Improvement of betulinic acid biosynthesis in yeast employing multiple strategies

De novo biosynthesis of betulinic acid in engineered Saccharomyces cerevisiae

Betulinic acid (BA) is a lupinane-type pentacyclic triterpenoid natural product derived from lupeol that has favorable anti-inflammatory and anti-tumor activities. Currently, BA is mainly produced via botanical extraction, which significantly limits its widespread use. In this study, we investigated the de novo synthesis of BA in Saccharomyces cerevisiae, and to facilitate the synthesis and storage of hydrophobic BA, we adopted a dual-engineering strategy involving peroxisomes and lipid droplets to construct the BA biosynthetic pathway. By expressing Betula platyphylla-derived lupeol C-28 oxidase (BPLO) and Arabidopsis-derived ATR1, we succeeded in developing a BA-producing strain and following multiple expression optimizations of the linker between BPLO and ATR1, the BA titer reached 77.53 mg/L in shake flasks and subsequently reached 205.74 mg/L via fed-batch fermentation in a 5-L bioreactor. In this study, we developed a feasible approach for the de novo synthesis of BA and its direct precursor lupeol in engineered S. cerevisiae.

Betulinic Acid for Glioblastoma Treatment: Reality, Challenges and Perspectives

Betulinic acid is a naturally occurring compound that can be obtained through methanolic or ethanolic extraction from plant sources, as well as through chemical synthesis or microbial biotransformation. Betulinic acid has been investigated for its potential therapeutic properties, and exhibits anti-inflammatory, antiviral, antimalarial, and antioxidant activities. Notably, its ability to cross the blood-brain barrier addresses a significant challenge in treating neurological pathologies. This review aims to compile information about the impact of betulinic acid as an antitumor agent, particularly in the context of glioblastoma. Importantly, betulinic acid demonstrates selective antitumor activity against glioblastoma cells by inhibiting proliferation and inducing apoptosis, consistent with observations in other cancer types. Compelling evidence published highlights the acid's therapeutic action in suppressing the Akt/NFκB-p65 signaling cascade and enhancing the cytotoxic effects of the chemotherapeutic agent temozolomide. Interesting findings with betulinic acid also suggest a focus on researching the reduction of glioblastoma's invasiveness and aggressiveness profile. This involves modulation of extracellular matrix components, remodeling of the cytoskeleton, and secretion of proteolytic proteins. Drawing from a comprehensive review, we conclude that betulinic acid formulations as nanoparticles and/or ionic liquids are promising drug delivery approaches with the potential for translation into clinical applications for the treatment and management of glioblastoma.

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.

Cytochrome P450 monooxygenase-mediated tailoring of triterpenoids and steroids in plants

The cytochrome P450 monooxygenase (CYP) superfamily comprises hemethiolate enzymes that perform remarkable regio- and stereospecific oxidative chemistry. As such, CYPs are key agents for the structural and functional tailoring of triterpenoids, one of the largest classes of plant natural products with widespread applications in pharmaceuticals, food, cosmetics, and agricultural industries. In this review, we provide a full overview of 149 functionally characterised CYPs involved in the biosynthesis of triterpenoids and steroids in primary as well as in specialised metabolism. We describe the phylogenetic distribution of triterpenoid- and steroid-modifying CYPs across the plant CYPome, present a structure-based summary of their reactions, and highlight recent examples of particular interest to the field. Our review therefore provides a comprehensive up-to-date picture of CYPs involved in the biosynthesis of triterpenoids and steroids in plants as a starting point for future research.

