A bifunctional delta-fatty acyl acetylenase/desaturase from the moss Ceratodon purpureus. A new member of the cytochrome b5 superfamily

Fatty Acid Profiles of Some Siberian Bryophytes and Prospects of Their Use in Chemotaxonomy

The composition of fatty acids (FAs) in gametophyte samples of 20 Siberian bryophyte species from four orders of mosses and four orders of liverworts collected in relatively cold months (April and/or October) was examined. FA profiles were obtained using gas chromatography. Thirty-seven FAs were found, from 12:0 to 26:0; they included mono-, polyunsaturated (PUFAs) and rare FAs, such as 22:5n-3 and two acetylenic FAs, 6a,9,12-18:3 and 6a,9,12,15-18:4 (dicranin). Acetylenic FAs were found in all examined species of the Bryales and Dicranales orders, dicranin being the predominant FA. The role of particular PUFAs in mosses and liverworts is discussed. Multivariate discriminant analysis (MDA) was performed to determine whether FAs can be used in the chemotaxonomy of bryophytes. Based on the MDA results, FA composition is related to the taxonomic status of species. Thus, several individual FAs were identified as chemotaxonomic markers at the level of bryophyte orders. These were 18:3n-3; 18:4n-3; 6a,9,12-18:3; 6a,9,12,15-18:4; 20:4n-3 and EPA in mosses and 16:3n-3; 16:2n-6; 18:2n-6; 18:3n-3 and EPA in liverworts. These findings indicate that further research into bryophyte FA profiles can shed light on phylogenetic relationships within this group of plants and the evolution of their metabolic pathways.

The Current Developments in Medicinal Plant Genomics Enabled the Diversification of Secondary Metabolites' Biosynthesis

Medicinal plants produce important substrates for their adaptation and defenses against environmental factors and, at the same time, are used for traditional medicine and industrial additives. Plants have relatively little in the way of secondary metabolites via biosynthesis. Recently, the whole-genome sequencing of medicinal plants and the identification of secondary metabolite production were revolutionized by the rapid development and cheap cost of sequencing technology. Advances in functional genomics, such as transcriptomics, proteomics, and metabolomics, pave the way for discoveries in secondary metabolites and related key genes. The multi-omics approaches can offer tremendous insight into the variety, distribution, and development of biosynthetic gene clusters (BGCs). Although many reviews have reported on the plant and medicinal plant genome, chemistry, and pharmacology, there is no review giving a comprehensive report about the medicinal plant genome and multi-omics approaches to study the biosynthesis pathway of secondary metabolites. Here, we introduce the medicinal plant genome and the application of multi-omics tools for identifying genes related to the biosynthesis pathway of secondary metabolites. Moreover, we explore comparative genomics and polyploidy for gene family analysis in medicinal plants. This study promotes medicinal plant genomics, which contributes to the biosynthesis and screening of plant substrates and plant-based drugs and prompts the research efficiency of traditional medicine.

Scedosporium Cell Wall: From Carbohydrate-Containing Structures to Host-Pathogen Interactions

Scedosporium species are filamentous fungi usually found in sewage and soil from human-impacted areas. They cause a wide range of diseases in humans, from superficial infections, such as mycetoma, to invasive and disseminated cases, especially associated in immunocompromised patients. Scedosporium species are also related to lung colonization in individuals presenting cystic fibrosis and are considered one of the most frequent fungal pathogens associated to this pathology. Scedosporium cell wall contains glycosylated molecules involved in important biological events related to virulence and pathogenicity and represents a significant source of antigens. Polysaccharides, peptidopolysaccharides, O-linked oligosaccharides and glycosphingolipids have been identified on the Scedosporium surface. Their primary structures were determined based on a combination of techniques including gas chromatography, ESI-MS, and ¹H and ¹³C nuclear magnetic resonance. Peptidorhamnnomannans are common cell wall components among Scedosporium species. Comparing different species, minor structural differences in the carbohydrate portions were detected which could be useful to understand variations in virulence observed among the species. N- and O-linked peptidorhamnomannans are major pathogen-associated molecular patterns and, along with α-glucans, play important roles in triggering host innate immunity. Glycosphingolipids, such as glucosylceramides, have highly conserved structures in Scedosporium species and are crucial for fungal growth and virulence. The present review presents current knowledge on structural and functional aspects of Scedosporium glycoconjugates that are relevant for understanding pathogenicity mechanisms and could contribute to the design of new agents capable of inhibiting growth and differentiation of Scedosporium species. Other cell components such as melanin and ectophosphatases will be also included.

Identification of a crucial amino acid implicated in the hydroxylation/desaturation ratio of CpFAH12 bifunctional hydroxylase

Claviceps purpurea bifunctional Δ12-hydroxylase/desaturase, CpFAH12, and monofunctional desaturase CpFAD2, share 86% of sequence identity. To identify the underlying determinants of the hydroxylation/desaturation specificity, chimeras of these two enzymes were tested for their fatty acid production in an engineered Yarrowia lipolytica strain. It reveals that transmembrane helices are not involved in the hydroxylation/desaturation specificity whereas all cytosolic domains have an impact on it. Especially, replacing the CpFAH12 cytosolic part near the second histidine-box by the corresponding CpFAD2 part annihilates all hydroxylation activity. Further mutagenesis experiments within this domain identified isoleucine 198 as the crucial element for the hydroxylation activity of CpFAH12. Monofunctional variants performing the only desaturation were obtained when this position was exchanged by the threonine of CpFAD2. Saturation mutagenesis at this position showed modulation in the hydroxylation/desaturation specificity in the different variants. The WT enzyme was demonstrated as the most efficient for ricinoleic acid production and some variants showed a better desaturation activity. A model based on the recently discovered membrane desaturase structures indicate that these changes in specificity are more likely due to modifications in the di-iron center geometry rather than changes in the substrate binding mode.

Sphingolipidomics: An Important Mechanistic Tool for Studying Fungal Pathogens

Sphingolipids form of a unique and complex group of bioactive lipids in fungi. Structurally, sphingolipids of fungi are quite diverse with unique differences in the sphingoid backbone, amide linked fatty acyl chain and the polar head group. Two of the most studied and conserved sphingolipid classes in fungi are the glucosyl- or galactosyl-ceramides and the phosphorylinositol containing phytoceramides. Comprehensive structural characterization and quantification of these lipids is largely based on advanced analytical mass spectrometry based lipidomic methods. While separation of complex lipid mixtures is achieved through high performance liquid chromatography, the soft - electrospray ionization tandem mass spectrometry allows a high sensitivity and selectivity of detection. Herein, we present an overview of lipid extraction, chromatographic separation and mass spectrometry employed in qualitative and quantitative sphingolipidomics in fungi.

