Chemoenzymatic synthesis of GDP-L-fucose and the Lewis X glycan derivatives

Lewis X (Le^x)-containing glycans play important roles in numerous cellular processes. However, the absence of robust, facile, and cost-effective methods for the synthesis of Le^x and its structurally related analogs has severely hampered the elucidation of the specific functions of these glycan epitopes. Here we demonstrate that chemically defined guanidine 5′-diphosphate-β-L-fucose (GDP-fucose), the universal fucosyl donor, the Le^x trisaccharide, and their C-5 substituted derivatives can be synthesized on preparative scales, using a chemoenzymatic approach. This method exploits L-fucokinase/GDP-fucose pyrophosphorylase (FKP), a bifunctional enzyme isolated from Bacteroides fragilis 9343, which converts L-fucose into GDP-fucose via a fucose-1-phosphate (Fuc-1-P) intermediate. Combining the activities of FKP and a Helicobacter pylori α1,3 fucosyltransferase, we prepared a library of Le^x trisaccharide glycans bearing a wide variety of functional groups at the fucose C-5 position. These neoglycoconjugates will be invaluable tools for studying Le^x-mediated biological processes.