Enzymatic O-Galactosylation of Protected Serine and Threonine by (beta)-Galactosidase Employing High Lactose Concentrations

Trans-galactosylation of protected serine and threonine by (beta)-galactosidase is the first rational step in the in vitro glycosylation of peptides and proteins. The experiments show the potential of trans-galactosylations with lactose for applications in which no product purification (such as that used in the food and life science industries) is needed. To examine these glycosylations systematically, the protected amino acids Boc-Ser-OMe, Boc-Thr-OMe, Cbz-Ser-OMe, and Cbz-Thr-OMe are used as substrates. The trans-monogalactosylation of serine with an excess of lactose yields 28% of N-tert-butoxycarbonyl-1-O-(beta)-D-galactopyranosyl-L-serinemethylester. The same transformational conditions, when applied to threonine, produced N-tert-butoxycarbonyl-1-O-(beta)-Dgalactopyranosyl-L-threonine-methylester in lower quantities. Mono-galactosylated serine and threonine are further galactosylated in the examined experimental setup to yield bi-galactosylated products also, especially at 50 °C with completely dissolved lactose.