Preparatory production of quercetin-3-β-D-glucopyranoside using alkali-tolerant thermostable α-L-rhamnosidase from Aspergillus terreus

Combinatorial metabolic engineering of Bacillus subtilis enables the efficient biosynthesis of isoquercitrin from quercetin

BACKGROUND

Isoquercitrin (quercetin-3-O-β-D-glucopyranoside) has exhibited promising therapeutic potentials as cardioprotective, anti-diabetic, anti-cancer, and anti-viral agents. However, its structural complexity and limited natural abundance make both bulk chemical synthesis and extraction from medical plants difficult. Microbial biotransformation through heterologous expression of glycosyltransferases offers a safe and sustainable route for its production. Despite several attempts reported in microbial hosts, the current production levels of isoquercitrin still lag behind industrial standards.

RESULTS

Herein, the heterologous expression of glycosyltransferase UGT78D2 gene in Bacillus subtilis 168 and reconstruction of UDP-glucose (UDP-Glc) synthesis pathway led to the synthesis of isoquercitrin from quercetin with titers of 0.37 g/L and 0.42 g/L, respectively. Subsequently, the quercetin catabolism blocked by disruption of a quercetin dioxygenase, three ring-cleavage dioxygenases, and seven oxidoreductases increased the isoquercitrin titer to 1.64 g/L. And the hydrolysis of isoquercitrin was eliminated by three β-glucosidase genes disruption, thereby affording 3.58 g/L isoquercitrin. Furthermore, UDP-Glc pool boosted by pgi (encoding glucose-6-phosphate isomerase) disruption increased the isoquercitrin titer to 10.6 g/L with the yield on quercetin of 72% and to 35.6 g/L with the yield on quercetin of 77.2% in a 1.3-L fermentor.

CONCLUSION

The engineered B. subtilis strain developed here holds great potential for initiating the sustainable and large-scale industrial production of isoquercitrin. The strategies proposed in this study provides a reference to improve the production of other flavonoid glycosides by engineered B. subtilis cell factories.

Effect of hot-melt extruded Morus alba leaves on intestinal microflora and epithelial cells

Although rutin and isoquercitrin have many effects, they are insoluble substances, making it difficult to obtain pure substances. This study was to investigate whether Morus alba leaves containing rutin and isoquercitrin could improve intestinal health by making a sustained-release formulation through a hot-melt extrusion (HME) process with improved stability and solubility and determine whether it could upregulate the balance of intestinal microorganisms and intestinal epithelial cells. A sustained-release formulation was prepared by the HME process using Morus alba leaves and a hydrophilic polymer matrix. Antibacterial activities of pathogenic microorganisms (Escherichia coli, Streptococcus aureus, Enterococcus faecalis) and proliferative effect of probiotics (Lactobacillus rhamnosus, Pediococcus pentosaceus) were tested against intestinal microorganisms. Regarding intestinal epithelial cells, a co-culture model of Caco-2 cells and RAW 264.7 cells was used. It was confirmed that the extrudate exhibited high antibacterial activities against pathogenic microorganisms and affected the proliferation of probiotics. Furthermore, after inducing inflammation through LPS, it recovered transepithelial electrical resistance-increased levels of tight junction proteins and decreased expression levels of pro-inflammatory cytokines. HME of Morus alba leaves containing rutin and isoquercitrin can upregulate intestinal microbial balance and intestinal epithelial cells.

Rutinosidase and other diglycosidases: Rising stars in biotechnology

Diglycosidases are a special class of glycosidases (EC 3.2.1) that catalyze the separation of intact disaccharide moieties from the aglycone part. The main diglycosidase representatives comprise rutinosidases that cleave rutinose (α-l-Rha-(1-6)-β-d-Glc) from rutin or other rutinosides, and (iso)primeverosidases processing (iso)primeverosides (d-Xyl-(1-6)-β-d-Glc), but other activities are known. Notably, some diglycosidases may be ranked as monoglucosidases with enlarged substrate specificity. Diglycosidases are found in various microorganisms and plants. Diglycosidases are used in the food industry for aroma enhancement and flavor modification. Besides their hydrolytic activity, they also possess pronounced synthetic (transglycosylating) capabilities. Recently, they have been demonstrated to glycosylate various substrates in a high yield, including peculiar species like inorganic azide or carboxylic acids, which is a unique feature in biocatalysis. Rhamnose-containing compounds such as rutinose are currently receiving increased attention due to their proven activity in anti-cancer and dermatological experimental studies. This review demonstrates the vast and yet underrated biotechnological potential of diglycosidases from various sources (plant, microbial), and reveals perspectives on the use of these catalysts as well as of their products in biotechnology.

