Isolation and characterization of two geometric allene oxide isomers synthesized from 9S-hydroperoxylinoleic acid by cytochrome P450 CYP74C3: stereochemical assignment of natural fatty acid allene oxides

Distinct Mechanistic Behaviour of Tomato CYP74C3 and Maize CYP74A19 Allene Oxide Synthases: Insights from Trapping Experiments and Allene Oxide Isolation

The product specificity and mechanistic peculiarities of two allene oxide synthases, tomato LeAOS3 (CYP74C3) and maize ZmAOS (CYP74A19), were studied. Enzymes were vortexed with linoleic acid 9-hydroperoxide in a hexane-water biphasic system (20-60 s, 0 °C). Synthesized allene oxide (9,10-epoxy-10,12-octadecadienoic acid; 9,10-EOD) was trapped with ethanol. Incubations with ZmAOS produced predominantly 9,10-EOD, which was converted into an ethanolysis product, (12Z)-9-ethoxy-10-oxo-12-octadecenoic acid. LeAOS3 produced the same trapping product and 9(R)-α-ketol at nearly equimolar yields. Thus, both α-ketol and 9,10-EOD appeared to be kinetically controlled LeAOS3 products. NMR data for 9,10-EOD (Me) preparations revealed that ZmAOS specifically synthesized 10(E)-9,10-EOD, whereas LeAOS3 produced a roughly 4:1 mixture of 10(E) and 10(Z) isomers. The cyclopentenone cis-10-oxo-11-phytoenoic acid (10-oxo-PEA) and the Favorskii-type product yields were appreciable with LeAOS3, but dramatically lower with ZmAOS. The 9,10-EOD (free acid) kept in hexane transformed into macrolactones but did not cyclize. LeAOS3 catalysis is supposed to produce a higher proportion of oxyallyl diradical (a valence tautomer of allene oxide), which is a direct precursor of both cyclopentenone and cyclopropanone. This may explain the substantial yields of cis-10-oxo-PEA and the Favorskii-type product (via cyclopropanone) with LeAOS3. Furthermore, 10(Z)-9,10-EOD may be produced via the reverse formation of allene oxide from oxyallyl diradical.

Detection of Unprecedented CYP74 Enzyme in Mammal: Hydroperoxide Lyase CYP74C44 of the Bat Sturnira hondurensis

The genome of the neotropical fruit bat Sturnira hondurensis was recently sequenced, revealing an unexpected gene encoding a plant-like protein, CYP74C44, which shares ca. 90% sequence identity with the putative CYP74C of Populus trichocarpa. The preparation and properties of the recombinant CYP74C44 are described in the present work. The CYP74C44 enzyme was found to be active against the 13- and 9-hydroperoxides of linoleic and α-linolenic acids (13-HPOD, 13-HPOT, 9-HPOD, and 9-HPOT, respectively), as well as the 15-hydroperoxide of eicosapentaenoic acid (15-HPEPE). All substrates studied were specifically transformed into chain cleavage products that are typical for hydroperoxide lyases (HPLs). The HPL chain cleavage reaction was validated by the identification of NaBH4-reduced products (Me/TMS) of 15-HPEPE and 13- and 9-hydroperoxides as (all-Z)-14-hydroxy-5,8,11-tetradecatrienoic, (9Z)-12-hydroxy-9-dodecenoic, and 9-hydroxynonanoic acids (Me/TMS), respectively. Thus, CYP74C44 possessed the HPL activity that is typical for the CYP74C subfamily proteins.

On the rearrangements of biologically-relevant vinyl allene oxides to cis-cyclopentenones, ketols, and Favorskii-type carboxylic acids

In addition to stereodefined cis-cyclopentenones, the rearrangement of naturally-occurring vinyl allene oxides can provide ketols, cyclopropylcarbinols, and Favorskii-type bis-(Z)-but-2-en-1-yl acetic acids. These processes have been studied by DFT computations using (Z)-but-1-en-1-yl allene oxides as model systems. Prior studies on the stepwise cascade process starting from (Z)-but-1-en-1-yl allene oxides established as key steps the ring opening of the oxirane to give oxidopentadienyl biradicals, and their isomerization through formation of alkenylcyclopropanone intermediates prior to the conrotatory electrocyclic ring closure to cis-configured cyclopentenones. Under neutral or under acidic conditions, the corresponding ketols and cyclopropylcarbinols have been computationally characterized as resulting from SN2, SN1 and SN1'-type processes, showing that the rearrangement of vinyl allene oxides is pH-dependent. Moreover, stereoconvergent base-induced Favorskii-type rearrangements to provide bis-(Z)-but-1-en-1-yl substituted acetic acids have also been justified. Since the model system captures the structural features of the vinyl allene oxides of biological relevance, our computations provide the most comprehensive overview of the complex reactivity of these natural species.

