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Identification and characterization of functional homologs of nitrogenase cofactor biosynthesis protein NifB from methanogens

机译:产甲烷菌中固氮酶辅因子生物合成蛋白NifB的功能同源物的鉴定与表征

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摘要

Nitrogenase biosynthesis protein NifB catalyzes the radical S-adenosyl-L-methionine (SAM)-dependent insertion of carbide into the M cluster, the cofactor of the molybdenum nitrogenase from Azotobacter vinelandii. Here, we report the identification and characterization of two naturally “truncated” homologs of NifB from Methanosarcina acetivorans (NifBMa) and Methanobacterium thermoautotrophicum (NifBMt), which contain a SAM-binding domain at the N terminus but lack a domain toward the C terminus that shares homology with NifX, an accessory protein in M cluster biosynthesis. NifBMa and NifBMt are monomeric proteins containing a SAM-binding [Fe4S4] cluster (designated the SAM cluster) and a [Fe4S4]-like cluster pair (designated the K cluster) that can be processed into an [Fe8S9] precursor to the M cluster (designated the L cluster). Further, the K clusters in NifBMa and NifBMt can be converted to L clusters upon addition of SAM, which corresponds to their ability to heterologously donate L clusters to the biosynthetic machinery of A. vinelandii for further maturation into the M clusters. Perhaps even more excitingly, NifBMa and NifBMt can catalyze the removal of methyl group from SAM and the abstraction of hydrogen from this methyl group by 5′-deoxyadenosyl radical that initiates the radical-based incorporation of methyl-derived carbide into the M cluster. The successful identification of NifBMa and NifBMt as functional homologs of NifB not only enabled classification of a new subset of radical SAM methyltransferases that specialize in complex metallocluster assembly, but also provided a new tool for further characterization of the distinctive, NifB-catalyzed methyl transfer and conversion to an iron-bound carbide.
机译:固氮酶生物合成蛋白NifB催化碳化物根据S-腺苷-L-蛋氨酸(SAM)依赖性插入M簇中,M簇是藤蔓固氮菌中钼固氮酶的辅助因子。在这里,我们报告鉴定和表征了两个自然的“截短的” NifB同源,分别来自乙酰甲烷单胞菌(NifB Ma )和嗜热甲烷菌(NifB Mt ),其中含有SAM N末端具有一个结合结构域,但缺乏一个与C末端共享与NifX(M簇生物合成中的辅助蛋白)同源的区域。 NifB Ma 和NifB Mt 是包含SAM结合[Fe4S4]簇(称为SAM簇)和[Fe4S4]样簇对(称为K)的单体蛋白。簇)可以处理为M簇(称为L簇)的[Fe8S9]前体。此外,添加SAM可以将NifB Ma 和NifB Mt 中的K个簇转化为L个簇,这对应于它们向生物合成机制异源捐赠L个簇的能力葡萄球菌进一步成熟成M簇。甚至更令人兴奋的是,NifB Ma 和NifB Mt 可以催化SAM中甲基的去除以及通过引发5'-脱氧腺苷基团从该甲基中提取氢基于自由基的甲基衍生碳化物结合到M团簇中。成功地将NifB Ma 和NifB Mt 鉴定为NifB的功能同源物,不仅能够分类专门用于复杂金属簇组装的自由基SAM甲基转移酶的新子集,而且还提供了一种新工具,用于进一步表征独特的NifB催化的甲基转移并转化为铁结合的碳化物。

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