Design and synthesis of oligosaccharides that interfere with glycoprotein quality-control systems

Calnexin (CNX) and its soluable homologue calreticulin (CRT) are lectin-like molecular chaperones that help newly synthesized glycoproteins to fold correctly in the rough endoplasmic reticulum (ER). To investigate the mechanism of glycoprotein-quality control, we have synthesized structurally defined high mannose-type oligosaccharides related to this system. The paper describes the synthesis of the non-natural undecsaccharide 2 and heptasaccharide 16, designed as potential inhibitors of the ER quality-control system. Each possesses the key tetrasaccharide element (Glc(1)Man(3)) critical for the CNX/CRT binding, while lacking the pentamannosyl branch required for glucosidase II recognition. These oligosaccharides were evaluated for their ability to bind CRT by isothermal titration colorimetry (ITC). As expected, each of them had a significant affinity towards CRT In addition, these compounds were shown to be resistant to glucosidose II digestion. Their activities in blocking the chaperone function of CRT wet I e next measured by using malate dehydrogenose (MDH) as a substrate. Their inhibitory effects were shown to correlate well with their CRT-binding affinities, both being critically dependent upon the presence of the terminal glucose (Glc) residue.