Chemoenzymatic Synthesis of DSGb5 and Sialylated Globo‐series Glycans

Sialic‐acid‐binding, immunoglobulin‐type lectin‐7 (Siglec‐7) is present on the surface of natural killer cells. Siglec‐7 shows preference for disialylated glycans, including α(2,8)‐α(2,3)‐disialic acids or internally branched α(2,6)‐NeuAc, such as disialosylglobopentaose (DSGb5). Herein, DSGb5 was synthesized by a one‐pot multiple enzyme method from Gb5 by α2,3‐sialylation (with PmST1) followed by α2,6‐sialylation (with Psp2,6ST) in 23 % overall yield. DSGb5 was also chemoenzymatically synthesized. The protection of the nonreducing‐end galactose of Gb5 as 3,4‐O‐acetonide, 3,4‐O‐benzylidene, and 4,6‐O‐benzylidene derivatives provided DSGb5 in overall yields of 26 %, 12 %, and 19 %, respectively. Gb3, Gb4, and Gb5 were enzymatically sialylated to afford a range of globo‐glycans. Surprisingly, DSGb5 shows a low affinity for Siglec‐7 in a glycan microarray binding affinity assay. Among the synthesized globo‐series glycans, α6α3DSGb4 shows the highest binding affinity for Siglec‐7.     

Selective Engineering of Linkage‐Specific α2,6‐N‐Linked Sialoproteins Using Sydnone‐Modified Sialic Acid Bioorthogonal Reporters

The metabolic oligosaccharide engineering (MOE) strategy using unnatural sialic acids has recently enabled the visualization of the sialome in living systems. However, MOE only reports on global sialylation and dissected information regarding subsets of sialosides is missing. Described here is the synthesis and utilization of sialic acids modified with a sydnone reporter for the metabolic labeling of sialoconjugates. The positioning of the reporter on the sugar significantly altered its metabolic fate. Further in vitro enzymatic assays revealed that the 9‐modified neuraminic acid is preferentially accepted by the sialyltransferase ST6Gal‐I over ST3Gal‐IV, leading to the favored incorporation of the reporter into linkage‐specific α2,6‐N‐linked sialoproteins. This sydnone sugar presents the possibility of investigating the roles of specific sialosides.