Metabolic Engineering Strategies for Improved Lipid Production and Cellular Physiological Responses in Yeast Saccharomyces cerevisiae

Oleaginous fungi: a promising source of biofuels and nutraceuticals with enhanced lipid production strategies

Oleaginous fungi have attracted a great deal of interest for their potency to accumulate high amounts of lipids (more than 20% of biomass dry weight) and polyunsaturated fatty acids (PUFAs), which have a variety of industrial and biological applications. Lipids of plant and animal origin are related to some restrictions and thus lead to attention towards oleaginous microorganisms as reliable substitute resources. Lipids are traditionally biosynthesized intra-cellularly and involved in the building structure of a variety of cellular compartments. In oleaginous fungi, under certain conditions of elevated carbon ratio and decreased nitrogen in the growth medium, a change in metabolic pathway occurred by switching the whole central carbon metabolism to fatty acid anabolism, which subsequently resulted in high lipid accumulation. The present review illustrates the bio-lipid structure, fatty acid classes and biosynthesis within oleaginous fungi with certain key enzymes, and the advantages of oleaginous fungi over other lipid bio-sources. Qualitative and quantitative techniques for detecting the lipid accumulation capability of oleaginous microbes including visual, and analytical (convenient and non-convenient) were debated. Factors affecting lipid production, and different approaches followed to enhance the lipid content in oleaginous yeasts and fungi, including optimization, utilization of cost-effective wastes, co-culturing, as well as metabolic and genetic engineering, were discussed. A better understanding of the oleaginous fungi regarding screening, detection, and maximization of lipid content using different strategies could help to discover new potent oleaginous isolates, exploit and recycle low-cost wastes, and improve the efficiency of bio-lipids cumulation with biotechnological significance.

Application of multiple strategies to enhance oleanolic acid biosynthesis by engineered Saccharomyces cerevisiae

Oleanolic acid and its derivatives are widely used in the pharmaceutical, agricultural, cosmetic and food industries. Previous studies have shown that oleanolic acid production levels in engineered cell factories are low, which is why oleanolic acid is still widely extracted from traditional medicinal plants. To construct a highly efficient oleanolic acid production strain, rate-limiting steps were regulated by inducible promoters and the expression of key genes in the oleanolic acid synthetic pathway was enhanced. Subsequently, precursor pool expansion, pathway refactoring and diploid construction were considered to harmonize cell growth and oleanolic acid production. The multi-strategy combination promoted oleanolic acid production of up to 4.07 g/L in a 100 L bioreactor, which was the highest level reported.

Tuning Fatty Acid Profile and Yield in Pichia pastoris

Fatty acids have been supplied for diverse non-food, industrial applications from plant oils and animal fats for many decades. Due to the massively increasing world population demanding a nutritious diet and the thrive to provide feedstocks for industrial production lines in a sustainable way, i.e., independent from food supply chains, alternative fatty acid sources have massively gained in importance. Carbohydrate-rich side-streams of agricultural production, e.g., molasses, lignocellulosic waste, glycerol from biodiesel production, and even CO2, are considered and employed as carbon sources for the fermentative accumulation of fatty acids in selected microbial hosts. While certain fatty acid species are readily accumulated in native microbial metabolic routes, other fatty acid species are scarce, and host strains need to be metabolically engineered for their high-level production. We report the metabolic engineering of Pichia pastoris to produce palmitoleic acid from glucose and discuss the beneficial and detrimental engineering steps in detail. Fatty acid secretion was achieved through the deletion of fatty acyl-CoA synthetases and overexpression of the truncated E. coli thioesterase 'TesA. The best strains secreted >1 g/L free fatty acids into the culture medium. Additionally, the introduction of C16-specific ∆9-desaturases and fatty acid synthases, coupled with improved cultivation conditions, increased the palmitoleic acid content from 5.5% to 22%.

Biotechnological production of omega-3 fatty acids: current status and future perspectives

Omega-3 fatty acids, including alpha-linolenic acids (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have shown major health benefits, but the human body's inability to synthesize them has led to the necessity of dietary intake of the products. The omega-3 fatty acid market has grown significantly, with a global market from an estimated USD 2.10 billion in 2020 to a predicted nearly USD 3.61 billion in 2028. However, obtaining a sufficient supply of high-quality and stable omega-3 fatty acids can be challenging. Currently, fish oil serves as the primary source of omega-3 fatty acids in the market, but it has several drawbacks, including high cost, inconsistent product quality, and major uncertainties in its sustainability and ecological impact. Other significant sources of omega-3 fatty acids include plants and microalgae fermentation, but they face similar challenges in reducing manufacturing costs and improving product quality and sustainability. With the advances in synthetic biology, biotechnological production of omega-3 fatty acids via engineered microbial cell factories still offers the best solution to provide a more stable, sustainable, and affordable source of omega-3 fatty acids by overcoming the major issues associated with conventional sources. This review summarizes the current status, key challenges, and future perspectives for the biotechnological production of major omega-3 fatty acids.

