Molecular Characterization of the Elaeis guineensis Medium-Chain Fatty Acid Diacylglycerol Acyltransferase DGAT1-1 by Heterologous Expression in Yarrowia lipolytica

Lipidomic Profiles of Lipid Biosynthesis in Oil Palm during Fruit Development

The fruit of the oil palm (Elaeis guineensis Jacq.) has fleshy mesocarpic tissue rich in lipids. This edible vegetable oil is economically and nutritionally significant across the world. The core concepts of oil biosynthesis in oil palms remain to be researched as the knowledge of oil biosynthesis in plants improves. In this study, we utilized a metabolite approach and mass spectral analysis to characterize metabolite changes and identify the sequences of protein accumulation during the physiological processes that regulate oil synthesis during oil palm fruit ripening. Here, we performed a comprehensive lipidomic data analysis in order to understand the role of lipid metabolism in oil biosynthesis mechanisms. The experimental materials were collected from the mesocarp of oil palm (Tenera) at 95 days (early accumulation of fatty acid, first stage), 125 days (rapid growth of fatty acid accumulation, second stage), and 185 days (stable period of fatty acid accumulation, third stage) after pollination. To gain a clear understanding of the lipid changes that occurred during the growth of the oil palm, the metabolome data were found using principal component analysis (PCA). Furthermore, the accumulations of diacylglycerols, ceramides, phosphatidylethanolamine, and phosphatidic acid varied between the developmental stages. Differentially expressed lipids were successfully identified and functionally classified using KEGG analysis. Proteins related to the metabolic pathway, glycerolipid metabolism, and glycerphospholipid metabolism were the most significantly changed proteins during fruit development. In this study, LC-MS analysis and evaluation of the lipid profile in different stages of oil palm were performed to gain insight into the regulatory mechanisms that enhance fruit quality and govern differences in lipid composition and biosynthesis.

Acyl-CoA-dependent and acyl-CoA-independent avocado acyltransferases positively influence oleic acid content in nonseed triacylglycerols

In higher plants, acyl-CoA:diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase (PDAT) catalyze the terminal step of triacylglycerol (TAG) synthesis in acyl-CoA-dependent and -independent pathways, respectively. Avocado (Persea americana) mesocarp, a nonseed tissue, accumulates significant amounts of TAG (~70% by dry weight) that is rich in heart-healthy oleic acid (18:1). The oil accumulation stages of avocado mesocarp development coincide with high expression levels for type-1 DGAT (DGAT1) and PDAT1, although type-2 DGAT (DGAT2) expression remains low. The strong preference for oleic acid demonstrated by the avocado mesocarp TAG biosynthetic machinery represents lucrative biotechnological opportunities, yet functional characterization of these three acyltransferases has not been explored to date. We expressed avocado PaDGAT1, PaDGAT2, and PaPDAT1 in bakers' yeast and leaves of Nicotiana benthamiana. PaDGAT1 complemented the TAG biosynthesis deficiency in the quadruple mutant yeast strain H1246, and substantially elevated total cellular lipid content. In vitro enzyme assays showed that PaDGAT1 prefers oleic acid compared to palmitic acid (16:0). Both PaDGAT1 and PaPDAT1 increased the lipid content and elevated oleic acid levels when expressed independently or together, transiently in N. benthamiana leaves. These results indicate that PaDGAT1 and PaPDAT1 prefer oleate-containing substrates, and their coordinated expression likely contributes to sustained TAG synthesis that is enriched in oleic acid. This study establishes a knowledge base for future metabolic engineering studies focused on exploitation of the biochemical properties of PaDGAT1 and PaPDAT1.

A yeast-based tool for screening mammalian diacylglycerol acyltransferase inhibitors

Dysregulation of lipid metabolism is associated with obesity and metabolic diseases but there is also increasing evidence of a relationship between lipid body excess and cancer. Lipid body synthesis requires diacylglycerol acyltransferases (DGATs) which catalyze the last step of triacylglycerol synthesis from diacylglycerol and acyl-coenzyme A. The DGATs and in particular DGAT2, are therefore considered potential therapeutic targets for the control of these pathologies. Here, the murine and the human DGAT2 were overexpressed in the oleaginous yeast Yarrowia lipolytica deleted for all DGAT activities, to evaluate the functionality of the enzymes in this heterologous host and DGAT activity inhibitors. This work provides evidence that mammalian DGATs expressed in Y. lipolytica are a useful tool for screening chemical libraries to identify potential inhibitors or activators of these enzymes of therapeutic interest.

