Oxylipin formation in Nostoc punctiforme (PCC73102)

Lipoxygenases at the Intersection of Infection and Carcinogenesis

The persisting presence of opportunistic pathogens like Pseudomonas aeruginosa poses a significant threat to many immunocompromised cancer patients with pulmonary infections. This review highlights the complexity of interactions in the host's defensive eicosanoid signaling network and its hijacking by pathogenic bacteria to their own advantage. Human lipoxygenases (ALOXs) and their mouse counterparts are integral elements of the innate immune system, mostly operating in the pro-inflammatory mode. Taking into account the indispensable role of inflammation in carcinogenesis, lipoxygenases have counteracting roles in this process. In addition to describing the structure-function of lipoxygenases in this review, we discuss their roles in such critical processes as cancer cell signaling, metastases, death of cancer and immune cells through ferroptosis, as well as the roles of ALOXs in carcinogenesis promoted by pathogenic infections. Finally, we discuss perspectives of novel oncotherapeutic approaches to harness lipoxygenase signaling in tumors.

Promising Anti-Inflammatory Tools: Biomedical Efficacy of Lipoxins and Their Synthetic Pathways

Lipoxins (LXs) have attracted widespread attention as a class of anti-inflammatory lipid mediators that are produced endogenously by the organism. LXs are arachidonic acid (ARA) derivatives that include four different structures: lipoxin A4 (LXA4), lipoxin B4 (LXB4), and the aspirin-induced differential isomers 15-epi-LXA4 and 15-epi-LXB4. Because of their unique biological activity of reducing inflammation in the body, LXs have great potential for neuroprotection, anti-inflammatory treatment of COVID-19, and other related diseases. The synthesis of LXs in vivo is achieved through the action of lipoxygenase (LO). As a kind of important enzyme, LO plays a major role in the physiological processes of living organisms in mammals and functions in some bacteria and fungi. This suggests new options for the synthesis of LXs in vitro. Meanwhile, there are other chemical and biochemical methods to synthesize LXs. In this review, the recent progress on physiological activity and synthetic pathways of LXs is summarized, and new insights into the synthesis of LXs in vitro are provided.

Computational Insights and In Silico Characterization of a Novel Mini-Lipoxygenase from Nostoc Sphaeroides and Its Application in the Quality Improvement of Steamed Bread

Lipoxygenase (EC1.13.11.12, LOX) has been potentially used in the food industry for food quality improvement. However, the low activity, poor thermal stability, narrow range of pH stability, as well as undesirable isoenzymes and off-flavors, have hampered the application of current commercial LOX. In this study, a putative mini-lipoxygenase gene from cyanobacteria, Nostoc sphaeroides (NsLOX), was cloned and expressed in E. coli BL21. NsLOX displayed only 26.62% structural identity with the reported LOX from Cyanothece sp., indicating it as a novel LOX. The purified NsLOX showed the maximum activity at pH 8.0 and 15 °C, with superior stability at a pH range from 6.0 to 13.0, retaining about 40% activity at 40 °C for 90 min. Notably, NsLOX exhibited the highest specific activity of 78,080 U/mg towards linoleic acid (LA), and the kinetic parameters-K_m, k_cat, and k_cat/K_m-attain values of 19.46 μM, 9199.75 s^-1, and 473.85 μM^-1 s^-1, respectively. Moreover, the activity of NsLOX was obviously activated by Ca²⁺, but it was completely inhibited by Zn²⁺ and Cu²⁺. Finally, NsLOX was supplied in steamed bread and contributed even better improved bread quality than the commercial LOX. These results suggest NsLOX as a promising substitute of current commercial LOX for application in the food industry.