Analysis of 12/15-lipoxygenase metabolism of EPA and DHA with special attention to authentication of docosatrienes

A proposed beneficial impact of highly unsaturated “fish oil” fatty acids is their conversion by lipoxygenase (LOX) enzymes to specialized proresolving lipid mediators, including 12/15-LOX products from EPA and DHA. The transformations of DHA include formation of docosatrienes, named for the distinctive conjugated triene of the double bonds. To further the understanding of biosynthetic pathways and mechanisms, herein we meld together biosynthesis and NMR characterization of the unstable leukotriene A (LTA)-related epoxide intermediates formed by recombinant human 15-LOX-1, along with identification of the stable enzymatic products, and extend the findings into the 12/15-LOX metabolism in resident murine peritoneal macrophages. Oxygenation of EPA by 15-LOX-1 converts the initial 15S-hydroperoxide to 14S,15S-trans-epoxy-5Z,8Z,10E,12E,17Z-EPA (appearing as its 8,15-diol hydrolysis products) and mixtures of dihydroperoxy fatty acids, while mainly the epoxide hydrolysis products are evident in the murine cells. DHA also undergoes transformations to epoxides and dihydroperoxides by 15-LOX-1, resulting in a mixture of 10,17-dihydro(pero)xy derivatives (docosatrienes) and minor 7S,17S- and 14,17S-dihydroperoxides. The 10,17S-dihydroxy hydrolysis products of the LTA-related epoxide intermediate dominate the product profile in mouse macrophages, whereas (neuro)protectin D1, the leukotriene B₄-related derivative with trans,trans,cis conjugated triene, was undetectable. In this study, we emphasize the utility of UV spectral characteristics for product identification, being diagnostic of the different double bond configurations and hydroxy fatty acid functionality versus hydroperoxide. LC-MS is not definitive for configurational isomers. Secure identification is based on chromatographic retention times, comparison with authentic standards, and the highly distinctive UV spectra.

Biosynthesis of Jasmonates from Linoleic Acid by the Fungus Fusarium oxysporum. Evidence for a Novel Allene Oxide Cyclase

Fusarium oxysporum f. sp. tulipae (FOT) secretes (+)-7-iso-jasmonoyl-(S)-isoleucine ((+)-JA-Ile) to the growth medium together with about 10 times less 9,10-dihydro-(+)-7-iso-JA-Ile. Plants and fungi form (+)-JA-Ile from 18:3n-3 via 12-oxophytodienoic acid (12-OPDA), which is formed sequentially by 13S-lipoxygenase, allene oxide synthase (AOS), and allene oxide cyclase (AOC). Plant AOC does not accept linoleic acid (18:2n-6)-derived allene oxides and dihydrojasmonates are not commonly found in plants. This raises the question whether 18:2n-6 serves as the precursor of 9,10-dihydro-JA-Ile in Fusarium, or whether the latter arises by a putative reductase activity operating on the n-3 double bond of (+)-JA-Ile or one of its precursors. Incubation of pentadeuterated (d₅ ) 18:3n-3 with mycelia led to the formation of d₅ -(+)-JA-Ile whereas d₅ -9,10-dihydro-JA-Ile was not detectable. In contrast, d₅ -9,10-dihydro-(+)-JA-Ile was produced following incubation of [17,17,18,18,18-² H₅ ]linoleic acid (d₅ -18:2n-6). Furthermore, 9(S),13(S)-12-oxophytoenoic acid, the 15,16-dihydro analog of 12-OPDA, was formed upon incubation of unlabeled or d₅ -18:2n-6. Appearance of the α-ketol, 12-oxo-13-hydroxy-9-octadecenoic acid following incubation of unlabeled or [¹³ C₁₈ ]-labeled 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid confirmed the involvement of AOS and the biosynthesis of the allene oxide 12,13(S)-epoxy-9,11-octadecadienoic acid. The lack of conversion of this allene oxide by AOC in higher plants necessitates the conclusion that the fungal AOC is distinct from the corresponding plant enzyme.

