Production of the sesquiterpenoid (+)-nootkatone by metabolic engineering of Pichia pastoris

Transportation engineering for enhanced production of plant natural products in microbial cell factories

Plant natural products (PNPs) exhibit a wide range of biological activities and have essential applications in various fields such as medicine, agriculture, and flavors. Given their natural limitations, the production of high-value PNPs using microbial cell factories has become an effective alternative in recent years. However, host metabolic burden caused by its massive accumulation has become one of the main challenges for efficient PNP production. Therefore, it is necessary to strengthen the transmembrane transport process of PNPs. This review introduces the discovery and mining of PNP transporters to directly mediate PNP transmembrane transportation both intracellularly and extracellularly. In addition to transporter engineering, this review also summarizes several auxiliary strategies (such as small molecules, environmental changes, and vesicles assisted transport) for strengthening PNP transportation. Finally, this review is concluded with the applications and future perspectives of transportation engineering in the construction and optimization of PNP microbial cell factories.

Multifactorial optimization enables the identification of a greener method to produce (+)-nootkatone

The natural aroma compound (+)-nootkatone was obtained in selective conversions of up to 74mol % from inexpensive (+)-valencene substrate by using a comparatively greener biocatalytic process developed based on modifications of the previously published Firmenich method. Buffer identity and concentration, pH, temperature and downstream work-up procedures were optimized to produce a crude product in which >90% of (+)-valencene had been converted, with high chemoselectivity observed for (+)-nootkatone production. Interestingly, the biotransformation was carried out efficiently at temperatures as low as 21 ºC. Surprisingly, the best results were obtained when an acidic pH in the range of 3-6 was applied, as compared to the previously published procedure in which it appeared to be necessary to buffer the pH optimally and fixed throughout at 8.5. Furthermore, there was no need to maintain a pure oxygen atmosphere to achieve good (+)-nootkatone yields. Instead, air bubbled continuously at a low rate through the reaction mixture via a submerged glass capillary was sufficient to enable the desired lipoxygenase-catalyzed oxidation reactions to occur efficiently. No valencene epoxide side-products were detected in the organic product extract by a standard GCMS protocol. Only traces of the anticipated corresponding α- and β-nootkatol intermediates were routinely observed.

Applications of the Methylotrophic Yeast Komagataella phaffii in the Context of Modern Biotechnology

Komagataella phaffii (formerly Pichia pastoris) is a methylotrophic yeast widely used in laboratories around the world to produce recombinant proteins. Given its advantageous features, it has also gained much interest in the context of modern biotechnology. In this review, we present the utilization of K. phaffii as a platform to produce several products of economic interest such as biopharmaceuticals, renewable chemicals, fuels, biomaterials, and food/feed products. Finally, we present synthetic biology approaches currently used for strain engineering, aiming at the production of new bioproducts.

Essential Oils: Chemistry and Pharmacological Activities-Part II

The importance of essential oils and their components in the industrial sector is attributed to their chemical characteristics and their application in the development of products in the areas of cosmetology, food, and pharmaceuticals. However, the pharmacological properties of this class of natural products have been extensively investigated and indicate their applicability for obtaining new drugs. Therefore, this review discusses the use of these oils as starting materials to synthesize more complex molecules and products with greater commercial value and clinic potential. Furthermore, the antiulcer, cardiovascular, and antidiabetic mechanisms of action are discussed. The main mechanistic aspects of the chemopreventive properties of oils against cancer are also presented. The data highlight essential oils and their derivatives as a strategic chemical group in the search for effective therapeutic agents against various diseases.

Identification of volatile compounds and their bioactivities from unpolar fraction of Alpinia oxyphylla Miq. and mining key genes of nootkatone biosynthesis

In this study, analysis of the chemical constituents and bioactivities of the unpolar fractions [petroleum ether (PE) and chloroform (C)] of fruits and leaves of Alpinia oxyphylla Miq. were carried out, as well as the bioactivities of the main compounds nootkatone and valencene. From PE and C fractions of the fruits, and PE fraction of the leaves, 95.80%, 59.30%, and 82.11% of the chemical constituents respectively were identified by GC-MS. Among these identified compounds, nootkatone was the main compound in all of three fractions, while valencene was the second main compound in the PE fractions of the fruits and leaves. The bioactivities results showed that all of the fractions and the major compound nootkatone showed tyrosinase inhibitory, as well as inhibitory effect on NO production in LPS-stimulated RAW264.7 cells. While valencene only presented inhibitory activity on NO production in RAW264.7 cells. The critical genes involved in nootkatone biosynthesis in A. oxyphylla were identified from the public transcriptome datasets, and protein sequences were preliminarily analyzed. Our studies develop the usage of the unpolar fractions of A. oxyphylla, especially its leaves as the waste during its production, and meanwhile provide the gene resources for nootkatone biosynthesis.

Biocatalyst for the synthesis of natural flavouring compounds as food additives: Bridging the gap for a more sustainable industrial future

Biocatalysis entails the use of purified enzymes in the manufacturing of flavouring chemicals food industry as well as at the laboratory level. These biocatalysts can significantly accelerate organic chemical processes and improve product stereospecificity. The unique characteristics of biocatalyst helpful in synthesizing the environmentally friendly flavour and aroma compounds used as a food additive in foodstuffs. With methods like enzyme engineering on biotechnological interventions the efficient tuning of produce will fulfil the needs of food industry. This review summarizes the biosynthesis of different flavour and aroma component through microbial catalysts and using advanced techniques which are available for enzyme improvement. Also pointing out their benefits and drawbacks for specific technological processes necessary for successful industrial application of biocatalysts. The article covers the market scenario, cost economics, environmental safety and regulatory framework for the production of food flavoured chemicals by the bioprocess engineering.

