Epoxidation Catalyzed by the Nonheme Iron(II)- and 2-Oxoglutarate-Dependent Oxygenase, AsqJ: Mechanistic Elucidation of Oxygen Atom Transfer by a Ferryl Intermediate

Abstract

Mechanisms of enzymatic epoxidation via oxygen atom transfer (OAT) to an olefin moiety is mainly derived from the studies on thiolate-heme containing epoxidases, such as cytochrome P450 epoxidases. The molecular basis of epoxidation catalyzed by non-heme-iron enzymes is much less explored. Herein, we present a detailed study on epoxidation catalyzed by the non-heme iron-and 2-oxoglutarate-dependent (Fe/2OG) oxygenase, AsqJ. The native substrate and analogs with different para substituents ranging from electron-donating groups (e.g. methoxy) to electronwithdrawing groups (e.g. trifluoromethyl) were used to probe the mechanism. The results derived from transient-state enzyme kinetics, Mössbauer spectroscopy, reaction product analysis, X-ray crystallography, density functional theory calculations and molecular dynamic simulations collectively revealed the following mechanistic insights: 1) The rapid O 2 addition to the AsqJ Fe(II) center occurs with the iron-bound 2OG adopting an online-binding mode in which the C1 carboxylate group of 2OG is trans to the proximal histidine (His134) of the 2-His-1-carboxylate facial triad, instead of assuming the offline-binding mode with the C1 carboxylate group trans to the distal histidine (His211); 2) The decay rate constant of the ferryl intermediate is not strongly affected by the nature of the para substituents of the substrate during the OAT step, a reactivity behavior that is drastically different from non-heme Fe(IV)-oxo synthetic model complexes; 3) The OAT step most likely proceeds through a step-wise process with the initial formation of *

Recognition of Surface Oxygen Intermediates on NiFe Oxyhydroxide Oxygen-Evolving Catalysts by Homogeneous Oxidation Reactivity

“…We took inspiration from the well-studied reactivities in homogeneous catalytic oxidation and utilized typical reactions of oxygen atom transfer (OAT) and hydrogen atom transfer (HAT). OAT probes can specifically target oxygen-terminated structures for abstracting oxygen atoms, often including phosphines, thioethers and alkenes [33][34][35] . HAT probes, often C-H-based molecules, can specifically target oxygen-associated high-valent metal species, such as metal oxo and metal peroxo species, for donating H atoms to generate the corresponding radical [36][37] .…”

“…It also supports that the oxygen intermediates on Fe sites are the main probe target sites. By combining with the aforementioned oxygen-terminated resting state, we hypothesis that the key OER intermediates involve the molecular-like Fe=O moiety that is often subject to OAT reactivity [34][35] .…”

Resolving Oxygenation Pathways in Manganese-Catalyzed C(sp3)–H Functionalization via Radical and Cationic Intermediates

“…In monoiron-dependent non-heme enzymes, the structural versatility of the metal coordination sphere enables alternative reactivity patterns; 19 for example, the transfer of halide and pseudohalide ligands adjacent to the hydroxyl defines the reactivity of iron-dependent halogenases in the ligand transfer step. 20 − 22 On the other hand, desaturation pathways initiated by HAT from C(sp 3 )–H bonds are seldom observed, for example, in the non-heme Fe II /α-ketoglutarate-dependent dioxygenase AsqJ, 23 with two divergent mechanisms, which may account for this reactivity pattern, currently under debate. The first one entails an electron transfer (ET) from the carbon radical to the metal to form a carbocation that then evolves via proton loss to the desaturation product.…”

Molecular insights into the unusually promiscuous and catalytically versatile Fe(II)/α-ketoglutarate-dependent oxygenase SptF

“…In plants and microorganisms, in addition to the typical hydroxylation reactions, Fe/αKG oxygenases catalyze a wide range of chemical transformations in the biosynthesis of natural products 2 – 27 , including complex skeletal rearrangement 11 , 12 , ring-expansion 15 , and C–C bond formation 21 – 23 . Given their intriguing chemical reactions, detailed biochemical, structural, and calculation studies of Fe/αKG oxygenases have been conducted over the past few decades 5 , 6 , 8 , 28 – 32 .…”