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

Analysis of Δ12-fatty acid desaturase function revealed that two distinct pathways are active for the synthesis of PUFAs in T. aureum ATCC 34304

Thraustochytrids are known to synthesize PUFAs such as docosahexaenoic acid (DHA). Accumulating evidence suggests the presence of two synthetic pathways of PUFAs in thraustochytrids: the polyketide synthase-like (PUFA synthase) and desaturase/elongase (standard) pathways. It remains unclear whether the latter pathway functions in thraustochytrids. In this study, we report that the standard pathway produces PUFA in Thraustochytrium aureum ATCC 34304. We isolated a gene encoding a putative Δ12-fatty acid desaturase (TauΔ12des) from T. aureum. Yeasts transformed with the tauΔ12des converted endogenous oleic acid (OA) into linoleic acid (LA). The disruption of the tauΔ12des in T. aureum by homologous recombination resulted in the accumulation of OA and a decrease in the levels of LA and its downstream PUFAs. However, the DHA content was increased slightly in tauΔ12des-disruption mutants, suggesting that DHA is primarily produced in T. aureum via the PUFA synthase pathway. The transformation of the tauΔ12des-disruption mutants with a tauΔ12des expression cassette restored the wild-type fatty acid profiles. These data clearly indicate that TauΔ12des functions as Δ12-fatty acid desaturase in the standard pathway of T. aureum and demonstrate that this thraustochytrid produces PUFAs via both the PUFA synthase and the standard pathways.

Enhancing the accumulation of omega-3 long chain polyunsaturated fatty acids in transgenic Arabidopsis thaliana via iterative metabolic engineering and genetic crossing

The synthesis and accumulation of long chain polyunsaturated fatty acids such as eicosapentaenoic acid has previously been demonstrated in the seeds of transgenic plants. However, the obtained levels are relatively low, indicating the need for further studies and the better definition of the interplay between endogenous lipid synthesis and the non-native transgene-encoded activities. In this study we have systematically compared three different transgenic configurations of the biosynthetic pathway for eicosapentaenoic acid, using lipidomic profiling to identify metabolic bottlenecks. We have also used genetic crossing to stack up to ten transgenes in Arabidopsis. These studies indicate several potential approaches to optimize the accumulation of target fatty acids in transgenic plants. Our data show the unexpected channeling of heterologous C20 polyunsaturated fatty acids into minor phospholipid species, and also the apparent negative metabolic regulation of phospholipid-dependent Δ6-desaturases. Collectively, this study confirms the benefits of iterative approaches to metabolic engineering of plant lipid synthesis.

The role of Δ6-desaturase acyl-carrier specificity in the efficient synthesis of long-chain polyunsaturated fatty acids in transgenic plants

The role of acyl-CoA-dependent Δ6-desaturation in the heterologous synthesis of omega-3 long-chain polyunsaturated fatty acids was systematically evaluated in transgenic yeast and Arabidopsis thaliana. The acyl-CoA Δ6-desaturase from the picoalga Ostreococcus tauri and orthologous activities from mouse (Mus musculus) and salmon (Salmo salar) were shown to generate substantial levels of Δ6-desaturated acyl-CoAs, in contrast to the phospholipid-dependent Δ6-desaturases from higher plants that failed to modify this metabolic pool. Transgenic plants expressing the acyl-CoA Δ6-desaturases from either O. tauri or salmon, in conjunction with the two additional activities required for the synthesis of C20 polyunsaturated fatty acids, contained higher levels of eicosapentaenoic acid compared with plants expressing the borage phospholipid-dependent Δ6-desaturase. The use of acyl-CoA-dependent Δ6-desaturases almost completely abolished the accumulation of unwanted biosynthetic intermediates such as γ-linolenic acid in total seed lipids. Expression of acyl-CoA Δ6-desaturases resulted in increased distribution of long-chain polyunsaturated fatty acids in the polar lipids of transgenic plants, reflecting the larger substrate pool available for acylation by enzymes of the Kennedy pathway. Expression of the O. tauriΔ6-desaturase in transgenic Camelina sativa plants also resulted in the accumulation of high levels of Δ6-desaturated fatty acids. This study provides evidence for the efficacy of using acyl-CoA-dependent Δ6-desaturases in the efficient metabolic engineering of transgenic plants with high value traits such as the synthesis of omega-3 LC-PUFAs.

Transgenic oilseed crops as an alternative to fish oils

Growing evidence suggests that omega-3 long chain polyunsaturated fatty acids (VLC-PUFAs), especially eicosapentaenoic acid (EPA; 20:5Δ5,8,11,14,17) and docosahexaenoic acid (DHA; 22:6Δ4,7,10,13,16,19) play critical roles in human health and development. VLC-PUFAs are mainly found in fish, some fungi, marine bacteria and microalgae. Currently, the predominant dietary sources of VLC-PUFAs are marine fish and seafood. However, the increasing demand for fish and fish oils is putting enormous pressure on marine ecosystems leading to a depletion of fish stocks while commercial cultivation of marine microorganisms and aquaculture are not sustainable and cannot compensate for the shortage in fish supply. Therefore, there is an obvious requirement for an alternative and sustainable source for VLC-PUFAs. Over the last decade, many genes encoding the primary VLC-PUFAs biosynthetic activities became available providing a toolkit for the "reverse-engineering" of transgenic plants to produce fish oils. In this review, we will describe the recent advances in this field and the insights they give us into the complexities of metabolic engineering of oil-seed crops producing VLC-PUFAs.

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.

Identification and functional characterisation of genes encoding the omega-3 polyunsaturated fatty acid biosynthetic pathway from the coccolithophore Emiliania huxleyi

The Prymnesiophyceae coccolithophore Emiliania huxleyi is one of the most abundant alga in our oceans and therefore plays a central role in marine foodwebs. E. huxleyi is notable for the synthesis and accumulation of the omega-3 long chain polyunsaturated fatty acid docosahexaenoic acid (DHA; 22:6Δ(4,7,10,13,16,19), n-3) which is accumulated in fish oils and known to have health-beneficial properties to humans, preventing cardiovascular disease and related pathologies. Here we describe the identification and functional characterisation of the five E. huxleyi genes which direct the synthesis of docosahexaenoic acid in this alga. Surprisingly, E. huxleyi does not use the conventional Δ6-pathway, instead using the alternative Δ8-desaturation route which has previously only been observed in a few unrelated microorganisms. Given that E. huxleyi accumulates significant levels of the Δ6-desaturated fatty acid stearidonic acid (18:4Δ(6,9,12,15), n-3), we infer that the biosynthesis of DHA is likely to be metabolically compartmentalised from the synthesis of stearidonic acid.

