Effects of different carbon sources on the growth, fatty acids production, and expression of three desaturase genes of Mortierella alpina ATCC 16266

Emerging prospects of microbial production of omega fatty acids: Recent updates

Healthy foods containing omega-3/omega-6 polyunsaturated fatty acids (PUFAs) have been in great demand because of their unique dietary and health properties. Reduction in chronic inflammatory and autoimmune diseases has shown their therapeutic and health-promoting effects when consumed under recommended ratio 1:1-1:4, however imbalanced ratios (>1:4, high omega-6) enhance these risks. The importance of omega-6 is apparent however microbial production favors larger production of omega-3. Current research focus is prerequisite to designing omega-6 production strategies for better application prospects, for which thraustochytrids could be promising due to higher lipid productivity. This review provides recent updates on essential fatty acids production from potential microbes and their application, especially major insights on omega research, also discussed the novel possible strategies to promote omega-3 and omega-6 accumulation via engineering and omics approaches. It covers strategies to block the conversion of omega-6 into omega-3 by enzyme inhibition, nanoparticle-mediated regulation and/or metabolic flux regulation, etc.

Catalytic mechanisms underlying fungal fatty acid desaturases activities

Polyunsaturated fatty acids (PUFAs) have beneficial roles in a variety of human pathologies and disorders. Owing to the limited source of PUFAs in animals and plants, microorganisms, especially fungi, have become a new source of PUFAs. In fungi, fatty acid desaturases (F-FADS) are the main enzymes that convert saturated fatty acids (SFAs) into PUFAs. Their catalytic activities and substrate specificities, which are directly dependent on the structure of the FADS proteins, determine their efficiency to convert SFAs to PUFAs. Catalytic mechanisms underlying F-FADS activities can be determined from the findings of the relationship between their structure and function. In this review, the advances made in the past decade in terms of catalytic activities and substrate specificities of the fungal FADS cluster are summarized. The relationship between the key domain(s) and site(s) in F-FADS proteins and their catalytic activity is highlighted, and the FADS cluster is analyzed phylogenetically. In addition, subcellular localization of F-FADS is discussed. Finally, we provide prospective crystal structures of F-FADSs. The findings may provide a reference for the resolution of the crystal structures of F-FADS proteins and facilitate the increase in fungal PUFA production for human health.

Genetic and non-genetic tailoring of microalgae for the enhanced production of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) - A review

The myriad health benefits associated with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) laid the path for their application in the functional foods and nutraceutical industries. Fish being primarily exploited for extraction of EPA and DHA are unsustainable sources; thus, oleaginous microalgae turn out to be an alternative sustainable source. This review paper aims to provide the recent developments in the context of enhancing EPA and DHA production by utilising non-genetic tailoring and genetic tailoring methods. We have also summarized the legislation, public perception, and possible risks associated with the usage of genetically modified microalgae focusing on EPA and DHA production.

Metabolic engineering and optimization of the fermentation medium for vitamin B12 production in Escherichia coli

Vitamin B₁₂ is a crucial fine chemical that is widely used in the pharmaceutical, food and chemical industries, and its production solely dependents on microbial fermentation. We previously constructed an artificial vitamin B₁₂ biosynthesis pathway in Escherichia coli, but the yield of the engineered strains was low. Here, we removed metabolic bottlenecks of the vitamin B₁₂ biosynthesis pathway in engineered E. coli strains. After screening cobB genes from different sources, optimizing the expression of cobN and customizing the ribosome binding sites of cobS and cobT, the vitamin B₁₂ yield increased to 152.29 μg/g dry cell weight (DCW). Optimization of the downstream module, which converts co(II)byrinic acid a,c-diamide into adenosylcobinamide phosphate, elevated the vitamin B₁₂ yield to 249.04 μg/g DCW. A comparison of a variety of equivalent components indicated that glucose and corn steep liquor are optimal carbon and nitrogen sources, respectively. Finally, an orthogonal array design was applied to determine the optimal concentrations of glucose and nitrogen sources including corn steep liquor and yeast extract, through which a vitamin B₁₂ yield of 530.29 μg/g DCW was obtained. The metabolic modifications and optimization of fermentation conditions achieved in this study offer a basis for further improving vitamin B₁₂ production in E. coli and will hopefully accelerate its industrial application.

