2.2 The development history and current situation of N-GlcNAc in China
N-glycosylation occurs on the amide nitrogen of the asparagine side chain, and the sugar chain is covalently linked to the free NH2 group of the aspartic acid of the protein. The synthesis of N-linked sugar chains starts in the endoplasmic reticulum (ER) and completes in the Golgi apparatus. The first step in the synthesis of N-glycans is to add a 14-glycan core oligosaccharide to the asparagine residue of the newly formed polypeptide chain with the characteristic sequence of Asn-Xaa-Ser/ Thr/Cys (Xaa can be any amino acid other than proline), asparagine acts as a sugar chain receptor. The core oligosaccharide is composed of 2 molecules of N-acetylglucosamine, 9 molecules of mannose and 3 molecules of glucose. The first N-acetylglucosamine binds to the phosphate group of the phosphopolyterpene alcohol on the ER bilayer membrane. When a new polypeptide is synthesized on the ER membrane, the entire sugar chain is transferred together. After transferring the oligosaccharide to the nascent peptide, it is further processed in the ER, and 3 molecules of glucose and 1 molecule of mannose are removed in turn. The glycoprotein formed by ER has similar sugar chains. After entering the Golgi apparatus from the Cis surface, most of the mannose on the original sugar chain is removed during the transport between the membrane vesicles, but a variety of glycosyltransferases are added in turn. Different types of sugar molecules form oligosaccharide chains with different structures. The glycan formed by N-glycosylation has only one glycosyl group, but has multiple branches. Proteins in body fluids such as plasma are mostly N-glycosylated, so N-glycoprotein is also known as plasma glycoprotein. Almost all such glycosylation modifications in animal cells are N-acetylglucosamine GlcNAc, and all are β-configuration[18].