Chromosome-level genome assembly and resequencing of camphor tree (Cinnamomum camphora) provides insight into phylogeny and diversification of terpenoid and triglyceride biosynthesis of Cinnamomum

Cinnamomum species attract attentions owing to their scents, medicinal properties, and ambiguous relationship in the phylogenetic tree. Here, we report a high-quality genome assembly of Cinnamomum camphora, based on which two whole-genome duplication (WGD) events were detected in the C. camphora genome: one was shared with Magnoliales, and the other was unique to Lauraceae. Phylogenetic analyses illustrated that Lauraceae species formed a compact sister clade to the eudicots. We then performed whole-genome resequencing on 24 Cinnamomum species native to China, and the results showed that the topology of Cinnamomum species was not entirely consistent with morphological classification. The rise and molecular basis of chemodiversity in Cinnamomum were also fascinating issues. In this study, six chemotypes were classified and six main terpenoids were identified as major contributors of chemodiversity in C. camphora by the principal component analysis. Through in vitro assays and subcellular localization analyses, we identified two key terpene synthase (TPS) genes (CcTPS16 and CcTPS54), the products of which were characterized to catalyze the biosynthesis of two uppermost volatiles (i.e. 1,8-cineole and (iso)nerolidol), respectively, and meditate the generation of two chemotypes by transcriptional regulation and compartmentalization. Additionally, the pathway of medium-chain triglyceride (MCT) biosynthesis in Lauraceae was investigated for the first time. Synteny analysis suggested that the divergent synthesis of MCT and long-chain triglyceride (LCT) in Lauraceae kernels was probably controlled by specific medium-chain fatty acyl-ACP thioesterase (FatB), type-B lysophosphatidic acid acyltransferase (type-B LPAAT), and diacylglycerol acyltransferase 2b (DGAT 2b) isoforms during co-evolution with retentions or deletions in the genome.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Engineering Yarrowia lipolytica to produce advanced biofuels: Current status and perspectives

Energy security and global climate change have necessitated the development of renewable energy with net-zero emissions. As alternatives to traditional fuels used in heavy-duty vehicles, advanced biofuels derived from fatty acids and terpenes have similar properties to current petroleum-based fuels, which makes them compatible with existing storage and transportation infrastructures. The fast development of metabolic engineering and synthetic biology has shown that microorganisms can be engineered to convert renewable feedstocks into these advanced biofuels. The oleaginous yeast Yarrowia lipolytica is rapidly emerging as a valuable chassis for the sustainable production of advanced biofuels derived from fatty acids and terpenes. Here, we provide a summary of the strategies developed in recent years for engineering Y. lipolytica to synthesize advanced biofuels. Finally, efficient biotechnological strategies for the production of these advanced biofuels and perspectives for future research are also discussed.

Lipid metabolism research in oleaginous fungus Mortierella alpina: Current progress and future prospects

The oleaginous fungus Mortierella alpina has distinct advantages in long-chain PUFAs production, and it is the only source for dietary arachidonic acid (ARA) certificated by FDA and European Commission. This review provides an overall introduction to M. alpina, including its major research methods, key factors governing lipid biosynthesis, metabolic engineering and omics studies. Currently, the research interests in M. alpina focus on improving lipid yield and fatty acid desaturation degree by enhancing fatty acid precursors and the reducing power NADPH, and genetic manipulation on PUFAs synthetic pathways is carried to optimise fatty acid composition. Besides, multi-omics studies have been applied to elucidate the global regulatory mechanism of lipogenesis in M. alpina. However, research challenges towards achieving a lipid cell factory lie in strain breeding and cost control due to the coenocytic mycelium, long fermentation period and insufficient conversion rate from carbon to lipid. We also proposed future research goals based on a multilevel regulating strategy: obtaining ideal chassis by directional evolution and high-throughput screening; rewiring central carbon metabolism and inhibiting competitive pathways by multi-gene manipulation system to enhance carbon to lipid conversion rate; optimisation of protein function based on post-translational modification; application of dynamic fermentation strategies suitable for different fermentation phases. By reviewing the comprehensive research progress of this oleaginous fungus, we aim to further comprehend the fungal lipid metabolism and provide reference information and guidelines for the exploration of microbial oils from the perspectives of fundamental research to industrial application.

