Sulfoxide Synthase versus Cysteine Dioxygenase Reactivity in a Nonheme Iron Enzyme
Sulfoxide Synthase versus Cysteine Dioxygenase Reactivity in a Nonheme Iron Enzyme
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DOI：
10.1021/jacs.7b04251
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发表时间：
2017-07-12
影响因子：
15
通讯作者：
de Visser, Sam P.
中科院分区：
文献类型：
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作者：
Faponle, Abayomi S.;Seebeck, Florian P.;de Visser, Sam P.
亚砜合成酶EgtB代表了一个独特的非血红素铁酶家族，它催化N-三甲基组氨酸和γ-谷氨酰半胱氨酸之间形成C-S键，这是麦角硫氨酸生物合成的关键步骤，麦角硫氨酸是一种与衰老相关的重要氨基酸。关于其催化机理与活性中心Tyr(377)残基的功能有关，一直存在争议。EgtB中麦角硫蛋白的生物合成与半胱氨酸双加氧酶的结构相似，半胱氨酸双加氧酶将两个氧原子转移到半胱氨酸的硫基上。因此，问题是EgtB酶如何在催化C-S成键反应的同时，防止其半胱氨酸底物发生双加氧反应？在这项工作中，我们用量子力学/分子力学的方法研究了亚砜合成酶和半胱氨酸双加氧酶的作用机制，并提出了EgtB中这两种反应通道的途径。我们发现EgtB含有一个保守的酪氨酸残基，它通过质子耦合电子转移与铁(III)-超氧物种反应，产生铁(III)-氢过氧基中间体，从而防止可能的硫代双加氧副反应。随之而来的是亲核C-S成键步骤，伴随着铁(II)-过氧化氢的质子传递回TYR(377)。这是反应循环中的速率决定步骤，紧随其后的是氢原子从三甲基组氨酸底物的CE1-H基团转移到铁(II)-超氧基。在最后一步，快速且几乎无障碍的亚硫氧化生成亚砜产物络合物。这项工作突出了驱动亚砜合成酶反应的酶的独特机制和活性部位设置。
The sulfoxide synthase EgtB represents a unique family of nonheme iron enzymes that catalyze the formation of a C-S bond between N-alpha-trimethyl histidine and gamma-glutamyl cysteine, which is the key step in the biosynthesis of ergothioneine, an important amino acid related to aging. A controversy has arisen regarding its catalytic mechanism related to the function of the active-site Tyr(377) residue. The biosynthesis of ergothioneine in EgtB shows structural similarities to cysteine dioxygenase which transfers two oxygen atoms to the thiolate group of cysteine. The question, therefore, is how do EgtB enzymes catalyze the C-S bond-formation reaction, while also preventing a dioxygenation of its cysteinate substrate? In this work we present a quantum mechanics/molecular mechanics study into the mechanism of sulfoxide synthase enzymes as compared to cysteine dioxygenase enzymes and present pathways for both reaction channels in EgtB. We show that EgtB contains a conserved tyrosine residue that reacts via proton-coupled electron transfer with the iron(III)-superoxo species and creates an iron(III)-hydroperoxo intermediate, thereby preventing the possible thiolate dioxygenation side reaction. The nucleophilic C-S bond-formation step happens subsequently concomitant to relay of the proton of the iron(II)-hydroperoxo back to Tyr(377). This is the rate-determining step in the reaction cycle and is followed by hydrogen-atom transfer from the CE1-H group of trimethyl histidine substrate to iron(II)-superoxo. In the final step, a quick and almost barrierless sulfoxidation leads to the sulfoxide product complexes. The work highlights a unique machinery and active-site setup of the enzyme that drives the sulfoxide synthase reaction.