Identification of Fatty Acid Glucose Esters as Os9BGlu31 Transglucosidase Substrates in Rice Flag Leaves.

Rice Os9BGlu31 transglucosidase transfers glucosyl moieties between various carboxylic acids and alcohols, including phenolic acids and flavonoids, in vitro. The role of Os9BGlu31 transglucosidase in rice plant metabolism has only been suggested to date. Methanolic extracts of rice bran and leaves were found to contain oleic acid and linoleic acid to which Os9BGlu31 could transfer glucose from the 4-nitrophenyl β-D-glucoside (4NPGlc) donor to form 1-O-acyl glucose esters.

Active site cleft mutants of Os9BGlu31 transglucosidase modify acceptor substrate specificity and allow production of multiple kaempferol glycosides.

Background: Rice Os9BGlu31 is a transglucosidase that can transfer glucose to phenolic acids, flavonoids, and phytohormones. Os9BGlu31 displays a broad specificity with phenolic 1-O-β-D-glucose esters acting as better glucose donors than glucosides, whereas the free phenolic acids of these esters are also excellent acceptor substrates.

Methods: Based on homology modeling of this enzyme, we made single point mutations of residues surrounding the acceptor binding region of the Os9BGlu31 active site.

Rice Os9BGlu31 is a transglucosidase with the capacity to equilibrate phenylpropanoid, flavonoid, and phytohormone glycoconjugates.

Glycosylation is an important mechanism of controlling the reactivities and bioactivities of plant secondary metabolites and phytohormones. Rice (Oryza sativa) Os9BGlu31 is a glycoside hydrolase family GH1 transglycosidase that acts to transfer glucose between phenolic acids, phytohormones, and flavonoids. The highest activity was observed with the donors feruloyl-glucose, 4-coumaroyl-glucose, and sinapoyl-glucose, which are known to serve as donors in acyl and glucosyl transfer reactions in the vacuole, where Os9BGlu31 is localized.