Plastidic membrane lipids are oxidized by a lipoxygenase in Lobosphaera incisa

Green microalgae can accumulate neutral lipids, as part of a general lipid remodeling mechanism under stress such as nitrogen starvation. Lobosphaera incisa is of special interest because of its unique TAG acyl chain composition, especially 20:4 (n-6) can reach up to 21% of dry weight after nitrogen starvation. In order to identify factors that may influence the accumulation of polyunsaturated fatty acids (PUFAs), we identified recently a linoleate 13-lipoxygenase (LiLOX). It shares highest identity with plastidic enzymes from vascular plants and is induced upon nitrogen starvation. Here, we confirmed the localization of LiLOX in the stroma of plastids via transient expression in epithelial onion cells. In order to further characterize this enzyme, we focused on the identification of the endogenous substrate of LiLOX. In this regard, an ex vivo enzymatic assay, coupled with non-targeted analysis via mass spectrometry allowed the identification of MGDG, DGDG and PC as three substrate candidates, later confirmed via in vitro assays. Further investigation revealed that LiLOX has preferences towards the lipid class MGDG, which seems in agreement with its localization in the galactolipid rich plastid. Altogether, this study shows the first characterization of plastidic LOX from green algae, showing preference for MGDGs. However, lipidomics analysis did neither reveal an endogenous LiLOX product nor the final end product of MGDG oxidation. Nevertheless, the latter is a key to understanding the role of this enzyme and since its expression is highest during the degradation of the plastidic membrane, it is tempting to assume its involvement in this process.

Aeroterrestrial and Extremophilic Microalgae as Promising Sources for Lipids and Lipid Nanoparticles in Dermal Cosmetics

Microscopic prokaryotic and eukaryotic algae (microalgae), which can be effectively grown in mass cultures, are gaining increasing interest in cosmetics. Up to now, the main attention was on aquatic algae, while species from aeroterrestrial and extreme environments remained underestimated. In these habitats, algae accumulate high amounts of some chemical substances or develop specific compounds, which cause them to thrive in inimical conditions. Among such biologically active molecules is a large family of lipids, which are significant constituents in living organisms and valuable ingredients in cosmetic formulations. Therefore, natural sources of lipids are increasingly in demand in the modern cosmetic industry and its innovative technologies. Among novelties in skin care products is the use of lipid nanoparticles as carriers of dermatologically active ingredients, which enhance their penetration and release in the skin strata. This review is an attempt to comprehensively cover the available literature on the high-value lipids from microalgae, which inhabit aeroterrestrial and extreme habitats (AEM). Data on different compounds of 87 species, subspecies and varieties from 53 genera (represented by more than 141 strains) from five phyla are provided and, despite some gaps in the current knowledge, demonstrate the promising potential of AEM as sources of valuable lipids for novel skin care products.

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.

Collection, Culturing, and Genome Analyses of Tropical Marine Filamentous Benthic Cyanobacteria

Decreasing sequencing costs has sparked widespread investigation of the use of microbial genomics to accelerate the discovery and development of natural products for therapeutic uses. Tropical marine filamentous cyanobacteria have historically produced many structurally novel natural products, and therefore present an excellent opportunity for the systematic discovery of new metabolites via the information derived from genomics and molecular genetics. Adequate knowledge transfer and institutional know-how are important to maintain the capability for studying filamentous cyanobacteria due to their unusual microbial morphology and characteristics. Here, we describe workflows, procedures, and commentary on sample collection, cultivation, genomic DNA generation, bioinformatics tools, and biosynthetic pathway analysis concerning filamentous cyanobacteria.

Biologically Active Oxylipins from Enzymatic and Nonenzymatic Routes in Macroalgae

Marine algae are rich and heterogeneous sources of great chemical diversity, among which oxylipins are a well-recognized class of natural products. Algal oxylipins comprise an assortment of oxygenated, halogenated, and unsaturated functional groups and also several carbocycles, varying in ring size and position in lipid chain. Besides the discovery of structurally diverse oxylipins in macroalgae, research has recently deciphered the role of some of these metabolites in the defense and innate immunity of photosynthetic marine organisms. This review is an attempt to comprehensively cover the available literature on the chemistry, biosynthesis, ecology, and potential bioactivity of oxylipins from marine macroalgae. For a better understanding, enzymatic and nonenzymatic routes were separated; however, both processes often occur concomitantly and may influence each other, even producing structurally related molecules.

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.

