Chemical syntheses of homogeneous glycoproteins were performed in order to understand the function of N-glycan. N-Glycan has three subgroups and their structure change dependent on their biosynthetic process of glycoproteins. High-mannose type glycan is formed and regulates glycoprotein folding in the endoplasmic reticulum (ER). A complex type of glycan is formed in the Golgi apparatus. The complex type glycan seems to regulate the circulation of glycoprotein in our body and immune response. The third type is a hybrid type structure including both high-mannose and complex type glycan structures. The glycoprotein quality control (GQC) system in the ER effectively uses chaperone-type enzymes and lectins such as UDP-glucose: glycoprotein glucosyltransferase (UGGT), calnexin, calreticulin, protein disulfide bond isomerases (ERp57 or PDIs), and glucosidases to generate natively folded glycoproteins from nascent glycosylpolypeptides. However, the individual processes of the GQC system at the molecular level are still not well understood. We chemically synthesized a series of several homogeneous glycoproteins bearing M9-high-mannose type oligosaccharides (M9-glycan), such as erythropoietin (EPO), interferon-beta and interleukin 8 and their misfolded counterparts, and used these glycoprotein probes to better understand the GQC process. Experiments using these probes demonstrated that the GQC system isolated from rat liver acts in a catalytic cycle regulated by the fast crosstalk of glucosylation/deglucosylation in order to accelerate the refolding of misfolded glycoproteins. In order to understand the role of complex type glycans that usually have acidic sialic acid at their terminal, we selected EPO as a target glycoprotein and designed an efficient synthetic strategy for the chemical syntheses of several kinds of homogeneous EPO glycoforms varying in glycosylation position and the number of human-type sialyloligosaccharides. All EPO glycoforms showed biological activity dependent on glycosylation number and their position in vivo. This presentation will discuss the function of N-glycan on protein along with a new chemical synthetic method of glycoproteins.

Functions of N-glycan on protein

2019 Fall National Meeting

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