α-L-rhamnosidase: production, properties, and applications

α-L-rhamnosidase [EC 3.2.1.40] belongs to glycoside hydrolase (GH) families (GH13, GH78, and GH106 families) in the carbohydrate-active enzymes (CAZy) database, which specifically hydrolyzes the non-reducing end of α-L-rhamnose. Αccording to the sites of catalytic hydrolysis, α-L-rhamnosidase can be divided into α-1, 2-rhamnosidase, α-1, 3-rhamnosidase, α-1, 4-rhamnosidase and α-1, 6-rhamnosidase. α-L-rhamnosidase is an important enzyme for various biotechnological applications, especially in food, beverage, and pharmaceutical industries. α-L-rhamnosidase has a wide range of sources and is commonly found in animals, plants, and microorganisms, and its microbial source includes a variety of bacteria, molds and yeasts (such as Lactobacillus sp., Aspergillus sp., Pichia angusta and Saccharomyces cerevisiae). In recent years, a series of advances have been achieved in various aspects of α-validates the above-described-rhamnosidase research. A number of α-L-rhamnosidases have been successfully recombinant expressed in prokaryotic systems as well as eukaryotic systems which involve Pichia pastoris, Saccharomyces cerevisiae and Aspergillus niger, and the catalytic properties of the recombinant enzymes have been improved by enzyme modification techniques. In this review, the sources and production methods, general and catalytic properties and biotechnological applications of α-L-rhamnosidase in different fields are summarized and discussed, concluding with the directions for further in-depth research on α-L-rhamnosidase.

In silico and in vitro studies of potential inhibitors against Dengue viral protein NS5 Methyl Transferase from Ginseng and Notoginseng

Background and aim

Dengue is a potentially deadly tropical infectious disease transmitted by mosquito vector Aedes aegypti with no antiviral drug available to date. Dengue NS5 protein is crucial for viral replication and is the most conserved among all four Dengue serotypes, making it an attractive drug target. Both Ginseng and Notoginseng extracts and isolates have been shown to be effective against various viral infections yet against Dengue Virus is understudied. We aim to identify potential inhibitors against Dengue NS5 Methyl transferase from small molecular compounds found in Ginseng and Notoginseng.

Experimental procedure

A molecular docking model of Dengue NS5 Methyl transferase (MTase) domain was tested with decoys and then used to screen 91 small molecular compounds found in Ginseng and Notoginseng followed by Molecular dynamics simulations and the per-residue free energy decompositions based on molecular mechanics/Poisson-Boltzmann (generalised Born) surface area (MM/PB(GB)SA) calculations of the hit. ADME predictions and drug-likeness analyses were discussed to evaluate the viability of the hit as a drug candidate. To confirm our findings, in vitro studies of antiviral activities against RNA and a E protein synthesis and cell toxicity were carried out.

Results and conclusion

The virtual screening resulted in Isoquercitrin as a single hit. Further analyses of the Isoquercitrin-MTase complex show that Isoquercitrin can reside within both of the NS5 Methyl Transferase active sites; the AdoMet binding site and the RNA capping site. The Isoquercitrin is safe for consumption and accessible on multikilogram scale. In vitro studies showed that Isoquercitrin can inhibit Dengue virus by reducing viral RNA and viral protein synthesis with low toxicity to cells (CC₅₀ > 20 μM). Our work provides evidence that Isoquercitrin can serve as an inhibitor of Dengue NS5 protein at the Methyl Transferase domain, further supporting its role as an anti-DENV agent.

Purification and Characterization of a Novel α-L-Rhamnosidase from Papiliotrema laurentii ZJU-L07 and Its Application in Production of Icariin from Epimedin C

Icariin is the most effective bioactive compound in Herba Epimedii. To enhance the content of icariin in the epimedium water extract, a novel strain, Papiliotrema laurentii ZJU-L07, producing an intracellular α-L-rhamnosidase was isolated from the soil and mutagenized. The specific activity of α-L-rhamnosidase was 29.89 U·mg⁻¹ through purification, and the molecular mass of the enzyme was 100 kDa, as assayed by SDS-PAGE. The characterization of the purified enzyme was determined. The optimal temperature and pH were 55 °C and 7.0, respectively. The enzyme was stable in the pH range 5.5–9.0 for 2 h over 80% and the temperature range 30–40 °C for 2 h more than 70%. The enzyme activity was inhibited by Ca²⁺, Fe²⁺, Cu²⁺, and Mg²⁺, especially Fe²⁺. The kinetic parameters of K_m and V_max were 1.38 mM and 24.64 μmol·mg⁻¹·min⁻¹ using pNPR as the substrate, respectively. When epimedin C was used as a nature substrate to determine the kinetic parameters of α-L-rhamnosidase, the values of K_m and V_max were 3.28 mM and 0.01 μmol·mg⁻¹·min⁻¹, respectively. The conditions of enzymatic hydrolysis were optimized through single factor experiments and response surface methodology. The icariin yield increased from 61% to over 83% after optimization. The enzymatic hydrolysis method could be used for the industrialized production of icariin. At the same time, this enzyme could also cleave the α-1,2 glycosidic linkage between glucoside and rhamnoside in naringin and neohesperidin, which could be applicable in other biotechnological processes.

Antioxidative effect of dietary flavonoid isoquercitrin on human ovarian granulosa cells HGL5 in vitro

This study aimed to examine the effect of dietary flavonoid isoquercitrin on ovarian granulosa cells using the immortalized human cell line HGL5. Cell viability, survival, apoptosis, release of steroid hormones 17beta-estradiol and progesterone, and human transforming growth factor-beta2 (TGF-beta2) and TGF-beta2 receptor as well as intracellular reactive oxygen species (ROS) generation were investigated after isoquercitrin treatment at the concentration range of 5-100 microg.ml-1. It did not cause any significant change (p>0.05) in cell viability as studied by AlamarBlue assay in comparison to control. No significant change was observed (p>0.05) in the proportion of live, dead and apoptotic cells as revealed by apoptotic assay using flow cytometry. Similarly, the release of 17beta-estradiol, progesterone, TGF-beta2 and its receptor were not affected significantly (p>0.05) by isoquercitrin as detected by ELISA, in comparison to control. Except for the highest concentration of 100 microg.ml-1, which led to oxidative stress, isoquercitrin exhibited antioxidative activity at lower concentration used in the study (5, 10, 25, and 50 microg.ml-1) by hampering the production of intracellular ROS, in comparison to control, as detected by chemiluminescence assay (p<0.05). Findings of the present study indicate an existence of the antioxidative pathway that involves inhibition of intracellular ROS generation by isoquercitrin in human ovarian granulosa cells.