Molecular Control of Oil Metabolism in the Endosperm of Seeds

In angiosperm seeds, the endosperm develops to varying degrees and accumulates different types of storage compounds remobilized by the seedling during early post-germinative growth. Whereas the molecular mechanisms controlling the metabolism of starch and seed-storage proteins in the endosperm of cereal grains are relatively well characterized, the regulation of oil metabolism in the endosperm of developing and germinating oilseeds has received particular attention only more recently, thanks to the emergence and continuous improvement of analytical techniques allowing the evaluation, within a spatial context, of gene activity on one side, and lipid metabolism on the other side. These studies represent a fundamental step toward the elucidation of the molecular mechanisms governing oil metabolism in this particular tissue. In particular, they highlight the importance of endosperm-specific transcriptional controls for determining original oil compositions usually observed in this tissue. In the light of this research, the biological functions of oils stored in the endosperm of seeds then appear to be more diverse than simply constituting a source of carbon made available for the germinating seedling.

Functional characterization of two type-1 diacylglycerol acyltransferase (DGAT1) genes from rice (Oryza sativa) embryo restoring the triacylglycerol accumulation in yeast

Two OsDGAT1 genes showed the ability to restore TAG and LB synthesis in yeast H1246. Alterations in the N-terminal region of OsDGAT1-1 gene revealed its regulatory role in gene function. Accumulation of triacylglycerol (TAG) or oil in vegetative tissues has emerged as a promising approach to meet the global needs of food, feed, and fuel. Rice (Oryza sativa) has been recognized as an important cereal crop containing nutritional rice bran oil with high economic value for renewable energy production. To identify the key component involved in storage lipid biosynthesis, two type-1 diacylglycerol acyltransferases (DGAT1) from rice were characterized for its in vivo function in the H1246 (dga1, lro1, are1 and are2) yeast quadruple mutant. The ectopic expression of rice DGAT1 (designated as OsDGAT1-1 and OsDGAT1-2) genes restored the capability of TAG synthesis and lipid body (LB) formation in H1246. OsDGAT1-1 showed nearly equal substrate preferences to C16:0-CoA and 18:1-CoA whereas OsDGAT1-2 displayed substrate selectivity for C16:0-CoA over 18:1-CoA, indicating that these enzymes have contrasting substrate specificities. In parallel, we have identified the intrinsically disordered region (IDR) at the N-terminal domains of OsDGAT1 proteins. The regulatory role of the N-terminal domain was dissected. Single point mutations at the phosphorylation sites and truncations of the N-terminal region highlighted reduced lipid accumulation capabilities among different OsDGAT1-1 variants.

Hepatic synthesis of triacylglycerols containing medium-chain fatty acids is dominated by diacylglycerol acyltransferase 1 and efficiently inhibited by etomoxir

Objective

Medium-chain fatty acids (MCFAs) play an increasing role in human nutrition. In the liver, one fraction is used for synthesis of MCFA-containing triacylglycerol (MCFA-TG), and the rest is used for oxidative energy production or ketogenesis. We investigated which enzymes catalyse the synthesis of MCFA-TG and how inhibition of MCFA-TG synthesis or fatty acid (FA) oxidation influences the metabolic fate of the MCFAs.

Methods

FA metabolism was followed by time-resolved tracing of alkyne-labelled FAs in freshly isolated mouse hepatocytes. Quantitative data were obtained by mass spectrometry of several hundred labelled lipid species. Wild-type hepatocytes and cells from diacylglycerol acyltransferase (DGAT)1^−/− mice were treated with inhibitors against DGAT1, DGAT2, or FA β-oxidation.

Results

Inhibition or deletion of DGAT1 resulted in a reduction of MCFA-TG synthesis by 70%, while long-chain (LC)FA-TG synthesis was reduced by 20%. In contrast, DGAT2 inhibition increased MCFA-TG formation by 50%, while LCFA-TG synthesis was reduced by 5–25%. Inhibition of β-oxidation by the specific inhibitor teglicar strongly increased MCFA-TG synthesis. In contrast, the widely used β-oxidation inhibitor etomoxir blocked MCFA-TG synthesis, phenocopying DGAT1 inhibition.

