Advances in plant-derived C-glycosides: Phytochemistry, bioactivities, and biotechnological production

Engineering regioselectivity of glycosyltransferase for efficient polydatin synthesis

Resveratrol is a promising functional ingredient applied in food products. However, low bioavailability and poor water solubility, which can be improved by glycosylation, hinder its application. A uridine diphosphate-dependent glycosyltransferase (UGT) from Bacillus subtilis 168 (named UGTBS) presents potential application for resveratrol glycosylation; nonetheless, imprecise regioselectivity renders the synthesis of resveratrol-3-O-β-D-glucoside (polydatin) difficult. Therefore, molecular evolution was applied to UGTBS. A triple mutant Y14I/I62G/M315W was developed for 3-OH glycosylation of resveratrol and polydatin accounted for 91% of the total product. Kinetic determination and molecular docking indicated that the enhancement of hydrogen bond interaction and altered conformation of the binding pocket increases the enzyme's affinity for the 3-OH group, stabilizing the enzyme-substrate intermediate and promoting polydatin formation. Furthermore, a fed-batch cascade reaction by periodic addition of resveratrol was conducted and nearly 20 mM polydatin was obtained. The mutant Y14I/I62G/M315W can be used for polydatin manufacture.

Multi-omics analysis reveals promiscuous O-glycosyltransferases involved in the diversity of flavonoid glycosides in Periploca forrestii (Apocynaceae)

Flavonoid glycosides are widespread in plants, and are of great interest owing to their diverse biological activities and effectiveness in preventing chronic diseases. Periploca forrestii, a renowned medicinal plant of the Apocynaceae family, contains diverse flavonoid glycosides and is clinically used to treat rheumatoid arthritis and traumatic injuries. However, the mechanisms underlying the biosynthesis of these flavonoid glycosides have not yet been elucidated. In this study, we used widely targeted metabolomics and full-length transcriptome sequencing to identify flavonoid diversity and biosynthetic genes in P. forrestii. A total of 120 flavonoid glycosides, including 21 C-, 96 O-, and 3 C/O-glycosides, were identified and annotated. Based on 24,123 full-length coding sequences, 99 uridine diphosphate sugar-utilizing glycosyltransferases (UGTs) were identified and classified into 14 groups. Biochemical assays revealed that four UGTs exhibited O-glycosyltransferase activity toward apigenin and luteolin. Among them, PfUGT74B4 and PfUGT92A8 were highly promiscuous and exhibited multisite O-glycosylation or consecutive glycosylation activities toward various flavonoid aglycones. These four glycosyltransferases may significantly contribute to the diversity of flavonoid glycosides in P. forrestii. Our findings provide a valuable genetic resource for further studies on P. forrestii and insights into the metabolic engineering of bioactive flavonoid glycosides.

The Nicotiana tabacum UGT89A2 enzyme catalyzes the glycosylation of di- and trihydroxylated benzoic acid derivatives

Glycosyltransferases catalyze the transfer of a glycoside group to a wide range of acceptor compounds to produce glycoconjugates with diverse biological and pharmacological activities. The present work reports the identification and biochemical characterization of Nicotiana tabacum UGT89A2 glycosyltransferase (NtUGT89A2). The enzyme is a monomer in solution that catalyzes the O-β-glucosylation of di- and tri-hydroxylated and chlorinated derivatives of benzoic acid. NtUGT89A2 has a preference for 2,5-dihydroxybenzoic acid (2,5-DHBA) over 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,4-dihydroxybenzoic acid (2,4-DHBA). Other substrates that can be used by NtUGT89A2 include 3,4,5-trihydroxybenzoic acid and chlorinated derivatives such as 2-chloro-5-hydroxybenzoic acid (2-Cl-5-HBA). The substrates of NtUGT89A2 were identified by thermal stability experiments, where we observed a maximum increase of the thermal denaturation midpoint (Tm) of 10 °C in the presence of 2,5-DHBA and UDP-glucose. On the other hand, the highest specific activity was obtained with 2,5-DHBA (225 ± 1.7 nkat/mg). Further characterization revealed that the enzyme has a micromolar affinity for its substrates. Notably, the enzyme retains full activity after incubation at 70 °C for 1 h. These results provide a basis for future functional and structural studies of NtUGT89A2.

