Complete biosynthesis of the phenylethanoid glycoside verbascoside

Efficient production of hydroxysalidroside in Escherichia coli via enhanced glycosylation and semi-rational design of UGT85A1

Hydroxysalidroside is an important natural phenylethanoid glycoside with broad application prospects in the food and pharmaceutical fields. However, its low concentration in plants and complex extraction hinder its production. Despite being a promising way to synthesize hydroxysalidroside in Escherichia coli, glycosylation remains the limiting factor for its production. A de novo biosynthetic pathway for hydroxysalidroside was successfully constructed in E. coli via the screening of glycosyltransferase, overexpressing phosphoglucomutase (pgm) and UDP-glucose pyrophosphorylase (galU) to ensure a sufficient supply of UDP-glucose (UDPG). Additionally, a semi-rational design of UGT85A1 was conducted to expand the acceptor-binding pocket to eliminate steric hindrance interfering with the binding of hydroxytyrosol. The endogenous genes ushA and otsA were knocked out to further reduce the consumption of UDPG. Finally, a titer of 5837.2 mg/L was achieved in a 5 L fermenter by optimizing the feeding times of carbon sources. This laid the foundation for the subsequent biosynthesis of phenylethanoid glycosides.

A systematic review of the traditional uses, chemistry, and curative aptitude of echinacoside-a phenylethanoid glycoside

Echinacoside (ECH), a naturally occurring water-soluble phenylethanoid glycoside, is one of the primary bioactive compounds present in several plant species, such as Echinacea, Cistanche, Plantago, Rosa, Buddleja, and Rehmannia. Research has revealed that these plants, rich in ECH, have diverse traditional uses and pharmacological activities, like anti-diabetic, anti-inflammatory, anti-fatigue, anti-allergic, anti-ageing, anti-skin glycation, analgesic, wound healing, and aphrodisiac properties. Among other activities, ophthalmic, haematopoiesis, pulmonary, anti-bacterial, anti-protozoal, anti-fungal, and anti-viral effects of ECH have been reported. Chemically, the compound comprises caffeic acid glycoside containing a trisaccharide that includes two glucose and one rhamnose unit. These units are linked through glycosidic bonds to a caffeic acid and a dihydroxyphenylethanol (hydroxytyrosol) residue, which are connected to the central rhamnose. The biosynthesis of ECH has been reported to start with forming L-phenylalanine and tyrosine precursors via the shikimic acid pathway. The structure-activity relationship of ECH has shown that various functional groups in the structure, particularly phenolic hydroxyl groups, are crucial for antioxidant activities. Similarly, in silico studies have revealed that ECH binds to different receptors, like Kelch-like ECH-associated protein 1 (Keap1), receptor for advanced glycation end products (RAGE), etc., to affect various pharmacological activities. The ECH contents in the reported plants often own these multifaceted properties, highlighting their importance in clinical research. Evident from its therapeutic efficacy, there is a huge potential for a comprehensive understanding of the mechanisms of actions of ECH, which underscores the need for more research in this area. Thus, this review is a compendium of the latest literature to analyse the existing knowledge on ECH, encompassing its distribution, traditional uses, extraction, chemical constituents, biosynthesis, pharmacological activities, structure-activity relationship, and in silico studies, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.

Complete pathway elucidation of echinacoside in Cistanche tubulosa and de novo biosynthesis of phenylethanoid glycosides

Echinacoside (ECH), one of the most representative phenylethanoid glycosides (PhGs), has considerable neuroprotective effects and is an effective ingredient in numerous commercial drugs. Here, we elucidate the complete ECH biosynthetic pathway in the medicinal plant Cistanche tubulosa. In total, 14 related genes are cloned and functionally characterized. Two upstream pathways for tyrosol biosynthesis from L-tyrosine are identified: one includes separate decarboxylation, deamination and reduction steps; the other uses microbial-like transamination, decarboxylation and reduction steps. In addition, a distinct downstream assembly process from tyrosol to ECH is revealed that includes sequential glucosylation, acylation, hydroxylation, and rhamnosylation to form acteoside, and ends with a final glucosylation converting acteoside to ECH. Furthermore, the de novo synthesis of 23 PhG derivatives is achieved via the heterologous expression of different combinations of the functional genes in tobacco. Our findings provide insights into the biosynthesis of ECH and a platform for alternative production of complex PhGs.

