Conversion of cucumber linoleate 13-lipoxygenase to a 9-lipoxygenating species by site-directed mutagenesis

The Biocontrol Functions of Bacillus velezensis Strain Bv-25 Against Meloidogyne incognita

Meloidogyne incognita is obligate parasitic nematode with a wide variety of hosts that causes huge economic losses every year. In an effort to identify novel bacterial biocontrols against M. incognita, the nematicidal activity of Bacillus velezensis strain Bv-25 obtained from cucumber rhizosphere soil was measured. Strain Bv-25 could inhibit the egg hatching of M. incognita and had strong nematicidal activity, with the mortality rate of second-stage M. incognita juveniles (J2s) at 100% within 12 h of exposure to Bv-25 fermentation broth. The M. incognita genes ord-1, mpk-1, and flp-18 were suppressed by Bv-25 fumigation treatment after 48 h. Strain Bv-25 could colonize cucumber roots, with 5.94 × 107 colony-forming units/g attached within 24 h, effectively reducing the infection rate with J2s by 98.6%. The bacteria up-regulated the expression levels of cucumber defense response genes pr1, pr3, and lox1 and induced resistance to M. incognita in split-root trials. Potted trials showed that Bv-25 reduced cucumber root knots by 73.8%. The field experiment demonstrated that disease index was reduced by 61.6%, cucumber height increased by 14.4%, and yield increased by 36.5% in Bv-25-treated plants compared with control. To summarize, B. velezensis strain Bv-25 strain has good potential to control root-knot nematodes both when colonizing the plant roots and through its volatile compounds.

Recombinant Porcine 12-Lipoxygenase Catalytic Domain: Effect of Inhibitors, Selectivity of Substrates and Specificity of Oxidation Products of Linoleic Acid

Lipoxygenase (LOX) is a major endogenous enzyme for the enzymatic oxidation of lipids during meat storage and meat product manufacturing. In the present work, some characteristics, i.e., effects of inhibitors, selectivity of substrates and specificity of oxidation products, were studied using recombinant porcine 12-lipoxygenase catalytic domain (12-LOXcd). Several familiar inhibitors were found inhibit the activity of recombinant porcine 12-LOXcd;nordihydroguaiaretic acid demonstrated the strongest inhibitory effect. The enzyme could oxygenate common polyunsaturated fatty acids, and showed the highest affinity to linoleic acid (LA), followed by arachidonic acid (AA), linolenic acid (LN) and docosahexaenoic acid (DHA). Under the action of porcine 12-LOXcd, LA was oxidized into four hydroxyoctadecadienoic acid (HODE) isomers, i.e., 13-Z,E-HODE, 13-E,E-HODE, 9-Z,E-HODE and 9-E,E-HODE. Variation of pH not only affected the yield of LA oxidation products, but also the distribution of HODE isomers. These results indicated that endogenous LOX activity and LOX-catalyzed lipid oxidation can be regulated during meat storage and meat product manufacturing.

Iron and manganese lipoxygenases of plant pathogenic fungi and their role in biosynthesis of jasmonates

Lipoxygenases (LOX) contain catalytic iron (FeLOX), but fungi also produce LOX with catalytic manganese (MnLOX). In this review, the 3D structures and properties of fungal LOX are compared and contrasted along with their associations with pathogenicity. The 3D structures and properties of two MnLOX (Magnaporthe oryzae, Geaumannomyces graminis) and the catalysis of five additional MnLOX have provided information on the metal center, substrate binding, oxygenation, tentative O₂ channels, and biosynthesis of exclusive hydroperoxides. In addition, the genomes of other plant pathogens also code for putative MnLOX. Crystals of the 13S-FeLOX of Fusarium graminearum revealed an unusual altered geometry of the Fe ligands between mono- and dimeric structures, influenced by a wrapping sequence extension near the C-terminal of the dimers. In plants, the enzymes involved in jasmonate synthesis are well documented whereas the fungal pathway is yet to be fully elucidated. Conversion of deuterium-labeled 18:3n-3, 18:2n-6, and their 13S-hydroperoxides to jasmonates established 13S-FeLOX of F. oxysporum in the biosynthesis, while subsequent enzymes lacked sequence homologues in plants. The Rice-blast (M. oryzae) and the Take-all (G. graminis) fungi secrete MnLOX to support infection, invasive hyphal growth, and cell membrane oxidation, contributing to their devastating impact on world production of rice and wheat.

Reduced expression of lipoxygenase genes improves flour processing quality in soft wheat

Lipoxygenases (Loxs) are dioxygenases that play an important role in plant growth and defense. Loxs affect flour processing quality in common wheat (Triticum aestivum). We conducted a genome-wide association study (GWAS) that identified 306 significant single-nucleotide polymorphisms (SNPs) related to Lox activity in Chinese wheat accessions. Among them, a novel lipoxygenase-encoding (Lpx) gene, TaLpx-B4, was detected on chromosome 3B in a biparental population. Analysis of mutant wheat lines induced using ethyl methanesulfonate confirmed the role of TaLpx-B4 in modulating Lox activity. A phylogenetic tree of various plant Lpx genes indicated the predominance of the 9-Lpx type in common wheat. Further analysis revealed conserved intron number, exon length, and motif number in the TaLpx gene family. GWAS, linkage mapping, and gene annotation collectively showed that 14 out of 29 annotated TaLpx genes played a critical role in regulating Lox activity in the Chinese wheat accessions. Transgenic wheat grains with knockdown of Lpx family genes by RNAi showed significantly lower Lox activity than the wild type. One TaLpx-RNAi line had significantly reduced starch content and dough stability, and thus possessed relatively superior biscuit quality in soft wheat. Further analysis of the transcriptome, lipid components, and other metabolites revealed that knockdown of TaLpx genes significantly increased biscuit quality via changes in unsaturated fatty acid content as well as in starch, sucrose, and galactose metabolism. Our results provide new insights into the role of the TaLpx gene family that will be beneficial in improving soft wheat flour quality.

Biocontrol Efficacy of Bacillus cereus Strain Bc-cm103 Against Meloidogyne incognita

Root-knot nematodes (Meloidogyne spp.) are soilborne pathogens that infect vegetable crops and cause major economic losses worldwide annually. Therefore, there is an urgent need for novel nematicides or biological control agents to reduce the damage caused by root-knot nematodes. In this study, we tested efficacy of the Bacillus cereus strain Bc-cm103, isolated from the rhizoplane of Cucumis metuliferus, against Meloidogyne incognita. Strain Bc-cm103 fermentation broth caused 100% mortality of the nematode second-stage juveniles within 12 h and decreased the egg hatching rate by 40.06% within 72 h compared with sterile water. Confocal laser-scanning microscopy revealed that strain Bc-cm103 formed a biofilm on cucumber (C. sativus) roots, which protected the roots from the infection of M. incognita. Additionally, strain Bc-cm103 activated the defense-responsive genes PR1, PR2, LOX1, and CTR1 in cucumber. Furthermore, strain Bc-cm103 significantly reduced the appearance of root galls in pot, split-root, and field tests. These results indicated that B. cereus strain Bc-cm103 had a strong suppressive effect on M. incognita and therefore could be used as a potential biocontrol agent against this pathogen.

