2.1 The development history and current situation of O-GlcNAc in China
O-glycoprotein mainly exists in mucus and immunoglobulin. The O-glycosylation process of forming O-glycoprotein is carried out in the Golgi apparatus. The sugar chain is covalently linked to the free OH group of serine or threonine. Usually, the first connected sugar unit is N-acetylgalactose, and then the sugar residue is successively transferred to form an oligosaccharide chain, and there is also a case where only monosaccharides are connected[17]. Sugar donors are also nucleoside sugars, such as UDP-galactose. The O-glycosylation site has no conserved sequence, and the sugar chain has no fixed core structure. Its composition can be either a monosaccharide or a huge sulfonated polysaccharide. The glycan formed by O-glycosylation has no sugar group, and there is one or no branch on the carbon skeleton. Since no specific glycosylation sequence has been found, the analysis of O-glycosylation is more complicated than other glycosylation.