Enzymatic C-glycosylation: Insights from the study of a complementary pair of plant O- and C-glucosyltransferases*

C-Glycosylation presents a rare mode of sugar attachment to the core structure of natural products and is catalyzed by a special type of Leloir C-glycosyltransferases (C-GTs). Elucidation of mechanistic principles for these glycosyltransferases (GTs) is of fundamental interest, and it could also contribute to the development of new biocatalysts for the synthesis of valuable C-glycosides, potentially serving as analogues of the highly hydrolysis-sensitive O-glycosides. Enzymatic glucosylation of the natural dihydrochalcone phloretin from UDP-D-glucose was applied as a model reaction in the study of a structurally and functionally homologous pair of plant glucosyltransferases, where the enzyme from rice (Oryza sativa) was specific for C-glycosylation and the enzyme from pear (Pyrus communis) was specific for O-glycosylation. We show that distinct active-site motifs are used by the two enzymes to differentiate between Cand O-glucosylation of the phloretin acceptor. An enzyme design concept is therefore developed where exchange of active-site motifs results in a reversible switch between C/O-glycosyltransferase (C/O-GT) activity. Mechanistic proposal for enzymatic C-glycosylation involves a single nucleophilic displacement at the glucosyl anomeric carbon, proceeding through an oxocarbenium ion-like transition state. Alternatively, the reaction could be described as Friedel–Crafts-like direct alkylation of the phenolic acceptor.