Heterologous biosynthesis of triterpenoid dammarenediol-II in engineered Escherichia coli

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.

Sustainable production of natural products using synthetic biology: Ginsenosides

Synthetic biology approaches offer potential for large-scale and sustainable production of natural products with bioactive potency, including ginsenosides, providing a means to produce novel compounds with enhanced therapeutic properties. Ginseng, known for its non-toxic and potent qualities in traditional medicine, has been used for various medical needs. Ginseng has shown promise for its antioxidant and neuroprotective properties, and it has been used as a potential agent to boost immunity against various infections when used together with other drugs and vaccines. Given the increasing demand for ginsenosides and the challenges associated with traditional extraction methods, synthetic biology holds promise in the development of therapeutics. In this review, we discuss recent developments in microorganism producer engineering and ginsenoside production in microorganisms using synthetic biology approaches.

Engineering amino acid residues of pentacyclic triterpene synthases for improving the activity

Pentacyclic triterpenoids exhibit a wide range of biological activities which have wide applications in the food, cosmetics, and pharmaceutical industries. High-performance chassis strains have been developed for the production of various pentacyclic triterpenoids, e.g., lupane-type and oleanane-type triterpenoids. The production of common pentacyclic triterpenes and their derivatives is limited by the poor activity of typical pentacyclic triterpene synthases (PTSs). However, a general strategy applicable to typical PTSs is still lacking. As typical pentacyclic triterpenes are derived from the baccharenyl cation, engineering the non-active-site residues in the MXXXXR motif might be beneficial for the catalytic efficiencies of typical PTSs by the stabilization of the baccharenyl cation. Here, we develop a general strategy for improving the activity of typical PTSs. As a proof of concept, the activity of three PTSs such as lupeol synthase, β-amyrin synthase, and α-amyrin synthases was significantly increased up to 7.3-fold by site-directed saturation mutagenesis. This strategy could be applied to improve the activity of various typical PTSs. KEY POINTS: • The strategy could be applied to typical PTSs for improving the activity. • The catalytic activity of typical PTSs was significantly increased.

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.

Natural products of pentacyclic triterpenoids: from discovery to heterologous biosynthesis

Covering: up to 2022Pentacyclic triterpenoids are important natural bioactive substances that are widely present in plants and fungi. They have significant medicinal efficacy, play an important role in reducing blood glucose and protecting the liver, and have anti-inflammatory, anti-oxidation, anti-fatigue, anti-viral, and anti-cancer activities. Pentacyclic triterpenoids are derived from the isoprenoid biosynthetic pathway, which generates common precursors of triterpenes and steroids, followed by cyclization with oxidosqualene cyclases (OSCs) and decoration via cytochrome P450 monooxygenases (CYP450s) and glycosyltransferases (GTs). Many biosynthetic pathways of triterpenoid saponins have been elucidated by studying their metabolic regulation network through the use of multiomics and identifying their functional genes. Unfortunately, natural resources of pentacyclic triterpenoids are limited due to their low content in plant tissues and the long growth cycle of plants. Based on the understanding of their biosynthetic pathway and transcriptional regulation, plant bioreactors and microbial cell factories are emerging as alternative means for the synthesis of desired triterpenoid saponins. The rapid development of synthetic biology, metabolic engineering, and fermentation technology has broadened channels for the accumulation of pentacyclic triterpenoid saponins. In this review, we summarize the classification, distribution, structural characteristics, and bioactivity of pentacyclic triterpenoids. We further discuss the biosynthetic pathways of pentacyclic triterpenoids and involved transcriptional regulation. Moreover, the recent progress and characteristics of heterologous biosynthesis in plants and microbial cell factories are discussed comparatively. Finally, we propose potential strategies to improve the accumulation of triterpenoid saponins, thereby providing a guide for their future biomanufacturing.

Multi-Level Optimization and Strategies in Microbial Biotransformation of Nature Products

Continuously growing demand for natural products with pharmacological activities has promoted the development of microbial transformation techniques, thereby facilitating the efficient production of natural products and the mining of new active compounds. Furthermore, due to the shortcomings and defects of microbial transformation, it is an important scientific issue of social and economic value to improve and optimize microbial transformation technology in increasing the yield and activity of transformed products. In this review, the aspects regarding the optimization of fermentation and the cross-disciplinary strategy, leading to the microbial transformation of increased levels of the high-efficiency process from natural products of a plant or microbial origin, were discussed. Additionally, due to the increasing craving for targeted and efficient methods for detecting transformed metabolites, analytical methods based on multiomics were also discussed. Such strategies can be well exploited and applied to the production of more efficient and more natural products from microbial resources.

