Family 1 Glycosyltransferase UGT706F8 from Zea mays Selectively Catalyzes the Synthesis of Silibinin 7-O-?-D-Glucoside

Regioselective glycosylation is a chemical challenge, leading tomultistep syntheses with protecting group manipulations, ultimately resultingin poor atom economy and compromised sustainability. Enzymes allow eco-friendly and regioselective bond formation with fully deprotected substratesin a single reaction. For the selective glucosylation of silibinin, apharmaceutical challenged with low solubility, enzyme engineering haspreviously been employed, but the resulting yields andkcatwere limited,prohibiting the application of the engineered catalyst. Here, we identified anaturally regioselective silibinin glucosyltransferase, UGT706F8, a family 1glycosyltransferase fromZea mays. It selectively and efficiently (kcat= 2.1 +/- 0.1 s-1;KM= 36.9 +/- 5.2 mu M; TTN = 768 +/- 22) catalyzes the quantitativesynthesis of silibinin 7-O-beta-D-glucoside. We solved the crystal structure ofUGT706F8 and investigated the molecular determinants of regioselectivesilibinin glucosylation. UGT706F8 was the only regioselective enzyme among 18 glycosyltransferases found to be active on silibinin.We found the temperature optimum of UGT706F8 to be 34 degrees C and the pH optimum to be 7-8. Our results indicate thatUGT706F8 is an efficient silibinin glycosyltransferase that enables biocatalytic production of silbinin 7-O-beta-D-glucoside