A Catalase-related Hemoprotein in Coral Is Specialized for Synthesis of Short-chain Aldehydes: DISCOVERY OF P450-TYPE HYDROPEROXIDE LYASE ACTIVITY IN A CATALASE

Oxylipin biosynthesis via an unprecedented 16-hydroperoxide lyase pathway in green tissues of cucumber (Cucumis sativus L.) plants

The plant lipoxygenase cascade is a source of various regulatory oxylipins that play a role in cell signalling, stress adaptation, and immune response. Recently, we detected an unprecedented 16(S)-lipoxygenase, CsLOX3, in the leaves and fruit pericarp of cucumber (Cucumis sativus L.). In the present work, an array of products biosynthesized through the conversions of α-linolenic acid 16-hydroperoxide (16-HPOT) was detected. Firstly, a prominent 15-hydroxy-9,12-pentadecadienoic acid (Me/TMS) was detected, the product of hydroperoxide lyase (HPL) chain cleavage of 16-HPOT and further reduction of aldehyde 15-oxo-9,12-pentadecadienoic acid to alcohol. Besides, the presence of dicarboxylic acid, 3,6-pentadecadiene-1,15-dioic acid, was deduced from the detection of its catalytic hydrogenation product, pentadecane-1,15-dioic acid. Finally, 12,15-dihydroxypentadecanoic acid (Me/TMS) was detected amongst the hydrogenated products, thus indicating the presence of the parent 12,15-dihydroxy-9,13-pentadecadienoic acid. To confirm the proposed HPL chain cleavage, the 16(S)-HPOT was prepared and incubated with the recombinant cucumber HPL CYP74B6 enzyme. The CYP74B6 possessed high activity towards 16-HPOT. Chain cleavage yields the (9Z,12Z)-15-oxo-9,12-pentadecadienoic acid, undergoing a spontaneous isomerization into (9Z,13E)-15-oxo-9,13-pentadecadienoic acid. Thus, the cucumber plants as well as the recombinant cucumber HPL CYP74B6 possessed unprecedented 16-HPL activity, cleaving 16-HPOT into a C15 fragment, 15-oxo-9,12-pentadecadienoic acid, and a complementary volatile C3 fragment, propionic aldehyde. The 16-LOX/16-HPL route of oxylipin biosynthesis presents a novel facet of the plant LOX pathway.

Epoxyalcohol Synthase Branch of Lipoxygenase Cascade

Oxylipins are one of the most important classes of bioregulators, biosynthesized through the oxidative metabolism of unsaturated fatty acids in various aerobic organisms. Oxylipins are bioregulators that maintain homeostasis at the cellular and organismal levels. The most important oxylipins are mammalian eicosanoids and plant octadecanoids. In plants, the main source of oxylipins is the lipoxygenase cascade, the key enzymes of which are nonclassical cytochromes P450 of the CYP74 family, namely allene oxide synthases (AOSs), hydroperoxide lyases (HPLs), and divinyl ether synthases (DESs). The most well-studied plant oxylipins are jasmonates (AOS products) and traumatin and green leaf volatiles (HPL products), whereas other oxylipins remain outside of the focus of researchers' attention. Among them, there is a large group of epoxy hydroxy fatty acids (epoxyalcohols), whose biosynthesis has remained unclear for a long time. In 2008, the first epoxyalcohol synthase of lancelet Branchiostoma floridae, BfEAS (CYP440A1), was discovered. The present review collects data on EASs discovered after BfEAS and enzymes exhibiting EAS activity along with other catalytic activities. This review also presents the results of a study on the evolutionary processes possibly occurring within the P450 superfamily as a whole.

