Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Chembiochem. 2010 Feb 15;11(3):426-33. doi: 10.1002/cbic.200900672.
Triterpenes, a diverse group of natural products comprising six isoprene units, are distributed across various organisms from bacteria to higher plants. Ferns are sporophytes that produce triterpenes and are lower on the evolutionary scale than higher plants. Among ferns that produce triterpenes analogous to bacterial hopanoids, Polypodiodes niponica produces migrated dammaranes and oleananes, which are also widely found in higher plants. Because the study of terpene-producing ferns could help us to understand the molecular basis of triterpene biosynthesis, cDNA cloning of squalene cyclases (SCs) from P. niponica was carried out. Two SCs (PNT and PNG) were obtained. The heterologously expressed PNT produces tirucalla-7,21-diene (67% major), and PNG produces germanicene (69%). Phylogenetic analysis revealed that PNT and PNG, which produce higher-plant-type migrated dammaranes and oleananes, are closely related to bacterial-type SCs. Furthermore, analysis of the minor products indicated that fern SCs gained the ability to directly form dammarenyl cations, which are key intermediates in oleanane formation during molecular evolution.
三萜类化合物是一类由六个异戊二烯单元组成的天然产物，分布于从细菌到高等植物等各种生物体中。蕨类植物是产生三萜类化合物的孢子体，在进化尺度上比高等植物低。在产生类似于细菌藿烷类的三萜类化合物的蕨类植物中，日本蹄盖蕨产生了迁移的达玛烷和齐墩果烷，这些化合物也广泛存在于高等植物中。由于研究产萜类化合物的蕨类植物可以帮助我们了解三萜类生物合成的分子基础，因此对日本蹄盖蕨的角鲨烯环化酶（SC）进行了 cDNA 克隆。获得了两种 SC（PNT 和 PNG）。异源表达的 PNT 产生了三萜烯-7,21-二烯（67%为主要产物），而 PNG 则产生了 germacrene（69%）。系统发育分析表明，产生高等植物型迁移达玛烷和齐墩果烷的 PNT 和 PNG 与细菌型 SC 密切相关。此外，对次要产物的分析表明，蕨类植物 SC 在分子进化过程中获得了直接形成达玛烯阳离子的能力，而达玛烯阳离子是齐墩果烷形成的关键中间体。
