Determinants of undesired α2-6-sialoside formation by PmST1 M144D

Sialyltransferases catalyze regioselective glycosidic bond formation between sialic acid and a glycan acceptor. Pasteurella multocida α2-3-sialyltransferase 1 (PmST1) is a widely used enzyme in chemoenzymatic synthesis. In particular, the PmST1 M144D mutant is routinely employed as an α2-3-sialyltransferase, although only low levels of α2-6-sialyltransferase activity have been reported. Here, we discover that for certain acceptors, the formation of the undesired α2-6-sialoside can reach up to 20% of the product. To elucidate the factors that influence this regioselectivity, we systematically examined the effects of (i) sulfation of the acceptor, (ii) the chemical nature of the aglycone, (iii) pH, and (iv) the extent of reaction completion. The results indicate that sulfation at the 6-position of GlcNAc or a β-ethyl-NHCbz aglycone is a factor that can increase the amount of α2-6 sialoside product. Surprisingly, pH had only a small impact, and the amount of α2-6 sialoside product did not differ over the course of the reaction. These findings provide insights into the enzymatic specificity of PmST1 M144D and inform its optimized use in chemo-enzymatic synthesis of defined sialosides.