Identification of amino acid residues that determine the substrate specificity of mammalian membrane-bound front-end fatty acid desaturases

Membrane-bound desaturases are physiologically and industrially important enzymes that are involved in the production of diverse fatty acids such as polyunsaturated fatty acids and their derivatives. Here, we identified amino acid residues that determine the substrate specificity of rat Δ6 desaturase (D6d) acting on linoleoyl-CoA by comparing its amino acid sequence with that of Δ5 desaturase (D5d), which converts dihomo-γ-linolenoyl-CoA. The N-terminal cytochrome b5-like domain was excluded as a determinant by domain swapping analysis. Substitution of eight amino acid residues (Ser209, Asn211, Arg216, Ser235, Leu236, Trp244, Gln245, and Val344) of D6d with the corresponding residues of D5d by site-directed mutagenesis switched the substrate specificity from linoleoyl-CoA to dihomo-γ-linolenoyl-CoA. In addition, replacement of Leu323 of D6d with Phe323 on the basis of the amino acid sequence of zebra fish Δ5/6 bifunctional desaturase was found to render D6d bifunctional. Homology modeling of D6d using recent crystal structure data of human stearoyl-CoA (Δ9) desaturase revealed that Arg216, Trp244, Gln245, and Leu323 are located near the substrate-binding pocket. To our knowledge, this is the first report on the structural basis of the substrate specificity of a mammalian front-end fatty acid desaturase, which will aid in efficient production of value-added fatty acids.

Modern plant metabolomics: advanced natural product gene discoveries, improved technologies, and future prospects

Plant metabolomics has matured and modern plant metabolomics has accelerated gene discoveries and the elucidation of a variety of plant natural product biosynthetic pathways. This review covers the approximate period of 2000 to 2014, and highlights specific examples of the discovery and characterization of novel genes and enzymes associated with the biosynthesis of natural products such as flavonoids, glucosinolates, terpenoids, and alkaloids. Additional examples of the integration of metabolomics with genome-based functional characterizations of plant natural products that are important to modern pharmaceutical technology are also reviewed. This article also provides a substantial review of recent technical advances in mass spectrometry imaging, nuclear magnetic resonance imaging, integrated LC-MS-SPE-NMR for metabolite identifications, and X-ray crystallography of microgram quantities for structural determinations. The review closes with a discussion on the future prospects of metabolomics related to crop species and herbal medicine.

Diversity of Δ12 fatty acid desaturases in santalaceae and their role in production of seed oil acetylenic fatty acids

Plants in the Santalaceae family, including the native cherry Exocarpos cupressiformis and sweet quandong Santalum acuminatum, accumulate ximenynic acid (trans-11-octadecen-9-ynoic acid) in their seed oil and conjugated polyacetylenic fatty acids in root tissue. Twelve full-length genes coding for microsomal Δ12 fatty acid desaturases (FADs) from the two Santalaceae species were identified by degenerate PCR. Phylogenetic analysis of the predicted amino acid sequences placed five Santalaceae FADs with Δ12 FADs, which include Arabidopsis thaliana FAD2. When expressed in yeast, the major activity of these genes was Δ12 desaturation of oleic acid, but unusual activities were also observed: i.e. Δ15 desaturation of linoleic acid as well as trans-Δ12 and trans-Δ11 desaturations of stearolic acid (9-octadecynoic acid). The trans-12-octadecen-9-ynoic acid product was also detected in quandong seed oil. The two other FAD groups (FADX and FADY) were present in both species; in a phylogenetic tree of microsomal FAD enzymes, FADX and FADY formed a unique clade, suggesting that are highly divergent. The FADX group enzymes had no detectable Δ12 FAD activity but instead catalyzed cis-Δ13 desaturation of stearolic acid when expressed in yeast. No products were detected for the FADY group when expressed recombinantly. Quantitative PCR analysis showed that the FADY genes were expressed in leaf rather than developing seed of the native cherry. FADs with promiscuous and unique activities have been identified in Santalaceae and explain the origin of some of the unusual lipids found in this plant family.

Cytochrome b₅ coexpression increases Tetrahymena thermophila Δ6 fatty acid desaturase activity in Saccharomyces cerevisiae

Very-long-chain polyunsaturated fatty acids such as arachidonic, eicosapentaenoic, and docosahexaenoic acids, are important to the physiology of many microorganisms and metazoans and are vital to human development and health. The production of these and related fatty acids depends on Δ6 desaturases, the final components of an electron transfer chain that introduces double bonds into 18-carbon fatty acid chains. When a Δ6 desaturase identified from the ciliated protist Tetrahymena thermophila was expressed in Saccharomyces cerevisiae cultures supplemented with the 18:2(Δ9,12) substrate, only 4% of the incorporated substrate was desaturated. Cytochrome b₅ protein sequences identified from the genome of T. thermophila included one sequence with two conserved cytochrome b₅ domains. Desaturation by the Δ6 enzyme increased as much as 10-fold when T. thermophila cytochrome b₅s were coexpressed with the desaturase. Coexpression of a cytochrome b₅ from Arabidopsis thaliana with the Δ6 enzyme also increased desaturation. A split ubiquitin growth assay indicated that the strength of interaction between cytochrome b₅ proteins and the desaturase plays a vital role in fatty acid desaturase activity, illustrating the importance of protein-protein interactions in this enzyme activity.

A global approach to analysis and interpretation of metabolic data for plant natural product discovery

Discovering molecular components and their functionality is key to the development of hypotheses concerning the organization and regulation of metabolic networks. The iterative experimental testing of such hypotheses is the trajectory that can ultimately enable accurate computational modelling and prediction of metabolic outcomes. This information can be particularly important for understanding the biology of natural products, whose metabolism itself is often only poorly defined. Here, we describe factors that must be in place to optimize the use of metabolomics in predictive biology. A key to achieving this vision is a collection of accurate time-resolved and spatially defined metabolite abundance data and associated metadata. One formidable challenge associated with metabolite profiling is the complexity and analytical limits associated with comprehensively determining the metabolome of an organism. Further, for metabolomics data to be efficiently used by the research community, it must be curated in publicly available metabolomics databases. Such databases require clear, consistent formats, easy access to data and metadata, data download, and accessible computational tools to integrate genome system-scale datasets. Although transcriptomics and proteomics integrate the linear predictive power of the genome, the metabolome represents the nonlinear, final biochemical products of the genome, which results from the intricate system(s) that regulate genome expression. For example, the relationship of metabolomics data to the metabolic network is confounded by redundant connections between metabolites and gene-products. However, connections among metabolites are predictable through the rules of chemistry. Therefore, enhancing the ability to integrate the metabolome with anchor-points in the transcriptome and proteome will enhance the predictive power of genomics data. We detail a public database repository for metabolomics, tools and approaches for statistical analysis of metabolomics data, and methods for integrating these datasets with transcriptomic data to create hypotheses concerning specialized metabolisms that generate the diversity in natural product chemistry. We discuss the importance of close collaborations among biologists, chemists, computer scientists and statisticians throughout the development of such integrated metabolism-centric databases and software.

Metabolic engineering of plant oils and waxes for use as industrial feedstocks

Society has come to rely heavily on mineral oil for both energy and petrochemical needs. Plant lipids are uniquely suited to serve as a renewable source of high-value fatty acids for use as chemical feedstocks and as a substitute for current petrochemicals. Despite the broad variety of acyl structures encountered in nature and the cloning of many genes involved in their biosynthesis, attempts at engineering economic levels of specialty industrial fatty acids in major oilseed crops have so far met with only limited success. Much of the progress has been hampered by an incomplete knowledge of the fatty acid biosynthesis and accumulation pathways. This review covers new insights based on metabolic flux and reverse engineering studies that have changed our view of plant oil synthesis from a mostly linear process to instead an intricate network with acyl fluxes differing between plant species. These insights are leading to new strategies for high-level production of industrial fatty acids and waxes. Furthermore, progress in increasing the levels of oil and wax structures in storage and vegetative tissues has the potential to yield novel lipid production platforms. The challenge and opportunity for the next decade will be to marry these technologies when engineering current and new crops for the sustainable production of oil and wax feedstocks.

