Metabolic Engineering of Saccharomyces cerevisiae for High-Level Production of Salidroside from Glucose

Two routes for tyrosol production by metabolic engineering of Corynebacterium glutamicum

BACKGROUND

The phenolic compound tyrosol is widely used in the pharmaceutical industry, owing to its beneficial effects on human health and its use as a precursor for key pharmaceuticals, including β1-receptor blockers. Tyrosol can be found in olive oil, but despite its natural biosynthesis in plants, low extraction efficiencies render microbial production a more viable alternative.

RESULTS

Here, we engineered the L-tyrosine overproducing Corynebacterium glutamicum strain AROM3 for the de novo production of tyrosol. Two routes were established and compared: one via 4-OH-phenylpyruvate as intermediate and the other via tyramine. We initially expected the first route to require heterologous expression of a prephenate dehydrogenase gene, given that C. glutamicum lacks this enzymatic function. However, heterologous expression of ARO10 from Saccharomyces cerevisiae (ARO10Sc), which encodes a phenylpyruvate decarboxylase, was sufficient to establish tyrosol production in strain AROM3. We identified that 4-OH-phenylpyruvate is synthesized from L-tyrosine by native aminotransferases, which is subsequently decarboxylated by Aro10Sc, and reduced to tyrosol by native alcohol dehydrogenases, leading to a titer of 9.4 ± 1.1 mM (1.30 ± 0.15 g/L). We identified the furfural dehydrogenase FudC as major enzyme involved in this pathway, as its gene deletion reduced tyrosol production by 75%. Given the instability of 4-OH-phenylpyruvate, the synthesis of tyrosol via the stable intermediate tyramine was pursued via the second route. Decarboxylation of L-tyrosine followed by oxidative deamination was accomplished by overexpression of the L-tyrosine decarboxylase gene tdc from Levilactobacillus brevis (tdcLb) and the tyramine oxidase gene tyo from Kocuria rhizophila (tyoKr). Using this route, tyrosol production was increased by 44% compared to the route via 4-OH-phenylpyruvate. With a division of labor approach by co-cultivating L-tyrosine producing strains that either express tdcLb or tyoKr, the highest titer of 14.1 ± 0.3 mM (1.95 ± 0.04 g/L) was achieved.

CONCLUSIONS

This study demonstrates the potential of endotoxin-free C. glutamicum as production host for the L-tyrosine-derived product tyrosol. Due to its L-arogenate pathway for L-tyrosine synthesis, the unstable 4-OH-phenylpyruvate could be excluded as intermediate in the Tdc-Tyo pathway, outcompeting the most often utilized production route via phenylpyruvate decarboxylases.

The gap-free genome assembly and multi-omics analyses illustrate the evolutionary history and the synthesis of medicinal components of Ligustrum lucidum

Ligustrum lucidum, an important ornamental and medicinal plant in the Oleaceae family, has become a globally notorious invasive species because of its high adaptability. However, the lack of a high quality genome has hindered the understanding of the genetic basis for its broad adaptability and the mechanisms of its medicinal component synthesis. In this study, we successfully assembled a gap free telomere to telomere genome of L. lucidum. Through the reconstruction of the phylogenetic tree, we presented evidence that Jasmineae diverged from Oleaceae earlier than Forsythieae, which challenges the existing phylogenetic relationships within Oleaceae. Comparative genomics has found that two rounds of gene family expansions in L. lucidum significantly increased the number of genes related to its biotic and abiotic stress resistances, which may lay the genetic foundation for its broad adaptability. Among the L. lucidum fruits from January to March in our investigation, the results of metabolome show that March may be the optimal harvesting time. By integrating genomic, transcriptomic, and metabolomic data, we identified candidate genes involved in the synthesis of compounds, such as specnuezhenide, salidroside, and oleanolic acid. This study provides valuable genomic resources for comparative genomics studies within Oleaceae and for the genetic research of L. lucidum.

Engineering Saccharomyces cerevisiae for medical applications

BACKGROUND

During the last decades, the advancements in synthetic biology opened the doors for a profusion of cost-effective, fast, and ecologically friendly medical applications priorly unimaginable. Following the trend, the genetic engineering of the baker's yeast, Saccharomyces cerevisiae, propelled its status from an instrumental ally in the food industry to a therapy and prophylaxis aid.

MAIN TEXT

In this review, we scrutinize the main applications of engineered S. cerevisiae in the medical field focusing on its use as a cell factory for pharmaceuticals and vaccines, a biosensor for diagnostic and biomimetic assays, and as a live biotherapeutic product for the smart in situ treatment of intestinal ailments. An extensive view of these fields' academic and commercial developments as well as main hindrances is presented.

CONCLUSION

Although the field still faces challenges, the development of yeast-based medical applications is often considered a success story. The rapid advances in synthetic biology strongly support the case for a future where engineered yeasts play an important role in medicine.

