What is the monomer of glycogen


Synonym: glycogen
English: glycogen
Molecular formula of the repeating unit: C6H10O5

1 definition

As Glycogen this is the name given to the storage form of carbohydrates present in the human and animal organism, which is comparable to vegetable starch.

2 synthesis

Glycogen is a highly cross-linked polymer made of 1,4- and 1,6-glycosidically linked D-glucose molecules, which is therefore also known as a dendrimer.

The synthesis of a glycogen molecule begins with the primer enzyme glycogenin. It's at the center of every glycogen molecule. Glycogenin attaches itself to short polysaccharide chains made of α-1,4-glycosidically bound glucose, which serve as a starting point for glycogen synthase. The glycogen synthase cannot determine this starting point itself - it always orientates itself on an existing chain of glucose molecules.

The glycogen synthase does not use free glucose for polymer formation, but activated UDP-glucose. It is obtained from glucose-6-phosphate by phosphoglucomutase and UTP-glucose-1-phosphate uridylyl transferase. Here, glucose-6-phosphate is firstly converted to glucose-1-phosphate isomerized and then UDP is added:

The glycogen synthase then attaches the UDP-glucose to the primer or the existing glycogen chain, whereby the UDP is released again:

Glycogen (n glucose) + UDP-glucose → glycogen (n + 1 glucose) + UDP

The growing glycogen chain does not continue in a strictly linear manner. Every 7 to 12 glucose units it is cut by the 1,4-α-glucan-branching enzyme ("branching enzyme") and the alpha-1,6-glycosidic fragment is attached to the side of the existing chain or an adjacent chain.

see main article:Glycogen synthesis

3 dismantling

The breakdown of glycogen is known as glycogenolysis. It behaves almost as a mirror image of the synthesis. The linear glucose chains of the glycogen are broken down by the enzyme glycogen phosphorylase. It catalyzes the bonding of a phosphate group on the C1 atom of glucose. The reaction is dependent on pyridoxal phosphate. The alpha-1,4-glycosidic bond between the glucose molecules is split and glucose-1-phosphate is released. Glucose-1-phosphate is then isomerized to glucose-6-phosphate by a mutase.

Due to the branched structure of the glycogen, another enzyme is necessary in order to also be able to dissolve glucose chains bound by alpha-1,6-glycosides. The glycogen phosphorylase can only break down glycogen up to the fourth glucose molecule before a 1,6 branch. This is the debranching enzyme. This "debranching enzyme" is a bifunctional enzyme that combines two different catalytic activities. On the one hand, it decouples 3 of 4 glucose residues from the remaining short chain and transfers them linearly to an adjacent chain. On the other hand, it splits off the remaining alpha-1,6-glycosidically bound glucose molecule in a second step, with free glucose being formed.

Due to these conversion reactions, around 90% glucose-1-phosphate and around 10% free glucose are produced during glycogen breakdown, because only about every tenth glucose molecule is at a branching point.

see main article: Glycogenolysis

4 physiology

Glycogen can be synthesized and stored in many tissues, including the liver, muscles, kidneys, brain, uterus, and vagina. However, due to the presence of the enzyme glucose-6-phosphatase, only the liver and kidneys are able to mobilize the carbohydrates (approx. 100 g) stored in their parenchyma and make them available to the body again.

5 pathophysiology

Pathophysiologically, increased glycogen storage plays a role in the context of glycogenoses.