Metabolism and synthesis of lipids in the polyunsaturated fatty acid-producing fungus Mortierella alliacea

The fungal strain Mortierella alliacea YN-15 is a promising industrial producer of polyunsaturated fatty acids (PUFAs), in particular arachidonic acid. In order to more efficiently produce PUFAs, the metabolism of an externally supplied plant oil, α-linolenic acid (ALA)-rich linseed triacylglycerol (TAG), was examined, and time-dependent changes in the composition of its lipid and fatty acid metabolites were traced. Addition of linseed TAG to growing cultures resulted in a transient increase in extracellular 1,2-diacylglycerol (DAG), and even more so of 1,3-DAG, in the mycelia. This was followed by a decrease in both DAGs and an increase in TAG. Eicosapentaenoic acid (EPA), a desaturated and elongated product of ALA, accumulated to a greater extent in cellular phospholipids than in neutral lipids. Moreover, the addition of ALA in free fatty acid form to the culture led to the generation of EPA. However, EPA production was not observed upon addition of ALA-rich 1,2- or 1,3-DAG, indicating that fatty acids released from exogenous lipids were used for resynthesis of mycelial TAG. These results suggested that TAG might be hydrolyzed by extracellular lipases, whereas its synthesis might be catalyzed by intracellular enzymes. Appropriate regulation of such enzymes might be an effective strategy to enhance PUFA production under plant oil supplementation.

Molecular analysis of ∆6 desaturase and ∆6 elongase from Conidiobolus obscurus in the biosynthesis of eicosatetraenoic acid, a ω3 fatty acid with nutraceutical potentials

Conidiobolus obscurus, an entomopathogenic fungus able to infect aphids, was previously reported to produce substantial amounts of very long chain polyunsaturated fatty acids (VLCPUFAs) that may mediate the insect infection. However, the genes involved in the biosynthesis of these VLCPUFAs from the order Entomophthorales have yet to be identified. Using degenerate reverse transcriptase-polymerase chain reaction and rapid amplification of the cDNA end methods, we cloned a ∆6 desaturase cDNA (CoD6) and a ∆6 elongase cDNA (CoE6) from C. obscurus. Expression of CoD6 and CoE6 in Saccharomyces cerevisiae revealed CoD6 could introduce a Δ6 double bond into α-linolenic acid (18:3n-3), and CoE6 preferentially elongated 18-carbon Δ6 desaturated fatty acid stearidonic acid (18:4n-3). When the fungus was grown under a temperature shift from 20 °C to 10 °C, the transcript level of CoD6 and CoE6 increased, whereas when the fungal culture was shifted from 20 °C to 30 °C, the transcript level of both genes decreased. The entire eicosatetraenoic acid biosynthetic pathway was reconstituted in yeast using four genes, CoD6 and CoE6 from C. obscurus, CpDes12 (a Δ12 desaturase) and CpDesX (a ω3 desaturase) from Claviceps purpurea. Yeast transformants expressing the four genes produced ten new fatty acids including the final product eicosatetraenoic acid (ETA). This represents the reconstitution of the entire ETA pathway in yeast without supplementation of any exogenous fatty acids.

Analysis of lipid content and quality in Arabidopsis plastids

Chloroplasts of plants contain an intricate membrane system, the thylakoids, which harbor the complexes of the photosynthetic machinery. Chloroplasts are confined by two membranes, the inner and outer envelope. The major glycerolipids of chloroplasts are the glycolipids monogalactosyl diacylglycerol (MGD), digalactosyl diacylglycerol (DGD), and sulfoquinovosyl diacylglycerol (SQD). Furthermore, two phospholipids, phosphatidyl glycerol (PG) and phosphatidyl choline (PC), are found in chloroplast membranes. The photosystems and light-harvesting complexes in the thylakoids are rich in photosynthetic pigments (chlorophyll, carotenoids, and xanthophylls) and contain a unique set of prenylquinol lipids (tocochromanol/vitamin E, plastoquinol, and phylloquinol/vitamin K1). In this chapter, methods for the isolation and quantification of chloroplast and leaf glycerolipids and prenylquinol lipids are presented. Glycerolipids are separated by thin-layer chromatography prior to conversion of the fatty acids into methyl esters. Fatty acid methyl esters are subsequently quantified by gas chromatography. Prenylquinol lipids are separated by HPLC and quantified by UV absorption (plastoquinol) or fluorescence (tocochromanol, phylloquinol).

Metabolic engineering of Saccharomyces cerevisiae for production of Eicosapentaenoic Acid, using a novel {Delta}5-Desaturase from Paramecium tetraurelia

Very-long-chain polyunsaturated fatty acids, such as arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have well-documented importance in human health and nutrition. Sustainable production in robust host organisms that do not synthesize them naturally requires the coordinated expression of several heterologous desaturases and elongases. In the present study we show production of EPA in Saccharomyces cerevisiae using glucose as the sole carbon source through expression of five heterologous fatty acid desaturases and an elongase. Novel Δ5-desaturases from the ciliate protozoan Paramecium tetraurelia and from the microalgae Ostreococcus tauri and Ostreococcus lucimarinus were identified via a BLAST search, and their substrate preferences and desaturation efficiencies were assayed in a yeast strain producing the ω6 and ω3 fatty acid substrates for Δ5-desaturation. The Δ5-desaturase from P. tetraurelia was up-to-2-fold more efficient than the microalgal desaturases and was also more efficient than Δ5-desaturases from Mortierella alpina and Leishmania major. In vivo investigation of acyl carrier substrate specificities showed that the Δ5-desaturases from P. tetraurelia, O. lucimarinus, O. tauri, and M. alpina are promiscuous toward the acyl carrier substrate but prefer phospholipid-bound substrates. In contrast, the Δ5-desaturase from L. major showed no activity on phospholipid-bound substrate and thus appears to be an exclusively acyl coenzyme A-dependent desaturase.

