Glycosylation

Glycosylation describes a group of enzymatic or chemical reactions in which the saccharides are linked to proteins, lipids or other aglycone. The resulting reaction product is referred to as a glycoside, in the case of proteins as glycoprotein or peptidoglycan. Glycation This is distinguished from a reaction between amine and reducing sugar. Involved in biological signaling processes glycosylation patterns are very specific and their biosynthetic pathway is one of the most important post-translational modifications of proteins. The majority of eukaryotic proteins is glycosylated. The case involved highly specific enzymes are called glycosyltransferases. For the laboratory chemical synthesis a wealth of Glykosylierungsmethoden was developed.

Function

Glycosylation serves very different functions. Firstly, it increases the stability of some proteins and protects against proteolytic degradation. Many proteins also fold correctly, if they have not previously glycosylated - that glycosylation is the physiologically functional protein conformation and also alters the affinity for binding partners (eg the insulin receptor). Next signal peptides is the glycosylation of the intracellular delivery ( targeting protein ) are transported to the cell membrane glycoproteins and in the extracellular matrix. Furthermore, the sugar chains also serve as structural components of cell membranes, as a lubricant ( for example, as a component of mucus ( phlegm ) ) and contribute to cell interaction in. Different glycosylation of blood proteins also leads to different blood groups in humans.

Glycosylation of proteins

→ Main article: glycoproteins

There are several, catalyzed by enzymes processes that lead to the glycosylation of proteins and carried out mostly in the rough endoplasmic reticulum ( rER ). Together these processes is that they are very specific, that is, only certain amino acids are glycosylated and the carbohydrate composition of each group is very specific. In some cases, the glycoproteins by glycosylation at the rER are not in their final form before, but are sometimes further modified at the Golgi apparatus. Complete glycosylation in the Golgi apparatus is not performed.

Glycosylation of lipids

→ Main article: glycolipids

In the enzymatically mediated glycosylation of lipids, glycolipids occur. The oligosaccharide portion of the glycolipids is generally presented on the outside of the cell membrane, where it plays a role in the interaction between cells or in signal transmission ( glycocalyx ).

Diseases caused by defective glycosylation

There are several hereditary diseases in which defective genes influence their own glycosylation. This can result in incorrect or missing glycosylation that can cause different diseases depending on the mutation. For example, a form of Marfan syndrome is caused by a defective glycosylation of fibrillin. But mutations can also affect the process of glycosylation itself.

Glycation

The reaction of reducing sugars with amines, such as amino acids, without enzyme participation called glycation. In this so-called Maillard reaction, which proceeds through a Schiff base to the so-called Amadori product, it is not to form a glycoside but by rearrangement to an α - aminoketone R-NH -CH2- C (O) -R '. Modifications of this type are analyzed in the biology in the glycomics.

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