Substrate Promiscuity, Crystal Structure, and Application of a Plant UDP-Glycosyltransferase UGT74AN3

Glycosyltransferases are effective enzymes for glycosylating natural products (NPs), and some of them have the unusual property of being exceedingly promiscuous catalytically toward a range of substrates. UGT74AN3 is a plant glycosyltransferase identified from Catharanthus roseus in our previous work. In this study, we found that UGT74AN3 exhibits high substrate promiscuity toward 78 acceptors and 6 sugar donors and also exhibits N-/S-glycosylation activity toward simple aromatic compounds. The crystal structures of UGT74AN3 in the complex with various NPs were solved. Sugar donor recognition of UGT74AN3 was altered by structure-based mutagenesis, and the T145V mutant shifted its sugar donor preference from UDP-Glc to UDP-Xyl. Structural analysis reveals that a spacious U-shaped hydrophobic binding pocket accounts for the high substrate promiscuity of UGT74AN3. The residues E85 and F193 might serve as gatekeepers of UGT74AN3 to control substrate binding. In addition, a rare substrate binding mode was discovered in the structure of UGT74AN3, and the process of substrate flipping in the pocket was charted by molecular dynamics simulations. Moreover, a cost-effective one-pot system by coupling UGT74AN3 with AtSuSy, a sucrose synthase, was established for in situ generating and recycling UDP-Glc from sucrose and UDP to glycosylate NPs. Our study reveals the structural basis underlying the substrate promiscuity of UGT74AN3 and provides an efficient and economical enzymatic synthesis strategy for producing valuable glycosides for drug discovery.