title: Origin of structural diversity in natural triterpenes: direct synthesis of seco-triterpene skeletons by oxidosqualene cyclase.
abstract: At1g78500, one of the oxidosqualene cyclase (OSC) homologues from Arabidopsis thaliana, was expressed in a lanosterol synthase-deficient yeast strain and the products were analyzed. In addition to the known triterpenes, this OSC was found to produce two new triterpenes, the structures of which were determined by NMR and MS analyses. The new triterpenes are C-ring-seco-beta-amyrin (1) and C-ring-seco-alpha-amyrin (2) and named beta-seco-amyrin and alpha-seco-amyrin, respectively. beta-seco-Amyrin is produced from the oleanyl cation through bond cleavage between C8 and C14, and alpha-seco-amyrin is produced from the ursanyl cation in the same manner. Together with Grob fragmentation catalyzed by another OSC (marneral synthase) from A. thaliana, the formation of seco-amyrins by this OSC revealed that OSCs not only catalyze carbon-carbon bond formations and Wagner-Meerwein rearrangements but also cleave preformed ring systems in cationic intermediates. Based on this information, direct production of other natural seco-triterpenes by OSCs is proposed.
authors.affiliation: Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Shibuya
authors.forename: Ting
authors.initials: T
authors.lastname: Xiang
authors.forename: Yuji
authors.initials: Y
authors.lastname: Katsube
authors.forename: Miyuki
authors.initials: M
authors.lastname: Otsuka
authors.forename: Hong
authors.initials: H
authors.lastname: Zhang
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Ebizuka
mesh_terms: D017360:Arabidopsis / Q000201:enzymology*; D002244:Carbon / Q000737:chemistry; D002384:Catalysis; D002412:Cations; D019751:Intramolecular Transferases / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D009828:Oleanolic Acid / Q000031:analogs & derivatives / Q000138:chemical synthesis* / Q000494:pharmacology; D021241:Spectrometry, Mass, Electrospray Ionization; D014315:Triterpenes / Q000138:chemical synthesis* / Q000494:pharmacology
chemical_list: D002412:Cations; D014315:Triterpenes; D009828:Oleanolic Acid; D002244:Carbon; D019751:Intramolecular Transferases; C021924:lanosterol synthase; C036380:beta-amyrin