Transglycosylations employing recombinant α- and β-galactosidases and novel donor substrates

•Chemoenzymatic synthesis.•Recombinant galactosidases.•Galactooligosaccharides. Recombinant α- and β-galactosidases could be prepared in larger amounts for chemoenzymatic syntheses of glycosylated oligosaccharides relevant in nutrition approaches. α-Galactosidase RafA from Escherichia coli, another thermophilic α-galactosidase AgaB from Geobacillus stearothermophilus KVE39, and also a thermophilic β-galactosidase BglT from Thermus thermophilus TH 125 could be employed in α- and in β-glycosylations, respectively. With model structures as well as sucrose, isomaltitol, and isomaltulose the stereo- and regiospecificities were studied. Further, a number of modified donor structures with structural variation and different leaving groups were synthesized, employed, and compared to classical donors for these transglycosylations.