Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 105, Issue 37, Pages 14204-14209Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0803876105
Keywords
expressed sequence tag; medicinal plant; secondary metabolite; monooxygenase; isoprenoid
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Funding
- New Energy and Industrial Technology Development Organization (NEDO), Japan
- Ministry of Education, Culture, Sports, Science and Technology [18780082]
- Grants-in-Aid for Scientific Research [18780082] Funding Source: KAKEN
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Glycyrrhizin, a major bioactive compound derived from the underground parts of Glycyrrhiza (licorice) plants, is a triterpene saponin that possesses a wide range of pharmacological properties and is used worldwide as a natural sweetener. Because of its economic value, the biosynthesis of glycyrrhizin has received considerable attention. Glycyrrhizin is most likely derived from the triterpene beta-amyrin, an initial product of the cyclization of 2,3-oxidosqualene. The subsequent steps in glycyrrhizin biosynthesis are believed to involve a series of oxidative reactions at the C-11 and C-30 positions, followed by glycosyl transfers to the C-3 hydroxyl group; however, no genes encoding relevant oxidases or glycosyltransferases have been identified. Here we report the successful identification of CYP88D6, a cytochrome P450 monooxygenase (111450) gene, as a glycyrrhizin-biosynthetic gene, by transcript profiling-based selection from a collection of licorice expressed sequence tags (ESTs). CYP88D6 was characterized by in vitro enzymatic activity assays and shown to catalyze the sequential two-step oxidation of p-amyrin at C-11 to produce 11-oxo-beta-amyrin, a possible biosynthetic intermediate between P-amyrin and glycyrrhizin. CYP88D6 coexpressed with beta-amyrin synthase in yeast also catalyzed in vivo oxidation of beta-amyrin to 11-oxo-beta-amyrin. CYP88D6 expression was detected in the roots and stolons by RT-PCR; however, no amplification was observed in the leaves or stems, which is consistent with the accumulation pattern of glycyrrhizin in planta. These results suggest a role for CYP88D6 as a beta-amyrin 11-oxidase in the glycyrrhizin pathway.
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