Mutagenesis and heterologous expression in yeast of a plant Delta6-fatty acid desaturase

A complete inventory of long-chain polyunsaturated fatty acid biosynthesis pathway enzymes in the miniaturized cyprinid Paedocypris micromegethes

The capacity for long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis activity in a species depends on the enzymatic activities of fatty acyl desaturase (Fads) and elongation of very long-chain fatty acid (Elovl). The miniaturized fish Paedocypris micromegethes is a developmentally truncated cyprinid living in highly acidic water conditions in tropical peat swamps. The capacity for LC-PUFA biosynthesis in this species, which has a reduced genome size, is unknown. A high-quality de novo transcriptome assembly enabled the identification of a putative Fads2 and four Elovl. The Fads2 was verified as a P. micromegethes Fads2 ortholog with in vitro Δ5 and Δ6 activities. The Elovl sequences were established as an Elovl5, Elovl2, and two Elovl4 paralogs, namely Elovl4a and Elovl4b. These Elovl enzymes, mainly Elovl5 and Elovl2, fulfill the necessary C18, C20, and C22 PUFA elongation steps for LC-PUFA biosynthesis. Collectively, these results validate the presence of a complete repertoire of LC-PUFA biosynthesis enzymes in a peat swamp miniatured freshwater fish.

Consensus mutagenesis and computational simulation provide insight into the desaturation catalytic mechanism for delta 6 fatty acid desaturase

Fatty acid desaturase (FADS) generates double bond at a certain position of the corresponding polyunsaturated fatty acids (PUFAs) with high selectivity, the enzyme activity and PUFAs products of which are essential to biological systems and are associated with a variety of physiological diseases. Little is known about the structure of FADSs and their amino acid residues related to catalytic activities. Identifying key residues of Micromonas pusilla delta 6 desaturase (MpFADS6) provides a point of departure for a better understanding of desaturation. In this study, conserved amino acids were anchored through gene consensus analysis, thereby generating corresponding variants by site-directed mutagenesis. To achieve stable and high-efficiency expression of MpFADS6 and its variants in Saccharomyces cerevisiae, the key points of induced expression were optimized. The contribution of conserved residues to the function of enzyme was determined by analyzing enzyme activity of the variants. Molecular modeling indicated that these residues are essential to catalytic activities, or substrate binding. Mutants MpFADS6_[Q409R] and MpFADS6_[M242P] abolished desaturation, while MpFADS6_[F419V] and MpFADS6_[A374Q] significantly reduced catalytic activities. Given that certain residues have been identified to have a significant impact on MpFADS6 activities, it is put forward that histidine-conserved region III of FADS6 is related to electronic transfer during desaturation, while histidine-conserved regions I and II are related to desaturation. These findings provide new insights and methods to determine the structure, mechanism and directed transformation of membrane-bound desaturases.

Functional characterisation of fatty acyl desaturase, Fads2, and elongase, Elovl5, in the Boddart's goggle-eyed goby Boleophthalmus boddarti (Gobiidae) suggests an incapacity for long-chain polyunsaturated fatty acid biosynthesis

The biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFA), a process to convert C18 polyunsaturated fatty acids into eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) or arachidonic acid (ARA), requires the concerted activities of two enzymes, the fatty acyl desaturase (Fads) and elongase (Elovl). This study highlights the cloning, functional characterisation and tissue expression pattern of a Fads and an Elovl from the Boddart's goggle-eyed goby (Boleophthalmus boddarti), a mudskipper species widely distributed in the Indo-Pacific region. Phylogenetic analysis revealed that the cloned fads and elovl are clustered with other teleost orthologs, respectively. The investigation of the genome of several mudskipper species, namely Boleophthalmus pectinirostris, Periophthalmus schlosseri and Periophthalmus magnuspinnatus, revealed a single Fads2 and two elongases, Elovl5 and Elovl4 for each respective species. A heterologous yeast assay indicated that the B. boddarti Fads2 possessed low desaturation activity on C18 PUFA and no desaturation on C20 and C22 PUFA substrates. In comparison, the Elovl5 showed a wide range of substrate specificity, with a capacity to elongate C18, C20 and C22 PUFA substrates. An amino acid residue that affects the capacity to elongate C22:5n-3 was identified in the B. boddarti Elovl5. Both genes are highly expressed in brain tissue. Among all tissues, DHA is highly concentrated in neuron-rich tissues, whereas EPA is highly deposited in gills. Taken together, the results showed that due to the inability to perform desaturation steps, B. boddarti is unable to biosynthesise LC-PUFA, relying on dietary intake to acquire these nutrients.

Comparative and functional analysis of desaturase FADS1 (∆5) and FADS2 (∆6) orthologues of marine organisms

Fatty acid desaturases are key enzymes involved in unsaturated fatty acid biosynthesis, which insert double bonds at specific positions of fatty acids, playing a pivotal role in unsaturated fatty acid synthesis required for membrane lipid fluidity. The ∆5 and ∆6 desaturases are responsible for producing long chain-polyunsaturated fatty acids (LC-PUFA) through their precursors α-linolenic acid and linoleic acid in organisms lacking or with very low ability to synthesize LC-PUFA by themselves. Extensive studies of fatty acid desaturases are available in model organisms, such as humans and mouse; however, the diversity of these genes in the marine biodiversity is less known. This study performed an exhaustive analysis to identify the ∆5 and ∆6 desaturases in the available marine genomes in databases, as well as transcriptomes and EST databases, and their coding sequences were compared to the well-characterized ∆5 and ∆6 desaturases from humans. The FADS1 and FADS2 genetic structures are well conserved among all the organisms analyzed. A common amino acid pattern was identified to discriminate between ∆5 and ∆6 desaturases. The analysis of the conserved motif involved in catalysis showed that 20% of the desaturases, ∆5 and ∆6, have lost motifs required for catalysis. Additionally, bifunctional ∆5/∆6 desaturases were able to be identified by amino acid sequence patterns found in previously described enzymes. A revision of the expression profiles and functional activity on sequences in databases and scientific literature provided information regarding the function of these marine organism enzymes.