The Aspartic Protease Yps3p and Cell Wall Glucanase Scw10p Are Novel Determinants That Enhance the Secretion of the Antitumor Triterpenoid GA-HLDOA in Saccharomyces cerevisiae

Efficient bioproduction of triterpenoids is gaining increasing interest because of their significant biological applications; however, the secretion and bioproduction of triterpenoids are hindered by untapped genetic determinants. In our previous study, we observed that different engineered Saccharomyces cerevisiae strains exhibit different abilities for secreting the antitumor triterpenoid ganoderic acid 3-hydroxy-lanosta-8,24-dien-26-oic acid (GA-HLDOA). In the present study, we performed comparative proteomics analyses of the engineered strains and identified two genes, encoding an aspartic protease, YPS3, and a cell wall glucanase, SCW10, as the most effective determinants that enhance the secretion of GA-HLDOA. Compared with this control strain, strain BJ5464-r demonstrated an overexpression of YPS3 and SCW10 resulting in 3.9-fold and 4.7-fold higher secretion of GA-HLDOA, respectively, and these increases were accompanied by an increase in cell permeability. Moreover, compared with the YPS3-overexpressing strain, the SCW10-overexpressing strain had a thinner outer mannan layer. Our findings offer valuable insights into designing microbial cell factories for the efficient secretion of triterpenoids.

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.

Molecular Cloning and Identification of NADPH Cytochrome P450 Reductase from Panax ginseng

Ginseng (Panax ginseng C.A. Mey.) is a precious Chinese traditional medicine, for which ginsenosides are the most important medicinal ingredients. Cytochrome P450 enzymes (CYP450) and their primary redox molecular companion NADPH cytochrome P450 reductase (CPR) play a key role in ginsenoside biosynthesis pathway. However, systematic studies of CPR genes in ginseng have not been reported. Numerous studies on ginsenoside synthesis biology still use Arabidopsis CPR (AtCPR1) as a reductase. In this study, we isolated two CPR genes (PgCPR1, PgCPR2) from ginseng adventitious roots. Phylogenetic tree analysis showed that both PgCPR1 and PgCPR2 are grouped in classⅡ of dicotyledonous CPR. Enzyme experiments showed that recombinant proteins PgCPR1, PgCPR2 and AtCPR1 can reduce cytochrome c and ferricyanide with NADPH as the electron donor, and PgCPR1 had the highest enzymatic activities. Quantitative real-time PCR analysis showed that PgCPR1 and PgCPR2 transcripts were detected in all examined tissues of Panax ginseng and both showed higher expression in stem and main root. Expression levels of the PgCPR1 and PgCPR2s were both induced after a methyl jasmonate (MeJA) treatment and its pattern matched with ginsenoside accumulation. The present investigation suggested PgCPR1 and PgCPR2 are associated with the biosynthesis of ginsenoside. This report will assist in future CPR family studies and ultimately improving ginsenoside production through transgenic engineering and synthetic biology.

De Novo Biosynthesis of the Anticancer Compound Euphol in Saccharomyces cerevisiae

Euphol is a euphane-type tetracyclic triterpene which is primarily found in the Euphorbia genus. Euphol has been renowned because of its great potential as a promising anticancer drug. Surprisingly, despite its diverse antitumor effects, the respective gene for euphol biosynthesis had not been identified until this study. In our experiments with Euphorbia tirucalli, euphol was detected predominantly in latex, the element that is often used for cancer treatments in Brazil. Two latex-specifically expressed oxidosqualene cyclases (OSCs) from E. tirucalli, designated as EtOSC5 and EtOSC6, were functionally characterized by expression in a lanosterol synthase knockout yeast strain GIL77. EtOSC5 produces euphol and its 20S-isomer tirucallol as two of the major products, while EtOSC6 produces taraxasterol and β-amyrin as the major products. These four compounds were also detected as the major triterpenes in the E. tirucalli latex, suggesting that EtOSC5 and EtOSC6 are the primary catalysts for the formation of E. tirucalli latex triterpene alcohols. Based on a model structure of EtOSC5 followed with site-mutagenesis experiments, the mechanism for the EtOSC5 activity was proposed. By applying state-of-the-art engineering techniques, the expression of EtOSC5 together with three other known precursor genes were chromosomally integrated into Saccharomyces cerevisiae. The resulting engineered yeast strain YS5E-1 produced 1.84 ± 0.17 mg/L of euphol in shake flasks.

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.

