Regioselectivity in β-Galactosidase-catalyzed Transglycosylation for the Enzymatic Assembly of D-Galactosyl-D-mannose

The regioselectivity of β-galactosidase derived from Bacillus circulans ATCC 31382 (β-1,3-galactosidase) in transgalactosylation reactions using D-mannose as an acceptor was investigated. This D-mannose associated regioselectivity was found to be different from reactions using either GlcNAc or GalNAc as acceptors, not only for β-1,3-galactosidase but also for β-galactosidases of different origins. The relative hydrolysis rate of Galβ-pNP and D-galactosyl-D-mannoses, of various linkages, was also measured in the presence of β-1,3-galactosidase and was found to correlate well with the ratio of disaccharides formed by transglycosylation. The unexpected regioselectivity using D-mannose can therefore be explained by an anomalous specificity in the hydrolysis reaction. By utilizing the identified characteristics of both regioselectivity and hydrolysis specificity using D-mannose, an efficient method for enzymatic synthesis of β-1,3-, β-1,4- and β-1,6-linked D-galactosyl-D-mannose was subsequently established.