How do Enzymes Utilize Reactive OH Radicals? Lessons from Nonheme HppE and Fenton Systems

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• 作者：Binju Wang ; Jiarui Lu ; Kshatresh Dutta Dubey ; Geng Dong ; Wenzhen Lai ; Sason Shaik
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：July 13, 2016
• 年：2016
• 卷：138
• 期：27
• 页码：8489-8496
• 全文大小：556K
• 年卷期：0
• ISSN：1520-5126

文摘

The iron(IV)–oxo (ferryl) intermediate has been amply established as the principal oxidant in nonheme enzymes and the key player in C–H bond activations and functionalizations. In contrast to this status, our present QM/MM calculations of the mechanism of fosfomycin biosynthesis (a broad range antibiotic) by the nonheme HppE enzyme rule out the iron(IV)–oxo as the reactive species in the hydrogen abstraction (H-abstraction) step of the pro-R hydrogen from the (S)-2-hydroxypropylphosphonic substrate. Moreover, the study reveals that the ferryl species is bypassed in HppE, while the actual oxidant is an HO^• radical hydrogen-bonded to a ferric-hydroxo complex, resulting via the homolytic dissociation of the hydrogen peroxide complex, Fe(II)–H₂O₂. The computed energy barrier of this pathway is 12.0 kcal/mol, in fair agreement with the experimental datum of 9.8 kcal/mol. An alternative mechanism involves the iron-complexed hydroxyl radical (Fe^III–(HO^•)) that is obtained by protonation of the iron(IV)–oxo group via the O–H group of the substrate. The barrier for this pathway, 23.0 kcal/mol, is higher than the one in the first mechanism. In both mechanisms, the HO^• radical is highly selective; its H-abstraction leading to the final cis-fosfomycin product. It appears that HppE is prone to usage of HO^• radicals for C–H bond activation, because the ferryl oxidant requires a specific H-abstraction trajectory (∠FeOH ∼ 180°) that cannot be met for intramolecular H-abstraction. Thus, this work broadens the landscape of nonheme iron enzymes and makes a connection to Fenton chemistry, with implications on new potential biocatalysts that may harness hydroxyl radicals for C–H bond functionalizations.