Enzymatic synthesis of a β-d-galactopyranosyl cyclic tetrasaccharide by β-galactosidases Original Research Article

The galactosyl transfer reaction to cyclo-{→ 6)-α-d-Glcp-(1 → 3)-α-d-Glcp-(1 → 6)-α-d-Glcp-(1 → 3)-α-d-Glcp-(1 →} (CTS) was examined using lactose as a donor and β-galactosidases from Aspergillus oryzae and Bacillus circulans. The A. oryzae β-galactosidase produced three galactosyl derivatives of CTS. The main galactosyl derivative produced by the A. oryzae enzyme was identified as 6-O-β-d-galactopyranosyl-CTS, cyclo-{→ 6)-α-d-Glcp-(1 → 3)-[β-d-Galp-(1 → 6)]-α-d-Glcp-(1 → 6)-α-d-Glcp-(1 → 3)-α-d-Glcp-(1 →}. The B. circulans β-galactosidase also synthesized three galactosyl-transfer products to CTS. The structure of main transgalactosylation product was 3-O-β-d-galactopyranosyl-CTS, cyclo-{→ 6)-α-d-Glcp-(1 → 3)-α-d-Glcp-(1 → 6)-[β-d-Galp-(1 → 3)]-α-d-Glcp-(1 → 3)-α-d-Glcp-(1 →}. These results showed that β-galactosidase transferred galactose directly to the ring glucose residue of CTS.