Screening and characterization of a GH78 α-l-rhamnosidase from Aspergillus terreus and its application in the bioconversion of icariin to icaritin with recombinant β-glucosidase

In this study, a GH78 α-L-rhamnosidase AtRha from Aspergillus terreus CCF3059 was screened and expressed in Pichia pastoris KM71H. The maximum enzyme activity of AtRha was 1000 U/mL after 12 days. AtRha was most active at 65 °C and pH 6.5, displaying excellent thermal stability and pH stability. The kinetic parameters Km, Vmax, kcat and kcat/Km values for pNPR were 0.481 mM, 659 μmol/min·mg, 1065 s-1 and 2214 s-1mM-1, respectively. AtRha could be inhibited by Fe2+, Hg2+ and Cu2+. Moreover, it displayed good tolerance to organic reagents with 52.6% activity in 15%(w/v) methanol. AtRha can hydrolyze icariin containing the α-1 rhamnoside linkage. Furthermore, AtRha and β-glucosidase TthBg3 showed excellent selectivity to cleave the rhamnose at the 3rd position and the glucosyl at the C-7 group of icariin, which established an effective and green method to produce the more pharmacological active icaritin. In addition, the optimal enzyme addition schemes and the reaction conditions were screened and optimized. After a two-stage transformation under optimized conditions, 0.5 g/L of icariin was transformed into 0.25 g/L of icaritin, with a corresponding molar conversion rate of 91.2%. Our findings provide a new, specific and cost-effective method for the production of icaritin in the industry.

Two fungal flavonoid-specific glucosidases/rutinosidases for rutin hydrolysis and rutinoside synthesis under homogeneous and heterogeneous reaction conditions

The glycosidases within GH5-23 cleave the glycosidic bond of β-glucosylated or rutinosylated flavonoids. Moreover, by virtue of their transglycosylation activity, glycoconjugates with glucosyl and rutinosyl moieties are accessible. Here we report the biochemical characterization and biotechnological assessment of two heterologously expressed members of GH5-23—McGlc from Mucor circinelloides and PcGlc from Penicillium chrysogenum. Both enzymes exhibited the highest hydrolytic activities with quercetin-3-β-O-glucopyranoside, whereas lower specificity constants were determined with the rutinosides narcissin, rutin and hesperidin. High stabilities against thermal, ethanol and dimethyl sulfoxide-induced inactivation, a very limited secondary hydrolysis of the formed transglycosylation products, and no detectable product inhibition were additional features appropriate for biotechnological applications. The enzymes were compared in their efficiencies to hydrolyze rutin and to synthesize 2-phenylethyl rutinoside under homogeneous and heterogeneous reaction conditions using high rutin concentrations of 100 and 300 mM. Highest transglycosylation efficiencies were achieved with fully dissolved rutin in reaction mixtures containing 25% dimethyl sulfoxide. Molecular docking and multiple sequence alignments suggest that the hydrophobic environment of aromatic residues within the + 1 subsite of GH5-23 glycosidases is very important for the binding of flavonoid glucosides and rutinosides.

Immobilization of GH78 α-L-Rhamnosidase from Thermotoga petrophilea with High-Temperature-Resistant Magnetic Particles Fe3O4-SiO2-NH2-Cellu-ZIF8 and Its Application in the Production of Prunin Form Naringin

To efficiently recycle GH78 thermostable rhamnosidase (TpeRha) and easily separate it from the reaction mixture and furtherly improve the enzyme properties, the magnetic particle Fe₃O₄-SiO₂-NH₂-Cellu-ZIF8 (FSNcZ8) was prepared by modifying Fe₃O₄-NH₂ with tetraethyl silicate (TEOS), microcrystalline cellulose and zinc nitrate hexahydrate. FSNcZ8 displayed better magnetic stability and higher-temperature stability than unmodified Fe₃O₄-NH₂ (FN), and it was used to adsorb and immobilize TpeRha from Thermotoga petrophilea 13995. As for properties, FSNcZ8-TpeRha showed optimal reaction temperature and pH of 90°C and 5.0, while its highest activity approached 714 U/g. In addition, FSNcZ8-TpeRha had better higher-temperature stability than FN. After incubation at 80°C for 3 h, the residual enzyme activities of FSNcZ8-TpeRha, FN-TpeRha and free enzyme were 93.5%, 63.32%, and 62.77%, respectively. The organic solvent tolerance and the monosaccharides tolerance of FSNcZ8-TpeRha, compared with free TpeRha, were greatly improved. Using naringin (1 mmol/l) as the substrate, the optimal conversion conditions were as follows: FSNcZ8-TpeRha concentration was 6 U/ml; induction temperature was 80°C; the pH was 5.5; induction time was 30 min, and the yield of products was the same as free enzyme. After repeating the reaction 10 times, the conversion of naringin remained above 80%, showing great improvement of the catalytic efficiency and repeated utilization of the immobilized α-L-rhamnosidase.