Biochemical characterization of allene oxide synthases from the liverwort Marchantia polymorpha and green microalgae Klebsormidium flaccidum provides insight into the evolutionary divergence of the plant CYP74 family

Allene oxide synthases (AOSs) were isolated from liverworts and charophytes. These AOSs exhibited enzymatic properties similar to those of angiosperms but formed a distinct phylogenetic clade. Allene oxide synthase (AOS) and hydroperoxide lyase (HPL) mediate the formation of precursors of jasmonates and carbon-six volatiles, respectively. AOS and HPL utilize fatty acid hydroperoxides and belong to the plant cytochrome P450 74 (CYP74) family that mediates plant defense against herbivores, pathogens, or abiotic stresses. Although members of the CYP74 family have been reported in mosses and other species, the evolution and function of multiple CYP74 genes in plants remain elusive. Here, we show that the liverwort Marchantia polymorpha belongs to a basal group in the evolution of land plants; has two closely related proteins (59% identity), MpAOS1 and MpAOS2, that are similar to moss PpAOS1 (49 and 47% identity, respectively); and exhibits AOS activity but not HPL activity. We also found that the green microalgae Klebsormidium flaccidum, consist of multicellular and non-branching filaments, contains an enzyme, KfAOS, that is similar to PpAOS1 (37% identity), and converts 13-hydroperoxide of linolenic acid to 12-oxo-phytodienoic acid in a coupled reaction with allene oxide cyclase. Phylogenetic analysis showed two evolutionarily distinct clusters. One cluster comprised AOS and HPL from charophytic algae, liverworts, and mosses, including MpAOSs and KfAOS. The other cluster was formed by angiosperm CYP74. Our results suggest that plant CYP74 enzymes with AOS, HPL, and divinyl ether synthase activities have arisen multiple times and in the two different clades, which occurred prior to the divergence of the flowering plant lineage.

Simultaneous silencing of five lipoxygenase genes increases the contents of α-linolenic and linoleic acids in tomato (Solanum lycopersicum L.) fruits

α-Linolenic and linoleic acids are essential fatty acids (EFAs) for humans and required for maintenance of optimal health, but they cannot be synthesized by the human body and must be obtained from dietary sources. Using TomloxC fragment, TomloxD fragment, and partial TomloxA sequence that is highly identical with TomloxB and TomloxE, a RNAi expression vector was constructed. The construct was used to transform tomato cotyledon explants with the Agrobacterium-mediated co-cultivation method. The real-time reverse transcription polymerase chain reaction analysis showed that the expression of TomloxA, TomloxB, TomloxC, TomloxD, and TomloxE in transgenic tomato plants was drastically repressed, which led to a marked decrease in the levels of lipoxygenase activity. Finally, higher accumulations of the endogenous α-linolenic and linoleic acids were detected in the transgenic tomato fruits, which were 1.65-3.99 and 2.91-4.98 times that of the non-transformed tomato fruits, respectively.

Preventive effect of the microalga Chlamydomonas debaryana on the acute phase of experimental colitis in rats

Inflammatory bowel diseases (IBD) are characterised by chronic uncontrolled inflammation of intestinal mucosa. Diet and nutritional factors have emerged as possible interventions for IBD. Microalgae are rich sources of n-3 PUFA and derived oxylipins. Oxylipins are lipid mediators involved in the resolution of many inflammatory disorders. The aim of the present study was to investigate the effects of the oxylipin-containing biomass of the microalga Chlamydomonas debaryana and its major oxylipin constituent, (9Z,11E,13S,15Z)-13-hydroxyoctadeca-9,11,15-trienoic acid ((13S)-HOTE), on acute 2,4,6-trinitrobenzenesulfonic acid (TNBS)-induced colitis in rats. Lyophilised microalgal biomass and (13S)-HOTE were administered by oral route 48, 24 and 1 h before the induction of colitis and 24 h later, and the rats were killed after 48 h. The treatment with the lyophilised microalga and (13S)-HOTE improved body-weight loss and colon shortening, as well as attenuated the extent of colonic damage and increased mucus production. Cellular neutrophil infiltration, with the subsequent increase in myeloperoxidase levels induced by TNBS, were also reduced after the administration of the lyophilised microalga or (13S)-HOTE. The anti-inflammatory effects of these treatments were confirmed by the inhibition of colonic TNF-α production. Moreover, lyophilised microalga or (13S)-HOTE down-regulated cyclo-oxygenase-2 and inducible nitric oxide synthase expression. The present study was the first to show the prophylactic effects of a lyophilised biomass sample of the microalga C. debaryana and the oxylipin (13S)-HOTE on TNBS-induced acute colitis in rats. Our findings suggest that the microalga C. debaryana or derived oxylipins could be used as nutraceuticals in the treatment of the active phase of IBD.

