Mechanisms of Radical-Dependent Biological Methylation

描述(申请人提供)：RLmN和CFR分别催化细菌核糖体C2和C8的23SRRNA的腺苷2503(A2503)的S依赖的腺苷蛋氨酸甲基化。C2甲基化在细菌中普遍存在，据信有助于多肽转移的效率。相比之下，C8甲基化是某些病原菌获得的一种活动，使它们对七种以上针对细菌核糖体大亚基的抗生素产生抗药性。C2和C8是亲电的sp2杂化碳，这使得它们对几乎所有其他已知的依赖SAM的甲基酶所使用的催化策略不起反应。事实上，我们已经证明了这些反应是通过自由基机制发生的，包括：i)通过标准的亲核置换机制将甲基从SAM转移到保守的半胱氨基残基；ii)通过第二个SAM分子的自由基碎裂得到的5‘-脱氧腺苷5’-dA从得到的甲基半胱氨基残基中提取氢原子；iii)将甲基半胱氨基中间体加成到核苷酸底物的C2或C8上；以及iv)拆分 由二硫键形成的蛋白质-核交联物。我们将使用各种动力学、光谱和生化技术进一步表征这个反应，并为反应中的每个假定中间体提供生化和/或结构证据。

DESCRIPTION (provided by applicant): RlmN and Cfr catalyze S-adenosylmethionine (SAM)-dependent methylation of adenosine 2503 (A2503) of 23S rRNA of the bacterial ribosome at C2 and C8, respectively. C2 methylation is found throughout bacteria, and is believed to aid in the efficiency of peptidyltransfer. By contrast, C8 methylation is an activity acquired by certain pathogenic bacteria that confers upon them resistance to over seven classes of antibiotics that target the large subunit of the bacterial ribosome. C2 and C8 are electrophilic sp2-hybridized carbons, which renders them unreactive toward the catalytic strategy used by almost all other known SAM-dependent methylases. In fact, we have shown that these reactions take place via radical mechanisms, involving i) initial transfer of a methyl group from SAM to a conserved cysteinyl residue via a standard nucleophilic displacement mechanism; ii) abstraction of a hydrogen atom from the resulting methylcysteinyl residue by a 5'-deoxyadenosyl 5'-radical (5'-dA) derived from radical fragmentation of a second SAM molecule; iii) addition of the methylcysteinyl radical intermediate to C2 or C8 of the nucleotide substrate; and iv) resolution of the resulting protein-nucleic cross-link by disulfide-bond formation. We will characterize this reaction further using a variety of kinetic, spectroscopic, and biochemical techniques, and provide biochemical and/or structural evidence for each of the postulated intermediates in the reaction.