A 13-Lipoxygenase, GhLOX2, positively regulates cotton tolerance against Verticillium dahliae through JA-mediated pathway

Verticillium wilt is a major limiting factor for sustainable production of cotton but the mechanism of controlling this disease is still poorly understood. Lipoxygenase (LOX)-derived oxylipins have been implicated in defense responses against diverse pathogens; however there is limited information about the functional characterization of LOXs in response to Verticillium dahliae infection. In this study, we report the characterization of a cotton LOX gene, GhLOX2, which phylogenetically clustered into 13-LOX subfamily and is closely related to Arabidopsis LOX2 gene. GhLOX2 was predominantly expressed in leaves and strongly induced following V. dahliae inoculation and treatment of methyl jasmonate (MeJA). RNAi-mediated knock-down of GhLOX2 enhanced cotton susceptibility to V. dahliae and was coupled with suppression of jasmonic acid (JA)-related genes both after inoculation with the cotton defoliating strain V991 or MeJA treatment. Interestingly, lignin contents, transcripts of lignin synthesis genes and H₂O₂ contents were also decreased in GhLOX2-silenced plants. This study suggests that GhLOX2 is involved in defense responses against infection of V. dahliae in cotton and supports that JA is one of the major defense hormones against this pathogen.

In Vitro Biosynthetic Pathway Investigations of Neuroprotectin D1 (NPD1) and Protectin DX (PDX) by Human 12-Lipoxygenase, 15-Lipoxygenase-1, and 15-Lipoxygenase-2

In this paper, human platelet 12-lipoxygenase [h12-LOX (ALOX12)], human reticulocyte 15-lipoxygenase-1 [h15-LOX-1 (ALOX15)], and human epithelial 15-lipoxygenase-2 [h15-LOX-2 (ALOX15B)] were observed to react with docosahexaenoic acid (DHA) and produce 17S-hydroperoxy-4Z,7Z,10Z,13Z,15E,19Z-docosahexaenoic acid (17S-HpDHA). The kcat/KM values with DHA for h12-LOX, h15-LOX-1, and h15-LOX-2 were 12, 0.35, and 0.43 s-1 μM-1, respectively, which demonstrate h12-LOX as the most efficient of the three. These values are comparable to their counterpart kcat/KM values with arachidonic acid (AA), 14, 0.98, and 0.24 s-1 μM-1, respectively. Comparison of their product profiles with DHA demonstrates that the three LOX isozymes produce 11S-HpDHA, 14S-HpDHA, and 17S-HpDHA, to varying degrees, with 17S-HpDHA being the majority product only for the 15-LOX isozymes. The effective kcat/KM values (kcat/KM × percent product formation) for 17S-HpDHA of the three isozymes indicate that the in vitro value of h12-LOX was 2.8-fold greater than that of h15-LOX-1 and 1.3-fold greater than that of h15-LOX-2. 17S-HpDHA was an effective substrate for h12-LOX and h15-LOX-1, with four products being observed under reducing conditions: protectin DX (PDX), 16S,17S-epoxy-4Z,7Z,10Z,12E,14E,19Z-docosahexaenoic acid (16S,17S-epoxyDHA), the key intermediate in neuroprotection D1 biosynthesis [NPD1, also known as protectin D1 (PD1)], 11,17S-diHDHA, and 16,17S-diHDHA. However, h15-LOX-2 did not react with 17-HpDHA. With respect to their effective kcat/KM values, h12-LOX was markedly less effective than h15-LOX-1 in reacting with 17S-HpDHA, with a 55-fold lower effective kcat/KM in producing 16S,17S-epoxyDHA and a 27-fold lower effective kcat/KM in generating PDX. This is the first direct demonstration of h15-LOX-1 catalyzing this reaction and reveals an in vitro pathway for PDX and NPD1 intermediate biosynthesis. In addition, epoxide formation from 17S-HpDHA and h15-LOX-1 was negatively affected via allosteric regulation by 17S-HpDHA (Kd = 5.9 μM), 12S-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12S-HETE) (Kd = 2.5 μM), and 17S-hydroxy-13Z,15E,19Z-docosatrienoic acid (17S-HDTA) (Kd = 1.4 μM), suggesting a possible regulatory pathway in reducing epoxide formation. Finally, 17S-HpDHA and PDX inhibited platelet aggregation, with EC50 values of approximately 1 and 3 μM, respectively. The in vitro results presented here may help advise in vivo PDX and NPD1 intermediate (i.e., 16S,17S-epoxyDHA) biosynthetic investigations and support the benefits of DHA rich diets.

Production of Aldehydes by Biocatalysis

The production of aldehydes, highly reactive and toxic chemicals, brings specific challenges to biocatalytic processes. Absence of natural accumulation of aldehydes in microorganisms has led to a combination of in vitro and in vivo strategies for both, bulk and fine production. Advances in genetic and metabolic engineering and implementation of computational techniques led to the production of various enzymes with special requirements. Cofactor synthesis, post-translational modifications and structure engineering are applied to prepare active enzymes for one-step or cascade reactions. This review presents the highlights in biocatalytical production of aldehydes with the potential to shape future industrial applications.

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.

Biocatalytic Synthesis of Natural Green Leaf Volatiles Using the Lipoxygenase Metabolic Pathway

In higher plants, the lipoxygenase enzymatic pathway combined actions of several enzymes to convert lipid substrates into signaling and defense molecules called phytooxylipins including short chain volatile aldehydes, alcohols, and esters, known as green leaf volatiles (GLVs). GLVs are synthesized from C18:2 and C18:3 fatty acids that are oxygenated by lipoxygenase (LOX) to form corresponding hydroperoxides, then the action of hydroperoxide lyase (HPL) produces C6 or C9 aldehydes that can undergo isomerization, dehydrogenation, and esterification. GLVs are commonly used as flavors to confer a fresh green odor of vegetable to perfumes, cosmetics, and food products. Given the increasing demand in these natural flavors, biocatalytic processes using the LOX pathway reactions constitute an interesting application. Vegetable oils, chosen for their lipid profile are converted in natural GLVs with high added value. This review describes the enzymatic reactions of GLVs biosynthesis in the plant, as well as the structural and functional properties of the enzymes involved. The various stages of the biocatalytic production processes are approached from the lipid substrate to the corresponding aldehyde or alcoholic aromas, as well as the biotechnological improvements to enhance the production potential of the enzymatic catalysts.

Oxygenation reactions catalyzed by the F557V mutant of soybean lipoxygenase-1: Evidence for two orientations of substrate binding

Plant lipoxygenases oxygenate linoleic acid to produce 13(S)-hydroperoxy-9Z,11E-octadecadienoic acid (13(S)-HPOD) or 9-hydroperoxy-10E,12Z-octadecadienoic acid (9(S)-HPOD). The manner in which these enzymes bind substrates and the mechanisms by which they control regiospecificity are uncertain. Hornung et al. (Proc. Natl. Acad. Sci. USA96 (1999) 4192-4197) have identified an important residue, corresponding to phe-557 in soybean lipoxygenase-1 (SBLO-1). These authors proposed that large residues in this position favored binding of linoleate with the carboxylate group near the surface of the enzyme (tail-first binding), resulting in formation of 13(S)-HPOD. They also proposed that smaller residues in this position facilitate binding of linoleate in a head-first manner with its carboxylate group interacting with a conserved arginine residue (arg-707 in SBLO-1), which leads to 9(S)-HPOD. In the present work, we have tested these proposals on SBLO-1. The F557V mutant produced 33% 9-HPOD (S:R = 87:13) from linoleic acid at pH 7.5, compared with 8% for the wild-type enzyme and 12% with the F557V,R707L double mutant. Experiments with 11(S)-deuteriolinoleic acid indicated that the 9(S)-HPOD produced by the F557V mutant involves removal of hydrogen from the pro-R position on C-11 of linoleic acid, as expected if 9(S)-HPOD results from binding in an orientation that is inverted relative to that leading to 13(S)-HPOD. The product distributions obtained by oxygenation of 10Z,13Z-nonadecadienoic acid and arachidonic acid by the F557V mutant support the hypothesis that ω6 oxygenation results from tail-first binding and ω10 oxygenation from head-first binding. The results demonstrate that the regiospecificity of SBLO-1 can be altered by a mutation that facilitates an alternative mode of substrate binding and adds to the body of evidence that 13(S)-HPOD arises from tail-first binding.

