Boosting the biosynthesis of betulinic acid and related triterpenoids in Yarrowia lipolytica via multimodular metabolic engineering

Comparative transcriptome analysis reveals the redirection of metabolic flux from cell growth to astaxanthin biosynthesis in Yarrowia lipolytica

Engineering Yarrowia lipolytica to produce astaxanthin provides a promising route. Here, Y. lipolytica M2 producing a titer of 181 mg/L astaxanthin was isolated by iterative atmospheric and room-temperature plasma mutagenesis and diphenylamine-mediated screening. Interestingly, a negative correlation was observed between cell biomass and astaxanthin production. To reveal the underlying mechanism, RNA-seq analysis of transcriptional changes was performed in high producer M2 and reference strain M1, and a total of 1379 differentially expressed genes were obtained. Data analysis revealed that carbon flux was elevated through lipid metabolism, acetyl-CoA and mevalonate supply, but restrained through central carbon metabolism in strain M2. Moreover, upregulation of other pathways such as ATP-binding cassette transporter and thiamine pyrophosphate possibly provided more cofactors for carotenoid hydroxylase and relieved cell membrane stress caused by astaxanthin insertion. These results suggest that balancing cell growth and astaxanthin production may be important to promote efficient biosynthesis of astaxanthin in Y. lipolytica.

Exploiting synthetic biology platforms for enhanced biosynthesis of natural products in Yarrowia lipolytica

With the rapid development of synthetic biology, researchers can design, modify, or even synthesize microorganisms de novo, and microorganisms endowed with unnatural functions can be considered "artificial life" and facilitate the development of functional products. Based on this concept, researchers can solve critical problems related to the insufficient supply of natural products, such as low yields, long production cycles, and cumbersome procedures. Due to its superior performance and unique physiological and biochemical characteristics, Yarrowia lipolytica is a favorable chassis cell used for green biomanufacturing by numerous researchers. This paper mainly reviews the development of synthetic biology techniques for Y. lipolytica and summarizes the recent research progress on the synthesis of natural products in Y. lipolytica. This review will promote the continued innovative development of Y. lipolytica by providing theoretical guidance for research on the biosynthesis of natural products.

Advances in the metabolic engineering of Saccharomyces cerevisiae and Yarrowia lipolytica for the production of -carotene

β-Carotene is one kind of the most important carotenoids. The major functions of β-carotene include the antioxidant and anti-cardiovascular properties, which make it a growing market. Recently, the use of metabolic engineering to construct microbial cell factories to synthesize β-carotene has become the latest model for its industrial production. Among these cell factories, yeasts including Saccharomyces cerevisiae and Yarrowia lipolytica have attracted the most attention because of the: security, mature genetic manipulation tools, high flux toward carotenoids using the native mevalonate pathway and robustness for large-scale fermentation. In this review, the latest strategies for β-carotene biosynthesis, including protein engineering, promoters engineering and morphological engineering are summarized in detail. Finally, perspectives for future engineering approaches are proposed to improve β-carotene production.

Two-Phase Fermentation Systems for Microbial Production of Plant-Derived Terpenes

Microbial cell factories, renowned for their economic and environmental benefits, have emerged as a key trend in academic and industrial areas, particularly in the fermentation of natural compounds. Among these, plant-derived terpenes stand out as a significant class of bioactive natural products. The large-scale production of such terpenes, exemplified by artemisinic acid-a crucial precursor to artemisinin-is now feasible through microbial cell factories. In the fermentation of terpenes, two-phase fermentation technology has been widely applied due to its unique advantages. It facilitates in situ product extraction or adsorption, effectively mitigating the detrimental impact of product accumulation on microbial cells, thereby significantly bolstering the efficiency of microbial production of plant-derived terpenes. This paper reviews the latest developments in two-phase fermentation system applications, focusing on microbial fermentation of plant-derived terpenes. It also discusses the mechanisms influencing microbial biosynthesis of terpenes. Moreover, we introduce some new two-phase fermentation techniques, currently unexplored in terpene fermentation, with the aim of providing more thoughts and explorations on the future applications of two-phase fermentation technology. Lastly, we discuss several challenges in the industrial application of two-phase fermentation systems, especially in downstream processing.

Efficient synthesis of squalene by cytoplasmic-peroxisomal engineering and regulating lipid metabolism in Yarrowia lipolytica

Squalene, a high-value acyclic triterpenoid compound, is broadly used in the food and medical industries. Although the large acetyl-CoA pool and hydrophobic space of Yarrowia lipolytica are suitable for the accumulation of squalene, the current production level in Y. lipolytica is still not sufficient for industrial production. In this study, two rounds of multicopy integration of genes encoding key enzymes were performed to enhance squalene anabolic flux in the cytoplasm. Furthermore, the mevalonate pathway was imported into peroxisomes through the compartmentalization strategy, and the production of squalene was significantly increased. By augmenting the acetyl-CoA supply in peroxisomes and the cytoplasm, the squalene was boosted to 2549.1 mg/L. Finally, the squalene production reached 51.2 g/L by fed-batch fermentation in a 5-L bioreactor. This is the highest squalene production reported to date for microbial production, and this study lays the foundation for the synthesis of steroids and squalene derivatives.

Building Yarrowia lipolytica Cell Factories for Advanced Biomanufacturing: Challenges and Solutions

Microbial cell factories have shown great potential for industrial production with the benefit of being environmentally friendly and sustainable. Yarrowia lipolytica is a promising and superior non-model host for biomanufacturing due to its cumulated advantages compared to model microorganisms, such as high fluxes of metabolic precursors (acetyl-CoA and malonyl-CoA) and its naturally hydrophobic microenvironment. However, although diverse compounds have been synthesized in Y. lipolytica cell factories, most of the relevant studies have not reached the level of industrialization and commercialization due to a number of remaining challenges, including unbalanced metabolic flux, conflict between cell growth and product synthesis, and cytotoxic effects. Here, various metabolic engineering strategies for solving the challenges are summarized, which is developing fast and extremely conducive to rational design and reconstruction of robust Y. lipolytica cell factories for advanced biomanufacturing. Finally, future engineering efforts for enhancing the production efficiency of this platform strain are highlighted.