Heterologous expression of C30 carotenoid biosynthetic gene crtNM from Lactiplantibacillus plantarum

BACKGROUND

Probiotic lactic acid bacterium Lactiplantibacillus plantarum is widely used in the dairy and other fermented food industries. L. plantarum AR113 harbors a C₃₀ carotenoid operon crtNM based on genomic analysis, but the yield of C₃₀ carotenoid is only 8.1 μg g^-1 DCW.

RESULTS

To improve the productivity of C₃₀ carotenoid, crtNM from L. plantarum AR113 was cloned and reconstructed in Escherichia coli BL21(DE3). The proteins crtN and crtM were successfully expressed based on a sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, and the carotenoid was detected using high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). In comparison with the constitutive promoter P₄₄ , the use of the inducible T7 promoter significantly increased the carotenoid content in E. coli. The fermentation conditions were also optimized with induction by 0.5 mmol/L IPTG at 20 °C for 7 h. The yield of C₃₀ carotenoid reached 154.5 μg g^-1 DCW, which was 18-fold higher than that of L. plantarum AR113. The 2,2-diphenyl-1-picryl (DPPH) and 2,2'-azino-bis (3-ethylbenzothiazoline-6sulfonic acid (ABTS) radical scavenging capacity of C₃₀ carotenoids synthesized by heterologous expression in E. coli was also higher than that of the antioxidant food additive butylated hydroxytoluene.

CONCLUSIONS

Our findings suggest that E. coli has strong potential as a basic chassis for the production of C₃₀ carotenoids from Lactiplantibacillus with high antioxidant activity. © 2022 Society of Chemical Industry.

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.

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.

A novel carotenoid biosynthetic route via oxidosqualene

Over 800 known carotenoids are synthesized from phytoene or 4,4'-diapophytoene (dehydrosqualene) characterized by three conjugated double bonds. In this paper, we report that carotenoid desaturase CrtN from Staphylococcus aureus and Methylomonas can accept oxidosqualene, which is the precursor for plant- or animal-type triterpenoids, yielding the yellow carotenoid pigments with 8, 9, or 10 conjugated double bonds. The resulting pathway is the second nonnatural route for carotenoid pigments and the first pathway for carotenoid pigments not biosynthesized via (diapo)phytoene.

Alternative metabolic pathways and strategies to high-titre terpenoid production in Escherichia coli

Covering: up to 2021Terpenoids are a diverse group of chemicals used in a wide range of industries. Microbial terpenoid production has the potential to displace traditional manufacturing of these compounds with renewable processes, but further titre improvements are needed to reach cost competitiveness. This review discusses strategies to increase terpenoid titres in Escherichia coli with a focus on alternative metabolic pathways. Alternative pathways can lead to improved titres by providing higher orthogonality to native metabolism that redirects carbon flux, by avoiding toxic intermediates, by bypassing highly-regulated or bottleneck steps, or by being shorter and thus more efficient and easier to manipulate. The canonical 2-C-methyl-D-erythritol 4-phosphate (MEP) and mevalonate (MVA) pathways are engineered to increase titres, sometimes using homologs from different species to address bottlenecks. Further, alternative terpenoid pathways, including additional entry points into the MEP and MVA pathways, archaeal MVA pathways, and new artificial pathways provide new tools to increase titres. Prenyl diphosphate synthases elongate terpenoid chains, and alternative homologs create orthogonal pathways and increase product diversity. Alternative sources of terpenoid synthases and modifying enzymes can also be better suited for E. coli expression. Mining the growing number of bacterial genomes for new bacterial terpenoid synthases and modifying enzymes identifies enzymes that outperform eukaryotic ones and expand microbial terpenoid production diversity. Terpenoid removal from cells is also crucial in production, and so terpenoid recovery and approaches to handle end-product toxicity increase titres. Combined, these strategies are contributing to current efforts to increase microbial terpenoid production towards commercial feasibility.

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.

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.

