Chlorinated Ethene Reactivity with Vitamin B12 Is Governed by Cobalamin Chloroethylcarbanions as Crossroads of Competing Pathways
Chlorinated Ethene Reactivity with Vitamin B12 Is Governed by Cobalamin Chloroethylcarbanions as Crossroads of Competing Pathways
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DOI：
10.1021/acscatal.7b02945
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发表时间：
2018-04-01
期刊：
影响因子：
12.9
通讯作者：
Elsner, Martin
中科院分区：
文献类型：
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作者：
Heckel, Benjamin;McNeill, Kristopher;Elsner, Martin
氯化乙烯是有毒的地下水污染物。虽然它们可以通过微生物、还原性脱卤酶和它们的类咕啉辅因子脱氯，但生化反应机制仍然没有解决。这项研究揭示了一个机制的转变，揭示了对比化合物特定的碳（C13 C-13）和氯（37）同位素效应与四氯乙烯，（三氯-37 = -4.0 ‰）和顺式二氯乙烯、顺式DCE（三氯Cl-37 = -1.5 ‰），三氯乙烯的pH值依赖性变化，TCE（从pH值12时的三氯-37 =-5.2ppm到pH值5时的三氯-37 =-1.2ppm）。不同的途径也支持pH值依赖的反应速率，三氯乙烯产品分布，和氢同位素效应。质量平衡赤字揭示了可逆和不可逆的钴胺素-底物协会，而高分辨率质谱缩小了可能的结构氯烷基和氯乙烯钴胺素复合物。结合实验证据是不一致的初始电子转移或烷基或乙烯基复合物作为两种途径的共享中间体。相反，它支持钴胺素氯碳负离子作为关键的中间体，其中Cl-消除产生乙烯基复合物（解释率和产品的TCE在高pH值），而质子化产生反应性较低的烷基复合物（解释率和产品的TCE在低pH值）。多元素同位素效应分析有望在真实的脱卤酶、微生物甚至受污染的含水层中识别这些竞争机制。
Chlorinated ethenes are toxic groundwater contaminants. Although they can be dechlorinated by microorganisms, reductive dehalogenases, and their corrinoid cofactor, biochemical reaction mechanisms remain unsolved. This study uncovers a mechanistic shift revealed by contrasting compound-specific carbon (epsilon C-13) and chlorine (epsilon Cl-37) isotope effects between perchloroethene, PCE (epsilon Cl-37 = -4.0 parts per thousand) and cis-dichloroethene, cis-DCE (epsilon Cl-37 = -1.5 parts per thousand), and a pH-dependent shift for trichloroethene, TCE (from epsilon Cl-37 = -5.2 parts per thousand at pH 12 to epsilon Cl-37 = -1.2 parts per thousand at pH 5). Different pathways are supported also by pH-dependent reaction rates, TCE product distribution, and hydrogen isotope effects. Mass balance deficits revealed reversible and irreversible cobalamin-substrate association, whereas high-resolution mass spectrometry narrowed down possible structures to chloroalkyl and chlorovinyl cobalamin complexes. Combined experimental evidence is inconsistent with initial electron transfer or alkyl or vinyl complexes as shared intermediates of both pathways. In contrast, it supports cobalamin chlorocarbanions as key intermediates from which Cl- elimination produces vinyl complexes (explaining rates and products of TCE at high pH), whereas protonation generates less reactive alkyl complexes (explaining rates and products of TCE at low pH). Multielement isotope effect analysis holds promise to identify these competing mechanisms also in real dehalogenases, microorganisms, and even contaminated aquifers.