Chemical Strategies towards the Synthesis of Betulinic Acid and Its More Potent Antiprotozoal Analogues

Betulinic acid (BA, 3β-hydroxy-lup-20(29)-en-28-oic acid) is a pentacyclic triterpene acid present predominantly in Betula ssp. (Betulaceae) and is also widely spread in many species belonging to different plant families. BA presents a wide spectrum of remarkable pharmacological properties, such as cytotoxic, anti-HIV, anti-inflammatory, antidiabetic and antimicrobial activities, including antiprotozoal effects. The present review first describes the sources of BA and discusses the chemical strategies to produce this molecule starting from betulin, its natural precursor. Next, the antiprotozoal properties of BA are briefly discussed and the chemical strategies for the synthesis of analogues displaying antiplasmodial, antileishmanial and antitrypanosomal activities are systematically presented. The antiplasmodial activity described for BA was moderate, nevertheless, some C-3 position acylated analogues showed an improvement of this activity and the hybrid models—with artesunic acid—showed the most interesting properties. Some analogues also presented more intense antileishmanial activities compared with BA, and, in addition to these, heterocycles fused to C-2/C-3 positions and amide derivatives were the most promising analogues. Regarding the antitrypanosomal activity, some interesting antitrypanosomal derivatives were prepared by amide formation at the C-28 carboxylic group of the lupane skeleton. Considering that BA can be produced either by isolation of different plant extracts or by chemical transformation of betulin, easily obtained from Betula ssp., it could be said that BA is a molecule of great interest as a starting material for the synthesis of novel antiprotozoal agents.

Cyclodextrins facilitate the efficient secretion of an anti-tumor triterpenoid ganoderic acid HLDOA by Saccharomyces cerevisiae

As a large group of natural product with significant biological activities, triterpenoid secretion is of particular importance towards its bioproduction. Due to the lack of specific transporters, most triterpenoids are naturally accumulated inside the cells. In this study, by taking an antitumor triterpenoid ganoderic acid 3-hydroxy-lanosta-8,24-dien-26 oic acid (GA-HLDOA) as example, we discovered that addition of 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) or 2,6-dimethyl-β-cyclodextrin (DM-β-CD) enable the fast and sufficient secretion of GA-HLDOA by the recombinant Saccharomyces cerevisiae strain as constructed in our previous study. In addition, these cyclodextrins (CDs) could not enter into cells, while no significant change of the cell membrane fluidity was observed after CDs treatment. This discovery provides a potential generally applicable method for triterpenoid secretion.

Off-Colony Screening of Biosynthetic Libraries by Rapid Laser-Enabled Mass Spectrometry

By leveraging advances in DNA synthesis and molecular cloning techniques, synthetic biology increasingly makes use of large construct libraries to explore large design spaces. For biosynthetic pathway engineering, the ability to screen these libraries for a variety of metabolites of interest is essential. If the metabolite of interest or the metabolic phenotype is not easily measurable, screening soon becomes a major bottleneck involving time-consuming culturing, sample preparation, and extraction. To address this, we demonstrate the use of automated laser-assisted rapid evaporative ionization mass spectrometry (LA-REIMS)-a form of ambient laser desorption ionization mass spectrometry-to perform rapid mass spectrometry analysis direct from agar plate yeast colonies without sample preparation or extraction. We use LA-REIMS to assess production levels of violacein and betulinic acid directly from yeast colonies at a rate of 6 colonies per minute. We then demonstrate the throughput enabled by LA-REIMS by screening over 450 yeast colonies within <4 h, while simultaneously generating recoverable glycerol stocks of each colony in real time. This showcases LA-REIMS as a prescreening tool to complement downstream quantification methods such as liquid chromatography-mass spectroscopy (LCMS). By prescreening several hundred colonies with LA-REIMS, we successfully isolate and verify a strain with a 2.5-fold improvement in betulinic acid production. Finally, we show that LA-REIMS can detect 20 out of a panel of 27 diverse biological molecules, demonstrating the broad applicability of LA-REIMS to metabolite detection. The rapid and automated nature of LA-REIMS makes this a valuable new technology to complement existing screening technologies currently employed in academic and industrial workflows.