Recent Advances in the Metabolic Engineering of Yeasts for Ginsenoside Biosynthesis

Ginsenosides are a group of glycosylated triterpenes isolated from Panax species. Ginsenosides are promising candidates for the prevention and treatment of cancer as well as food additives. However, owing to a lack of efficient approaches for ginsenoside production from plants and chemical synthesis, ginsenosides may not yet have reached their full potential as medicinal resources. In recent years, an alternative approach for ginsenoside production has been developed using the model yeast Saccharomyces cerevisiae and non-conventional yeasts such as Yarrowia lipolytica and Pichia pastoris. In this review, various metabolic engineering strategies, including heterologous gene expression, balancing, and increasing metabolic flux, and enzyme engineering, have been described as recent advanced engineering techniques for improving ginsenoside production. Furthermore, the usefulness of a systems approach and fermentation strategy has been presented. Finally, the present challenges and future research direction for industrial cell factories have been discussed.

Identifying and engineering the ideal microbial terpenoid production host

More than 70,000 different terpenoid structures are known so far; many of them offer highly interesting applications as pharmaceuticals, flavors and fragrances, or biofuels. Extraction of these compounds from their natural sources or chemical synthesis is-in many cases-technically challenging with low or moderate yields while wasting valuable resources. Microbial production of terpenoids offers a sustainable and environment-friendly alternative starting from simple carbon sources and, frequently, safeguards high product specificity. Here, we provide an overview on employing recombinant bacteria and yeasts for heterologous de novo production of terpenoids. Currently, Escherichia coli and Saccharomyces cerevisiae are the two best-established production hosts for terpenoids. An increasing number of studies have been successful in engineering alternative microorganisms for terpenoid biosynthesis, which we intend to highlight in this review. Moreover, we discuss the specific engineering challenges as well as recent advances for microbial production of different classes of terpenoids. Rationalizing the current stages of development for different terpenoid production hosts as well as future prospects shall provide a valuable decision basis for the selection and engineering of the cell factory(ies) for industrial production of terpenoid target molecules.

Production of Triterpene Ginsenoside Compound K in the Non-conventional Yeast Yarrowia lipolytica

Compound K (CK) is a rare, tetracyclic, triterpenoid compound with important medical properties, such as antitumor and anti-inflammatory activities. Herein, an efficient biosynthetic pathway of CK was constructed in metabolically engineered Yarrowia lipolytica for the first time, and the engineered strain, YL-CK0, produced 5.1 mg/L CK. The production of CK was further increased by 5.96-fold to 30.4 mg/L with overexpression of key genes in the MVA pathway and fusion of cytochrome P450 monooxygenase (PPDS) and NADPH-P450 reductase. Finally, 161.8 mg/L CK production was achieved by fed-batch fermentation in a 5 L fermenter using the strain YL-MVA-CK. To the best of our knowledge, this is the first report on heterologous CK synthesis with the highest titer in Y. lipolytica. This study demonstrates the feasibility of producing high-value triterpenoid compounds using Y. lipolytica as a platform.

Biosynthesis of cycloartenol by expression of plant and bacterial oxidosqualene cyclases in engineered Rhodobacter capsulatus

Cyclic triterpenes are a large group of secondary metabolites produced by plants, fungi and bacteria. They have diverse biological functions, and offer potential health benefits for humans. Although various terpenes from the mono-, sesqui- and diterpene classes are easy to produce in engineered bacteria, heterologous synthesis of cyclic triterpenes is more challenging. We have recently shown that the triterpene cycloartenol can be produced in Rhodobacter capsulatus SB1003 but initial titers were low with 0.34mgL^-1. To assess, if this phototrophic α-proteobacterium can be engineered for enhanced triterpene production, we followed two alternative strategies by comparing the performance of the R. capsulatus SB1003 wildtype strain with two recombinant strains carrying either a mevalonate pathway implemented from Paracoccus zeaxanthinifaciens or a deletion in the intrinsic carotenoid biosynthesis gene crtE. These strains are thus engineered for an enhanced isoprenoid biosynthesis or a suppressed precursor conversion by the competing carotenoid pathway. Moreover, three different cycloartenol synthase (CAS) genes from Arabidopsis thaliana or the myxobacterial strains Stigmatella aurantiaca Sga15 and DW4/3-1 were tested for heterologous cycloartenol synthesis. We found that the heterologous expression of mevalonate pathway enzymes had little impact on cycloartenol levels irrespective of the chosen CAS. In contrast, the use of the newly constructed carotenoid-deficient crtE deletion strain showed threefold increased cycloartenol product titers. We conclude that R. capsulatus is a promising alternative host for the functional expression of triterpene biosynthetic enzymes from plants and microbes. Apparently, product titers can also be improved by suppression of competing precursor consumption.