Multi-modular metabolic engineering and efflux engineering for enhanced lycopene production in recombinant Saccharomyces cerevisiae

Lycopene has been widely used in the food industry and medical field due to its antioxidant, anti-cancer, and anti-inflammatory properties. However, achieving efficient manufacture of lycopene using chassis cells on an industrial scale remains a major challenge. Herein, we attempted to integrate multiple metabolic engineering strategies to establish an efficient and balanced lycopene biosynthetic system in Saccharomyces cerevisiae. First, the lycopene synthesis pathway was modularized to sequentially enhance the metabolic flux of the mevalonate pathway, the acetyl-CoA supply module, and lycopene exogenous enzymatic module. The modular operation enabled the efficient conversion of acetyl-CoA to downstream pathway of lycopene synthesis, resulting in a 3.1-fold increase of lycopene yield. Second, we introduced acetate as an exogenous carbon source and utilized an acetate-repressible promoter to replace the natural ERG9 promoter. This approach not only enhanced the supply of acetyl-CoA but also concurrently diminished the flux toward the competitive ergosterol pathway. As a result, a further 42.3% increase in lycopene production was observed. Third, we optimized NADPH supply and mitigated cytotoxicity by overexpressing ABC transporters to promote lycopene efflux. The obtained strain YLY-PDR11 showed a 12.7-fold increase in extracellular lycopene level compared to the control strain. Finally, the total lycopene yield reached 343.7 mg/L, which was 4.3 times higher than that of the initial strain YLY-04. Our results demonstrate that combining multi-modular metabolic engineering with efflux engineering is an effective approach to improve the production of lycopene. This strategy can also be applied to the overproduction of other desirable isoprenoid compounds with similar synthesis and storage patterns in S. cerevisiae.

ONE-SENTENCE SUMMARY

In this research, lycopene production in yeast was markedly enhanced by integrating a multi-modular approach, acetate signaling-based down-regulation of competitive pathways, and an efflux optimization strategy.

Engineering yeast for the production of plant terpenoids using synthetic biology approaches

Covering: 2011-2022The low amounts of terpenoids produced in plants and the difficulty in synthesizing these complex structures have stimulated the production of terpenoid compounds in microbial hosts by metabolic engineering and synthetic biology approaches. Advances in engineering yeast for terpenoid production will be covered in this review focusing on four directions: (1) manipulation of host metabolism, (2) rewiring and reconstructing metabolic pathways, (3) engineering the catalytic activity, substrate selectivity and product specificity of biosynthetic enzymes, and (4) localizing terpenoid production via enzymatic fusions and scaffolds, or subcellular compartmentalization.

Improved antioxidant activities of spice require enrichment of distinct yet closely-related metabolic pathways

Improved biosynthesis of commercially and pharmacologically relevant phytometabolites through genetic and metabolic engineering is a lucrative strategy for crop improvement. However, identifying appropriate biosynthetic pathways pertaining to specific bioactivities has been challenging since the major metabolic pathways remain closely interconnected. Here we propose a reverse association strategy in which, based on the phytochemical profile, putative target metabolic pathways could be identified for increased production of phytochemicals. Dried seed fruits of Coriandrum sativum, Trachyspermum ammi, Cuminum cyminum, and Foeniculum vulgare (family Apiaceae) were subjected to untargeted gas chromatography-mass spectrometry-based phytochemical profiling followed by evaluation of the overall antioxidant profile using multiple antioxidant assays. Using bioinformatics approaches, specific phytochemical classes and the enrichment of their respective biosynthetic pathways were identified. Collectively, the data suggest enrichment of isoprenoids and fatty acids biosynthetic pathways. The close association of metabolic pathways with antioxidant capacities indicated a need for enrichment of specific yet closely-related metabolic pathways to achieve an improved quality of spices for better antioxidant effects.

Recent advances in triterpenoid pathway elucidation and engineering

Triterpenoids are among the most assorted class of specialized metabolites found in all the taxa of living organisms. Triterpenoids are the leading active ingredients sourced from plant species and are utilized in pharmaceutical and cosmetic industries. The triterpenoid precursor 2,3-oxidosqualene, which is biosynthesized via the mevalonate (MVA) pathway is structurally diversified by the oxidosqualene cyclases (OSCs) and other scaffold-decorating enzymes such as cytochrome P450 monooxygenases (P450s), UDP-glycosyltransferases (UGTs) and acyltransferases (ATs). A majority of the bioactive triterpenoids are harvested from the native hosts using the traditional methods of extraction and occasionally semi-synthesized. These methods of supply are time-consuming and do not often align with sustainability goals. Recent advancements in metabolic engineering and synthetic biology have shown prospects for the green routes of triterpenoid pathway reconstruction in heterologous hosts such as Escherichia coli, Saccharomyces cerevisiae and Nicotiana benthamiana, which appear to be quite promising and might lead to the development of alternative source of triterpenoids. The present review describes the biotechnological strategies used to elucidate complex biosynthetic pathways and to understand their regulation and also discusses how the advances in triterpenoid pathway engineering might aid in the scale-up of triterpenoid production in engineered hosts.

De Novo Biosynthesis of Free Vaccenic Acid with a Low Content of Oleic Acid in Saccharomyces cerevisiae

Omega-7 (ω-7) fatty acids have potential application in the fields of nutraceutical, agricultural, and food industry. The natural ω-7 fatty acids are currently from plants or vegetable oils, which are unsustainable and limited by the availability of plant sources. Here, we developed an innovative biosynthetic route to produce vaccenic acid (C18:1 ω-7) while minimizing oleic acid (C18:1 ω-9) content in Saccharomyces cerevisiae. We have engineered S. cerevisiaeto produce C18:1 ω-7 by expressing a fatty acid elongase from Rattus norvegicus. To reduce the content of C18:1 ω-9, the endogenous desaturase Ole1 was replaced by the desaturase, which has specific activity on palmitoyl-coenzyme A (C16:0-CoA). Finally, the production of free C18:1 ω-7 was improved by optimizing the source of cytochrome b5 and overexpressing endoplasmic reticulum chaperones. After combining these strategies, the yield of C18:1 ω-7 was increased from 0 to 9.3 mg/g DCW and C18:1 ω-9 was decreased from 25.2 mg/g DCW to 1.6 mg/g DCW. This work shows a de novo synthetic pathway to produce the highest amount of free C18:1 ω-7 with a low content of C18:1 ω-9 in S. cerevisiae.

Enhanced acetate utilization for value-added chemicals production in Yarrowia lipolytica by integration of metabolic engineering and microbial electrosynthesis

The limited supply of reducing power restricts the efficient utilization of acetate in Yarrowia lipolytica. Here, microbial electrosynthesis (MES) system, enabling direct conversion of inward electrons to NAD(P)H, was used to improve the production of fatty alcohols from acetate based on pathway engineering. First, the conversion efficiency of acetate to acetyl-CoA was reinforced by heterogenous expression of ackA-pta genes. Second, a small amount of glucose was used as cosubstrate to activate the pentose phosphate pathway and promote intracellular reducing cofactors synthesis. Third, through the employment of MES system, the final fatty alcohols production of the engineered strain YLFL-11 reached 83.8 mg/g dry cell weight (DCW), which was 6.17-fold higher than the initial production of YLFL-2 in shake flask. Furthermore, these strategies were also applied for the elevation of lupeol and betulinic acid synthesis from acetate in Y. lipolytica, demonstrating that our work provides a practical solution for cofactor supply and the assimilation of inferior carbon sources.

