Sugar Auxiliary Group Assisted Diversity-Oriented Enzymatic Modular Synthesis of 0-Series Ganglioside Glycans.

Owing to the inaccessibility of β1-4-N-acetylgalactosaminyltransferase for direct glycan chain elongation, the enzymatic synthesis of 0-series gangliosides with extended backbones has not been explored. In this study, sialic acid was enzymatically introduced as an auxiliary group to overcome the limitation of substrate specificity of Campylobacter jejuni β1-4-N-acetylgalactosaminyltransferase (CjCgtA) to achieve the synthesis of desired extended 0-series ganglioside core structures, and the sialic acid auxiliary group could be removed by sialidase at appropriate stages. A bacterial α2-6-sialyltransferase from Photobacterium damselae (Pd2,6ST) exhibited unexpected acceptor substrate specificity for 0-series ganglioside core structures, providing ready access to complex gangliosides bearing the sialyl N-acetylgalactosamine unit. The 0-series ganglioside core structures as the key acceptor substrates were further diversified by sequential enzymatic modular assembly to generate a collection of 31 complex 0-series ganglioside glycans after removal of the sugar auxiliary group of sialic acid at the appropriate stage.