Detection of unprecedented allene oxide synthase member of CYP74B subfamily: CYP74B33 of carrot (Daucus carota)

Enzymes of CYP74 family widespread in higher plants control the metabolism of fatty acid hydroperoxides to numerous bioactive oxylipins. Hydroperoxide lyases (HPLs, synonym: hemiacetal synthases) of CYP74B subfamily belong to the most common CYP74 enzymes. HPLs isomerize the hydroperoxides to the short-lived hemiacetals, which are spontaneously decomposed to aldehydes and aldoacids. All CYP74Bs studied yet except the CYP74B16 (flax divinyl ether synthase, LuDES) possessed the 13-HPL activity. Present work reports the cloning of the expressed CYP74B33 gene of carrot (Daucus carota L.) and studies of catalytic properties of the recombinant CYP74B33 protein. In contrast to all CYP74B proteins studied yet, CYP74B33 behaved differently in few respects. Firstly, the preferred substrates of CYP74B33 are 9-hydroperoxides. Secondly and most importantly, CYP74B33 exhibits the 9-allene oxide synthase (AOS) activity. For example, the 9(S)-hydroperoxide of linoleic acid (9-HPOD) underwent the conversion to α-ketol via the short-lived allene oxide. Uncommonly, the 9-HPOD conversion affords a minority of cis-10-oxo-11-phytoenoic acid, which is also produced by CYP74C but not the CYP74A AOSs. The similar product patterns were observed upon the incubations of CYP74B33 with 9(S)-hydroperoxide of α-linolenic acid. The enzyme possessed a mixed HPL, AOS, and the epoxyalcohol synthase activity toward the 13-hydroperoxides, but the total activity was much lower than toward 9-hydroperoxides. Thus, the obtained results show that CYP74B33 is an unprecedented 9-AOS within the CYP74B subfamily.

Rearrangement of vinyl allene oxide geometric isomers to cyclopentenones. Further computational insights with biologically relevant model systems

Pathways for the rearrangement of the E and Z isomers of allyl- and methyl-substituted vinyl allene oxides to stereodefined cyclopentenones have been studied by DFT computations. Regardless of the reactant geometry, cis-configured cyclopentenones are found to be formed in a stepwise cascade comprising as key steps the ring opening of the oxirane to give an oxidopentadienyl diradical, its isomerization, and electrocyclization. An allyl substituent at the C_sp³ atom of the starting vinyl allene oxide induces opposite effects on the activation energies for ring opening: a decrease owing to assistance by homoconjugation for the out motion and an increase due to the stereoelectronic stabilization of the reactant. As a result, allyl- and methyl-substituted vinyl allene oxides exhibit comparable activation energies. Only model systems with crotyl substituents afford lower activation energies than the methyl counterparts due to the additional stabilization of the forming charge deficiency at a secondary carbon by homoconjugation. Moreover, upon homoconjugative interaction reactants of Z geometry are predicted to undergo cyclization more readily than the E isomers. The results with Z-crotyl substituent are congruent with the spontaneous rearrangement of natural vinyl allene oxide derived from α-linolenic acid to a racemic cis-cyclopentenone (12-oxo-PDA).

High Resolution NMR Spectroscopy as a Structural and Analytical Tool for Unsaturated Lipids in Solution

Mono- and polyunsaturated lipids are widely distributed in Nature, and are structurally and functionally a diverse class of molecules with a variety of physicochemical, biological, medicinal and nutritional properties. High resolution NMR spectroscopic techniques including 1H-, 13C- and 31P-NMR have been successfully employed as a structural and analytical tool for unsaturated lipids. The objective of this review article is to provide: (i) an overview of the critical 1H-, 13C- and 31P-NMR parameters for structural and analytical investigations; (ii) an overview of various 1D and 2D NMR techniques that have been used for resonance assignments; (iii) selected analytical and structural studies with emphasis in the identification of major and minor unsaturated fatty acids in complex lipid extracts without the need for the isolation of the individual components; (iv) selected investigations of oxidation products of lipids; (v) applications in the emerging field of lipidomics; (vi) studies of protein-lipid interactions at a molecular level; (vii) practical considerations and (viii) an overview of future developments in the field.