Triacetic acid lactone production using 2-pyrone synthase expressing Yarrowia lipolytica via targeted gene deletion

An environmentally sustainable world can be realized by using microorganisms to produce value-added materials from renewable biomass. Triacetic acid lactone (TAL) is a high-value-added compound that is used as a precursor of various organic compounds such as food additives and pharmaceuticals. In this study, we used metabolic engineering to produce TAL from glucose using an oleaginous yeast Yarrowia lipolytica. We first introduced TAL-producing gene 2-pyrone synthase into Y. lipolytica, which enabled TAL production. Next, we increased TAL production by engineering acetyl-CoA and malonyl-CoA biosynthesis pathways by redirecting carbon flux to glycolysis. Finally, we optimized the carbon and nitrogen ratios in the medium, culminating in the production of 4078 mg/L TAL. The strategy presented in this study had the potential to improve the titer and yield of polyketide biosynthesis.

Betulinic acid in the treatment of breast cancer: Application and mechanism progress

Betulinic acid (BA) is a pentacyclic triterpene compound, which can be obtained by separation, chemical synthesis and biotransformation. BA has excellent biological activities, especially its role in the treatment of breast cancer deserves attention. Its mechanisms mainly include inducing mitochondrial oxidative stress, regulating specific protein (Sp) transcription factors, inhibiting breast cancer metastasis, inhibiting glucose metabolism and NF-κB pathway. In addition, BA can also increase the sensitivity of breast cancer cells to other chemotherapy drugs such as paclitaxel and reduce its toxic side effects. This article reviews the application and possible mechanism of BA in the treatment of breast cancer.

Boosting the epoxidation of squalene to produce triterpenoids in Saccharomyces cerevisiae

BACKGROUND

Polycyclic triterpenoids (PTs) are common in plants, and have attracted considerable interest due to their remarkable biological activities. Currently, engineering the ergosterol synthesis pathway in Saccharomyces cerevisiae is a safe and cost-competitive way to produce triterpenoids. However, the strict regulation of ERG1 involved in the epoxidation of squalene limits the triterpenoid production.

RESULTS

In this study, we found that the decrease in ERG7 protein level could dramatically boost the epoxidation of squalene by improving the protein stability of ERG1. We next explored the potential factors that affected the degradation process of ERG1 and confirmed that ERG7 was involved in the degradation process of ERG1. Subsequently, expression of four different triterpene cyclases utilizing either 2,3-oxidosqualene or 2,3:22,23-dioxidosqualene as the substrate in ERG7-degraded strains showed that the degradation of ERG7 to prompt the epoxidation of squalene could significantly increase triterpenoid production. To better display the potential of the strategy, we increased the supply of 2,3-oxidosqualene, optimized flux distribution between ergosterol synthesis pathway and β-amyrin synthesis pathway, and modified the GAL-regulation system to separate the growth stage from the production stage. The best-performing strain ultimately produced 4216.6 ± 68.4 mg/L of β-amyrin in a two-stage fed-fermentation (a 47-fold improvement over the initial strain).

CONCLUSIONS

This study showed that deregulation of the native restriction in ergosterol pathway was an effective strategy to increase triterpenoid production in yeast, which provided a new insight into triterpenoids biosynthesis.

Metabolic Engineering of Yarrowia lipolytica for Terpenoid Production: Tools and Strategies

Terpenoids are a diverse group of compounds with isoprene units as basic building blocks. They are widely used in the food, feed, pharmaceutical, and cosmetic industries due to their diverse biological functions such as antioxidant, anticancer, and immune enhancement. With an increase in understanding the biosynthetic pathways of terpenoids and advances in synthetic biology techniques, microbial cell factories have been built for the heterologous production of terpenoids, with the oleaginous yeast Yarrowia lipolytica emerging as an outstanding chassis. In this paper, recent progress in the development of Y. lipolytica cell factories for terpenoid production with a focus on the advances in novel synbio tools and metabolic engineering strategies toward enhanced terpenoid biosynthesis is reviewed.

Recent Advances in Yeast Recombinant Biosynthesis of the Triterpenoid Protopanaxadiol and Glycosylated Derivatives Thereof

Plant natural products are a seemingly endless resource for novel chemical structures. However, their extraction often results in high prices, fluctuation in both quantity and quality, and negative environmental impact. The latter might result from the extraction procedure but more often from the high amount of plant biomass required. With the advent of synthetic biology, producing natural plant products in large quantities using yeasts as hosts has become possible. Here, we focus on the recent advances in metabolic engineering of the yeasts species Saccharomyces cerevisiae and Yarrowia lipolytica for the synthesis of ginsenoside triterpenoids, namely, dammarenediol-II, protopanaxadiol, protopanaxatriol, compound K, ginsenoside Rh1, ginsenoside Rh2, ginsenoside Rg3, and ginsenoside F1. A discussion is provided on advanced synthetic biology, bioprocess strategies, and current challenges for the biosynthesis of ginsenoside triterpenoids. Finally, future directions in metabolic and process engineering are summarized and may help reify sustainable ginsenoside production.

Cytochrome P450s in plant terpenoid biosynthesis: discovery, characterization and metabolic engineering

As the largest family of natural products, terpenoids play valuable roles in medicine, agriculture, cosmetics and food. However, the traditional methods that rely on direct extraction from the original plants not only produce low yields, but also result in waste of resources, and are not applicable at all to endangered species. Modern heterologous biosynthesis is considered a promising, efficient, and sustainable production method, but it relies on the premise of a complete analysis of the biosynthetic pathway of terpenoids, especially the functionalization processes involving downstream cytochrome P450s. In this review, we systematically introduce the biotech approaches used to discover and characterize plant terpenoid-related P450s in recent years. In addition, we propose corresponding metabolic engineering approaches to increase the effective expression of P450 and improve the yield of terpenoids, and also elaborate on metabolic engineering strategies and examples of heterologous biosynthesis of terpenoids in Saccharomyces cerevisiae and plant hosts. Finally, we provide perspectives for the biotech approaches to be developed for future research on terpenoid-related P450.

