记者日前从中国科学院植物研究所获悉，该所研究员漆小泉团队通过分析比较来自水稻的两个亚种及其野生近缘种的同源基因及其功能，揭示了植物如何利用相同的“原料”产生构象和结构各异的三萜类化合物的催化机制。相关成果在线发表在《新植物学家（New Phytologist）》杂志上。

研究人员在籼稻中鉴定出了一类名叫氧化鲨烯环化酶的酶家族中的一个新成员——籼稻醇合酶，该酶与前期在粳稻中鉴定出的帕克醇合酶为直系同源关系，帕克醇合酶是粳稻在驯化过程中由其近缘野生稻新产生的单功能三萜环化酶。

研究人员利用分子生物学手段，在籼稻醇合酶中鉴别出3个对产物的构象和结构变化具有重要影响的关键氨基酸位点，这些位点通过影响第4个氨基酸残基的空间方向，决定了其产物的类型和空间构象。这一成果为今后利用生物学手段生产具有商业价值的三萜类活性化合物奠定了基础。

据了解，萜类代谢物是植物合成的一类重要代谢产物，多达2万种以上，其中许多是中药的主要有效活性成分，有着极高的应用价值。在萜类合成过程中，氧化鲨烯环化酶家族起到关键作用，能够合成100多种构象和结构各不相同的三萜骨架，但其机制并不清楚。（来源：科学网 丁佳）

Identification of key amino acid residues determining product specificity of 2,3‐oxidosqualene cyclase in Oryza species

Abstract  Triterpene synthases, also known as 2,3‐oxidosqualene cyclases (OSCs), synthesize diverse triterpene skeletons that form the basis of an array of functionally divergent steroids and triterpenoids. Tetracyclic and pentacyclic triterpene skeletons are synthesized via protosteryl and dammarenyl cations, respectively. The mechanism of conversion between two scaffolds is not well understood. Here, we report a promiscuous OSC from rice (Oryza sativa) (OsOS) that synthesizes a novel pentacyclic triterpene orysatinol as its main product. The OsOS gene is widely distributed in indica subspecies of cultivated rice and in wild rice accessions. Previously, we have characterized a different OSC, OsPS, a tetracyclic parkeol synthase found in japonica subspecies. Phylogenetic and protein structural analyses identified three key amino acid residues (#732, #365, #124) amongst 46 polymorphic sites that determine functional conversion between OsPS and OsOS, specifically, the chair–semi(chair)–chair and chair–boat–chair interconversions. The different orientation of a fourth amino acid residue Y257 was shown to be important for functional conversion. The discovery of orysatinol unlocks a new path to triterpene diversity in nature. Our findings also reveal mechanistic insights into the cyclization of oxidosqualene into tetra‐ and pentacyclic skeletons, and provide a new strategy to identify key residues determining OSC specificity.

原文链接：https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.15080