Recovery of Bioactive Constituents from Olive Leaf Pruning Waste of Five Different Cultivars: A Comparison of Green Extraction Techniques to Maximize Health Benefits

Sustainable agro-waste revaluation is critical to enhance the profitability and environmental footprint of the olive oil industry. Herein, the valorization of olive leaf pruning waste from five cultivars ('Caiazzana', 'Carolea', 'Itrana', 'Leccino', and 'Frantoio') employed green extraction methods to recover compounds with potential health benefits. Sequential ultrasound-assisted maceration (UAM) in n-hexane and ethanol was compared with a compressed fluid extraction strategy consisting of supercritical fluid extraction (SFE) and pressurized liquid extraction (PLE) for their efficiency in recovering distinct classes of bioactives. Chemical profiling by UHPLC-HR-MS/MS (ultra-high-performance liquid chromatography high-resolution tandem mass spectrometry) and GC-MS (gas chromatography mass spectrometry) showed that UAM-EtOH effectively extracted polyphenols (especially luteolin derivatives) and triterpenes (notably maslinic acid), while PLE yielded the highest amount of secoiridoids (e.g., secologanoside). PLE extracts showed better antiradical activities, putatively due to a higher content of flavonoids, secoiridoids, and HCA derivatives than UAM-EtOH ones, as these latter also contained 20-40% (cultivar-dependent) of triterpenes. SFE extracts with a higher concentration of fatty acids and triterpenes showed moderate antioxidant activities but very high AChE inhibition. This study highlights the importance of selecting appropriate extraction methodologies based on the target bioactive compounds and underscores the potential of olive leaf extracts for sustainable bio-products.

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.

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.

Distribution, biosynthesis, and synthetic biology of phenylethanoid glycosides in the order Lamiales

Phenylethanoid glycosides (PhGs), with a C6-C2 glucoside unit as the basic skeleton, are specialized (secondary) metabolites found in several medicinal plants. As PhGs exhibit various pharmacological activities, they are expected to be used as lead compounds in drug discovery. However, mass-production systems have not yet been established even for acteoside, a typical PhG that is widely distributed in nature (more than 150 species). This review focuses on recent studies on the accumulation and distribution of PhGs in plants, biosynthetic pathways of PhGs, and the bioproduction of PhGs.

Engineering an Ancestral Glycosyltransferase for Biosynthesis of 2-Phenylethyl-β-d-Glucopyranoside and Salidroside

Phenylethanoid glycosides (PhGs) are naturally occurring glycosides derived from plants with various biological activities. Glycosyltransferases catalyze the production of PhGs from phenylethanols via a transglycosylation reaction. The low activity and stability of glycosyltransferase limit its industrial application. An ancestral glycosyltransferase, UGTAn85, with heat resistance, alkali resistance, and high stability was resurrected using ancestral sequence reconstruction technology. This enzyme can efficiently convert phenylethanols to PhGs. The optimal reaction temperature and pH for UGTAn85 were found to be 70 °C and pH 10.0, respectively. This study employed a combination of structure-guided rational design and co-evolution analysis to enhance its catalytic activity. Potential mutation sites were identified through computer-aided design, including homology modeling, molecular docking, Rosetta dock design, molecular dynamics simulation, and co-evolution analysis. By targeted mutagenesis, the UGTAn85 mutant Q23E/N65D exhibited a 2.2-fold increase in enzyme activity (11.85 U/mg) and elevated affinity (Km = 0.11 mM) for 2-phenylethanol compared to UGTAn85. Following a fed-batch reaction, 36.16 g/L 2-phenylethyl-β-d-glucopyranoside and 51.49 g/L salidroside could be produced within 24 h, respectively. The findings in this study provide a new perspective on enhancing the stability and activity of glycosyltransferases, as well as a potential biocatalyst for the industrial production of PhGs.

