Enzyme-Catalyzed Glycosylation of Curcumin and Its Analogues by Glycosyltransferases from Bacillus subtilis ATCC 6633

Curcumin is a naturally occurring polyphenolic compound that is commonly used in both medicine and food additives, but its low aqueous solubility and poor bioavailability hinder further clinical applications. For assessing the effect of the glycosylation of curcumin on its aqueous solubility, two glycosyltransferase genes (BsGT1 and BsGT2) were cloned from the genome of the strain Bacillus subtilis ATCC 6633 and over-expressed in Escherichia coli. Then, the two glycosyltransferases were purified, and their glycosylation capacity toward curcumin and its two analogues was verified. The results showed that both BsGT1 and BsGT2 could convert curcumin and its two analogues into their glucosidic derivatives. Then, the structures of the derivatives were characterized as curcumin 4'-O-beta-D-glucoside and two new curcumin analogue monoglucosides namely, curcumoid-O-alpha-D-glucoside (2a) and 3-pentadienone-O-alpha-D-glucoside (3a) by nuclear magnetic resonance (NMR) spectroscopy. Subsequently, the dissolvability of curcumin 4'-O-beta-D-glucoside was measured to be 18.78 mg/L, while its aglycone could not be determined. Furthermore, the optimal catalyzing conditions and kinetic parameters of BsGT1 and BsGT2 toward curcumin were determined, which showed that the Kcat value of BsGT1 was about 2.6-fold higher than that of BsGT2, indicating that curcumin is more favored for BsGT2. Our findings effectively apply the enzymatic approach to obtain glucoside derivatives with enhanced solubility.