Unravelling triterpene biosynthesis through functional characterization of an oxidosqualene cyclase (OSC) from Cleome arabica L

Cleome arabica is a medicinal plant contains diverse bioactive compounds and terpenoids are the major components. However, the isolation and purification of the active triterpenes from this plant involve long and complicated procedures. The present work investigates the triterpenes profiles of different tissues, besides that, describes the isolation, heterologous expression and functional characterization of C. arabica gene coding for triterpenes synthases. The phytochemical investigation through GC-MS revealed significant accumulation of pentacyclic triterpenes in leaves and siliques at mature stage compared to the stems and roots of C. arabica. Among the pentacyclic triterpenes, the lupeol reached the highest level of 320 μg/g DW in leaves at maturity stage compared to the other tissues. The biosynthesis of a pentacyclic triterpene was investigated through isolation and cloning of a full-length oxidosqualene cyclase cDNA (CaOSC) from mature leaves of C. arabica. The bioinformatic analyses revealed that CaOSC was highly homologous with the characterized lupeol synthases and shared 79.3% identity to camelliol C synthase from A. thaliana. Heterologous expression of CaOSC gene in Saccharomyces cerevisiae synthesized lupeol as a single product. The lupeol biosynthesis was exponentially increased after induction through the fermentation process reaching the maximum of 2.33 μg/ml for 240 h. Furthermore, organ-specific expression of lupeol gene was exactly matched the accumulation pattern in different tissues of C. arabica during phenological cycle. Thus, the identified CaOSC will be useful in enhancing triterpene yield for industrial purposes.

Cannabimimetic plants: are they new cannabinoidergic modulators?

Phytochemicals and secondary metabolites able to interact with the endocannabinoid system (Cannabimimetics) have been recently described in a broad range of plants and fruits. These findings can open new alternative avenues to explore for the development of novel therapeutic compounds. The cannabinoids regulate many physiological and pathological functions in both animals and plants. Cannabis sativa is the main plant that produces phytocannabinoids inside resins capable to defend the plant from the aggression of parasites and herbivores. Animals produce anandamide and 2-arachidonoyl glycerol, which thanks to binding with main receptors such as type-1 cannabinoid receptor (CB1R) and the type-2 cannabinoid receptor (CB2R) are involved in inflammation processes and several brain functions. Endogenous cannabinoids, enzymes for synthesis and degradation of cannabinoids, and CB1R and CB2R constitute the endocannabinoid system (ECS). Other plants can produce cannabinoid-like molecules such as perrottetinene extracted from Radula perrottetii, or anandamide and 2-arachidonoyl glycerol extracted from some bryophytes. Moreover, several other secondary metabolites can also interact with the ECS of animals and take the name of cannabimimetics. These phytoextracts not derived from Cannabis sativa can act as receptor agonists or antagonist, or enzyme inhibitors of ECS and can be involved in the inflammation, oxidative stress, cancer, and neuroprotection. Finally, given the evolutionary heterogeneity of the cannabimimetic plants, some authors speculated on the fascinating thesis of the evolutionary convergence between plants and animals regarding biological functions of ECS. The review aims to provide a critical and complete assessment of the botanical, chemical and therapeutic aspects of cannabimimetic plants to evaluate their spread in the world and medicinal potentiality.

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

BACKGROUND

Betulinic acid is a pentacyclic lupane-type triterpenoid and a potential antiviral and antitumor drug, but the amount of betulinic acid in plants is low and cannot meet the demand for this compound. Yarrowia lipolytica, as an oleaginous yeast, is a promising microbial cell factory for the production of highly hydrophobic compounds due to the ability of this organism to accumulate large amounts of lipids that can store hydrophobic products and supply sufficient precursors for terpene synthesis. However, engineering for the heterologous production of betulinic acid and related triterpenoids has not developed as systematically as that for the production of other terpenoids, thus the production of betulinic acid in microbes remains unsatisfactory.

