Tropinone is the first intermediate in the biosynthesis of the pharmacologically important tropane alkaloids that possesses the 8-azabicyclo[3.2.1]octane core bicyclic structure that defines this alkaloid class. Chemical synthesis of tropinone was achieved in 1901 but the mechanism of tropinone biosynthesis has remained elusive. In this study, we identify a root-expressed type III polyketide synthase from Atropa belladonna (AbPYKS) that catalyzes the formation of 4-(1-methyl-2-pyrrolidinyl)-3-oxobutanoic acid. This catalysis proceeds through a non-canonical mechanism that directly utilizes an unconjugated N-methyl-Δ¹-pyrrolinium cation as the starter substrate for two rounds of malonyl-Coenzyme A mediated decarboxylative condensation. Subsequent formation of tropinone from 4-(1-methyl-2-pyrrolidinyl)-3-oxobutanoic acid is achieved through cytochrome P450-mediated catalysis by AbCYP82M3. Silencing of AbPYKS and AbCYP82M3 reduces tropane levels in A. belladonna. This study reveals the mechanism of tropinone biosynthesis, explains the in planta co-occurrence of pyrrolidines and tropanes, and demonstrates the feasibility of tropane engineering in a non-tropane producing plant.

中文翻译：

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通过非典型聚酮合酶和P450介导的环化作用合成托品酮。

Tropinone是具有重要药理学意义的托烷生物碱的生物合成中的第一个中间体，该生物碱具有定义该生物碱类别的8-氮杂双环[3.2.1]辛烷核心双环结构。肌钙蛋白的化学合成于1901年完成，但是肌钙蛋白的生物合成机理仍然难以捉摸。在这项研究中，我们从Atropa belladonna（AbPYKS）鉴定了根表达的III型聚酮化合物合酶，它催化了4-（1-甲基-2-吡咯烷基）-3-氧代丁酸的形成。此催化通过非规范机构，其直接利用非共轭N-甲基Δ前进¹-吡咯啉阳离子作为两轮丙二酰辅酶A介导的脱羧缩合反应的起始底物。随后通过AbCYP82M3通过细胞色素P450介导的催化作用，由4-（1-甲基-2-吡咯烷基）-3-氧代丁酸形成肌钙蛋白。沉默AbPYKS和AbCYP82M3会降低颠茄农杆菌中的托烷含量。这项研究揭示了肌钙蛋白生物合成的机制，解释了吡咯烷和托烷在植物中的共生现象，并证明了在非托烷生产工厂中进行托烷工程的可行性。