The Biosynthesis of 1-octene-3-ol by a Multifunctional Fatty Acid Dioxygenase and Hydroperoxide Lyase in Agaricus bisporus

The biosynthetic pathway from linoleic acid to 1-octen-3-ol in Agaricus bisporus has long been established, in which linoleic acid is converted to 10-hydroperoxide (10-HPOD) by deoxygenation, and 10-HPOD is subsequently cleaved to yield 1-octene-3-ol and 10-oxodecanoic acid. However, the corresponding enzymes have not been identified and cloned. In the present study, four putative genes involved in oxylipid biosynthesis, including one lipoxygenase gene named AbLOX, two linoleate diol synthase genes named AbLDS1 and AbLDS2, and one hydroperoxide lyase gene named AbHPL were retrieved from the A. bisporus genome by a homology search and cloned and expressed prokaryotically. AbLOX, AbLDS1, and AbLDS2 all exhibited fatty acid dioxygenase activity, catalyzing the conversion of linoleic acid to generate hydroperoxide, and AbHPL showed a cleaving hydroperoxide activity, as was determined by the KI-starch method. AbLOX and AbHPL catalyzed linoleic acid to 1-octen-3-ol with an optimum temperature of 35 °C and an optimum pH of 7.2, whereas AbLDS1, AbLDS2, and AbHPL catalyzed linoleic acid without 1-octen-3-ol. Reduced AbLOX expression in antisense AbLOX transformants was correlated with a decrease in the yield of 1-octen-3-ol. AbLOX and AbHPL were highly homologous to the sesquiterpene synthase Cop4 of Coprinus cinerea and the yeast sterol C-22 desaturase, respectively. These results reveal that the enzymes for the oxidative cleavage of linoleic acid to synthesize 1-octen-3-ol in A. bisporus are the multifunctional fatty acid dioxygenase AbLOX and hydroperoxide lyase AbHPL.

Hydroperoxide bicyclase CYP50918A1 of Plasmodiophora brassicae (Rhizaria, SAR): Detection of novel enzyme of oxylipin biosynthesis

The genome of the cabbage clubroot pathogen Plasmodiophora brassicae Woronin 1877 (Cercozoa, Rhizaria, SAR), possesses two expressed genes encoding the P450s that are phylogenetically related to the enzymes of oxylipin biosynthesis of the CYP74 clan. The cDNA of one of these genes (CYP50918A1) has been expressed in E. coli. The preferred substrate for the recombinant protein, the 13-hydroperoxide of α-linolenic acid (13-HPOT), was converted to the novel heterobicyclic oxylipins, plasmodiophorols A and B (1 and 2) at the ratio ca. 12:1. Compounds 1 and 2 were identified as the substituted 6-oxabicyclo[3.1.0]hexane and 2-oxabicyclo[2.2.1]heptane (respectively) using the MS and NMR spectroscopy, as well as the chemical treatments. The ¹⁸O labelling experiments revealed the incorporation of a single ¹⁸O atom from [¹⁸O₂]13-HPOT into the epoxide and ether functions of products 1 and 2 (respectively), but not into their OH groups. In contrast, the ¹⁸O from [¹⁸O₂]water was incorporated only into the hydroxyl functions. One more minor polar product, plasmodiophorol C (3), identified as the cyclopentanediol, was formed through the hydrolysis of compounds 1 and 2. Plasmodiophorols A-C are the congeners of egregiachlorides, hybridalactone, ecklonialactones and related bicyclic oxylipins detected before in some brown and red algae. The mechanism of 13-HPOT conversions to plasmodiophorols A and B involving the epoxyallylic cation intermediate is proposed. The hydroperoxide bicyclase CYP50918A1 is the first enzyme controlling this kind of fatty acid hydroperoxide conversion.

Examining the Effect of Heat Stress on Montastraea cavernosa (Linnaeus 1767) from a Mesophotic Coral Ecosystem (MCE)

Coral reefs are under increasing pressure from global warming. Little knowledge, however, exists regarding heat induced stress on deeper mesophotic coral ecosystems (MCEs). Here, we examined the effect of acute (72 h) and chronic (480 h) heat stress on the host coral Montastraea cavernosa (Linnaeus 1767) collected from an upper MCE (~30 m) in Florida, USA. We examined six immune/stress-related genes: ribosomal protein L9 (RpL9), ribosomal protein S7 (RpS7), B-cell lymphoma 2 apoptosis regulator (BCL-2), heat shock protein 90 (HSP90), catalase, and cathepsin L1, as a proxy for coral response to heat stress. Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to evaluate the gene expression. Overall, both acute and chronic heat stress treatments elicited a response in gene expression relative to control samples. Acute heat exposure resulted in up-regulation of catalase, BCL-2, and HSP90 at all time points from hour 24 to 48, suggesting the activation of an oxidative protective enzyme, molecular chaperone, and anti-apoptotic protein. Fewer genes were up-regulated in the chronic experiment until hour 288 (30 °C) where catalase, RpL9, and RpS7 were significantly up-regulated. Chronic heat exposure elicited a physiological response at 30 °C, which we propose as a heat-stress threshold for Montastraea cavernosa (M. cavernosa) collected from an MCE.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