Complete pathway elucidation and heterologous reconstitution of (+)-nootkatone biosynthesis from Alpinia oxyphylla

(+)-Nootkatone is a natural sesquiterpene ketone widely used in food, cosmetics, pharmaceuticals, and agriculture. It is also regarded as one of the most valuable terpenes used commercially. However, plants contain trace amounts of (+)-nootkatone, and extraction from plants is insufficient to meet market demand. Alpinia oxyphylla is a well-known medicinal plant in China, and (+)-nootkatone is one of the main components within the fruits. By transcriptome mining and functional screening using a precursor-providing yeast chassis, the complete (+)-nootkatone biosynthetic pathway in Alpinia oxyphylla was identified. A (+)-valencene synthase (AoVS) was identified as a novel monocot-derived valencene synthase; three (+)-valencene oxidases AoCYP6 (CYP71BB2), AoCYP9 (CYP71CX8), and AoCYP18 (CYP701A170) were identified by constructing a valencene-providing yeast strain. With further characterisation of a cytochrome P450 reductase (AoCPR1) and three dehydrogenases (AoSDR1/2/3), we successfully reconstructed the (+)-nootkatone biosynthetic pathway in Saccharomyces cerevisiae, representing a basis for its biotechnological production. Identifying the biosynthetic pathway of (+)-nootkatone in A. oxyphylla unravelled the molecular mechanism underlying its formation in planta and also supported the bioengineering production of (+)-nootkatone. The highly efficient yeast chassis screening method could be used to elucidate the complete biosynthetic pathway of other valuable plant natural products in future.

Artificial carbon assimilation: From unnatural reactions and pathways to synthetic autotrophic systems

Synthetic biology is being increasingly used to establish novel carbon assimilation pathways and artificial autotrophic strains that can be used in low-carbon biomanufacturing. Currently, artificial pathway design has made significant progress from advocacy to practice within a relatively short span of just over ten years. However, there is still huge scope for exploration of pathway diversity, operational efficiency, and host suitability. The accelerated research process will bring greater opportunities and challenges. In this paper, we provide a comprehensive summary and interpretation of representative one-carbon assimilation pathway designs and artificial autotrophic strain construction work. In addition, we propose some feasible design solutions based on existing research results and patterns to promote the development and application of artificial autotrophy.

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%.

Metabolic Engineering of Pichia pastoris for High-Level Production of Lycopene

Lycopene is widely used in cosmetics, food, and nutritional supplements. Microbial production of lycopene has been intensively studied. However, few metabolic engineering studies on Pichia pastoris have been aimed at achieving high-yield lycopene production. In this study, the CRISPR/Cpf1-based gene repression system was developed and the gene editing system was optimized, which were applied to improve lycopene production successfully. In addition, the sterol regulatory element-binding protein SREBP (Sre) was used for the regulation of lipid metabolic pathways to promote lycopene overproduction in P. pastoris for the first time. The final engineered strain produced lycopene at 7.24 g/L and 75.48 mg/g DCW in fed-batch fermentation, representing the highest lycopene yield in P. pastoris reported to date. These findings provide effective strategies for extended metabolic engineering assisted by the CRISPR/Cpf1 system and new insights into metabolic engineering through transcriptional regulation of related metabolic pathways to enhance carotenoid production in P. pastoris.

Biosynthesis of High-Active Hemoproteins by the Efficient Heme-Supply Pichia Pastoris Chassis

Microbial synthesis of valuable hemoproteins has become a popular research topic, and Pichia pastoris is a versatile platform for the industrial production of recombinant proteins. However, the inadequate supply of heme limits the synthesis of high-active hemoproteins. Here a strategy for enhancing intracellular heme biosynthesis to improve the titers and functional activities of hemoproteins is reported. After selecting a suitable expressional strategy for globins, the efficient heme-supply P. pastoris chassis is established by removing the spatial segregation during heme biosynthesis, optimizing precursor synthesis, assembling rate-limiting enzymes using protein scaffolds, and inhibiting heme degradation. This robust chassis produces several highly active hemoproteins, including porcine myoglobin, soy hemoglobin, Vitreoscilla hemoglobin, and P450-BM3, which can be used in the development of artificial meat, high-cell-density fermentation, and whole-cell catalytic synthesis of high-value-added compounds. Furthermore, the engineered chassis strain has great potential for producing and applying other hemoproteins with high activities in various fields.

Reviewing the Source, Physiological Characteristics, and Aroma Production Mechanisms of Aroma-Producing Yeasts

Flavor is an essential element of food quality. Flavor can be improved by adding flavoring substances or via microbial fermentation to impart aroma. Aroma-producing yeasts are a group of microorganisms that can produce aroma compounds, providing a strong aroma to foods and thus playing a great role in the modern fermentation industry. The physiological characteristics of aroma-producing yeast, including alcohol tolerance, acid tolerance, and salt tolerance, are introduced in this article, beginning with their origins and biological properties. The main mechanism of aroma-producing yeast is then analyzed based on its physiological roles in the fermentation process. Functional enzymes such as proteases, lipases, and glycosidase are released by yeast during the fermentation process. Sugars, fats, and proteins in the environment can be degraded by these enzymes via pathways such as glycolysis, methoxylation, the Ehrlich pathway, and esterification, resulting in the production of various aromatic esters (such as ethyl acetate and ethyl caproate), alcohols (such as phenethyl alcohol), and terpenes (such as monoterpenes, sesquiterpenes, and squalene). Furthermore, yeast cells can serve as cell synthesis factories, wherein specific synthesis pathways can be introduced into cells using synthetic biology techniques to achieve high-throughput production. In addition, the applications of aroma yeast in the food, pharmaceutical, and cosmetic industries are summarized, and the future development trends of aroma yeasts are discussed to provide a theoretical basis for their application in the food fermentation industry.

Methanol-based biomanufacturing of fuels and chemicals using native and synthetic methylotrophs

Methanol has recently gained significant attention as a potential carbon substrate for the production of fuels and chemicals, owing to its high degree of reduction, abundance, and low price. Native methylotrophic yeasts and bacteria have been investigated for the production of fuels and chemicals. Alternatively, synthetic methylotrophic strains are also being developed by reconstructing methanol utilization pathways in model microorganisms, such as Escherichia coli. Owing to the complex metabolic pathways, limited availability of genetic tools, and methanol/formaldehyde toxicity, the high-level production of target products for industrial applications are still under development to satisfy commercial feasibility. This article reviews the production of biofuels and chemicals by native and synthetic methylotrophic microorganisms. It also highlights the advantages and limitations of both types of methylotrophs and provides an overview of ways to improve their efficiency for the production of fuels and chemicals from methanol.

