Controllable Enzymatic Synthesis of Natural Asymmetric Human Milk Oligosaccharides

Among human milk oligosaccharides (HMOs), linear HMOs are synthesized through mature but varied routes. Although branched HMOs can be synthesized by chemical, enzymatic, or chemoenzymatic methods, these methods cannot be easily applied to the synthesis of asymmetric multiantennary oligosaccharides. Herein, we developed a controllable method to synthesize asymmetric biantennary HMOs. In our synthetic route, GlcNAcβ1,3(GlcN3β1,6)Glaβ1,4Glc was first chemically synthesized as the core tetrasaccharide, which contains β1,6GlcN3 as the "stop" sugar in transferase-catalyzed glycosylation. The desired sugars at the GlcNAcβ1-3Gal arm can be assembled using galactosyltransferase, -acetylglucosaminyltransferase, and fucosyltransferase. Then, the Staudinger reduction and acetylation were used to transform GlcN3 to GlcNAc and assemble sugars by initiating the "go" process. By manipulating transferase-catalyzed glycosylations, 22 natural asymmetric biantennary oligosaccharides were synthesized.