Assessment of Fatty Acid Desaturase (Fads2) Structure-Function Properties in Fish in the Context of Environmental Adaptations and as a Target for Genetic Engineering

Fatty acid desaturase 2 (Fads2) is the key enzyme of long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis. Endogenous production of these biomolecules in vertebrates, if present, is insufficient to meet demand. Hence, LC-PUFA are considered as conditionally essential. At present, however, LC-PUFA are globally limited nutrients due to anthropogenic factors. Research attention has therefore been paid to finding ways to maximize endogenous LC-PUFA production, especially in production species, whereby deeper knowledge on molecular mechanisms of enzymatic steps involved is being generated. This review first briefly informs about the milestones in the history of LC-PUFA essentiality exploration before it focuses on the main aim-to highlight the fascinating Fads2 potential to play roles fundamental to adaptation to novel environmental conditions. Investigations are summarized to elucidate on the evolutionary history of fish Fads2, providing an explanation for the remarkable plasticity of this enzyme in fish. Furthermore, structural implications of Fads2 substrate specificity are discussed and some relevant studies performed on organisms other than fish are mentioned in cases when such studies have to date not been conducted on fish models. The importance of Fads2 in the context of growing aquaculture demand and dwindling LC-PUFA supply is depicted and a few remedies in the form of genetic engineering to improve endogenous production of these biomolecules are outlined.

Identification and functional characterization of Buglossoides arvensis microsomal fatty acid desaturation pathway genes involved in polyunsaturated fatty acid synthesis in seeds

Buglossoides arvensis seed oil is the richest natural source of stearidonic acid (SDA), an ω-3 fatty acid with nutraceutical potential superior to α-linolenic acid (ALA). The molecular basis of polyunsaturated fatty acid synthesis in B. arvensis is unknown. Here, we describe the identification of B. arvensis fatty acid desaturase2 (BaFAD2), fatty acid desaturase3 (BaFAD3), and Delta-6-desaturase (BaD6D-1 and BaD6D-2) genes by mining the transcriptome of developing seeds and their functional characterization by heterologous expression in Saccharomyces cerevisiae. In silico analysis of their encoded protein sequences showed conserved histidine-boxes and signature motifs essential for desaturase activity. Expression profiling of these genes showed higher transcript abundance in reproductive tissues than in vegetative tissues, and their expression varied with temperature stress treatments. Yeast expressing BaFAD2 was found to desaturate both oleic acid and palmitoleic acid into linoleic acid (LA) and hexadecadienoic acid, respectively. Fatty acid supplementation studies in yeast expressing BaFAD3 and BaD6D-1 genes revealed that the encoded enzyme activities of BaFAD3 efficiently converted LA to ALA, and BaD6D-1 converted LA to γ-linolenic acid and ALA to SDA, but with an apparent preference to LA. BaD6D-2 did not show the encoded enzyme activity and is not a functional D6D. Our results provide an insight into SDA biosynthesis in B. arvensis and expand the repository of fatty acid desaturase targets available for biotechnological production of SDA in traditional oilseed crops.

High accumulation of γ-linolenic acid and Stearidonic acid in transgenic Perilla (Perilla frutescens var. frutescens) seeds

BACKGROUND

Polyunsaturated fatty acids such as linoleic acid (LA) and α-linolenic acid (ALA) are abundant in vegetable oils and are important for human health. In the body, LA and ALA are respectively converted to the omega-6 fatty acid γ-linolenic acid (GLA) and the omega-3 fatty acid stearidonic acid (SDA) by Δ6 desaturase (D6DES). Currently, dietary GLA and SDA are mainly obtained from marine organisms, but given their benefits to human health, many studies have aimed to enhance their accumulation in transgenic crops. Perilla frutescens (perilla) accumulates more ALA in its seed oil compared to other oilseed crops, making it a good candidate for the production of fatty acids via the fatty acid desaturase D6DES.

RESULTS

In this study, we cloned the D6DES gene from Phytophthora citrophthora and confirmed its function in budding yeast. We then transformed the functional D6DES gene under the control of the seed-specific vicilin promoter into the perilla cultivar Yeobsil. The resulting transgenic perilla seeds accumulated significant levels of GLA and SDA, as well as putative C18:2Δ^6,9 at minor levels. Developing seeds and leaves also accumulated GLA and SDA, although PcD6DES expression and GLA and SDA levels were much lower in leaves compared to developing seeds. GLA and SDA accumulated in both polar lipids and neutral lipids in mature perilla seeds expressing PcD6DES, especially in neutral lipids. Although the seed weight in PcD6DES perilla was 87-96% that of wild type, the total oil content per seed weight was similar between lines. The PcD6DES perilla plants contained very high content (over 45%) of both GLA and SDA in seed oil.

CONCLUSIONS

Thus, PcD6DES perilla plants may represent a feasible alternative to traditional marine sources for the production of omega-3 oil capsules and to evening primrose seed oil for GLA as health food. In addition, these plants can be used to create other transgenic lines harboring additional genes to produce other desirable fish-oil like oils.

