Glycosylation describes a series of enzymatic or chemical reactions in which carbohydrates are bound to proteins , lipids or other aglycones . The resulting reaction product is referred to as a glycoside , in the case of proteins as a glycoprotein or peptidoglycan .
Glycosylation occurs in all areas of life, but in a different sequence and number of carbohydrates. Glycosylation patterns involved in biological signaling processes are very specific and their biosynthesis is an important post-translational modification of proteins. The majority of eukaryotic proteins are glycosylated. The highly specific enzymes involved are known as glycosyltransferases . Post-translational modifications of this kind are examined in biochemistry within the framework of glycomics and in biology within the framework of glycobiology . Many intestinal infectious agents bind to glycosylations to attach to a cell when infected.
Chemical glycosylation methods have also been developed for laboratory synthesis. A distinction is made between glycation and glycation as a chemical reaction between amine and reducing sugar without the participation of enzymes.
Glycosylation serves very different functions. On the one hand, it increases the stability of some proteins and protects against proteolytic degradation. Many proteins do not fold correctly if they have not been glycosylated beforehand - glycosylation can therefore serve the physiologically functional protein conformation . It can also change the affinity for binding partners (e.g. the insulin receptor ). In addition to signal peptides , glycosylation is also used for intracellular transport ( protein targeting ) and for exocytosis : glycoproteins are transported to the cell membrane . Non-glycosylated proteins cannot be released into the extracellular matrix . The saccharides also serve as structural components of cell membranes and contribute to cell interaction . Different glycosylation of the surface proteins of erythrocytes leads to different blood groups in many mammals . Glycosylation can also function as a lubricant in the mucous membrane or mucus .
Glycosylation of Proteins
Various processes catalyzed by enzymes in the rough endoplasmic reticulum (rER) and partly subsequently in the Golgi apparatus lead to the glycosylation of proteins, as N- glycosylation on asparagine or O -glycosylation on serine or threonine . What these processes have in common is that they are very specific: only certain amino acids are glycosylated and the composition of the respective carbohydrate group is very specific. In O- glycosylation, the glycoproteins are not in their final form after glycosylation at the rER, but are sometimes still modified in the Golgi apparatus. Complete glycosylation does not take place in the Golgi apparatus.
Glycosylation of Lipids
Glycolipids are formed during the enzymatically mediated glycosylation of lipids. The oligosaccharide part of the glycolipids is usually presented on the outside of the cell membrane , where it plays a role in the interaction between individual cells or in signal transmission ( glycocalyx ).
Various hereditary diseases lead to incorrect or absent glycosylation, which is physiologically less effective. Depending on the mutation , the biopolymer to be glycosylated or the glycosylating enzyme may be defective. For example, one form of Marfan's syndrome is caused by improper glycosylation of fibrillin .
Bacterial expression systems have been genetically modified so that they generally do not glycosylate proteins. Through pathway design , glycosylation in different non-human expression systems can be adapted to human glycosylation patterns.
A chemical reaction without enzyme participation between reducing carbohydrates and amines (e.g. in amino acids ) is called glycation. This Maillard reaction , which proceeds via a Schiff base to the Amadori product , does not result in the formation of a glycoside, but rather through rearrangement to an α-aminoketone R-NH-CH 2 -C (O) -R´.
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