Defined Glycan Ligands for Detecting Rare l-Sugar-Binding Proteins.

Most cells are decorated with distinct sugar sequences that can be recognized by carbohydrate-binding proteins (CBPs), such as antibodies and lectins. While humans utilize ten monosaccharide building blocks, bacteria biosynthesize hundreds of activated sugars to assemble diverse glycans. Monosaccharides absent in mammals are termed "rare" and are enriched in deoxy l-sugars beyond the "common" sugar l-fucose (l-Fuc) found across species. While immune proteins recognize microbial surfaces, there are limited probes to identify CBPs for the many rare sugars that may mediate these interactions. Here, we devise chemoenzymatic strategies to defined glycoconjugates containing l-Fuc and its structural analog l-colitose (l-Col), a bacterial dideoxysugar believed to bind immune proteins. We report a concise synthesis of l-Col and semisynthetic routes to several activated l-sugars. Incorporation of these sugars into glycans is evaluated using bacterial and mammalian glycosyltransferases (GTs) annotated to transfer l-Col or l-Fuc, respectively. We find that each GT can transfer both l-sugars, along with the rare hexose l-galactose (l-Gal), onto various glycan acceptors. Incorporation of these l-sugars into the resulting glycoconjugates is confirmed using known CBPs. Finally, these glycan ligands are employed to detect rare sugar-binding proteins in human serum. Overall, this work reveals similarities between bacterial and mammalian GTs that may be exploited for