Newman Adam G, Townsend Craig A
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
J Am Chem Soc. 2016 Mar 30;138(12):4219-28. doi: 10.1021/jacs.6b00633. Epub 2016 Mar 16.
Perylenequinones are a class of photoactivated polyketide mycotoxins produced by fungal plant pathogens that notably produce reactive oxygen species with visible light. The best-studied perylenequinone is cercosporin-a product of the Cercospora species. While the cercosporin biosynthetic gene cluster has been described in the tobacco pathogen Cercospora nicotianae, little is known of the metabolite's biosynthesis. Furthermore, in vitro investigations of the polyketide synthase central to cercosporin biosynthesis identified the naphthopyrone nor-toralactone as its direct product-an observation in conflict with published biosynthetic proposals. Here, we present an alternative biosynthetic pathway to cercosporin based on metabolites characterized from a series of biosynthetic gene knockouts. We show that nor-toralactone is the key polyketide intermediate and the substrate for the unusual didomain protein CTB3. We demonstrate the unique oxidative cleavage activity of the CTB3 monooxygenase domain in vitro. These data advance our understanding of perylenequinone biosynthesis and expand the biochemical repertoire of flavin-dependent monooxygenases.
苝醌类是一类由真菌植物病原体产生的光活化聚酮类霉菌毒素，这类病原体在可见光照射下能产生活性氧。研究最深入的苝醌类物质是尾孢菌素——尾孢菌属的产物。虽然烟草病原体烟草尾孢菌中的尾孢菌素生物合成基因簇已被描述，但对该代谢产物的生物合成了解甚少。此外，对尾孢菌素生物合成核心的聚酮合酶进行的体外研究确定萘并吡喃酮去甲托酚酮为其直接产物——这一观察结果与已发表的生物合成假说相矛盾。在此，我们基于一系列生物合成基因敲除所鉴定的代谢产物，提出了一条尾孢菌素的替代生物合成途径。我们表明去甲托酚酮是关键的聚酮中间体，也是不寻常的双结构域蛋白CTB3的底物。我们在体外证明了CTB3单加氧酶结构域独特的氧化裂解活性。这些数据增进了我们对苝醌类生物合成的理解，并扩展了黄素依赖性单加氧酶的生化功能范围。