Increasing medium chain fatty acids production in Yarrowia lipolytica by metabolic engineering

BACKGROUND

Oleaginous yeast Yarrowia lipolytica is an organism of choice for the development of biofuel and oleochemicals. It has become a chassis for metabolic engineering in order to produce targeted lipids. Understanding the function of key-enzymes involved in lipid metabolism is essential to design better routes for enhanced lipid production and for strains producing lipids of interest. Because medium chain fatty acids (MCFA) are valuable compounds for biokerosene production, we previously generated strains capable of producing MCFA up to 12% of total lipid content (Rigouin et al. in ACS Synth Biol 6:1870-1879, 2017). In order to improve accumulation and content of C14 fatty acid (FA), the elongation, degradation and accumulation of these MCFA in Yarrowia lipolytica were studied.

RESULTS

We brought evidence of the role of YALI0F0654 (YlELO1) protein in the elongation of exogenous or de novo synthesized C14 FA into C16 FA and C18 FA. YlELO1 deletion into a αFAS_I1220W expressing strain leads to the sole production of C14 FA. However, because this strain does not provide the FA essential for its growth, it requires being cultivated with essential fatty acids and C14 FA yield is limited. To promote MCFA accumulation in Y. lipolytica without compromising the growth, we overexpressed a plant diglyceride acyltransferase specific for MCFA and reached an accumulation of MCFA up to 45% of total lipid content.

CONCLUSION

We characterized the role of YlELO1 in Y. lipolytica by proving its involvement in Medium chain fatty acids elongation. We showed that MCFA content can be increased in Yarrowia lipolytica by promoting their accumulation into a stable storage form (triacylglycerides) to limit their elongation and their degradation.

Structural determinant of functionality in acyl lipid desaturases

Little is known about the structure-function relationship of membrane-bound lipid desaturases. Using a domain-swapping strategy, we found that the N terminus (comprising the two first transmembrane segments) region of Bacillus cereus DesA desaturase improves Bacillus subtilis Des activity. In addition, the replacement of the first two transmembrane domains from Bacillus licheniformis inactive open reading frame (ORF) BL02692 with the corresponding domain from DesA was sufficient to resurrect this enzyme. Unexpectedly, we were able to restore the activity of ORF BL02692 with a single substitution (Cys40Tyr) of a cysteine localized in the first transmembrane domain close to the lipid-water interface. Substitution of eight residues (Gly90, Trp104, Lys172, His228, Pro257, Leu275, Tyr282, and Leu284) by site-directed mutagenesis produced inactive variants of DesA. Homology modeling of DesA revealed that His228 is part of the metal binding center, together with the canonical His boxes. Trp104 shapes the hydrophobic tunnel, whereas Gly90 and Lys172 are probably involved in substrate binding/recognition. Pro257, Leu275, Tyr282, and Leu284 might be relevant for the structural arrangement of the active site or interaction with electron donors. This study reveals the role of the N-terminal region of Δ5 phospholipid desaturases and the individual residues necessary for the activity of this class of enzymes.

Terminal Alkyne Biosynthesis in Marine Microbes

The terminal alkyne is a readily derivatized functionality valued for its diverse applications in material synthesis, pharmaceutical science, and chemical biology. The synthetic biology routes to terminal alkynes are highly desired and yet underexplored. Some marine natural products contain a terminal alkyne functionality, and the discovery of the biosynthetic gene clusters for jamaicamide B and carmabin A marked the beginning of a new era in the understanding and engineering of terminal alkyne biosynthesis. In this chapter, we will overview recent advances in understanding the biosynthetic machinery for terminal alkyne synthesis. We will first describe how to elucidate terminal alkyne biosynthetic mechanism through heterologous expression, purification, and in vitro biochemical assays of individual pathway proteins. This will be followed by the description of an in vivo reporting system for the characterization of a membrane-bound bifunctional desaturase/acetylenase involved in terminal alkyne formation. The chapter will also cover the strategies for discovering additional protein homologs for terminal alkyne synthesis from microbes as well as the applications of click chemistry to identify and quantify terminal alkyne-bearing metabolites from microbial cultures. We will conclude this chapter with current challenges and future directions in this field.

Isolation and functional analysis of fatty acid desaturase genes from peanut (Arachis hypogaea L.)

BACKGROUND

Fatty acid desaturases are enzymes that introduce double bonds into fatty acyl chains. Extensive studies of fatty acid desaturases have been done in many plants. However, less is known about the diversity of this gene family in peanut (Arachis hypogaea L.), an important oilseed crop that is cultivated worldwide.

RESULTS

In this study, twelve novel AhFADs genes were identified and isolated from peanut. Quantitative real-time PCR analysis indicated that the transcript abundances of AhFAB2-2 and AhFAD3-1 were higher in seeds than in other tissues examined, whereas the AhADS and AhFAD7-1 transcripts were more abundant in leaves. AhFAB2-3, AhFAD3-2, AhFAD4, AhSLD-4, and AhDES genes were highly expressed in flowers, whereas AhFAD7-2, AhSLD-2, and AhSLD-3 were expressed most strongly in stems. During seed development, the expressions of AhFAB2-2, AhFAD3-1, AhFAD7-1, and AhSLD-3 gradually increased in abundance, reached a maximum expression level, and then decreased. The AhFAB2-3, AhFAD3-2, AhFAD4, AhADS, and AhDES transcript levels remained relatively high at the initial stage of seed development, but decreased thereafter. The AhSLD-4 transcript level remained relatively low at the initial stage of seed development, but showed a dramatic increase in abundance at the final stage. The AhFAD7-2 and AhSLD-2 transcript levels remained relatively high at the initial stage of seed development, but then decreased, and finally increased again. The AhFAD transcripts were differentially expressed following exposure to abiotic stresses or abscisic acid. Moreover, the functions of one AhFAD6 and four AhSLD genes were confirmed by heterologous expression in Synechococcus elongates or Saccharomyces cerevisiae.