Conclusions

DGAT1 is the major enzyme for hepatic MCFA-TG synthesis. Its loss can only partially be compensated by DGAT2. Specific inhibition of β-oxidation leads to a compensatory increase in MCFA-TG synthesis, whereas etomoxir blocks both β-oxidation and MCFA-TG synthesis, indicating a strong off-target effect on DGAT1.

Identification and functional characterization of two acyl CoA:diacylglycerol acyltransferase 1 (DGAT1) genes from forage sorghum (Sorghum bicolor) embryo

Elevating the lipid content in high-biomass forage crops has emerged as a new research platform for increasing energy density and improving livestock production efficiency associated with improved human health beneficial meat and milk quality. To gain insights of triacylglycerol (TAG) biosynthesis in forage sorghum, two type-1 diacylglycerol acyltransferase (designated as SbDGAT1-1 and SbDGAT1-2) were characterized for its in vivo function. SbDGAT1-2 is more abundantly expressed in embryo and bran during the early stage of the grain development in comparison to SbDGAT1-1. Heterologous expression of SbDGAT1 genes in TAG deficient H1246 strain restored the TAG accumulation capability with high substrate predilection towards 16:0, 16:1 and 18:1 fatty acids (FA). In parallel, we have identified N-terminal intrinsically disordered region (IDR) in SbDGAT1 proteins. To test the efficacy of the N-terminal region, truncated variants of SbDGAT1-1 (designated as SbDGAT1-1_(39-515) and SbDGAT1-1_(89-515)) were generated and expressed in yeast H1246 strain. Deletion in the N-terminal region resulted in decreased accumulation of TAG and FA (16:0 and 18:0) when compared to the SbDGAT1-1 variant expressed in yeast H1246 strain. The present study provides significant insight in forage sorghum DGAT1 gene function, useful for enhancing the green-forage TAG content through metabolic engineering.

Characterization and heterologous expression of three DGATs from oil palm (Elaeis guineensis) mesocarp in Saccharomyces cerevisiae

Oil palm (Elaeis guineensis) can accumulate up to 88% oil in fruit mesocarp. A previous transcriptome study of oil palm fruits indicated that genes coding for three diacylglycerol acyltransferases (DGATs), designated as EgDGAT1_3, EgDGAT2_2 and EgWS/DGAT_1 (according to Rosli et al., 2018) were highly expressed in mesocarp during oil accumulation. In the present study, the corresponding open reading frames were isolated, and characterized by heterologous expression in the mutant yeast H1246, which is devoid of neutral lipid synthesis. Expression of EgDGAT1_3 or EgDGAT2_2 could restore TAG synthesis, confirming that both proteins are true DGAT. In contrast, expression of EgWS/DGAT_1 resulted in the synthesis of fatty acid isoamyl esters (FAIEs) with saturated long-chain and very-long-chain fatty acids. In the presence of exogenously supplied fatty alcohols, EgWS/DGAT_1 was able to produce wax esters, indicating that EgWS/DGAT_1 codes for an acyltransferase with wax ester synthase but no DGAT activity. Finally, the complete wax ester biosynthetic pathway was reconstituted in yeast by coexpressing EgWS/DGAT_1 with a fatty acyl reductase from Tetrahymena thermophila. Altogether, our results characterized two novel DGATs from oil palm as well as a putative wax ester synthase that preferentially using medium chain fatty alcohols and saturated very-long chain fatty acids as substrates.

Genome-wide analysis and functional characterization of Acyl-CoA:diacylglycerol acyltransferase from soybean identify GmDGAT1A and 1B roles in oil synthesis in Arabidopsis seeds

Acyl-CoA:diacylglycerol acyltransferase (DGAT) is a key enzyme in the Kennedy pathway of triacylglycerol (TAG) synthesis. It catalyzes the acyl-CoA-dependent acylation of sn-1, 2-diacylglycerol to form TAG. DGATs in soybean (Glycine max) have been reported, but their functions are largely unclear. Here we cloned three members of DGAT1 and four members of DGAT2 family from soybean, named GmDGAT1A to GmDGAT1C, and GmDGAT2A to GmDGAT2D, respectively. GmDGAT1A and GmDGAT1C were expressed at a high level in immature seeds, GmDGAT2B in mature seeds, and GmDGAT2C in older leaves. The seven genes were transformed into the H1246 quadruple mutant yeast strain, in which GmDGAT1A, GmDGAT1B, GmDGAT1C, GmDGAT2A, and GmDGAT2B had the ability to produce TAG. Six genes were transformed into Arabidopsis respectively, and constitutive expression of GmDGAT1A and GmDGAT1B resulted in an increase in oil content at the cost of reduced protein content in seeds. Overexpression of GmDGAT1A produced heavier weight of individual seed, but did not affect the weight of total seeds from a plant. Our results reveal the functions of soybean DGATs in seed oil synthesis using transgenic Arabidopsis. The implications for the biotechnological modification of the oil contents in soybeans by altering DGAT expression are discussed.

