Advances and Challenges in Enzymatic C-glycosylation of Flavonoids in Plants

Re-evaluation of the C-Glucosyltransferase IroB Illuminates Its Ability to C-Glucosylate Non-native Triscatecholate Enterobactin Mimics

The pathogen-associated C-glucosyltransferase IroB is involved in the biosynthesis of salmochelins, C-glucosylated derivatives of enterobactin (Ent), which is a triscatecholate siderophore of enteric bacteria including Salmonella enterica and Escherichia coli. Here, we reassess the ability of IroB to C-glucosylate non-native triscatecholate mimics of Ent, which may have utility in the design and development of siderophore-based therapeutics and diagnostics. We establish TRENCAM (TC) and MECAM (MC), synthetic Ent analogs with tris(2-aminoethyl)amine- or mesitylene-derived backbones replacing the trilactone core of Ent, respectively, and their monoglucosylated congeners as substrates of IroB. Time course analyses and steady-state kinetic studies, which were performed under conditions that provide enhanced activity relative to prior studies, inform the substrate selectivity and catalytic efficiencies of this enzyme. We extend these findings to the preparation of a siderophore-antibiotic conjugate composed of monoglucosylated TC and ampicillin (MGT-Amp). Examination of its antibacterial activity and receptor specificity demonstrates that MGT-Amp targets pathogenicity because it shows specificty for the pathogen-associated outer membrane receptor IroN. Overall, our findings extend the biochemical characterization of IroB and its substrate scope and illustrate the ability to leverage a bacterial C-glucosyltransferase for non-native chemoenzymatic transformations along with potential applications of salmochelin mimics.

Mining and Engineering the Di-O-glycosylation Pattern of UGT72B1 for the Highly Efficient O-Glycosylation of Endogenous Quercetin

Compared with mono-O-glycosylation, di-O-glycosylation endows the precursor with better performance. However, the mining and engineering of di-O-glycosylation patterns of glycosyltransferases are limited, hindering their synthetic applications. Here, an Arabidopsis xenobiotic-transforming glycosyltransferase, UGT72B1, was found to catalyze the glycosylation of endogenous quercetin and its monoglycosides, generating di-O-glucosides. Mutating M17/G18/Y315 into L/T/Q in UGT72B1 altered its regioselectivity toward quercetin 7-O-glucoside, enzymatically generating another 3,7-di-O-glycoside with up to a 100% conversion rate, and increased the sugar donor preference. Altering the regiospecificity of glycosyltransferases likely required coordination between the entrance and the active site, where the orientations of the sugar acceptors and donors shift to adopt a lower binding energy state. Moreover, quercetin 3,4'-di-O-β-d-glucoside and quercetin 3,7-di-O-β-d-glucoside synthesized were found to have the highest anti-inflammatory activities. Overall, this work presents an efficient strategy to engineer glycosylation patterns for the synthesis of quercetin di-O-β-d-glucosides to be used as food additives, therapeutics, and nutraceuticals.

Advances in glycosyltransferase-mediated glycodiversification of small molecules

Currently, numerous glycosides have been synthesized and used in clinical applications, neutraceuticals, cosmetics, and food processing. Structurally, a glycoside is composed of aglycone attaching to one or several sugar moieties so-called glycone. It is found that biochemical or biopharmaceutical properties of glycoside are mainly determined by its sugar part and thereby alternation of this glycone resulting in novel structure and characteristics as well. The use of traditional production methods of glycosides such as direct extraction and purification from plants, animals, or microorganisms is very challenging (laborious, time-consuming, technique, high price, low yield, etc.). Alternatively, the use of enzymatic methods for the biosynthesis of glycosides has become a highly promising tool. Particularly, the diverse structure of glycosides can be obtained using the promiscuous catalytic activity of glycosyltransferases (GT) mined from bioresources (plants, fungi, microorganisms, etc.). In addition, the exploration of GT catalytic promiscuity toward diverse aglycones, and glycones has indeed been interesting and played a key role in the production of novel glycosides. This review described the recent advances in glycosyltransferase-mediated glycodiversification of small molecules (flavonoids, steroids, terpenoids, etc.). Mostly, references were collected from 2014 to 2023.