Three Arabidopsis fatty acyl-coenzyme A reductases, FAR1, FAR4, and FAR5, generate primary fatty alcohols associated with suberin deposition

Suberin is a protective hydrophobic barrier consisting of phenolics, glycerol, and a variety of fatty acid derivatives, including C18:0-C22:0 primary fatty alcohols. An eight-member gene family encoding alcohol-forming fatty acyl-coenzyme A reductases (FARs) has been identified in Arabidopsis (Arabidopsis thaliana). Promoter-driven expression of the beta-glucuronidase reporter gene indicated that three of these genes, FAR1(At5g22500), FAR4(At3g44540), and FAR5(At3g44550), are expressed in root endodermal cells. The three genes were transcriptionally induced by wounding and salt stress. These patterns of gene expression coincide with known sites of suberin deposition. We then characterized a set of mutants with T-DNA insertions in FAR1, FAR4, or FAR5 and found that the suberin compositions of roots and seed coats were modified in each far mutant. Specifically, C18:0-OH was reduced in far5-1, C20:0-OH was reduced in far4-1, and C22:0-OH was reduced in far1-1. We also analyzed the composition of polymer-bound lipids of leaves before and after wounding and found that the basal levels of C18:0-C22:0 primary alcohols in wild-type leaves were increased by wounding. In contrast, C18:0-OH and C22:0-OH were not increased by wounding in far5-1 and far1-1 mutants, respectively. Heterologous expression of FAR1, FAR4, and FAR5 in yeast confirmed that they are indeed active alcohol-forming FARs with distinct, but overlapping, chain length specificities ranging from C18:0 to C24:0. Altogether, these results indicate that Arabidopsis FAR1, FAR4, and FAR5 generate the fatty alcohols found in root, seed coat, and wound-induced leaf tissue.

The sunflower plastidial omega3-fatty acid desaturase (HaFAD7) contains the signalling determinants required for targeting to, and retention in, the endoplasmic reticulum membrane in yeast but requires co-expressed ferredoxin for activity

Although plant plastidial omega3-desaturases are closely related to microsomal desaturases, heterologous expression in yeast of the Helianthus annuus FAD7 omega3-desaturase showed low activity in contrast to similar expression of microsomal FAD3 omega3-desaturases. However, the removal of the plastidial transit peptide and the incorporation of a KKNL motif to the C-terminus of HaFAD7 increased the activity by 10-fold compared to the native protein. N-terminal fusion of transmembrane-domains from either the yeast microsomal ELO3, (a type III signal anchor domain), or FAE1, an endoplasmic reticulum membrane anchoring domain, resulted in moderate increases in enzyme activity (5- and 7-fold, respectively), suggesting that the first, most hydrophobic transmembrane domain of HaFAD7 is sufficient to direct targeting to, and insertion into, the endoplasmic reticulum membrane. Furthermore, fusing a hemagglutinin (HA) epitope tag upstream of an endogenous C-terminal KEK motif resulted in a significant loss of activity compared to the un-tagged construct, indicating that the endogenous KEK C-terminal di-lysine motif is capable of directing in yeast the ER-retention of this normally plastidial-located protein. Western blotting analysis of constructs with internal HA epitope revealed that in whole cell extracts, with the exception of the one bound to C-terminal, it did not display a reduced level of protein accumulation. Whilst ferredoxin was shown to be required for HaFAD7 activity in yeast, it appears not necessary for protein stability and accumulation of this plastidial desaturase in the endoplasmic reticulum.

Fetal bovine serum concentration affects delta9 desaturase activity of Trypanosoma cruzi

Fetal bovine serum (FBS) is an important factor in the culture of Trypanosoma cruzi, since this parasite obtains and metabolizes fatty acids (FAs) from the culture medium, and changes in FBS concentration reduce the degree of unsaturation of FAs in phosphoinositides. When T. cruzi epimastigotes were cultured with 5% instead of 10% FBS, and stearic acid was used as the substrate, (9) desaturase activity decreased by 50%. Apparent K (m) and V (m) values for stearic acid, determined from Lineaweaver-Burk plots, were 2 microM and 219 pmol/min/mg of protein, respectively. In studies of the requirement for reduced pyridine nucleotide, (9) desaturase activity reached a maximum with 8 microM NADH and then remained constant; the apparent K (m) and V (m) were 4.3 microM and 46.8 pmol/min/mg of protein, respectively. The effect of FBS was observed only for (9) desaturase activity; (12) desaturase activity was not affected. The results suggest that decreased FBS in culture medium is a signal that modulates (9) desaturase activity in T. cruzi epimastigotes.

A leaf-based assay using interchangeable design principles to rapidly assemble multistep recombinant pathways

The assembly of multistep recombinant pathways in stably transformed plants is a cornerstone of crops producing new products yet can be a laborious and time-consuming process. Any heterologous expression platform capable of providing a rapid estimation of the functional assembly of an entire pathway would guide the design of such transgenic traits. In this study, we use a Nicotiana benthamiana transient leaf expression system to simultaneously express five genes, from five independent T(DNA) binary vectors, to assemble a complete recombinant pathway in five days. In this study, we demonstrate the production of long-chain polyunsaturated fatty acids (LC-PUFA) requiring five transgene-encoded reactions to convert endogenous fatty acids to LC-PUFA. The addition of a triacylglycerol assembly enzyme, Arabidopsis thaliana diacylglyceride-O-acyltransferase, and fractionation of the total lipid profile demonstrated that leaf oils contained 37% newly synthesised LC-PUFA, including 7% arachidonic acid (AA), 6% eicosopentaenoic acid and 3% docosahexaenoic acid. The calculation of enzymatic conversion efficiencies at each step of LC-PUFA synthesis suggests that this transient assembly of a complicated multistep pathway is highly efficient. Unlike experiments using stably transformed plants our assembly of an intricate pathway maintained full gene-for-gene interchangeability and required a fraction of the time and glasshouse space. Furthermore, an exogenous LC-PUFA fatty acid substrate, AA, was fed and metabolised by a transiently expressed Delta17-desaturase enzyme, and provided results similar to those obtained in yeast feeding experiments. Although the assay was ideal for LC-PUFA pathways, this assay format may become a powerful tool for the characterisation and step-wise improvement of other recombinant pathways and multigenic traits.

Metabolic engineering of omega3-very long chain polyunsaturated fatty acid production by an exclusively acyl-CoA-dependent pathway

omega3-Very long chain polyunsaturated fatty acids (VLCPUFA) are essential for human development and brain function and, thus, are indispensable components of the human diet. The current main source of VLCPUFAs is represented by ocean fish stocks, which are in severe decline, and the development of alternative, sustainable sources of VLCPUFAs is urgently required. Our research aims at exploiting the powerful infrastructure available for the large scale culture of oilseed crops, such as rapeseed, to produce VLCPUFAs such as eicosapentaenoic acid in transgenic plants. VLCPUFA biosynthesis requires repeated desaturation and repeated elongation of long chain fatty acid substrates. In previous experiments the production of eicosapentaenoic acid in transgenic plants was found to be limited by an unexpected bottleneck represented by the acyl exchange between the site of desaturation, endoplasmic reticulum-associated phospholipids, and the site of elongation, the cytosolic acyl-CoA pool. Here we report on the establishment of a coordinated, exclusively acyl-CoA-dependent pathway, which avoids the rate-limiting transesterification steps between the acyl lipids and the acyl-CoA pool during VLCPUFA biosynthesis. The pathway is defined by previously uncharacterized enzymes, encoded by cDNAs isolated from the microalga Mantoniella squamata. The conceptual enzymatic pathway was established and characterized first in yeast to provide proof-of-concept data for its feasibility and subsequently in seeds of Arabidopsis thaliana. The comparison of the acyl-CoA-dependent pathway with the known lipid-linked pathway for VLCPUFA biosynthesis showed that the acyl-CoA-dependent pathway circumvents the bottleneck of switching the Delta6-desaturated fatty acids between lipids and acyl-CoA in Arabidopsis seeds.

