Journal Article

SAM-dependent enzyme-catalysed pericyclic reactions in natural product biosynthesis

Tang, Man-cheng, Hai, Yang, Yang, Zhongyue, Watanabe, Kenji, Chen, Mengbin, Ohashi, Masao, Liu, Fang, Sato, Michio, Houk, K. N., Tang, Yi

2017

Overview

The enzyme LepI is found to be capable of catalysing several natural-product pericyclic transformations, including a hetero-Diels–Alder reaction and a retro-Claisen rearrangement. A (cyclo)addition to nature's toolbox Although common in synthesis, naturally occurring pericyclic reactions, in which two fragments combine to form a cyclic molecule, are rare. Several examples of cyclohexene-forming enzymes called Diels–Alderases have been discovered. However, biosynthetic inverse electron demand Diels–Alder reactions are still unknown. These reactions often involve heteroatoms (non-carbon atoms) in the cycloaddition step, so are important in the synthesis of both heterocyclic and natural products. Here, the authors report the versatile S -adenosyl-L-methionine (SAM)-dependent enzyme, LepI, which is capable of catalysing several pericyclic transformations, including a hetero-Diels–Alder reaction. The biosynthesis of the cytotoxic leporin B proceeds via a bifurcated reaction pathway regulated by LepI, a direct hetero-Diels–Alder reaction and an indirect Diels–Alder/retro-Claisen rearrangement sequence, converging to give the heterocyclic pyran product. 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 1 . They have been widely applied to the synthesis of biologically active complex natural products containing contiguous stereogenic carbon centres 2 , 3 , 4 , 5 , 6 . Despite the prominence of pericyclic reactions in total synthesis, only three naturally existing enzymatic examples (the intramolecular Diels–Alder reaction 7 , and the Cope 8 and the Claisen rearrangements 9 ) 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 10 . 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’ 11 . The new role of the versatile cofactor SAM is likely to be found in other examples of enzyme catalysis.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 119/118 / 38/22 / 42/44 / 631/45/173 / 639/638/60 / 82/80 / 82/83 / Aspergillus nidulans - enzymology / Aspergillus nidulans - genetics / Benzopyrans - chemistry / Benzopyrans - metabolism / Biocatalysis / Biological activity / Biological Products - chemistry / Biological Products - metabolism / Biosynthesis / Biosynthetic Pathways / Carbon / Catalysis / Chemical properties / Chemical synthesis / Chromatography, High Pressure Liquid / Coenzymes - metabolism / Cycloaddition Reaction / Dehydration / Diels-Alder reactions / Enzymes / Escherichia coli - genetics / Humanities and Social Sciences / letter / Methionine / Methods / multidisciplinary / Natural products / Pyrans - chemistry / Pyrans - metabolism / Pyridones - chemistry / Pyridones - metabolism / S-Adenosylmethionine - metabolism / Saccharomyces cerevisiae / Science / Stereoselectivity