Engineering acetyl-CoA metabolism to enhance stress tolerance of yeast by regulating membrane functionality

Zygosaccharomyces rouxii has excellent fermentation performance and good tolerance to osmotic stress. Acetyl-CoA is a crucial intermediate precursor in the central carbon metabolic pathway of yeast. This study investigated the effect of engineering acetyl-CoA metabolism on the membrane functionality and stress tolerance of yeast. Firstly, exogenous supplementation of acetyl-CoA improved the biomass and the ability of unsaturated fatty acid synthesis of Z. rouxii under salt stress. Q-PCR results suggested that the gene ACSS (coding acetyl-CoA synthetase) was significantly up-expressed. Subsequently, the gene ACSS from Z. rouxii was transformed and heterologously expressed in S. cerevisiae. The recombinant cells exhibited better multiple stress (salt, acid, heat, and cold) tolerance, higher fatty acid contents, membrane integrity, and fluidity. Our findings may provide a suitable means to enhance the stress tolerance and fermentation efficiency of yeast under harsh fermentation environments.

Valorisation of Biomass Waste for Sustainable Bioenergy and Biofuel Production

Although the rapid development of industrialisation has brought great benefits to our societies, waste accumulation and energy depletion have inevitably grown to be critical issues in recent decades [...].

Applications of Diacylglycerol Acyltransferase for Triacylglycerol Production in Mortierella alpina

Triacylglycerol (TG) with high-value long-chain polyunsaturated fatty acids is beneficial to human health; consequently, there is an urgent need to broaden its sources due to the current growing demand. Mortierella alpina, one of the most representative oleaginous fungi, is the only certificated source of dietary arachidonic acid-rich oil supplied in infant formula. This study was conducted to improve TG production in M. alpina by homologous overexpression of diacylglycerol acyltransferase (DGAT) and linseed oil (LSO) supplementation. Our results showed that the homologous overexpression of MaDGAT1B and MaDGAT2A strengthened TG biosynthesis and significantly increased the TG content compared to the wild-type by 12.24% and 14.63%, respectively. The supplementation with an LSO concentration of 0.5 g/L elevated the TG content to 83.74% and total lipid yield to 4.26 ± 0.38 g/L in the M. alpina-MaDGAT2A overexpression strain. Our findings provide an effective strategy for enhancing TG production and highlight the role of DGAT in TG biosynthesis in M. alpina.

Linoleic acid functions as a quorum-sensing molecule in Monascus purpureus-Saccharomyces cerevisiae co-culture

When Monascus purpureus was co-cultured with Saccharomyces cerevisiae, we noted significant changes in the secondary metabolism and morphological development of Monascus. In yeast co-culture, although the pH was not different from that of a control, the Monascus mycelial biomass increased during fermentation, and the Monacolin K yield was significantly enhanced (up to 58.87% higher). However, pigment production did not increase. Co-culture with S. cerevisiae significantly increased the expression levels of genes related to Monacolin K production (mokA-mokI), especially mokE, mokF, and mokG. Linoleic acid, that has been implicated in playing a regulating role in the secondary metabolism and morphology of Monascus, was hypothesized to be the effector. Linoleic acid was detected in the co-culture, and its levels changed during fermentation. Addition of linoleic acid increased Monacolin K production and caused similar morphological changes in Monascus spores and mycelia. Exogenous linoleic acid also significantly upregulated the transcription levels of all nine genes involved in the biosynthesis of Monacolin K (up to 69.50% higher), consistent with the enhanced Monacolin K yield. Taken together, our results showed the effect of S. cerevisiae co-culture on M. purpureus and suggested linoleic acid as a specific quorum-sensing molecule in Saccharomyces-Monascus co-culture.

Recent Development of Advanced Biotechnology in the Oleaginous Fungi for Arachidonic Acid Production

Arachidonic acid is an essential ω-6 polyunsaturated fatty acid, which plays a significant role in cardiovascular health and neurological development, leading to its wide use in the food and pharmaceutical industries. Traditionally, ARA is obtained from deep-sea fish oil. However, this source is limited by season and is depleting the already threatened global fish stocks. With the rapid development of synthetic biology in recent years, oleaginous fungi have gradually attracted increasing attention as promising microbial sources for large-scale ARA production. Numerous advanced technologies including metabolic engineering, dynamic regulation of fermentation conditions, and multiomics analysis were successfully adapted to increase ARA synthesis. This review summarizes recent advances in the bioengineering of oleaginous fungi for ARA production. Finally, perspectives for future engineering approaches are proposed to further improve the titer yield and productivity of ARA.