BvCGT1-mediated differential distribution of flavonoid C-glycosides contributes to plant's response to UV-B stress

C-glycosides are a predominant class of flavonoids that demonstrate diverse medical properties and plant physiological functions. The chemical stability, structural diversity, and differential aboveground distribution of these compounds in plants make them ideal protectants. However, little is known about the transcriptional regulatory mechanisms that play these diverse roles in plant physiology. In this study, chard was selected from 69 families for its significantly different flavonoid C-glycosides distributions between the aboveground and underground parts to investigate the role and regulatory mechanism of flavonoid C-glycosides in plants. Our results indicate that flavonoid C-glycosides are affected by various stressors, especially UV-B. Through cloning and validation of key biosynthetic genes of flavonoid C-glycosides in chard (BvCGT1), we observed significant effects induced by UV-B radiation. This finding was further confirmed by resistance testing in BvCGT1 silenced chard lines and in Arabidopsis plants with BvCGT1 overexpression. Yeast one-hybrid and dual-luciferase assays were employed to determine the underlying regulatory mechanisms of BvCGT1 in withstanding UV-B stress. These results indicate a potential regulatory role of BvDof8 and BvDof13 in modulating flavonoid C-glycosides content, through their influence on BvCGT1. In conclusion, we have effectively demonstrated the regulation of BvCGT1 by BvDof8 and BvDof13, highlighting their crucial role in plant adaptation to UV-B radiation. Additionally, we have outlined a comprehensive transcriptional regulatory network involving BvDof8 and BvDof13 in response to UV-B radiation.

Polarity-Matched Initiation of Radical-Polar Crossover Reactions for the Synthesis of C-Allyl Glycosides with gem-Difluoroalkene Groups

Herein, we disclose a general method for the assembly of C-allyl glycosides containing gem-difluoroalkene groups via a radical-polar crossover strategy. Central to the success of this process is the polarity matching between the benzenesulfinate radical and the glycosyl donor, which facilitates the initiation of the glycosyl radical and the subsequent formation of gem-difluoroalkene sugar derivatives. This method demonstrated good compatibility with various glycosyl donors and functional groups. Furthermore, we showcase the utility of this method in modifying amino acids, potentially paving the way for analogous modifications to peptides.

Fragmentation Patterns of Phenolic C-Glycosides in Mass Spectrometry Analysis

BACKGROUND

Many phenolic C-glycosides possess nutritional benefits and pharmacological efficacies. However, the MS/MS fragmentation pattern of phenolic C-glycosides analysis is understudied. This paper aims to determine the MS/MS fragmentation patterns of phenolic C-glycosides.

METHOD

Ten compounds with different sugar moieties, aglycones, and substitutes were analyzed to determine the impact of these structural features on MS/MS fragmentation using UPLC-QTOF-MS analysis.

RESULTS

The results showed that water loss followed by RDA reaction and alpha cleavage in the C-C bonded sugar moieties are the major fragmentation pathways. Additionally, the sugar cleavage was not affected by the skeleton and the substitute of the aglycones. These results suggested that the fragmentation patterns of phenolic C-glycosides differ from those in the O-glycosides, where the O-C glycosidic bond is the most cleavage-liable bond in MS/MS analysis.

CONCLUSIONS

These MS/MS fragmentation patterns can be used for the identification of C-glycosides from dietary components and herbal medicine as well as developing robust methods using MRM methods to quantify C-glycosides.

The high-quality genome of Grona styracifolia uncovers the genomic mechanism of high levels of schaftoside, a promising drug candidate for treatment of COVID-19