CONCLUSIONS

The present study provides valuable information that improves understanding of the biological roles of FAD genes in fatty acid synthesis, and will help peanut breeders improve the quality of peanut oil via molecular design breeding.

The Linoleic Acid: Dihomo-γ-Linolenic Acid Ratio (LA:DGLA)-An Emerging Biomarker of Zn Status

Zinc (Zn) deficiency is a common aliment predicted to affect 17% of the world's population. Zinc is a vital micronutrient used for over 300 enzymatic reactions and multiple biochemical and structural processes in the body. Although whole blood, plasma, and urine zinc decrease in severe zinc deficiency, accurate assessment of zinc status, especially in mild to moderate deficiency, is difficult as studies with these biomarkers are often contradictory and inconsistent. Hence, as suggested by the World Health Organization, sensitive and specific biological markers of zinc status are still needed. In this review, we provide evidence to demonstrate that the LA:DGLA ratio (linoleic acid:dihomo-γ-linolenic acid ratio) may be a useful additional indicator for assessing Zn status more precisely. However, this biomarker needs to be tested further in order to determine its full potential.

Microalgae as a Source for VLC-PUFA Production

Microalgae present a huge and still insufficiently tapped resource of very long-chain omega-3 and omega-6 polyunsaturated fatty acids (VLC-PUFA) for human nutrition and medicinal applications. This chapter describes the diversity of unicellular eukaryotic microalgae in respect to VLC-PUFA biosynthesis. Then, we outline the major biosynthetic pathways mediating the formation of VLC-PUFA by sequential desaturation and elongation of C18-PUFA acyl groups. We address the aspects of spatial localization of those pathways and elaborate on the role for VLC-PUFA in microalgal cells. Recent progress in microalgal genetic transformation and molecular engineering has opened the way to increased production efficiencies for VLC-PUFA. The perspectives of photobiotechnology and metabolic engineering of microalgae for altered or enhanced VLC-PUFA production are also discussed.

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.

Modifying the lipid content and composition of plant seeds: engineering the production of LC-PUFA

Omega-3 fatty acids are characterized by a double bond at the third carbon atom from the end of the carbon chain. Latterly, long chain polyunsaturated omega-3 fatty acids such as eicosapentaenoic acid (EPA; 20:5Δ5,8,11,14,17) and docosahexanoic acid (DHA; 22:6 Δ4,7,10,13,16,19), which typically only enter the human diet via the consumption of oily fish, have attracted much attention. The health benefits of the omega-3 LC-PUFAs EPA and DHA are now well established. Given the desire for a sustainable supply of omega-LC-PUFA, efforts have focused on enhancing the composition of vegetable oils to include these important fatty acids. Specifically, EPA and DHA have been the focus of much study, with the ultimate goal of producing a terrestrial plant-based source of these so-called fish oils. Over the last decade, many genes encoding the primary LC-PUFA biosynthetic activities have been identified and characterized. This has allowed the reconstitution of the LC-PUFA biosynthetic pathway in oilseed crops, producing transgenic plants engineered to accumulate omega-3 LC-PUFA to levels similar to that found in fish oil. In this review, we will describe the most recent developments in this field and the challenges of overwriting endogenous seed lipid metabolism to maximize the accumulation of these important fatty acids.

Metabolic engineering of microorganisms to produce omega-3 very long-chain polyunsaturated fatty acids

Omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs) have received growing attention due to their significant roles in human health. Currently the main source of these nutritionally and medically important fatty acids is marine fish, which has not met ever-increasing global demand. Microorganisms are an important alternative source also being explored. Although many microorganisms accumulate omega-3 LC-PUFAs naturally, metabolic engineering might still be necessary for significantly improving their yields. Here, we review recent research involving the engineering of microorganisms for production of omega-3 LC-PUFAs, including eicospentaenoic acid and docosohexaenoic acid. Both reconstitution of omega-3 LC-PUFA biosynthetic pathways and modification of existing pathways in microorganisms have demonstrated the potential to produce high levels of omega-3 LC-PUFAs. However, the yields of omega-3 LC-PUFAs in host systems have been substantially limited by potential metabolic bottlenecks, which might be caused partly by inefficient flux of fatty acid intermediates between the acyl-CoA and different lipid class pools. Although fatty acid flux in both native and heterologous microbial hosts might be controlled by several acyltransferases, evidence has suggested that genetic manipulation of one acyltransferase alone could significantly increase the accumulation of LC-PUFAs. The number of oleaginous microorganisms that can be genetically transformed is increasing, which will advance engineering efforts to maximize LC-PUFA yields in microbial strains.

