Regioselectivity of the enzymatic transgalactosidation of d- and l-xylose catalysed by β-galactosidases

The regioselectivity of enzymatic transgalactosidation depends on the source of the β-galactosidase used. When the galactosyl acceptor only contains secondary hydroxyl groups, e.g., d- or l-xylose, it is possible to find an enzyme that catalyses preferentially the synthesis of any of the three regioisomers 4-, 3- and 2-O-(β-d-galactopyranosyl-d-xylose (1, 2 and 3, respectively) or 4-, 3- and 2-O-β-d-galactopyranosyl-l-xylose (4, 5 and 6, respectively). Enriched mixtures in 1, 2 or 3 were obtained using β-galactosidases from Escherichia coli, bovine testes or Aspergillus oryzae, respectively, by transgalactosidation reaction of O-nitrophenyl-β-d-galactopyranoside and d-xylose, and enriched mixtures in 4, 5 or 6 were obtained in a similar way using β-galactosidases from Aspergillus oryzae, lamb small-intestine (intestinal lactase-phloridzin hydrolase) or Saccharomyces fragilis, respectively, using l-xylose as acceptor.