Recent study has evidenced that traditional Chinese medicinal (TCM) plant-derived schaftoside shows promise as a potential drug candidate for COVID-19 treatment. However, the biosynthetic pathway of schaftoside in TCM plants remains unknown. In this study, the genome of the TCM herb Grona styracifolia (Osbeck) H.Ohashi & K.Ohashi (GSO), which is rich in schaftoside, was sequenced, and a high-quality assembly of GSO genome was obtained. Our findings revealed that GSO did not undergo recent whole genome duplication (WGD) but shared an ancestral papilionoid polyploidy event, leading to the gene expansion of chalcone synthase (CHS) and isoflavone 2'-hydroxylase (HIDH). Furthermore, GSO-specific tandem gene duplication resulted in the gene expansion of C-glucosyltransferase (CGT). Integrative analysis of the metabolome and transcriptome identified 13 CGTs and eight HIDHs involved in the biosynthetic pathway of schaftoside. Functional studies indicated that CGTs and HIDHs identified here are bona fide responsible for the biosynthesis of schaftoside in GSO, as confirmed through hairy root transgenic system and in vitro enzyme activity assay. Taken together, the ancestral papilionoid polyploidy event expanding CHSs and HIDHs, along with the GSO-specific tandem duplication of CGT, contributes, partially if not completely, to the robust biosynthesis of schaftoside in GSO. These findings provide insights into the genomic mechanisms underlying the abundant biosynthesis of schaftoside in GSO, highlighting the potential of GSO as a source of bioactive compounds for pharmaceutical development.

The Combination of Bioactive Herbal Compounds with Biomaterials for Regenerative Medicine

Regenerative medicine aims to restore the function of diseased or damaged tissues and organs by cell therapy, gene therapy, and tissue engineering, along with the adjunctive application of bioactive molecules. Traditional bioactive molecules, such as growth factors and cytokines, have shown great potential in the regulation of cellular and tissue behavior, but have the disadvantages of limited source, high cost, short half-life, and side effects. In recent years, herbal compounds extracted from natural plants/herbs have gained increasing attention. This is not only because herbal compounds are easily obtained, inexpensive, mostly safe, and reliable, but also owing to their excellent effects, including anti-inflammatory, antibacterial, antioxidative, proangiogenic behavior and ability to promote stem cell differentiation. Such effects also play important roles in the processes related to tissue regeneration. Furthermore, the moieties of the herbal compounds can form physical or chemical bonds with the scaffolds, which contributes to improved mechanical strength and stability of the scaffolds. Thus, the incorporation of herbal compounds as bioactive molecules in biomaterials is a promising direction for future regenerative medicine applications. Herein, an overview on the use of bioactive herbal compounds combined with different biomaterial scaffolds for regenerative medicine application is presented. We first introduce the classification, structures, and properties of different herbal bioactive components and then provide a comprehensive survey on the use of bioactive herbal compounds to engineer scaffolds for tissue repair/regeneration of skin, cartilage, bone, neural, and heart tissues. Finally, we highlight the challenges and prospects for the future development of herbal scaffolds toward clinical translation. Overall, it is believed that the combination of bioactive herbal compounds with biomaterials could be a promising perspective for the next generation of regenerative medicine.

Agave angustifolia Haw. Leaves as a Potential Source of Bioactive Compounds: Extraction Optimization and Extract Characterization

The leaves of Agave angustifolia Haw. are the main agro-waste generated by the mezcal industry and are becoming an important source of bioactive compounds, such as phenolic compounds, that could be used in the food and pharmaceutical industries. Therefore, the extraction and identification of these phytochemicals would revalorize these leaf by-products. Herein, maceration and supercritical carbon dioxide (scCO2) extractions were optimized to maximize the phenolic and flavonoid contents and the antioxidant capacity of vegetal extracts of A. angustifolia Haw. In the maceration process, the optimal extraction condition was a water-ethanol mixture (63:37% v/v), which yielded a total phenolic and flavonoid content of 27.92 ± 0.90 mg EAG/g DL and 12.85 ± 0.53 µg QE/g DL, respectively, and an antioxidant capacity of 32.67 ± 0.91 (ABTS assay), 17.30 ± 0.36 (DPPH assay), and 13.92 ± 0.78 (FRAP assay) µM TE/g DL. Using supercritical extraction, the optimal conditions for polyphenol recovery were 60 °C, 320 bar, and 10% v/v. It was also observed that lower proportions of cosolvent decreased the polyphenol extraction more than pressure and temperature. In both optimized extracts, a total of 29 glycosylated flavonoid derivatives were identified using LC-ESI-QTof/MS. In addition, another eight novel compounds were identified in the supercritical extracts, showing the efficiency of the cosolvent for recovering new flavonoid derivatives.

