The C-Glycosylation of Flavonoids in Cereals

• Published in: The Journal of biological chemistry 2009-07, Vol.284 (27), p.17926-17934
• Main Authors: Brazier-Hicks, Melissa, Evans, Kathryn M., Gershater, Markus C., Puschmann, Horst, Steel, Patrick G., Edwards, Robert
• Format: Article
• Language: English
• Subjects: Carbon Radioisotopes; Enzyme Activation - physiology; Enzyme Catalysis and Regulation; Flavonoids - metabolism; Glucose - pharmacokinetics; Glucosides - biosynthesis; Glucosyltransferases - genetics; Glucosyltransferases - isolation & purification; Glucosyltransferases - metabolism; Glycosylation; Hydro-Lyases - genetics; Hydro-Lyases - isolation & purification; Hydro-Lyases - metabolism; Molecular Sequence Data; Oryza - enzymology; Oryza - genetics; Plant Proteins - genetics; Plant Proteins - isolation & purification; Plant Proteins - metabolism; Recombinant Proteins - genetics; Recombinant Proteins - metabolism; Seedlings - enzymology; Seedlings - genetics
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M109.009258

Flavonoids normally accumulate in plants as O-glycosylated derivatives, but several species, including major cereal crops, predominantly synthesize flavone-C-glycosides, which are stable to hydrolysis and are biologically active both in planta and as dietary components. An enzyme (OsCGT) catalyzing the UDP-glucose-dependent C-glucosylation of 2-hydroxyflavanone precursors of flavonoids has been identified and cloned from rice (Oryza sativa ssp. indica), with a similar protein characterized in wheat (Triticum aestivum L.). OsCGT is a 49-kDa family 1 glycosyltransferase related to known O-glucosyltransferases. The recombinant enzyme C-glucosylated 2-hydroxyflavanones but had negligible O-glucosyltransferase activity with flavonoid acceptors. Enzyme chemistry studies suggested that OsCGT preferentially C-glucosylated the dibenzoylmethane tautomers formed in equilibrium with 2-hydroxyflavanones. The resulting 2-hydroxyflavanone-C-glucosides were unstable and spontaneously dehydrated in vitro to yield a mixture of 6C- and 8C-glucosyl derivatives of the respective flavones. In contrast, in planta, only the respective 6C-glucosides accumulated. Consistent with this selectivity in glycosylation product, a dehydratase activity that preferentially converted 2-hydroxyflavanone-C-glucosides to the corresponding flavone-6C-glucosides was identified in both rice and wheat. Our results demonstrate that cereal crops synthesize C-glucosylated flavones through the concerted action of a CGT and dehydratase acting on activated 2-hydroxyflavanones, as an alternative means of generating flavonoid metabolites.