Identification and function analysis of a type 2 diacylglycerol acyltransferase (DGAT2) from the endosperm of coconut (Cocos nucifera L.)

Coconut (Cocos nucifera L.) is one of the most characteristic plants of tropical areas. Coconut oil and its derivatives have been widely used in various industries. In this paper, a type 2 diacylglycerol acyltransferase (DGAT2), which is one of the key enzymes involved in triacylglycerol (TAG) biosynthesis, was first characterized in coconut pulp (endosperm). The results indicated that CoDGAT2 was highly expressed in coconut pulp approximately 7 months after pollination. The heterologous expression of CoDGAT2 in the mutant yeast H1246 restored TAG biosynthesis in the yeast, which exhibited substrate preference for two unsaturated fatty acids (UFAs), palmitoleic acid (C16:1) and oleic acid (C18:1). Moreover, the seed-specific overexpression of CoDGAT2 in Arabidopsis thaliana led to a significant increase in the linoleic acid (C18:2) content (approximately 6%) compared with that in the wild type. In contrast, the proportions of eicosadienoic acid (C20:1) and arachidic acid (C20:0) were decreased. These results offer new insights on the function of CoDGAT2 in coconut and provide a novel molecular target for lipid genetic modification to change the fatty acid (FA) composition of oils.

Identification of Genes Involved in Lipid Biosynthesis through de novo Transcriptome Assembly from Cocos nucifera Developing Endosperm

Cocos nucifera (coconut), a member of the Arecaceae family, is an economically important woody palm that is widely grown in tropical and subtropical regions. The coconut palm is well known for its ability to accumulate large amounts of oil, approximately 63% of the seed weight. Coconut oil varies significantly from other vegetable oils as it contains a high proportion of medium-chain fatty acids (MCFA; 85%). The unique composition of coconut oil raises interest in understanding how the coconut palm produces oil of a high saturated MCFA content, and if such an oil profile could be replicated via biotechnology interventions. Although some gene discovery work has been performed there is still a significant gap in the knowledge associated with coconut's oil production pathways. In this study, a de novo transcriptome was assembled for developing coconut endosperm to identify genes involved in the synthesis of lipids, particularly triacylglycerol. Of particular interest were thioesterases, acyltransferases and oleosins because of their involvement in the processes of releasing fatty acids for assembly, esterification of fatty acids into glycerolipids and protecting oils from degradation, respectively. It is hypothesized that some of these genes may exhibit a strong substrate preference for MCFA and hence may assist the future development of vegetable oils with an enriched MCFA composition. In this study, we identified and confirmed functionality of five candidate genes from the gene families of interest. This study will benefit future work in areas of increasing vegetable oil production and the tailoring of oil fatty acid compositions.

Arabidopsis thaliana DGAT3 is a [2Fe-2S] protein involved in TAG biosynthesis

Acyl-CoA:diacylglycerol acyltransferases 3 (DGAT3) are described as plant cytosolic enzymes synthesizing triacylglycerol. Their protein sequences exhibit a thioredoxin-like ferredoxin domain typical of a class of ferredoxins harboring a [2Fe-2S] cluster. The Arabidopsis thaliana DGAT3 (AtDGAT3; At1g48300) protein is detected in germinating seeds. The recombinant purified protein produced from Escherichia coli, although very unstable, exhibits DGAT activity in vitro. A shorter protein version devoid of its N-terminal putative chloroplast transit peptide, Δ46AtDGAT3, was more stable in vitro, allowing biochemical and spectroscopic characterization. The results obtained demonstrate the presence of a [2Fe-2S] cluster in the protein. To date, AtDGAT3 is the first metalloprotein described as a DGAT.

