Chemoenzymatic synthesis of the Salmonella group E1 core trisaccharide using a recombinant β-(1→4)-mannosyltransferase

The chemical synthesis of the bacterial O-antigen from Salmonella serogroup E1, 3-O-(4-O-β-d-mannopyranosyl-α-l-rhamnopyranosyl)-α-d-galactose, presents a particular challenge because it contains a β-(1→4) mannosidic linkage to l-rhamnose. We report a chemoenzymatic synthesis of this crucial antigenic material which culminates in the enzymatic formation of the critical β-mannosyl connection catalyzed by Salmonella GDP-α-d-Man:αRha1→3αGal-PP-Und β-(1→4)-mannosyltransferase (ManTβ4). In comparison with previous synthetic routes, this method is advantageous since it utilizes intermediates, available in significant yield, which can be readily derivatized from the reducing end to present flexibility for analog construction, while the enzymatic construction of the Man1→4Rha glycosidic bond is both rapid and occurs in high yield. Furthermore, the reported spectroscopic and enzymatic structural characterization of the trisaccharide product furnishes the first indisputable functional link between wbaO and ManTβ4 and clearly sets the stage for the future mechanistic study and exploitation of this fascinating glycocatalyst.