High-level sustainable production of complex phenylethanoid glycosides from glucose through engineered yeast cell factories

Complex phenylethanoid glycosides (PhGs), such as verbascoside and echinacoside, comprise a vital family of natural products with renowned nutraceutical and pharmaceutical significance. Despite the high demand for these compounds across various industries, traditional plant extraction methods yield insufficient quantities, highlighting the need for alternative production methods. Therefore, this paper reports the successful engineering of Saccharomyces cerevisiae cell factories for the efficient production of complex PhGs from glucose. First, key pathway enzymes with enhanced catalytic activities in yeast were primarily screened from various verbascoside-producing plants. Second, intermediate osmanthuside B was produced with a titer of 21.5 ± 1.5 mg/L from glucose by overexpressing several enzymes, including glucosyltransferase RrUGT33 from Rhdiola rosea, acyltransferase SiAT, and 1,3-rhamnosyltransferase SiRT from Sesamum indicum, UDP-L-rhamnose synthase AtRHM2, and 4-coumarate: coenzyme A ligase At4CL1 from Arabidopsis thaliana in a p-coumaric acid-overproducing S. cerevisiae strain. Third, the production of osmanthuside B was further enhanced by increasing the copy number of SiAT and AtRHM2 in genome and diverting L-tyrosine into tyrosol biosynthesis by introducing an aromatic aldehyde synthase PcAAS from Petroselinum crispum with a titer of 320.6 ± 59.3 mg/L. Fourth, the biosynthesis of verbascoside was accomplished by integrating genes CYP98A20 and AtCPR1 into the chromosomes of the osmanthuside B-producing strain, the titer reached 184.7 ± 5.7 mg/L. Furthermore, the overexpression of the glucose-6-phosphate dehydrogenase (ZWF1) led to significantly enhanced verbascoside production to 230.6 ± 11.8 mg/L. The strains were further engineered to produce echinacoside with a titer of 184.2 ± 11.2 mg/L. Finally, the fed-batch fermentation in a 5-L bioreactor yielded 4497.9 ± 285.2 mg/L of verbascoside or 3617.4 ± 117.4 mg/L of echinacoside. This work provides a crucial foundation for the green, industrial, and sustainable production of verbascoside and echinacoside and sets an initial point for the microbial production of other complex PhG derivatives.

Engineering Escherichia coli for Efficient De Novo Synthesis of Salidroside

Salidroside is a high-value plant-derived glycoside with diverse biological activities, but the main industrial salidroside production method, extraction from Rhodiola plants, is insufficient to meet the growing market demand. The biosynthetic route via microbial fermentation is a sustainable and eco-friendly alternative method. De novo synthesis of the precursor tyrosol was established by introducing the ARO10 and ADH6 genes. Systematic metabolic engineering resulted in 3.0 g/L tyrosol, but accumulated tyrosol inhibited cell growth. Adaptive evolution produced an evolved strain with a 10.0% higher OD600 and a 3.3 g/L tyrosol titer. Introducing glucosyltransferase AtUGT85A1, and overexpressing phosphoglucose mutase pgm and UDP-glucose pyrophosphorylase galU, achieved de novo synthesis of salidroside. Furthermore, AtUGT85A1 was semirationally engineered, resulting in the A21G mutation, which enhanced salidroside production by 31.2%. The optimally engineered strain produced 16.8 g/L salidroside with 0.4 g/(L h) productivity in a 5 L bioreactor. This study laid a foundation for future industrial production of salidroside and provided important guidance for efficient biosynthesis of other tyrosol derivatives.

Salidroside: An Overview of Its Promising Potential and Diverse Applications

Salidroside, a phenolic compound isolated from various Rhodiola plants, is the principal active constituent of Traditional Chinese Medicine known for its adaptogenic properties. Due to the challenging environment of Rhodiola species, such as high altitude, high radiation, drought, and hypoxia, the source of salidroside is scarce. However, numerous studies have shown that salidroside has a range of biological activities, including cardiovascular and central nervous system activity, and anti-hypoxia, anti-inflammatory, and anti-aging activities. Although previous studies have partially summarized the pharmacological effects of salidroside, the overall pharmacological effects have not been analyzed. Hence, this review will systematically summarize the isolation, purification, synthesis, derivatization, pharmacological activity, pharmacokinetics, clinical application, and safety of salidroside. It is expected to provide new insights for the further research and pharmaceutical development of salidroside.

Functional activities of three Rehmannia glutinosa enzymes: Elucidation of the Rehmannia glutinosa salidroside biosynthesis pathway in Saccharomyces cerevisiae

Rehmannia glutinosa produces many phenylethanoid glycoside (PhG) compounds, including salidroside, which not only possesses various biological activities but also is a core precursor of some medicinal PhGs, so it is very important to elucidate the species' salidroside biosynthesis pathway to enhance the production of salidroside and its derivations. Although some plant copper-containing amine oxidases (CuAOs), phenylacetaldehyde reductases (PARs) and UDP-glucose glucosyltransferases (UGTs) are thought to be vital catalytic enzymes involved in the downstream salidroside biosynthesis pathways, to date, none of these proteins or the associated genes in R. glutinosa have been characterized. To verify a postulated R. glutinosa salidroside biosynthetic pathway starting from tyrosine, this study identified and characterized a set of R. glutinosa genes encoding RgCuAO, RgPAR and RgUGT enzymes for salidroside biosynthesis. The functional activities of these proteins were tested in vitro by heterologous expression of these genes in Escherichia coli, confirming these catalytic abilities in these corresponding reaction steps of the biosynthetic pathway. Importantly, four enzyme-encoding genes (including the previously reported RgTyDC2 encoding tyrosine decarboxylase and the RgCuAO1, RgPAR1 and RgUGT2 genes) were cointegrated into Saccharomyces cerevisiae to reconstitute the R. glutinosa salidroside biosynthetic pathway, achieving an engineered strain that produced salidroside and validating these enzymes' catalytic functions. This study elucidates the complete R. glutinosa salidroside biosynthesis pathway from tyrosine metabolism in S. cerevisiae, establishing a basic platform for the efficient production of salidroside and its derivatives.

