The C-Glycosylation of Flavonoids in Cereals

Melissa Brazier-Hicks, Kathryn M. Evans, Markus C. Gershater, Horst Puschmann, Patrick G. Steel, Robert Edwards

Research output: Contribution to journalArticlepeer-review

Abstract

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-glucosedependent 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.

Original languageEnglish
Pages (from-to)17926-17934
Number of pages9
JournalJournal of Biological Chemistry
Volume284
Issue number27
DOIs
Publication statusPublished - 3 Jul 2009

Keywords

  • ANTIBIOTIC URDAMYCIN
  • SECONDARY METABOLISM
  • ARABIDOPSIS-THALIANA
  • IN-VITRO
  • BIOSYNTHESIS
  • IDENTIFICATION
  • GLYCOSYLTRANSFERASE
  • RICE
  • GLUCOSYLTRANSFERASE
  • GLYCOSIDES

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