Glycoprotein quality control system exists in the endoplasmic reticulum and discriminates and excludes misfolded glycoproteins. The key component of this system is UDP-glucose : glycoprotein glucosyltransferase (UGGT), which serves as a folding sensor as it can only monoglucosylate misfolded glycoproteins. Monoglucosylation serves as a tag for refolding assisted by lectin chaperones calnexin/calreticulin. To elucidate the recognition mechanism of misfolding by UGGT, various sophisticated misfolded glycoprotein models have been reported. Variety of model glycoproteins can be prepared by biological approaches although those are usually heterogeneous in both glycan and protein structures. Recently, we introduced a chemical approach for the synthesis of homogeneous misfolded glycoprotein. Chemical method can provide small glycoproteins but with homogeneous glycan and with intentionally manipulated protein 3D-structure. In this review, both chemical and biological approaches for the preparation of misfolded glycoprotein probes are discussed, which will give us an ability to gain further insights into the glycoprotein quality control system.
This article provides an overview of chemical methods, strategies and applications that have been introduced by our group during the past decade. The major discussion point is the development of new methods for chemical glycosylation. The methods overviewed here include: new leaving and protecting groups, metal-coordination assisted synthesis, picolinyl approach to stereoselective glycosylation, etc. Also discussed is the invention of strategies for efficient oligosaccharide assembly. Innovative tools for oligosaccharide synthesis including the temporary deactivation concept, the inverse armed-disarmed strategy, thioimidate-only orthogonal and active–atent activation, and O-2/O-5 cooperative effect are discussed in light of recent results. Our recent attempts to develop new automated technologies, i.e. Surface-Tethered Iterative Carbohydrate Synthesis (STICS) and HPLC-assisted oligosaccharide synthesis are also introduced.
Recently, imaging of glycans on the cell surface has attracted much attention. There has been a good deal of research on glycan labeling for mammalian cells in vitro and also for some vertebrates (mice and zebrafish) in vivo, but little on bacterial cells. In this mini review, recent examples of chemical glycobiology, including various modification methods of bacterial surface carbohydrates which can be divided into genetic modification and non-genetic modification (metabolic labeling), are described.