Semi-rational design and modification of phosphoketolase to improve the yield of tyrosol in Saccharomyces cerevisiae

Tyrosol is an important component of pharmaceuticals, nutraceuticals, and cosmetics, and their biosynthetic pathways are currently a hot research topic. d-Erythrose 4-phosphate is a key precursor for the biosynthesis of tyrosol in Saccharomyces cerevisiae. Hence, the flux of d-Erythrose 4-phosphate determined the yield of tyrosol synthesis. In this study, we first obtained an S. cerevisiae strain S19 with a tyrosol yield of 247.66 mg/L by metabolic engineering strategy. To increase the production of d-Erythrose 4-phosphate, highly active phosphoketolase BA-C was obtained by bioinformatics combined with tyrosol yield assay. The key residue sites 183, 217, and 320 were obtained by molecular docking, kinetic simulation, and tyrosol yield verification. After mutation, the highly efficient phosphoketolase BA-CHis320Met was obtained, with a 37.32 % increase in enzyme activity. The tyrosol production of strain S26 with BA-CHis320Arg increased by 43.05 % than strain S25 with BA-C and increased by 151.19 % compared with the strain S19 without phosphoketolase in a 20 L fermenter. The mining and modification of phosphoketolase will provide strong support for the de novo synthesis of aromatic compounds.

Inverse metabolic engineering based on metabonomics for efficient production of hydroxytyrosol by Saccharomyces cerevisiae

Metabolic engineering provides a powerful approach to efficiently produce valuable compounds, with the aid of emerging gene editing tools and diverse metabolic regulation strategies. However, apart from the current known biochemical pathway information, a variety of unclear constraints commonly limited the optimization space of cell phenotype. Hydroxytyrosol is an important phenolic compound that serves various industries with prominent health-beneficial properties. In this study, the inverse metabolic engineering based on metabolome analysis was customized and implemented to disclose the hidden rate-limiting steps and thus to improve hydroxytyrosol production in Saccharomyces cerevisiae (S. cerevisiae). The potential rate-limiting steps involved three modules that were eliminated individually via reinforcing and balancing metabolic flow, optimizing cofactor supply, and weakening the competitive pathways. Ultimately, a 118.53 % improvement in hydroxytyrosol production (639.84 mg/L) was achieved by inverse metabolic engineering.

Facilitating secretory expression of apple seed β-glucosidase in Komagataella phaffii for the efficient preparation of salidroside

Plant-derived β-glucosidases hold promise for glycoside biosynthesis via reverse hydrolysis because of their excellent glucose tolerance and robust stability. However, their poor heterologous expression hinders the development of large-scale production and applications. In this study, we overexpressed apple seed β-glucosidase (ASG II) in Komagataella phaffii and enhanced its production from 289 to 4322 U L-1 through expression cassette engineering and protein engineering. Upon scaling up to a 5-L high cell-density fermentation, the resultant mutant ASG IIV80A achieved a maximum protein concentration and activity in the secreted supernatant of 2.3 g L-1 and 41.4 kU L-1, respectively. The preparative biosynthesis of salidroside by ASG IIV80A exhibited a high space-time yield of 33.1 g L-1 d-1, which is so far the highest level by plant-derived β-glucosidase. Our work addresses the long-standing challenge of the heterologous expression of plant-derived β-glucosidase in microorganisms and presents new avenues for the efficient production of salidroside and other natural glycosides.

Development of a gene-coded biosensor to establish a high-throughput screening platform for salidroside production

Metabolic engineering reconfigures cellular networks to produce value-added compounds from renewable substrates efficiently. However, identifying strains with desired phenotypes from large libraries through rational or random mutagenesis remains challenging. To overcome this bottleneck, an effective high-throughput screening (HTS) method must be developed to detect and analyze target candidates rapidly. Salidroside is an aromatic compound with broad applications in food, healthcare, medicine, and daily chemicals. However, there currently needs to be HTS methods available to monitor salidroside levels or to screen enzyme variants and strains for high-yield salidroside biosynthesis, which severely limits the development of microbial cell factories capable of efficiently producing salidroside on an industrial scale. This study developed a gene-encoded whole-cell biosensor that is specifically responsive to salidroside. The biosensor was created by screening a site-saturated mutagenic library of uric acid response regulatory protein binding bags. This work demonstrates the feasibility of monitoring metabolic flux with whole-cell biosensors for critical metabolites. It provides a promising tool for building salidroside high-yielding strains for high-throughput screening and metabolic regulation to meet industrial needs.

Efficient Biosynthesis of Salidroside via Artificial in Vivo enhanced UDP-Glucose System Using Cheap Sucrose as Substrate

Salidroside, a valuable phenylethanoid glycoside, is obtained from plants belonging to the Rhodiola genus, known for its diverse biological properties. At present, salidroside is still far from large-scale industrial production due to its lower titer and higher process cost. In this study, we have for the first time increased salidroside production by enhancing UDP-glucose supply in situ. We constructed an in vivo UDP-glucose regeneration system that works in conjunction with UDP-glucose transferase from Rhodiola innovatively to improve UDP-glucose availability. And a coculture was formed in order to enable de novo salidroside synthesis. Confronted with the influence of tyrosol on strain growth, an adaptive laboratory evolution strategy was implemented to enhance the strain's tolerance. Similarly, salidroside production was optimized through refinement of the fermentation medium, the inoculation ratio of the two microbes, and the inoculation size. The final salidroside titer reached 3.8 g/L. This was the highest titer achieved at the shake flask level in the existing reports. And this marked the first successful synthesis of salidroside in an in situ enhanced UDP-glucose system using sucrose. The cost was reduced by 93% due to the use of inexpensive substrates. This accomplishment laid a robust foundation for further investigations into the synthesis of other notable glycosides and natural compounds.