Isolation, purification, and mass spectrometry identification of the enzyme involved in citrus flavor (+)-valencene biotransformation to (+)-nootkatone by Yarrowia lipolytica

BACKGROUND

(+)-Nootkatone is a highly valuable sesquiterpene compound that can be used as an aromatic in the food industry because of its grapefruit flavor and low sensory threshold. The unconventional yeast Yarrowia lipolytica has many unique physical and chemical properties, metabolic characteristics, and genetic structure, which has aroused the interest of researchers. Previous research showed that Y. lipolytica possesses the ability to transform the sesquiterpene (+)-valencene to (+)-nootkatone. The aim of this study was to isolate, purify, and identify the enzyme involved in the (+)-valencene bioconversion to (+)-nootkatone by Y. lipolytica.

RESULTS

In this study, ultrasonic-assisted extraction, ammonium sulfate precipitation, anion-exchange chromatography, and gel-filtration chromatography were used to separate and purify the enzyme involved in the (+)-valencene bioconversion by Y. lipolytica. The protein was identified as aldehyde dehydrogenase (ALDH) (gene0658) using sodium dodecyl sulfate polyacrylamide gel electrophoresis and liquid chromatography-tandem mass spectrometry analysis. The ALDH had the highest activity when the pH value was 6.0 and the temperature was 30 °C. The activity of ALDH was significantly stimulated by ferrous ions and inhibited by barium, calcium, and magnesium ions.

CONCLUSION

This is the first time that ALDH was found to participate in (+)-valencene biotransformation by Y. lipolytica. It may be involved in regulating the microbial transformation of (+)-valencene to (+)-nootkatone through redox characteristics. This study provides a theoretical basis and reference for the biological synthesis of citrus flavor (+)-nootkatone. © 2023 Society of Chemical Industry.

Development of a Pichia pastoris cell factory for efficient production of germacrene A: a precursor of β-elemene

β-Elemene, an active ingredient found in medicinal plants like turmeric and zedoary, is a sesquiterpene compound with antitumor activity against various cancers. However, its current mode of production through plant extraction suffers from low efficiency and limited natural resources. Recently, there has been an increased interest in establishing microbial cell factories to produce germacrene A, which can be converted to β-elemene by a one-step reaction in vitro. In this study, we constructed an engineered Pichia pastoris cell factory for producing germacrene A. We rerouted the fluxes towards germacrene A biosynthesis through the optimization of the linker sequences between germacrene A synthase (GAS) and farnesyl pyrophosphate synthase (ERG20), overexpression of important pathway genes (i.e., IDI1, tHMG1, and ACS), and multi-copy integration of related expression cassettes. In combination with medium optimization and bioprocess engineering, the final titer of germacrene A in a 1 L fermenter reached 1.9 g/L through fed-batch fermentation. This represents the first report on the production of germacrene A in P. pastoris and demonstrates its advantage in producing terpenoids and other value-added natural products.

A high-quality assembled genome of a representative peach landrace, 'Feichenghongli', and analysis of distinct late florescence and narrow leaf traits

BACKGROUND

Peach (Prunus persica L. Batsch) is one of the most popular fruits worldwide. Although the reference genome of 'Lovell' peach has been released, the diversity of genome-level variations cannot be explored with one genome. To detect these variations, it is necessary to assemble more genomes.

RESULTS

We sequenced and de novo assembled the genome of 'Feichenghongli' (FCHL), a representative landrace with strict self-pollination, which maintained the homozygosity of the genome as much as possible. The chromosome-level genome of FCHL was 239.06 Mb in size with a contig N50 of 26.93 Mb and only 4 gaps at the scaffold level. The alignment of the FCHL genome with the reference 'Lovell' genome enabled the identification of 432535 SNPs, 101244 insertions and deletions, and 7299 structural variants. Gene family analysis showed that the expanded genes in FCHL were enriched in sesquiterpenoids and triterpenoid biosynthesis. RNA-seq analyses were carried out to investigate the two distinct traits of late florescence and narrow leaves. Two key genes, PpDAM4 and PpAGL31, were identified candidates for the control of flower bud dormancy, and an F-box gene, PpFBX92, was identified as a good candidate gene in the regulation of leaf size.

CONCLUSIONS

The assembled high-quality genome could deepen our understanding of variations among diverse genomes and provide valuable information for identifying functional genes and improving the molecular breeding process.

Focus and Insights into the Synthetic Biology-Mediated Chassis of Economically Important Fungi for the Production of High-Value Metabolites

Substantial progress has been achieved and knowledge gaps addressed in synthetic biology-mediated engineering of biological organisms to produce high-value metabolites. Bio-based products from fungi are extensively explored in the present era, attributed to their emerging importance in the industrial sector, healthcare, and food applications. The edible group of fungi and multiple fungal strains defines attractive biological resources for high-value metabolites comprising food additives, pigments, dyes, industrial chemicals, and antibiotics, including other compounds. In this direction, synthetic biology-mediated genetic chassis of fungal strains to enhance/add value to novel chemical entities of biological origin is opening new avenues in fungal biotechnology. While substantial success has been achieved in the genetic manipulation of economically viable fungi (including Saccharomyces cerevisiae) in the production of metabolites of socio-economic relevance, knowledge gaps/obstacles in fungal biology and engineering need to be remedied for complete exploitation of valuable fungal strains. Herein, the thematic article discusses the novel attributes of bio-based products from fungi and the creation of high-value engineered fungal strains to promote yield, bio-functionality, and value-addition of the metabolites of socio-economic value. Efforts have been made to discuss the existing limitations in fungal chassis and how the advances in synthetic biology provide a plausible solution.

Biotechnologies in Perfume Manufacturing: Metabolic Engineering of Terpenoid Biosynthesis

The fragrance industry is increasingly turning to biotechnology to produce sustainable and high-quality fragrance ingredients. Microbial-based approaches have been found to be particularly promising, as they offer a more practical, economical and sustainable alternative to plant-based biotechnological methods for producing terpene derivatives of perfumery interest. Among the evaluated works, the heterologous expression of both terpene synthase and mevalonate pathway into Escherichia coli has shown the highest yields. Biotechnology solutions have the potential to help address the growing demand for sustainable and high-quality fragrance ingredients in an economically viable and responsible manner. These approaches can help compensate for supply issues of rare or impermanent raw materials, while also meeting the increasing demand for sustainable ingredients and processes. Although scaling up biotransformation processes can present challenges, they also offer advantages in terms of safety and energy savings. Exploring microbial cell factories for the production of natural fragrance compounds is a promising solution to both supply difficulties and the demand for sustainable ingredients and processes in the fragrance industry.