The green microalga Lobosphaera incisa harbours an arachidonate 15S-lipoxygenase

The green microalga Lobosphaera incisa is an oleaginous eukaryotic alga that is rich in arachidonic acid (20:4). Being rich in this polyunsaturated fatty acid (PUFA), however, makes it sensitive to oxidation. In plants, lipoxygenases (LOXs) are the major enzymes that oxidise these molecules. Here, we describe, to our best knowledge, the first characterisation of a cDNA encoding a LOX (LiLOX) from a green alga. To obtain first insights into its function, we expressed it in E. coli, purified the recombinant enzyme and analysed its enzyme activity. The protein sequence suggests that LiLOX and plastidic LOXs from bryophytes and flowering plants may share a common ancestor. The fact that LiLOX oxidises all PUFAs tested with a consistent oxidation on the carbon n-6, suggests that PUFAs enter the substrate channel through their methyl group first (tail first). Additionally, LiLOX form the fatty acid hydroperoxide in strict S configuration. LiLOX may represent a good model to study plastid LOX, because it is stable after heterologous expression in E. coli and highly active in vitro. Moreover, as the first characterised LOX from green microalgae, it opens the possibility to study endogenous LOX pathways in these organisms.

Genome-wide identification of tomato (Solanum lycopersicum L.) lipoxygenases coupled with expression profiles during plant development and in response to methyl-jasmonate and wounding

Lipoxygenases (LOXs) (EC 1.13.11.12) catalyze the oxygenation of fatty acids and produce oxylipins including the plant hormone jasmonate (jasmonic acid/methyl jasmonate; MeJA). Little is known about the tomato LOX gene family members that impact tomato growth and development, and less so about their feed-back regulation in response to MeJA. We present genome wide identification of 14 LOX gene family members in tomato which map unevenly on 12 chromosomes. The characteristic structural features of 9-LOX and 13-LOX tomato gene family, their protein domains/features, and divergence are presented. Quantification of the expression patterns of all the 14 SlLOX gene members segregated the members based on differential association with growth, development, or fruit ripening. We also identified those SlLOX genes whose transcription responds to exogenous MeJA and/or wounding stress. MeJA-based feedback regulation that involves activation of specific members of LOX genes is defined. Specific nature of SlLOX gene regulation in tomato is defined. The novel data on dynamics of SlLOX gene expression should help catalyze future strategies to elucidate role(s) of each gene member in planta and for crop biotechnological intervention.

Genome-wide identification, classification and expression of lipoxygenase gene family in pepper

KEY MESSAGE

Lipoxygenases mediate important biological processes. Through comparative genomics, domain-scan analysis, sequence analysis, phylogenetic analysis, homology modelling and transcriptional analysis the lipoxygenase gene family of pepper (Capsicum annuum) has been identified. Lipoxygenases (LOXs) are non-heme, iron-containing dioxygenases playing a pivotal role in diverse biological processes in plants, including defence and development. Here, we exploited the recent sequencing of the pepper genome to investigate the LOX gene family in pepper. Two LOX classes are recognized, the 9- and 13-LOXs that oxygenate lipids at the 9th and 13th carbon atom, respectively. Using two main in-silico approaches, we identified a total of eight LOXs in pepper. Phylogenetic analysis classified four LOXs (CaLOX1, CaLOX3, CaLOX4 and CaLOX5) as 9-LOXs and four (CaLOX2, CaLOX6, CaLOX7 and CaLOX8) as 13-LOXs. Furthermore, sequence similarity/identity and subcellular localization analysis strengthen the classification predicted by phylogenetic analysis. Pivotal amino acids together with all domains and motifs are highly conserved in all pepper LOXs. Expression of 13-LOXs appeared to be more dynamic compared to 9-LOXs both in response to exogenous JA application and to thrips feeding. Bioinformatic and expression analyses predict the putative functions of two 13-LOXs, CaLOX6 and CaLOX7, in the biosynthesis of Green Leaf Volatiles, involved in indirect defence. The data are discussed in the context of LOX families in solanaceous plants and plants of other families.

Genome-wide identification of lipoxygenase gene family in cotton and functional characterization in response to abiotic stresses

BACKGROUND

Plant lipoxygenase (LOX) genes are members of the non-haeme iron-containing dioxygenase family that catalyze the oxidation of polyunsaturated fatty acids into functionally diverse oxylipins. The LOX family genes have been extensively studied under biotic and abiotic stresses, both in model and non-model plant species; however, information on their roles in cotton is still limited.

RESULTS

A total of 64 putative LOX genes were identified in four cotton species (Gossypium (G. hirsutum, G. barbadense, G. arboreum, and G. raimondii)). In the phylogenetic tree, these genes were clustered into three categories (9-LOX, 13-LOX type I, and 13-LOX type II). Segmental duplication of putative LOX genes was observed between homologues from A2 to At and D5 to Dt hinting at allopolyploidy in cultivated tetraploid species (G. hirsutum and G. barbadense). The structure and motif composition of GhLOX genes appears to be relatively conserved among the subfamilies. Moreover, many cis-acting elements related to growth, stresses, and phytohormone signaling were found in the promoter regions of GhLOX genes. Gene expression analysis revealed that all GhLOX genes were induced in at least two tissues and the majority of GhLOX genes were up-regulated in response to heat and salinity stress. Specific expressions of some genes in response to exogenous phytohormones suggest their potential roles in regulating growth and stress responses. In addition, functional characterization of two candidate genes (GhLOX12 and GhLOX13) using virus induced gene silencing (VIGS) approach revealed their increased sensitivity to salinity stress in target gene-silenced cotton. Compared with controls, target gene-silenced plants showed significantly higher chlorophyll degradation, higher H2O2, malondialdehyde (MDA) and proline accumulation but significantly reduced superoxide dismutase (SOD) activity, suggesting their reduced ability to effectively degrade accumulated reactive oxygen species (ROS).

CONCLUSION

This genome-wide study provides a systematic analysis of the cotton LOX gene family using bioinformatics tools. Differential expression patterns of cotton LOX genes in different tissues and under various abiotic stress conditions provide insights towards understanding the potential functions of candidate genes. Beyond the findings reported here, our study provides a basis for further uncovering the biological roles of LOX genes in cotton development and adaptation to stress conditions.