Plant-Derived Caffeic Acid and Its Derivatives: An Overview of Their NMR Data and Biosynthetic Pathways

In recent years, caffeic acid and its derivatives have received increasing attention due to their obvious physiological activities and wide distribution in nature. In this paper, to clarify the status of research on plant-derived caffeic acid and its derivatives, nuclear magnetic resonance spectroscopy data and possible biosynthetic pathways of these compounds were collected from scientific databases (SciFinder, PubMed and China Knowledge). According to different types of substituents, 17 caffeic acid and its derivatives can be divided into the following classes: caffeoyl ester derivatives, caffeyltartaric acid, caffeic acid amide derivatives, caffeoyl shikimic acid, caffeoyl quinic acid, caffeoyl danshens and caffeoyl glycoside. Generalization of their 13C-NMR and 1H-NMR data revealed that acylation with caffeic acid to form esters involves acylation shifts, which increase the chemical shift values of the corresponding carbons and decrease the chemical shift values of the corresponding carbons of caffeoyl. Once the hydroxyl group is ester, the hydrogen signal connected to the same carbon shifts to the low field (1.1~1.6). The biosynthetic pathways were summarized, and it was found that caffeic acid and its derivatives are first synthesized in plants through the shikimic acid pathway, in which phenylalanine is deaminated to cinnamic acid and then transformed into caffeic acid and its derivatives. The purpose of this review is to provide a reference for further research on the rapid structural identification and biofabrication of caffeic acid and its derivatives.

Optimization of tyrosol-producing pathway with tyrosine decarboxylase and tyramine oxidase in high-tyrosine-producing Escherichia coli

Tyrosol (4-hydroxyphenylethanol) is a phenolic compound used in the pharmaceutical and chemical industries. However, current supply methods, such as extraction from natural resources and chemical synthesis, have disadvantages from the viewpoint of cost and environmental protection. Here, we developed a tyrosol-producing Escherichia coli cell factory from a high-tyrosine-producing strain by expressing selected tyrosine decarboxylase-, tyramine oxidase (TYO)-, and medium-chain dehydrogenase/reductase (YahK)-encoding genes. The genes were controlled by the strong T7 promoter and integrated into the chromosome because of the advantages over plasmid-based systems. The strain produced a melanin-like pigment as a by-product, which is suggested to be formed from 4-hydroxyphenylacetaldehyde (a TYO product/YahK substrate). By using a culture medium containing a high concentration of glycerol, which was reported to enhance NADH supply required for YahK activity, the final titer of tyrosol reached 2.42 g/L in test tube-scale cultivation with a concomitant decrease in the amount of pigment. These results indicate that chromosomally integrated and T7 promoter-controlled gene expression system in E. coli is useful for high production of heterologous enzymes and might be applied for industrial production of useful compounds including tyrosine and tyrosol.

Construction of an Escherichia coli cell factory for de novo synthesis of tyrosol through semi-rational design based on phenylpyruvate decarboxylase ARO10 engineering

Tyrosol (2-(4-hydroxyphenyl) ethanol) is extensively used in the pharmaceutical industry as an important natural product from plants. In previous research, we constructed a recombinant Escherichia coli strain capable of de novo synthesis of tyrosol by integrating the phenylpyruvate decarboxylase ARO10 derived from Saccharomyces cerevisiae. Nevertheless, the insufficient catalytic efficiency of ARO10 required the insertion of multiple gene copies into the genome to attain enhanced tyrosol production. In this study, we constructed a mutation library of ARO10 based on a computer-aided semi-rational design strategy and developed a high-throughput screening method for selecting high-yield tyrosol mutants by introducing the heterologous hydroxylase complex HpaBC. Through multiple rounds of screening and site-saturation mutagenesis, we ultimately identified the two optimal ARO10 mutants, ARO10D331V and ARO10D331C, which respectively achieved a tyrosol titer of 2.02 g/L and 2.04 g/L in shake flasks, both representing more than 50 % improvement compared to the wild-type. Our study demonstrates the great potential of computer-based semi-rational enzyme design strategy in metabolic engineering. The high-throughput screening method for target compound derivative possesses a certain level of generality. Ultimately, we obtained promising mutants capable of achieving industrial-scale production of tyrosol, which also lays a solid foundation for the efficient synthesis of tyrosol derivatives.

Promoting FADH2 Regeneration of Hydroxylation for High-Level Production of Hydroxytyrosol from Glycerol in Escherichia coli

Hydroxytyrosol is a natural polyphenolic compound widely used in the food and drug industries. The current commercial production of hydroxytyrosol relies mainly on plant extracts, which involve long extraction cycles and various raw materials. Microbial fermentation has potential value as an environmentally friendly and low-cost method. Here, a de novo biosynthetic pathway of hydroxytyrosol has been designed and constructed in an Escherichia coli strain with released tyrosine feedback inhibition. By introduction of hpaBC from E. coli and ARO10 and ADH6 from Saccharomyces cerevisiae, the de novo biosynthesis of hydroxytyrosol was achieved. An important finding in cofactor engineering is that the introduction of L-amino acid deaminase (LAAD) promotes not only cofactor regeneration but also metabolic flow redistribution. To further enhance the hydroxylation process, different 4-hydroxyphenylacetate 3-monooxygenase (hpaB) mutants and HpaBC proteins from different sources were screened. Finally, after optimization of the carbon source, pH, and seed medium, the optimum engineered strain produced 9.87 g/L hydroxytyrosol in a 5 L bioreactor. This represents the highest titer reported to date for de novo biosynthesis of hydroxytyrosol in microorganisms.