Functional characterization of polyphenol oxidase OfPPO2 supports its involvement in parallel biosynthetic pathways of acteoside

Acteoside is a bioactive phenylethanoid glycoside widely distributed throughout the plant kingdom. Because of its two catechol moieties, acteoside displays a variety of beneficial activities. The biosynthetic pathway of acteoside has been largely elucidated, but the assembly logic of two catechol moieties in acteoside remains unclear. Here, we identified a novel polyphenol oxidase OfPPO2 from Osmanthus fragrans, which could hydroxylate various monophenolic substrates, including tyrosine, tyrosol, tyramine, 4-hydroxyphenylacetaldehyde, salidroside, and osmanthuside A, leading to the formation of corresponding catechol-containing intermediates for acteoside biosynthesis. OfPPO2 could also convert osmanthuside B into acteoside, creating catechol moieties directly via post-modification of the acteoside skeleton. The reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis and subcellular localization assay further support the involvement of OfPPO2 in acteoside biosynthesis in planta. These findings suggest that the biosynthesis of acteoside in O. fragrans may follow "parallel routes" rather than the conventionally considered linear route. In support of this hypothesis, the glycosyltransferase OfUGT and the acyltransferase OfAT could direct the flux of diphenolic intermediates generated by OfPPO2 into acteoside. Significantly, OfPPO2 and its orthologs constitute a functionally conserved enzyme family that evolved independently from other known biosynthetic enzymes of acteoside, implying that the substrate promiscuity of this PPO family may offer acteoside-producing plants alternative ways to synthesize acteoside. Overall, this work expands our understanding of parallel pathways plants may employ to efficiently synthesize acteoside, a strategy that may contribute to plants' adaptation to environmental challenges.

Construct Phenylethanoid Glycosides Harnessing Biosynthetic Networks, Protein Engineering and One-Pot Multienzyme Cascades

Phenylethanoid glycosides (PhGs) exhibit a multitude of structural variations linked to diverse pharmacological activities. Assembling various PhGs via multienzyme cascades represents a concise strategy over traditional synthetic methods. However, the challenge lies in identifying a comprehensive set of catalytic enzymes. This study explores biosynthetic PhG reconstruction from natural precursors, aiming to replicate and amplify their structural diversity. We discovered 12 catalytic enzymes, including four novel 6'-OH glycosyltransferases and three new polyphenol oxidases, revealing the intricate network in PhG biosynthesis. Subsequently, the crystal structure of CmGT3 (2.62 Å) was obtained, guiding the identification of conserved residue 144# as a critical determinant for sugar donor specificity. Engineering this residue in PhG glycosyltransferases (FsGT61, CmGT3, and FsGT6) altered their sugar donor recognition. Finally, a one-pot multienzyme cascade was established, where the combined action of glycosyltransferases and acyltransferases boosted conversion rates by up to 12.6-fold. This cascade facilitated the reconstruction of 26 PhGs with conversion rates ranging from 5-100 %, and 20 additional PhGs detectable by mass spectrometry. PhGs with extra glycosyl and hydroxyl modules demonstrated notable liver cell protection. This work not only provides catalytic tools for PhG biosynthesis, but also serves as a proof-of-concept for cell-free enzymatic construction of diverse natural products.

Cutting-edge plant natural product pathway elucidation

Plant natural products (PNPs) play important roles in plant physiology and have been applied across diverse fields of human society. Understanding their biosynthetic pathways informs plant evolution and meanwhile enables sustainable production through metabolic engineering. However, the discovery of PNP biosynthetic pathways remains challenging due to the diversity of enzymes involved and limitations in traditional gene mining approaches. In this review, we will summarize state-of-the-art strategies and recent examples for predicting and characterizing PNP biosynthetic pathways, respectively, with multiomics-guided tools and heterologous host systems and share our perspectives on the systematic pipelines integrating these various bioinformatic and biochemical approaches.

A α-L-rhamnosidase from Echinacea purpurea endophyte Simplicillium sinense EFF1 and its application in production of Calceorioside B

Calceorioside B, a multifunctional phenylethanol glycosides (PhGs) derivative, exhibits a variety of notable properties, such as antithrombotic, anti-tumorigenic, anti-neocoronavirus, anti-inflammatory, and neuroprotective effects. However, the large-scale production of calceorioside B is routinely restricted by its existence as an intermediary compound derived from plants, and still unachieved through excellent and activity chemical synthesis. Here, a total of 51 fungal endophytes were isolated from four PhGs-producing plants, and endophyte Simplicillium sinense EFF1 from Echinacea purpurea was identified with the ability to de-rhamnosing isoacteoside to generate calceorioside B. According to the RNA-transcription of EFF1 under the various substrates, a key gene CL1206.Contig2 that undertakes the hydrolysis function was screened out and charactered by heterologous expression. The sequence alignment, phylogenetic tree construction and substrate specificity analysis revealed that CL1206 was a novel α-L-rhamnosidase that belongs to the glycosyl hydrolase family 78 (GH78). The optimum catalytic conditions for CL1206 were at pH 6.5 and 55 °C. Finally, the enzyme-catalyzed approach to produce calceorioside B from 50 % crude isoacteoside extract was explored and optimized, with the maximum conversion rate reaching 69.42 % and the average producing rate reaching 0.37 g-1.L-1.h-1, which offered a great biocatalyst for potential industrial calceorioside B production. This is the first case for microorganism and rhamnosidase to show the hydrolysis ability to caffeic acid-modified PhGs.

