Fig. 2.

Proposed catalytic mechanisms of plant UGTs. A. The Histidine-Aspartate dyad of VvGT1 (PDB ID: 2C1Z). The NTD and CTD are represented in pink and tan, respectively. Proposed hydrogen bonds are depicted as dashed lines. B. Schematic diagram illustrating the SN2-like direct displacement catalytic mechanisms of plant UGTs. In O- and C-glycosylation, the Histidine-Aspartate dyad deprotonates the hydroxyl group and the ortho-aromatic carbon of the acceptor molecules, respectively. In S- and N-glycosylation, the deprotonation of acceptor molecules occurs spontaneously. Subsequently, the deprotonated acceptor molecules undergo nucleophilic attack on the anomeric carbon of the sugar donors, ultimately forming the inverted glycoside products.