Identification of the substrate recognition region in the Δ⁶-fatty acid and Δ⁸-sphingolipid desaturase by fusion mutagenesis

Δ⁸-sphingolipid desaturase and Δ⁶-fatty acid desaturase share high protein sequence identity. Thus, it has been hypothesized that Δ⁶-fatty acid desaturase is derived from Δ⁸-sphingolipid desaturase; however, there is no direct proof. The substrate recognition regions of Δ⁶-fatty acid desaturase and Δ⁸-sphingolipid desaturase, which aid in understanding the evolution of these two enzymes, have not been reported. A blackcurrant Δ⁶-fatty acid desaturase and a Δ⁸-sphingolipid desaturase gene, RnD6C and RnD8A, respectively, share more than 80 % identity in their coding protein sequences. In this study, a set of fusion genes of RnD6C and RnD8A were constructed and expressed in yeast. The Δ⁶- and Δ⁸-desaturase activities of the fusion proteins were characterized. Our results indicated that (1) the exchange of the C-terminal 172 amino acid residues can lead to a significant decrease in both desaturase activities; (2) amino acid residues 114-174, 206-257, and 258-276 played important roles in Δ⁶-substrate recognition, and the last two regions were crucial for Δ⁸-substrate recognition; and (3) amino acid residues 114-276 of Δ⁶-fatty acid desaturase contained the substrate recognition site(s) responsible for discrimination between ceramide (a substrate of Δ⁸-sphingolipid desaturase) and acyl-PC (a substrate of Δ⁶-fatty acid desaturase). Substituting the amino acid residues 114-276 of RnD8A with those of RnD6C resulted in a gain of Δ⁶-desaturase activity in the fusion protein but a loss in Δ⁸-sphingolipid desaturase activity. In conclusion, several regions important for the substrate recognition of Δ⁸-sphingolipid desaturase and Δ⁶-fatty acid desaturase were identified, which provide clues in understanding the relationship between the structure and function in desaturases.

Four amino acid residues influence the substrate chain-length and regioselectivity of Siganus canaliculatus Δ4 and Δ5/6 desaturases

Although ω3- and ω6- desaturases have been well studied in terms of substrate preference and regiospecificity, relatively little is known about the membrane-bound, "front-end" long chain fatty acid desaturases, such as ∆4, Δ5 or Δ6 desaturases. The first vertebrate ∆4 desaturase was recently identified in the marine teleost fish Siganus canaliculatus (S. canaliculatus), which also possesses a bifunctional Δ5/6 desaturase. These two long chain polyunsaturated fatty acid desaturases are very different in terms of regiospecificity and substrate chain-length, but share an unusually high degree of amino acid identity (83 %). We took advantage of this similarity by constructing a series of chimeric enzymes, replacing regions of one enzyme with the corresponding sequence of the other. Heterologous expression of the chimeric series of enzymes in yeast indicated that the substitution of a four amino acid region was sufficient to convert a ∆4 desaturase to an enzyme with ∆6 desaturase activity, and convert a ∆5/6 desaturase to an enzyme with a low level of ∆4 desaturase activity. In addition, enzymes having both ∆4 and ∆6 desaturase activities were produced by single or double amino acid substitutions within this four-amino acid region.

Site-directed mutagenesis of a fatty acid elongase ELO-like condensing enzyme

The condensation step of fatty acid elongation is the addition of a C2 unit from malonyl-CoA to an acyl primer catalyzed by one of two families of enzymes, the 3-ketoacyl-CoA synthases and the ELO-like condensing enzymes. 3-Ketoacyl-CoA synthases use a Claisen-like reaction mechanism while the mechanism of the ELO-catalyzed condensation reaction is unknown. We have used site-directed mutagenesis of Dictyostelium discoideum EloA to identify residues important to catalytic activity and/or structure. Mutation of highly conserved polar residues to alanine resulted in an inactive enzyme strongly suggesting that these residues play a role in the condensation reaction.

Trans-vaccenate is Δ13-desaturated by FADS3 in rodents

Fatty acid desaturases play critical roles in regulating the biosynthesis of unsaturated fatty acids in all biological kingdoms. As opposed to plants, mammals are so far characterized by the absence of desaturases introducing additional double bonds at the methyl-end site of fatty acids. However, the function of the mammalian fatty acid desaturase 3 (FADS3) gene remains unknown. This gene is located within the FADS cluster and presents a high nucleotide sequence homology with FADS1 (Δ5-desaturase) and FADS2 (Δ6-desaturase). Here, we show that rat FADS3 displays no common Δ5-, Δ6- or Δ9-desaturase activity but is able to catalyze the unexpected Δ13-desaturation of trans-vaccenate. Although there is no standard for complete conclusive identification, structural characterization strongly suggests that the Δ11,13-conjugated linoleic acid (CLA) produced by FADS3 from trans-vaccenate is the trans11,cis13-CLA isomer. In rat hepatocytes, knockdown of FADS3 expression specifically reduces trans-vaccenate Δ13-desaturation. Evidence is presented that FADS3 is the first "methyl-end" fatty acid desaturase functionally characterized in mammals.

Identification and functional analysis of a Δ6-desaturase gene and the effects of temperature and wounding stresses on its expression in Microula sikkimensis leaves

A Δ6-desaturase gene was isolated from Microula sikkimensis. Sequence analysis indicated that the gene, designated MsD6DES, had an open reading frame of 1,357 bp and encoded 448 amino acids. Heterologous expression in tobacco indicated that MsD6DES can use endogenous substrates to synthesize γ-linolenic acid (GLA, 18:3(Δ 6,9,12)) and octadecatetraenoic acid (OTA, 18:4(Δ 6,9,12,15)). MsD6DES transcripts were distributed in all tested tissues, with high expression levels in seeds and young leaves. The effects of temperature and wounding stresses on MsD6DES expression were analyzed. The results indicated that temperature regulates MsD6DES at the transcriptional level. MsD6DES expression increased first, reaching a maximum 4 h after low-temperature treatment. A slight increase in MsD6DES transcript levels was also observed under high temperature. We found that the response of MsD6DES to temperature stress was different from those of fungi and algae. In addition, MsD6DES was found to be wound-inducible.