RESULTS

In this study, we applied a multimodular strategy to systematically improve the biosynthesis of betulinic acid and related triterpenoids in Y. lipolytica by engineering four functional modules, namely, the heterogenous CYP/CPR, MVA, acetyl-CoA generation, and redox cofactor supply modules. First, by screening 25 combinations of cytochrome P450 monooxygenases (CYPs) and NADPH-cytochrome P450 reductases (CPRs), each of which originated from 5 different sources, we selected two optimal betulinic acid-producing strains. Then, ERG1, ERG9, and HMG1 in the MVA module were overexpressed in the two strains, which dramatically increased betulinic acid production and resulted in a strain (YLJCC56) that exhibited the highest betulinic acid yield of 51.87 ± 2.77 mg/L. Then, we engineered the redox cofactor supply module by introducing NADPH- or NADH-generating enzymes and the acetyl-CoA generation module by directly overexpressing acetyl-CoA synthases or reinforcing the β-oxidation pathway, which further increased the total triterpenoid yield (the sum of the betulin, betulinic acid, betulinic aldehyde yields). Finally, we engineered these modules in combination, and the total triterpenoid yield reached 204.89 ± 11.56 mg/L (composed of 65.44% betulin, 23.71% betulinic acid and 10.85% betulinic aldehyde) in shake flask cultures.

CONCLUSIONS

Here, we systematically engineered Y. lipolytica and achieved, to the best of our knowledge, the highest betulinic acid and total triterpenoid yields reported in microbes. Our study provides a suitable reference for studies on heterologous exploitation of P450 enzymes and manipulation of triterpenoid production in Y. lipolytica.

Oxidosqualene cyclase and CYP716 enzymes contribute to triterpene structural diversity in the medicinal tree banaba

Pentacyclic triterpenes (PCTs) represent a major class of bioactive metabolites in banaba (Lagerstroemia speciosa) leaves; however, biosynthetic enzymes and their involvement in the temporal accumulation of PCTs remain to be studied. We use an integrated approach involving transcriptomics, metabolomics and gene function analysis to identify oxidosqualene cyclases (OSCs) and cytochrome P450 monooxygenases (P450s) that catalyzed sequential cyclization and oxidative reactions towards PCT scaffold diversification. Four monofunctional OSCs (LsOSC1,3-5) converted the triterpene precursor 2,3-oxidosqualene to either lupeol, β-amyrin or cycloartenol, and a multifunctional LsOSC2 formed α-amyrin as a major product along with β-amyrin. Two CYP716 family P450s (CYP716A265, CYP716A266) catalyzed C-28 oxidation of α-amyrin, β-amyrin and lupeol to form ursolic acid, oleanolic acid and betulinic acid, respectively. However, CYP716C55 catalyzed C-2α hydroxylation of ursolic acid and oleanolic acid to produce corosolic acid and maslinic acid, respectively. Besides, combined transcript and metabolite analysis suggested major roles for the LsOSC2, CYP716A265 and CYP716C55 in determining leaf ursane and oleanane profiles. Combinatorial expression of OSCs and CYP716s in Saccharomyces cerevisiae and Nicotiana benthamiana led to PCT pathway reconstruction, signifying the utility of banaba enzymes for bioactive PCT production in alternate plant/microbial hosts that are more easily tractable than the tree species.

Engineering the unicellular alga Phaeodactylum tricornutum for high-value plant triterpenoid production

Plant triterpenoids constitute a diverse class of organic compounds that play a major role in development, plant defence and environmental interaction. Several triterpenes have demonstrated potential as pharmaceuticals. One example is betulin, which has shown promise as a pharmaceutical precursor for the treatment of certain cancers and HIV. Major challenges for triterpenoid commercialization include their low production levels and their cost-effective purification from the complex mixtures present in their natural hosts. Therefore, attempts to produce these compounds in industrially relevant microbial systems such as bacteria and yeasts have attracted great interest. Here, we report the production of the triterpenes betulin and its precursor lupeol in the photosynthetic diatom Phaeodactylum tricornutum, a unicellular eukaryotic alga. This was achieved by introducing three plant enzymes in the microalga: a Lotus japonicus oxidosqualene cyclase and a Medicago truncatula cytochrome P450 along with its native reductase. The introduction of the L. japonicus oxidosqualene cyclase perturbed the mRNA expression levels of the native mevalonate and sterol biosynthesis pathway. The best performing strains were selected and grown in a 550-L pilot-scale photobioreactor facility. To our knowledge, this is the most extensive pathway engineering undertaken in a diatom and the first time that a sapogenin has been artificially produced in a microalga, demonstrating the feasibility of the photo-bio-production of more complex high-value, metabolites in microalgae.