Oxidative Cyclization in Natural Product Biosynthesis

Oxidative cyclizations are important transformations that occur widely during natural product biosynthesis. The transformations from acyclic precursors to cyclized products can afford morphed scaffolds, structural rigidity, and biological activities. Some of the most dramatic structural alterations in natural product biosynthesis occur through oxidative cyclization. In this Review, we examine the different strategies used by nature to create new intra(inter)molecular bonds via redox chemistry. This Review will cover both oxidation- and reduction-enabled cyclization mechanisms, with an emphasis on the former. Radical cyclizations catalyzed by P450, nonheme iron, α-KG-dependent oxygenases, and radical SAM enzymes are discussed to illustrate the use of molecular oxygen and S-adenosylmethionine to forge new bonds at unactivated sites via one-electron manifolds. Nonradical cyclizations catalyzed by flavin-dependent monooxygenases and NAD(P)H-dependent reductases are covered to show the use of two-electron manifolds in initiating cyclization reactions. The oxidative installations of epoxides and halogens into acyclic scaffolds to drive subsequent cyclizations are separately discussed as examples of "disappearing" reactive handles. Last, oxidative rearrangement of rings systems, including contractions and expansions, will be covered.

Stable bromoallene oxides

The first stable bromoallene oxides were obtained by the DMDO epoxidation of 1-bromo-1,3-di-tert-alkylallenes, producing the first crystalline allene oxide of any kind. The epoxidations are regioselective for the bromine-bearing Δ^1,2 alkene, and also face selective producing single diastereomer E-olefin products.

A new class of fatty acid allene oxide formed by the DOX-P450 fusion proteins of human and plant pathogenic fungi, C. immitis and Z. tritici

Linoleate dioxygenase-cytochrome P450 (DOX-CYP) fusion enzymes are common in pathogenic fungi. The DOX domains form hydroperoxy metabolites of 18:2n-6, which can be transformed by the CYP domains to 1,2- or 1,4-diols, epoxy alcohols, or to allene oxides. We have characterized two novel allene oxide synthases (AOSs), namely, recombinant 8R-DOX-AOS of Coccidioides immitis (causing valley fever) and 8S-DOX-AOS of Zymoseptoria tritici (causing septoria tritici blotch of wheat). The 8R-DOX-AOS oxidized 18:2n-6 sequentially to 8R-hydroperoxy-9Z,12Z-octadecadienoic acid (8R-HPODE) and to an allene oxide, 8R(9)-epoxy-9,12Z-octadecadienoic acid, as judged from the accumulation of the α-ketol, 8S-hydroxy-9-oxo-12Z-octadecenoic acid. The 8S-DOX-AOS of Z. tritici transformed 18:2n-6 sequentially to 8S-HPODE and to an α-ketol, 8R-hydroxy-9-oxo-12Z-octadecenoic acid, likely formed by hydrolysis of 8S(9)-epoxy-9,12Z-octadecadienoic acid. The 8S-DOX-AOS oxidized [8R-(2)H]18:2n-6 to 8S-HPODE with retention of the (2)H-label, suggesting suprafacial hydrogen abstraction and oxygenation in contrast to 8R-DOX-AOS. Both enzymes oxidized 18:1n-9 and 18:3n-3 to α-ketols, but the catalysis of the 8R- and 8S-AOS domains differed. 8R-DOX-AOS transformed 9R-HPODE to epoxy alcohols, but 8S-DOX-AOS converted 9S-HPODE to an α-ketol (9-hydroxy-10-oxo-12Z-octadecenoic acid) and epoxy alcohols in a ratio of ∼1:2. Whereas all fatty acid allene oxides described so far have a conjugated diene impinging on the epoxide, the allene oxides formed by 8-DOX-AOS are unconjugated.

Investigation into 9(S)-HPODE-derived allene oxide to cyclopentenone cyclization mechanism via diradical oxyallyl intermediates