Highly Efficient Verbascoside Production from Olive (Olea europea L. var. Cellina di Nardò) In Vitro Cell Cultures

Olive (Olea europea L.) is one of the oldest and most important fruit tree species cultivated in the Mediterranean region. Various plant tissues, drupes, and olive oil contain several phenolics (including verbascoside, although it is present in the plant at a low level) that are well-known for their highly beneficial effects on human health. An in vitro olive cell suspension culture (cultivar Cellina di Nardò, "CdN") was established, characterized for its growth and morphological features. Furthermore, a vital and relatively uniform population of protoplasts was generated from the olive suspension culture to investigate their cellular characteristics during growth. The polyphenolic extract of the in vitro "CdN" olive cells contained almost exclusively verbascoside, as revealed by the UPLC-ESI-MS analysis. The content of verbascoside reached up to 100 mg/g DW, with an average production rate of approximately 50 mg/g DW over one year of culture. This level of production has not been previously reported in a limited number of previous studies. This remarkable production of verbascoside was associated with an exceptionally high antioxidant capacity. The high level of verbascoside production and purity of the extract make this system a promising tool for secondary metabolite production.

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.

Plant Growth Regulators and Activated Charcoal Selectively Affect Phenylethanoid and Flavone Glycoside Accumulation in Sideritis scardica Griseb. Tissue Culture

Sideritis scardica Griseb. is a Balkan endemic species traditionally used for the treatment of pulmonary emphysema and angina pectoris. Recent research has also shown its phytotherapeutic potential as an anticancer and neuroprotective agent. These findings, as well as the endangered status of the species in its wild habitats, have motivated the present research on application of plant cell tissue and organ culture for the purposes of both valuable germplasm conservation and secondary metabolites production. Shoot cultures of the plant were initiated from sterile germinated seeds and the effects of activated charcoal (AC), as well benzyl adenine and 1-naphthaleneacetic acid treatments, were experimented. The phenolic profile analysis was performed by HPLC/DAD/MSⁿ. Comparison with samples collected from wild plants in their natural habitat was performed. It was established that in vitro multiplication induced by plant growth regulators (PGRs) was accompanied by a higher impairment of leaf morphology and trichome formation, as well as by the occurrence of plantlet hyperhydricity and callus formation, as compared with the AC treatments. Shoot culture-derived plant material was shown to produce two phenylethanoids and five flavone glycosides, not detected in the wild collected plant material. In addition, the two types of in vitro culture treatments led to the stimulation of either flavone glycosides or phenylethanoids in the in vitro cultivated plants. Thus, AC stimulated, to a higher extent, flavone glycosides' accumulation, leading to an elevated flavone/phenylethanoid ratio, as compared with PGR treatments.

Identification of a highly promiscuous glucosyltransferase from Penstemon barbatus for natural product glycodiversification

Glycosylation reactions mediated by UDP-glycosyltransferases (UGTs) are common post-modifications involved in plant secondary metabolism and significantly improve the solubility and bioactivity of aglycones. Penstemon barbatus is rich in phenylethanoid glycosides (PhGs), such as echinacoside and verbascoside. In this study, a promiscuous glycosyltransferase UGT84A95 was identified from P. barbatus. In vitro enzyme assays showed that UGT84A95 catalyzed the glucosylation of the phenol hydroxyl group of PhGs efficiently as well as other structurally diverse phenolic glycosides, including flavonoids, terpenoids, stilbene glycosides, coumarins, and simple polyphenols. By using UGT84A95, 12 glycosylated products were prepared and structurally identified by NMR spectroscopy, among which 7 are new compounds. These findings suggest that UGT84A95 could be a potential biocatalyst to synthesize multi-glycosylated glycosides.