Characterization of a novel strain of Aspergillus aculeatinus: From rhamnogalacturonan type I pectin degradation to improvement of fruit juice filtration

Aspergillus spp. are well-known producers of pectinases commonly used in the industry. Aspergillus aculeatinus is a recently identified species but poorly characterized. This study aimed at giving a comprehensive characterization of the enzymatic potential of the O822 strain to produce Rhamnogalacturonan type I (RGI)-degrading enzymes. Proteomic analysis identified cell wall degrading enzymes (cellulases, hemicellulases, and pectinases) that accounted for 92 % of total secreted proteins. Twelve out of fifty proteins were identified as RGI-degrading enzymes. NMR and enzymatic assays revealed high levels of arabinofuranosidase, arabinanase, galactanase, rhamnogalacturonan hydrolases and rhamnogalacturonan acetylesterase activities in aqueous extracts. Viscosity assays carried out with RGI-rich camelina mucilage confirmed the efficiency of enzymes secreted by O822 to hydrolyze RGI, by decreasing viscosity by 70 %. Apple juice trials carried out at laboratory and pilot scale showed an increase in filtration flow rate and yield, paving the way for an industrial use of enzymes derived from A. aculeatinus.

pH-promoted O-α-glucosylation of flavonoids using an engineered α-glucosidase mutant

Retaining glycosidase mutants lacking its general acid/base catalytic residue are originally termed thioglycoligases which synthesize thio-linked disaccharides using sugar acceptor bearing a nucleophilic thiol group. A few thioglycoligases derived from retaining α-glycosidases have been classified into a new class of catalysts, O-glycoligases which transfer sugar moiety to a hydroxy group of sugar acceptors, resulting in the formation of O-linked glycosides or oligosaccharides. In this study, an efficient O-α-glucosylation of flavonoids was developed using an O-α-glycoligase derived from a thermostable α-glucosidase from Sulfolobus solfataricus (MalA-D416A). The O-glycoligase exhibited efficient transglycosylation activity with a broad substrate spectrum for all kinds of tested flavonoids including flavone, flavonol, flavanone, flavanonol, flavanol and isoflavone classes in yields of higher than 90%. The glucosylation by MalA-D416A preferred alkaline conditions, suggesting that pH-promoted deprotonation of hydroxyl groups of the flavonoids would accelerate turnover of covalent enzyme intermediate via transglucosylation. More importantly, the glucosylation of flavonoids by MalA-D416A was exclusively regioselective, resulting in the synthesis of flavonoid 7-O-α-glucosides as the sole product. Kinetic analysis and molecular dynamics simulations provided insights into the acceptor specificity and the regiospecificity of O-α-glucosylation by MalA-D416A. This pH promoted transglycosylation using O-α-glycoligases may prove to be a general synthesis route to flavonoid O-α-glycosides.

Flavonoids, terpenoids, and polyketide antibiotics: Role of glycosylation and biocatalytic tactics in engineering glycosylation

Flavonoids, terpenoids, and polyketides are structurally diverse secondary metabolites used widely as pharmaceuticals and nutraceuticals. Most of these molecules exist in nature as glycosides, in which sugar residues act as a decisive factor in their architectural complexity and bioactivity. Engineering glycosylation through selective trimming or extension of the sugar residues in these molecules is a prerequisite to their commercial production as well to creating novel derivatives with specialized functions. Traditional chemical glycosylation methods are tedious and can offer only limited end-product diversity. New in vitro and in vivo biocatalytic tools have emerged as outstanding platforms for engineering glycosylation in these three classes of secondary metabolites to create a large repertoire of versatile glycoprofiles. As knowledge has increased about secondary metabolite-associated promiscuous glycosyltransferases and sugar biosynthetic machinery, along with phenomenal progress in combinatorial biosynthesis, reliable industrial production of unnatural secondary metabolites has gained momentum in recent years. This review highlights the significant role of sugar residues in naturally occurring flavonoids, terpenoids, and polyketide antibiotics. General biocatalytic tools used to alter the identity and pattern of sugar molecules are described, followed by a detailed illustration of diverse strategies used in the past decade to engineer glycosylation of these valuable metabolites, exemplified with commercialized products and patents. By addressing the challenges involved in current bio catalytic methods and considering the perspectives portrayed in this review, exceptional drugs, flavors, and aromas from these small molecules could come to dominate the natural-product industry.

Efficient bioconversion of epimedin C to icariin by a glycosidase from Aspergillus nidulans

Herba Epimedii is a traditional Chinese herbal medicine that contains a mixture of bioactive flavonoid glycosides. Among them, icariin has the most outstanding bioactive functions, while epimedin C exhibits substantial toxicity. A recombinant α-L-rhamnosidase (^synAnRhaE) from Aspergillus nidulans was expressed in Escherichia coli to promote the efficient bioconversion of epimedin C to icariin. A hydrolase activity of 574.5 U L^-1 was acquired via optimized fed-batch fermentation in a 5-L bioreactor. The enzyme proved to be stable in an acidulous pH range below 55 °C with an optimal pH of 4.5 and optimal temperature of 55 °C. Epimedin C (1 g L^-1) was 100% converted to icariin within 90 min using recombinant cells. The resting cells proved to be selective for epimedin C and 2″-O-rhamnosylicariside II in crude extracts of the epimedium plant. This work provides an original and efficient biocatalyst system that can be applied in industrialized production of icariin.

