Parametric Analysis of Donor Activation for Glycosylation Reactions.

The chemical synthesis of complex oligosaccharides relies on efficient and highly reproducible glycosylation reactions. The outcome of a glycosylation is contingent upon several environmental factors, such as temperature, acidity, the presence of residual moisture, as well as the steric, electronic, and conformational aspects of the reactants. Each glycosylation proceeds rapidly and with a high yield within a rather narrow temperature range. For better control over glycosylations and to ensure fast and reliable reactions, a systematic analysis of 18 glycosyl donors revealed the effect of reagent concentration, water content, protecting groups, and structure of the glycosyl donors on the activation temperature. With these insights, we parametrize the first step of the glycosylation reaction to be executed reliably and efficiently.

A Modular Chemoenzymatic Synthesis of Disialosyl Globopentaosylceramide (DSGb5Cer) Glycan.

The total synthesis of the oligosaccharide moiety of disialosyl globopentaosylceramide (DSGb5 Cer), a dominant ganglioside isolated from malignant renal cell carcinoma tissues, is reported. The synthetic strategy relies on a chemical α(2,6)-sialylation at the internal GalNAc unit of a Gb5 pentasaccharide backbone that furnishes a Neu5Acα(2,6)GalNAc-linked hexasaccharide, suitable for an enzymatic α(2,3)-sialylation of the terminal Gal residue to construct a heptasaccharide glycan. Convergent access to this key α(2,6)-sialylated hexasaccharide was also achieved through a [3+3] glycosylation building upon a Galß(1,3)[Neu5Acα(2,6)]GalNAc-based trisaccharide donor and a Gb3 acceptor. The synthetic DSGb5 glycan bearing a 6-azidohexyl aglycon at the reducing end could undergo further regioselective functionalization. This approach represents a viable chemoenzymatic method for accessing complex ganglioside glycans and should be useful for the synthesis and biological investigation of DSGb5 derivatives.

Sugar nucleotide regeneration system for the synthesis of Bi- and triantennary N-glycans and exploring their activities against siglecs.

Enzymatic synthesis that is commenced by the sugar nucleotide regeneration system (SNRS) protocol can minimize 1) the consumption of exorbitant sugar nucleotides, 2) the amount of transferases required, and 3) byproduct feedback inhibition. In this study, LacNAc extensions/modifications of the N-linked mannose core were carried out efficiently with SNRS with high yields and purities on all branches in a uniform manner. In addition, we demonstrate that with SNRS, bacterial glycosyltransferases exhibit a wide acceptor tolerance for bi- and triantennary mannose core structures as substrates for target oligosaccharides. The synthesized small library of mannose core-based glycans and linear O-glycans were screened for their binding affinity against h-Siglecs 2, 4, 7, 9, 14, 15, and m-Siglec-15 to explore their structure-based binding preferences. Microarray data revealed that each Siglec showed few distinct yet overlapping specificities. An increase in branching from mono to di or tri antennary did not necessarily lead to increasing affinity. Glycans with the disialoside sequence α(2,3)α(2,8)/α(2,6)α(2,8) showed high specificity and affinity for Siglec-7, and sLex α(2,3) exhibited a strong affinity for Siglec-9. Explicit recognition of α(2,6)α(2,3)- linear and α(2,3)α(2,6)-branched glycans by Siglecs-2, 4, and 15, respectively, suggests that these structures can act as potential candidates for the further development of high-affinity ligands.