For a set of flavonoid O- and C-β-glycosyltransferases, we show that β-glucosyl fluoride can function as substrate for an enzymatic reaction wherein uridine 5′-diphosphate (UDP) α-glucose is synthesized in the presence of UDP. In pH and mutagenesis studies of the C-glycosyltransferase from rice, we show that reaction with the β-glucosyl fluoride can serve to identify the acid-base catalytic residue of the enzyme (His24). We also show that β-glucosyl fluoride can rescue activity in an enzyme variant (Ile121Asp) strongly impaired in the canonical reaction wherein flavonoid acceptor is glucosylated from UDP-glucose. Coupling of this variant with the wildtype C-glycosyltransferase in a one-pot reaction enabled efficient 3′-β-C-glucosylation of phloretin from β-glucosyl fluoride in the presence of substochiometric amounts of UDP.
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