A large and functionally diverse family of Fad2 genes in safflower (Carthamus tinctorius L.)

BACKGROUND

The application and nutritional value of vegetable oil is highly dependent on its fatty acid composition, especially the relative proportion of its two major fatty acids, i.e oleic acid and linoleic acid. Microsomal oleoyl phosphatidylcholine desaturase encoded by FAD2 gene is known to introduce a double bond at the Δ12 position of an oleic acid on phosphatidylcholine and convert it to linoleic acid. The known plant FAD2 enzymes are encoded by small gene families consisting of 1-4 members. In addition to the classic oleate Δ12-desaturation activity, functional variants of FAD2 that are capable of undertaking additional or alternative acyl modifications have also been reported in a limited number of plant species. In this study, our objective was to identify FAD2 genes from safflower and analyse their differential expression profile and potentially diversified functionality.

RESULTS

We report here the characterization and functional expression of an exceptionally large FAD2 gene family from safflower, and the temporal and spatial expression profiles of these genes as revealed through Real-Time quantitative PCR. The diversified functionalities of some of the safflower FAD2 gene family members were demonstrated by ectopic expression in yeast and transient expression in Nicotiana benthamiana leaves. CtFAD2-1 and CtFAD2-10 were demonstrated to be oleate desaturases specifically expressed in developing seeds and flower head, respectively, while CtFAD2-2 appears to have relatively low oleate desaturation activity throughout the plant. CtFAD2-5 and CtFAD2-8 are specifically expressed in root tissues, while CtFAD2-3, 4, 6, 7 are mostly expressed in the cotyledons and hypocotyls in young safflower seedlings. CtFAD2-9 was found to encode a novel desaturase operating on C16:1 substrate. CtFAD2-11 is a tri-functional enzyme able to introduce a carbon double bond in either cis or trans configuration, or a carbon triple (acetylenic) bond at the Δ12 position.

CONCLUSIONS

In this study, we isolated an unusually large FAD2 gene family with 11 members from safflower. The seed expressed FAD2 oleate Δ12 desaturase genes identified in this study will provide candidate targets to manipulate the oleic acid level in safflower seed oil. Further, the divergent FAD2 enzymes with novel functionality could be used to produce rare fatty acids, such as crepenynic acid, in genetically engineered crop plants that are precursors for economically important phytoalexins and oleochemical products.

The front-end desaturase: structure, function, evolution and biotechnological use

Very long chain polyunsaturated fatty acids such as arachidonic acid (ARA, 20:4n-6), eicosapentaenoic acid (EPA, 20:5n-3), docosapentaenoic acid (DPA, 22:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) are essential components of cell membranes, and are precursors for a group of hormone-like bioactive compounds (eicosanoids and docosanoids) involved in regulation of various physiological activities in animals and humans. The biosynthesis of these fatty acids involves an alternating process of fatty acid desaturation and elongation. The desaturation is catalyzed by a unique class of oxygenases called front-end desaturases that introduce double bonds between the pre-existing double bond and the carboxyl end of polyunsaturated fatty acids. The first gene encoding a front-end desaturase was cloned in 1993 from cyanobacteria. Since then, front-end desaturases have been identified and characterized from a wide range of eukaryotic species including algae, protozoa, fungi, plants and animals including humans. Unlike front-end desaturases from bacteria, those from eukaryotes are structurally characterized by the presence of an N-terminal cytochrome b₅-like domain fused to the main desaturation domain. Understanding the structure, function and evolution of front-end desaturases, as well as their roles in the biosynthesis of very long chain polyunsaturated fatty acids offers the opportunity to engineer production of these fatty acids in transgenic oilseed plants for nutraceutical markets.

Functional diversity in fungal fatty acid synthesis: the first acetylenase from the Pacific golden chanterelle, Cantharellus formosus

Acetylenic specialized metabolites containing one or more carbon-carbon triple bonds are widespread, being found in fungi, vascular and lower plants, marine sponges and algae, and insects. Plants, moss, and most recently, insects, have been shown to employ an energetically difficult, sequential dehydrogenation mechanism for acetylenic bond formation. Here, we describe the cloning and heterologous expression in yeast of a linoleoyl 12-desaturase (acetylenase) and a bifunctional desaturase with Delta(12)-/Delta(14)-regiospecificity from the Pacific golden chanterelle. The acetylenase gene, which is the first identified from a fungus, is phylogenetically distinct from known plant and fungal desaturases. Together, the bifunctional desaturase and the acetylenase provide the enzymatic activities required to drive oleate through linoleate to crepenynate and the conjugated enyne (14Z)-dehydrocrepenynate, the branchpoint precursors to a major class of acetylenic natural products.

Identification and functional analysis of the genes encoding Delta6-desaturase from Ribes nigrum

Gamma-linolenic acid (gamma-linolenic acid, GLA; C18:3 Delta(6, 9, 12)) belongs to the omega-6 family and exists primarily in several plant oils, such as evening primrose oil, blackcurrant oil, and borage oil. Delta(6)-desaturase is a key enzyme involved in the synthesis of GLA. There have been no previous reports on the genes encoding Delta(6)-desaturase in blackcurrant (Ribes nigrum L.). In this research, five nearly identical copies of Delta(6)-desaturase gene-like sequences, named RnD8A, RnD8B, RnD6C, RnD6D, and RnD6E, were isolated from blackcurrant. Heterologous expression in Saccharomyces cerevisiae and/or Arabidopsis thaliana confirmed that RnD6C/D/E were Delta(6)-desaturases that could use both alpha-linolenic acids (ALA; C18:3 Delta(9,12,15)) and linoleic acid (LA; C18:2 Delta(9,12)) precursors in vivo, whereas RnD8A/B were Delta(8)-sphingolipid desaturases. Expression of GFP tagged with RnD6C/D/E showed that blackcurrant Delta(6)-desaturases were located in the mitochondrion (MIT) in yeast and the endoplasmic reticulum (ER) in tobacco. GC-MS results showed that blackcurrant accumulated GLA and octadecatetraenoic acids (OTA; C18:4 Delta(6,9,12,15)) mainly in seeds and a little in other organs and tissues. RT-PCR results showed that RnD6C and RnD6E were expressed in all the tissues at a low level, whereas RnD6D was expressed at a high level only in seeds, leading to the accumulation of GLA and OTA in seeds. This research provides new insights to our understanding of GLA synthesis and accumulation in plants and the evolutionary relationship of this class of desaturases, and new clues as to the amino acid determinants which define precise enzyme activity.