Product specificity of fungal 8R- and 9S-dioxygenases of the peroxidase-cyclooxygenase superfamily with amino acid derivatized polyenoic fatty acids

Pathogenic fungi express fatty acid dioxygenases (DOX) fused to cytochromes P450 with diol or allene oxide synthase activities. The orientation of the fatty acids in the active sites of DOX was investigated with amino acid conjugates of 18:3n-3 and 18:2n-6. 9S-DOX-allene oxide synthase (AOS) oxidized the Gly, Ile, and Trp derivatives at C-9, which suggests that these conjugates enter the substrate recognition site with the omega end in analogy with fatty acids bound to cyclooxygenases and coral 8R-lipoxygenase (8R-LOX). In contrast, 7,8-diol synthases (7,8-LDS), 5,8-LDS, and 8R-DOX-AOS oxidized the Gly conjugates in most case only to small amounts of metabolites, but with retention of hydrogen abstraction at C-8 and relatively minor hydrogen abstraction at C-11. The Ile and Trp conjugates were not oxidized at C-8, and often insignificantly at C-9/C-13. The 8-DOX domains of these enzymes likely position the carboxyl group of substrates at the end of the active site in analogy with plant α-DOX and 9-LOX. Tyr radicals of the 9S-DOX and 8R-DOX domains catalyze antarafacial hydrogen abstraction and oxygen insertion in 18:3n-3. This occurs by abstraction of the proR and proS hydrogens at C-11 and C-8, respectively, in agreement with different "head to tail" orientation in the active site.

Heterologous Expression and Biochemical Characterization of Two Lipoxygenases in Oriental Melon, Cucumis melo var. makuwa Makino

Lipoxygenases (LOXs) are a class of non-heme iron-containing dioxygenases that catalyse oxidation of polyunsaturated fatty acids to produce hydroperoxidation that are in turn converted to oxylipins. Although multiple isoforms of LOXs have been detected in several plants, LOXs in oriental melon have not attracted much attention. Two full-length LOX cDNA clones, CmLOX10 and CmLOX13 which have been isolated from oriental melon (Cucumis melo var. makuwa Makino) cultivar “Yumeiren”, encode 902 and 906 amino acids, respectively. Bioinformatics analysis showed that CmLOX10 and CmLOX13 included all of the typical LOX domains and shared 58.11% identity at the amino acid level with each other. The phylogenetic analysis revealed that CmLOX10 and CmLOX13 were members of the type 2 13-LOX subgroup which are known to be involved in biotic and abiotic stress. Heterologous expression of the full-length CmLOX10 and truncated CmLOX13 in Escherichia coli revealed that the encoded exogenous proteins were identical to the predicted molecular weights and possessed the lipoxygenase activities. The purified CmLOX10 and CmLOX13 recombinant enzymes exhibited maximum activity at different temperature and pH and both had higher affinity for linoleic acid than linolenic acid. Chromatogram analysis of reaction products from the CmLOX10 and CmLOX13 enzyme reaction revealed that both enzymes produced 13S-hydroperoxides when linoleic acid was used as substrate. Furthermore, the subcellular localization analysis by transient expression of the two LOX fusion proteins in tobacco leaves showed that CmLOX10 and CmLOX13 proteins were located in plasma membrane and chloroplasts respectively. We propose that the two lipoxygenases may play different functions in oriental melon during plant growth and development.

Crystal structure of a lipoxygenase from Cyanothece sp. may reveal novel features for substrate acquisition

In eukaryotes, oxidized PUFAs, so-called oxylipins, are vital signaling molecules. The first step in their biosynthesis may be catalyzed by a lipoxygenase (LOX), which forms hydroperoxides by introducing dioxygen into PUFAs. Here we characterized CspLOX1, a phylogenetically distant LOX family member from Cyanothece sp. PCC 8801 and determined its crystal structure. In addition to the classical two domains found in plant, animal, and coral LOXs, we identified an N-terminal helical extension, reminiscent of the long α-helical insertion in Pseudomonas aeruginosa LOX. In liposome flotation studies, this helical extension, rather than the β-barrel domain, was crucial for a membrane binding function. Additionally, CspLOX1 could oxygenate 1,2-diarachidonyl-sn-glycero-3-phosphocholine, suggesting that the enzyme may act directly on membranes and that fatty acids bind to the active site in a tail-first orientation. This binding mode is further supported by the fact that CspLOX1 catalyzed oxygenation at the n-10 position of both linoleic and arachidonic acid, resulting in 9R- and 11R-hydroperoxides, respectively. Together these results reveal unifying structural features of LOXs and their function. While the core of the active site is important for lipoxygenation and thus highly conserved, peripheral domains functioning in membrane and substrate binding are more variable.

A dual positional specific lipoxygenase functions in the generation of flavor compounds during climacteric ripening of apple

Lipoxygenase (LOX) is an important contributor to the formation of aroma-active C6 aldehydes in apple (Malus × domestica) fruit upon tissue disruption but little is known about its role in autonomously produced aroma volatiles from intact tissue. We explored the expression of 22 putative LOX genes in apple throughout ripening, but only six LOXs were expressed in a ripening-dependent manner. Recombinant LOX1:Md:1a, LOX1:Md:1c, LOX2:Md:2a and LOX2:Md:2b proteins showed 13/9-LOX, 9-LOX, 13/9-LOX and 13-LOX activity with linoleic acid, respectively. While products of LOX1:Md:1c and LOX2:Md:2b were S-configured, LOX1:Md:1a and LOX2:Md:2a formed 13(R)-hydroperoxides as major products. Site-directed mutagenesis of Gly567 to an alanine converted the dual positional specific LOX1:Md:1a to an enzyme with a high specificity for 9(S)-hydroperoxide formation. The high expression level of the corresponding MdLOX1a gene in stored apple fruit, the genetic association with a quantitative trait locus for fruit ester and the remarkable agreement in regio- and stereoselectivity of the LOX1:Md:1a reaction with the overall LOX activity found in mature apple fruits, suggest a major physiological function of LOX1:Md:1a during climacteric ripening of apples. While LOX1:Md:1c, LOX2:Md:2a and LOX2:Md:2b may contribute to aldehyde production in immature fruit upon cell disruption our results furnish additional evidence that LOX1:Md:1a probably regulates the availability of precursors for ester production in intact fruit tissue.

The novel monocot-specific 9-lipoxygenase ZmLOX12 is required to mount an effective jasmonate-mediated defense against Fusarium verticillioides in maize

Fusarium verticillioides is a major limiting factor for maize production due to ear and stalk rot and the contamination of seed with the carcinogenic mycotoxin fumonisin. While lipoxygenase (LOX)-derived oxylipins have been implicated in defense against diverse pathogens, their function in maize resistance against F. verticillioides is poorly understood. Here, we functionally characterized a novel maize 9-LOX gene, ZmLOX12. This gene is distantly related to known dicot LOX genes, with closest homologs found exclusively in other monocot species. ZmLOX12 is predominantly expressed in mesocotyls in which it is strongly induced in response to F. verticillioides infection. The Mutator transposon-insertional lox12-1 mutant is more susceptible to F. verticillioides colonization of mesocotyls, stalks, and kernels. The infected mutant kernels accumulate a significantly greater amount of the mycotoxin fumonisin. Reduced resistance to the pathogen is accompanied by diminished levels of the jasmonic acid (JA) precursor 12-oxo phytodienoic acid, JA-isoleucine, and expression of jasmonate-biosynthetic genes. Supporting the strong defense role of jasmonates, the JA-deficient opr7 opr8 double mutant displayed complete lack of immunity to F. verticillioides. Unexpectedly, the more susceptible lox12 mutant accumulated higher levels of kauralexins, suggesting that F. verticillioides is tolerant to this group of antimicrobial phytoalexins. This study demonstrates that this unique monocot-specific 9-LOX plays a key role in defense against F. verticillioides in diverse maize tissues and provides genetic evidence that JA is the major defense hormone against this pathogen.