In silico characterization and expression profiling of the diacylglycerol acyltransferase gene family (DGAT1, DGAT2, DGAT3 and WS/DGAT) from oil palm, Elaeis guineensis

The diacylglycerol acyltransferases (DGAT) (diacylglycerol:acyl-CoA acyltransferase, EC 2.3.1.20) are a key group of enzymes that catalyse the final and usually the most important rate-limiting step of triacylglycerol biosynthesis in plants and other organisms. Genes encoding four distinct functional families of DGAT enzymes have been characterised in the genome of the African oil palm, Elaeis guineensis. The contrasting features of the various isoforms within the four families of DGAT genes, namely DGAT1, DGAT2, DGAT3 and WS/DGAT are presented both in the oil palm itself and, for comparative purposes, in 12 other oil crop or model/related plants, namely Arabidopsis thaliana, Brachypodium distachyon, Brassica napus, Elaeis oleifera, Glycine max, Gossypium hirsutum, Helianthus annuus, Musa acuminata, Oryza sativa, Phoenix dactylifera, Sorghum bicolor, and Zea mays. The oil palm genome contains respectively three, two, two and two distinctly expressed functional copies of the DGAT1, DGAT2, DGAT3 and WS/DGAT genes. Phylogenetic analyses of the four DGAT families showed that the E. guineensis genes tend to cluster with sequences from P. dactylifera and M. acuminata rather than with other members of the Commelinid monocots group, such as the Poales which include the major cereal crops such as rice and maize. Comparison of the predicted DGAT protein sequences with other animal and plant DGATs was consistent with the E. guineensis DGAT1 being ER located with its active site facing the lumen while DGAT2, although also ER located, had a predicted cytosol-facing active site. In contrast, DGAT3 and some (but not all) WS/DGAT in E. guineensis are predicted to be soluble, cytosolic enzymes. Evaluation of E. guineensis DGAT gene expression in different tissues and developmental stages suggests that the four DGAT groups have distinctive physiological roles and are particularly prominent in developmental processes relating to reproduction, such as flowering, and in fruit/seed formation especially in the mesocarp and endosperm tissues.

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.

Seeds as oil factories

Studying seed oil metabolism. The seeds of higher plants represent valuable factories capable of converting photosynthetically derived sugars into a variety of storage compounds, including oils. Oils are the most energy-dense plant reserves and fatty acids composing these oils represent an excellent nutritional source. They supply humans with much of the calories and essential fatty acids required in their diet. These oils are then increasingly being utilized as renewable alternatives to petroleum for the chemical industry and for biofuels. Plant oils therefore represent a highly valuable agricultural commodity, the demand for which is increasing rapidly. Knowledge regarding seed oil production is extensively exploited in the frame of breeding programs and approaches of metabolic engineering for oilseed crop improvement. Complementary aspects of this research include (1) the study of carbon metabolism responsible for the conversion of photosynthetically derived sugars into precursors for fatty acid biosynthesis, (2) the identification and characterization of the enzymatic actors allowing the production of the wide set of fatty acid structures found in seed oils, and (3) the investigation of the complex biosynthetic pathways leading to the production of storage lipids (waxes, triacylglycerols). In this review, we outline the most recent developments in our understanding of the underlying biochemical and molecular mechanisms of seed oil production, focusing on fatty acids and oils that can have a significant impact on the emerging bioeconomy.

Substrate preferences of long-chain acyl-CoA synthetase and diacylglycerol acyltransferase contribute to enrichment of flax seed oil with α-linolenic acid

Seed oil from flax (Linum usitatissimum) is enriched in α-linolenic acid (ALA; 18:3Δ^{9cis,12cis,15cis} ), but the biochemical processes underlying the enrichment of flax seed oil with this polyunsaturated fatty acid are not fully elucidated. Here, a potential process involving the catalytic actions of long-chain acyl-CoA synthetase (LACS) and diacylglycerol acyltransferase (DGAT) is proposed for ALA enrichment in triacylglycerol (TAG). LACS catalyzes the ATP-dependent activation of free fatty acid to form acyl-CoA, which in turn may serve as an acyl-donor in the DGAT-catalyzed reaction leading to TAG. To test this hypothesis, flax LACS and DGAT cDNAs were functionally expressed in Saccharomyces cerevisiae strains to probe their possible involvement in the enrichment of TAG with ALA. Among the identified flax LACSs, LuLACS8A exhibited significantly enhanced specificity for ALA over oleic acid (18:1Δ^9cis ) or linoleic acid (18:2Δ^9cis,12cis ). Enhanced α-linolenoyl-CoA specificity was also observed in the enzymatic assay of flax DGAT2 (LuDGAT2-3), which displayed ∼20 times increased preference toward α-linolenoyl-CoA over oleoyl-CoA. Moreover, when LuLACS8A and LuDGAT2-3 were co-expressed in yeast, both in vitro and in vivo experiments indicated that the ALA-containing TAG enrichment process was operative between LuLACS8A- and LuDGAT2-3-catalyzed reactions. Overall, the results support the hypothesis that the cooperation between the reactions catalyzed by LACS8 and DGAT2 may represent a route to enrich ALA production in the flax seed oil.