Oxylipins from the microalgae Chlamydomonas debaryana and Nannochloropsis gaditana and their activity as TNF-α inhibitors

The chemical study of the microalgae Chlamydomonas debaryana and Nannochloropsis gaditana has led to the isolation of oxylipins. The samples of C. debaryana have yielded the compounds (4Z,7Z,9E,11S,13Z)-11-hydroxyhexadeca-4,7,9,13-tetraenoic acid (1), (4Z,7E,9E,13Z)-11-hydroxyhexadeca-4,7,9,13-tetraenoic acid (2), (4Z,6E,10Z,13Z)-8-hydroxyhexadeca-4,6,10,13-tetraenoic acid (3), (4Z,8E,10Z,13Z)-7-hydroxyhexadeca-4,8,10,13-tetraenoic acid (4), and (5E,7Z,10Z,13Z)-4-hydroxyhexadeca-5,7,10,13-tetraenoic acid (5), which are derived from the fatty acid 16:4Δ(4,7,10,13) together with the compound (5Z,9Z,11E,15Z)-13-hydroxyoctadeca-5,9,11,15-tetraenoic acid (7) derived from coniferonic acid (18:4Δ(5,9,12,15)). In addition, the known polyunsaturated hydroxy acids 11-HHT (6), (5Z,9Z,11E)-13-hydroxyoctadeca-5,9,11-trienoic acid (8), (13S)-HOTE (9), (9E,11E,15Z)-13-hydroxyoctadeca-9,11,15-trienoic acid (10), 9-HOTE (11), 12-HOTE (12), 16-HOTE (13) and (13S)-HODE (14) have also been obtained. The chemical study of N. gaditana has led to the isolation of the hydroxy acid (15S)-HEPE (15) derived from EPA (20:5Δ(5,8,11,14,17)). The structures of the isolated compounds were established by spectroscopic means. The optical activity displayed by oxylipins 1, 2, 6, 7, 9, 10, 14, and 15 suggests the occurrence of LOX-mediated pathways in C. debaryana and N. gaditana. In anti-inflammatory assays, all the tested compounds inhibited the TNF-α production in LPS-stimulated THP-1 macrophages. The most active oxylipin was the C-16 hydroxy acid 1, which at 25μM caused a 60% decrease of the TNF-α level.

Hydrogen tunneling in a prokaryotic lipoxygenase

A bacterial lipoxygenase (LOX) shows a deuterium kinetic isotope effect (KIE) that is similar in magnitude and temperature dependence to the very large KIE of eukaryotic LOXs. This occurs despite the evolutionary distance, an ~25% smaller catalytic domain, and an increase in Ea of ~11 kcal/mol. Site-specific mutagenesis leads to a protein variant with an Ea similar to that of the prototypic plant LOX, providing possible insight into the origin of evolutionary differences. These findings, which extend the phenomenon of hydrogen tunneling to a prokaryotic LOX, are discussed in the context of a role for protein size and/or flexibility in enzymatic hydrogen tunneling.

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.

The lipoxygenase-dependent oxygenation of lipid body membranes is promoted by a patatin-type phospholipase in cucumber cotyledons

Oilseed germination is characterized by the mobilization of storage lipids as a carbon and energy source for embryonic growth. In addition to storage lipid degradation in germinating oilseeds via the direct action of a triacylglycerol lipase (TGL) on the storage lipids, a second degradation pathway that is dependent on a specific lipid body trilinoleate 13-lipoxygenase (13-LOX) has been proposed in several plant species. The activity of this specific 13-LOX leads first to the formation of ester lipid hydroperoxides. These hydroperoxy fatty acids are then preferentially cleaved off by a TGL and serve as a substrate for glyoxysomal β-oxidation. As a prerequisite for triacylglycerol (TAG) mobilization, a partial degradation of the phospholipid monolayer and/or membrane proteins of the oil body has been discussed. Evidence has now been found for both processes: partial degradation of the proteins caleosin and oleosin was observed and simultaneously a patatin-like protein together with transient phospholipase (PLase) activity could be detected at the oil body membranes during germination. Moreover, in vitro experiments with isolated oil bodies from mature seeds revealed that the formation of 13-LOX-derived lipid peroxides in lipid body membranes is increased after incubation with the purified recombinant patatin-like protein. These experiments suggest that in vivo the degradation of storage lipids in cucumber cotyledons is promoted by the activity of a specific oil body PLase, which leads to an increased decomposition of the oil body membrane by the 13-LOX and thereby TAGs may be better accessible to LOX and TGL.