Heterologous production of dihomo-gamma-linolenic acid in Saccharomyces cerevisiae

To make dihomo-gamma-linolenic acid (DGLA) (20:3n-6) in Saccharomyces cerevisiae, we introduced Kluyveromyces lactis Delta12 fatty acid desaturase, rat Delta6 fatty acid desaturase, and rat elongase genes. Because Fad2p is able to convert the endogenous oleic acid to linoleic acid, this allowed DGLA biosynthesis without the need to supply exogenous fatty acids on the media. Medium composition, cultivation temperature, and incubation time were examined to improve the yield of DGLA. Fatty acid content was increased by changing the medium from a standard synthetic dropout medium to a nitrogen-limited minimal medium (NSD). Production of DGLA was higher in the cells grown at 15 degrees C than in those grown at 20 degrees C, and no DGLA production was observed in the cells grown at 30 degrees C. In NSD at 15 degrees C, fatty acid content increased up until day 7 and decreased after day 10. When the cells were grown in NSD for 7 days at 15 degrees C, the yield of DGLA reached 2.19 microg/mg of cells (dry weight) and the composition of DGLA to total fatty acids was 2.74%. To our knowledge, this is the first report describing the production of polyunsaturated fatty acids in S. cerevisiae without supplying the exogenous fatty acids.

Engineering oilseeds for sustainable production of industrial and nutritional feedstocks: solving bottlenecks in fatty acid flux

Oilseeds provide a unique platform for the production of high-value fatty acids that can replace non-sustainable petroleum and oceanic sources of specialty chemicals and aquaculture feed. However, recent efforts to engineer the seeds of crop and model plant species to produce new types of fatty acids, including hydroxy and conjugated fatty acids for industrial uses and long-chain omega-3 polyunsaturated fatty acids for farmed fish feed, have met with only modest success. The collective results from these studies point to metabolic 'bottlenecks' in the engineered plant seeds that substantially limit the efficient or selective flux of unusual fatty acids between different substrate pools and ultimately into storage triacylglycerol. Evidence is emerging that diacylglycerol acyltransferase 2, which catalyzes the final step in triacylglycerol assembly, is an important contributor to the synthesis of unusual fatty acid-containing oils, and is likely to be a key target for future oilseed metabolic engineering efforts.

Cloning and characterization of unusual fatty acid desaturases from Anemone leveillei: identification of an acyl-coenzyme A C20 Delta5-desaturase responsible for the synthesis of sciadonic acid

The seed oil of Anemone leveillei contains significant amounts of sciadonic acid (20:3Delta(5,11,14); SA), an unusual non-methylene-interrupted fatty acid with pharmaceutical potential similar to arachidonic acid. Two candidate cDNAs (AL10 and AL21) for the C(20) Delta(5cis)-desaturase from developing seeds of A. leveillei were functionally characterized in transgenic Arabidopsis (Arabidopsis thaliana) plants. The open reading frames of both Delta(5)-desaturases showed some similarity to presumptive acyl-coenzyme A (CoA) desaturases found in animals and plants. When expressed in transgenic Arabidopsis, AL21 showed a broad range of substrate specificity, utilizing both saturated (16:0 and 18:0) and unsaturated (18:2, n-6 and 18:3, n-3) substrates. In contrast, AL10 did not show any activity in wild-type Arabidopsis. Coexpression of AL10 or AL21 with a C(18) Delta(9)-elongase in transgenic Arabidopsis plants resulted in the production of SA and juniperonic fatty acid (20:4Delta(5,11,14,17)). Thus, AL10 acted only on C(20) polyunsaturated fatty acids in a manner analogous to "front-end" desaturases. However, neither AL10 nor AL21 contain the cytochrome b(5) domain normally present in this class of enzymes. Acyl-CoA profiling of transgenic Arabidopsis plants and developing A. leveillei seeds revealed significant accumulation of Delta(5)-unsaturated fatty acids as acyl-CoAs compared to the accumulation of these fatty acids in total lipids. Positional analysis of triacylglycerols of A. leveillei seeds showed that Delta(5)-desaturated fatty acids were present in both sn-2 and sn-1 + sn-3 positions, although the majority of 16:1Delta(5), 18:1Delta(5), and SA was present at the sn-2 position. Our data provide biochemical evidence for the A. leveillei Delta(5)-desaturases using acyl-CoA substrates.

Elongation of polyunsaturated fatty acids in trypanosomatids

Leishmania major synthesizes polyunsaturated fatty acids by using Delta6, Delta5 and Delta4 front-end desaturases, which have recently been characterized [Tripodi KE, Buttigliero LV, Altabe SG & Uttaro AD (2006) FEBS J273, 271-280], and two predicted elongases specific for C18 Delta6 and C20 Delta5 polyunsaturated fatty acids, respectively. Trypanosoma brucei and Trypanosoma cruzi lack Delta6 and Delta5 desaturases but contain Delta4 desaturases, implying that trypanosomes use exogenous polyunsaturated fatty acids to produce C22 Delta4 fatty acids. In order to identify putative precursors of these C22 fatty acids and to completely describe the pathways for polyunsaturated fatty acid biosynthesis in trypanosomatids, we have performed a search in the three genomes and identified four different elongase genes in T. brucei, five in T. cruzi and 14 in L. major. After a phylogenetic analysis of the encoded proteins together with elongases from a variety of other organisms, we selected four candidate polyunsaturated fatty acid elongases. Leishmania major CAJ02037, T. brucei AAX69821 and T. cruzi XP_808770 share 57-52% identity, and group together with C20 Delta5 polyunsaturated fatty acid elongases from algae. The predicted activity was corroborated by functional characterization after expression in yeast. T. brucei elongase was also able to elongate Delta8 and Delta11 C20 polyunsaturated fatty acids. L. major CAJ08636, which shares 33% identity with Mortierella alpinaDelta6 elongase, showed a high specificity for C18 Delta6 polyunsaturated fatty acids. In all cases, a preference for n6 polyunsaturated fatty acids was observed. This indicates that L. major has, as predicted, Delta6 and Delta5 elongases and a complete pathway for polyunsaturated fatty acid biosynthesis. Trypanosomes contain only Delta5 elongases, which, together with Delta4 desaturases, allow them to use eicosapentaenoic acid and arachidonic acid, a precursor that is relatively abundant in the host, for C22 polyunsaturated fatty acid biosynthesis.