Dietary stearidonic acid is a long chain (n-3) polyunsaturated fatty acid with potential health benefits

The therapeutic and health-promoting effects of (n-3) long-chain PUFA (LCPUFA) from fish are well known, although these same benefits may not be shared by their precursor, alpha-linolenic acid (ALA). World-wide agencies and scientific organizations (i.e. FDA, AHA, International Society for the Study of Fatty Acids and Lipids, Institute of Medicine, WHO, etc.) have made similar dietary recommendations for (n-3) LCPUFA; however, due to concerns regarding the safety of consuming fish, alternative sources of (n-3) LCPUFA are being investigated. One such lipid is stearidonic acid (SDA), a naturally occurring (n-3) PUFA that may have similar biological properties to eicosapentaenoic acid (EPA), a major (n-3) PUFA in fish oil. Existing and novel plant sources rich in SDA are being cultivated and promoted as potential alternatives to marine-based (n-3) PUFA. This critical review provides a direct comparison of SDA with other dietary (n-3) PUFA under similar experimental conditions. The comparative results suggest that SDA shares many of the biological effects of (n-3) LCPUFA and functions most similarly to dietary EPA compared with ALA when consumed in a typical Western diet. Therefore, although SDA may not replace fish as a major dietary source of (n-3) LCPUFA, it could become a prominent surrogate for EPA in the commercial development of foods fortified with (n-3) PUFA.

Biosynthesis and function of polyacetylenes and allied natural products

Polyacetylenic natural products are a substantial class of often unstable compounds containing a unique carbon-carbon triple bond functionality, that are intriguing for their wide variety of biochemical and ecological functions, economic potential, and surprising mode of biosynthesis. Isotopic tracer experiments between 1960 and 1990 demonstrated that the majority of these compounds are derived from fatty acid and polyketide precursors. During the past decade, research into the metabolism of polyacetylenes has swiftly advanced, driven by the cloning of the first genes responsible for polyacetylene biosynthesis in plants, moss, fungi, and actinomycetes and the initial characterization of the gene products. The current state of knowledge of the biochemistry and molecular genetics of polyacetylenic secondary metabolic pathways will be presented together with an up-to-date survey of new terrestrial and marine natural products, their known biological activities, and a discussion of their likely metabolic origins.

A multifunctional desaturase involved in the biosynthesis of the processionary moth sex pheromone

The sex pheromone of the female processionary moth, Thaumetopoea pityocampa, is a unique C16 enyne acetate that is biosynthesized from palmitic acid. Three consecutive desaturation reactions transform this saturated precursor into the triunsaturated fatty acyl intermediate: formation of (Z)-11-hexadecenoic acid, acetylenation to 11-hexadecynoic acid, and final Delta(13) desaturation to (Z)-13-hexadecen-11-ynoic acid. By using degenerate primers common to all reported insect desaturases, a single cDNA sequence was isolated from total RNA of T. pityocampa female pheromone glands. The full-length transcript of this putative desaturase was expressed in elo1Delta/ole1Delta yeast mutants (both elongase 1 and Delta(9) desaturase-deficient) for functional assays. The construct fully rescued the Deltaole1 yeast phenotype, confirming its desaturase activity. Analysis of the unsaturated products from transformed yeast extracts demonstrated that the cloned enzyme showed Delta(11) desaturase, Delta(11) acetylenase, and Delta(13) desaturase activities. Therefore, this single desaturase may account for the three desaturation steps involved in the sex pheromone biosynthetic pathway of the processionary moth.

Identification and functional characterization of the delta 6-fatty acid desaturase gene from Thamnidium elegans

A cDNA sequence was cloned from the filamentous fungus Thamnidium elegans As3.2806 using reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends method (RACE). Sequence analysis indicated that this cDNA sequence has an open reading frame of 1,380 bp, which encodes a 52.4 kDa peptide of 459 amino acids. The designated amino acid sequence has high similarity with that found in fungal delta 6-fatty acid desaturases: it shows three conserved histidine-rich motifs and two hydrophobic domains. A cytochrome b5-like domain was observed at the N-terminus. To elucidate the function of this novel putative desaturase, the open reading frame was cloned into the intracellular expression vector pPIC3.5K and the gene was expressed heterologously in Pichia pastoris. Accumulation of gamma-linolenic acid to the level of 6.83% in total fatty acid demonstrated that the deduced amino acid sequence possesses of delta 6-fatty acid desaturase activity.

An ancient genome duplication contributed to the abundance of metabolic genes in the moss Physcomitrella patens

Background:

Analyses of complete genomes and large collections of gene transcripts have shown that most, if not all seed plants have undergone one or more genome duplications in their evolutionary past.

Results:

In this study, based on a large collection of EST sequences, we provide evidence that the haploid moss Physcomitrella patens is a paleopolyploid as well. Based on the construction of linearized phylogenetic trees we infer the genome duplication to have occurred between 30 and 60 million years ago. Gene Ontology and pathway association of the duplicated genes in P. patens reveal different biases of gene retention compared with seed plants.

Conclusion:

Metabolic genes seem to have been retained in excess following the genome duplication in P. patens. This might, at least partly, explain the versatility of metabolism, as described for P. patens and other mosses, in comparison to other land plants.

A linkage map reveals a complex basis for segregation distortion in an interpopulation cross in the moss Ceratodon purpureus

We report the construction of a linkage map for the moss Ceratodon purpureus (n = 13), based on a cross between geographically distant populations, and provide the first experimental confirmation of maternal chloroplast inheritance in bryophytes. From a mapping population of 288 recombinant haploid gametophytes, genotyped at 121 polymorphic AFLP loci, three gene-based nuclear loci, one chloroplast marker, and sex, we resolved 15 linkage groups resulting in a map length of approximately 730 cM. We estimate that the map covers more than three-quarters of the C. purpureus genome. Approximately 35% of the loci were sex linked, not including those in recombining pseudoautosomal regions. Nearly 45% of the loci exhibited significant segregation distortion (alpha = 0.05). Several pairs of unlinked distorted loci showed significant deviations from multiplicative genotypic frequencies, suggesting that distortion arises from genetic interactions among loci. The distorted autosomal loci all exhibited an excess of the maternal allele, suggesting that these interactions may involve nuclear-cytoplasmic factors. The sex ratio of the progeny was significantly male biased, and the pattern of nonrandom associations among loci indicates that this results from interactions between the sex chromosomes. These results suggest that even in interpopulation crosses, multiple mechanisms act to influence segregation ratios.

A small membrane-peripheral region close to the active center determines regioselectivity of membrane-bound fatty acid desaturases from Aspergillus nidulans

Fatty acid desaturases catalyze the introduction of double bonds at specific positions of an acyl chain and are categorized according to their substrate specificity and regioselectivity. The current understanding of membrane-bound desaturases is based on mutant studies, biochemical topology analysis, and the comparison of related enzymes with divergent functionality. Because structural information is lacking, the principles of membrane-bound desaturase specificity are still not understood despite of substantial research efforts. Here we compare two membrane-bound fatty acid desaturases from Aspergillus nidulans: a strictly monofunctional oleoyl-Delta12 desaturase and a processive bifunctional oleoyl-Delta12/linoleoyl-omega3 desaturase. The high similarities in the primary sequences of the enzymes provide an ideal starting point for the systematic analysis of factors determining substrate specificity and bifunctionality. Based on the most current topology models, both desaturases were divided into nine domains, and the domains of the monofunctional Delta12 desaturase were systematically exchanged for their respective corresponding matches of the bifunctional sister enzyme. Catalytic capacities of hybrid enzymes were tested by heterologous expression in yeast, followed by biochemical characterization of the resulting fatty acid patterns. The individual exchange of two domains of a length of 18 or 49 amino acids each resulted in bifunctional Delta12/omega3 activity of the previously monofunctional parental enzyme. Sufficient determinants of fatty acid desaturase substrate specificity and bifunctionality could, thus, be narrowed down to a membrane-peripheral region close to the catalytic site defined by conserved histidine-rich motifs in the topology model.