Flavonoids and ischemic stroke-induced neuroinflammation: Focus on the glial cells

Ischemic stroke is one of the most cases worldwide, with high rate of morbidity and mortality. In the pathological process of ischemic stroke, neuroinflammation is an essential process that defines the functional prognosis. After stroke onset, microglia, astrocytes and the infiltrating immune cells contribute to a complicated neuroinflammation cascade and play the complicated roles in the pathophysiological variations of ischemic stroke. Both microglia and astrocytes undergo both morphological and functional changes, thereby deeply participate in the neuronal inflammation via releasing pro-inflammatory or anti-inflammatory factors. Flavonoids are plant-specific secondary metabolites and can protect against cerebral ischemia injury via modulating the inflammatory responses. For instances, quercetin can inhibit the expression and release of pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-α, IL-6 and IL-1β, in the cerebral nervous system (CNS). Apigenin and rutin can promote the polarization of microglia to anti-inflammatory genotype and then inhibit neuroinflammation. In this review, we focused on the dual roles of activated microglia and reactive astrocyte in the neuroinflammation following ischemic stroke and discussed the anti-neuroinflammation of some flavonoids. Importantly, we aimed to reveal the new strategies for alleviating the cerebral ischemic stroke.

Efficient production of the glycosylated derivatives of baicalein in engineered Escherichia coli

Baicalein-7-O-glucoside and baicalein-7-O-rhamnoside have been proven to possess many pharmacological activities and are potential candidate drug leads and herb supplements. However, their further development is largely limited due to low content in host plants. Few studies reported that both bioactive plant components are prepared through the bioconversion of baicalein that is considered as the common biosynthetic precursor of both compounds. Herein, we constructed a series of the engineered whole-cell bioconversion systems in which the deletion of competitive genes and the introduction of exogenous UDP-glucose supply pathway, glucosyltransferase, rhamnosyltransferase, and the UDP-rhamnose synthesis pathway are made. Using these engineered strains, the precursor baicalein is able to be transformed into baicalein-7-O-glucoside and baicalein-7-O-rhamnoside, with high-titer production, respectively. The further optimization of fermentation conditions led to the final production of 568.8 mg/L and 877.0 mg/L for baicalein-7-O-glucoside and baicalein-7-O-rhamnoside, respectively. To the best of our knowledge, it is the highest production in preparation of baicalein-7-O-glucoside from baicalein so far, while the preparation of baicalein-7-O-rhamnoside is the first reported via bioconversion approach. Our study provides a reference for the industrial production of high-value products baicalein-7-O-glucoside and baicalein-7-O-rhamnoside using engineered E. coli. KEY POINTS: • Integrated design for improving the intracellular UDP-glucose pool • High production of rare baicalein glycosides in the engineered E. coli • Baicalein-7-O-glucoside and baicalein-7-O-rhamnoside.

The (Poly)phenol-Carbohydrate Combination for Diabetes: Where Do We Stand?

The type 2 diabetes epidemic is real and hardly coming to an end in the upcoming years. The efforts of the scientific community to develop safer and more effective compounds for type 2 diabetes based on the structure of natural (poly)phenols are remarkable and have indeed proven worthwhile after the introduction of gliflozins in clinical practice. However, low-quality reports on the antidiabetic potential of plant-derived lipophilic (poly)phenols continue to pile up in the literature. Many of these compounds continue to be published as promising functional nutrients and antidiabetic pharmaceutical leads without consideration of their Pan-Assay Interference Compounds (PAINS) profile. This evidence-based opinion article conveys the authors' perspectives on the natural (poly)phenol artillery as a valuable and reliable source of bioactive compounds for diabetes. Ultimately, in light of the already established membrane-perturbing behavior of lipophilic (poly)phenols, together with the multiple benefits that may come with the introduction of a C-glucosyl moiety in bioactive compounds, we aim to raise awareness of the importance of contemplating the shift to (poly)phenol-carbohydrate combinations in the development of functional nutrients, as well as in the early stages of antidiabetic drug discovery.

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.