Glycoproteins (the older term mucoproteins) are proteins that contain a sugar ( saccharide ) component in their molecule, which is attached in varying amounts, as a short or long branch or an unbranched chain (oligosaccharide chain). The weight share of carbohydrates in the molecule is 1% to 85% . They have a mostly neutral character
The protein carrier is synthesized on the rough ER , and carbohydrates are attached to it in the GA in two ways:
O-glycosidic bond to the OH group of Serine or Threonine of the protein using N-acetylglucosamine of the carbohydrate chain
By N-glycosidic linkage to the NH 2 (N-amide) group of the Asparagine protein by means of N-acetylglucosamine, to which the carbohydrate chain has been transferred from the dolichol pyrophosphate carrier, the group of N-linked glycoproteins includes membrane-bound glycoproteins as well as circulating glycoproteins
degradation in lysosomes by endoglycosidases (fucosidase, aspartylglucosaminidase) and exoglycosidases (galactosidase, neuraminidase, hexosaminidase, mannosidase)
Glycoproteins have a very wide range of roles, whether they are structural, receptor or enzymatic functions.
They participate in cellular recognition as specific receptors. It is located on the surface of all eukaryotic cells , while their protein component anchors the glycoprotein to the membrane and the carbohydrate component serves as a receptor .
Glucose does not occur in mature glycoproteins (except collagen ). The monitoring of glycosylated proteins in plasma helps in the long-term monitoring of diabetes therapy.