Integrated pathway engineering and transcriptome analysis for improved astaxanthin biosynthesis in Yarrowia lipolytica

Astaxanthin is a high value carotenoid with a broad range of commercial applications due to its superior antioxidant properties. In this study, β-carotene-producing Yarrowia lipolytica XK17 constructed in the lab was employed for astaxanthin biosynthesis. The catalytic effects of β-carotene ketolase CrtW and β-carotene hydroxylase CrtZ from various species were investigated. The PspCrtW from Paracoccus sp. and HpCrtZ^# from Haematococcus pluvialis were confirmed to be the best combination in converting β-carotene. Several key bottlenecks in biomass and astaxanthin biosynthesis were effectively eliminated by optimizing the expression of the above enzymes and restoring uracil/leucine biosynthesis. In addition, the effects of astaxanthin biosynthesis on cell metabolism were investigated by integrated analysis of pathway modification and transcriptome information. After further optimization, strain DN30 was able to synthesize up to 730.3 mg/L astaxanthin in laboratory 5-L fermenter. This study provides a good metabolic strategy and a sustainable development platform for high-value carotenoid production.

Engineering of Yarrowia lipolytica for terpenoid production

Terpenoids are a group of chemicals of great importance for human health and prosperity. Terpenoids can be used for human and animal nutrition, treating diseases, enhancing agricultural output, biofuels, fragrances, cosmetics, and flavouring. However, due to the rapid depletion of global natural resources and manufacturing practices relying on unsustainable petrochemical synthesis, there is a need for economic alternatives to supply the world's demand for these essential chemicals. Microbial biosynthesis offers the means to develop scalable and sustainable bioprocesses for terpenoid production. In particular, the non-conventional yeast Yarrowia lipolytica demonstrates excellent potential as a chassis for terpenoid production due to its amenability to industrial production scale-up, genetic engineering, and high accumulation of terpenoid precursors. This review aims to illustrate the scientific progress in developing Y. lipolytica terpenoid cell factories, focusing on metabolic engineering approaches for strain improvement and cultivation optimization.

Separation and purification of nootkatone from fermentation broth of Yarrowia lipolytica with high-speed counter-current chromatography

Nootkatone is an important functional sesquiterpene, which can be obtained by the biotransformation of valencene. It is increasingly important because of its pleasant citrus aroma and physiological effects. Yarrowia lipolytica is beneficial for biotechnology applications and has ability to transform valencene to nootkatone. High-speed counter-current chromatography (HSCCC) was used to isolate and purify the product of nootkatone in this study. The suitable two-phase solvent system was selected and the optimum separation conditions were determined. The partition coefficients of nootkatone and the separation factor between nootkatone and valencene were considered as the indexes. The results showed that there were numerous products during the transformation of valencene by Yarrowia lipolytica, and the content of nootkatone was 13.75%. The obtained nootkatone was separated by HSCCC with a solvent system n-hexane/methanol/water (5/4/1, v/v). The final purity of nootkatone was 91.61 ± 0.20% and the elution time was 290-310 min. The structure of nootkatone was identified by gas chromatography-mass spectrometry (GC-MS), infrared spectrum and nuclear magnetic resonance hydrogen spectroscopy (1H NMR). This was the first report on the separation of nootkatone from the fermentation broth by HSCCC. This study proved that HSCCC could be used as an effective method to separate and purify the nootkatone from valencene transformed by Yarrowia lipolytica with n-hexane/methanol/water (5/4/1, v/v).

Applications of protein engineering in the microbial synthesis of plant triterpenoids

Triterpenoids are a class of natural products widely used in fields related to medicine and health due to their biological activities such as hepatoprotection, anti-inflammation, anti-viral, and anti-tumor. With the advancement in biotechnology, microorganisms have been used as cell factories to produce diverse natural products. Despite the significant progress that has been made in the construction of microbial cell factories for the heterogeneous biosynthesis of triterpenoids, the industrial production of triterpenoids employing microorganisms has been stymied due to the shortage of efficient enzymes as well as the low expression and low catalytic activity of heterologous proteins in microbes. Protein engineering has been demonstrated as an effective way for improving the specificity, catalytic activity, and stability of the enzyme, which can be employed to overcome these challenges. This review summarizes the current progress in the studies of Oxidosqualene cyclases (OSCs), cytochrome P450s (P450s), and UDP-glycosyltransferases (UGTs), the key enzymes in the triterpenoids synthetic pathway. The main obstacles restricting the efficient catalysis of these key enzymes are analyzed, the applications of protein engineering for the three key enzymes in the microbial synthesis of triterpenoids are systematically reviewed, and the challenges and prospects of protein engineering are also discussed.

Identification of key amino acid residues toward improving the catalytic activity and substrate specificity of plant-derived cytochrome P450 monooxygenases CYP716A subfamily enzyme for triterpenoid production in Saccharomyces cerevisiae

Triterpenoids constitute a group of specialized plant metabolites with wide structural diversity and high therapeutic value for human health. Cytochrome P450 monooxygenases (CYP) are a family of enzymes important for generating the structural diversity of triterpenoids by catalyzing the site-specific oxidization of the triterpene backbone. The CYP716 enzyme family has been isolated from various plant families as triterpenoid oxidases; however, their experimental crystal structures are not yet available and the detailed catalytic mechanism remains elusive. Here, we address this challenge by integrating bioinformatics approaches with data from other CYP families. Medicago truncatula CYP716A12, the first functionally characterized CYP716A subfamily enzyme, was chosen as the model for this study. We performed homology modeling, structural alignment, in silico site-directed mutagenesis, and molecular docking analysis to search and screen key amino acid residues relevant to the catalytic activity and substrate specificity of the CYP716A subfamily enzyme in triterpenoid biosynthesis. An in vivo functional analysis using engineered yeast that endogenously produced plant-derived triterpenes was performed to elucidate the results. When the amino acids in the signature region and substrate recognition sites (SRSs) were substituted, the product profile of CYP716A12 was modified. We identified amino acid residues that control the substrate contraction of the enzyme (D292) and engineered the enzyme to improve its catalytic activity and substrate specificity (D122, I212, and Q358) for triterpenoid biosynthesis. In addition, we demonstrated the versatility of this strategy by changing the properties of key residues in SRSs to improve the catalytic activity of Arabidopsis thaliana CYP716A1 (S356) and CYP716A2 (M206, F210) at C-28 on the triterpene backbone. This research has the potential to help in the production of desired triterpenoids in engineered yeast by increasing the catalytic activity and substrate specificity of plant CYP716A subfamily enzymes.

