beta-1,4-galactosyltransferase-catalyzed glycosylation of sugar derivatives: Modulation of the enzyme activity by alpha-lactalbumin, immobilization and solvent tolerance

The influence of different parameters on the activity of the beta-1,4-galactosyltransferase (beta-1,4-GalT) from bovine milk has been investigated using various acceptor and donor substrates. It was found that the "specifier" protein alpha-lactalbumin (alpha-LA), which interacts with beta-1,4-GalT forming the lactose synthase (LS) complex, is not necessary when the acceptors are different glucopyranosides, and, in some cases, it can even have an inhibitory effect, like with the complex glucosides ginsenoside Rg 1 (1) and colchicoside (2). By optimization of the reaction conditions, the galactosylated and glucosylated derivatives of 2 were prepared, using UDP-Gal and UDP-Glc as sugar donors, respectively, and characterized. Moreover, beta-1,4-GalT was covalently immobilized on Eupergit C 250 L in the absence of alpha-LA, and the synthetic performances of this immobilized biocatalyst were evaluated. Finally, the best organic cosolvents to be used both with beta-1,4-GalT and the LS complex were identified.