Dataset Information

Substrate-Triggered Formation of a Peroxo-Fe2(III/III) Intermediate during Fatty Acid Decarboxylation by UndA.

ABSTRACT: The iron-dependent oxidase UndA cleaves one C3-H bond and the C1-C2 bond of dodecanoic acid to produce 1-undecene and CO2. A published X-ray crystal structure showed that UndA has a heme-oxygenase-like fold, thus associating it with a structural superfamily that includes known and postulated non-heme diiron proteins, but revealed only a single iron ion in the active site. Mechanisms proposed for initiation of decarboxylation by cleavage of the C3-H bond using a monoiron cofactor to activate O2 necessarily invoked unusual or potentially unfeasible steps. Here we present spectroscopic, crystallographic, and biochemical evidence that the cofactor of Pseudomonas fluorescens Pf-5 UndA is actually a diiron cluster and show that binding of the substrate triggers rapid addition of O2 to the Fe2(II/II) cofactor to produce a transient peroxo-Fe2(III/III) intermediate. The observations of a diiron cofactor and substrate-triggered formation of a peroxo-Fe2(III/III) intermediate suggest a small set of possible mechanisms for O2, C3-H and C1-C2 activation by UndA; these routes obviate the problematic steps of the earlier hypotheses that invoked a single iron.

SUBMITTER: Zhang B

PROVIDER: S-EPMC6800569 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Publications

Substrate-Triggered Formation of a Peroxo-Fe<sub>2</sub>(III/III) Intermediate during Fatty Acid Decarboxylation by UndA.

Zhang Bo B Rajakovich Lauren J LJ Van Cura Devon D Blaesi Elizabeth J EJ Mitchell Andrew J AJ Tysoe Christina R CR Zhu Xuejun X Streit Bennett R BR Rui Zhe Z Zhang Wenjun W Boal Amie K AK Krebs Carsten C Bollinger J Martin JM

Journal of the American Chemical Society 20190910 37

The iron-dependent oxidase UndA cleaves one C3-H bond and the C1-C2 bond of dodecanoic acid to produce 1-undecene and CO<sub>2</sub>. A published X-ray crystal structure showed that UndA has a heme-oxygenase-like fold, thus associating it with a structural superfamily that includes known and postulated non-heme diiron proteins, but revealed only a single iron ion in the active site. Mechanisms proposed for initiation of decarboxylation by cleavage of the C3-H bond using a monoiron cofactor to ac ...[more]

PMID: 31487162

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Project description:An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.

Project description:Aliphatic halogenases activate O(2), cleave alpha-ketoglutarate (alphaKG) to CO(2) and succinate, and form haloferryl [X-Fe(IV)O; X = Cl or Br] complexes that cleave aliphatic C-H bonds to install halogens during the biosynthesis of natural products by non-ribosomal peptide synthetases (NRPSs). For the related alphaKG-dependent dioxygenases, it has been shown that reaction of the Fe(II) cofactor with O(2) to form the C-H bond-cleaving ferryl complex is "triggered" by binding of the target substrate. In this study, we have tested for and defined structural determinants of substrate triggering (ST) in the halogenase, SyrB2, from the syringomycin E biosynthetic NRPS of Pseudomonas syringae B301D. As for other halogenases, the substrate of SyrB2 is complex, consisting of l-Thr tethered via a thioester linkage to a covalently bound phosphopantetheine (PPant) cofactor of a carrier protein, SyrB1. Without an appended amino acid, SyrB1 does not trigger formation of the chloroferryl intermediate state in SyrB2, even in the presence of free l-Thr or its analogues, but SyrB1 charged either by l-Thr (l-Thr-S-SyrB1) or by any of several non-native amino acids does trigger the reaction by as much as 8000-fold (for the native substrate). Triggering efficacy is sensitive to the structures of both the amino acid and the carrier protein, being diminished by 5-24-fold when the native l-Thr is replaced with another amino acid and by approximately 40-fold when SyrB1 is replaced with the heterologous carrier protein, CytC2. The directing effect of the carrier protein and consequent tolerance for profound modifications to the target amino acid allow the chloroferryl state to be formed in the presence of substrates that perturb the ratio of its two putative coordination isomers, lack the target C-H bond (l-Ala-S-SyrB1), or contain a C-H bond of enhanced strength (l-cyclopropylglycyl-S-SyrB1). For the latter two cases, the SyrB2 chloroferryl state so formed exhibits unprecedented stability (t(1/2) = 30-110 min at 0 degree C), can be trapped at high concentration and purity by manual freezing without a cryosolvent, and represents an ideal target for structural characterization. As initial steps toward this goal, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to determine the Fe-O and Fe-Cl distances and density functional theory (DFT) calculations have been used to confirm that the measured distances are consistent with the anticipated structure of the intermediate.

Dataset Information

Substrate-Triggered Formation of a Peroxo-Fe2(III/III) Intermediate during Fatty Acid Decarboxylation by UndA.

Publications

Substrate-Triggered Formation of a Peroxo-Fe<sub>2</sub>(III/III) Intermediate during Fatty Acid Decarboxylation by UndA.

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