The production of very-long-chain PUFA biosynthesis in transgenic plants: towards a sustainable source of fish oils

There is now considerable evidence of the importance of n-3 long-chain PUFA in human health and development. At the same time, the marine fish stocks that serve as the primary sources of these fatty acids are threatened by continued over-exploitation. Thus, there is an urgent need to provide a sustainable alternative source of the n-3 long-chain PUFA normally found in fish oils. The possibility of using transgenic plants genetically engineered to synthesise these important fatty acids has recently been demonstrated. The approaches taken to realise this outcome will be discussed, as will their prospects for providing a sustainable resource for the future.

The production of unusual fatty acids in transgenic plants

The ability to genetically engineer plants has facilitated the generation of oilseeds synthesizing non-native fatty acids. Two particular classes of fatty acids are considered in this review. First, so-called industrial fatty acids, which usually contain functional groups such as hydroxyl, epoxy, or acetylenic bonds, and second, very long chain polyunsaturated fatty acids normally found in fish oils and marine microorganisms. For industrial fatty acids, there has been limited progress toward obtaining high-level accumulation of these products in transgenic plants. For very long chain polyunsaturated fatty acids, although they have a much more complex biosynthesis, accumulation of some target fatty acids has been remarkably successful. In this review, we consider the probable factors responsible for these different outcomes, as well as the potential for further optimization of the transgenic production of unusual fatty acids in transgenic plants.

Fatty acid production in Schizochytrium sp.: Involvement of a polyunsaturated fatty acid synthase and a type I fatty acid synthase

Schizochytrium sp. is a marine microalga that has been developed as a commercial source for docosahexaenoic acid (DHA, C22:6 (omega-3), enriched biomass, and oil. Previous work suggested that the DHA, as well as docosapentaenoic acid (DPA, C22:5 omega-6), that accumulate in Schizochytrium are products of a multi-subunit polyunsaturated fatty acid (PUFA) synthase (1). Here we show data to support this view and also provide information on other aspects of fatty acid synthesis in this organism. Three genes encoding subunits of the PUFA synthase were isolated from genomic DNA and expressed in E. coli along with an essential accessory gene encoding a phosphopantetheinyl transferase (PPTase). The resulting transformants accumulated both DHA and DPA. The ratio of DHA to DPA was approximately the same as that observed in Schizochytrium. Treatment of Schizochytrium cells with certain levels of cerulenin resulted in inhibition of 14C acetate incorporation into short chain fatty acids without affecting labeling of PUFAs, indicating distinct biosynthetic pathways. A single large gene encoding the presumed short chain fatty acid synthase (FAS) was cloned and sequenced. Based on sequence homology and domain organization, the Schizochytrium FAS resembles a fusion of fungal FAS beta and alpha subunits.

A Bifunctional Δ12,Δ15-Desaturase from Acanthamoeba castellanii Directs the Synthesis of Highly Unusual n-1 Series Unsaturated Fatty Acids

The free-living soil protozoon Acanthamoeba castellanii synthesizes a range of polyunsaturated fatty acids, the balance of which can be altered by environmental changes. We have isolated and functionally characterized in yeast a microsomal desaturase from A. castellanii, which catalyzes the sequential conversion of C(16) and C(18) Delta9-monounsaturated fatty acids to di- and tri-unsaturated forms. In the case of C(16) substrates, this bifunctional A. castellanii Delta12,Delta15-desaturase generated a highly unusual fatty acid, hexadecatrienoic acid (16:3Delta(9,12,15)(n-1)). The identification of a desaturase, which can catalyze the insertion of a double bond between the terminal two carbons of a fatty acid represents a new addition to desaturase functionality and plasticity. We have also co-expressed in yeast the A. castellanii bifunctional Delta12,Delta15-desaturase with a microsomal Delta6-desaturase, resulting in the synthesis of the highly unsaturated C(16) fatty acid hexadecatetraenoic acid (16:4Delta(6,9,12,15)(n-1)), previously only reported in marine microorganisms. Our work therefore demonstrates the feasibility of the heterologous synthesis of polyunsaturated fatty acids of the n-1 series. The presence of a bifunctional Delta12,Delta15-desaturase in A. castellanii is also considered with reference to the evolution of desaturases and the lineage of this protist.

Mice raised on milk transgenically enriched with n-3 PUFA have increased brain docosahexaenoic acid

The brain contains high levels of the long-chain n-3 FA DHA (22:6n-3), mainly in the gray matter and synaptosomes. Adequate intake of DHA is crucial for optimal nervous system function, particularly in infants. Supplementation of infant formulas with DHA at levels similar to human breast milk is recommended for biochemical and functional benefits to neonates. We generated transgenic mice that produce elevated levels of n-3 PUFA in their milk by expressing the Caenorhabditis elegans n-3 FA desaturase under the control of a lactation-induced goat beta-casein promoter. To examine the postnatal effects of consuming the n-3-enriched milk, we compared the growth and brain and plasma FA composition of mouse pups raised on milk from transgenic dams with those observed for pups raised on milk from nontransgenic dams. A significant decrease in arachidonic acid (ARA, 20:4n-6) and concomitant increases in n-3 PUFA were observed in the phospholipid fraction of transgenic mouse milk. The n-6:n-3 FA ratios were 4.7 and 34.5 for the transgenic and control milk phospholipid fractions, respectively. DHA and DPA (22:5n-6) comprised 15.1% and 2.8% of brain FA from weanling mice nursed on transgenic dams, as compared with 6.9% and 9.2% for weanling mice nursed on control dams, respectively. This transgenic mouse model offers a unique approach to disassociate the effects and fetal programming resulting from a high n-6:n-3 FA ratio gestational environment from the postnatal nutritional effects of providing milk with differing n-6:n-3 FA ratios.

