Abstract
Flavonoids are usually conjugated with various sugar moieties by UDP-sugar glycosyltransferases (UGT) in a species-specific manner. It is unclear how the sugar-donor specificity of UGTs evolved although it significantly impacts on large structural diversification of phytochemicals. We isolated flavonoid 7-O-glucuronosyltransferases (F7GAT) from different Lamiales plants. All of the F7GATs were members of the UGT88-related cluster and specifically utilized UDP-glucuronic acid (UDPGA). By homology modeling, we identified a unique Arg residue in the PSPG-box of Lamiales F7GATs. Substitution of this Arg with Trp was sufficient to convert the sugar donor-specificity from UDPGA to UDP-glucose, indicating that the Arg recognizes the carboxylate group of UDPGA by interaction of the cationic guanidinium moiety with the anionic carboxylate oxygen of the glucuronic acid. Our results suggest that differentiation of sugar-donor specificity of UGTs occurred locally in specific plant families after establishment of regiospecificity on the sugar acceptor.
