The synthesis of N-linked sugar chains starts from the endoplasmic reticulum and completes in the Golgi apparatus. The glycoproteins formed in the endoplasmic reticulum have similar sugar chain After entering Golgi apparatus from Cis surface, a series of orderly processing and modification took place in the process of transport between membrane capsules. Most of the mannose in the original sugar chain was cut off, but many Glycosyltransferase Different types of sugar molecules were added in turn to form oligosaccharide chains with different structures. The spatial structure of glycoprotein determines which glycosyltransferase it can combine with to produce specific glycosylation modification. Many glycoproteins have both N-linked and O-linked sugar chains. O-linked glycosylation is carried out in Golgi apparatus. Generally, the first linked sugar unit is N-acetylgalactose, and the linked position is the hydroxyl group of Ser, Thr and Hyp, and then the sugar group is transferred to it successively to form an oligosaccharide chain. The sugar donor is also a nucleoside sugar, such as UDP galactose. Glycosylation results in different proteins being labeled differently, changing the conformation of polypeptides and increasing the stability of proteins. [1]
On the Golgi apparatus, one or more aminoglycan chains can also be installed on the serine residue of the core protein through xylose to form proteoglycans. Some of these proteins are secreted out of cells to form Extracellular matrix Or mucus layer, some anchored to the membrane. [2] according to Glycoside Chain type, protein glycosylation can be divided into four categories, namely serine , threonine, hydroxylysine and hydroxyproline The hydroxyl group of is the connection point, forming the - O-glycoside bond. with Asparagine N-terminal amino acid α - Amino and lysine or arginine ω - The amino group is the connecting point, forming the N-glycoside bond; The free carboxyl group of aspartic acid or glutamic acid is used as the connection point to form the lipopolysaccharide bond and Cysteine Is the glycopeptide bond of the connection point. Glycosylation often plays an important role in the function of membrane proteins, mediating specific biological functions:
1. Yes Cells It has many functions such as protection, stability, organization and barrier; 2. It can be used as exogenous receptor Specificity Ligands, Some sugar chain It can be used as a specific receptor for various viruses, bacteria and parasites; 3， sugar chain It can also be used as an endogenous receptor Specificity Ligands are involved in mediating clearance, turnover and intracellular passage; 4， sugar chain It plays an important role in the process of fertilization. [3] Methods of deglycosylation in living cells:
1. Tunamycin blocks N-linked sugar in vivo;
2. Cut in Neuraminidase Injected developing retina It is suggested that the specific role of polysialic acid is to inject high-purity enzyme into cells+appropriate control; 3. Make the cDNA of glycosyl modifying enzyme live Cells Or expressed in animals; 4. Add lectin or antibody Block off specific glycans. 