Synthesis and use of deuterated palmitic acids to decipher the cryptoregiochemistry of a Delta13 desaturation

The synthesis of two hexadeuterated palmitic acids differing in the position of the diagnostic labels, and their use to decipher the cryptoregiochemistry of a Delta13 desaturation are described. A dithiane and a triple bond functionalities were used to introduce the diagnostic (C13 or C14) and tagging (C8 and C9) labels, respectively, in the palmitic acid skeleton. Using these probes, the cryptoregiochemistry of the Delta13 desaturation involved in the biosynthesis of Thaumetopoea pityocampa sex pheromone was studied by means of kinetic isotope effect determinations. Transformation of both (Z)-11-hexadecenoic and 11-hexadecynoic acids into (Z, Z)-11,13-hexadecadienoic and (Z)-13-hexadecen-11-ynoic acids, respectively, is initiated by abstraction of the hydrogen atom at the C13 position, followed by the fast elimination of the C14 hydrogen to give the double bond.

Channeling of eukaryotic diacylglycerol into the biosynthesis of plastidial phosphatidylglycerol

Plastidial glycolipids contain diacylglycerol (DAG) moieties, which are either synthesized in the plastids (prokaryotic lipids) or originate in the extraplastidial compartment (eukaryotic lipids) necessitating their transfer into plastids. In contrast, the only phospholipid in plastids, phosphatidylglycerol (PG), contains exclusively prokaryotic DAG backbones. PG contributes in several ways to the functions of chloroplasts, but it is not known to what extent its prokaryotic nature is required to fulfill these tasks. As a first step toward answering this question, we produced transgenic tobacco plants that contain eukaryotic PG in thylakoids. This was achieved by targeting a bacterial DAG kinase into chloroplasts in which the heterologous enzyme was also incorporated into the envelope fraction. From lipid analysis we conclude that the DAG kinase phosphorylated eukaryotic DAG forming phosphatidic acid, which was converted into PG. This resulted in PG with 2-3 times more eukaryotic than prokaryotic DAG backbones. In the newly formed PG the unique Delta3-trans-double bond, normally confined to 3-trans-hexadecenoic acid, was also found in sn-2-bound cis-unsaturated C18 fatty acids. In addition, a lipidomics technique allowed the characterization of phosphatidic acid, which is assumed to be derived from eukaryotic DAG precursors in the chloroplasts of the transgenic plants. The differences in lipid composition had only minor effects on measured functions of the photosynthetic apparatus, whereas the most obvious phenotype was a significant reduction in growth.

Synthesis and Use of Probes to Investigate the Cryptoregiochemistry of the First Animal Acetylenase

Thaumetopoea pityocampa pheromone glands contain an unusual Delta(11) acetylenase that produces an alkynoic fatty acid intermediate in the sex pheromone biosynthetic pathway of this species. In this article, we describe the synthesis and use of the deuterated (Z)-11-hexadecenoic acid probes required to decipher the cryptoregiochemistry of this enzyme. The label in the olefinic bonds was introduced by Wittig reaction between the appropriate deuterated reagents. Besides the vinyl deuterium atoms, for reliable GC-MS analyses these compounds bear a tetradeuterium tag, which was introduced by deuteration of an alkyne intermediate in the presence of the Wilkinson catalyst. Pheromone gland metabolization studies of these probes provided experimental evidence that the transformation of (Z)-11-hexadecenoic acid into 11-hexadecynoic acid by the Delta(11) acetylenase takes place by a stepwise mechanism, in which a significant perturbation of the strong vinyl C11-H bond occurs prior to a fast elimination of the vinyl hydrogen at C-12.

Agravitropic mutants of the moss Ceratodon purpureus do not complement mutants having a reversed gravitropic response

New mutants of the moss Ceratodon purpureus have been isolated, which showed abnormal gravitropic responses. The apical cells of protonemal filaments of wild-type strains respond to gravity by growing upwards and are well aligned to the gravity vector. This response only occurs in darkness. Mutants show a range of phenotypes. Some are insensitive to gravity, showing symmetrical growth, while others align to the gravity vector but orient growth downwards. A further class grows in darkness as though it were in light, showing insensitivity to gravity and continued chlorophyll synthesis. Somatic hybrids between mutants and wild-type strains and between pairs of mutants have been selected using transgenic antibiotic resistance as selective markers. Hybrids between wild-type strains and all of the mutants have a wild-type phenotype, and so all mutants therefore have recessive phenotypes. Mutants comprise three complementation groups. One group has a single member, while another has three members. The third has at least 16 members and shows a complex pattern of complementation consistent with a single gene product functioning in both orientation and alignment to gravity, as well as contributing more than one subunit to the mature product.

Identification of fungal sphingolipid C9-methyltransferases by phylogenetic profiling

Fungal glucosylceramides play an important role in plant-pathogen interactions enabling plants to recognize the fungal attack and initiate specific defense responses. A prime structural feature distinguishing fungal glucosylceramides from those of plants and animals is a methyl group at the C9-position of the sphingoid base, the biosynthesis of which has never been investigated. Using information on the presence or absence of C9-methylated glucosylceramides in different fungal species, we developed a bioinformatics strategy to identify the gene responsible for the biosynthesis of this C9-methyl group. This phylogenetic profiling allowed the selection of a single candidate out of 24-71 methyltransferase sequences present in each of the fungal species with C9-methylated glucosylceramides. A Pichia pastoris knock-out strain lacking the candidate sphingolipid C9-methyltransferase was generated, and indeed, this strain contained only non-methylated glucosylceramides. In a complementary approach, a Saccharomyces cerevisiae strain was engineered to produce glucosylceramides suitable as a substrate for C9-methylation. C9-methylated sphingolipids were detected in this strain expressing the candidate from P. pastoris, demonstrating its function as a sphingolipid C9-methyltransferase. The enzyme belongs to the superfamily of S-adenosylmethionine-(SAM)-dependent methyltransferases and shows highest sequence similarity to plant and bacterial cyclopropane fatty acid synthases. An in vitro assay showed that sphingolipid C9-methylation is membrane-bound and requires SAM and Delta4,8-desaturated ceramide as substrates.

Fatty acids from lipids of marine organisms: molecular biodiversity, roles as biomarkers, biologically active compounds, and economical aspects

Because of their characteristic living environments, marine organisms produce a variety of lipids. Fatty acids constitute the essential part of triglycerides and wax esters, which are the major components of fats and oils. Nevertheless, phospholipids and glycolipids have considerable importance and will be taken into account, especially the latter compounds that excite increasing interest regarding their promising biological activities. Thus, in addition to the major polyunsaturated fatty acids (PUFA) such as eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, a great number of various fatty acids occur in marine organisms, e.g. saturated, mono- and diunsaturated, branched, halogenated, hydroxylated, methoxylated, non-methylene-interrupted. Various unprecedented chemical structures of fatty acids, and lipid-containing fatty acids, have recently been discovered, especially from the most primitive animals such as sponges and gorgonians. This review of marine lipidology deals with recent advances in the field of fatty acids since the end of the 1990s. Different approaches will be followed, mainly developing biomarkers of trophic chains in marine ecosystems and of chemotaxonomic interest, reporting new structures, especially those with biological activities or biosynthetic interest. An important part of this review will be devoted to the major PUFA, their relevance to health and nutrition, their biosynthesis, their sources (usual and promising) and market.