A trichome-specific linoleate lipoxygenase expressed during pyrethrin biosynthesis in pyrethrum

The lipid precursor alcohols of pyrethrins-jasmolone, pyrethrolone and cinerolone-have been proposed as sharing parts of the oxylipin pathway with jasmonic acid. This implies that one of the first committed steps of pyrethrin biosynthesis is catalyzed by a lipoxygenase, catalyzing the hydroperoxidation of linolenic acid at position 13. Previously, we showed that the expression and activity of chrysanthemyl diphosphate synthase (TcCDS), the enzyme catalyzing the first committed step in the biosynthesis of the acid moiety of pyrethrins, is trichome-specific and developmentally regulated in flowers. In the present study we characterized the expression pattern of 25 lipoxygenase EST contigs, and subsequently carried out the molecular cloning of two pyrethrum lipoxygenases, TcLOX1 and TcLOX2, that have a similar pattern to TcCDS. Only recombinant TcLOX1 catalyzed the peroxidation of the linolenic acid substrate. Just as TcCDS, TcLOX1, are exclusively expressed in trichomes. Phylogenetic analysis showed that the enzyme shared the highest homology with chloroplast-localized 13-type-lipoxygenases that are involved in maintaining basal levels of jasmonate.

Production of hydroxy fatty acids by microbial fatty acid-hydroxylation enzymes

Hydroxy fatty acids are widely used in chemical, food, and cosmetic industries as starting materials for the synthesis of polymers and as additives for the manufacture of lubricants, emulsifiers, and stabilizers. They have antibiotic, anti-inflammatory, and anticancer activities and therefore can be applied for medicinal uses. Microbial fatty acid-hydroxylation enzymes, including P450, lipoxygenase, hydratase, 12-hydroxylase, and diol synthase, synthesize regio-specific hydroxy fatty acids. In this article, microbial fatty acid-hydroxylation enzymes, with a focus on region-specificity and diversity, are summarized and the production of mono-, di-, and tri-hydroxy fatty acids is introduced. Finally, the production methods of regio-specific and diverse hydroxy fatty acids, such as gene screening, protein engineering, metabolic engineering, and combinatory biosynthesis, are suggested.

An iron 13S-lipoxygenase with an α-linolenic acid specific hydroperoxidase activity from Fusarium oxysporum

Jasmonates constitute a family of lipid-derived signaling molecules that are abundant in higher plants. The biosynthetic pathway leading to plant jasmonates is initiated by 13-lipoxygenase-catalyzed oxygenation of α-linolenic acid into its 13-hydroperoxide derivative. A number of plant pathogenic fungi (e.g. Fusarium oxysporum) are also capable of producing jasmonates, however, by a yet unknown biosynthetic pathway. In a search for lipoxygenase in F. oxysporum, a reverse genetic approach was used and one of two from the genome predicted lipoxygenases (FoxLOX) was cloned. The enzyme was heterologously expressed in E. coli, purified via affinity chromatography, and its reaction mechanism characterized. FoxLOX was found to be a non-heme iron lipoxygenase, which oxidizes C₁₈-polyunsaturated fatty acids to 13S-hydroperoxy derivatives by an antarafacial reaction mechanism where the bis-allylic hydrogen abstraction is the rate-limiting step. With α-linolenic acid as substrate FoxLOX was found to exhibit a multifunctional activity, because the hydroperoxy derivatives formed are further converted to dihydroxy-, keto-, and epoxy alcohol derivatives.

Transcriptomic analysis of oxylipin biosynthesis genes and chemical profiling reveal an early induction of jasmonates in chickpea roots under drought stress

Drought is one of the major constraints in subtropical agriculture. Therefore improving water stress tolerance is of great importance to breed for drought tolerance in future. The first plant organ sensing dehydration is the root. Aim of the present work was to clarify the potential impact of the phyto-oxylipins pathway on drought tolerance of chickpea (Cicer arietinum), the third important legume crop worldwide. Therefore, we measured the expression of key genes involved in oxylipins metabolism by qPCR on samples from stressed and non-stressed roots of a drought-tolerant and a drought-sensitive chickpea variety using commercially available TaqMan assays. We demonstrate that the drought tolerant variety reacts to drought with sustained and earlier activation of a specific lipoxygenase (Mt-LOX 1) gene, two hydroperoxide lyases (Mt-HPL 1 and Mt-HPL 2), an allene oxide synthase (Mt-AOS), and an oxo-phytodienoate reductase (Mt-OPR). We further show that gene over-expression positively correlates with the levels of major oxylipin metabolites from the AOS branch of the pathway, which finally leads to the synthesis of jasmonates. Higher levels of jasmonic acid (JA), its precursor 12-oxophytodienoic acid (OPDA) and the active form JA-isoleucine (JA-Ile) were especially detected in the root tissues of the tolerant variety, prompting us to assume a role of jasmonates in the early signalling of drought stress in chickpea and its involvement in the tolerance mechanism of the drought-tolerant variety.

Catalytic convergence of manganese and iron lipoxygenases by replacement of a single amino acid

Lipoxygenases (LOXs) contain a hydrophobic substrate channel with the conserved Gly/Ala determinant of regio- and stereospecificity and a conserved Leu residue near the catalytic non-heme iron. Our goal was to study the importance of this region (Gly(332), Leu(336), and Phe(337)) of a lipoxygenase with catalytic manganese (13R-MnLOX). Recombinant 13R-MnLOX oxidizes 18:2n-6 and 18:3n-3 to 13R-, 11(S or R)-, and 9S-hydroperoxy metabolites (∼80-85, 15-20, and 2-3%, respectively) by suprafacial hydrogen abstraction and oxygenation. Replacement of Phe(337) with Ile changed the stereochemistry of the 13-hydroperoxy metabolites of 18:2n-6 and 18:3n-3 (from ∼100% R to 69-74% S) with little effect on regiospecificity. The abstraction of the pro-S hydrogen of 18:2n-6 was retained, suggesting antarafacial hydrogen abstraction and oxygenation. Replacement of Leu(336) with smaller hydrophobic residues (Val, Ala, and Gly) shifted the oxygenation from C-13 toward C-9 with formation of 9S- and 9R-hydroperoxy metabolites of 18:2n-6 and 18:3n-3. Replacement of Gly(332) and Leu(336) with larger hydrophobic residues (G332A and L336F) selectively augmented dehydration of 13R-hydroperoxyoctadeca-9Z,11E,15Z-trienoic acid and increased the oxidation at C-13 of 18:1n-6. We conclude that hydrophobic replacements of Leu(336) can modify the hydroperoxide configurations at C-9 with little effect on the R configuration at C-13 of the 18:2n-6 and 18:3n-3 metabolites. Replacement of Phe(337) with Ile changed the stereospecific oxidation of 18:2n-6 and 18:3n-3 with formation of 13S-hydroperoxides by hydrogen abstraction and oxygenation in analogy with soybean LOX-1.