Multi-omics data-driven investigations of metabolic diversity of plant triterpenoids

The vast majority of structurally diverse metabolites play essential roles in mediating the interactions between plant and environment, and constitute a valuable resource for industrial applications. Recent breakthroughs in sequencing technology have greatly accelerated metabolic studies of natural plant products, providing opportunities to investigate the molecular basis underlying the diversity of specialized plant metabolites through large-scale analysis. Here, we focus on the biosynthesis of plant triterpenoids, especially the three diversifying reactions (cyclization, oxidation and glycosylation) that largely contribute to the structural diversity of triterpenoids. Gene mining through large-scale omics data and functional characterization of metabolic genes including enzymes, transcription factors and transporters could provide important insights into the evolution of specialized plant metabolism and pave the way for the production of high-value metabolites or derivatives using synthetic biology approaches.

Biosynthesis of Plant Triterpenoid Saponins in Microbial Cell Factories

Triterpenoid saponins are triterpenoid glycoside compounds which have been widely used in pharmaceutical, agricultural, and food industries. Traditionally, they are extracted from plants, which is time-consuming and environmentally unfriendly. Recently, de novo synthesis of triterpenoid saponins in microbial cell factories was realized, which provides a promising and green approach to alter the traditional supply way. However, the complex biosynthetic pathway and the poor suitability between the endogenous and heterogeneous pathways tremendously limit the yield of triterpenoid saponins. We introduce the biosynthetic pathways of triterpenoid saponins first, and we then summarize the microbial cell factories developed to produce these compounds. Further, we discuss the strategies applied to enhance the production. This paper systematically illustrates the biosynthesis of plant triterpenoid saponins in microbial cell factories.

Comparative analysis of CYP716A subfamily enzymes for the heterologous production of C-28 oxidized triterpenoids in transgenic yeast

Several enzymes of the CYP716A subfamily have been reported to be involved in triterpenoid biosynthesis. Members of this subfamily oxidize various positions along the triterpenoid backbone and the majority of them catalyze a three-step oxidation at the C-28 position. Interestingly, C-28 oxidation is a common feature in oleanolic acid, ursolic acid, and betulinic acid, which are widely distributed in plants and exhibit important biological activities. In this work, three additional CYP716A enzymes isolated from olive, sugar beet, and coffee, were characterized as multifunctional C-28 oxidases. Semi-quantitative comparisons of in vivo catalytic activity were made against the previously characterized enzymes CYP716A12, CYP716A15, and CYP716A52v2. When heterologously expressed in yeast, the isolated enzymes differed in both catalytic activity and substrate specificity. This study indicates that the screening of enzymes from different plants could be a useful means of identifying enzymes with enhanced catalytic activity and desired substrate specificity. Furthermore, we show that "naturally-evolved" enzymes can be useful in the heterologous production of pharmacologically and industrially important triterpenoids.

Upregulating the mevalonate pathway and repressing sterol synthesis in Saccharomyces cerevisiae enhances the production of triterpenes

Pentacyclic triterpenes are diverse plant secondary metabolites derived from the mevalonate (MVA) pathway. Many of these molecules are potentially valuable, particularly as pharmaceuticals, and research has focused on their production in simpler and more amenable heterologous systems such as the yeast Saccharomyces cerevisiae. We have developed a new heterologous platform for the production of pentacyclic triterpenes in S. cerevisiae based on a combinatorial engineering strategy involving the overexpression of MVA pathway genes, the knockout of negative regulators, and the suppression of a competing pathway. Accordingly, we overexpressed S. cerevisiae ERG13, encoding 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, and a truncated and deregulated variant of the rate-limiting enzyme HMG-CoA reductase 1 (tHMGR). In the same engineering step, we deleted the ROX1 gene, encoding a negative regulator of the MVA pathway and sterol biosynthesis, resulting in a push-and-pull strategy to enhance metabolic flux through the system. In a second step, we redirected this enhanced metabolic flux from late sterol biosynthesis to the production of 2,3-oxidosqualene, the direct precursor of pentacyclic triterpenes. In yeast cells transformed with a newly isolated sequence encoding lupeol synthase from the Russian dandelion (Taraxacum koksaghyz), we increased the yield of pentacyclic triterpenes by 127-fold and detected not only high levels of lupeol but also a second valuable pentacyclic triterpene product, β-amyrin.

