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

Abstract

The galactosyl transfer reaction to cyclo-{→ 6)-α- d-Glc p-(1 → 3)-α- d-Glc p-(1 → 6)-α- d-Glc p-(1 → 3)-α- d-Glc p-(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-Glc p-(1 → 3)-[β- d-Gal p-(1 → 6)]-α- d-Glc p-(1 → 6)-α- d-Glc p-(1 → 3)-α- d-Glc p-(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-Glc p-(1 → 3)-α- d-Glc p-(1 → 6)-[β- d-Gal p-(1 → 3)]-α- d-Glc p-(1 → 3)-α- d-Glc p-(1 →}. These results showed that β-galactosidase transferred galactose directly to the ring glucose residue of CTS.