Functional expression of Spirulina-Delta6 desaturase gene in yeast, Saccharomyces cerevisiae

Spirulina-acyl-lipid desaturases are membrane-bound enzymes found in thylakoid and plasma membranes. These enzymes carry out the fatty acid desaturation process of Spirulina to yield gamma-linolenic acid (GLA) as the final desaturation product. In this study, Spirulina-Delta(6) desaturase encoded by the desD gene was heterologously expressed and characterized in Saccharomyces cerevisiae. We then conducted site-directed mutagenesis of the histidine residues in the three histidine boxes to determine the role of these amino acid residues in the enzyme function. Our results showed that while four mutants showed complete loss of Delta(6)-desaturase activity and two mutants showed only trace of the activity, the enzyme activity could be partially restored by chemical rescue using exogenously provided imidazole. This study reveals that the histidine residues (which have imidazole as their functional group) in the conserved clusters play a critical role in Delta(6)-desaturase activity, possibly by providing a di-iron catalytic center. In our previous study, this enzyme was expressed in Escherichia coli. The results reveal that the enzyme can function only in the presence of an exogenous cofactor, ferredoxin, provided in vitro. This evidence suggests that baker's yeast has a cofactor that can complement ferredoxin, thought to act as an electron donor for the Delta(6) desaturation in cyanobacteria, including Spirulina. The electron donor of the Spirulina-Delta(6) desaturation in yeast is more likely to be cytochrome b(5), which is absent in E. coli. This means that the enzyme expressed in S. cerevisiae can catalyze the biosynthesis of the product, GLA, in vivo.

Regulation of long chain unsaturated fatty acid synthesis in yeast

Saccharomyces cerevisiae forms monounsaturated fatty acids using the ER membrane-bound Delta-9 fatty acid desaturase, Ole1p, an enzyme system that forms a double bond in saturated fatty acyl CoA substrates. Ole1p is a chimeric protein consisting of an amino terminal desaturase domain fused to cytochrome b5. It catalyzes the formation of the double bond through an oxygen-dependent mechanism that requires reducing equivalents from NADH. These are transferred to the enzyme via NADH cytochrome b5 reductase to the Ole1p cytochrome b5 domain and then to the diiron-oxo catalytic center of the enzyme. The control of OLE1 gene expression appears to mediated through the ER membrane proteins Spt23p and Mga2p. N-terminal fragments of these proteins are released by an ubiquitin/proteasome mediated proteolysis system and translocated to the nucleus where they appear to act as transcription coactivators of OLE1. OLE1 is regulated through Spt23p and Mga2p by multiple systems that control its transcription and mRNA stability in response to diverse stimuli that include nutrient fatty acids, carbon source, metal ions and the availability of oxygen.

Biosynthesis of isoprenoids, polyunsaturated fatty acids and flavonoids in Saccharomyces cerevisiae

Industrial biotechnology employs the controlled use of microorganisms for the production of synthetic chemicals or simple biomass that can further be used in a diverse array of applications that span the pharmaceutical, chemical and nutraceutical industries. Recent advances in metagenomics and in the incorporation of entire biosynthetic pathways into Saccharomyces cerevisiae have greatly expanded both the fitness and the repertoire of biochemicals that can be synthesized from this popular microorganism. Further, the availability of the S. cerevisiae entire genome sequence allows the application of systems biology approaches for improving its enormous biosynthetic potential. In this review, we will describe some of the efforts on using S. cerevisiae as a cell factory for the biosynthesis of high-value natural products that belong to the families of isoprenoids, flavonoids and long chain polyunsaturated fatty acids. As natural products are increasingly becoming the center of attention of the pharmaceutical and nutraceutical industries, the use of S. cerevisiae for their production is only expected to expand in the future, further allowing the biosynthesis of novel molecular structures with unique properties.

Ypr140wp, 'the yeast tafazzin', displays a mitochondrial lysophosphatidylcholine (lyso-PC) acyltransferase activity related to triacylglycerol and mitochondrial lipid synthesis

When the yeast protein Ypr140w was expressed in Escherichia coli, a lyso-PC [lysophosphatidylcholine (1-acylglycerophosphorylcholine)] acyltransferase activity was found associated with the membranes of the bacteria. To our knowledge, this is the first identification of a protein capable of catalysing the acylation of lyso-PC molecules to form PC. Fluorescence microscopy analysis of living yeasts revealed that the fusion protein Ypr140w-green fluorescent protein is targeted to the mitochondria. Moreover, in contrast with wild-type cells, in the absence of acyl-CoA, the yeast mutant deleted for the YPR140w gene has no lyso-PC acyltransferase activity associated with the mitochondrial fraction. When yeast cells were grown in the presence of lactate, the mutant synthesized 2-fold more triacylglycerols when compared with the wild-type. Moreover, its mitochondrial membranes contained a lesser amount of PC and cardiolipin, and the fatty acid composition of these latter was greatly changed. These modifications were accompanied by a 2-fold increase in the respiration rates (states 3 and 4) of the mitochondria. The relationship between the deletion of the YPR140w gene and the lipid composition of the ypr140wDelta cells is discussed.

Metabolic Engineering of Plants to Produce Very Long-chain Polyunsaturated Fatty Acids

Very long-chain polyunsaturated fatty acids (VLCPUFAs) are essential for human health and well-being. However, the current sources of these valuable compounds are limited and may not be sustainable in the long term. Recently, considerable progress has been made in identifying genes involved in the biosynthesis of VLCPUFAs. The co-expression of these genes in model systems such as plant embryos or yeast provided many valuable insights into the mechanisms of VLCPUFA synthesis. The recent successful reconstitution of pathways leading to the synthesis of arachidonic acid, eicosapentaenoic acid and finally docosahexaenoic acid in oil-seed plants indicates the feasibility of using transgenic crops as alternative sources of VLCPUFAs. The various approaches used to attain these results and the specific constraints associated with each approach are discussed.

Revealing the complementation of ferredoxin by cytochrome b (5) in the Spirulina- (6)-desaturation reaction by N-terminal fusion and co-expression of the fungal-cytochrome b (5) domain and Spirulina- (6)-acyl-lipid desaturase

Spirulina-acyl-lipid desaturases are integral membrane proteins found in thylakoid and plasma membranes. These enzymes catalyze the fatty acid desaturation process of Spirulina to yield gamma-linolenic acid (GLA) as the final desaturation product. It has been reported that the cyanobacterial desaturases use ferredoxin as an electron donor, whereas the acyl-lipid desaturase in plant cytoplasm and the acyl-CoA desaturase of animals and fungi use cytochrome b (5). The low level of ferredoxin present in Escherichia coli cells leads to an inability to synthesize GLA when the cells are transformed with the Spirulina-(6) desaturase, desD, and grown in the presence of the reaction substrate, linoleic acid. In this study, Spirulina-(6) desaturase, encoded by the desD gene, was N-terminally fused and co-expressed with the cytochrome b (5) domain from Mucor rouxii. The product, GLA, made heterologously in E. coli and Saccharomyces cerevisiae, was then detected and analyzed. The results revealed the production of GLA by Spirulina-(6) desaturase fused or co-expressed with cytochrome b (5) in E. coli cells, in which GLA production by this gene cannot occur in the absence of cytochrome b (5). Moreover, the GLA production ability in the E. coli host cells was lost after the single substitution mutation was introduced to H52 in the HPGG motif of the cytochrome b (5) domain. These results revealed the complementation of the ferredoxin requirement by the fusion or co-expression of the fungal-cytochrome b (5) domain in the desaturation process of Spirulina-(6) desaturase. Furthermore, the free form of cytochrome b (5) domain can also enhance GLA production by the Spirulina-desD gene in yeast cells.