A multifunctional acyl-acyl carrier protein desaturase from Hedera helix L. (English ivy) can synthesize 16- and 18-carbon monoene and diene products

A desaturase with 83% sequence identity to the coriander delta(4)-16:0-ACP desaturase was isolated from developing seeds of Hedera helix (English ivy). Expression of the ivy desaturase in Arabidopsis resulted in the accumulation of 16:1delta(4) and its expected elongation product 18:1delta(6) (petroselinic acid). Expression in Escherichia coli resulted in the accumulation of soluble, active protein that was purified to apparent homogeneity. In vitro assays confirmed delta(4) desaturation with 16:0-ACP; however, with 18:0-acyl acyl carrier protein (ACP) desaturation occurred at the delta(9) position. The ivy desaturase also converted 16:1delta(9)-ACP and 18:1delta(9)-ACP to the corresponding delta(4,9) dienes. These data suggest at least two distinct substrate binding modes, one placing C4 at the diiron active site and the other placing C9 at the active site. In the latter case, 18:0 would likely bind in an extended conformation as described for the castor desaturase with 9-carbons accommodated in the cavity beyond the dirron site. However, delta(4) desaturation would require the accommodation of 12 carbons for C16 substrates or 14 carbons for C18 substrates. The amino acids lining the substrate binding cavity of ivy and castor desaturases are conserved except for T117R and P179I (castor/ivy). Paradoxically, both substitutions, when introduced into the castor desaturase, favored the binding of shorter acyl chains. Thus, it seems likely that delta(4) desaturation would require a non-extended, perhaps U-shaped, substrate conformation. A cis double bond may facilitate the initiation of such a non-extended conformation in the monounsaturated substrates. The multifunctional properties of the ivy desaturase make it well suited for further dissection of the determinants of regiospecificity.

Substrate specificity and preference of Δ6-desaturase ofMucor rouxii

The Delta(6)-fatty acid desaturase is a key enzyme in the synthesis of an important fatty acid, gamma-linolenic acid. We have characterized, by heterologous expression in Saccharomyces cerevisiae, substrate specificity and preference of Delta(6)-desaturase of Mucor rouxii. Fatty acid supplementation was carried out based on the predicted enzyme topology, fatty acid phenotype and the corresponding metabolic pathway in M. rouxii. The enzyme has a broad substrate specificity as based on C15-C18. The result also supported classification of the M. rouxii Delta(6)-desaturase into a front-end desaturase. Interestingly, a relatively rare activity based on odd acyl chains and not described previously in other eukaryotic Delta(6)-desaturases was also observed.

Genetic modification of essential fatty acids biosynthesis inHansenula polymorpha

The Delta(6)-desaturase gene isoform II involved in the formation of gamma-linolenic acid (GLA) was identified from Mucor rouxii. To study the possibility of alteration of the synthetic pathway of essential fatty acids in the methylotrophic yeast, Hansenula polymorpha, the cloned gene of M. rouxii under the control of the methanol oxidase (MOX) promoter of H. polymorpha, was used for genetic modification of this yeast. Changes in flux through the n-3 and n-6 pathways in the transgenic yeast were observed. The proportion of GLA varied dramatically depending on the growth temperature and media composition. This can be explained by the effects of either substrate availability or enzymatic activity. In addition to the potential application for manipulating the fatty acid profile, this study provides an attractive model system of H. polymorpha for investigating the deviation of fatty acid metabolism in eukaryotes.

Lipid metabolism in arbuscular mycorrhizal roots of Medicago truncatula

The peroxidation of polyunsaturated fatty acids, common to all eukaryotes, is mostly catalyzed by members of the lipoxygenase enzyme family of non-heme iron containing dioxygenases. Lipoxygenase products can be metabolized further in the oxylipin pathway by several groups of CYP74 enzymes. One prominent oxylipin is jasmonic acid (JA), a product of the 13-allene oxide synthase branch of the pathway and known as signaling substance that plays a role in vegetative and propagative plant development as well as in plant responses to wounding and pathogen attack. In barley roots, JA level increases upon colonization by arbuscular mycorrhizal fungi. Apart from this first result regarding JA, no information is available on the relevance of lipidperoxide metabolism in arbuscular mycorrhizal symbiosis. Thus we analyzed fatty acid and lipidperoxide patterns in roots of Medicago truncatula during mycorrhizal colonization. Levels of fungus-specific fatty acids as well as palmitic acid (16:0) and oleic acid (18:1 n - 9) were increased in mycorrhizal roots. Thus the degree of arbuscular mycorrhizal colonization of roots can be estimated via analysis of fungal specific esterified fatty acids. Otherwise, no significant changes were found in the profiles of esterified and free fatty acids. The 9- and 13-LOX products of linoleic and alpha-linolenic acid were present in all root samples, but did not show significant differences between mycorrhizal and non-mycorrhizal roots, except JA which showed elevated levels in mycorrhizal roots. In both types of roots levels of 13-LOX products were higher than those of 9-LOX products. In addition, three cDNAs encoding CYP74 enzymes, two 9/13-hydroperoxide lyases and a 13-allene oxide synthase, were isolated and characterized. The transcript accumulation of these three genes, however, was not increased in mycorrhizal roots of M. truncatula.

Novel fatty acid elongases and their use for the reconstitution of docosahexaenoic acid biosynthesis

In algae, the biosynthesis of docosahexaenoic acid (22:6omega3; DHA) proceeds via the elongation of eicosapentaenoic acid (20:5omega3; EPA) to 22:5omega3, which is required as a substrate for the final Delta4 desaturation. To isolate the elongase specific for this step, we searched expressed sequence tag and genomic databases from the algae Ostreococcus tauri and Thalassiosira pseudonana, from the fish Oncorhynchus mykiss, from the frog Xenopus laevis, and from the sea squirt Ciona intestinalis using as a query the elongase sequence PpPSE1 from the moss Physcomitrella patens. The open reading frames of the identified elongase candidates were expressed in yeast for functional characterization. By this, we identified two types of elongases from O. tauri and T. pseudonana: one specific for the elongation of (Delta6-)C18-PUFAs and one specific for (Delta5-)C20-PUFAs, showing highest activity with EPA. The clones isolated from O. mykiss, X. laevis, and C. intestinalis accepted both C18- and C20-PUFAs. By coexpression of the Delta6- and Delta5-elongases from T. pseudonana and O. tauri, respectively, with the Delta5- and Delta4-desaturases from two other algae we successfully implemented DHA synthesis in stearidonic acid-fed yeast. This may be considered an encouraging first step in future efforts to implement this biosynthetic sequence into transgenic oilseed crops.