Preparation of isoquercitrin by biotransformation of rutin using α-L-rhamnosidase from Aspergillus niger JMU-TS528 and HSCCC purification

Isoquercitrin is a flavonoid with important applications in the pharmaceutical and nutraceutical industries. However, a low yield and high production cost hinders the industrial preparation of isoquercitrin. In the present study, isoquercitrin was prepared by biotransformation of rutin using α-L-rhamnosidase from Aspergillus niger JMU-TS528 combined with high-speed countercurrent chromatography (HSCCC) purification. As a result, the optimum transformation pH, temperature, and time were pH 4.0, 60 °C, and 60 min, respectively. The K_m and V_max were 0.36 mM and 0.460 mmol/min, respectively. For isoquercitrin production, the optimal rutin concentration and transformation time were approximately 1000 mg/L and 60 min. The rutin transformation rate reached 96.44%. The isoquercitrin was purified to a purity of 97.83% using one-step purification with HSCCC. The isoquercitrin was identified using UPLC-Q-TOF-MS. The comprehensive results indicated that the biotransformation procedure using the α-L-rhamnosidase from A. niger JMU-TS528 combined with HSCCC was a simple and effective process to prepare isoquercitrin, which might facilitate the production of isoquercitrin for industrial use.

Natural Product Glycosylation: Biocatalytic Synthesis of Quercetin-3,4'-O-diglucoside

Flavonoids have gained much attention for their proposed positive effects for human health. Glycosylation is a significant method for the structural modification of various flavanols, resulting in glycosides with increased solubility, stability, and bioavailability compared with the corresponding aglycone. Natural product glycosylation by using enzymes has emerged as a topic of interest as it offers a sustainable and economical alternative source so as to address supply scalability limitations associated with plant-based production. Quercetin-3,4'-O-diglucoside, as one of the major but trace bioactive flavonoids in onion (Allium cepa), is superior or at least equal to quercetin aglycone in its bioavailability. In the present study, the onion-derived enzyme, UGT73G1, coupled with sucrose synthase, StSUS1, from Solanum tuberosum formed a circulatory system to produce quercetin-3,4'-O-diglucoside from quercetin, which preferred sucrose as a sugar donor and quercetin as a sugar acceptor. The optimal conditions were determined in order to increase the production of quercetin-3,4'-O-diglucoside. The maximum concentration of quercetin-3,4'-O-diglucoside achieved in a 10-mL reaction was 427.11 mg/L, from the conversion of 1 g/L of quercetin for 16 h at 40 °C and pH 7.2.

In vitro response of human ovarian cancer cells to dietary bioflavonoid isoquercitrin

Isoquercitrin is a dietary bioflavonoid used as a food supplement. We studied the mechanism underlying its effect in human ovarian cancer cells using OVCAR-3 cell line. Viability, survival, apoptosis, release of human transforming growth factor-β1 (TGF-β1) and TGF-β1 receptor, and intracellular reactive oxygen species (ROS) generation by OVCAR-3 cells were examined after isoquercitrin treatment at concentrations 5, 10, 25, 50, and 100 μg mL^-1. AlamarBlue assay revealed that isoquercitrin did not cause any significant change (P > 0.05) in cell viability as compared to control. Apoptotic assay using flow cytometry did not find any significant change (P > 0.05) in the proportion of live, dead and apoptotic cells as compared to control. ELISA also showed that the release of human TGF-β1 and TGF-β1 receptor were not significantly (P > 0.05) affected by isoquercitrin as compared to control. Chemiluminescence assay demonstrated that lower concentrations (5, 10, and 25 μg mL^-1) were able to exhibit beneficial effects by inhibiting the generation of intracellular ROS. In contrast, elevated concentrations of 50 and 100 μg mL^-1 led to oxidative stress (P < 0.05). We concluded that the beneficial effect of isoquercitrin on ovarian cancer cells may be mediated by an antioxidative pathway that involves inhibition of intracellular ROS generation, thereby limiting oxidative stress.

A spectrophotometric method for high-throughput screening of α-l-rhamnosidase activity on rutin coupled with a β-d-glucosidase assay

α-l-Rhamnosidase may biotransform rutin into isoquercetin with better bioavailability and bioactivity. To date, the high-throughput screening for the activity of α-l-rhamnosidases on rutin could not be achieved. Herein, based on the spectral differences between rutin and its aglycone quercetin in alkaline pH 10.0, we have developed a novel and simple spectrophotometric method for high-throughput screening of α-l-rhamnosidase activity on rutin by combining with a highly active β-d-glucosidase. Quercetin showed the maximum absorbance at 320 nm in alkaline pH 10.0, and could be considered as the characteristic peak of quercetin because rutin had low absorption at 320 nm. Meanwhile, rutin exhibited the maximum absorption at 400 nm and quercetin showed low absorption at 400 nm in pH 10.0. With this novel spectrophotometric method, the relative abilities of nine different α-l-rhamnosidases on rutin had been evaluated by monitoring the absorption values of the reaction mixture in alkaline pH 10.0 at 320 nm and 400 nm, and the trend in the activity on rutin was consistent with that obtained by HPLC. Moreover, the library from site-directed saturation mutagenesis at the residue Val338 in the α-l-rhamnosidase BtRha78A from Bacteroides thetaiotaomicron was constructed for high-throughput screening by this novel spectrophotometric method, and the mutant V338S with improved activity on rutin was obtained. The conversion rate of the mutant V338S on rutin increased by 21.7% and 16.8% than wild type when using whole cells and purified enzymes, respectively. Our findings demonstrated that this novel spectrophotometric method coupled with the β-d-glucosidase assay might be applied for high-throughput screening of different α-l-rhamnosidases and a great number of mutants from semi-rational design and directed evolution for α-l-rhamnosidase.