Biosynthesis of Epipyrone A Reveals a Highly Specific Membrane-Bound Fungal C-Glycosyltransferase for Pyrone Galactosylation

Epipyrone A is a unique C-galactosylated 4-hydroxy-2-pyrone derivative with an antifungal potential from the fungus Epicoccum nigrum. We elucidated its biosynthesis via heterologous expression and characterized an unprecedented membrane-bound pyrone C-glycosyltransferase biochemically. Molecular docking and mutagenesis experiments suggested a possible mechanism for the heterocyclic C-glycosylation and the importance of a transmembrane helix for its catalysis. These results expand the repertoire of C-glycosyltransferases and provide new insights into the formation of C-glycosides in fungi.

New insights of flavonoid glycosidases and their application in food industry

Flavonoids are significant natural nutraceuticals and a key component of dietary supplements. Given that flavonoid glycosides are more plentiful in nature and less beneficial to human health than their aglycone counterparts, they serve as potential precursors for flavonoid production. Glycosidases have shown substantial potential within the food industry, particularly in enhancing the organoleptic properties of juice, wine, and tea. When applied to food resources, glycosidases can amplify their biological activities, thereby improving the performance of functional foods. This review provides up-to-date information on flavonoid glycosidases, including their catalytic mechanisms, biochemical properties, and natural sources, as well as their applications within the food industry. The use of flavonoid glycosidases in improving food quality is also reviewed.

Mechanistic insights into glycoside 3-oxidases involved in C-glycoside metabolism in soil microorganisms

C-glycosides are natural products with important biological activities but are recalcitrant to degradation. Glycoside 3-oxidases (G3Oxs) are recently identified bacterial flavo-oxidases from the glucose-methanol-coline (GMC) superfamily that catalyze the oxidation of C-glycosides with the concomitant reduction of O2 to H2O2. This oxidation is followed by C-C acid/base-assisted bond cleavage in two-step C-deglycosylation pathways. Soil and gut microorganisms have different oxidative enzymes, but the details of their catalytic mechanisms are largely unknown. Here, we report that PsG3Ox oxidizes at 50,000-fold higher specificity (kcat/Km) the glucose moiety of mangiferin to 3-keto-mangiferin than free D-glucose to 2-keto-glucose. Analysis of PsG3Ox X-ray crystal structures and PsG3Ox in complex with glucose and mangiferin, combined with mutagenesis and molecular dynamics simulations, reveal distinctive features in the topology surrounding the active site that favor catalytically competent conformational states suitable for recognition, stabilization, and oxidation of the glucose moiety of mangiferin. Furthermore, their distinction to pyranose 2-oxidases (P2Oxs) involved in wood decay and recycling is discussed from an evolutionary, structural, and functional viewpoint.

Enzymatic β-elimination in natural product O- and C-glycoside deglycosylation

Biological degradation of natural product glycosides involves, alongside hydrolysis, β-elimination for glycosidic bond cleavage. Here, we discover an O-glycoside β-eliminase (OGE) from Agrobacterium tumefaciens that converts the C3-oxidized O-β-D-glucoside of phloretin (a plant-derived flavonoid) into the aglycone and the 2-hydroxy-3-keto-glycal elimination product. While unrelated in sequence, OGE is structurally homologous to, and shows effectively the same Mn2+ active site as, the C-glycoside deglycosylating enzyme (CGE) from a human intestinal bacterium implicated in β-elimination of 3-keto C-β-D-glucosides. We show that CGE catalyzes β-elimination of 3-keto O- and C-β-D-glucosides while OGE is specific for the O-glycoside substrate. Substrate comparisons and mutagenesis for CGE uncover positioning of aglycone for protonic assistance by the enzyme as critically important for C-glycoside cleavage. Collectively, our study suggests convergent evolution of active site for β-elimination of 3-keto O-β-D-glucosides. C-Glycoside cleavage is a specialized feature of this active site which is elicited by substrate through finely tuned enzyme-aglycone interactions.