Properties of two multifunctional plant fatty acid acetylenase/desaturase enzymes

The properties of the Delta6 desaturase/acetylenase from the moss Ceratodon purpureus and the Delta12 acetylenase from the dicot Crepis alpina were studied by expressing the encoding genes in Arabidopsis thaliana and Saccharomyces cerevisiae. The acetylenase from C. alpinaDelta12 desaturated both oleate and linoleate with about equal efficiency. The desaturation of oleate gave rise to 9(Z),12(E)- and 9(Z),12(Z)-octadecadienoates in a ratio of approximately 3 : 1. Experiments using stereospecifically deuterated oleates showed that the pro-R hydrogen atoms were removed from C-12 and C-13 in the introduction of the 12(Z) double bond, whereas the pro-R and pro-S hydrogen atoms were removed from these carbons during the formation of the 12(E) double bond. The results suggested that the Delta12 acetylenase could accommodate oleate having either a cisoid or transoid conformation of the C(12)-C(13) single bond, and that these conformers served as precursors of the 12(Z) and 12(E) double bonds, respectively. However, only the 9(Z),12(Z)-octadecadienoate isomer could be further desaturated to 9(Z)-octadecen-12-ynoate (crepenynate) by the enzyme. The evolutionarily closely related Delta12 epoxygenase from Crepis palaestina had only weak desaturase activity but could also produce 9(Z),12(E)-octadecadienoate from oleate. The Delta6 acetylenase/desaturase from C. purpureus, on the other hand, produced only the 6(Z) isomers using C16 and C18 acyl groups possessing a Delta9 double bond as substrates. The Delta6 double bond was efficiently further converted to an acetylenic bond by a second round of desaturation but only if the acyl substrate had a Delta12 double bond and that this was in the Z configuration.

Production of very long chain polyunsaturated omega-3 and omega-6 fatty acids in plants

We report the production of two very long chain polyunsaturated fatty acids, arachidonic acid (AA) and eicosapentaenoic acid (EPA), in substantial quantities in a higher plant. This was achieved using genes encoding enzymes participating in the omega3/6 Delta8 -desaturation biosynthetic pathways for the formation of C20 polyunsaturated fatty acids. Arabidopsis thaliana was transformed sequentially with genes encoding a Delta9 -specific elongating activity from Isochrysis galbana, a Delta8 -desaturase from Euglena gracilis and a Delta5 -desaturase from Mortierella alpina. Instrumental in the successful reconstitution of these C20 polyunsaturated fatty acid biosynthetic pathways was the I. galbana C18-Delta9 -elongating activity, which may bypass rate-limiting steps present in the conventional Delta6 -desaturase/elongase pathways. The accumulation of EPA and AA in transgenic plants is a breakthrough in the search for alternative sustainable sources of fish oils.

Eicosapentaenoic acid: biosynthetic routes and the potential for synthesis in transgenic plants

Long chain polyunsaturated fatty acids are now known to play important roles in human health. In particular, eicosapentaenoic acid (20:5Delta(5,8,11,14,17); n-3: EPA) is implicated as a protective agent in a range of pathologies such as cardiovascular disease and Metabolic Syndrome (Syndrome X). Eicosapentaenoic acid is currently sourced from fish oils, the presence of this fatty acid being due to the dietary piscine consumption of EPA-synthesising micro-algae. The biosynthetic pathway of EPA has been elucidated, and contains several alternative metabolic routes. Progress in using "reverse engineering" to transgenically mobilize the trait(s) for EPA are considered. In particular, the prospect of producing this important polyunsaturated fatty acid in transgenic oilseeds is highlighted, as is the urgent need for a sustainable replacement for diminishing fish stocks.

Identification and characterization of a novel Δ6 -fatty acid desaturase gene from Rhizopus arrhizus

A cDNA sequence putatively encoding a Delta(6)-fatty acid desaturase was isolated from Rhizopus arrhizus using reverse transcription polymerase chain reaction and rapid amplification of cDNA ends methods. Sequence analysis indicated that this cDNA sequence had an open reading frame of 1377 bp encoding 458 amino acids of 52 kDa. The deduced amino acid sequence showed high similarity to those of fungal Delta(6)-fatty acid desaturases which comprised the characteristics of membrane-bound desaturases, including three conserved histidine-rich motifs and hydropathy profile. A cytochrome b(5)-like domain was observed at the N-terminus. To elucidate the function of this novel putative desaturase, the coding sequence was expressed heterologously in Saccharomyces cerevisiae strain INVScl. The result demonstrated that the coding product of the sequence exhibited Delta(6)-fatty acid desaturase activity by the accumulation of gamma-linolenic acid.

Acyl carriers used as substrates by the desaturases and elongases involved in very long-chain polyunsaturated fatty acids biosynthesis reconstituted in yeast

The health benefits attributed to very long-chain polyunsaturated fatty acids and the long term goal to produce them in transgenic oilseed crops have led to the cloning of all the genes coding for the desaturases and elongases involved in their biosynthesis. The encoded activities have been confirmed in vivo by heterologous expression, but very little is known about the actual acyl substrates involved in these pathways. Using a Delta 6-elongase and front-end desaturases from different organisms, we have reconstituted in Saccharomyces cerevisiae the biosynthesis of arachidonic acid from exogenously supplied linoleic acid in order to identify these acyl carriers. Acyl-CoA measurements strongly suggest that the elongation step involved in polyunsaturated fatty acids biosynthesis is taking place within the acyl-CoA pool. In contrast, detailed analyses of lipids revealed that the two desaturation steps (Delta 5 and Delta 6) occur predominantly at the sn-2 position of phosphatidylcholine when using Delta 5- and Delta 6-desaturases from lower plants, fungi, worms, and algae. The specificity of these Delta 6-desaturases for the fatty acid acylated at this particular position as well as a limiting re-equilibration with the acyl-CoA pool result in the accumulation of gamma-linolenic acid at the sn-2 position of phosphatidylcholine and prevent efficient arachidonic acid biosynthesis in yeast. We confirm by using a similar experimental approach that, in contrast, the human Delta 6-desaturase uses linoleoyl-CoA as substrate, which results in high efficiency of the subsequent elongation step. In addition, we report that Delta 12-desaturases have no specificity toward the lipid polar headgroup or the sn-position.

Mechanistic study of an improbable reaction: alkene dehydrogenation by the delta12 acetylenase of Crepis alpina

The mechanism by which the fatty acid acetylenase of Crepis alpina catalyzes crepenynic acid ((9Z)-octadeca-9-en-12-ynoic acid) production from linoleic acid has been probed through the use of kinetic isotope effect (KIE) measurements. This was accomplished by incubating appropriate mixtures of regiospecifically deuterated isotopomers with a strain of Saccharomyces cerevisiae expressing a functional acetylenase. LC/MS analysis of crepenynic acid obtained in these experiments showed that the oxidation of linoleate occurs in two discrete steps, since the cleavage of the C12-H bond is very sensitive to isotopic substitution (k(H)/k(D) = 14.6 +/- 3.0) while a minimal isotope effect (k(H)/k(D) = 1.25 +/- 0.08) was observed for the C13-H bond breaking step. These data suggest that crepenynic acid is produced via initial H-atom abstraction at C12 of a linoleoyl substrate. The relationship between the mechanism of enzymatic acetylenation and epoxidation is discussed.