The photosynthetic bacteria Rhodobacter capsulatus and Synechocystis sp. PCC 6803 as new hosts for cyclic plant triterpene biosynthesis

Cyclic triterpenes constitute one of the most diverse groups of plant natural products. Besides the intriguing biochemistry of their biosynthetic pathways, plant triterpenes exhibit versatile bioactivities, including antimicrobial effects against plant and human pathogens. While prokaryotes have been extensively used for the heterologous production of other classes of terpenes, the synthesis of cyclic triterpenes, which inherently includes the two-step catalytic formation of the universal linear precursor 2,3-oxidosqualene, is still a major challenge. We thus explored the suitability of the metabolically versatile photosynthetic α-proteobacterium Rhodobacter capsulatus SB1003 and cyanobacterium Synechocystis sp. PCC 6803 as alternative hosts for biosynthesis of cyclic plant triterpenes. Therefore, 2,3-oxidosqualene production was implemented and subsequently combined with different cyclization reactions catalyzed by the representative oxidosqualene cyclases CAS1 (cycloartenol synthase), LUP1 (lupeol synthase), THAS1 (thalianol synthase) and MRN1 (marneral synthase) derived from model plant Arabidopsis thaliana. While successful accumulation of 2,3-oxidosqualene could be detected by LC-MS analysis in both hosts, cyclase expression resulted in differential production profiles. CAS1 catalyzed conversion to only cycloartenol, but expression of LUP1 yielded lupeol and a triterpenoid matching an oxidation product of lupeol, in both hosts. In contrast, THAS1 expression did not lead to cyclic product formation in either host, whereas MRN1-dependent production of marnerol and hydroxymarnerol was observed in Synechocystis but not in R. capsulatus. Our findings thus indicate that 2,3-oxidosqualene cyclization in heterologous phototrophic bacteria is basically feasible but efficient conversion depends on both the respective cyclase enzyme and individual host properties. Therefore, photosynthetic α-proteo- and cyanobacteria are promising alternative candidates for providing new bacterial access to the broad class of triterpenes for biotechnological applications.

A literature update elucidating production of Panax ginsenosides with a special focus on strategies enriching the anti-neoplastic minor ginsenosides in ginseng preparations

Ginseng, an oriental gift to the world of healthcare and preventive medicine, is among the top ten medicinal herbs globally. The constitutive triterpene saponins, ginsenosides, or panaxosides are attributed to ginseng's miraculous efficacy towards anti-aging, rejuvenating, and immune-potentiating benefits. The major ginsenosides such as Rb1, Rb2, Rc, Rd., Re, and Rg1, formed after extensive glycosylations of the aglycone "dammaranediol," dominate the chemical profile of this genus in vivo and in vitro. Elicitations have successfully led to appreciable enhancements in the production of these major ginsenosides. However, current research on ginseng biotechnology has been focusing on the enrichment or production of the minor ginsenosides (the less glycosylated precursors of the major ginsenosides) in ginseng preparations, which are either absent or are produced in very low amounts in nature or via cell cultures. The minor ginsenosides under current scientific scrutiny include diol ginsenosides such as Rg3, Rh2, compound K, and triol ginsenosides Rg2 and Rh1, which are being touted as the next "anti-neoplastic pharmacophores," with better bioavailability and potency as compared to the major ginsenosides. This review aims at describing the strategies for ginsenoside production with special attention towards production of the minor ginsenosides from the major ginsenosides via microbial biotransformation, elicitations, and from heterologous expression systems.

β-Amyrin biosynthesis: catalytic mechanism and substrate recognition

In the past five years, there have been remarkable advances in the study of β-amyrin synthase. This review outlines the catalytic mechanism and substrate recognition in β-amyrin biosynthesis, which have been attained by the site-directed mutagenesis and substrate analog experiments.