Near-perfect control of the regioselective glucosylation enabled by rational design of glycosyltransferases

Regioselective syntheses of glycosides using modern chemical methods suffer from the need to invoke multiple steps of protection and deprotection, and even then product mixtures may occur. Discovering and tuning by mutagenesis glycosyltransferases (GTs) with considerable substrate promiscuity is an alternative challenge. We tuned by rational design based on extended Focused Rational Iterative Site-specific Mutagenesis (FRISM) a newly identified GT from Siraitia grosvenorii (UGT74AC2) as the catalyst in the targeted regioselective glucosylation of the polyhydroxy substrate silybin and derivatives. A handful of mutants was constructed with a restricted set of rationally chosen amino acids, providing variants showing 94%, > 99% and > 99% selectivity on the 3-OH, 7-OH and 3,7-O-diglycoside, respectively, compared with the wildtype resulting in a 22%:39%:39% product mixture. Remarkably, even N- and S-glucosylation was achieved. Docking and molecular dynamics (MD) simulations studies shed light on the origin of regioselectivity. These findings can be invoked to guide future enzymatic tailoring of UGTs in the production of flavonoids as potent pharmaceuticals and of other useful compounds.