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.

A bisallylic mini-lipoxygenase from cyanobacterium Cyanothece sp. that has an iron as cofactor

Lipoxygenases are enzymes that are found ubiquitously in higher animals and plants, but have only recently been identified in a number of bacteria. The genome of the diazotrophic unicellular cyanobacterium Cyanothece sp. harbors two genes with homology to lipoxygenases. Here we describe the isolation of one gene, formerly named csplox2. It was cloned, and the protein was expressed in Escherichia coli and purified. The purified enzyme belongs to the group of prokaryotic mini lipoxygenases, because it had a molecular mass of 65 kDa. Interestingly, it catalyzed the conversion of linoleic acid, the only endogenously found polyunsaturated fatty acid, primarily to the bisallylic hydroperoxide 11R-hydroperoxyoctadecadienoic acid. This product had previously only been described for the manganese lipoxygenase from the take all fungus, Gaeumannomyces graminis. By contrast, CspLOX2 was shown to be an iron lipoxygenase. In addition, CspLOX2 formed a mixture of typical conjugated lipoxygenase products, e.g. 9R- and 13S-hydroperoxide. The conversion of linoleic acid took place with a maximum reaction rate of 31 s(-1). Incubation of the enzyme with [(11S)-(2)H]linoleic acid led to the formation of hydroperoxides that had lost the deuterium label, thus suggesting that CspLOX2 catalyzes antarafacial oxygenation as opposed to the mechanism of manganese lipoxygenase. CspLOX2 could also oxidize diarachidonylglycerophosphatidylcholine with similar specificity as the free fatty acid, indicating that binding of the substrate takes place with a "tail-first" orientation. We conclude that CspLOX2 is a novel iron mini-lipoxygenase that catalyzes the formation of bisallylic hydroperoxide as the major product.

Physcomitrella patens has lipoxygenases for both eicosanoid and octadecanoid pathways

Mosses have substantial amounts of long chain C20 polyunsaturated fatty acids, such as arachidonic and eicosapentaenoic acid, in addition to the shorter chain C18 alpha-linolenic and linoleic acids, which are typical substrates of lipoxygenases in flowering plants. To identify the fatty acid substrates used by moss lipoxygenases, eight lipoxygenase genes from Physcomitrella patens were heterologously expressed in Escherichia coli, and then analyzed for lipoxygenase activity using linoleic, alpha-linolenic and arachidonic acids as substrates. Among the eight moss lipoxygenases, only seven were found to be enzymatically active in vitro, two of which selectively used arachidonic acid as the substrate, while the other five preferred alpha-linolenic acid. Based on enzyme assays using a Clark-type oxygen electrode, all of the active lipoxygenases had an optimum pH at 7.0, except for one with highest activity at pH 5.0. HPLC analyses indicated that the two arachidonic acid lipoxygenases form (12S)-hydroperoxy eicosatetraenoic acid as the main product, while the other five lipoxygenases produce mainly (13S)-hydroperoxy octadecatrienoic acid from alpha-linolenic acid. These results suggest that mosses may have both C20 and C18 based oxylipin pathways.