Distinct characteristics of the substrate binding between highly homologous catalase-related allene oxide synthase and hydroperoxide lyase

A catalase-related allene oxide synthase (cAOS) or a hydroperoxide lyase (cHPL) fused together with an 8R-lipoxygenase is involved in the stress signaling of corals via an arachidonic acid pathway. cAOS gives rise to α-ketol and cyclopentenone, while cHPL catalyzes the cleavage of 8R-hydroperoxyeicosatetraenoic acid (8R-HpETE) to C8-oxo acid and C12 aldehyde. In silico analysis of the substrate entry sites of highly identical coral cAOS and cHPL indicated that two positively charged residues of cAOS, K60 and K107, and the corresponding residues of cHPL, E60 and K107, may be involved in the anchoring of the carboxy group of polyunsaturated fatty acid (PUFA) hydroperoxides. A mutational analysis of cAOS and cHPL revealed that K60 or E60 and K107 were not necessary in the tethering of 8R-HpETE, however, the E60 of cHPL was essential in the productive binding of PUFA hydroperoxides. The substrate preferences of cAOS and cHPL were determined with hydroperoxy derivatives of C18, C20, C22 PUFAs, anandamide (AEA), 1-arachidonoyl glycerol (1-AG) and selected methylated substrates. Although cAOS and cHPL were able to metabolize different free PUFA substrates and arachidonoyl derivatives, only cHPL catalyzed the reaction with methylated PUFA hydroperoxides. The differences in the substrate binding and preferences between cAOS and cHPL can be explained by the distinct properties of their substrate entry sites. The current study demonstrated that homologous PUFA metabolizing enzymes may contribute to the versatile usage of the substrate pool.

The CYP443C1 (CYP74 clan) Cytochrome of Sea Anemone Nematostella vectensis-the First Metazoan Enzyme Possessing Hydroperoxide Lyase/Epoxyalcohol Synthase Activity

A novel CYP74 clan gene CYP443С1 of the starlet sea anemone (Nematostella vectensis, Cnidaria) has been cloned, and the properties of the corresponding recombinant protein have been studied. Depending on the substrate, CYP443С1 exhibited double function hydroperoxide lyase/epoxyalcohol synthase activity.

ALLENE OXIDE SYNTHASE and HYDROPEROXIDE LYASE, Two Non-Canonical Cytochrome P450s in Arabidopsis thaliana and Their Different Roles in Plant Defense

The channeling of metabolites is an essential step of metabolic regulation in all living organisms. Multifunctional enzymes with defined domains for metabolite compartmentalization are rare, but in many cases, larger assemblies forming multimeric protein complexes operate in defined metabolic shunts. In Arabidopsis thaliana, a multimeric complex was discovered that contains a 13-lipoxygenase and allene oxide synthase (AOS) as well as allene oxide cyclase. All three plant enzymes are localized in chloroplasts, contributing to the biosynthesis of jasmonic acid (JA). JA and its derivatives act as ubiquitous plant defense regulators in responses to both biotic and abiotic stresses. AOS belongs to the superfamily of cytochrome P450 enzymes and is named CYP74A. Another CYP450 in chloroplasts, hydroperoxide lyase (HPL, CYP74B), competes with AOS for the common substrate. The products of the HPL reaction are green leaf volatiles that are involved in the deterrence of insect pests. Both enzymes represent non-canonical CYP450 family members, as they do not depend on O₂ and NADPH-dependent CYP450 reductase activities. AOS and HPL activities are crucial for plants to respond to different biotic foes. In this mini-review, we aim to summarize how plants make use of the LOX2–AOS–AOC2 complex in chloroplasts to boost JA biosynthesis over volatile production and how this situation may change in plant communities during mass ingestion by insect pests.

