Ohashi Masao, Liu Fang, Hai Yang, Chen Mengbin, Tang Man-Cheng, Yang Zhongyue, Sato Michio, Watanabe Kenji, Houk K N, Tang Yi
Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California, USA.
Department of Pharmaceutical Sciences, University of Shizuoka, Shizuoka 422-8526, Japan.
Nature. 2017 Sep 28;549(7673):502-506. doi: 10.1038/nature23882. Epub 2017 Sep 13.
Pericyclic reactions-which proceed in a concerted fashion through a cyclic transition state-are among the most powerful synthetic transformations used to make multiple regioselective and stereoselective carbon-carbon bonds. They have been widely applied to the synthesis of biologically active complex natural products containing contiguous stereogenic carbon centres. Despite the prominence of pericyclic reactions in total synthesis, only three naturally existing enzymatic examples (the intramolecular Diels-Alder reaction, and the Cope and the Claisen rearrangements) have been characterized. Here we report a versatile S-adenosyl-l-methionine (SAM)-dependent enzyme, LepI, that can catalyse stereoselective dehydration followed by three pericyclic transformations: intramolecular Diels-Alder and hetero-Diels-Alder reactions via a single ambimodal transition state, and a retro-Claisen rearrangement. Together, these transformations lead to the formation of the dihydropyran core of the fungal natural product, leporin. Combined in vitro enzymatic characterization and computational studies provide insight into how LepI regulates these bifurcating biosynthetic reaction pathways by using SAM as the cofactor. These pathways converge to the desired biosynthetic end product via the (SAM-dependent) retro-Claisen rearrangement catalysed by LepI. We expect that more pericyclic biosynthetic enzymatic transformations remain to be discovered in naturally occurring enzyme 'toolboxes'. The new role of the versatile cofactor SAM is likely to be found in other examples of enzyme catalysis.
周环反应——通过环状过渡态以协同方式进行——是用于构建多个区域选择性和立体选择性碳-碳键的最强大的合成转化反应之一。它们已被广泛应用于合成含有相邻手性碳中心的生物活性复杂天然产物。尽管周环反应在全合成中很突出，但仅鉴定出三个天然存在的酶促反应实例（分子内狄尔斯-阿尔德反应、科普重排和克莱森重排）。在此，我们报道了一种多功能的依赖S-腺苷-L-甲硫氨酸（SAM）的酶LepI，它可以催化立体选择性脱水，随后进行三种周环转化：通过单一的双功能过渡态进行分子内狄尔斯-阿尔德反应和杂狄尔斯-阿尔德反应，以及逆克莱森重排。这些转化共同导致真菌天然产物leporein的二氢吡喃核心的形成。体外酶促表征和计算研究相结合，深入了解了LepI如何以SAM作为辅因子来调节这些分支生物合成反应途径。这些途径通过LepI催化的（依赖SAM的）逆克莱森重排汇聚到所需的生物合成终产物。我们预计在天然存在的酶“工具箱”中仍有待发现更多的周环生物合成酶促转化反应。多功能辅因子SAM的新作用可能会在酶催化的其他实例中被发现。
