A Mononuclear Iron-Dependent Methyltransferase Catalyzes Initial Steps in Assembly of the Apratoxin A Polyketide Starter Unit
A Mononuclear Iron-Dependent Methyltransferase Catalyzes Initial Steps in Assembly of the Apratoxin A Polyketide Starter Unit
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DOI：
10.1021/acschembio.7b00746
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发表时间：
2017-12-01
影响因子：
4
通讯作者：
Smith, Janet L.
中科院分区：
文献类型：
--
作者：
Skiba, Meredith A.;Sikkema, Andrew P.;Smith, Janet L.
天然产物生物合成途径包含大量的酶工具来进行困难的生物合成转化。在这里，我们发现了一种不寻常的单核铁依赖性甲基转移酶，它在阿普拉毒素A生物合成的起始步骤中起作用（AprA MT1）。富Fe3+的AprA MT1在底物丙二酰acp（酰基载体蛋白）上催化一个或两个甲基转移反应，而Co2+、Fe2+、Mn2+和Ni2+只支持一个甲基转移反应。存在MT1同源物；在几种具有丙酰、异丁基或戊酰起始单元的模块化生物合成途径的“GNAT”（gcns相关的n -乙酰转移酶）装载模块中。GNAT结构域被认为催化丙二酰辅酶a脱羧和乙酰转移到载体蛋白。在AprA中，GNAT结构域缺乏脱羧和酰基转移活性。AprA MT1-GNAT双结构域结合Mn2+、丙二酸盐和甲基供体s -腺苷甲硫氨酸（SAM）的晶体结构表明，丙二醇基底物是双齿金属配体，表明金属作为刘易斯酸促进丙二醇基α -碳的甲基化。GNAT结构域相对于功能同源物被截断。这些结果扩大了对MT1-GNAT结构和活性的理解，并允许对同源GNAT加载模块进行功能注释，包括甲基转移酶和不甲基转移酶，另外揭示了它们在不同生物合成环境下的快速进化适应。
Natural product biosynthetic pathways contain a plethora of enzymatic tools to carry out difficult biosynthetic transformations. Here, we discover an unusual mononuclear iron dependent methyltransferase that acts in the initiation steps of apratoxin A biosynthesis (AprA MT1). Fe3+-replete AprA MT1 catalyzes one or two methyl transfer reactions on the substrate malonyl-ACP (acyl carrier protein), whereas Co2+, Fe2+, Mn2+, and Ni2+ support only a single methyl transfer. MT1 homologues' exist;Within the "GNAT" (GCNS-related N-acetyltransferase) loading modules of several modular biosynthetic pathways with propionyl, isobutyryt or pivaloyl starter units. GNAT domains are thought to catalyze decarboXylation of malonyl-CoA and acetyl transfer to a carrier protein. In AprA, the GNAT domain lacks both decarboxylation and acyl transfer activity. A crystal structure of the AprA MT1-GNAT di-domain with bound Mn2+, malonate, and the methyl donor S-adenosylmethionine (SAM) reveals that the malonyl substrate is a bidentate metal ligand, indicating that the metal acts as a Lewis acid to promote methylation of the malonyl alpha-carbon. The GNAT domain is truncated relative to functional homologues. These results afford an expanded understanding of MT1-GNAT structure and activity arid permit the functional annotation of homologous GNAT loading modules both with and without methyltransferases, additionally revealing their rapid evolutionary adaptation in different biosynthetic contexts.