trans-Sialidase catalyzed sialylation of beta-galactosyldisaccharide with an introduction of beta-galactosidase.

Introduction of beta-galactosidase into a trans-sialidase reaction, i.e. sialic acid transfer reaction from a donor substrate (alpha2,3-sialyllactose) to an acceptor substrate (beta-galactosyldisaccharide), could improve the yield of desired sialylated trisaccharide by hydrolyzing lactose, a byproduct from the donor. When trans-sialidase reaction was performed with stoichiometric amounts (2 mM) of alpha2,3-sialyllactose and Galbeta(1,3)GlcNAc, the yield of NeuAcalpha(2,3)Galbeta(1,3)GlcNAc increased from 45% to 75% by the coupling of Escherichia coli beta-galactosidase. Furthermore, by changing the substrate ratio in the coupled reaction, i.e. two-fold excess of alpha2,3-sialyllactose to Galbeta(1,3)GlcNAc, above 95% of yield was achieved based on the amount of Galbeta(1,3)GlcNAc. However, two-fold excess of Galbeta(1,3)GlcNAc to alpha2,3-sialyllactose in this reaction was more desirable for the purification of NeuAcalpha(2,3)Galbeta(1,3)GlcNAc, since complete consumption of alpha2,3-sialyllactose was achieved. Efficiency of the coupled reaction was affected by the specificity of beta-galactosidase for acceptor substrate. When Galbeta(1,6)GlcNAc was used as the acceptor, E. coli beta-galactosidase hydrolyzed Galbeta(1,6)GlcNAc as well as lactose in the coupled reaction, resulting in a significant decrease in the yield of desired sialylated trisaccharide. The conversion yield of the sialylation of Galbeta(1,6)GlcNAc could be improved by employing Bacillus circulans beta-galactosidase.