Bioproduction of Quercetin and Rutinose Catalyzed by Rutinosidase: Novel Concept of "Solid State Biocatalysis"

Quercetin is a flavonoid largely employed as a phytochemical remedy and a food or dietary supplement. We present here a novel biocatalytic methodology for the preparation of quercetin from plant-derived rutin, with both substrate and product being in mostly an undissolved state during biotransformation. This "solid-state" enzymatic conversion uses a crude enzyme preparation of recombinant rutinosidase from Aspergillus niger yielding quercetin, which precipitates from virtually insoluble rutin. The process is easily scalable and exhibits an extremely high space-time yield. The procedure has been shown to be robust and was successfully tested with rutin concentrations of up to 300 g/L (ca 0.5 M) at various scales. Using this procedure, pure quercetin is easily obtained by mere filtration of the reaction mixture, followed by washing and drying of the filter cake. Neither co-solvents nor toxic chemicals are used, thus the process can be considered environmentally friendly and the product of "bio-quality." Moreover, rare disaccharide rutinose is obtained from the filtrate at a preparatory scale as a valuable side product. These results demonstrate for the first time the efficiency of the "Solid-State-Catalysis" concept, which is applicable virtually for any biotransformation involving substrates and products of low water solubility.

Biochemical Characterization of the α-l-Rhamnosidase DtRha from Dictyoglomus thermophilum: Application to the Selective Derhamnosylation of Natural Flavonoids

α-l-Rhamnosidases are catalysts of industrial tremendous interest, but their uses are still somewhat limited by their poor thermal stabilities and selectivities. The thermophilic DtRha from Dictyoglomus thermophilum was cloned, and the recombinant protein was easily purified to homogeneity to afford 4.5 mg/L culture of biocatalyst. Michaelis-Menten parameters demonstrated it to be fully specific for α-l-rhamnose. Most significantly, DtRha demonstrated to have a stronger preference for α(1 → 2) linkage rather than α(1 → 6) linkage when removing rhamnosyl moiety from natural flavonoids. This selectivity was fully explained by the difference of binding of the corresponding substrates in the active site of the protein.

An alkali tolerant α-l-rhamnosidase from Fusarium moniliforme MTCC-2088 used in de-rhamnosylation of natural glycosides

Analkali tolerant α-l-rhamnosidase has been purified to homogeneity from the culture filtrate of a new fungal strain, Fusarium moniliforme MTCC-2088, using concentration by ultrafiltration and cation exchange chromatography on CM cellulose column. The molecular mass of the purified enzyme has been found to be 36.0 kDa using SDS-PAGE analysis. The K_m value using p-nitrophenyl-α-l-rhamnopyranoside as the variable substrate in 0.2 M sodium phosphate buffer pH10.5 at50 °C was 0.50 mM. The catalytic rate constant was15.6 s^-1giving the values of k_cat/K_m is 3.12 × 10⁴M^-1 s^-1. The pH and temperature optima of the enzyme were 10.5 and 50 °C, respectively. The purified enzyme had better stability at 10 °C in basic pH medium. The enzyme derhamnosylated natural glycosides like naringin to prunin, rutin to isoquercitrin and hesperidin to hesperetin glucoside. The purified α-l-rhamnosidase has potential for enhancement of wine aroma.

Crystal structure of native α-L-rhamnosidase from Aspergillus terreus

α-L-Rhamnosidases cleave terminal nonreducing α-L-rhamnosyl residues from many natural rhamnoglycosides. This makes them catalysts of interest for various biotechnological applications. The X-ray structure of the GH78 family α-L-rhamnosidase from Aspergillus terreus has been determined at 1.38 Å resolution using the sulfur single-wavelength anomalous dispersion phasing method. The protein was isolated from its natural source in the native glycosylated form, and the active site contained a glucose molecule, probably from the growth medium. In addition to its catalytic domain, the α-L-rhamnosidase from A. terreus contains four accessory domains of unknown function. The structural data suggest that two of these accessory domains, E and F, might play a role in stabilizing the aglycon portion of the bound substrate.

