Substrate structure and incubation-parameter-dependent selectivities in chiral discrimination of galactopyranosides by β-galactosidase hydrolysis

Abstract

Mono-β-galactopyranosides of (±)-propane-1,2-diol, (±)-butane-1,3-diol, (±)-pentane-1,4-diol, (±)-butan-2-ol, (±)-pentan-2-ol and (±)-1,2-O-isopropylideneglycerol were synthesized by the Koenigs–Knorr reaction using hydroxycarbonyl compounds as precursors for the diolic substructures. Hydrolysis of 3-hydroxybutyl β-D-galactopyranoside 18 by β-galactosidases from Escherichia coli, Aspergillus oryzae, Kluyveromyces lactis and Bacillus circulans, respectively, resulted in each case in an enantiomeric enrichment of the released diol. This was most significant with the E. coli enzyme and increased with higher reaction temperature and shorter incubation periods. Under standardized conditions, cleavage of all synthesized galactopyranosides by this enzyme showed the highest stereoselectivity for butane-1,3-diol, butan-2-ol and isopropylideneglycerol with enantiomeric excesses in the range 60–75%. For compounds with substructural similarity to the natural substrate lactose, enhanced stereodiscriminations were expected. However, this could not be confirmed and instead a specific hydrophobic interaction is suggested to play a crucial role.