Tyrosol induces multiple drug resistance in yeast Saccharomyces cerevisiae

In yeast, multiple (pleiotropic) drug resistance (MDR) transporters efflux xenobiotics from the cytoplasm to the environment. Additionally, upon the accumulation of xenobiotics in the cells, MDR genes are induced. At the same time, fungal cells can produce secondary metabolites with physico-chemical properties similar to MDR transporter substrates. Nitrogen limitation in yeast Saccharomyces cerevisiae leads to the accumulation of phenylethanol, tryptophol, and tyrosol, which are products of aromatic amino acid catabolism. In this study, we investigated whether these compounds could induce or inhibit MDR in yeast. Double deletion of PDR1 and PDR3 genes, which are transcription factors that upregulate the expression of PDR genes, reduced yeast resistance to high concentrations of tyrosol (4-6 g/L) but not to the other two tested aromatic alcohols. PDR5 gene, but not other tested MDR transporter genes (SNQ2, YOR1, PDR10, PDR15) contributed to yeast resistance to tyrosol. Tyrosol inhibited the efflux of rhodamine 6G (R6G), a substrate for MDR transporters. However, preincubating yeast cells with tyrosol induced MDR, as evidenced by increased Pdr5-GFP levels and reduced yeast ability to accumulate Nile red, another fluorescent MDR-transporter substrate. Moreover, tyrosol inhibited the cytostatic effect of clotrimazole, the azole antifungal. Our results demonstrate that a natural secondary metabolite can modulate yeast MDR. We speculate that intermediates of aromatic amino acid metabolites coordinate cell metabolism and defense mechanisms against xenobiotics.

Yeast: A platform for the production of L -tyrosine derivatives

L -Tyrosine derivatives are widely applied in the pharmaceutical, food, and chemical industries. Their production is mainly confined to chemical synthesis and plant extract. Microorganisms, as cell factories, exhibit promising advantages for valuable chemical production to fulfill the increase in the demand of global markets. Yeast has been used to produce natural products owing to its robustness and genetic maneuverability. Focusing on the progress of yeast cell factories for the production of L -tyrosine derivatives, we summarized the emerging metabolic engineering approaches in building L -tyrosoine-overproducing yeast and constructing cell factories of three typical chemicals and their derivatives: tyrosol, p-coumaric acid, and L -DOPA. Finally, the challenges and opportunities of L -tyrosine derivatives production in yeast cell factories were also discussed.

A Novel Microbial Consortia Catalysis Strategy for the Production of Hydroxytyrosol from Tyrosine

Hydroxytyrosol, a valuable plant-derived phenolic compound, is increasingly produced from microbial fermentation. However, the promiscuity of the key enzyme HpaBC, the two-component flavin-dependent monooxygenase from Escherichia coli, often leads to low yields. To address this limitation, we developed a novel strategy utilizing microbial consortia catalysis for hydroxytyrosol production. We designed a biosynthetic pathway using tyrosine as the substrate and selected enzymes and overexpressing glutamate dehydrogenase GdhA to realize the cofactor cycling by coupling reactions catalyzed by the transaminase and the reductase. Additionally, the biosynthetic pathway was divided into two parts and performed by separate E. coli strains. Furthermore, we optimized the inoculation time, strain ratio, and pH to maximize the hydroxytyrosol yield. Glycerol and ascorbic acid were added to the co-culture, resulting in a 92% increase in hydroxytyrosol yield. Using this approach, the production of 9.2 mM hydroxytyrosol was achieved from 10 mM tyrosine. This study presents a practical approach for the microbial production of hydroxytyrosol that can be promoted to produce other value-added compounds.

De Novo Production of Hydroxytyrosol by Saccharomyces cerevisiae-Escherichia coli Coculture Engineering

Hydroxytyrosol is a valuable plant-derived phenolic compound with excellent pharmacological activities for application in the food and health care industries. Microbial biosynthesis provides a promising approach for sustainable production of hydroxytyrosol via metabolic engineering. However, its efficient production is limited by the machinery and resources available in the commonly used individual microbial platform, for example, Escherichia coli, Saccharomyces cerevisiae. In this study, a S. cerevisiae-E. coli coculture system was designed for de novo biosynthesis of hydroxytyrosol by taking advantage of their inherent metabolic properties, whereby S. cerevisiae was engineered for de novo production of tyrosol based on an endogenous Ehrlich pathway, and E. coli was dedicated to converting tyrosol to hydroxytyrosol by use of native hydroxyphenylacetate 3-monooxygenase (EcHpaBC). To enhance hydroxytyrosol production, intra- and intermodule engineering was employed in this microbial consortium: (I) in the upstream S. cerevisiae strain, multipath regulations combining with a glucose-sensitive GAL regulation system were engineered to enhance the precursor supply, resulting in significant increase of tyrosol production (from 17.60 mg/L to 461.07 mg/L); (II) Echpabc was overexpressed in the downstream E. coli strain, improving the conversion rate of tyrosol to hydroxytyrosol from 0.03% to 86.02%; (III) and last, intermodule engineering with this coculture system was performed by optimization of the initial inoculation ratio of each population and fermentation conditions, achieving 435.32 mg/L of hydroxytyrosol. This S. cerevisiae-E. coli coculture strategy provides a new opportunity for de novo production of hydroxytyrosol from inexpensive feedstock.