Cloning and expression of three lipoxygenase genes from liverwort, Marchantia polymorpha L., in Escherichia coli

Three genes homologous to plant lipoxygenase genes were identified from the EST libraries of Marchantia polymorpha, in order to clarify the function of LOXs in bryophytes. Full-length genes were isolated using 5'- and 3'-RACE methods and named MpLOX1, MpLOX2, and MpLOX3, respectively. To investigate the enzymatic activities of liverwort LOXs, recombinant MpLOX1, MpLOX2, and MpLOX3 proteins were prepared from Escherichia coli cells expressing the corresponding gene. LC-MS/MS analyses and chiral column chromatography of their reaction products showed that MpLOX1 codes for 11S/15S-lipoxygenase against eicosapentaenoic acid and for 15S-lipoxygenase against arachidonic acid, and that MpLOX2 and MpLOX3 code for 15S-lipoxygenase against eicosapentaenoic and arachidonic acids. Phylogenetic analysis showed that the liverwort lipoxygenase genes separated from the ancestor of higher plants in the early stages of plant evolution. Quantification analyses suggested that arachidonic acid and eicosapentaenoic acid were preferred substrates. Furthermore, each liverwort lipoxygenase exhibited highest activity at pH 7.0 and dependency on Ca(2+) ion in the oxygenation reaction.

A chitosan induced 9-lipoxygenase in Adelostemma gracillimum seedlings

Oxylipins generated by the lipoxygenase (LOX) pathway play an important role in plant defense against biotic and abiotic stress. In chitosan-treated Adelostemma gracillimum seedlings, obvious accumulation of 9-LOX-derived oxylipins, namely 9,10,11-trihydroxy-12-octadecenoic acid, was detected. Using degenerate primers, a LOX-specific fragment putatively encoding LOX was obtained by RT-PCR, and a 2.9-kb full-length cDNA named AgLOX1 was isolated by RACE from chitosan-induced A. gracillimum seedlings. Genomic Southern analysis implied that there was only one copy of AgLOX1 in the A. gracillimum genome. AgLOX1 was expressed in Escherichia coli and the recombinant protein was partially purified. The enzyme converted linoleic and linolenic acids almost exclusively to their 9-hydroperoxides. AgLOX1 encoded a 9-lipoxygenase. Northern blot analysis indicated that chitosan-induced AgLOX1 transcript accumulation peaked at 8 h after initiation of treatment, whereas trihydroxy derivatives accumulation was highest at 24 h after elicitation. Results showed that chitosan-induced AgLOX1 encoded a 9-lipoxygenase potentially involved in the defense response through 9-LOX pathway leading to biosynthesis of antimicrobial compounds in A. gracillimum seedlings.

Molecular cloning, functional characterization and transcriptional regulation of a 9-lipoxygenase gene from olive

A lipoxygenase (LOX) cDNA clone (Oep2LOX1) has been isolated from olive fruit (Olea europaea cv. Picual). The deduced amino acid sequence displayed significant similarity to known plant LOX1 sequences. Genomic Southern blot analysis suggests that only one copy of Oep2LOX1 is present in the olive genome. Linolenic acid was the preferred substrate for the recombinant Oep2LOX1, which produced almost exclusively 9-hydroperoxide when linolenic acid was used as substrate, whereas a mixture of 9- and 13-hydroperoxides in a ratio 4:1 was formed from linoleic acid. Expression levels were measured in different tissues of Picual and Arbequina cultivars, including the mesocarp and seed during development and ripening of olive fruit. The results showed that Oep2LOX1 transcript level is spatially and temporally regulated. Besides, the transcriptional regulation of the Oep2LOX1 gene in response to different abiotic stresses was also investigated. Temperature, light and wounding regulate Oep2LOX1 gene expression in olive fruit mesocarp. The physiological role of the Oep2LOX1 gene during olive fruit ripening and in the stress response is discussed.

Synthesis of green note aroma compounds by biotransformation of fatty acids using yeast cells coexpressing lipoxygenase and hydroperoxide lyase

Green notes are substances that characterize the aroma of freshly cut grass, cucumbers, green apples, and foliage. In plants, they are synthesized by conversion of linolenic or linoleic acid via the enzymes lipoxygenase (LOX) and hydroperoxide lyase (HPL) to short-chained aldehydes. Current processes for production of natural green notes rely on plant homogenates as enzyme sources but are limited by low enzyme concentration and low specificity. In an alternative approach, soybean LOX2 and watermelon HPL were overexpressed in Saccharomyces cerevisiae. After optimization of the expression constructs, a yeast strain coexpressing LOX and HPL was applied in whole cell biotransformation experiments. Whereas addition of linolenic acid to growing cultures of this strain yielded no products, we were able to identify high green note concentrations when resting cells were used. The primary biotransformation product was 3(Z)-hexenal, a small amount of which isomerized to 2(E)-hexenal. Furthermore, both aldehydes were reduced to the corresponding green note alcohols by endogenous yeast alcohol dehydrogenase to some extent. As the cosolvent ethanol was the source of reducing equivalents for green note alcohol formation, the hexenal/hexenol ratio could be influenced by the use of alternative cosolvents. Further investigations to identify the underlying mechanism of the rather low biocatalyst stability revealed a high toxicity of linolenic acid to yeast cells. The whole cell catalyst containing LOX and HPL enzyme activity described here can be a promising approach towards a highly efficient microbial green note synthesis process.

A peroxygenase pathway involved in the biosynthesis of epoxy fatty acids in oat

While oat (Avena sativa) has long been known to produce epoxy fatty acids in seeds, synthesized by a peroxygenase pathway, the gene encoding the peroxygenase remains to be determined. Here we report identification of a peroxygenase cDNA AsPXG1 from developing seeds of oat. AsPXG1 is a small protein with 249 amino acids in length and contains conserved heme-binding residues and a calcium-binding motif. When expressed in Pichia pastoris and Escherichia coli, AsPXG1 catalyzes the strictly hydroperoxide-dependent epoxidation of unsaturated fatty acids. It prefers hydroperoxy-trienoic acids over hydroperoxy-dienoic acids as oxygen donors to oxidize a wide range of unsaturated fatty acids with cis double bonds. Oleic acid is the most preferred substrate. The acyl carrier substrate specificity assay showed phospholipid and acyl-CoA were not effective substrate forms for AsPXG1 and it could only use free fatty acid or fatty acid methyl esters as substrates. A second gene, AsLOX2, cloned from oat codes for a 9-lipoxygenase catalyzing the synthesis of 9-hydroperoxy-dienoic and 9-hydroperoxy-trienoic acids, respectively, when linoleic (18:2-9c,12c) and linolenic (18:3-9c,12c,15c) acids were used as substrates. The peroxygenase pathway was reconstituted in vitro using a mixture of AsPXG1 and AsLOX2 extracts from E. coli. Incubation of methyl oleate and linoleic acid or linolenic acid with the enzyme mixture produced methyl 9,10-epoxy stearate. Incubation of linoleic acid alone with a mixture of AsPXG1 and AsLOX2 produced two major epoxy fatty acids, 9,10-epoxy-12-cis-octadecenoic acid and 12,13-epoxy-9-cis-octadecenoic acid, and a minor epoxy fatty acid, probably 12,13-epoxy-9-hydroxy-10-transoctadecenoic acid. AsPXG1 predominately catalyzes intermolecular peroxygenation.