Cell-Free Production of Pentacyclic Triterpenoid Compound Betulinic Acid from Betulin by the Engineered Saccharomyces cerevisiae

Betulinic acid is a product of plant secondary metabolism which has shown various bioactivities. Several CYP716A subfamily genes were recently characterized encoding multifunctional oxidases capable of C-28 oxidation. CYP716A12 was identified as betulin C-28 oxidase, capable of modifying betulin. This study aimed to induce the transformation of betulin to betulinic acid by co-expressing enzymes CYP716A12 from Medicago truncatula and ATR1 from Arabidopsis thaliana in Saccharomyces cerevisiae. The microsome protein extracted from the transgenic yeast successfully catalyzed the transformation of betulin to betulinic acid. We also characterized the optimization of cell fragmentation, protein extraction method, and the conversion conditions. Response surface methodology was implemented, and the optimal yield of betulinic acid reached 18.70%. After optimization, the yield and the conversion rate of betulin were increased by 83.97% and 136.39%, respectively. These results may present insights and strategies for the sustainable production of betulinic acid in multifarious transgenic microbes.

Functional Characterization of CYP716 Family P450 Enzymes in Triterpenoid Biosynthesis in Tomato

Triterpenoids are a group of structurally diverse specialized metabolites that frequently show useful bioactivities. These chemicals are biosynthesized from the common precursor 2,3-oxidosqualene in plants. The carbon skeletons produced by oxidosqualene cyclase (OSC) are usually modified by cytochrome P450 monooxygenases (P450s) and UDP-dependent glycosyltransferases. These biosynthetic enzymes contribute to the structural diversification of plant triterpenoids. Until now, many P450 enzymes have been characterized as triterpenoid oxidases. Among them, the CYP716 family P450 enzymes, which have been isolated from a wide range of plant families, seem to contribute to the triterpenoid structural diversification. Many CYP716 family P450 enzymes have been characterized as the multifunctional triterpene C-28 oxidases, which oxidize α-amyrin and β-amyrin to the widely distributed triterpenoids ursolic and oleanolic acids, respectively. Tomato (Solanum lycopersicum) is one of the most important solanaceous crops in the world. However, little information is known regarding its triterpenoid biosynthesis. To understand the mechanism of triterpenoid biosynthesis in tomato, we focused on the function of CYP716 family enzymes as triterpenoid oxidases. We isolated all six CYP716 family genes from the Micro-Tom cultivar of tomato, and functionally characterized them in the heterologous yeast expression system. The in vivo enzymatic assays showed that CYP716A44 and CYP716A46 exhibited the ordinary C-28 oxidation activity against α-amyrin and β-amyrin to produce ursolic and oleanolic acids, respectively. Interestingly, one CYP716E subfamily enzyme, CYP716E26, exhibited the previously unreported C-6β hydroxylation activity against β-amyrin to produce a rare bioactive triterpenoid, daturadiol (olean-12-ene-3β,6β-diol). To determine the roles of the CYP716 family genes in tomato triterpenoid biosynthesis, we analyzed the gene expression and triterpenoid accumulation patterns in different plant tissues by performing the quantitative real-time polymerase chain reaction (qPCR) and gas chromatography-mass spectrometry (GC-MS) analyses, respectively. High levels of the CYP716A44 gene expression and the accumulation of C-28-oxidized triterpenoids, ursolic acid, and oleanolic acid were observed in the roots, indicating a significant contribution of the CYP716A44 gene in the triterpenoid biosynthesis in tomato. Thus, our study partially elucidated the mechanism of triterpenoid biosynthesis in tomato, and identified CYP716E26 as a novel C-6β hydroxylase for its subsequent use in the combinatorial biosynthesis of bioactive triterpenoids.