Engineering Critical Amino Acid Residues of Lanosterol Synthase to Improve the Production of Triterpenoids in Saccharomyces cerevisiae

Triterpenoids are a subgroup of terpenoids and have wide applications in the food, cosmetics, and pharmaceutical industries. The heterologous production of various triterpenoids in Saccharomyces cerevisiae, as well as other microbes, has been successfully implemented as these production hosts not only produce the precursor of triterpenoids 2,3-oxidosqualene by the mevalonate pathway but also allow simple expression of plant membrane-anchored enzymes. Nevertheless, 2,3-oxidosqualene is natively converted to lanosterol catalyzed by the endogenous lanosterol synthase (Erg7p), causing low production of recombinant triterpenoids. While simple deletion of ERG7 was not effective, in this study, the critical amino acid residues of Erg7p were engineered to lower this critical enzyme activity. The engineered S. cerevisiae indeed accumulated 2,3-oxidosqualene up to 180 mg/L. Engineering triterpenoid synthesis into the ERG7-modified strain resulted in 7.3- and 3-fold increases in the titers of dammarane-type and lupane-type triterpenoids, respectively. This study presents an efficient inducer-free strategy for lowering Erg7p activity, thereby providing 2,3-oxidosqualene for the enhanced production of various triterpenoids.

Advances in Metabolic Engineering Paving the Way for the Efficient Biosynthesis of Terpenes in Yeasts

Terpenes are a large class of secondary metabolites with diverse structures and functions that are commonly used as valuable raw materials in food, cosmetics, and medicine. With the development of metabolic engineering and emerging synthetic biology tools, these important terpene compounds can be sustainably produced using different microbial chassis. Currently, yeasts such as Saccharomyces cerevisiae and Yarrowia lipolytica have received extensive attention as potential hosts for the production of terpenes due to their clear genetic background and endogenous mevalonate pathway. In this review, we summarize the natural terpene biosynthesis pathways and various engineering strategies, including enzyme engineering, pathway engineering, and cellular engineering, to further improve the terpene productivity and strain stability in these two widely used yeasts. In addition, the future prospects of yeast-based terpene production are discussed in light of the current progress, challenges, and trends in this field. Finally, guidelines for future studies are also emphasized.

Dual cytoplasmic-peroxisomal engineering for high-yield production of sesquiterpene α-humulene in Yarrowia lipolytica

The sesquiterpene α-humulene is an important plant natural product, which has been used in pharmaceutical industry due to the anti-inflammatory and anticancer activities. Although phytoextraction and chemical synthesis have previously been applied into α-humulene production, the low efficiency and high costs limit the development. In this study, Y. lipolytica was engineered as the robust cell factory for sustainable α-humulene production. First, a chassis with high α-humulene output in the cytoplasm was constructed by integrating α-humulene synthases with high catalytic activity, optimizing the flux of MVA and acetyl-CoA pathways. Subsequently, the strategy of dual cytoplasmic-peroxisomal engineering was adopted in Y. lipolytica, the best strain GQ3006 generated by introducing 31 copies of 12 different genes could produce 2280.3 ± 38.2 mg/L (98.7 ± 4.2 mg/g DCW) α-humulene, a 100-fold improvement relative to the baseline strain. In order to further improve the titer, a novel strategy for downregulation of squalene biosynthesis based on Cu²⁺ -repressible promoters was firstly established, which significantly improved the α-humulene titer by 54.2 % to 3516.6 ± 34.3 mg/L. Finally, the engineered strain could produce 21.7 g/L α-humulene in 5-L bioreactor, 6.8-fold higher than the highest α-humulene titer reported prior to this study. Overall, system metabolic engineering strategies used in this study provide a valuable reference for highly sustainable production of terpenoids in Y. lipolytica. This article is protected by copyright. All rights reserved.

Engineering of Yarrowia lipolytica for the production of plant triterpenoids: Asiatic, madecassic, and arjunolic acids

Several plant triterpenoids have valuable pharmaceutical properties, but their production and usage is limited since extraction from plants can burden natural resources, and result in low yields and purity. Here, we engineered oleaginous yeast Yarrowia lipolytica to produce three valuable plant triterpenoids (asiatic, madecassic, and arjunolic acids) by fermentation. First, we established the recombinant production of precursors, ursolic and oleanolic acids, by expressing plant enzymes in free or fused versions in a Y. lipolytica strain previously optimized for squalene production. Engineered strains produced up to 11.6 mg/g DCW ursolic acid or 10.2 mg/g DCW oleanolic acid. The biosynthetic pathway from ursolic acid was extended by expressing the Centella asiatica cytochrome P450 monoxygenases CaCYP716C11p, CaCYP714E19p, and CaCYP716E41p, resulting in the production of trace amounts of asiatic acid and 0.12 mg/g DCW madecassic acid. Expressing the same C. asiatica cytochromes P450 in oleanolic acid-producing strain resulted in the production of oleanane triterpenoids. Expression of CaCYP716C11p in the oleanolic acid-producing strain yielded 8.9 mg/g DCW maslinic acid. Further expression of a codon-optimized CaCYP714E19p resulted in 4.4 mg/g DCW arjunolic acid. Lastly, arjunolic acid production was increased to 9.1 mg/g DCW by swapping the N-terminal domain of CaCYP714E19p with the N-terminal domain from a Kalopanax septemlobus cytochrome P450. In summary, we have demonstrated the production of asiatic, madecassic, and arjunolic acids in a microbial cell factory. The strains and fermentation processes need to be further improved before the production of these molecules by fermentation can be industrialized.

Non-homologous End Joining-Mediated Insertional Mutagenesis Reveals a Novel Target for Enhancing Fatty Alcohols Production in Yarrowia lipolytica

Non-homologous end joining (NHEJ)-mediated integration is effective in generating random mutagenesis to identify beneficial gene targets in the whole genome, which can significantly promote the performance of the strains. Here, a novel target leading to higher protein synthesis was identified by NHEJ-mediated integration that seriously improved fatty alcohols biosynthesis in Yarrowia lipolytica. One batch of strains transformed with fatty acyl-CoA reductase gene (FAR) showed significant differences (up to 70.53-fold) in fatty alcohol production. Whole-genome sequencing of the high-yield strain demonstrated that a new target YALI0_A00913g ("A1 gene") was disrupted by NHEJ-mediated integration of partial carrier DNA, and reverse engineering of the A1 gene disruption (YlΔA1-FAR) recovered the fatty alcohol overproduction phenotype. Transcriptome analysis of YlΔA1-FAR strain revealed A1 disruption led to strengthened protein synthesis process that was confirmed by sfGFP gene expression, which may account for enhanced cell viability and improved biosynthesis of fatty alcohols. This study identified a novel target that facilitated synthesis capacity and provided new insights into unlocking biosynthetic potential for future genetic engineering in Y. lipolytica.

Current state and future perspectives of cytochrome P450 enzymes for C–H and C=C oxygenation

Cytochrome P450 enzymes (CYPs) catalyze a series of C–H and C=C oxygenation reactions, including hydroxylation, epoxidation, and ketonization. They are attractive biocatalysts because of their ability to selectively introduce oxygen into inert molecules under mild conditions. This review provides a comprehensive overview of the C–H and C=C oxygenation reactions catalyzed by CYPs and the various strategies for achieving higher selectivity and enzymatic activity. Furthermore, we discuss the application of C–H and C=C oxygenation catalyzed by CYPs to obtain the desired chemicals or pharmaceutical intermediates in practical production. The rapid development of protein engineering for CYPs provides excellent biocatalysts for selective C–H and C=C oxygenation reactions, thereby promoting the development of environmentally friendly and sustainable production processes.

