Flavonoid glucosylation by non-Leloir glycosyltransferases: formation of multiple derivatives of 3,5,7,3′,4′-pentahydroxyflavane stereoisomers

Flavonoids are known to possess a multitude of biological activities. Therefore, diversification of the core structures is of considerable interest. One of nature's important tailoring reactions in the generation of bioactive compounds is glycosylation, which is able to influence numerous molecular properties. Here, we examined two non-Leloir glycosyltransferases that use sucrose as an inexpensive carbohydrate donor, glycosyltransferase R from Streptococcus oralis (GtfR) and amylosucrase from Neisseria polysaccharea (Ams), for the glucosylation of flavonoids. Flavones generally were poor substrates. Several inhibited Ams. In contrast, flavanes were well accepted by both enzymes. All glucose attachments occurred via α1 linkages. Comparison of the three available stereoisomers of 3,5,7,3',4'-pentahydroxyflavane revealed significant differences in glycoside formation between them as well as between the two enzymes. The latter were shown to possess largely complementary product ranges. Altogether, three of the four hydroxy substituents of the terminal flavonoid rings were glycosylated. Typically, Ams glucosylated the B ring at position 3', whereas GtfR glucosylated this ring at position 4' and/or the A ring at position 7. In several instances, short carbohydrate chains were attached to the aglycones. These contained α 1-4 linkages when formed by Ams, but α 1-3 bonds when generated by GtfR. The results show that both enzymes are useful catalysts for the glucodiversification of flavanes. In total, more than 16 products were formed, of which seven have previously not been described.