Radical-mediated enzymatic methylation: a tale of two SAMS.
Radical-mediated enzymatic methylation: a tale of two SAMS.
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自由基介导的酶促甲基化：两个 SAMS 的故事
DOI：
10.1021/ar200202c
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发表时间：
2012-04-17
影响因子：
18.3
通讯作者：
Liu, Wen
中科院分区：
文献类型：
--
作者：
Zhang, Qi;van der Donk, Wilfred A.;Liu, Wen
甲基化是一种基本的、普遍存在的反应，在广泛的生物过程中起着重要作用。大多数生物甲基化使用s -腺苷蛋氨酸（SAM）作为甲基供体，并通过SN2位移机制进行。然而，研究人员发现越来越多的甲基化涉及到自由基化学。已知催化这些反应的酶都属于自由基SAM超家族。该酶家族利用一个特殊的[4Fe-4S]簇对SAM进行还原裂解，产生高活性的5'-脱氧腺苷（dAdo）自由基。然后将自由基化学作用于各种有机底物上，导致各种各样的转化。直到最近，研究人员还没有完全理解这些酶是如何利用自由基化学来介导甲基转移反应的。序列分析表明，目前已鉴定的自由基SAM甲基转移酶（RSMTs）可分为三类，它们在蛋白结构和机制上表现出不同。A类rsmt主要包括来自不同来源的rRNA甲基转移酶RlmN和Cfr。以大肠杆菌RlmN为例，这些蛋白具有一个单一的典型自由基SAM核心结构域，包括一个（βα）6部分桶状结构域，与丙酮酸甲酸裂解酶激活酶的结构域最相似。最近关于RlmN和Cfr的令人兴奋的研究开始为A类rsmt的有趣化学提供见解。这些酶利用在独特的甲基化半胱氨酸残基上产生的亚甲基自由基。然而，基于其他类rsmt使用的底物的多样性，可能会发现替代机制。B类rsmt除了在c端含有一个自由基SAM结构域外，还含有一个n端钴胺素结合结构域。这类蛋白在惰性sp3碳、芳香杂环和膦酸盐上甲基化各种底物，可能涉及钴胺介导的甲基转移过程。C类RSMTs与卟啉原III氧化酶HemN具有显著的序列相似性。尽管甲基化相似的底物（芳香族杂环），但C类rsmt可能采用与a类不同的机制，因为a类rsmt所需的两个保守半胱氨酸通常在C类rsmt中找不到。A类和B类酶可能共用两个SAM分子：一个产生dAdo自由基，一个向底物提供甲基。在A类中，半胱氨酸将作为甲基的通道，而在B类中，钴胺素可能起到这一作用。目前尚不清楚C类rsmt的机制，但其成员与HemN的序列相似性以及HemN结合两个SAM分子的观察表明，C类酶可以利用两个SAM分子进行催化。使用两种SAM分子的不同策略反映了自由基介导的甲基化反应的丰富化学性质以及自由基SAM超家族的显著多功能性。
Methylation is an essential and ubiquitous reaction that plays an important role in a wide range of biological processes. Most biological methylations use S-adenosylmethionine (SAM) as the methyl donor and proceed via an SN2 displacement mechanism. However, researchers have discovered an increasing number of methylations that involve radical chemistry. The enzymes known to catalyze these reactions all belong to the radical SAM superfamily. This family of enzymes utilizes a specialized [4Fe-4S] cluster for reductive cleavage of SAM to yield a highly reactive 5'-deoxyadenosyl (dAdo) radical. Radical chemistry is then imposed on a variety of organic substrates, leading to a diverse array of transformations. Until recently, researchers had not fully understood how these enzymes employ radical chemistry to mediate a methyl transfer reaction. Sequence analyses reveal that the currently identified radical SAM methyltransferases (RSMTs) can be grouped into three classes, which appear distinct in protein architecture and mechanism. Class A RSMTs mainly include the rRNA methyltransferases RlmN and Cfr from various origins. As exemplified by Escherichia coli RlmN, these proteins have a single canonical radical SAM core domain that includes an (βα)6 partial barrel most similar to that of pyruvate formate lyase-activase. The exciting recent studies on RlmN and Cfr are beginning to provide insights into the intriguing chemistry of class A RSMTs. These enzymes utilize a methylene radical generated on a unique methylated cysteine residue. However, based on the variety of substrates used by the other classes of RSMTs, alternative mechanisms are likely to be discovered. Class B RSMTs contain a proposed N-terminal cobalamin binding domain in addition to a radical SAM domain at the C-terminus. This class of proteins methylates diverse substrates at inert sp3 carbons, aromatic heterocycles, and phosphinates, possibly involving a cobalamin-mediated methyl transfer process. Class C RSMTs share significant sequence similarity with coproporphyrinogen III oxidase HemN. Despite methylating similar substrates (aromatic heterocycles), class C RSMTs likely employ a mechanism distinct from that of class A because two conserved cysteines that are required for class A are typically not found in class C RSMTs. Class A and class B enzymes probably share the use of two molecules of SAM: one to generate a dAdo radical and one to provide the methyl group to the substrate. In class A, a cysteine would act as a conduit of the methyl group whereas in class B cobalamin may serve this purpose. Currently no clues are available regarding the mechanism of class C RSMTs, but the sequence similarities between its members and HemN and the observation that HemN binds two SAM molecules suggest that class C enzymes could use two SAM molecules for catalysis. The diverse strategies for using two SAM molecules reflect the rich chemistry of radical-mediated methylation reactions and the remarkable versatility of the radical SAM superfamily.
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自由基 SAM 酶进行甲基转移的结构基础。
DOI：
10.1126/science.1205358
发表时间：
2011-05-27
期刊：
Science (New York, N.Y.)
影响因子：
--
作者：
Boal AK;Grove TL;McLaughlin MI;Yennawar NH;Booker SJ;Rosenzweig AC
通讯作者：
Rosenzweig AC
影响因子：
7.8
作者：
Booker, Squire J.
通讯作者：
Booker, Squire J.
影响因子：
3.3
作者：
Kudo, Furnitaka;Kasama, Yuko;Eguchi, Tadashi
通讯作者：
Eguchi, Tadashi
影响因子：
15
作者：
HOUCK, DR;CHEN, LC;FLOSS, HG
通讯作者：
FLOSS, HG
影响因子：
2.9
作者：
Lee, Kyung-Hoon;Saleh, Lana;Anton, Brian P.;Madinger, Catherine L.;Benner, Jack S.;Iwig, David F.;Roberts, Richard J.;Krebs, Carsten;Booker, Squire J.
通讯作者：
Booker, Squire J.