Gamma-linolenic and stearidonic acids are required for basal immunity in Caenorhabditis elegans through their effects on p38 MAP kinase activity

Polyunsaturated fatty acids (PUFAs) form a class of essential micronutrients that play a vital role in development, cardiovascular health, and immunity. The influence of lipids on the immune response is both complex and diverse, with multiple studies pointing to the beneficial effects of long-chain fatty acids in immunity. However, the mechanisms through which PUFAs modulate innate immunity and the effects of PUFA deficiencies on innate immune functions remain to be clarified. Using the Caenorhabditis elegans-Pseudomonas aeruginosa host-pathogen system, we present genetic evidence that a Delta6-desaturase FAT-3, through its two 18-carbon products--gamma-linolenic acid (GLA, 18:3n6) and stearidonic acid (SDA, 18:4n3), but not the 20-carbon PUFAs arachidonic acid (AA, 20:4n6) and eicosapentaenoic acid (EPA, 20:5n3)--is required for basal innate immunity in vivo. Deficiencies in GLA and SDA result in increased susceptibility to bacterial infection, which is associated with reduced basal expression of a number of immune-specific genes--including spp-1, lys-7, and lys-2--that encode antimicrobial peptides. GLA and SDA are required to maintain basal activity of the p38 MAP kinase pathway, which plays important roles in protecting metazoan animals from infections and oxidative stress. Transcriptional and functional analyses of fat-3-regulated genes revealed that fat-3 is required in the intestine to regulate the expression of infection- and stress-response genes, and that distinct sets of genes are specifically required for immune function and oxidative stress response. Our study thus uncovers a mechanism by which these 18-carbon PUFAs affect basal innate immune function and, consequently, the ability of an organism to defend itself against bacterial infections. The conservation of p38 MAP kinase signaling in both stress and immune responses further encourages exploring the function of GLA and SDA in humans.

Properties of a mini 9R-lipoxygenase from Nostoc sp. PCC 7120 and its mutant forms

Lipoxygenases (LOXs) consist of a class of enzymes that catalyze the regio- and stereospecific dioxygenation of polyunsaturated fatty acids. Current reports propose that a conserved glycine residue in the active site of R-lipoxygenases and an alanine residue at the corresponding position in S-lipoxygenases play a crucial role in determining the stereochemistry of the product. Recently, a bifunctional lipoxygenase with a linoleate diol synthase activity from Nostoc sp. PCC7120 with R stereospecificity and the so far unique feature of carrying an alanine instead of the conserved glycine in the position of the sequence determinant for chiral specificity was identified. The recombinant carboxy-terminal domain was purified after expression in Escherichia coli. The ability of the enzyme to use linoleic acid esterified to a bulky phosphatidylcholine molecule as a substrate suggested a tail-fist binding orientation of the substrate. Site directed mutagenesis of the alanine to glycine did not cause alterations in the stereospecificity of the products, while mutation of the alanine to valine or isoleucine modified both regio- and enantioselectivity of the enzyme. Kinetic measurements revealed that substitution of Ala by Gly or Val did not significantly influence the reaction characteristics, while the A162I mutant showed a reduced vmax. Based on the mutagenesis data obtained, we suggest that the existing model for stereocontrol of the lipoxygenase reaction may be expanded to include enzymes that seem to have in general a smaller amino acid in R and a bulkier one in S lipoxygenases at the position that controls stereospecificity.

Transcription of the extended hyp-operon in Nostoc sp. strain PCC 7120

BACKGROUND

The maturation of hydrogenases into active enzymes is a complex process and e.g. a correctly assembled active site requires the involvement of at least seven proteins, encoded by hypABCDEF and a hydrogenase specific protease, encoded either by hupW or hoxW. The N2-fixing cyanobacterium Nostoc sp. strain PCC 7120 may contain both an uptake and a bidirectional hydrogenase. The present study addresses the presence and expression of hyp-genes in Nostoc sp. strain PCC 7120.

RESULTS

RT-PCRs demonstrated that the six hyp-genes together with one ORF may be transcribed as a single operon. Transcriptional start points (TSPs) were identified 280 bp upstream from hypF and 445 bp upstream of hypC, respectively, demonstrating the existence of several transcripts. In addition, five upstream ORFs located in between hupSL, encoding the small and large subunits of the uptake hydrogenase, and the hyp-operon, and two downstream ORFs from the hyp-genes were shown to be part of the same transcript unit. A third TSP was identified 45 bp upstream of asr0689, the first of five ORFs in this operon. The ORFs are annotated as encoding unknown proteins, with the exception of alr0692 which is identified as a NifU-like protein. Orthologues of the four ORFs asr0689-alr0692, with a highly conserved genomic arrangement positioned between hupSL, and the hyp genes are found in several other N2-fixing cyanobacteria, but are absent in non N2-fixing cyanobacteria with only the bidirectional hydrogenase. Short conserved sequences were found in six intergenic regions of the extended hyp-operon, appearing between 11 and 79 times in the genome.

