Lodeiro Silvia, Xiong Quanbo, Wilson William K, Kolesnikova Mariya D, Onak Carl S, Matsuda Seiichi P T
Department of Chemistry, Rice University, Houston, Texas 77005, USA.
J Am Chem Soc. 2007 Sep 12;129(36):11213-22. doi: 10.1021/ja073133u. Epub 2007 Aug 18.
The genome of the model plant Arabidopsis thaliana encodes 13 oxidosqualene cyclases, 9 of which have been characterized by heterologous expression in yeast. Here we describe another cyclase, baruol synthase (BARS1), which makes baruol (90%) and 22 minor products (0.02-3% each). This represents as many triterpenes as have been reported for all other Arabidopsis cyclases combined. By accessing an extraordinary repertoire of mechanistic pathways, BARS1 makes numerous skeletal types and deprotonates the carbocation intermediates at 14 different sites around rings A, B, C, D, and E. This undercurrent of structural and mechanistic diversity in a superficially accurate enzyme is incompatible with prevailing concepts of triterpene biosynthesis, which posit tight control over the mechanistic pathway through cation-pi interactions, with a single proton acceptor in a hydrophobic active site. Our findings suggest that mechanistic diversity is the default for triterpene biosynthesis and that product accuracy results from exclusion of alternative pathways.
模式植物拟南芥的基因组编码13种氧化鲨烯环化酶，其中9种已通过在酵母中的异源表达进行了表征。在此，我们描述了另一种环化酶——巴鲁醇合酶（BARS1），它产生巴鲁醇（90%）和22种次要产物（每种0.02 - 3%）。这代表的三萜类化合物数量与已报道的所有其他拟南芥环化酶产生的三萜类化合物数量总和相同。通过采用一系列独特的机制途径，BARS1生成了多种骨架类型，并在环A、B、C、D和E周围的14个不同位点使碳正离子中间体去质子化。这种表面上精确的酶所具有的结构和机制多样性与三萜生物合成的主流概念相悖，主流概念认为通过阳离子 - π相互作用对机制途径进行严格控制，在疏水活性位点有单一质子受体。我们的研究结果表明，机制多样性是三萜生物合成的默认状态，而产物精确性是通过排除替代途径实现的。