Plant hydroperoxide-cleaving enzymes (CYP74 family) function as hemiacetal synthases: Structural proof of hemiacetals by NMR spectroscopy

Hydroperoxide lyases (HPLs) of the CYP74 family (P450 superfamily) are widely distributed enzymes in higher plants and are responsible for the stress-initiated accumulation of short-chain aldehydes. Fatty acid hydroperoxides serve as substrates for HPLs; however, details of the HPL-promoted conversion are still incompletely understood. In the present work, we report first time the micropreparative isolation and the NMR structural studies of fatty acid hemiacetal (TMS/TMS), the short-lived HPL product. With this aim, linoleic acid 9(S)‑hydroperoxide (9(S)‑HPOD) was incubated with recombinant melon hydroperoxide lyase (CmHPL, CYP74C2) in a biphasic system of water/hexane for 60 s at 0 °C, pH 4.0. The hexane layer was immediately decanted and vortexed with a trimethylsilylating mixture. Analysis by GC-MS revealed a major product, i.e. the bis-TMS derivative of a hemiacetal which was conclusively identified as 9‑hydroxy‑9‑[(1'E,3'Z)‑nonadienyloxy]‑nonanoic acid by NMR-spectroscopy. Further support for the hemiacetal structure was provided by detailed NMR-spectroscopic analysis of the bis-TMS hemiacetal generated from [¹³C₁₈]9(S)‑HPOD in the presence of CmHPL. The results obtained provide incontrovertible evidence that the true products of the HPL group of enzymes are hemiacetals, and that the short-chain aldehydes are produced by their rapid secondary chain breakdown. Therefore, we suggest replacing the name "hydroperoxide lyase", which does not reflect the factual isomerase (intramolecular oxidoreductase) activity, with "hemiacetal synthase" (HAS).

Catalase-Related Allene Oxide Synthase, on a Biosynthetic Route to Fatty Acid Cyclopentenones: Expression and Assay of the Enzyme and Preparation of the 8R-HPETE Substrate

Catalase-related allene oxide synthase (cAOS) is a hemoprotein that converts a specific fatty acid hydroperoxide to an unstable allene oxide intermediate at turnover rates in the order of 1000 per second. Fatty acid allene oxides are intermediates in the formation of cyclopentenone or hydrolytic products in marine systems, most notably the prostanoid-related clavulones. Although the key catalytic amino acid residues around the active site of cAOS are the same as in true catalases, cAOS does not react with hydrogen peroxide. cAOS occurs exclusively as the N-terminal domain of a naturally occurring fusion protein with a C-terminal lipoxygenase (LOX) domain that supplies the hydroperoxide substrate. In marine invertebrates, an 8R-LOX domain converts arachidonic acid to 8R-hydroperoxyeicosatetraenoic acid (8R-HPETE) and the cAOS domain forms an 8,9-epoxy allene oxide. The fusion protein from the sea whip octocoral Plexaura homomalla is the prototypical model with crystal structures of the individual domains. The cAOS (43kDa) expresses exceptionally well in Escherichia coli, with yields of up to 100mg/L. This article describes in detail expression and assay of the P. homomalla cAOS and two methods for the preparation of its 8R-HPETE substrate. Another article in this volume focuses on the P. homomalla 8R-LOX (Gilbert, Neau, & Newcomer, 2018).

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Eicosanoid Diversity of Stony Corals

Oxylipins are well-established lipid mediators in plants and animals. In mammals, arachidonic acid (AA)-derived eicosanoids control inflammation, fever, blood coagulation, pain perception and labor, and, accordingly, are used as drugs, while lipoxygenases (LOX), as well as cyclooxygenases (COX) serve as therapeutic targets for drug development. In soft corals, eicosanoids are synthesized on demand from AA by LOX, COX, and catalase-related allene oxide synthase-lipoxygenase (cAOS-LOX) and hydroperoxide lyase-lipoxygenase (cHPL-LOX) fusion proteins. Reef-building stony corals are used as model organisms for the stress-related genomic studies of corals. Yet, the eicosanoid synthesis capability and AA-derived lipid mediator profiles of stony corals have not been determined. In the current study, the genomic and transcriptomic data about stony coral LOXs, AOS-LOXs, and COXs were analyzed and the eicosanoid profiles and AA metabolites of three stony corals, Acropora millepora, A. cervicornis, and Galaxea fascicularis, were determined by reverse-phase high-performance liquid chromatography (RP-HPLC) coupled with MS-MS and a radiometric detector. Our results confirm that the active LOX and AOS-LOX pathways are present in Acropora sp., which correspond to the genomic/sequence data reported earlier. In addition, LOX, AOS-LOX, and COX products were detected in the closely related species G. fascicularis. In conclusion, the functional 8R-LOX and/or AOS-LOX pathways are abundant among corals, while COXs are restricted to certain soft and stony coral lineages.

