Biosynthesis of glycoproteins in the endoplasmic reticulum employs a quality control (QC) system, which discriminates and excludes misfolded malfunctional glycoproteins from correctly folded one. In the QC system, UDP-glucose:glycoprotein glucosyltransferase (UGGT) recognizes misfolded glycoprotein bearing high-mannose type N-glycan and glucosylate it so that the misfolded proteins can interact with molecular chaperones calnexin/calreticulin to attain correctly folded structure. As chemical tools to study glycoprotein quality control system at molecular level, we systematically synthesized misfolded and correctly folded homogeneous glycoproteins such as erythropoietin bearing high-mannose type oligosaccharide.

In order to synthesize the target glycoprotein, full-length glycosylpolypeptide chains was designed to divide into 6 segments and these were prepared by tert-Boc-SPPS.　In    terms of M9-highmannose oligosaccharide, corresponding glycopeptide was prepared   from natural source and then converted into tert-Boc-Asn-M9-highmannose for tert-Boc-SPPS. Glycopeptide-^athioester and peptide-^athioesters thus obtained were successfully coupled by repetitive native chemical ligation. Subsequent folding experiments of chemically synthesized homogeneous glycopeptide yielded correctly folded glycoprotein with native disulfide bond patterns as well as misfolded glycoproteins with non-native disulfide bond patterns. Using these misfolded and correctly folded glycoproteins, we evaluated the ability of UGGT to discriminate between correctly and misfolded glycoproteins.

In this presentation, I would like to discuss the chemical synthesis of homogeneous glycoproteins and the ability of glycoprotein folding sensor enzyme, UGGT.