Progress and perspectives for microbial production of farnesene

Farnesene is increasingly used in industry, agriculture, and other fields due to its unique and excellent properties, necessitating its efficient synthesis. Microbial synthesis is an ideal farnesene production method. Recently, researchers have used several strategies to optimize the production performance of microorganisms. This review summarized these strategies, including regulation of farnesene synthesis pathways, and proposed some emerging tools and methods in stain engineering. Meanwhile, new farnesene biosynthetic pathways and effective farnesene production from cheap or waste substrates were emphatically introduced. Finally, future farnesene biosynthesis challenges were discussed.

Harnessing Yarrowia lipolytica Peroxisomes as a Subcellular Factory for α-Humulene Overproduction

The sesquiterpene α-humulene has been shown to have anti-inflammatory and anticancer activities, which has led to its vast application potential in medicine. However, α-humulene production methods including phytoextraction and chemical synthesis currently were limited to low yield, high costs, and expensive catalysts, which cannot meet the increasing market demand. In this study, Yarrowia lipolytica was developed as a robust cell factory for α-humulene production. The peroxisome in Y. lipolytica was first engineered to boost the synthesis of the sesquiterpene α-humulene. By compartmentalization of the α-humulene biosynthesis pathway, improving ATP and acetyl-CoA supply, and optimizing the gene copy numbers of rate-limiting enzymes, the engineered strain GQ2012 could produce 3.2 g/L α-humulene in a 5 L bioreactor, the highest α-humulene titer reported so far. Our study provides a valuable reference for highly sustainable production of terpenoids by peroxisome engineering in Y. lipolytica.

Redox metabolism for improving whole-cell P450-catalysed terpenoid biosynthesis

The growing preference for producing cytochrome P450-mediated natural products in microbial systems stems from the challenging nature of the organic chemistry approaches. The P450 enzymes are redox-dependent proteins, through which they source electrons from reducing cofactors to drive their activities. Widely researched in biochemistry, most of the previous studies have extensively utilised expensive cell-free assays to reveal mechanistic insights into P450 functionalities in presence of commercial redox partners. However, in the context of microbial bioproduction, the synergic activity of P450- reductase proteins in microbial systems have not been largely investigated. This is mainly due to limited knowledge about their mutual interactions in the context of complex systems. Hence, manipulating the redox potential for natural product synthesis in microbial chassis has been limited. As the potential of redox state as crucial regulator of P450 biocatalysis has been greatly underestimated by the scientific community, in this review, we re-emphasize their pivotal role in modulating the in vivo P450 activity through affecting the product profile and yield. Particularly, we discuss the applications of widely used in vivo redox engineering methodologies for natural product synthesis to provide further suggestions for patterning on P450-based terpenoids production in microbial platforms.

High-efficiency production of bisabolene from waste cooking oil by metabolically engineered Yarrowia lipolytica

The natural plant product bisabolene serves as a precursor for the production of a wide range of industrially relevant chemicals. However, the low abundance of bisabolene in plants renders its isolation from plant sources non-economically viable. Therefore, creation of microbial cell factories for bisabolene production supported by synthetic biology and metabolic engineering strategies presents a more competitive and environmentally sustainable method for industrial production of bisabolene. In this proof-of-principle study, for the first time, we engineered the oleaginous yeast Yarrowia lipolytica to produce α-bisabolene, β-bisabolene and γ-bisabolene through heterologous expression of the α-bisabolene synthase from Abies grandis, the β-bisabolene synthase gene from Zingiber officinale and the γ-bisabolene synthase gene from Helianthus annuus respectively. Subsequently, two metabolic engineering approaches, including overexpression of the endogenous mevalonate pathway genes and introduction of heterologous multidrug efflux transporters, were employed in order to improve bisabolene production. Furthermore, the fermentation conditions were optimized to maximize bisabolene production by the engineered Y. lipolytica strains from glucose. Finally, we explored the potential of the engineered Y. lipolytica strains for bisabolene production from the waste cooking oil. To our knowledge, this is the first report of bisabolene production in Y. lipolytica using metabolic engineering strategies. These findings provide valuable insights into the engineering of Y. lipolytica for a higher-level production of bisabolene and its utilization in converting waste cooking oil into various industrially valuable products.

Volatile Profiling Aided in the Isolation of Anti-Proliferative Lupeol from the Roots of Clinacanthus nutans (Burm. f.) Lindau

Isolation of anti-proliferative compounds from plants is always hindered by the complexities of the plant’s nature and tedious processes. Clinacanthus nutans (Burm. f.) Lindau is a medicinal plant with reported anti-proliferative activities. Our study aimed to isolate potential anti-proliferative compounds present in C. nutans plant. To start with, for our study, we came up with a strategy by first profiling the volatile compounds present in the leaf, stem and root of C. nutans using GC-MS. Comparing the plant’s volatile profiles greatly narrowed down our target of study. We decided to start with the isolation and characterization of a pentacyclic terpenoid, i.e., lupeol from the roots of C. nutans, as this compound was found to present abundantly in the roots compared to the leaf or stem. We developed a simple maceration and re-crystallization method, without the necessity to go through the fractionation or column chromatography for the isolation of lupeol. Characterizations of the isolated compound identified the compound as lupeol. The anti-proliferative activity of the isolated lupeol was further investigated against the MCF-7 cell line, which showed comparable anti-proliferative activity with the authentic lupeol and camptothecin. Our strategy to profile every part of the plant first, followed by selection of the most suitable plant part and targeted compound proved useful for further isolation and characterization bioactive compound from C. nutans.

Metabolic engineering of Yarrowia lipolytica for terpenoids production: advances and perspectives

Terpenoids are a large family of natural products with diversified structures and functions that are widely used in the food, pharmaceutical, cosmetic, and agricultural fields. However, the traditional methods of terpenoids production such as plant extraction and chemical synthesis are inefficient due to the complex processes, high energy consumption, and low yields. With progress in metabolic engineering and synthetic biology, microbial cell factories provide an interesting alternative for the sustainable production of terpenoids. The non-conventional yeast, Yarrowia lipolytica, is a promising host for terpenoid biosynthesis due to its inherent mevalonate pathway, high fluxes of acetyl-CoA and NADPH, and the naturally hydrophobic microenvironment. In this review, we highlight progress in the engineering of Y. lipolytica as terpenoid biomanufacturing factories, describing the different terpenoid biosynthetic pathways and summarizing various metabolic engineering strategies, including progress in genetic manipulation, dynamic regulation, organelle engineering, and terpene synthase variants.