A Specialized Diacylglycerol Acyltransferase Contributes to the Extreme Medium-Chain Fatty Acid Content of Cuphea Seed Oil

Seed oils of many Cuphea sp. contain >90% of medium-chain fatty acids, such as decanoic acid (10:0). These seed oils, which are among the most compositionally variant in the plant kingdom, arise from specialized fatty acid biosynthetic enzymes and specialized acyltransferases. These include lysophosphatidic acid acyltransferases (LPAT) and diacylglycerol acyltransferases (DGAT) that are required for successive acylation of medium-chain fatty acids in the sn-2 and sn-3 positions of seed triacylglycerols (TAGs). Here we report the identification of a cDNA for a DGAT1-type enzyme, designated CpuDGAT1, from the transcriptome of C. avigera var pulcherrima developing seeds. Microsomes of camelina (Camelina sativa) seeds engineered for CpuDGAT1 expression displayed DGAT activity with 10:0-CoA and the diacylglycerol didecanoyl, that was approximately 4-fold higher than that in camelina seed microsomes lacking CpuDGAT1. In addition, coexpression in camelina seeds of CpuDGAT1 with a C. viscosissima FatB thioesterase (CvFatB1) that generates 10:0 resulted in TAGs with nearly 15 mol % of 10:0. More strikingly, expression of CpuDGAT1 and CvFatB1 with the previously described CvLPAT2, a 10:0-CoA-specific Cuphea LPAT, increased 10:0 amounts to 25 mol % in camelina seed TAG. These TAGs contained up to 40 mol % 10:0 in the sn-2 position, nearly double the amounts obtained from coexpression of CvFatB1 and CvLPAT2 alone. Although enriched in diacylglycerol, 10:0 was not detected in phosphatidylcholine in these seeds. These findings are consistent with channeling of 10:0 into TAG through the combined activities of specialized LPAT and DGAT activities and demonstrate the biotechnological use of these enzymes to generate 10:0-rich seed oils.

Characterization and Functional Analysis of a Type 2 Diacylglycerol Acyltransferase (DGAT2) Gene from Oil Palm (Elaeis guineensis Jacq.) Mesocarp in Saccharomyces cerevisiae and Transgenic Arabidopsis thaliana

Oil palm (Elaeis guineensis Jacq.) is the highest oil-yielding plant in the world, storing 90 and 60% (dry weight) oil in its mesocarp and kernel, respectively. To gain insights into the oil accumulation mechanism, one of the key enzymes involved in triacylglycerol (TAG) biosynthesis, a Type 2 diacylglycerol acyltransferase (DGAT2) from oil palm, was characterized for its in vivo activity. EgDGAT2 is highly expressed in mesocarp during the last two developmental stages while large amounts of oil are accumulated at the highest rate during ripening. Heterologous expression of EgDGAT2 in mutant yeast H1246 restored TAG biosynthesis with substrate preference toward unsaturated fatty acids (FAs) (16:1 and 18:1). Furthermore, seed-specific overexpression of EgDGAT2 in Arabidopsis thaliana enhanced the content of polyunsaturated FAs 18:2 and 18:3 (each by 6 mol%) in seed TAGs, when compared to that from wild-type Arabidopsis. In turn, the proportion of 18:0 and 20:0 FAs in seed TAGs from EgDGAT2 transgenic lines decreased accordingly. These results provide new insights into understanding the in vivo activity of EgDGAT2 from oil palm mesocarp, which will be of importance for metabolic enhancement of unsaturated FAs production.