Department of Chemistry, Fudan University, Shanghai, 200433, China.
School of Life Sciences, Lanzhou University, Lanzhou, 730000, China.
Angew Chem Int Ed Engl. 2017 Mar 27;56(14):3857-3861. doi: 10.1002/anie.201609948. Epub 2017 Jan 23.
S-Adenosylmethionine (SAM) is one of the most common co-substrates in enzyme-catalyzed methylation reactions. Most SAM-dependent reactions proceed through an S 2 mechanism, whereas a subset of them involves radical intermediates for methylating non-nucleophilic substrates. Herein, we report the characterization and mechanistic investigation of NosN, a class C radical SAM methyltransferase involved in the biosynthesis of the thiopeptide antibiotic nosiheptide. We show that, in contrast to all known SAM-dependent methyltransferases, NosN does not produce S-adenosylhomocysteine (SAH) as a co-product. Instead, NosN converts SAM into 5'-methylthioadenosine as a direct methyl donor, employing a radical-based mechanism for methylation and releasing 5'-thioadenosine as a co-product. A series of biochemical and computational studies allowed us to propose a comprehensive mechanism for NosN catalysis, which represents a new paradigm for enzyme-catalyzed methylation reactions.
S-腺苷甲硫氨酸（SAM）是酶促甲基化反应中最常见的共底物之一。大多数依赖 SAM 的反应通过 S 2 机制进行，而其中一部分涉及自由基中间体，用于甲基化非亲核底物。本文报道了 NosN 的表征和机制研究，NosN 是一种参与硫肽抗生素诺西肽生物合成的 C 类自由基 SAM 甲基转移酶。我们表明，与所有已知的依赖 SAM 的甲基转移酶不同，NosN 不会产生 S-腺苷同型半胱氨酸（SAH）作为副产物。相反，NosN 将 SAM 转化为 5'-甲基硫代腺苷作为直接甲基供体，采用基于自由基的机制进行甲基化，并释放 5'-硫代腺苷作为副产物。一系列生化和计算研究使我们能够提出 NosN 催化的综合机制，这代表了酶促甲基化反应的新范例。