Molecular enzymology of lipoxygenases

Lipoxygenases (LOXs) are lipid peroxidizing enzymes, implicated in the pathogenesis of inflammatory and hyperproliferative diseases, which represent potential targets for pharmacological intervention. Although soybean LOX1 was discovered more than 60years ago, the structural biology of these enzymes was not studied until the mid 1990s. In 1993 the first crystal structure for a plant LOX was solved and following this protein biochemistry and molecular enzymology became major fields in LOX research. This review focuses on recent developments in molecular enzymology of LOXs and summarizes our current understanding of the structural basis of LOX catalysis. Various hypotheses explaining the reaction specificity of different isoforms are critically reviewed and their pros and cons briefly discussed. Moreover, we summarize the current knowledge of LOX evolution by profiling the existence of LOX-related genomic sequences in the three kingdoms of life. Such sequences are found in eukaryotes and bacteria but not in archaea. Although the biological role of LOXs in lower organisms is far from clear, sequence data suggests that this enzyme family might have evolved shortly after the appearance of atmospheric oxygen on earth.

Recombinant maize 9-lipoxygenase: expression, purification, and properties

Expression of maize 9-lipoxygenase was performed and optimized in Escherichia coli Rosetta(DE3)pLysS. The purity of recombinant protein obtained during Q-Sepharose and Octyl-Sepharose chromatographies in an LP system at 4 degrees C was >95%. Maximum activity of the lipoxygenase reaction was observed at pH 7.5. Enzyme stability was studied at pH 4.5 to 9.5 and in the presence of different compounds: phenylmethanesulfonyl fluoride, beta-mercaptoethanol, ammonium sulfate, and glycerol. HPLC and GC-MS analysis showed that enzyme produced 99% 9S-hydroperoxide from linoleic acid. 13-Hydroperoxide (less than 1%) consisted of S- and R-enantiomers in ratio 2 : 3.

Comparative molecular and biochemical characterization of segmentally duplicated 9-lipoxygenase genes ZmLOX4 and ZmLOX5 of maize

Lipoxygenases (LOXs) catalyze hydroperoxidation of polyunsaturated fatty acids (PUFAs) to form structurally and functionally diverse oxylipins. Precise physiological and biochemical functions of individual members of plant multigene LOX families are largely unknown. Herein we report on molecular and biochemical characterization of two closely related maize 9-lipoxygenase paralogs, ZmLOX4 and ZmLOX5. Recombinant ZmLOX5 protein displayed clear 9-LOX regio-specificity at both neutral and slightly alkaline pH. The genes were differentially expressed in various maize organs and tissues as well as in response to diverse stress treatments. The transcripts of ZmLOX4 accumulated predominantly in roots and shoot apical meristem, whereas ZmLOX5 was expressed in most tested aboveground organs. Both genes were not expressed in untreated leaves, but displayed differential induction by defense-related hormones. While ZmLOX4 was only induced by jasmonic acid (JA), the transcripts of ZmLOX5 were increased in response to JA and salicylic acid treatments. ZmLOX5 was transiently induced both locally and systemically by wounding, which was accompanied by increased levels of 9-oxylipins, and fall armyworm herbivory, suggesting a putative role for this gene in defense against insects. Surprisingly, despite of moderate JA- and wound-inducibility of ZmLOX4, the gene was not responsive to insect herbivory. These results suggest that the two genes may have distinct roles in maize adaptation to diverse biotic and abiotic stresses. Both paralogs were similarly induced by virulent and avirulent strains of the fungal leaf pathogen Cochliobolus carbonum. Putative physiological roles for the two genes are discussed in the context of their biochemical and molecular properties.

The pepper 9-lipoxygenase gene CaLOX1 functions in defense and cell death responses to microbial pathogens

Lipoxygenases (LOXs) are crucial for lipid peroxidation processes during plant defense responses to pathogen infection. A pepper (Capsicum annuum) 9-LOX gene, CaLOX1, which encodes a 9-specific lipoxygenase, was isolated from pepper leaves. Recombinant CaLOX1 protein expressed in Escherichia coli catalyzed the hydroperoxidation of linoleic acid, with a K(m) value of 113. 9 mum. Expression of CaLOX1 was differentially induced in pepper leaves not only during Xanthomonas campestris pv vesicatoria (Xcv) infection but also after exposure to abiotic elicitors. Transient expression of CaLOX1 in pepper leaves induced the cell death phenotype and defense responses. CaLOX1-silenced pepper plants were more susceptible to Xcv and Colletotrichum coccodes infection, which was accompanied by reduced expression of defense-related genes, lowered lipid peroxidation, as well as decreased reactive oxygen species and lowered salicylic acid accumulation. Infection with Xcv, especially in an incompatible interaction, rapidly stimulated LOX activity in unsilenced, but not CaLOX1-silenced, pepper leaves. Furthermore, overexpression of CaLOX1 in Arabidopsis (Arabidopsis thaliana) conferred enhanced resistance to Pseudomonas syringae pv tomato, Hyaloperonospora arabidopsidis, and Alternaria brassicicola. In contrast, mutation of the Arabidopsis CaLOX1 ortholog AtLOX1 significantly increased susceptibility to these three pathogens. Together, these results suggest that CaLOX1 and AtLOX1 positively regulate defense and cell death responses to microbial pathogens.

A lipoxygenase with dual positional specificity is expressed in olives (Olea europaea L.) during ripening

Plant lipoxygenases (LOXs) are a class of widespread dioxygenases catalysing the hydroperoxidation of polyunsaturated fatty acids. Although multiple isoforms of LOX have been detected in a wide range of plants, their physiological roles remain to be clarified. With the aim to clarify the occurrence of LOXs in olives and their contribution to the elaboration of the olive oil aroma, we cloned and characterized the first cDNA of the LOX isoform which is expressed during olive development. The open reading frame encodes a polypeptide of 864 amino acids. This olive LOX is a type-1 LOX which shows a high degree of identity at the peptide level towards hazelnut (77.3%), tobacco (76.3%) and almond (75.5%) LOXs. The recombinant enzyme shows a dual positional specificity, as it forms both 9- and 13-hydroperoxide of linoleic acid in a 2:1 ratio, and would be defined as 9/13-LOX. Although a LOX activity was detected throughout the olive development, the 9/13-LOX is mainly expressed at late developmental stages. Our data suggest that there are at least two Lox genes expressed in black olives, and that the 9/13-LOX is associated with the ripening and senescence processes. However, due to its dual positional specificity and its expression pattern, its contribution to the elaboration of the olive oil aroma might be considered.

Lipoxygenases - Structure and reaction mechanism

Lipid oxidation is a common metabolic reaction in all biological systems, appearing in developmentally regulated processes and as response to abiotic and biotic stresses. Products derived from lipid oxidation processes are collectively named oxylipins. Initial lipid oxidation may either occur by chemical reactions or is derived from the action of enzymes. In plants this reaction is mainly catalyzed by lipoxygenase (LOXs) enzymes and during recent years analysis of different plant LOXs revealed insights into their enzyme mechanism. This review aims at giving an overview of concepts explaining the catalytic mechanism of LOXs as well as the different regio- and stereo-specificities of these enzymes.

Specificity of oxidation of linoleic acid homologs by plant lipoxygenases

The lipoxygenase-catalyzed oxidation of linoleic acid homologs was studied. While the linoleic acid oxidation by maize 9-lipoxygenase (9-LO) specifically produced (9S)-hydroperoxide, the dioxygenation of (11Z,14Z)-eicosadienoic (20:2) and (13Z,16Z)-docosadienoic (22:2) acids by the same enzyme lacked regio- and stereospecificity. The oxidation of 20:2 and 22:2 by 9-LO afforded low yields of racemic 11-, 12-, 14-, and 15-hydroperoxides or 13- and 17-hydroperoxides, respectively. Soybean 13-lipoxygenase-1 (13-LO) specifically oxidized 20:2, 22:2, and linoleate into (omega6S)-hydroperoxides. Dioxygenation of (9Z,12Z)-hexadecadienoic acid (16:2) by both 9-LO and 13-LO occurred specifically, affording (9S)- and (13S)-hydroperoxides, respectively. The data are consistent with the "pocket theory of lipoxygenase catalysis" (i.e. with the penetration of a substrate into the active center with the methyl end first). Our findings also demonstrate that the distance between carboxyl group and double bonds substantially determines the positioning of substrates within the active site.