"Sweet Flavonoids": Glycosidase-Catalyzed Modifications

Natural flavonoids, especially in their glycosylated forms, are the most abundant phenolic compounds found in plants, fruit, and vegetables. They exhibit a large variety of beneficial physiological effects, which makes them generally interesting in a broad spectrum of scientific areas. In this review, we focus on recent advances in the modifications of the glycosidic parts of various flavonoids employing glycosidases, covering both selective trimming of the sugar moieties and glycosylation of flavonoid aglycones by natural and mutant glycosidases. Glycosylation of flavonoids strongly enhances their water solubility and thus increases their bioavailability. Antioxidant and most biological activities are usually less pronounced in glycosides, but some specific bioactivities are enhanced. The presence of l-rhamnose (6-deoxy-α-l-mannopyranose) in rhamnosides, rutinosides (rutin, hesperidin) and neohesperidosides (naringin) plays an important role in properties of flavonoid glycosides, which can be considered as "pro-drugs". The natural hydrolytic activity of glycosidases is widely employed in biotechnological deglycosylation processes producing respective aglycones or partially deglycosylated flavonoids. Moreover, deglycosylation is quite commonly used in the food industry aiming at the improvement of sensoric properties of beverages such as debittering of citrus juices or enhancement of wine aromas. Therefore, natural and mutant glycosidases are excellent tools for modifications of flavonoid glycosides.

Glycosyltransferase cascades made fit for chemical production: Integrated biocatalytic process for the natural polyphenol C-glucoside nothofagin

Glycosyltransferase cascades are promising tools of biocatalysis for natural product glycosylation, but their suitability for actual production remains to be shown. Here, we demonstrate at a scale of 100 g isolated product the integrated biocatalytic production of nothofagin, the natural 3'-C-β-D-glucoside of the polyphenol phloretin. A parallel reaction cascade involving coupled C-glucosyltransferase and sucrose synthase was optimized for the one-pot glucosylation of phloretin from sucrose via an UDP/UDP-glucose shuttle. Inclusion complexation with the highly water soluble 2-hydroxypropyl-β-cyclodextrin pushed the phloretin solubility to its upper practical limit (∼120 mM) and so removed the main bottleneck on an efficient synthesis of nothofagin. The biotransformation thus intensified had excellent performance metrics of 97% yield and ∼50 gproduct /L at a space-time yield of 3 g/L/hr. The UDP-glucose was regenerated up to ∼220 times. A scalable downstream process for efficient recovery of nothofagin (≥95% purity; ≥65% yield) was developed. A tailored anion-exchange chromatography at pH 8.5 was used for capture and initial purification of the product. Recycling of the 2-hydroxypropyl-β-cyclodextrin would also be possible at this step. Product precipitation at a lowered pH of 6.0 and re-dissolution in acetone effectively replaced desalting by size exclusion chromatography in the final step of nothofagin purification. This study therefore, reveals the potential for process intensification in the glycosylation of polyphenol acceptors by glycosyltransferase cascades. It demonstrates that, with up- and downstream processing carefully optimized and suitably interconnected, a powerful biocatalytic technology becomes available for the production of an important class of glycosides difficult to prepare otherwise.

Characterization of a glycoside hydrolase family 78 α-l-rhamnosidase from Bacteroides thetaiotaomicron VPI-5482 and identification of functional residues

A putative glycoside hydrolase family 78 α-l-rhamnosidase BtRha78A from Bacteroides thetaiotaomicron VPI-5482 was heterologously over-expressed in Escherichia coli. Enzymatic properties of recombinant BtRha78A were characterized in detail. Recombinant BtRha78A might efficiently hydrolyze p-nitrophenyl α-l-rhamnopyranoside. BtRha78A displayed the highest activity at 60 °C in pH 6.5. BtRha78A exhibited a good pH stability and relatively high thermostability. BtRha78A could be tolerant of a low concentration of alcohols. These attractive advantages made it a promising alternative biocatalyst for industrial applications. The catalytic general acid Asp335 and general base Glu595 of BtRha78A were confirmed by site-directed mutagenesis. Alanine scanning mutagenesis based on sequence alignment and structural analysis revealed that the conserved residues Asp330, Arg334, Trp339, Asp342, Tyr383, Trp440, and His620 were crucial for enzyme catalysis. Most functional residues located at the conserved general acid motif (Asp330-Asp342) and were completely conserved in the subfamily I Rha78s.

Biotransformation of quercetin by Gliocladium deliquescens NRRL 1086

With an attempt to synthesize high-value isoquercitrin (quercetin-3-O-β-D-glucopyranoside), we carried out the biotransformation of quercetin (1) by Gliocladium deliquescens NRRL 1086. Along with the aimed product quercetin 3-O-β-D-glycoside (2), three additional metabolites, 2-protocatechuoyl-phlorogucinol carboxylic acid (3), 2,4,6-trihydroxybenzoic acid (4), and protocatechuic acid (5), were also isolated. The time-course experiments revealed that there were two metabolic routes, regio-selectivity glycosylation and quercetin 2,3-dioxygenation, co-existing in the culture. Both glycosylation and oxidative cleavage rapidly took place after quercetin feeding; about 98% quercetin were consumed within the initial 8 h and the oxdized product (2-protocatechuoyl-phlorogucinol carboxylic acid) was hydrolyzed into two phenolic compounds (2,4,6-trihydroxybenzoic acid and protocatechuic acid). We also investigated the impact of glucose content and metal ions on the two reactions and found that high concentrations of glucose significantly inhibited the oxidative cleavage and improved the yield of isoquercitrin and that Ca²⁺, Fe²⁺, Mn²⁺, Mg²⁺, and Zn²⁺ inhibited glycosylation. To test the promiscuity of this culture, we selected other four flavonols as substrates; the results demonstrated its high regio-selectivity glycosylation ability towards flavonols at C-3 hydroxyl. In conclusion, our findings indicated that the versatile microbe of G. deliquescens NRRL 1086 maitained abundant enzymes, deserving further research.