Global metabolic rewiring of the nonconventional yeast Ogataea polymorpha for biosynthesis of the sesquiterpenoid β-elemene

Bioproduction of natural products via microbial cell factories is a promising alternative to traditional plant extraction. Recently, nonconventional microorganisms have emerged as attractive chassis hosts for biomanufacturing. One such microorganism, Ogataea polymorpha is an industrial yeast used for protein expression with numerous advantages, such as thermal-tolerance, a wide substrate spectrum and high-density fermentation. Here, we systematically rewired the cellular metabolism of O. polymorpha to achieve high-level production of the sesquiterpenoid β-elemene by optimizing the mevalonate pathway, enhancing the supply of NADPH and acetyl-CoA, and downregulating competitive pathways. The engineered strain produced 509 mg/L and 4.7 g/L of β-elemene under batch and fed-batch fermentation, respectively. Therefore, this study identified the potential industrial application of O. polymorpha as a good microbial platform for producing sesquiterpenoids.

Engineering Komagataella phaffii to biosynthesize cordycepin from methanol which drives global metabolic alterations at the transcription level

Cordycepin has the potential to be an alternative to the disputed herbicide glyphosate. However, current laborious and time-consuming production strategies at low yields based on Cordyceps militaris lead to extremely high cost and restrict its application in the field of agriculture. In this study, Komagataella phaffii (syn. Pichia pastoris) was engineered to biosynthesize cordycepin from methanol, which could be converted from CO₂. Combined with fermentation optimization, cordycepin content in broth reached as high as 2.68 ± 0.04 g/L within 168 h, around 15.95 mg/(L·h) in productivity. Additionally, a deaminated product of cordycepin was identified at neutral or weakly alkaline starting pH during fermentation. Transcriptome analysis found the yeast producing cordycepin was experiencing severe inhibition in methanol assimilation and peroxisome biogenesis, responsible for delayed growth and decreased carbon flux to pentose phosphate pathway (PPP) which led to lack of precursor supply. Amino acid interconversion and disruption in RNA metabolism were also due to accumulation of cordycepin. The study provided a unique platform for the manufacture of cordycepin based on the emerging non-conventional yeast and gave practical strategies for further optimization of the microbial cell factory.

Overproduction of Patchoulol in Metabolically Engineered Komagataella phaffii

Patchoulol, a plant-derived sesquiterpene compound, is widely used in perfumes, cosmetics, and pharmaceuticals. Microbial production provides a promising alternative approach for the efficient and sustainable production of patchoulol. However, there are no systematic engineering studies on Komagataella phaffii aimed at achieving high-yield patchoulol production. Herein, by fusion overexpression of FPP synthase and patchoulol synthase (ERG20LPTS), increasing the precursor supply, adjusting the copy number of ERG20LPTS and PTS, and combined with adding auxiliary carbon source and methanol concentration optimization, we constructed a high-yield patchoulol-producing strain P6H53, which produced 149.64 mg/L patchoulol in shake-flask fermentation with methanol as the substrate. In fed-batch fermentation, strain P6H53 achieved the highest production (2.47 g/L, 21.48 mg/g DCW, and 283.25 mg/L/d) to date in a 5 L fermenter. This study will lay a good foundation for the development of K. phaffii as a promising chassis microbial cell for the synthesis of patchoulol and other sesquiterpenes with methanol as the carbon source.

Production, Function, and Applications of the Sesquiterpenes Valencene and Nootkatone: a Comprehensive Review

Valencene and nootkatone, two sesquiterpenes, extracted from natural sources, have great market potential with diverse applications. This paper aims to comprehensively review the recent advances in valencene and nootkatone, including source, production, physicochemical and biological properties, safety and pharmacokinetics evaluation, potential uses, and their industrial applications as well as future research directions. Microbial biosynthesis offers a promising alternative approach for sustainable production of valencene and nootkatone. Both compounds exert various beneficial activities, including antimicrobial, insecticidal, antioxidant, anti-inflammatory, anticancer, cardioprotective, neuroprotective, hepatoprotective, and nephroprotective and other activities. However, most of the studies are performed in animals and in vitro, making it difficult to give a conclusive description about their health benefits and extend their application. Hence, more attention should be paid to in vivo and long-term clinical studies in the future. Moreover, valencene and nootkatone are considered safe for consumption and show great promise in the applications of food, cosmetic, pharmaceutical, chemical, and agricultural industries.

Systematic Optimization of HPO-CPR to Boost (+)-Nootkatone Synthesis in Engineered Saccharomyces cerevisiae

As an important and expensive natural sesquiterpene compound in grapefruit, the interest in (+)-nootkatone is stimulated by its strong grapefruit-like odor and physiological activities, which induce efforts for its microbial production. However, the low catalytic efficiency of the cytochrome P450-P450 reductase (HPO-CPR) system is the main challenge. We developed a high-throughput screening (HTS) method using the principle of the color reaction between carbonyl compounds and 2,4-dinitrophenylhydrazine (DNPH), which could rapidly screen the activity of candidate HPO mutants. After optimizing the pairing of HPO and CPR and through semirational design, the optimal mutant HPO_M18 with catalytic performance 2.54 times that of the initial was obtained. An encouraging (+)-nootkatone titer of 2.39 g/L was achieved through two-stage fed-batch fermentation after metabolic engineering and endoplasmic reticulum engineering, representing the highest titer reported to date. Our findings lay the foundation for the development of an economically viable bioprocess for (+)-nootkatone.