Efficient whole cell biotransformation of tyrosol from L-tyrosine by engineered Escherichia coli

An engineered Escherichia coli was constructed by co-expressing L-amino acid deaminase, α-keto acid decarboxylase, alcohol dehydrogenase, and glucose dehydrogenase through two plasmids for tyrosol production. The activity of the rate-limiting enzyme L-amino acid deaminase from Cosenzaea myxofaciens (CmAAD) toward tyrosine was improved by structure-guided modification. The enzyme activity of triple mutant CmAAD V438G/K147V/R151E toward tyrosine was ~5.12-fold higher than that of the wild-type CmAAD. Secondly, the plasmid copy numbers and the gene orders were optimized to improve the titer of tyrosol. Finally, the recombinant strain CS-6 transformed 10 mM tyrosine into 9.56 ± 0.64 mM tyrosol at 45 ℃, and the space-time yield reached 0.478 mM·L^-1·h^-1. This study proposes a novel idea for the efficient and natural production of tyrosol, which has great potential for industrial application.

De Novo Production of Hydroxytyrosol by Metabolic Engineering of Saccharomyces cerevisiae

Hydroxytyrosol is an olive-derived phenolic compound of increasing commercial interest due to its health-promoting properties. In this study, a high-yield hydroxytyrosol-producing Saccharomyces cerevisiae cell factory was established via a comprehensive metabolic engineering scheme. First, de novo biosynthetic pathway of hydroxytyrosol was constructed in yeast by gene screening and overexpression of different phenol hydroxylases, among which paHD (from Pseudomonas aeruginosa) displayed the best catalytic performance. Next, hydroxytyrosol precursor supply was enhanced via a multimodular engineering approach: elimination of tyrosine feedback inhibition through genomic integration of aro4^K229L and aro7^G141S, construction of an aromatic aldehyde synthase (AAS)-based tyrosine metabolic pathway, and redistribution of metabolic flux between glycolytic pathway and pentose phosphate pathway (PPP) by introducing the exogenous gene Bbxfpk^opt. As a result, the titer of hydroxytyrosol was improved by 6.88-fold. Finally, a glucose-responsive dynamic regulation system based on GAL80 deletion was implemented, resulting in the final hydroxytyrosol yields of 308.65 mg/L and 167.98 mg/g cell mass, the highest known from de novo production in S. cerevisiae to date.

Whole-Cell Biocatalyst for Rubusoside Production in Saccharomyces cerevisiae

Rubusoside (Rub) is a highly sweet diterpene glycoside mainly isolated from the leaves of Rubus suavissimus (Rosaceae). It has been used as a low-calorie natural sweetener for decades and was recently found to be a potential drug lead. In this study, we designed a whole-cell biocatalyst to achieve the glycosylation of steviol to Rub in Saccharomyces cerevisiae. The sucrose synthases were applied to construct a uridine diphosphate glucose regeneration system, which were coupled with optimal combinations of different uridine diphosphate (UDP) glycosyltransferases from multiple plant species. After optimization of reaction conditions, the residues in SrUGT74G1 probably influencing glycosylation efficiency were subjected to site-directed mutagenesis. Double mutations of S84A/E87A reduced the accumulation of intermediates, finally glucosylating 1.27 g/L steviol to 0.45 ± 0.06 g/L steviolmonoside (conversion rate = 23.3%) and 1.92 ± 0.17 g/L Rub (conversion rate = 74.9%). A high efficiency of Rub biosynthesis could be achieved without supply of additional UDPG. This work provided the first example of multi-step glycosylation reactions in whole-cell biocatalysis, which laid a foundation of scalable production of the value-added diterpene sweetener in the future.

Multi-modular engineering of Saccharomyces cerevisiae for high-titre production of tyrosol and salidroside

Tyrosol and its glycosylated product salidroside are important ingredients in pharmaceuticals, nutraceuticals and cosmetics. Despite the ability of Saccharomyces cerevisiae to naturally synthesize tyrosol, high yield from de novo synthesis remains a challenge. Here, we used metabolic engineering strategies to construct S. cerevisiae strains for high-level production of tyrosol and salidroside from glucose. First, tyrosol production was unlocked from feedback inhibition. Then, transketolase and ribose-5-phosphate ketol-isomerase were overexpressed to balance the supply of precursors. Next, chorismate synthase and chorismate mutase were overexpressed to maximize the aromatic amino acid flux towards tyrosol synthesis. Finally, the competing pathway was knocked out to further direct the carbon flux into tyrosol synthesis. Through a combination of these interventions, tyrosol titres reached 702.30 ± 0.41 mg l-1 in shake flasks, which were approximately 26-fold greater than that of the WT strain. RrU8GT33 from Rhodiola rosea was also applied to cells and maximized salidroside production from tyrosol in S. cerevisiae. Salidroside titres of 1575.45 ± 19.35 mg l-1 were accomplished in shake flasks. Furthermore, titres of 9.90 ± 0.06 g l-1 of tyrosol and 26.55 ± 0.43 g l-1 of salidroside were achieved in 5 l bioreactors, both are the highest titres reported to date. The synergistic engineering strategies presented in this study could be further applied to increase the production of high value-added aromatic compounds derived from the aromatic amino acid biosynthesis pathway in S. cerevisiae.