The alternative pathway C20 Delta8-desaturase from the non-photosynthetic organism Acanthamoeba castellanii is an atypical cytochrome b5-fusion desaturase

A cDNA encoding a C20 Delta8-desaturase was isolated from the free-living soil amoeba, Acanthamoeba castellanii and functionally characterised by heterologous expression. The open reading frame of the A. castellanii C20 Delta8-desaturase showed similarity to other microsomal front-end desaturases, but the N-terminal domain contained a variant form of the conserved heme-binding motif in which H-P-G-G is replaced by H-P-A-G. Co-expression of the A. castellani Delta8-desaturase with the Isochrysis galbana Delta9-elongase in transgenic Arabidopsis plants confirmed the activity observed in yeast and its role in the alternative pathway for C20 polyunsaturated fatty acid synthesis. Acyl-CoA profiles of these transgenic plants revealed an unexpected accumulation of C20 fatty acids in the acyl-CoA pool. This is the first report of an alternative pathway C20 Delta8-desaturase from a non-photosynthetic organism, and also the first report of a front-end desaturase lacking the canonical cytochrome b5 domain.

Engineering cells for cell culture bioprocessing--physiological fundamentals

In the past decade, we have witnessed a tremendous increase in the number of mammalian cell-derived therapeutic proteins with clinical applications. The success of making these life-saving biologics available to the public is partly due to engineering efforts to enhance process efficiency. To further improve productivity, much effort has been devoted to developing metabolically engineered producing cells, which possess characteristics favorable for large-scale bioprocessing. In this article we discuss the fundamental physiological basis for cell engineering. Different facets of cellular mechanisms, including metabolism, protein processing, and the balancing pathways of cell growth and apoptosis, contribute to the complex traits of favorable growth and production characteristics. We present our assessment of the current state of the art by surveying efforts that have already been undertaken in engineering cells for a more robust process. The concept of physiological homeostasis as a key determinant and its implications on cell engineering is emphasized. Integrating the physiological perspective with cell culture engineering will facilitate attainment of dream cells with superlative characteristics.

A front-end desaturase from Chlamydomonas reinhardtii produces pinolenic and coniferonic acids by omega13 desaturation in methylotrophic yeast and tobacco

Pinolenic acid (PA; 18:3Delta(5,9,12)) and coniferonic acid (CA; 18:4Delta(5,9,12,15)) are Delta(5)-unsaturated bis-methylene-interrupted fatty acids (Delta(5)-UBIFAs) commonly found in pine seed oil. They are assumed to be synthesized from linoleic acid (LA; 18:2Delta(9,12)) and alpha-linolenic acid (ALA; 18:3Delta(9,12,15)), respectively, by Delta(5)-desaturation. A unicellular green microalga Chlamydomonas reinhardtii also accumulates PA and CA in a betain lipid. The expressed sequence tag (EST) resource of C. reinhardtii led to the isolation of a cDNA clone that encoded a putative fatty acid desaturase named as CrDES containing a cytochrome b5 domain at the N-terminus. When the coding sequence was expressed heterologously in the methylotrophic yeast Pichia pastoris, PA and CA were newly detected and comparable amounts of LA and ALA were reduced, demonstrating that CrDES has Delta(5)-desaturase activity for both LA and ALA. CrDES expressed in the yeast showed Delta(5)-desaturase activity on 18:1Delta(9) but not 18:1Delta(11). Unexpectedly, CrDES also showed Delta(7)-desaturase activity on 20:2Delta(11,14) and 20:3Delta(11,14,17) to produce 20:3Delta(7,11,14) and 20:4Delta(7,11,14,17), respectively. Since both the Delta(5) bond in C18 and the Delta(7) bond in C20 fatty acids are 'omega13' double bonds, these results indicate that CrDES has omega13 desaturase activity for omega9 unsaturated C18/C20 fatty acids, in contrast to the previously reported front-end desaturases. In order to evaluate the activity of CrDES in higher plants, transgenic tobacco plants expressing CrDES were created. PA and CA accumulated in the leaves of transgenic plants. The highest combined yield of PA and CA was 44.7% of total fatty acids, suggesting that PA and CA can be produced in higher plants on a large scale.

In vivo characterization of the first acyl-CoA Delta6-desaturase from a member of the plant kingdom, the microalga Ostreococcus tauri

Genomic DNA of Ostreococcus tauri, a fully sequenced marine unicellular alga from the phytoplankton, was used to amplify a gene coding for a typical front-end desaturase involved in polyunsaturated fatty acid biosynthesis. Heterologous expression in Saccharomyces cerevisiae revealed very high desaturation activity with Delta6-regioselectivity. Short-time kinetic experiments showed that the desaturase product was detected in the acyl-CoA pool 5 min after addition of the exogenous substrate to the yeast medium and long before its appearance in the total fatty acids. When this desaturase was co-expressed with the acyl-CoA Delta6-elongase from Physcomitrella patens and the lipid-linked Delta5-desaturase from Phaeodactylum tricornutum, high proportions of arachidonic or eicosapentaenoic acid were obtained, because nearly all of the Delta6-desaturated products were elongated. Furthermore, the product/educt ratios calculated in each glycerolipid for the Delta6-desaturase or for the acyl-CoA Delta6-elongase were in about the same range, whereas this ratio showed a very uneven profile in the case of the lipid-linked Delta5-desaturase. Finally, a sequence-based comparison of all the functionally characterized Delta6-desaturases showed that this enzyme was not related to any previously described sequence. Altogether, our data suggest that this desaturase from O. tauri is an acyl-CoA Delta6-desaturase, the first one cloned from a photosynthetically active organism.