Developing methylotrophic microbial platforms for a methanol-based bioindustry

Methanol, a relatively cheap and renewable single-carbon feedstock, has gained considerable attention as a substrate for the bio-production of commodity chemicals. Conventionally produced from syngas, along with emerging possibilities of generation from methane and CO2, this C1 substrate can serve as a pool for sequestering greenhouse gases while supporting a sustainable bio-economy. Methylotrophic organisms, with the inherent ability to use methanol as the sole carbon and energy source, are competent candidates as platform organisms. Accordingly, methanol bioconversion pathways have been an attractive target for biotechnological and bioengineering interventions in developing microbial cell factories. This review summarizes the recent advances in methanol-based production of various bulk and value-added chemicals exploiting the native and synthetic methylotrophic organisms. Finally, the current challenges and prospects of streamlining these methylotrophic platforms are discussed.

Natural products of medicinal plants: biosynthesis and bioengineering in post-genomic era

Globally, medicinal plant natural products (PNPs) are a major source of substances used in traditional and modern medicine. As we human race face the tremendous public health challenge posed by emerging infectious diseases, antibiotic resistance and surging drug prices etc., harnessing the healing power of medicinal plants gifted from mother nature is more urgent than ever in helping us survive future challenge in a sustainable way. PNP research efforts in the pre-genomic era focus on discovering bioactive molecules with pharmaceutical activities, and identifying individual genes responsible for biosynthesis. Critically, systemic biological, multi- and inter-disciplinary approaches integrating and interrogating all accessible data from genomics, metabolomics, structural biology, and chemical informatics are necessary to accelerate the full characterization of biosynthetic and regulatory circuitry for producing PNPs in medicinal plants. In this review, we attempt to provide a brief update on the current research of PNPs in medicinal plants by focusing on how different state-of-the-art biotechnologies facilitate their discovery, the molecular basis of their biosynthesis, as well as synthetic biology. Finally, we humbly provide a foresight of the research trend for understanding the biology of medicinal plants in the coming decades.

Advances in the synthesis of three typical tetraterpenoids including β-carotene, lycopene and astaxanthin

Carotenoids are natural pigments that widely exist in nature. Due to their excellent antioxidant, anticancer and anti-inflammatory properties, carotenoids are commonly used in food, medicine, cosmetic and other fields. At present, natural carotenoids are mainly extracted from plants, algae and microorganisms. With the rapid development of metabolic engineering and molecular biology as well as the continuous in-depth study of carotenoids synthesis pathways, industrial microorganisms have showed promising applications in the synthesis of carotenoids. In this review, we introduced the properties of several carotenoids and their biosynthetic metabolism process. Then, the microorganisms synthesizing carotenoids through the natural and non-natural pathways and the extraction methods of carotenoids were summarized and compared. Meanwhile, the influence of substrates on the carotenoids production was also listed. The methods and strategies for achieving high carotenoid production are categorized to help with future research.

Methanol biotransformation toward high-level production of fatty acid derivatives by engineering the industrial yeast Pichia pastoris

Methanol-based biorefinery is a promising strategy to achieve carbon neutrality goals by linking CO₂ capture and solar energy storage. As a typical methylotroph, Pichia pastoris shows great potential in methanol biotransformation. However, challenges still remain in engineering methanol metabolism for chemical overproduction. Here, we present the global rewiring of the central metabolism for efficient production of free fatty acids (FFAs; 23.4 g/L) from methanol, with an enhanced supply of precursors and cofactors, as well as decreased accumulation of formaldehyde. Finally, metabolic transforming of the fatty acid cell factory enabled overproduction of fatty alcohols (2.0 g/L) from methanol. This study demonstrated that global metabolic rewiring released the great potential of P. pastoris for methanol biotransformation toward chemical overproduction.

Establishing Komagataella phaffii as a Cell Factory for Efficient Production of Sesquiterpenoid α-Santalene

Santalene, a major component of the sandalwood essential oil, is a typical representative of sesquiterpenes and has important applications in medicine, food, flavors, and other fields. Due to the limited supply of natural sandalwood resources, there is a growing interest in engineering microbial cell factories for the mass production of santalene. In the present study, Komagataella phaffii (also known as Pichia pastoris) was established as a cell factory for high-level production of α-santalene for the first time. The metabolic fluxes were rewired toward α-santalene biosynthesis through the optimization of promoters to drive the expression of the α-santalene synthase (SAS) gene, overexpression of the key mevalonate pathway genes (i.e., tHMG1, IDI1, and ERG20), and multi-copy integration of the SAS expression cassette. In combination with medium optimization and bioprocess engineering, the optimal strain (STE-9) was able to produce α-santalene with a titer as high as 829.8 ± 70.6 mg/L, 4.4 ± 0.3 g/L, and 21.5 ± 1.6 g/L in a shake flask, batch fermenter, and fed-batch fermenter, respectively. These represented the highest production of α-santalene ever reported, highlighting the advantages of K. phaffii cell factories for the production of terpenoids and other natural products.

Global Metabolic Rewiring of Yeast Enables Overproduction of Sesquiterpene (+)-Valencene

(+)-Valencene is a bioactive sesquiterpene that can be used for flavoring and fragrances, and microbial production provides an alternative sustainable access. However, the complexity of cellular metabolism makes it challenging for its high-level production. Here, we report the global rewiring cellular metabolism for de novo production of (+)-valencene in yeast Saccharomyces cerevisiae by engineering central metabolism, mevalonate pathway, and sesquiterpenoid synthase. In particular, we show that metabolic transformation can help accelerate the strain construction process and multiple copy expression of sesquiterpenoid synthase is essential for boosting the metabolic flux for product synthesis with enhanced supply of precursors. The engineered strain produced 1.2 g/L (+)-valencene under fed-batch fermentation in shake flasks, which was increased by 549-fold and demonstrated great potential of the yeast cell factory for (+)-valencene production.