Biosynthesis of docosahexaenoic acid in Euglena gracilis: biochemical and molecular evidence for the involvement of a Delta4-fatty acyl group desaturase

Docosahexaenoic acid (DHA) can be synthesized via alternative routes from which only the omega3/omega6-pathways involve the action of a Delta4-fatty acid desaturase. We examined the suitability of Euglena gracilis, Thraustochytrium sp., Schizochytrium sp., and Crypthecodinium cohnii to serve as sources for cloning a cDNA encoding a Delta4-fatty acid desaturase. For this purpose we carried out in vivo labeling studies with radiolabeled C22 polyunsaturated fatty acid substrates. Schizochytrium sp. was unable to convert exogenously supplied [2-(14)C]-docosapentaenoic acid (DPA, 22:5(Delta)(7,10,13,16,19)) to DHA, while E. gracilis and Thraustochytrium sp. carried out this desaturation very efficiently. Hydrogenation and alpha-oxidation of the labeled DHA isolated from these two organisms showed that it was the result of direct Delta4-desaturation and not of substrate breakdown and resynthesis. To clone the desaturase gene, a cDNA library of E. gracilis was subjected to mass sequencing. A full-length clone with highest homology to the Delta4-desaturase of Thraustochytrium sp. was isolated, and its function was verified by heterologous expression in yeast. The desaturase efficiently converted DPA to DHA. Analysis of the substrate specificity demonstrated that the enzyme activity was not limited to C22 fatty acids, since it also efficiently desaturated C16 fatty acids. The enzyme showed strict Delta4-regioselectivity and required the presence of a Delta7-double bond in the substrate. Positional analysis of phosphatidylcholine revealed that the proportion of the Delta4-desaturated products was up to 20 times higher in the sn-2 position than in the sn-1 position.

Plant sphingolipids: structural diversity, biosynthesis, first genes and functions

In mammals and Saccharomyces cerevisiae, sphingolipids have been a subject of intensive research triggered by the interest in their structural diversity and in mammalian pathophysiology as well as in the availability of yeast mutants and suppressor strains. More recently, sphingolipids have attracted additional interest, because they are emerging as an important class of messenger molecules linked to many different cellular functions. In plants, sphingolipids show structural features differing from those found in animals and fungi, and much less is known about their biosynthesis and function. This review focuses on the sphingolipid modifications found in plants and on recent advances in the functional characterization of genes gaining new insight into plant sphingolipid biosynthesis. Recent studies indicate that plant sphingolipids may be also involved in signal transduction, membrane stability, host-pathogen interactions and stress responses.

Identification of primula fatty acid delta 6-desaturases with n-3 substrate preferences

Fatty acid Delta(6)-desaturation, the first committed step in C(20) polyunsaturated fatty acid biosynthesis, is generally considered not to discriminate between n-3 and n-6 substrates. We previously identified higher plant species that showed preferential Delta(6)-desaturation of n-3 C(18) fatty acid substrates. A polymerase chain reaction-based approach was used to isolate 'front-end' cytochrome b(5) fusion desaturases from Primula vialii Franchet and Primula farinosa L. Functional analysis in Saccharomyces cerevisiae identified fatty acid Delta(6)-desaturases with a strong specificity for the n-3 substrate alpha-linolenic acid (18:3 Delta(9,12,15)). These results indicate that the accumulation of octadecatetraenoic acid (18:4 Delta(6,9,12,15)) in planta is due to the activity of a novel n-3-specific fatty acid Delta(6)-desaturase.

New insight into Phaeodactylum tricornutum fatty acid metabolism. Cloning and functional characterization of plastidial and microsomal delta12-fatty acid desaturases

In contrast to 16:3 plants like rapeseed (Brassica napus), which contain alpha-linolenic acid (18:3(Delta9,12,15)) and hexadecatrienoic acid (16:3(Delta7,10,13)) as major polyunsaturated fatty acids in leaves, the silica-less diatom Phaeodactylum tricornutum contains eicosapentaenoic acid (EPA; 20:5(Delta5,8,11,14,17)) and a different isomer of hexadecatrienoic acid (16:3(Delta6,9,12)). In this report, we describe the characterization of two cDNAs having sequence homology to Delta12-fatty acid desaturases from higher plants. These cDNAs were shown to code for a microsomal and a plastidial Delta12-desaturase (PtFAD2 and PtFAD6, respectively) by heterologous expression in yeast (Saccharomyces cerevisiae) and Synechococcus, respectively. Using these systems in the presence of exogenously supplied fatty acids, the substrate specificities of the two desaturases were determined and compared with those of the corresponding rapeseed enzymes (BnFAD2 and BnFAD6). The microsomal desaturases were similarly specific for oleic acid (18:1(Delta9)), suggesting that PtFAD2 is involved in the biosynthesis of EPA. In contrast, the plastidial desaturase from the higher plant and the diatom clearly differed. Although the rapeseed plastidial desaturase showed high activity toward the omega9-fatty acids 18:1(Delta9) and 16:1(Delta7), in line with the fatty acid composition of rapeseed leaves, the enzyme of P. tricornutum was highly specific for 16:1(Delta9). Our results indicate that in contrast to EPA, which is synthesized in the microsomes, the hexadecatrienoic acid isomer found in P. tricornutum (16:3(Delta6,9,12)) is of plastidial origin.

Comparative sex pherome biosynthesis in Thaumetopoea pityocampa and T. processionea: a rationale for the phenotypic variation in the sex pherome within the genus Thaumetopoea

The female sex pheromones of the Mediterranean processionary moths (Thaumetopoea sp.) are conjugated dienes or enynes of 16 carbon atoms with the unsaturations located at C11 and C13. To investigate the biochemical basis of this phenotypic variation, the biosynthetic pathway of T. processionea sex pheromone, a diene acetate, has been elucidated and compared to that reported for the enyne-producing species T. pityocampa. Mass labeling experiments showed that T. processionea sex pheromone is biosynthesized from palmitic acid, by subsequent (Z)-11 and (Z)-13 desaturations and final reduction and acetylation. The Pheromone Biosynthesis Activating Neuropeptide (PBAN) activates this biosynthetic pathway downstream of the dienoate intermediate. When either 11-hexadecynoic acid or (Z)-13-hexadecen-11-ynoic acid were administered to T. processionea, this species was able to produce the enyne sex pheromone of T. pityocampa upon PBAN stimulation. In contrast, T. pityocampa does not produce either 11-hexadecynyl acetate or (Z,Z)-11,13-hexadecadienyl acetate, despite having the corresponding precursors in the pheromone gland. However, both acetates are detected after administration of the corresponding alcohols. These overall results suggest that the absence of delta(11) acetylenase and the existence of an enynoate specific reductase in the diene and enyne-producing Thaumetopeae, respectively, account for the different sex pheromones produced by the two groups.

The role of cytochrome b5 fusion desaturases in the synthesis of polyunsaturated fatty acids

The biosynthetic pathway of polyunsaturated fatty acids (PUFAs) has been the subject of much interest over the last few years. Significant progress has been made in the identification of the enzymes required for PUFA synthesis; in particular, the fatty acid desaturases which are central to this pathway have now all been identified. These "front-end" desaturases are all members of the cytochrome b(5) fusion desaturase superfamily, since they contain an N-terminal domain that is orthologous to the microsomal cytochrome b(5). Examination of the primary sequence relationships between the various PUFA-specific cytochrome b(5) fusion desaturases and related fusion enzymes allows inferences regarding the evolution of this important enzyme class. More importantly, this knowledge helps underpin our understanding of polyunsaturated fatty acid biosynthesis.