The thermostability of two kinds of recombinant ∆6-fatty acid desaturase with different N-terminal sequence lengths in low temperature

Two recombinant Rhizopus stolonifer ∆6-fatty acid desaturase enzymes with different-length N-termini were cloned and expressed in Saccharomyces cerevisiae strain INVScl: LRsD6D begins with the sequence of the N-terminal of the R. stolonifer ∆6-fatty acid desaturase native, encoding a deduced polypeptide of 459 amino acids (M-S-T-L-D-R-Q-S-I-F-T-I-K-E-L-E-S-I-S-Q-R-I-H-D-G-D-E-E-A-M-K-F), whereas SRsD6D begins with the amino acid sequence of the predicted ORF, encoding a deduced polypeptide of 430 amino acids (M-K-F) and LRsD6D is longer than SRsD6D by 29 amino acids (M-S-T-L-D-R-Q-S-I-F-T-I-K-E-L-E-S-I-S-Q-R-I-H-D-G-D-E-E-A). Bioinformatic analysis characterized the two recombinant ∆6-fatty acid desaturase enzymes with different-length N-termini, including three conserved histidine-rich motifs, hydropathy profile, and a cytochrome b5-like domain in the N-terminus. When the coding sequence was expressed in S. cerevisiae strain INVScl, the coding produced ∆6-fatty acid desaturase activity exhibited by RsD6D, leading to a novel peak corresponding to γ-linolenic acid methyl ester standards, which was detected with the same retention time. The residual activity of LRsD6D was 74 % at 15 °C for 4 h and that of SRsD6D was 43 %. Purified recombinant LRsD6D was more stable than SRsD6D, indicating that the N-terminal extension, containing mostly hydrophobic residues, affected the overall stability of recombinant LRsD6D.

The role of C-terminal amino acid residues of a Δ⁶-fatty acid desaturase from blackcurrant

Δ⁶-fatty acid desaturase is an important enzyme in the catalytic synthesis of polyunsaturated fatty acids. Using domain swapping and a site-directed mutagenesis strategy, we found that the region of the C-terminal 67 amino acid residues of Δ⁶-fatty acid desaturase RnD6C from blackcurrant was essential for its catalytic activity and that seven different residues between RnD6C and RnD8A in that region were involved in the desaturase activity. Compared with RnD6C, the activity of the following mutations, V394A, K395I, F411L, S436P, VK3945AI and IS4356VP, was significantly decreased, whereas the activity of I417T was significantly increased. The amino acids N, T and Y in the last four residues also play a certain role in the desaturase activity.

A cytochrome b5-containing plastid-located fatty acid desaturase from Chlamydomonas reinhardtii

Monogalactosyldiacylglycerol (MGDG) in Chlamydomonas reinhardtii and other green algae contains hexadeca-4,7,10,13-tetraenoic acid (16:4) in the glycerol sn-2 position. While many genes necessary for the introduction of acyl chain double bonds have been functionally characterized, the Δ4-desaturase remained unknown. Using a phylogenetic comparison, a candidate gene encoding the MGDG-specific Δ4-desaturase from Chlamydomonas (CrΔ4FAD) was identified. CrΔ4FAD shows all characteristic features of a membrane-bound desaturase, including three histidine boxes and a transit peptide for chloroplast targeting. But it also has an N-terminal cytochrome b(5) domain, distinguishing it from other known plastid desaturases. Cytochrome b(5) is the primary electron donor for endoplasmic reticulum (ER) desaturases and is often fused to the desaturase domain in desaturases modifying the carboxyl end of the acyl group. Difference absorbance spectra of the recombinant cytochrome b(5) domain of CrΔ4FAD showed that it is functional in vitro. Green fluorescent protein fusions of CrΔ4FAD localized to the plastid envelope in Chlamydomonas. Interestingly, overproduction of CrΔ4FAD in Chlamydomonas not only increased levels of 16:4 acyl groups in cell extracts but specifically increased the total amount of MGDG. Vice versa, the amount of MGDG was lowered in lines with reduced levels of CrΔ4FAD. These data suggest a link between MGDG molecular species composition and galactolipid abundance in the alga, as well as a specific function for this fatty acid in MGDG.

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.

Isolation and characterization of fatty acid desaturase genes from peanut (Arachis hypogaea L.)

Fatty acid desaturases are responsible for the insertion of double bonds into pre-formed fatty acid chains in reactions that require oxygen and reducing equivalents. In this study, genes for FAB2, FAD2-2, FAD6 and SLD1, were cloned from peanut (Arachis hypogaea L.). The ORFs of the four genes were 1,221, 1,152, 1,329 and 1,347 bp in length, encoding 406, 383, 442 and 448 amino acids, respectively. The predicted amino acid sequences of AhFAB2, AhFAD2-2, AhFAD6, AhSLD1 shared high sequence identity of 79, 76.2, 73.4 and 61% to the corresponding ones in Arabidopsis, respectively. Heterologous expression in yeast was used to confirm the regioselectivity and the function of AhFAD2-2 and AhFAD6. Linoleic acid (18:2), normally not present in wild-type yeast cells, was detected in transformants of these two genes. Quantitative real-time RT-PCR analysis indicated that the transcript abundances of AhFAB2 and AhFAD2-1 were higher in seed than that in other tissues examined. On the other hand, transcript of AhFAD2-2, AhFAD6 and AhSLD1 showed higher abundances in leaves. In addition, these five genes showed different expression patterns during seed development. These results indicated that the five genes may have different biochemical functions during vegetative growth and seed development.

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.