De Novo Production of Plant 4'-Deoxyflavones Baicalein and Oroxylin A from Ethanol in Crabtree-Negative Yeast

Baicalein and oroxylin A are well-known medicinal 4'-deoxyflavones found mainly in the roots of traditional medicinal plant Scutellaria baicalensis Georgi. However, extraction from plants is time-consuming, environmentally unfriendly, and insufficient. Although microbial synthesis of flavonoids has been extensively reported, synthesis of downstream modified 4'-deoxyflavones has not, and their yields are extremely low. Here, we reassembled the S. baicalensis 4'-deoxyflavone biosynthetic pathway in a Crabtree-negative yeast, Pichia pastoris, with activity analysis and combinatorial expression of eight biosynthetic genes, allowing production of 4'-deoxyflavones like baicalein, oroxylin A, wogonin, norwogonin, 6-methoxywogonin, and the novel 6-methoxynorwogonin. De novo baicalein synthesis was then achieved by complete pathway assembly. Toxic intermediates highly impaired the cell production capacity; hence, we alleviated cinnamic acid growth inhibition by culturing the cells at near-neutral pH and using alcoholic carbon sources. To achieve pathway balance and improve baicalein and oroxylin A synthesis, we further divided the pathway into five modules. A series of ethanol-induced and constitutive transcriptional amplification devices were constructed to adapt to the modules. This fine-tuning pathway control considerably reduced byproduct and intermediate accumulation and achieved high-level de novo baicalein (401.9 mg/L with a total increase of 1182-fold, the highest titer reported) and oroxylin A (339.5 mg/L, for the first time) production from ethanol. This study provides new strategies for the microbial synthesis of 4'-deoxyflavones and other flavonoids.

Artificial fusions between P450 BM3 and an alcohol dehydrogenase for efficient (+)-nootkatone production

Multi‐enzyme cascades enable the production of valuable chemical compounds, and fusion of the enzymes that catalyze these reactions can improve the reaction outcome. In this work, P450 BM3 from Bacillus megaterium and an alcohol dehydrogenase from Sphingomonas yanoikuyae were fused to bifunctional constructs to enable cofactor regeneration and improve the in vitro two‐step oxidation of (+)‐valencene to (+)‐nootkatone. An up to 1.5‐fold increased activity of P450 BM3 was achieved with the fusion constructs compared to the individual enzyme. Conversion of (+)‐valencene coupled to cofactor regeneration and performed in the presence of the solubilizing agent cyclodextrin resulted in up to 1080 mg L⁻¹ (+)‐nootkatone produced by the fusion constructs as opposed to 620 mg L⁻¹ produced by a mixture of the separate enzymes. Thus, a two‐step (+)‐valencene oxidation was considerably improved through the simple method of enzyme fusion.

Probing the Synergistic Ratio of P450/CPR To Improve (+)-Nootkatone Production in Saccharomyces cerevisiae

(+)-Nootkatone is an expensive sesquiterpene substance found in grapefruit peels and the heartwood of yellow cedar. It can be used as a food additive, perfume, and insect repellent; therefore, its highly efficient production is greatly needed. However, the low catalytic efficiency of the membrane-anchored cytochrome P450/P450 reductase system (HPO/AtCPR) is the main challenge and limits the production of (+)-nootkatone. We developed an effective high-throughput screening system based on cell wall destruction to probe the optimal ratio of HPO/AtCPR, which achieved a twofold elevation in (+)-valencene oxidation in Saccharomyces cerevisiae. An engineered strain PK2RI-AtC/H_m6A was constructed to realize de novo (+)-nootkatone production by a series of metabolic engineering strategies. In biphasic fed-batch fermentation, maximum titers of 3.73 and 1.02 g/L for (+)-valencene and (+)-nootkatone, respectively, were achieved. The dramatically improved performance of the constructed S. cerevisiae provides an excellent approach for economical production of (+)-nootkatone from glucose.

Plasmid-Based Gene Knockout Strategy with Subsequent Marker Recycling in Pichia pastoris

Gene knockout is a key technology in the development of cell factories and basic research alike. The methylotrophic yeast Pichia pastoris is typically employed as a producer of proteins and of fine chemicals, due to its ability to accumulate high cell densities in conjunction with a set of strong inducible promoters. However, protocols for genome engineering in this host are still cumbersome and time-consuming. Moreover, extensive genome engineering raises the need for a multitude of selection markers, which are limited in P. pastoris. In this chapter, we describe a fast and efficient method for gene disruption in P. pastoris that utilizes marker recycling to enable repetitive genome engineering cycles. A set of ready-to-use knockout vectors simplifies cloning procedures and facilitates quick knockout generation.

Diversifying Isoprenoid Platforms via Atypical Carbon Substrates and Non-model Microorganisms

Isoprenoid compounds are biologically ubiquitous, and their characteristic modularity has afforded products ranging from pharmaceuticals to biofuels. Isoprenoid production has been largely successful in Escherichia coli and Saccharomyces cerevisiae with metabolic engineering of the mevalonate (MVA) and methylerythritol phosphate (MEP) pathways coupled with the expression of heterologous terpene synthases. Yet conventional microbial chassis pose several major obstacles to successful commercialization including the affordability of sugar substrates at scale, precursor flux limitations, and intermediate feedback-inhibition. Now, recent studies have challenged typical isoprenoid paradigms by expanding the boundaries of terpene biosynthesis and using non-model organisms including those capable of metabolizing atypical C1 substrates. Conversely, investigations of non-model organisms have historically informed optimization in conventional microbes by tuning heterologous gene expression. Here, we review advances in isoprenoid biosynthesis with specific focus on the synergy between model and non-model organisms that may elevate the commercial viability of isoprenoid platforms by addressing the dichotomy between high titer production and inexpensive substrates.

Aerobic Utilization of Methanol for Microbial Growth and Production

Methanol is a reduced one-carbon (C1) compound. It supports growth of aerobic methylotrophs that gain ATP from reduced redox equivalents by respiratory phosphorylation in their electron transport chains. Notably, linear oxidation of methanol to carbon dioxide may yield three reduced redox equivalents if methanol oxidation is NAD-dependent as, e.g., in Bacillus methanolicus. Methanol has a higher degree of reduction per carbon than glucose (6 vs. 4), and thus, lends itself as an ideal carbon source for microbial production of reduced target compounds. However, C-C bond formation in the RuMP or serine cycle, a prerequisite for production of larger molecules, requires ATP and/or reduced redox equivalents. Moreover, heat dissipation and a high demand for oxygen during catabolic oxidation of methanol may pose challenges for fermentation processes. In this chapter, we summarize metabolic pathways for aerobic methanol utilization, aerobic methylotrophs as industrial production hosts, strain engineering, and methanol bioreactor processes. In addition, we provide technological and market outlooks.