Engineering the oleaginous yeast Yarrowia lipolytica for β-farnesene overproduction

β-farnesene is a sesquiterpenoid with various industrial applications which is now commercially produced by a Saccharomyces cerevisiae strain obtained by random mutagenesis and genetic engineering. We rationally designed a genetically defined Yarrowia lipolytica through recovery of L-leucine biosynthetic route, gene dosage optimization of β-farnesene synthase and disruption of the competition pathway. The resulting β-farnesene titer was improved from 8 to 345 mg L^-1 . Finally, the strategy for decreasing the lipid accumulation by individually and iteratively knocking out four acyltransferases encoding genes was adopted. The result displayed that β-farnesene titer in the engineered strain CIBT6304 in which acyltransferases (DGA1 and DGA2) were deleted increased by 45% and reached 539 mg L^-1 (88 mg g^-1 DCW). Using fed-batch fermentation, CIBT6304 could produce the highest β-farnesene titer (22.8 g L^-1 ) among the genetically defined strains. This study will provide the foundation of engineering Y. lipolytica to produce other terpenoids more cost-efficiently.

Progress in heterologous biosynthesis of forskolin

Forskolin, a class of labdane-type diterpenoid, has significant medicinal value in anticancer, antiasthmatic, antihypertensive, and heart-strengthening treatments. The main source of natural forskolin is its extraction from the cork tissue of the root of Coleus forskohlii. However, conventional modes of extraction pose several challenges. In recent years, the construction of microbial cell factories to produce medicinal natural products via synthetic biological methods has effectively solved the current problems and is a research hotspot in this field. This review summarizes the recent progress in the heterologous synthesis of forskolin via synthetic biological technology, analyzes the current challenges, and proposes corresponding strategies.

Biocatalysis in the Chemistry of Lupane Triterpenoids

Pentacyclic triterpenes are important representatives of natural products that exhibit a wide variety of biological activities. These activities suggest that these compounds may represent potential medicines for the treatment of cancer and viral, bacterial, or protozoal infections. Naturally occurring triterpenes usually have several drawbacks, such as limited activity and insufficient solubility and bioavailability; therefore, they need to be modified to obtain compounds suitable for drug development. Modifications can be achieved either by methods of standard organic synthesis or with the use of biocatalysts, such as enzymes or enzyme systems within living organisms. In most cases, these modifications result in the preparation of esters, amides, saponins, or sugar conjugates. Notably, while standard organic synthesis has been heavily used and developed, the use of the latter methodology has been rather limited, but it appears that biocatalysis has recently sparked considerably wider interest within the scientific community. Among triterpenes, derivatives of lupane play important roles. This review therefore summarizes the natural occurrence and sources of lupane triterpenoids, their biosynthesis, and semisynthetic methods that may be used for the production of betulinic acid from abundant and inexpensive betulin. Most importantly, this article compares chemical transformations of lupane triterpenoids with analogous reactions performed by biocatalysts and highlights a large space for the future development of biocatalysis in this field. The results of this study may serve as a summary of the current state of research and demonstrate the potential of the method in future applications.

Advanced Strategies for the Synthesis of Terpenoids in Yarrowia lipolytica

Terpenoids are an important class of secondary metabolites that play an important role in food, agriculture, and other fields. Microorganisms are rapidly emerging as a promising source for the production of terpenoids. As an oleaginous yeast, Yarrowia lipolytica contains a high lipid content which indicates that it must produce high amounts of acetyl-CoA, a necessary precursor for the biosynthesis of terpenoids. Y. lipolytica has a complete eukaryotic mevalonic acid (MVA) pathway but it has not yet seen commercial use due to its low productivity. Several metabolic engineering strategies have been developed to improve the terpenoids production of Y. lipolytica, including developing the orthogonal pathway for terpenoid synthesis, increasing the catalytic efficiency of terpenoids synthases, enhancing the supply of acetyl-CoA and NADPH, expressing rate-limiting genes, and modifying the branched pathway. Moreover, most of the acetyl-CoA is used to produce lipid, so it is an effective strategy to strike a balance of precursor distribution by rewiring the lipid biosynthesis pathway. Lastly, the latest developed non-homologous end-joining strategy for improving terpenoid production is introduced. This review summarizes the status and metabolic engineering strategies of terpenoids biosynthesis in Y. lipolytica and proposes new insights to move the field forward.

Endophytes, biotransforming microorganisms, and engineering microbial factories for triterpenoid saponins production

Triterpenoid saponins are structurally diverse secondary metabolites. They are the main active ingredient of many medicinal plants and have a wide range of pharmacological effects. Traditional production of triterpenoid saponins, directly extracted from cultivated plants, cannot meet the rapidly growing demand of pharmaceutical industry. Microorganisms with triterpenoid saponins production ability (especially Agrobacterium genus) and biotransformation ability, such as fungal species in Armillaria and Aspergillus genera and bacterial species in Bacillus and Intestinal microflora, represent a valuable source of active metabolites. With the development of synthetic biology, engineering microorganisms acquired more potential in terms of triterpenoid saponins production. This review focusses on potential mechanisms and the high yield strategies of microorganisms with inherent production or biotransformation ability of triterpenoid saponins. Advances in the engineering of microorganisms, such as Saccharomyces cerevisiae, Yarrowia lipolytica, and Escherichia coli, for the biosynthesis triterpenoid saponins de novo have also been reported. Strategies to increase the yield of triterpenoid saponins in engineering microorganisms are summarized following four aspects, that is, introduction of high efficient gene, optimization of enzyme activity, enhancement of metabolic flux to target compounds, and optimization of fermentation conditions. Furthermore, the challenges and future directions for improving the yield of triterpenoid saponins biosynthesis in engineering microorganisms are discussed.

Comparative Analysis of NADPH-Cytochrome P450 Reductases From Legumes for Heterologous Production of Triterpenoids in Transgenic Saccharomyces cerevisiae

Triterpenoids are plant specialized metabolites with various pharmacological activities. They are widely distributed in higher plants, such as legumes. Because of their low accumulation in plants, there is a need for improving triterpenoid production. Cytochrome P450 monooxygenases (CYPs) play critical roles in the structural diversification of triterpenoids. To perform site-specific oxidations, CYPs require the electrons that are transferred by NADPH-cytochrome P450 reductase (CPR). Plants possess two main CPR classes, class I and class II. CPR classes I and II have been reported to be responsible for primary and specialized (secondary) metabolism, respectively. In this study, we first analyzed the CPR expression level of three legumes species, Medicago truncatula, Lotus japonicus, and Glycyrrhiza uralensis, showing that the expression level of CPR class I was lower and more stable, while that of CPR class II was higher in almost all the samples. We then co-expressed different combinations of CYP716As and CYP72As with different CPR classes from these three legumes in transgenic yeast. We found that CYP716As worked better with CPR-I from the same species, while CYP72As worked better with any CPR-IIs. Using engineered yeast strains, CYP88D6 paired with class II GuCPR produced the highest level of 11-oxo-β-amyrin, the important precursor of high-value metabolites glycyrrhizin. This study provides insight into co-expressing genes from legumes for heterologous production of triterpenoids in yeast.