The Therapeutic Effects and Mechanisms of Salidroside on Cardiovascular and Metabolic Diseases: An Updated Review

The increasing incidence of metabolic and cardiovascular diseases has severely affected global human health and life safety. In recent years, some effective drugs with remarkable curative effects and few side effects found in natural compounds have attracted attention. Salidroside (SAL), a phenylpropane glycoside, is the main active ingredient of the plateau plant Rhodiola. So far, many animal experiments proved that SAL has good biological activity against some metabolic and cardiovascular diseases. However, most of these reports are scattered. This review systematically summarizes the pharmacological progress of SAL in the treatment of several metabolic (e. g., diabetes and non-alcoholic fatty liver disease) and cardiovascular (e. g., atherosclerosis) diseases in a timely manner to promote the clinical application and basic research of SAL. Accumulating evidence proves that SAL has beneficial effects on these diseases. It can improve glucose tolerance, insulin sensitivity, and β-cell and liver functions, and inhibit adipogenesis, inflammation and oxidative stress. Overall, SAL may be a valuable and potential drug candidate for the treatment of metabolic and cardiovascular diseases. However, more studies especially clinical trials are needed to further confirm its therapeutic effects and molecular mechanisms.

Metabolic engineering of Saccharomyces cerevisiae for high-level production of gastrodin from glucose

BACKGROUND

The natural phenolic glycoside gastrodin is the major bioactive ingredient in the well-known Chinese herb Tianma and is widely used as a neuroprotective medicine in the clinic. Microbial production from sustainable resources is a promising method to replace plant extraction and chemical synthesis which were currently used in industrial gastrodin production. Saccharomyces cerevisiae is considered as an attractive host to produce natural plant products used in the food and pharmaceutical fields. In this work, we intended to explore the potential of S. cerevisiae as the host for high-level production of gastrodin from glucose.

RESULTS

Here, we first identified the plant-derived glucosyltransferase AsUGT to convert 4-hydroxybenzyl alcohol to gastrodin with high catalytic efficiency in yeast. Then, we engineered de novo production of gastrodin by overexpressing codon-optimized AsUGT^syn, the carboxylic acid reductase gene CAR^syn from Nocardia species, the phosphopantetheinyl transferase gene PPTcg-1^syn from Corynebacterium glutamicum, the chorismate pyruvate-lyase gene UbiC^syn from Escherichia coli, and the mutant ARO4^K229L. Finally, we achieved an improved product titer by a chromosomal multiple-copy integration strategy and enhancement of metabolic flux toward the aglycon 4-hydroxybenzyl alcohol. The best optimized strain produced 2.1 g/L gastrodin in mineral medium with glucose as the sole carbon source by flask fermentation, which was 175 times higher than that of the original gastrodin-producing strain.

CONCLUSIONS

The de novo high-level production of gastrodin was first achieved. Instead of chemical synthesis or plants extraction, our work provides an alternative strategy for the industrial production of gastrodin by microbial fermentation from a sustainable resource.

Potential role of two cytochrome P450s obtained from Lithospermum erythrorhizon in catalyzing the oxidation of geranylhydroquinone during Shikonin biosynthesis

Shikonin is a natural naphthoquinone derivative that specifically occurs in boraginaceous plants, and the major active ingredient of the medicinal plant Lithospermum erythrorhizon. Previously, a cytochrome P450 oxygenase (CYP) CYP76B74 catalyzing 3″-hydroxylation of geranylhydroquinone (GHQ) - a key intermediate of shikonin biosynthesis, was identified from cultured cells of Arnebia euchroma. However, the enzymes catalyzing oxidation of the geranyl side-chain of GHQ from L. erythrorhizon remain unknown. In this study, we performed transcriptome analysis of different tissues (red roots and green leaves/stems) from L. erythrorhizon using RNA sequencing technology. Highly expressed CYP genes found in the roots were then heterologously expressed in Saccharomyces cerevisiae and functionally screened with GHQ as the substrate. As the result, two CYPs of CYP76B subfamily catalyzing the oxidation of GHQ were characterized. CYP76B100 catalyzed the hydroxylation of the geranyl side-chain of GHQ at the C-3″ position to form 3″-hydroxyl geranylhydroquinone (GHQ-3″-OH). The enzyme CYP76B101 carried out oxidation reaction of GHQ at the C-3″ position to produce a 3″-carboxylic acid derivative of GHQ (GHQ-3″-COOH) as well as GHQ-3″-OH. This enzyme-catalyzed oxidation reaction with GHQ as the substrate is reported for the first time. This study implicates CYP76B100 and CYP76B101 as having a potential role in shikonin biosynthesis in L. erythrorhizon.

Recent Advances in Metabolically Engineered Microorganisms for the Production of Aromatic Chemicals Derived From Aromatic Amino Acids

Aromatic compounds derived from aromatic amino acids are an important class of diverse chemicals with a wide range of industrial and commercial applications. They are currently produced via petrochemical processes, which are not sustainable and eco-friendly. In the past decades, significant progress has been made in the construction of microbial cell factories capable of effectively converting renewable carbon sources into value-added aromatics. Here, we systematically and comprehensively review the recent advancements in metabolic engineering and synthetic biology in the microbial production of aromatic amino acid derivatives, stilbenes, and benzylisoquinoline alkaloids. The future outlook concerning the engineering of microbial cell factories for the production of aromatic compounds is also discussed.