Establishment of a co-culture system using Escherichia coli and Pichia pastoris (Komagataella phaffii) for valuable alkaloid production

Background

Plants produce a variety of specialized metabolites, many of which are used in pharmaceutical industries as raw materials. However, certain metabolites may be produced at markedly low concentrations in plants. This problem has been overcome through metabolic engineering in recent years, and the production of valuable plant compounds using microorganisms such as Escherichia coli or yeast cells has been realized. However, the development of complicated pathways in a single cell remains challenging. Additionally, microbial cells may experience toxicity from the bioactive compounds produced or negative feedback effects exerted on their biosynthetic enzymes. Thus, co-culture systems, such as those of E. coli–E. coli and E. coli-Saccharomyces cerevisiae, have been developed, and increased production of certain compounds has been achieved. Recently, a co-culture system of Pichia pastoris (Komagataella phaffii) has gained considerable attention due to its potential utility in increased production of valuable compounds. However, its co-culture with other organisms such as E. coli, which produce important intermediates at high concentrations, has not been reported.

Results

Here, we present a novel co-culture platform for E. coli and P. pastoris. Upstream E. coli cells produced reticuline from a simple carbon source, and the downstream P. pastoris cells produced stylopine from reticuline. We investigated the effect of four media commonly used for growth and production of P. pastoris, and found that buffered methanol-complex medium (BMMY) was suitable for P. pastoris cells. Reticuline-producing E. coli cells also showed better growth and reticuline production in BMMY medium than that in LB medium. De novo production of the final product, stylopine from a simple carbon source, glycerol, was successful upon co-culture of both strains in BMMY medium. Further analysis of the initial inoculation ratio showed that a higher ratio of E. coli cells compared to P. pastoris cells led to higher production of stylopine.

Conclusions

This is the first report of co-culture system established with engineered E. coli and P. pastoris for the de novo production of valuable compounds. The co-culture system established herein would be useful for increased production of heterologous biosynthesis of complex specialized plant metabolites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01687-z.

Engineering Plant Sesquiterpene Synthesis into Yeasts: A Review

Sesquiterpenes are natural compounds composed of three isoprene units. They represent the largest class of terpene compounds found in plants, and many have remarkable biological activities. Furthermore, sesquiterpenes have broad applications in the flavor, pharmaceutical and biofuel industries due to their complex structures. With the development of metabolic engineering and synthetic biology, the production of different sesquiterpenes has been realized in various chassis microbes. The microbial production of sesquiterpenes provides a promising alternative to plant extraction and chemical synthesis, enabling us to meet the increasing market demand. In this review, we summarized the heterologous production of different plant sesquiterpenes using the eukaryotic yeasts Saccharomyces cerevisiae and Yarrowia lipolytica, followed by a discussion of common metabolic engineering strategies used in this field.

Advances on (+)-nootkatone microbial biosynthesis and its related enzymes

(+)-Nootkatone is an important functional sesquiterpene and is comprehensively used in pharmaceutical, cosmetic, agricultural and food flavor industries. However, (+)-nootkatone is accumulated trace amounts in plants, and the demand for industry is mainly met by chemical methods which is harmful to the environment. The oxygen-containing sesquiterpenes prepared using microbial methods can be considered as "natural." Microbial transformation has the advantages of mild reaction conditions, high efficiency, environmental protection, and strong stereoselectivity, and has become an important method for the production of natural spices. The microbial biosynthesis of (+)-nootkatone from the main precursor (+)-valencene is summarized in this paper. Whole-cell systems of fungi, bacteria, microalgae, and plant cells have been employed. It was described that the enzymes involved in the microbial biosynthesis of (+)-nootkatone, including cytochrome p450 enzymes, laccase, lipoxygenase, and so on. More recently, the related enzymes were expressed in microbial hosts to heterologous produce (+)-nootkatone, such as Escherichia coli, Pichia pastoris, Yarrowia lipolytica, and Saccharomyces cerevisiae. Finally, the development direction of research for realizing industrialization of microbial transformation was summarized and it provided many options for future improved bioprocesses.

Expression of proteins in Pichia pastoris

The methylotrophic yeast Pichia pastoris is currently one of the most versatile and popular hosts for the production of heterologous proteins, including industrial enzymes. The popularity of P. pastoris stems from its ability to grow to high cell densities, producing high titers of secreted heterologous protein with very low amounts of endogenous proteins. Its ability to express correctly folded proteins with post-translational modifications makes it an excellent candidate for the production of biopharmaceuticals. In addition, production in P. pastoris typically uses the strong, methanol-inducible and tightly regulated promoter (P_AOX1), which can result in heterologous protein that constitutes up to 30% of total cell protein upon growth in methanol. In this chapter, we present methodology for the production of secreted recombinant proteins in P. pastoris, and we discuss alternatives to enhance protein production with the desired yield and quality.

Screening and evaluation of the strong endogenous promoters in Pichia pastoris

Background

Due to its ability to perform fast and high-density fermentation, Pichia pastoris is not only used as an excellent host for heterologous protein expression but also exhibits good potential for efficient biosynthesis of small-molecule compounds. However, basic research on P. pastoris lags far behind Saccharomyces cerevisiae, resulting in a lack of available biological elements. Especially, fewer strong endogenous promoter elements available for foreign protein expression or construction of biosynthetic pathways were carefully evaluated in P. pastoris. Thus, it will be necessary to identify more available endogenous promoters from P. pastoris.

Results

Based on RNA-seq and LacZ reporter system, eight strong endogenous promoters contributing to higher transcriptional expression levels and β-galactosidase activities in three frequently-used media were screened out. Among them, the transcriptional expression level contributed by P₀₀₁₉, P₀₁₀₇, P₀₂₃₀, P₀₃₉₂, or P₀₇₈₅ was basically unchanged during the logarithmic phase and stationary phase of growth. And the transcriptional level contributed by P₀₂₀₈ or P₀₆₂₇ exhibited a growth-dependent characteristic (a lower expression level during the logarithmic phase and a higher expression level during the stationary phase). After 60 h growth, the β-galactosidase activity contributed by P₀₂₀₈, P₀₆₂₇, P₀₀₁₉, P₀₄₀₇, P₀₃₉₂, P₀₂₃₀, P₀₇₈₅, or P₀₁₀₇ was relatively lower than P_GAP but higher than P_ACT1. To evaluate the availability of these promoters, several of them were randomly applied to a heterogenous β-carotene biosynthetic pathway in P. pastoris, and the highest yield of β-carotene from these mutants was up to 1.07 mg/g. In addition, simultaneously using the same promoter multiple times could result in a notable competitive effect, which might significantly lower the transcriptional expression level of the target gene.