Structural basis for pH-dependent alterations of reaction specificity of vertebrate lipoxygenase isoforms

Lipoxygenases have been classified according to their specificity of fatty acid oxygenation and for several plant enzymes pH-dependent alterations in the product patterns have been reported. Assuming that the biological role of mammalian lipoxygenases is based on the formation of specific reaction products, pH-dependent alterations would impact enzymes' functionality. In this study we systematically investigated the pH-dependence of vertebrate lipoxygenases and observed a remarkable stability of the product pattern in the near physiological range for the wild-type enzyme species. Site-directed mutagenesis of selected amino acids and alterations in the substrate concentrations induced a more pronounced pH-dependence of the reaction specificity. For instance, for the V603H mutant of the human 15-lipoxygenase-2 8-lipoxygenation was dominant at acidic pH (65%) whereas 15-H(p)ETE was the major oxygenation product at pH 8. Similarly, the product pattern of the wild-type mouse 8-lipoxygenase was hardly altered in the near physiological pH range but H604F exchange induced strong pH-dependent alterations in the positional specificity. Taken together, our data suggest that the reaction specificities of wild-type vertebrate lipoxygenase isoforms are largely resistant towards pH alterations. However, we found that changes in the assay conditions (low substrate concentration) and introduction/removal of a critical histidine at the active site impact the pH-dependence of reaction specificity for some lipoxygenase isoforms.

tasselseed1 is a lipoxygenase affecting jasmonic acid signaling in sex determination of maize

Sex determination in maize is controlled by a developmental cascade leading to the formation of unisexual florets derived from an initially bisexual floral meristem. Abortion of pistil primordia in staminate florets is controlled by a tasselseed-mediated cell death process. We positionally cloned and characterized the function of the sex determination gene tasselseed1 (ts1). The TS1 protein encodes a plastid-targeted lipoxygenase with predicted 13-lipoxygenase specificity, which suggests that TS1 may be involved in the biosynthesis of the plant hormone jasmonic acid. In the absence of a functional ts1 gene, lipoxygenase activity was missing and endogenous jasmonic acid concentrations were reduced in developing inflorescences. Application of jasmonic acid to developing inflorescences rescued stamen development in mutant ts1 and ts2 inflorescences, revealing a role for jasmonic acid in male flower development in maize.

On the substrate binding of linoleate 9-lipoxygenases

Lipoxygenases (LOX; linoleate:oxygen oxidoreductase EC 1.13.11.12) consist of a class of enzymes that catalyze the regio- and stereo specific dioxygenation of polyunsaturated fatty acids. Here we characterize two proteins that belong to the less studied class of 9-LOXs, Solanum tuberosum StLOX1 and Arabidopsis thaliana AtLOX1. The proteins were recombinantly expressed in E. coli and the product specificity of the enzymes was tested against different fatty acid substrates. Both enzymes showed high specificity against all tested C18 fatty acids and produced (9S)-hydroperoxides. However, incubation of the C20 fatty acid arachidonic acid with AtLOX1 gave a mixture of racemic hydroperoxides. On the other hand, with StLOX1 we observed the formation of a mixture of products among which the (5S)-hydroperoxy eicosatetraenoic acid (5S-H(P)ETE) was the most abundant. Esterified fatty acids were no substrates. We used site directed mutagenesis to modify a conserved valine residue in the active site of StLOX1 and examine the importance of space within the active site, which has been shown to play a role in determining the positional specificity. The Val576Phe mutant still catalyzed the formation of (9S)-hydroperoxides with C18 fatty acids, while it exhibited altered specificity against arachidonic acid and produced mainly (11S)-H(P)ETE. These data confirm the model that in case of linoleate 9-LOX binding of the substrate takes place with the carboxyl-group first.

Identification of an amino acid determinant of pH regiospecificity in a seed lipoxygenase from Momordica charantia

Lipoxygenases (LOX) form a heterogeneous family of lipid peroxidizing enzymes, which catalyze specific dioxygenation of polyunsaturated fatty acids. According to their positional specificity of linoleic acid oxygenation plant LOX have been classified into linoleate 9- and linoleate 13-LOX and recent reports identified a critical valine at the active site of 9-LOX. In contrast, more bulky phenylalanine or histidine residues were found at this position in 13-LOX. We have recently cloned a LOX-isoform from Momordica charantia and multiple amino acid alignments indicated the existence of a glutamine (Gln599) at the position were 13-LOX usually carry histidine or phenylalanine residues. Analyzing the pH-dependence of the positional specificity of linoleic acid oxygenation we observed that at pH-values higher than 7.5 this enzyme constitutes a linoleate 13-LOX whereas at lower pH, 9-H(P)ODE was the major reaction product. Site-directed mutagenesis of glutamine 599 to histidine (Gln599His) converted the enzyme to a pure 13-LOX. These data confirm previous observation suggesting that reaction specificity of certain LOX-isoforms is not an absolute enzyme property but may be impacted by reaction conditions such as pH of the reaction mixture. We extended this concept by identifying glutamine 599 as sequence determinant for such pH-dependence of the reaction specificity. Although the biological relevance for this alteration switch remains to be investigated it is of particular interest that it occurs at near physiological conditions in the pH-range between 7 and 8.

Diversity of the enzymatic activity in the lipoxygenase gene family of Arabidopsis thaliana

Lipoxygenases (LOX) catalyze the oxygenation of polyunsaturated fatty acids, the first step in the biosynthesis of a large group of biologically active fatty acid metabolites collectively named oxylipins. In the present study we report the characterization of the enzymatic activity of the six lipoxygenases found in the genome of the model plant Arabidopsis thaliana. Recombinant expressed AtLOX-1 and AtLOX-5 had comparable oxygenase activity with either linoleic acid or linolenic acid. AtLOX-2, AtLOX-3, AtLOX-4 and AtLOX-6 displayed a selective oxygenation of linolenic acid. Analyses by high-performance liquid chromatography and gas chromatography-mass spectrometry demonstrated that AtLOX-1 and AtLOX-5 are 9S-lipoxygenases, and AtLOX-2, AtLOX-3, AtLOX-4 and AtLOX-6 are 13S-lipoxygenases. None of the enzymes had dual positional specificity. The determined activities correlated with that predicted by their phylogenetic relationship to other biochemically-characterized plant lipoxygenases.

Characterisation of lipoxygenase isoforms from olive callus cultures

Two lipoxygenase isoforms from olive callus cultures were separated from each other. Acetone powders were made to stabilise activity and remove lipids. Separation was then achieved by salt precipitation and ion-exchange chromatography. Both isoforms had comparable activity with linoleic and alpha-linolenic acid substrates, a basic pH optimum and had molecular masses of around 95 kDa. The callus extracts preferentially formed the 13-hydroperoxy products, in keeping with the pattern of volatile derivatives found in olive tissues and oils derived therefrom.