Current advances in acteoside biosynthesis pathway elucidation and biosynthesis

Acteoside is an important bioactive natural product distributed in many plant species, composed of four moieties such as caffeic acid, glucose, rhamnose and phenylethyl alcohol, and possesses some bioactivities such as anti-inflammatory, anti-oxidant, neuro-protective, anti-tumor and so on. However, acteoside content in medicinal plants is low, and acteoside stability is bad, so acteoside biosynthesis is a problem. Recent years, acteoside biosynthesis pathway elucidation and bio-production have been widely investigated, so many achievements have been made up to now. In this study, we reviewed current advances in both the elucidation and bio-production such as the putative methods and enzymatic determination of acteoside biosynthesis pathway, functional analyses of the roles of some candidate genes for verbascoside biosynthesis by transgenic technology, acteoside production via metabolic engineering and synthetic biology approaches and plant tissue culture. Moreover, we first established a combined putative acteoside biosynthesis pathway based on its recent studies in animals, plants and microbes. Meanwhile, we pointed out both problems to shortcomings, and highlighted its future development trend. These results will provide references for the complete elucidation of acteoside biosynthesis pathway and the improvement of acteoside content in medicinal plants and acteoside production via microbial and plant metabolic engineering and synthetic biology approaches, and inform the readers critically of the latest developments of them.

Advanced Strategies for Production of Natural Products in Yeast

Natural products account for more than 50% of all small-molecule pharmaceutical agents currently in clinical use. However, low availability often becomes problematic when a bioactive natural product is promising to become a pharmaceutical or leading compound. Advances in synthetic biology and metabolic engineering provide a feasible solution for sustainable supply of these compounds. In this review, we have summarized current progress in engineering yeast cell factories for production of natural products, including terpenoids, alkaloids, and phenylpropanoids. We then discuss advanced strategies in metabolic engineering at three different dimensions, including point, line, and plane (corresponding to the individual enzymes and cofactors, metabolic pathways, and the global cellular network). In particular, we comprehensively discuss how to engineer cofactor biosynthesis for enhancing the biosynthesis efficiency, other than the enzyme activity. Finally, current challenges and perspective are also discussed for future engineering direction.

Rational Engineering of Chorismate-Related Pathways in Saccharomyces cerevisiae for Improving Tyrosol Production

Tyrosol is extensively used in the pharmaceutical industry as an important natural product from plants. In this study, an exogenous pathway involved in catalyzing tyrosine to tyrosol was introduced into Saccharomyces cerevisiae. Furthermore, The pyruvate decarboxylase gene pdc1 was deleted to redirect the flux distribution at the pyruvate node, and a bifunctional NAD⁺-dependent fused chorismate mutase/prephenate dehydrogenase from E. coli (EcTyrA) and its' tyrosine inhibition resistant mutant (EcTyrA^M53I/A354V) were heterologously expression in S. cerevisiae to tuning up the chorismate metabolism effectively directed the metabolic flux toward tyrosol production. Finally, the tyrosol yield of the engineered strain GFT-4 was improved to 126.74 ± 6.70 mg/g DCW at 48 h, increased 440 times compared with that of the control strain GFT-0 (0.28 ± 0.01 mg/g DCW). The new synergetic engineering strategy developed in this study can be further applied to increase the production of high value-added aromatic compounds derived from aromatic amino acid or shikimate in S. cerevisiae.

Identification of a novel UDP-glycosyltransferase gene from Rhodiola rosea and its expression during biotransformation of upstream precursors in callus culture

Roseroot (Rhodiola rosea L.) is a medicinal plant with adaptogenic properties and several pharmaceutically important metabolites. In this study, a full length cDNA encoding a UDPG gene of roseroot was identified, cloned and characterized. Its ORF (1425 bp) was transferred into E. coli, where the expression of the recombinant enzyme was confirmed. To monitor the enzyme activity, 3 precursors (tyramine, 4-hydroxyphenylpyruvate & tyrosol) of salidroside biosynthesis pathway were added to roseroot callus cultures and samples were harvested after 1, 6, 12, 24, 48 & 96 h. Along with the controls (without precursor feeding), each sample was subjected to HPLC and qRT-PCR for phytochemical and relative UDP-glycosyltransferase gene expression analysis, respectively. The HPLC analysis showed that the salidroside content significantly increased; reaching 0.5% of the callus dry weight (26-fold higher than the control) after 96 h when 2 mM tyrosol was given to the media. The expression of the UDP-glycosyltransferase increased significantly being the highest at 12 h after the feeding. The effect of tyramine and 4-hydroxyphenylpyruvate was not as pronounced as of tyrosol. Here, we introduce a R. rosea specific UDPG gene and its expression pattern after biotransformation of intermediate precursors in in vitro roseroot callus cultures.

Enzymatic Glucosylation of Salidroside from Starch by α-Amylase

α-Amylases are among the most important and widely used industrial enzymes for starch processing. In this work, an α-amylase from Bacillus subtilis XL8 was purified and found to possess both hydrolysis and transglycosylation activities. The optimal pH and temperature for starch hydrolysis were pH 5.0 and 70 °C, respectively. The enzyme could degrade soluble starch into beneficial malto-oligosaccharides ranging from dimer to hexamer. More importantly, it was able to catalyze α-glycosyl transfer from the soluble starch to salidroside, a medicinal plant-derived component with broad pharmacological properties. The transglycosylation reaction catalyzed by the enzyme generated six derivatives in a total high yield of 73.4% when incubating with 100 mg/mL soluble starch and 50 mM salidroside (pH 7.5) at 50 °C for 2 h. These derivatives were identified as α-1,4-glucosyl, maltosyl, maltotriosyl, maltotetraosyl, maltopentaosyl, and maltohexaosyl salidrosides, respectively. They were novel promising compounds that might integrate the bioactive functions of malto-oligosaccharides and salidroside.

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.