FATTY ACID DESATURASE4 of Arabidopsis encodes a protein distinct from characterized fatty acid desaturases

Polar membrane glycerolipids occur in a mixture of molecular species defined by a polar head group and characteristic acyl groups esterified to a glycerol backbone. A molecular species of phosphatidylglycerol specific to chloroplasts of plants carries a Delta(3-trans) hexadecenoic acid in the sn-2 position of its core glyceryl moiety. The fad4-1 mutant of Arabidopsis thaliana missing this particular phosphatidylglycerol molecular species lacks the necessary fatty acid desaturase, or a component thereof. The overwhelming majority of acyl groups associated with membrane lipids in plants contains double bonds with a cis configuration. However, FAD4 is unusual because it is involved in the formation of a trans double bond introduced close to the carboxyl group of palmitic acid, which is specifically esterified to the sn-2 glyceryl carbon of phosphatidylglycerol. As a first step towards the analysis of this unusual desaturase reaction, the FAD4 gene was identified by mapping of the FAD4 locus and coexpression analysis with known lipid genes. FAD4 encodes a predicted integral membrane protein that appears to be unrelated to classic membrane bound fatty acid desaturases based on overall sequence conservation. However, the FAD4 protein contains two histidine motifs resembling those of metalloproteins such as fatty acid desaturases. FAD4 is targeted to the plastid. Overexpression of the cDNA in transgenic Arabidopsis led to increased accumulation of the Delta(3-trans) hexadecanoyl group in phosphatidylglycerol relative to wild type. Taken together these results are consistent with the hypothesis that FAD4 is the founding member of a novel class of fatty acid desaturases.

Cloning and expression analysis of candidate genes involved in wax deposition along the growing barley (Hordeum vulgare) leaf

The aim of the present study was to isolate clones of genes which are likely to be involved in wax deposition on barley leaves. Of particular interest were those genes which encode proteins that take part in the synthesis and further modification of very long chain fatty acids (VLCFAs), the precursors of waxes. Previously, it had been shown that wax deposition commences within a spatially well-defined developmental zone along the growing barley leaf (Richardson et al. in Planta 222:472-483, 2005). In the present study, a barley microarray approach was used to screen for candidate contig-sequences (www.barleybase.org) that are expressed particularly in those leaf zones where wax deposition occurs and which are expressed specifically within the epidermis, the site of wax synthesis. Candidate contigs were used to screen an established in-house cDNA library of barley. Six full-length coding sequences clones were isolated. Based on sequence homologies, three clones were related to Arabidopsis CER6/CUT1, and these clones were termed HvCUT1;1, HvCUT1;2 and HvCUT1;3. A fourth clone, which was related to Arabidopsis Fiddlehead (FDH), was termed HvFDH1;1. These clones are likely to be involved in synthesis of VLCFAs. A fifth and sixth clone were related to Arabidopsis CER1, and were termed HvCER1;1 and HvCER1;2. These clones are likely to be involved in the decarbonylation pathway of VLCFAs. Semi-quantitative RT-PCR confirmed microarray expression data. In addition, expression analyses at 10-mm resolution along the blade suggest that HvCUT1;1 (and possibly HvCUT1;2) and HvCER1;1 are involved in commencement of wax deposition during barley leaf epidermal cell development.

A defect in the activity of Delta6 and Delta5 desaturases may be a factor in the initiation and progression of atherosclerosis

Atherosclerosis is a dynamic process. Dyslipidemia, diabetes mellitus, hypertension, obesity, and shear stress of blood flow, the risk factors for the development of atherosclerosis, are characterized by abnormalities in the metabolism of essential fatty acids (EFAs). Gene expression profiling studies revealed that at the sites of atheroslcerosis-prone regions, endothelial cells showed upregulation of pro-inflammatory genes as well as antioxidant genes, and endothelial cells themselves showed changes in cell shape and proliferation. Uncoupled respiration (UCP-1) precedes atherosclerosis at lesion-prone sites but not at the sites that are resistant to atherosclerosis. UCP-1 expression in aortic smooth muscle cells causes hypertension, enhanced superoxide anion production and decreased the availability of NO, suggesting that inefficient metabolism in blood vessels causes atherosclerosis without affecting cholesterol levels. Thus, mitochondrial dysfunction triggers atherosclerosis. Atherosclerosis-free aortae have abundant concentrations of the EFA-linoleate, whereas fatty streaks (an early stage of atherosclerosis) are deficient in EFAs. EFA deficiency promotes respiratory uncoupling and atherosclerosis. I propose that a defect in the activity of Delta6 and Delta5 desaturases decreases the formation of gamma-linolenic acid (GLA), dihomo-DGLA (DGLA), arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) from dietary linoleic acid (LA) and alpha-linolenic acid (ALA). This, in turn, leads to inadequate formation of prostaglandin E1 (PGE1), prostacyclin (PGI2), PGI3, lipoxins (LXs), resolvins, neuroprotectin D1 (NPD1), NO, and nitrolipids that have anti-inflammatory and platelet anti-aggregatory actions, inhibit leukocyte activation and augment wound healing and resolve inflammation and thus, lead to the initiation and progression atheroslcerosis. In view of this, it is suggested that Delta6 and Delta5 desaturases could serve as biological target(s) for the discovery and development of pharmaceuticals to treat atherosclerosis.