The cyclopentane core is ubiquitous among a large number of biologically relevant natural products. Cyclopentenones have been shown to be versatile intermediates for the stereoselective preparation of highly substituted cyclopentane derivatives. Allene oxides are oxygenated fatty acids which are involved in the pathways of cyclopentenone biosynthesis in plants and marine invertebrates; however, their cyclization behavior is not well understood. Recent work by Brash and co-workers (J. Biol. Chem., 2013, 288, 20797) revealed an unusual cyclization property of the 9(S)-HPODE-derived allene oxides: the previously unreported 10Z-isomer cyclizes to a cis-dialkylcyclopentenone in hexane/isopropyl alcohol (100 : 3, v/v), but the known 10E-isomer does not yield cis-cyclopentenone under the same conditions. The mechanism for cyclization has been investigated for unsubstituted and methyl substituted vinyl allene oxide using a variety of methods including CASSCF, ωB97xD, and CCSD(T) and basis sets up to cc-pVTZ. The lowest energy pathway proceeds via homolytic cleavage of the epoxide ring, formation of an oxyallyl diradical, which closes readily to a cyclopropanone intermediate. The cyclopropanone opens to the requisite oxyallyl which closes to the experimentally observed product, cis-cyclopentenone. The calculations show that the open shell, diradical pathway is lower in energy than the closed shell reactions of allene oxide to cyclopropanone, and cyclopropanone to cyclopentenone.

A unifying mechanism for the rearrangement of vinyl allene oxide geometric isomers to cyclopentenones

Z-Vinyl allene oxides are predicted to rearrange with high fidelity to stereodefined cyclopentenones through intermediate cyclopropanones.

Discovery of a linoleate 9S-dioxygenase and an allene oxide synthase in a fusion protein of Fusarium oxysporum

Fusarium oxysporum is a devastating plant pathogen that oxidizes C₁₈ fatty acids sequentially to jasmonates. The genome codes for putative dioxygenase (DOX)-cytochrome P450 (CYP) fusion proteins homologous to linoleate diol synthases (LDSs) and the allene oxide synthase (AOS) of Aspergillus terreus, e.g., FOXB_01332. Recombinant FOXB_01332 oxidized 18:2n-6 to 9S-hydroperoxy-10(E),12(Z)-octadecadienoic acid by hydrogen abstraction and antarafacial insertion of molecular oxygen and sequentially to an allene oxide, 9S(10)-epoxy-10,12(Z)-octadecadienoic acid, as judged from nonenzymatic hydrolysis products (α- and γ-ketols). The enzyme was therefore designated 9S-DOX-AOS. The 9S-DOX activity oxidized C₁₈ and C₂₀ fatty acids of the n-6 and n-3 series to hydroperoxides at the n-9 and n-7 positions, and the n-9 hydroperoxides could be sequentially transformed to allene oxides with only a few exceptions. The AOS activity was stereospecific for 9- and 11-hydroperoxides with S configurations. FOXB_01332 has acidic and alcoholic residues, Glu⁹⁴⁶-Val-Leu-Ser⁹⁴⁹, at positions of crucial Asn and Gln residues (Asn-Xaa-Xaa-Gln) of the AOS and LDS. Site-directed mutagenesis studies revealed that FOXB_01332 and AOS of A. terreus differ in catalytically important residues suggesting that AOS of A. terreus and F. oxysporum belong to different subfamilies. FOXB_01332 is the first linoleate 9-DOX with homology to animal heme peroxidases and the first 9-DOX-AOS fusion protein.

Thermal conversions of fatty acid peroxides to cyclopentenones: a biomimetic model for allene oxide synthase pathway

The trimethylsilyl (TMS) peroxides of linoleic acid 9(S)-hydroperoxide (TMS or Me esters) were subjected to gas chromatography-mass spectrometry (GC-MS) analyses. The cyclopentenones, trans- and cis-10-oxo-11-phytoenoic acid (10-oxo-PEA, Me or TMS esters) were first time detected as the products of TMS-peroxide thermal conversions. The major products were ketodienes, epoxyalcohols, hemiacetals and decadienals. For further study of thermal cyclopentenone formation, 9(S)- or 13(S)-hydroperoxides of linoleic acid (Me esters) were sealed in ampoules and heated at 230 °C for 15 or 30 min. The products were separated by HPLC. The cyclopentenone fractions were collected and analyzed by GC-MS. Trans-10-oxo-PEA (Me) and 10-oxo-9(13)-PEA (Me) were formed during the thermal conversion of 9-hydroperoxide (Me ester). Similarly, the cyclopentenones trans-12-oxo-PEA (Me) and 12-oxo-9(13)-PEA (Me) were detected after the heating of 13-hydroperoxide (Me ester). Thermal formation of cyclopentenones can be considered as a biomimetic model of AOS pathway, providing new insights into the mechanisms of allene oxide formation and cyclization.