Stereoconvergent
and Chemoenzymatic Synthesis of Tumor-Associated
Glycolipid Disialosyl Globopentaosylceramide for Probing the Binding
Affinity of Siglec‑7
Disialosyl globopentaosylceramide
(DSGb5) is a tumor-associated
complex glycosphingolipid. However, the accessibility of structurally
well-defined DSGb5 for precise biological functional studies remains
challenging. Herein, we describe the first total synthesis of DSGb5
glycolipid by an efficient chemoenzymatic approach. A Gb5 pentasaccharide-sphingosine
was chemically synthesized by a convergent and stereocontrolled [2
+ 3] method using an oxazoline disaccharide donor to exclusively form
β-anomeric linkage. After investigating the substrate specificity
of different sialyltransferases, regio- and stereoselective installment
of two sialic acids was achieved by two sequential enzyme-catalyzed
reactions using α2,3-sialyltransferase Cst-I and α2,6-sialyltransferase
ST6GalNAc5. A unique aspect of the approach is that methyl-β-cyclodextrin-assisted
enzymatic α2,6-sialylation of glycolipid substrate enables installment
of the challenging internal α2,6-linked sialoside to synthesize
DSGb5 glycosphingolipid. Surface plasmon resonance studies indicate
that DSGb5 glycolipid exhibits better binding affinity for Siglec-7
than the oligosaccharide moiety of DSGb5. The binding results suggest
that the ceramide moiety of DSGb5 facilitates its binding by presenting
multivalent interactions of glycan epitope for the recognition of
Siglec-7.