain and erythrocytes. When the liver is filled up with glycogen, it represents the organ with the highest concentration of glycogen (100mg glycogen per g liver). Glycogen stored in the muscles, on the other hand, is only for personal use, and about 250g of glycogen can be stored here.
Formation, occurrence, properties and optimal values
When starch is ingested with food, the enzyme alpha-amylase breaks it down into isomaltose and maltose in the mouth and duodenum, respectively. These are then converted into glucose by further enzymes. Glycogen synthesis requires glycogenin, a protein found in the center of a glycogen molecule. With the help of this enzyme and glucose-6-phosphate, elongation of the sugar side chains occurs. Before that, glucose-6-phosphate is still converted to glucose-1-phosphate. Glycogen is degraded with the help of glycogen phosphorylase. When the terminal glucose molecules are broken down, glucose-1-phosphate is formed. In total, 400g of glucose can be stored in the human body in the form of glycogen. It is not possible to store individual glucose molecules, as these would draw too much water into a cell, causing it to burst. All cells – with the exception of erythrocytes – can produce or build up glycogen. Two organ systems in particular are important for glycogen storage:
- The musculature: it stores glycogen only for itself.
- The liver: it stores glycogen to be able to supply the organism with it in further consequence
Glycogen metabolism has different tasks in the muscles and in the liver and for this reason is also regulated in different ways. The regulation is either hormonal or allosteric. Allosteric regulation begins within a cell. Increased AMP occurs in the muscles, which activates phosphorylase. Here, a lack of ATP is compensated by an increased degradation of glycogen. On the other hand, glucose-6-phosphate and ATP act as inhibitors. They indicate that sufficient glucose and energy are available and therefore no degradation is required. Such regulation is not possible in the liver, where glucose is provided for other organs and not consumed by the body itself. In the liver, glucagon signals a need for glucose, while in the muscles, adrenaline is responsible for this. In both cases, there is an increase in cAMP levels, which causes glycogen to be broken down. Insulin, on the other hand, lowers the cAMP level and subsequently leads to a build-up of glycogen.
Diseases and disorders
A very rare disorder is the so-called glycogenoses, which are genetic. This is a congenital storage disease characterized by abnormal glycogen content. In glycogen storage diseases, the body is unable to convert glycogen to glucose, and sometimes glycogen production or glycogen utilization is disturbed. A distinction can be made between liver and muscle glycogenoses. One symptom of hepatic glycogenosis is an enlarged liver in which increased fat or glycogen is stored. Patients are also prone to short stature and hypoglycemia. In muscle glycogenosis, muscle wasting, muscle cramps, or muscle weakness occur. Thirteen different types of glycogenesis are now known, which are classified into muscle or liver forms and designated by Roman numerals. Glycogen storage disorders are due to an autosomal recessive inheritance and can be detected in the womb by amniocentesis. Glycogeneses are mainly treated symptomatically. Therapy includes, for example, breathing and physiotherapy or special dietary recommendations. In the further course, artificial nutrition or ventilation often also become necessary.