Structural and functional insights into the reaction specificity of catalase-related hydroperoxide lyase: A shift from lyase activity to allene oxide synthase by site-directed mutagenesis

Two highly identical fusion proteins, an allene oxide synthase-lipoxygenase (AOS-LOX) and a hydroperoxide lyase-lipoxygenase (HPL-LOX), were identified in the soft coral Capnella imbricata. Both enzymes initially catalyze the formation of 8R-hydroperoxy-eicosatetraenoic acid (8R-HpETE) from arachidonic acid by the C-terminal lipoxygenase (LOX) domain. Despite the fact that the defined catalytically important residues of N-terminal catalase-related allene oxide synthase (cAOS) domain are also conserved in C. imbricata hydroperoxide lyase (cHPL), their reaction specificities differ. In the present study, we tested which of the amino acid substitutions around the active site of cHPL are responsible for a control in the reaction specificity. The possible candidates were determined via comparative sequence and structural analysis of the substrate channel and the heme region of coral cAOSs and C. imbricata cHPL. The amino acid replacements in cHPL—R56G, ME59-60LK, P65A, F150L, YS176-177NL, I357V, and SSSAGE155-160PVKEGD—with the corresponding residues of cAOS were conducted by site-directed mutagenesis. Although all these mutations influenced the catalytic efficiency of cHPL, only F150L and YS176-177NL substitutions caused a shift in the reaction specificity from HPL to AOS. The docking analysis of P. homomalla cAOS with 8R-HpETE substrate revealed that the Leu150 of cAOS interacts with the C5-C6 double bond and the Leu177 with the hydrophobic tail of 8R-HpETE. We propose that the corresponding residues in cHPL, Phe150 and Ser177, are involved in a proper coordination of the epoxy allylic radical intermediate necessary for aldehyde formation in the hydroperoxide lyase reaction.

Identification of CYP443D1 (CYP74 clan) of Nematostella vectensis as a first cnidarian epoxyalcohol synthase and insights into its catalytic mechanism

The CYP74 clan enzymes are responsible for the biosynthesis of numerous bioactive oxylipins in higher plants, some Proteobacteria, brown and green algae, and Metazoa. A novel putative CYP74 clan gene CYP443D1 of the starlet sea anemone (Nematostella vectensis, Cnidaria) has been cloned, and the properties of the corresponding recombinant protein have been studied in the present work. The recombinant CYP443D1 was incubated with the 9- and 13-hydroperoxides of linoleic and α-linolenic acids (9-HPOD, 13-HPOD, 9-HPOT, and 13-HPOT, respectively), as well as with the 9-hydroperoxide of γ-linolenic acid (γ-9-HPOT) and 15-hydroperoxide of eicosapentaenoic acid (15-HPEPE). The enzyme was active towards all C₁₈-hydroperoxides with some preference to 9-HPOD. In contrast, 15-HPEPE was a poor substrate. The CYP443D1 specifically converted 9-HPOD into the oxiranyl carbinol 1, (9S,10R,11S,12Z)-9,10-epoxy-11-hydroxy-12-octadecenoic acid. Both ¹⁸O atoms from [¹⁸O₂-hydroperoxy]9-HPOD were virtually quantitatively incorporated into product 1. Thus, the CYP443D1 exhibited epoxyalcohol synthase (EAS) activity. The ¹⁸O labelling data demonstrated that the reaction mechanism included three sequential steps: (1) hydroperoxyl homolysis, (2) oxy radical rearrangement into epoxyallylic radical, (3) hydroxyl rebound, resulting in oxiranyl carbinol formation. The 9-HPOT and γ-9-HPOT were also specifically converted into the oxiranyl carbinols, 15,16- and 6,7-dehydro analogues of compound 1, respectively. The 13-HPOD was converted into erythro- and threo-isomers of oxiranyl carbinol, as well as oxiranyl vinyl carbinols. The obtained results allow assignment of the name "N. vectensis EAS" (NvEAS) to CYP443D1. The NvEAS is a first EAS detected in Cnidaria.