Insights into the missing apiosylation step in flavonoid apiosides biosynthesis of Leguminosae plants

Apiose is a natural pentose containing an unusual branched-chain structure. Apiosides are bioactive natural products widely present in the plant kingdom. However, little is known on the key apiosylation reaction in the biosynthetic pathways of apiosides. In this work, we discover an apiosyltransferase GuApiGT from Glycyrrhiza uralensis. GuApiGT could efficiently catalyze 2″-O-apiosylation of flavonoid glycosides, and exhibits strict selectivity towards UDP-apiose. We further solve the crystal structure of GuApiGT, determine a key sugar-binding motif (RLGSDH) through structural analysis and theoretical calculations, and obtain mutants with altered sugar selectivity through protein engineering. Moreover, we discover 121 candidate apiosyltransferase genes from Leguminosae plants, and identify the functions of 4 enzymes. Finally, we introduce GuApiGT and its upstream genes into Nicotiana benthamiana, and complete de novo biosynthesis of a series of flavonoid apiosides. This work reports an efficient phenolic apiosyltransferase, and reveals mechanisms for its sugar donor selectivity.

Flavonoids as Aglycones in Retaining Glycosidase-Catalyzed Reactions: Prospects for Green Chemistry

Flavonoids and their glycosides are abundant in many plant-based foods. The (de)glycosylation of flavonoids by retaining glycoside hydrolases has recently attracted much interest in basic and applied research, including the possibility of altering the glycosylation pattern of flavonoids. Research in this area is driven by significant differences in physicochemical, organoleptic, and bioactive properties between flavonoid aglycones and their glycosylated counterparts. While many flavonoid glycosides are present in nature at low levels, some occur in substantial quantities, making them readily available low-cost glycosyl donors for transglycosylations. Retaining glycosidases can be used to synthesize natural and novel glycosides, which serve as standards for bioactivity experiments and analyses, using flavonoid glycosides as glycosyl donors. Engineered glycosidases also prove valuable for the synthesis of flavonoid glycosides using chemically synthesized activated glycosyl donors. This review outlines the bioactivities of flavonoids and their glycosides and highlights the applications of retaining glycosidases in the context of flavonoid glycosides, acting as substrates, products, or glycosyl donors in deglycosylation or transglycosylation reactions.

Solvent Engineering for Nonpolar Substrate Glycosylation Catalyzed by the UDP-Glucose-Dependent Glycosyltransferase UGT71E5: Intensification of the Synthesis of 15-Hydroxy Cinmethylin β-d-Glucoside

Sugar nucleotide-dependent glycosyltransferases are powerful catalysts of the glycosylation of natural products and xenobiotics. The low solubility of the aglycone substrate often limits the synthetic efficiency of the transformation catalyzed. Here, we explored different approaches of solvent engineering for reaction intensification of β-glycosylation of 15HCM (a C15-hydroxylated, plant detoxification metabolite of the herbicide cinmethylin) catalyzed by safflower UGT71E5 using UDP-glucose as the donor substrate. Use of a cosolvent (DMSO, ethanol, and acetonitrile; ≤50 vol %) or a water-immiscible solvent (n-dodecane, n-heptane, n-hexane, and 1-hexene) was ineffective due to enzyme activity and stability, both impaired ≥10-fold compared to a pure aqueous solvent. Complexation in 2-hydroxypropyl-β-cyclodextrin enabled dissolution of 50 mM 15HCM while retaining the UGT71E5 activity (∼0.32 U/mg) and stability. Using UDP-glucose recycling, 15HCM was converted completely, and 15HCM β-d-glucoside was isolated in 90% yield (∼150 mg). Collectively, this study highlights the requirement for a mild, enzyme-compatible strategy for aglycone solubility enhancement in glycosyltransferase catalysis applied to glycoside synthesis.

A highly selective C-rhamnosyltransferase from Viola tricolor and insights into its mechanisms

C-Glycosides are important natural products with various bioactivities. In plant biosynthetic pathways, the C-glycosylation step is usually catalyzed by C-glycosyltransferases (CGTs), and most of them prefer to accept uridine 5'-diphosphate glucose (UDP-Glc) as sugar donor. No CGTs favoring UDP-rhamnose (UDP-Rha) as sugar donor has been reported, thus far. Herein, we report the first selective C-rhamnosyltransferase VtCGTc from the medicinal plant Viola tricolor. VtCGTc could efficiently catalyze C-rhamnosylation of 2-hydroxynaringenin 3-C-glucoside, and exhibited high selectivity towards UDP-Rha. Mechanisms for the sugar donor selectivity of VtCGTc were investigated by molecular dynamics (MD) simulations and molecular mechanics with generalized Born and surface area solvation (MM/GBSA) binding free energy calculations. Val144 played a vital role in recognizing UDP-Rha, and the V144T mutant could efficiently utilize UDP-Glc. This work provides a new and efficient approach to prepare flavonoid C-rhamnosides such as violanthin and iso-violanthin.