Characterization of an Omega-3 Desaturase From Phytophthora parasitica and Application for Eicosapentaenoic Acid Production in Mortierella alpina

Omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs) have important therapeutic and nutritional benefits in humans. In the biosynthesis pathways of these LC-PUFAs, omega-3 desaturase plays a critical role. In this study, we report a new omega-3 desaturase (PPD17) from Phytophthora parasitica. This desaturase shares high similarities with the known omega-3 desaturases and was expressed in Saccharomyces cerevisiae for the activity and substrate specificity research. The desaturase has a wide omega-6 fatty acid substrate, containing both 18C and 20C fatty acids, and exhibits a strong activity of delta-17 desaturase but a weak activity of delta-15 desaturase. The new desaturase converted the omega-6 arachidonic acid (AA, C20:4) to EPA (an omega-3 LC-PUFA, C20:5) with a substrate conversion rate of 70%. To obtain a high EPA-producing strain, we transformed PPD17 into Mortierella alpina, an AA-producing filamentous fungus. The EPA content of the total fatty acids in reconstruction strains reached 31.5% and was followed by the fermentation optimization of the EPA yield of up to 1.9 g/L. This research characterized a new omega-3 desaturase and provides a possibility of industrially producing EPA using M. alpina.

Differential expression of desaturase genes and changes in fatty acid composition of Mortierella sp. AGED in response to environmental factors

BACKGROUND

Some oleaginous fungi can produce large amounts of polyunsaturated fatty acids (PUFAs) which serve many physiological functions. Numerous desaturases are critical for the synthesis of PUFAs. This study aimed to investigate the regulation of lipid production and desaturase gene expression in Mortierella sp. AGED in response to different environmental factors, and the relationships between lipid production and desaturase gene expression.

RESULTS

The fatty acid composition and mRNA levels of desaturase genes were significantly changed under low temperatures. With the exception of Δ5-desaturase, the transcript levels of all desaturase genes increased at a temperature of 20 °C. Changes in content of lipid and PUFAs responding to low temperature were consistent with desaturase gene expression. Time course studies on gene expression showed that mRNA levels of four desaturase genes increased rapidly after transferring the cells to low temperature. Ethanol (1.5% v/v) increased the transcript levels of Δ9-, Δ6- and Δ5-desaturase genes significantly and of Δ12-desaturase gene slightly. Different metal ions such as Ca²⁺ , Zn²⁺ and Fe³⁺ could stimulate PUFA synthesis and up-regulate desaturase gene transcription, while Cu²⁺ inhibited desaturase gene expression and lipid accumulation.

CONCLUSION

This study should enable us to understand the regulatory mechanism of desaturase gene expression and lipid synthesis. It is helpful to improve PUFA productivity in Mortierella sp. AGED. © 2016 Society of Chemical Industry.

Enhanced polyunsaturated fatty acids production in Mortierella alpina by SSF and the enrichment in chicken breasts

BACKGROUND

Distiller's dried grains with solubles (DDGS) and soybean meal were used as the substrates for the production of polyunsaturated fatty acids (PUFA) in solid-state fermentation (SSF) by Mortierella alpine. These fermented products were fed to laying hens. PUFA enrichment from chicken breasts was studied.

METHODS

The maximum productivity of PUFA was achieved under optimized process condition, including 1% w/w yeast extract as additive, an incubation period of 5 days at 12°C, 10% v/w inoculum level, 75% moisture content, and pH 6.0. The hens were then fed with ration containing soybean DDGS, rapeseed oil, soybean oil, and peanut oil. The control group was fed with basal ration.