Identification and functional analysis of a delta-6 desaturase from the marine microalga Glossomastix chrysoplasta

A delta-6 (Delta6) desaturase gene was isolated from the marine microalga Glossomastix chrysoplasta, a stramenopile that produces large amounts of eicosapentaenoic acid (EPA). A functional analysis of this gene was performed in Saccharomyces cerevisiae. Isolation of the Delta6 fatty acid desaturase was achieved via reverse transcriptase-polymerase chain reaction (RT-PCR) with degenerate primers designed from conserved histidine motifs and 5' and 3' RACE. Two almost identical copies of Delta6 desaturase were found, differing by nine silent mutations. The existence of two such genes may be a result of a recent gene duplication event, or may have arisen from the possible diploid nature of vegetative algae. This appears to be the first instance of two Delta6 desaturase mRNA sequences existing in the same organism. The isolated mRNA sequences and their corresponding hypothetical protein, GcDES6, were found to contain features characteristic of a membrane-bound Delta6 desaturase, including membrane-spanning regions separating conserved histidine boxes and N-terminal cytochrome b5 fusion. Heterologous expression in S. cerevisiae was used to confirm Delta6 regioselectivity and the function of GcDES6. Both omega3(18:3Delta9,12,15) and omega6(18:2Delta9,12) precursors can be used by GcDES6 in vivo with similar desaturase activity. One intron site was found in the cytochrome b5 fusion region of GcDES6. Although the conservation of intron-exon junctions has been found for several desaturases in humans and in Caenorhabditis elegans, a comparison of introns in GcDES6 and other Delta6 desaturases has not revealed any strong similarities.

Identification and functional characterization of the moss Physcomitrella patens delta5-desaturase gene involved in arachidonic and eicosapentaenoic acid biosynthesis

The moss Physcomitrella patens contains high levels of arachidonic acid and lesser amounts of eicosapentaenoic acid. Here we report the identification and characterization of a delta5-desaturase from P. patens that is associated with the synthesis of these fatty acids. A full-length cDNA for this desaturase was identified by data base searches based on homology to sequences of known delta5-desaturase cDNAs from fungal and algal species. The resulting P. patens cDNA encodes a 480-amino acid polypeptide that contains a predicted N-terminal cytochrome b5-like domain as well as three histidine-rich domains. Expression of the enzyme in Saccharomyces cerevisiae resulted in the production of the delta5-containing fatty acid arachidonic acid in cells that were provided di-homo-gamma-linolenic acid. In addition, the expressed enzyme generated delta5-desaturation products with the C20 substrates omega-6 eicosadienoic and omega-3 eicosatrienoic acids, but no products were detected with the C18 fatty acid linoleic and alpha-linolenic acids or with the C22 fatty acid adrenic and docosapentaenoic acids. When the corresponding P. patens genomic sequence was disrupted by replacement through homologous recombination, a dramatic alteration in the fatty acid composition was observed, i.e. an increase in di-homo-gamma-linolenic and eicosatetraenoic acids accompanied by a concomitant disappearance of the delta5-fatty acid arachidonic and eicosapentaenoic acids. In addition, overexpression of the P. patens cDNA in protoplasts isolated from a disrupted line resulted in the restoration of arachidonic acid synthesis.

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.

Identification of mutation sites on Δ5 desaturase genes from Mortierella alpina 1S-4 mutants

The mutation sites on delta5 desaturase genes in delta5 desaturase-defective mutants derived from arachidonic acid-producing Mortierella alpina 1S-4 were identified. The mutations resulted in an amino acid replacement (G189E or W301Stop) and uncorrected transcription caused by recognition of an AG-terminal newly created on C207A gene mutation, resulting in low or no delta5 desaturase activity in these mutants.

Domain swapping localizes the structural determinants of regioselectivity in membrane-bound fatty acid desaturases of Caenorhabditis elegans

Most fatty acid desaturases are members of a large superfamily of integral membrane, O(2)-dependent, iron-containing enzymes that catalyze a variety of oxidative modifications to lipids. Sharing a similar primary structure and membrane topology, these enzymes are broadly categorized according to their positional specificity or regioselectivity, which designates the preferred position for substrate modification. To investigate the structural basis of regioselectivity in membrane-bound desaturases, the Caenorhabditis elegans omega-3 (FAT-1) and "Delta12" (FAT-2) desaturases were used as a model system. With the use of unnatural substrates, the regioselectivity of C. elegans FAT-2 was clearly defined as nu+3, i.e. it "measures" three carbons from an existing double bond. The structural basis for nu+3 and omega-3 regioselectivities was examined through construction and expression of chimeric DNA sequences based on FAT-1 and FAT-2. Each sequence was divided into seven domains, and chimeras were constructed in which specific domains were replaced with sequence from the other desaturase. When tested by expression in yeast using exogenously supplied substrates, chimeric sequences were found in which domain swapping resulted in a change of regioselectivity from nu+3 to omega-3 and vice versa. In this way, the structural determinants of regioselectivity in FAT-1 and FAT-2 have been localized to two interdependent regions: a relatively hydrophobic region between the first two histidine boxes and the carboxyl-terminal region.

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.

The variant 'his-box' of the C18-Delta9-PUFA-specific elongase IgASE1 from Isochrysis galbana is essential for optimum enzyme activity

IgASE1, a C18-Delta9-polyunsaturated fatty acid-specific fatty acid elongase component from Isochrysis galbana, contains a variant histidine box (his-box) with glutamine replacing the first histidine of the conserved histidine-rich motif present in all other known equivalent proteins. The importance of glutamine and other variant amino acid residues in the his-box of IgASE1 was determined by site-directed mutagenesis. Results showed that all the variation in amino acid sequence between this motif in IgASE1 and the consensus sequences of other elongase components was required for optimum enzyme activity. The substrate specificity was shown to be unaffected by these changes suggesting that components of the his-box are not directly responsible for substrate specificity.

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.

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.