CONCLUSION

This study demonstrated that five ORFs upstream of the hyp-gene cluster are co-transcribed with the hyp-genes, and identified three TSPs in the extended hyp-gene cluster in Nostoc sp. strain PCC 7120. This may indicate a function related to the assembly of a functional uptake hydrogenase, hypothetically in the assembly of the small subunit of the enzyme.

A lipoxygenase with linoleate diol synthase activity from Nostoc sp. PCC 7120

The dioxygenation of PUFAs (polyunsaturated fatty acids) in plants is mainly catalysed by members of the LOX (lipoxygenase) enzyme family. LOX products may be further metabolized, and are known as signalling substances in plant development and in responses to wounding and pathogen attack. In contrast with the situation in eukaryotes, information on the relevance of lipid peroxide metabolism in prokaryotic organisms is scarce. Therefore, we aimed to analyse LOXs and oxylipin patterns of cyanobacterial origin. A search of the genomic sequence of the cyanobacterium Nostoc sp. PCC 7120 suggested an open reading frame encoding a putative LOX named NspLOX that harboured an N-terminal extension. Individual analysis of recombinant C-terminal domain revealed enzymatic activity as a linoleate (9R)-LOX. Analysis of the full-length NspLOX protein, however, revealed linoleate diol synthase activity, generating (10E,12E)-9,14-dihydroxy-10,12-octadecadienoic acid as the main product from LA (linoleic acid) and (10E,12E,14E)-9,16-dihydroxy-10,12,14-octadecatrienoic acid as the main product from ALA (α-LA) substrates respectively, with ALA as preferred substrate. The enzyme exhibited a broad pH optimum between pH 7 and pH 10. Soluble extracts of Nostoc sp. contain more 9-LOX-derived hydroperoxides in sonified than in non-sonified cells, but products of full-length NspLOX were not detectable under the conditions used. As no other LOX-like sequence was identified in the genome of Nostoc sp. PCC 7120, the results presented suggest that (9R)-LOX-derived oxylipins may represent the endogenous products of NspLOX. Based on the biochemical results of NspLOX, we suggest that this bifunctional enzyme may represent a more ancient way to control the intracellular amount of oxylipins in this cyanobacterium.

A 49-kDa mini-lipoxygenase from Anabaena sp. PCC 7120 retains catalytically complete functionality

Anabaena sp. PCC 7120 is one of the few prokaryotes harboring a lipoxygenase (LOX) gene. The sequence resides in an open reading frame encoding a fusion protein of a catalase-like hemoprotein with an unusually short LOX (approximately 49 kDa) at the C terminus. The recombinant mini-LOX contains a non-heme iron in the active site and is highly active with linoleic and alpha-linolenic acids (which occur naturally in Anabaena) giving the respective 9R-hydroperoxides, the mirror image of the 9S-LOX products of plants. Using stereospecifically labeled [11-(3)H]linoleic acids we show that reaction is catalyzed via a typical antarafacial relationship of initial hydrogen abstraction and oxygenation. The mini-LOX oxygenated C16/C18:2-phosphatidylcholine with 9R specificity, suggesting a "tail first" mode of fatty acid binding. Site-directed mutagenesis of an active site Ala (Ala215), typically conserved as Gly in R-LOX, revealed that substitution with Gly retained 9R specificity, whereas the larger Val substitution switched oxygenation to 13S, implying that Ala215 represents the functional equivalent of the Gly in other R-LOX. Metabolism studies using a synthetic fatty acid with extended double bond conjugation, 9E,11Z,14Z-20:3omega6, showed that the mini-LOX can control oxygenation two positions further along the fatty acid carbon chain. We conclude that the mini-LOX, despite lacking the beta-barrel domain and much additional sequence, is catalytically complete. Interestingly, animal and plant LOX, which undoubtedly share a common ancestor, are related in sequence only in the catalytic domain; it is possible that the prokaryotic LOX represents a common link and that the beta-barrel domain was then acquired independently in the animal and plant kingdoms.