Conclusions

The novel strong endogenous promoter identified in this study adds to the number of promoter elements available in P. pastoris. And the competitive effect observed here suggests that a careful pre-evaluation is needed when simultaneously and multiply using the same promoter in one yeast strain. This work also provides an effective strategy to identify more novel biological elements for engineering applications in P. pastoris.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01648-6.

Dual Regulation of Cytoplasm and Peroxisomes for Improved Α-Farnesene Production in Recombinant Pichia pastoris

Microbial production of α-farnesene from renewable raw materials is a feasible alternative to traditional petroleum craft. Recently, the research on improving α-farnesene production in Pichia pastoris mainly focused on cytoplasmic engineering, while comprehensive engineering of multiple subcellular compartments is rarely reported. Here, we first sought to confirm that the isopentenol utilization pathway (IUP) could act as a two-step shortcut for IPP synthesis in P. pastoris peroxisomes. In addition, we proposed dual regulation of cytoplasm and peroxisomes to boost α-farnesene synthesis in P. pastoris X33, thus the resultant strain produced 2.18 ± 0.04 g/L, which was 1.3 times and 2.1 times than that of the strain only with peroxisomal or cytoplasmic engineering, respectively. The α-farnesene production achieved 2.56 ± 0.04 g/L in shake flasks after carbon source cofeeding, which was the highest reported production in worldwide literatures to the best of my knowledge. Therefore, we propose these strategies as efficient approaches to enhancing α-farnesene production in P. pastoris, which might bring new ideas for the biosynthesis of high-value compounds.

Development of synthetic biology tools to engineer Pichia pastoris as a chassis for the production of natural products

The methylotrophic yeast Pichia pastoris (a.k.a. Komagataella phaffii) is one of the most commonly used hosts for industrial production of recombinant proteins. As a non-conventional yeast, P. pastoris has unique biological characteristics and its expression system has been well developed. With the advances in synthetic biology, more efforts have been devoted to developing P. pastoris into a chassis for the production of various high-value compounds, such as natural products. This review begins with the introduction of synthetic biology tools for the engineering of P. pastoris, including vectors, promoters, and terminators for heterologous gene expression as well as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated System (CRISPR/Cas) for genome editing. This review is then followed by examples of the production of value-added natural products in metabolically engineered P. pastoris strains. Finally, challenges and outlooks in developing P. pastoris as a synthetic biology chassis are prospected.

Engineering Nootkatone Biosynthesis in Artemisia annua

Nootkatone is a valuable sesquiterpene widely used in the food, fragrance, and flavor industries. Its price is very high due to its limited production in grapefruit peels or Alaska cypress heartwoods. Chemical synthesis of nootkatone uses heavy metals, highly flammable compounds, and strong oxidants, which cause severe damage to the environment. In this study, nootkatone is synthesized in Artemisia annua, using synthetic biology methods. Engineered Artemisia annua coexpressing valencene synthase (VS) and valencene oxidase (VO) in the cytosol produced nootkatone ranging from 0.89 to 8.52 μg/g fresh weight (FW). Furthermore, transgenic Artemisia annua coexpressing farnesyl diphosphate synthase (FPS), VS, and VO in plastids produced nootkatone ranging from 12.11 to 47.80 μg/g FW. These results indicated that engineering nootkatone biosynthesis in plastids was superior to that in the cytosol. Meanwhile, artemisinin production was unaltered in nootkatone-producing Artemisia annua. Our study developed a green approach for producing nootkatone in Artemisia annua with great market potential.

Recent advances in systems and synthetic biology approaches for developing novel cell-factories in non-conventional yeasts

Microbial bioproduction of chemicals, proteins, and primary metabolites from cheap carbon sources is currently an advancing area in industrial research. The model yeast, Saccharomyces cerevisiae, is a well-established biorefinery host that has been used extensively for commercial manufacturing of bioethanol from myriad carbon sources. However, its Crabtree-positive nature often limits the use of this organism for the biosynthesis of commercial molecules that do not belong in the fermentative pathway. To avoid extensive strain engineering of S. cerevisiae for the production of metabolites other than ethanol, non-conventional yeasts can be selected as hosts based on their natural capacity to produce desired commodity chemicals. Non-conventional yeasts like Kluyveromyces marxianus, K. lactis, Yarrowia lipolytica, Pichia pastoris, Scheffersomyces stipitis, Hansenula polymorpha, and Rhodotorula toruloides have been considered as potential industrial eukaryotic hosts owing to their desirable phenotypes such as thermotolerance, assimilation of a wide range of carbon sources, as well as ability to secrete high titers of protein and lipid. However, the advanced metabolic engineering efforts in these organisms are still lacking due to the limited availability of systems and synthetic biology methods like in silico models, well-characterised genetic parts, and optimized genome engineering tools. This review provides an insight into the recent advances and challenges of systems and synthetic biology as well as metabolic engineering endeavours towards the commercial usage of non-conventional yeasts. Particularly, the approaches in emerging non-conventional yeasts for the production of enzymes, therapeutic proteins, lipids, and metabolites for commercial applications are extensively discussed here. Various attempts to address current limitations in designing novel cell factories have been highlighted that include the advances in the fields of genome-scale metabolic model reconstruction, flux balance analysis, 'omics'-data integration into models, genome-editing toolkit development, and rewiring of cellular metabolisms for desired chemical production. Additionally, the understanding of metabolic networks using ¹³C-labelling experiments as well as the utilization of metabolomics in deciphering intracellular fluxes and reactions have also been discussed here. Application of cutting-edge nuclease-based genome editing platforms like CRISPR/Cas9, and its optimization towards efficient strain engineering in non-conventional yeasts have also been described. Additionally, the impact of the advances in promising non-conventional yeasts for efficient commercial molecule synthesis has been meticulously reviewed. In the future, a cohesive approach involving systems and synthetic biology will help in widening the horizon of the use of unexplored non-conventional yeast species towards industrial biotechnology.

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.