Glycoengineering of cyanobacterial thylakoid membranes for future studies on the role of glycolipids in photosynthesis

The lipid composition of thylakoid membranes is conserved from cyanobacteria to angiosperms. The predominating components are monogalactosyl- and digalactosyldiacylglycerol. In cyanobacteria, thylakoid membrane biosynthesis starts with the formation of monoglucosyldiacylglycerol which is C4-epimerized to the corresponding galactolipid, whereas in plastids monogalactosyldiacylglycerol is formed at the beginning. This suggests that galactolipids have specific functions in thylakoids. We wanted to investigate whether galactolipids can be replaced by glycosyldiacylglycerols with headgroups differing in their epimeric and anomeric details as well as the attachment point of the terminal hexose in diglycosyldiacylglycerols. For this purpose putative glycosyltransferase sequences were identified in databases to be used for functional expression in various host organisms. From 18 newly identified sequences, four turned out to encode glycosyltransferases catalyzing final steps in glycolipid biosynthesis: two alpha-glucosyltransferases, one beta-galactosyltransferase and one beta-glucosyltransferase. Their functional annotation was based on detailed structural characterization of the new glycolipids formed in the transformant hosts as well as on in vitro enzymatic assays. The expression of alpha-glucosyltransferases in the cyanobacterium Synechococcus resulted in the accumulation of the new alpha-galactosyldiacylglycerol which is ascribed to epimerization of the corresponding glucolipid. The expression of the beta-glucosyltransferase led to a high proportion of new beta-glucosyl-(1-->6)-beta-galactosyldiacylglycerol almost entirely replacing the native digalactosyldiacylglycerol. These results demonstrate that modifications of the glycolipid pattern in thylakoids are possible.

Stepwise engineering to produce high yields of very long-chain polyunsaturated fatty acids in plants

Very long chain polyunsaturated fatty acids (VLCPUFAs) such as arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are valuable commodities that provide important human health benefits. We report the transgenic production of significant amounts of AA and EPA in Brassica juncea seeds via a stepwise metabolic engineering strategy. Using a series of transformations with increasing numbers of transgenes, we demonstrate the incremental production of VLCPUFAs, achieving AA levels of up to 25% and EPA levels of up to 15% of total seed fatty acids. Both fatty acids were almost exclusively found in triacylglycerols, with AA located preferentially at sn-2 and sn-3 positions and EPA distributed almost equally at all three positions. Moreover, we reconstituted the DHA biosynthetic pathway in plant seeds, demonstrating the practical feasibility of large-scale production of this important omega-3 fatty acid in oilseed crops.

Enzymes for transgenic biosynthesis of long-chain polyunsaturated fatty acids

Polyunsaturated fatty acids (PUFAs) are important for the normal development and function of all organisms, and are essential in maintaining human health. Impaired PUFA metabolism is thought to be associated with pathogenesis of many chronic diseases. Dietary supplementation of PUFAs, such as gamma-linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid, which bypass the defective or dysfunctional steps of the biosynthetic pathway has been found to significantly alleviate the symptoms of the disease. These findings have drawn a great deal of interest from general public and food manufacturers. As the demand of these beneficial PUFAs has drastically increased in recent years, there are also increasing efforts in finding the alternate sources of PUFAs that are more economical and sustainable. One option is to modify the oil-seed crops to produce PUFAs through genetic engineering technique. This review examines the isolation, identification and expression of genes encoding the enzymes required for the biosynthesis of the above mentioned PUFAs in plants.

A micromethod for the stereochemical analysis of fatty acid desaturase-mediated sulfoxidation reactions

The stereochemistry of fatty acid desaturase-mediated sulfoxidation can be evaluated at micromolar concentrations of analyte using (19)F NMR in combination with a chiral shift reagent: (S)-(+)-MPAA.

Progress toward the production of long-chain polyunsaturated fatty acids in transgenic plants

Long-chain PUFA such as eicosapentaenoic and docosahexaenoic acids are prevalent in fish oils, and these compounds have been demonstrated to play important roles in human health and nutrition. In particular, these n-3/omega-3 long-chain PUFA provide protection from cardiovascular disease and a collection of symptoms (termed metabolic syndrome) associated with progression toward type 2 diabetes and obesity. Within Western populations, a large increase in the occurrence of these conditions represents a major public health concern. Unfortunately, both marine fish stocks and (consequentially) consumption of fish oils are in steep decline, limiting the protective role of long-chain PUFA in human health. One alternative approach to the provision of these health-beneficial FA is via their synthesis in transgenic plants. This review will describe recent advances in the production of transgenic plant oils nutritionally enhanced to produce long-chain PUFA.

Identification of two novel microalgal enzymes involved in the conversion of the omega3-fatty acid, eicosapentaenoic acid, into docosahexaenoic acid

Marine microalgae such as Pavlova and Isochrysis produce abundant amounts of the omega3-PUFAs (polyunsaturated fatty acids), EPA (eicosapentaenoic acid, 20:5n-3) and DHA (docosahexaenoic acid, 22:6n-3). The pathway leading to the conversion of EPA into DHA in these lower eukaryotes is not well established although it is predicted to involve an elongation step, catalysed by an elongating enzyme complex, leading to the conversion of EPA into omega3-DPA (omega-3-docosapentaenoic acid, 22:5n-3); followed by a desaturation step, catalysed by a Delta4-desaturase, which results in the conversion of DPA into DHA. To date, the enzymes involved in the elongation of EPA have not been identified from any lower eukaryote. In the present study, we describe the identification of microalgal genes involved in the two-step conversion of EPA into DHA. By expressed sequence tag analysis, a gene (pavELO) encoding a novel elongase was identified from Pavlova, which catalysed the conversion of EPA into omega3-DPA in yeast. Unlike any previously identified elongase from higher or lower eukaryotes, this enzyme displayed unique substrate specificity for both n-6 and n-3 C20-PUFA substrates, with no activity towards any C18- or C22-PUFA substrates. In addition, a novel Delta4-desaturase gene (IgD4) was isolated from Isochrysis, which was capable of converting omega3-DPA into DHA, as well as adrenic acid (22:4n-6) into omega6-DPA. Yeast co-expression studies, with pavELO and IgD4, revealed that these genes were capable of functioning together to carry out the two-step conversion of EPA into DHA.

Identification of a long-chain polyunsaturated fatty acid acyl-coenzyme A synthetase from the diatom Thalassiosira pseudonana

The draft genome of the diatom Thalassiosira pseudonana was searched for DNA sequences showing homology with long-chain acyl-coenzyme A synthetases (LACSs), since the corresponding enzyme may play a key role in the accumulation of health-beneficial polyunsaturated fatty acids (PUFAs) in triacylglycerol. Among the candidate genes identified, an open reading frame named TplacsA was found to be full length and constitutively expressed during cell cultivation. The predicted amino acid sequence of the corresponding protein, TpLACSA, exhibited typical features of acyl-coenzyme A (acyl-CoA) synthetases involved in the activation of long-chain fatty acids. Feeding experiments carried out in yeast (Saccharomyces cerevisiae) transformed with the algal gene showed that TpLACSA was able to activate a number of PUFAs, including eicosapentaenoic acid and docosahexaenoic acid (DHA). Determination of acyl-CoA synthetase activities by direct measurement of acyl-CoAs produced in the presence of different PUFA substrates showed that TpLACSA was most active toward DHA. Heterologous expression also revealed that TplacsA transformants were able to incorporate more DHA in triacylglycerols than the control yeast.