The glycosylation of small hydrophobic compounds is catalyzed by uridine diphosphate glycosyltransferases (UGTs). Because glycosylation is an invaluable tool for improving the stability and water solubility of hydrophobic compounds, UGTs have attracted attention for their application in the food, cosmetics, and pharmaceutical industries. However, the ability of UGTs to accept and glycosylate a wide range of substrates is not clearly understood due to the existence of a large number of UGTs. PaGT2, a UGT from Phytolacca americana, can regioselectively glycosylate piceatannol but has low activity toward other stilbenoids. To elucidate the substrate specificity and catalytic mechanism, we determined the crystal structures of PaGT2 with and without substrates and performed molecular docking studies. The structures have revealed key residues involved in substrate recognition and suggest the presence of a nonconserved catalytic residue (His81) in addition to the highly conserved catalytic histidine in UGTs (His18). The role of the identified residues in substrate recognition and catalysis is elucidated with the mutational assay. Additionally, the structure-guided mutation of Cys142 to other residues, Ala, Phe, and Gln, allows PaGT2 to glycosylate resveratrol with high regioselectivity, which is negligibly glycosylated by the wild-type enzyme. These results provide a basis for tailoring an efficient glycosyltransferase.

The health benefits of polyphenols have attracted their use as potential therapeutic agents, food additives, and cosmetics. However, low water solubility of polyphenols limits their cell absorbability, obscuring further exploration. Glycosylation is known to enhance the solubility of polyphenols preserving their pharmacological properties. Here, we show that a uridine diphosphate (UDP) glucosyltransferase from Phytolacca americana (PaGT2) regioselectively catalyzes the transfer of glucose from UDP-glucose to stilbenoids such as piceatannol and flavonoids such as kaempferol. To understand the structure-function relationship of PaGT2, we determined the crystal structure of PaGT2 as well as PaGT2 complexed with donor analogue UDP-2-fluoro glucose and stilbenoid acceptor analogues. While only one conserved histidine residue is recognized as a catalytic residue in known UGTs, the crystal structures of PaGT2 suggested the presence of two catalytically active residues (His18 and His81) at two sides of the catalytic pocket. Although the single catalytic residue mutant His18Ala or His81Ala did not completely impair the glucosylation activity of the enzyme, the double mutant His18Ala/His81Ala failed to form glucoside products. These results showed that both catalytic residues in PaGT2 actively and independently catalyze glucosylation, hence we called PaGT2 as an ambidextrous UGT. The information from PaGT2 will be advantageous for the engineering of efficient biocatalysts for production of therapeutic polyphenols.