Reaction intensification for biocatalytic production of polyphenolic natural product di-C-β-glucosides

Polyphenolic aglycones featuring two sugars individually attached via C-glycosidic linkage (di-C-glycosides) represent a rare class of plant natural products with unique physicochemical properties and biological activities. Natural scarcity of such di-C-glycosides limits their use-inspired exploration as pharmaceutical ingredients. Here, we show a biocatalytic process technology for reaction-intensified production of the di-C-β-glucosides of two representative phenol substrates, phloretin (a natural flavonoid) and phenyl-trihydroxyacetophenone (a phenolic synthon for synthesis), from sucrose. The synthesis proceeds via an iterative two-fold C-glycosylation of the respective aglycone, supplied as inclusion complex with 2-hydroxypropyl β-cyclodextrin for enhanced water solubility of up to 50 mmol/L, catalyzed by a kumquat di-C-glycosyltransferase (di-CGT), and it uses UDP-Glc provided in situ from sucrose by a soybean sucrose synthase, with catalytic amounts (≤3 mol%) of UDP added. Time course analysis reveals the second C-glycosylation as rate-limiting (0.4-0.5 mmol/L/min) for the di-C-glucoside production. With internal supply from sucrose keeping the UDP-Glc at a constant steady-state concentration (≥50% of the UDP added) during the reaction, the di-C-glycosylation is driven to completion (≥95% yield). Contrary to the mono-C-glucoside intermediate which is stable, the di-C-glucoside requires the addition of reducing agent (10 mmol/L 2-mercaptoethanol) to prevent its decomposition during the synthesis. Both di-C-glucosides are isolated from the reaction mixtures in excellent purity (≥95%), and their expected structures are confirmed by NMR. Collectively, this study demonstrates efficient glycosyltransferase cascade reaction for flexible use in natural product di-C-β-glucoside synthesis from expedient substrates.

Identification of a flavonoid C-glycosyltransferase from fern species Stenoloma chusanum and the application in synthesizing flavonoid C-glycosides in Escherichia coli

BACKGROUND

Flavonoid C-glycosides have many beneficial effects and are widely used in food and medicine. However, plants contain a limited number of flavonoid C-glycosides, and it is challenging to create these substances chemically.

RESULTS

To screen more robust C-glycosyltransferases (CGTs) for the biosynthesis of flavonoid C-glycosides, one CGT enzyme from Stenoloma chusanum (ScCGT1) was characterized. Biochemical analyses revealed that ScCGT1 showed the C-glycosylation activity for phloretin, 2-hydroxynaringenin, and 2-hydroxyeriodictyol. Structure modeling and mutagenesis experiments indicated that the glycosylation of ScCGT1 may be initiated by the synergistic action of conserved residue His26 and Asp14. The P164T mutation increased C-glycosylation activity by forming a hydrogen bond with the sugar donor. Furthermore, when using phloretin as a substrate, the extracellular nothofagin production obtained from the Escherichia coli strain ScCGT1-P164T reached 38 mg/L, which was 2.3-fold higher than that of the wild-type strain. Finally, it is proved that the coupling catalysis of CjFNS I/F2H and ScCGT1-P164T could convert naringenin into vitexin and isovitexin.

CONCLUSION

This is the first time that C-glycosyltransferase has been characterized from fern species and provides a candidate gene and strategy for the efficient production of bioactive C-glycosides using enzyme catalysis and metabolic engineering.

Functional characterization of a C-glycosyltransferase from Pueraria lobata with dual-substrate selectivity

We reported a C-glycosyltransferase PlCGT with dual-substrate selectivity. An Asn16–Asp124 dyad may mediate the SN2-like mechanism in the C-glycosylation.