Fig. 3: Structure of OsUGT91C1 and the binding modes to recognize substrate.

a Overall structure of OsUGT91C1. The structure is shown in cartoon representation with the N-terminus domain colored cyan and the C-terminus white. The bound UDP is shown in stick-ball mode with carbon colored orange, oxygen red, and nitrogen blue. The steviol compound derived from Reb E is shown in stick with carbons of the steviol aglycone colored white, while carbons of the glycan are colored differently, carbons of glucose 1-R2 in cyan, glucose 1-R1 in dark cyan, glucose 2-1-R1 in yellow, and glucose 6-1-R1 in pink. b Binding of UDP mainly involves the C-terminal domain with the hydrogen bonds shown by dashed lines. A cluster of water molecules, shown as red balls, fills the space most likely occupied by the glucose of UDP-glucose. c, d Binding of the steviol compounds Reb E (c) and ST (d) with the R2 end at the active site and the R1 end at the “out” site. e Binding of the steviol compound STB with the R1 end at the active site and the R2 end at the “out” site. The catalytic His27 is marked with a red asterisk in c–e. f Comparison of two binding modes using ST (the R2 end at the active site, denoted as R2 “in”) and STB (the R1 end at the active site, denoted as R1 “in”).