Recent updates on immunological, pharmacological, and alternative approaches to combat COVID-19

The pandemic coronavirus disease 2019 (COVID-19) is instigated by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that is mainly transmitted via the inhalation route and characterized by fever, coughing and shortness of breath. COVID-19 affects all age groups with no single cure. The drug discovery, manufacturing, and safety studies require extensive time and sources and, therefore, struggled to match the exponential spread of COVID-19. Yet, various repurposed drugs (antivirals, immune-modulators, nucleotide analogues), and convalescent plasma therapy have been authorized for emergency use against COVID-19 by Food and Drug Administration under certain limits and conditions. The discovery of vaccine is the biggest milestone achieved during the current pandemic era. About nine vaccines were developed for human use with varying claims of efficacy. The rapid emergence of mutations in SARS-CoV-2, suspected adverse drug reactions of current therapies in special population groups and limited availability of drugs in developing countries necessitate the development of more efficacious, safe and cheap drugs/vaccines for treatment and prevention of COVID-19. Keeping in view these limitations, the current review provides an update on the efficacy and safety of the repurposed, and natural drugs to treat COVID-19 as well as the vaccines used for its prophylaxis.

Current Challenges and Opportunities in Non-native Chemical Production by Engineered Yeasts

Yeasts are promising industrial hosts for sustainable production of fuels and chemicals. Apart from efficient bioethanol production, yeasts have recently demonstrated their potential for biodiesel production from renewable resources. The fuel-oriented product profiles of yeasts are now expanding to include non-native chemicals with the advances in synthetic biology. In this review, current challenges and opportunities in yeast engineering for sustainable production of non-native chemicals will be discussed, with a focus on the comparative evaluation of a bioethanol-producing Saccharomyces cerevisiae strain and a biodiesel-producing Yarrowia lipolytica strain. Synthetic pathways diverging from the distinctive cellular metabolism of these yeasts guide future directions for product-specific engineering strategies for the sustainable production of non-native chemicals on an industrial scale.

Genetic and bioprocess engineering to improve squalene production in Yarrowia lipolytica

Squalene is the precursor for triterpene-based natural products and steroids-based drugs. It has been widely used as pharmaceutical intermediates and personal care products. The aim of this work is to test the feasibility of engineering Yarrowia lipolytica as a potential host for squalene production. The bottleneck of the pathway was removed by overexpressing native HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase. With the recycling of NADPH from the mannitol cycle, the engineered strain produced about 180.3 mg/L and 188.2 mg/L squalene from glucose or acetate minimal media. By optimizing the C/N ratio, controlling the media pH and mitigating acetyl-CoA flux competition from lipogenesis, the engineered strain produced 502.7 mg/L squalene, a 28-fold increase over the parental strain (17.2 mg/L). This work may serve as a baseline to harness Y. lipolytica as an oleaginous cell factory for sustainable production of squalene or terpenoids-based chemicals and natural products.

Functional expression of eukaryotic cytochrome P450s in yeast

Cytochrome P450 enzymes (P450s) are a superfamily of heme-thiolate proteins widely existing in various organisms. Due to their key roles in secondary metabolism, degradation of xenobiotics, and carcinogenesis, there is a great demand to heterologously express and obtain a sufficient amount of active eukaryotic P450s. However, most eukaryotic P450s are endoplasmic reticulum-localized membrane proteins, which is the biggest challenge for functional expression to high levels. Furthermore, the functions of P450s require the cooperation of cytochrome P450 reductases for electron transfer. Great efforts have been devoted to the heterologous expression of eukaryotic P450s, and yeasts, particularly Saccharomyces cerevisiae are frequently considered as the first expression systems to be tested for this challenging purpose. This review discusses the strategies for improving the expression and activity of eukaryotic P450s in yeasts, followed by examples of P450s involved in biosynthetic pathway engineering.

Engineering Critical Enzymes and Pathways for Improved Triterpenoid Biosynthesis in Yeast

Triterpenoids represent a diverse group of phytochemicals that are widely distributed in the plant kingdom and have many biological activities. The heterologous production of triterpenoids in Saccharomyces cerevisiae has been successfully implemented by introducing various triterpenoid biosynthetic pathways. By engineering related enzymes as well as through yeast metabolism, the yield of various triterpenoids is significantly improved from the milligram per liter scale to the gram per liter scale. This achievement demonstrates that engineering critical enzymes is considered a potential strategy to overcome the main hurdles of the industrial application of these potent natural products. Here, we review strategies for designing enzymes to improve the yield of triterpenoids in S. cerevisiae in terms of three main aspects: 1, elevating the supply of the precursor 2,3-oxidosqualene; 2, optimizing triterpenoid-involved reactions; and 3, lowering the competition of the native sterol pathway. Then, we provide challenges and prospects for further enhancing triterpenoid production in S. cerevisiae.

Yarrowia lipolytica Strains Engineered for the Production of Terpenoids

Terpenoids are a diverse group of over 55,000 compounds with potential applications as advanced fuels, bulk and fine chemicals, pharmaceutical ingredients, agricultural chemicals, etc. To facilitate their bio-based production, there is a need for plug-and-play hosts, capable of high-level production of different terpenoids. Here we engineer Yarrowia lipolytica platform strains for the overproduction of mono-, sesqui-, di-, tri-, and tetraterpenoids. The monoterpene platform strain was evaluated by expressing Perilla frutescens limonene synthase, which resulted in limonene titer of 35.9 mg/L and was 100-fold higher than when the same enzyme was expressed in the strain without mevalonate pathway improvement. Expression of Callitropsis nootkatensis valencene synthase in the sesquiterpene platform strain resulted in 113.9 mg/L valencene, an 8.4-fold increase over the control strain. Platform strains for production of squalene, complex triterpenes, or diterpenes and carotenoids were also constructed and resulted in the production of 402.4 mg/L squalene, 22 mg/L 2,3-oxidosqualene, or 164 mg/L β-carotene, respectively. The presented terpenoid platform strains can facilitate the evaluation of terpenoid biosynthetic pathways and are a convenient starting point for constructing efficient cell factories for the production of various terpenoids. The platform strains and exemplary terpenoid strains can be obtained from Euroscarf.