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.

A fungal catalase reacts selectively with the 13S fatty acid hydroperoxide products of the adjacent lipoxygenase gene and exhibits 13S-hydroperoxide-dependent peroxidase activity

The genome of the fungal plant pathogen Fusarium graminearum harbors six catalases, one of which has the sequence characteristics of a fatty acid peroxide-metabolizing catalase. We cloned and expressed this hemoprotein (designated as Fg-cat) along with its immediate neighbor, a 13S-lipoxygenase (cf. Brodhun et al., PloS One, e64919, 2013) that we considered might supply a fatty acid hydroperoxide substrate. Indeed, Fg-cat reacts abruptly with the 13S-hydroperoxide of linoleic acid (13S-HPODE) with an initial rate of 700-1300s^-1. By comparison there was no reaction with 9R- or 9S-HPODEs and extremely weak reaction with 13R-HPODE (~0.5% of the rate with 13S-HPODE). Although we considered Fg-cat as a candidate for the allene oxide synthase of the jasmonate pathway in fungi, the main product formed from 13S-HPODE was identified by UV, MS, and NMR as 9-oxo-10E-12,13-cis-epoxy-octadecenoic acid (with no traces of AOS activity). The corresponding analog is formed from the 13S-hydroperoxide of α-linolenic acid along with novel diepoxy-ketones and two C13 aldehyde derivatives, the reaction mechanisms of which are proposed. In a peroxidase assay monitoring the oxidation of ABTS, Fg-cat exhibited robust activity (kcat 550s^-1) using the 13S-hydroperoxy-C₁₈ fatty acids as the oxidizing co-substrate. There was no detectable peroxidase activity using the corresponding 9S-hydroperoxides, nor with t-butyl hydroperoxide, and very weak activity with H₂O₂ or cumene hydroperoxide at micromolar concentrations of Fg-cat. Fg-cat and the associated lipoxygenase gene are present together in fungal genera Fusarium, Metarhizium and Fonsecaea and appear to constitute a partnership for oxidations in fungal metabolism or defense.

n-Hexanal and (Z)-3-hexenal are generated from arachidonic acid and linolenic acid by a lipoxygenase in Marchantia polymorpha L

Most terrestrial plants form green leaf volatiles (GLVs), which are mainly composed of six-carbon (C6) compounds. In our effort to study the distribution of the ability of lipoxygenase (LOX) to form GLVs, we found that a liverwort, Marchantia polymorpha, formed n-hexanal and (Z)-3-hexenal. Some LOXs execute a secondary reaction to form short chain volatiles. One of the LOXs from M. polymorpha (MpLOX7) oxygenized arachidonic and α-linolenic acids at almost equivalent efficiency and formed C6-aldehydes during its catalysis; these are likely formed from hydroperoxides of arachidonic and α-linolenic acids, with a cleavage of the bond between carbon at the base of the hydroperoxy group and carbon of double bond, which is energetically unfavorable. These lines of evidence suggest that one of the LOXs in liverwort employs an unprecedented reaction to form C6 aldehydes as by-products of its reaction with fatty acid substrates.

The Thr-His Connection on the Distal Heme of Catalase-Related Hemoproteins: A Hallmark of Reaction with Fatty Acid Hydroperoxides

This review focuses on a group of heme peroxidases that retain the catalase fold in structure, yet show little or no reaction with hydrogen peroxide. Instead of having a role in oxidative defense, these enzymes are involved in secondary metabolite biosynthesis. The prototypical enzyme is catalase-related allene oxide synthase, an enzyme that converts a specific fatty acid hydroperoxide to the corresponding allene oxide (epoxide). Other catalase-related enzymes form allylic epoxides, aldehydes, or a bicyclobutane fatty acid. In all catalases (including these relatives), a His residue on the distal face of the heme is absolutely required for activity. Its immediate neighbor in sequence as well as in 3 D space is conserved as Val in true catalases and Thr in the fatty acid hydroperoxide-metabolizing enzymes. Thr-His on the distal face of the heme is critical in switching the substrate specificity from H2 O2 to fatty acid hydroperoxide.