RESULTS

Under the optimal condition, M. alpine produced total fatty acids (TFA) of 182.34 mg/g dry substrate. It has better mycelial growth when soybean meal was added to DDGS (SDDGS). PUFA in fermentation product increased with higher soybean meal content. The addition of 70% soybean meal to DDGS substrate yielded 175.16 mg of TFA, including 2.49 mg eicosapentaenoic acid (EPA) and 5.26 mg docosahexaenoic acid (DHA). The ratios of ω-6/ω-3 found in chicken breasts fat were all lower than that found in control by 36.98, 31.51, 18.15, and 12.63% for SDDGS, rapeseed oil, soybean oil, and peanut oil, respectively.

CONCLUSIONS

This study identified an optimized SSF process to maximize PUFA productivity by M. alpine as the strain. This PUFA-enriched feed increased the PUFA contents as well as the proportions of ω-6 and ω-3 in chicken breasts and liver.

Metabolic engineering of Mortierella alpina for arachidonic acid production with glycerol as carbon source

Background

Although some microorganisms can convert glycerol into valuable products such as polyunsaturated fatty acids, the yields are relative low due primarily to an inefficient assimilation of glycerol. Mortierella alpina is an oleaginous fungus which preferentially uses glucose over glycerol as the carbon source for fatty acid synthesis.

Results

In the present study, we metabolically engineered M. alpina to increase the utilization of glycerol. Glycerol kinase and glycerol-3-phosphate dehydrogenase control the first two steps of glycerol decomposition. GK overexpression increased the total fatty acid content by 35 %, whereas G3PD1, G3PD2 and G3PD3 had no significant effect. Overexpression of malic enzyme (ME1) but not glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase or isocitrate dehydrogenase significantly increased fatty acid content when glycerol was used as carbon source. Simultaneous overexpression of GK and ME1 enabled M. alpina to accumulate fatty acids efficiently, with a 44 % increase in fatty acid content (% of dry weight), a 57 % increase in glycerol to fatty acid yield (g/g glycerol) and an 81 % increase in fatty acid production (g/L culture). A repeated batch process was applied to relieve the inhibitory effect of raw glycerol on arachidonic acid synthesis, and under these conditions, the yield reached 52.2 ± 1.9 mg/g.

Conclusions

This study suggested that GK is a rate-limiting step in glycerol assimilation in M. alpina. Another restricting factor for fatty acid accumulation was the supply of cytosolic NADPH. We reported a bioengineering strategy by improving the upstream assimilation and NADPH supply, for oleaginous fungi to efficiently accumulate fatty acid with glycerol as carbon source.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0392-4) contains supplementary material, which is available to authorized users.

Reconstruction and analysis of a genome-scale metabolic model of the oleaginous fungus Mortierella alpina

Background

Mortierella alpina is an oleaginous fungus used in the industrial scale production of arachidonic acid (ARA). In order to investigate the metabolic characteristics at a systems level and to explore potential strategies for enhanced lipid production, a genome-scale metabolic model of M. alpina was reconstructed.

Results

This model included 1106 genes, 1854 reactions and 1732 metabolites. On minimal growth medium, 86 genes were identified as essential, whereas 49 essential genes were identified on yeast extract medium. A series of sequential desaturase and elongase catalysed steps are involved in the synthesis of polyunsaturated fatty acids (PUFAs) from acetyl-CoA precursors, with concomitant NADPH consumption, and these steps were investigated in this study. Oxygen is known to affect the degree of unsaturation of PUFAs, and robustness analysis determined that an oxygen uptake rate of 2.0 mmol gDW⁻¹ h⁻¹ was optimal for ARA accumulation. The flux of 53 reactions involving NADPH was significantly altered at different ARA levels. Of these, malic enzyme (ME) was confirmed as a key component in ARA production and NADPH generation. When using minimization of metabolic adjustment, a knock-out of ME led to a 38.28% decrease in ARA production.

Conclusions

The simulation results confirmed the model as a useful tool for future research on the metabolism of PUFAs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-014-0137-8) contains supplementary material, which is available to authorized users.