Improved soluble bacterial expression and properties of the recombinant flavonoid glucosyltransferase UGT73G1 from Allium cepa

Glycosylation of quercetin using flavonol-specific glycosyltransferases offers an alternate method for isoquercitrin production. Obtaining sufficient quantities of bioactive enzymes is an important prerequisite for highly effective biocatalysis and biotransformation. In this study, a codon-optimized gene for the flavonoid glucosyltransferase UGT73G1 from Allium cepa was heterologously expressed in the preferred prokaryotic expression host Escherichia coli. By combining expression as a fusion protein with 6-histidine tags with coexpression with molecular chaperones, increased soluble expression of UGT73G1 was achieved in E. coli. Two-terminal 6-histidine tags contributed more to the expression than molecular chaperones, as demonstrated by comparison of specific activities in crude extracts obtained from the recombinant E. coli strains. Studies of the catalytic properties of purified UGT73G1 indicated that its activity was significantly promoted by Mn²⁺ and Mg²⁺, while it was strongly inhibited by Cu²⁺. These expression strategies enhanced the solubility and activity of the overexpressed protein and enabled characterization of this plant-derived glucosyltransferase expressed in a prokaryotic host.

Comparison in antioxidant and antitumor activities of pine polyphenols and its seven biotransformation extracts by fungi

Microbial transformation can strengthen the antioxidant and antitumor activities of polyphenols. Polyphenols contents, antioxidant and antitumor activities of pine polyphenols and its biotransformation extracts by Aspergillus niger, Aspergillus oryzae, Aspergillus carbonarius, Aspergillus candidus, Trichodermas viride, Mucor wutungkiao and Rhizopus sp were studied. Significant differences were noted in antioxidant and antitumor activities. The highest antioxidant activities in Trolox equivalent antioxidant capacity (TEAC), DPPH radical scavenging activity, superoxide anion radical scavenging activity, hydroxyl radical scavenging activity, reducing power assay and antitumor activity against LoVo cells were biotransformation extract of Aspergillus carbonarius (BAC), biotransformation extract of Mucor wutungkiao (BMW), biotransformation extract of Aspergillus carbonarius (BAC), biotransformation extract of Aspergillus niger (BAN), biotransformation extract of Aspergillus oryzae (BAO) and BMW, respectively. Correlation analysis found that antioxidant and antitumor activities were associated with polyphenols contents and types of free radicals and tumors. A. carbonarius can make polyphenol oxidation, hydroxylation and methylation, and form new polyphenols. In conclusion, A. carbonarius, A. niger and M. wutungkiao are valuable microorganisms used for polyphenols biotransformation and enhance the antioxidant and antitumor activities of polyphenols.

Synthesis and Antiradical Activity of Isoquercitrin Esters with Aromatic Acids and Their Homologues

Isoquercitrin, (IQ, quercetin-3-O-β-d-glucopyranoside) is known for strong chemoprotectant activities. Acylation of flavonoid glucosides with carboxylic acids containing an aromatic ring brings entirely new properties to these compounds. Here, we describe the chemical and enzymatic synthesis of a series of IQ derivatives at the C-6″. IQ benzoate, phenylacetate, phenylpropanoate and cinnamate were prepared from respective vinyl esters using Novozym 435 (Lipase B from Candida antarctica immobilized on acrylic resin). The enzymatic procedure gave no products with “hydroxyaromatic” acids, their vinyl esters nor with their benzyl-protected forms. A chemical protection/deprotection method using Steglich reaction yielded IQ 4-hydroxybenzoate, vanillate and gallate. In case of p-coumaric, caffeic, and ferulic acid, the deprotection lead to the saturation of the double bonds at the phenylpropanoic moiety and yielded 4-hydroxy-, 3,4-dihydroxy- and 3-methoxy-4-hydroxy-phenylpropanoates. Reducing capacity of the cinnamate, gallate and 4-hydroxyphenylpropanoate towards Folin-Ciocalteau reagent was significantly lower than that of IQ, while other derivatives displayed slightly better or comparable capacity. Compared to isoquercitrin, most derivatives were less active in 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, but they showed significantly better 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid, ABTS) scavenging activity and were substantially more active in the inhibition of tert-butylhydroperoxide induced lipid peroxidation of rat liver microsomes. The most active compounds were the hydroxyphenylpropanoates.

A Sustainable One-Pot, Two-Enzyme Synthesis of Naturally Occurring Arylalkyl Glucosides

A sustainable, convenient, scalable, one-pot, two-enzyme method for the glucosylation of arylalkyl alcohols was developed. The reaction scheme is based on a transrutinosylation catalyzed by a rutinosidase from A. niger using the cheap commercially available natural flavonoid rutin as glycosyl donor, followed by selective "trimming" of the rutinoside unit catalyzed by a rhamnosidase from A. terreus. The process was validated with the syntheses of several natural bioactive glucosides, which could be isolated in up to 75 % yield without silica-gel chromatography.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Enhancing the thermostability of α-L-rhamnosidase from Aspergillus terreus and the enzymatic conversion of rutin to isoquercitrin by adding sorbitol

Background

Thermally stable α-L-rhamnosidase with cleaving terminal α-L-rhamnose activity has great potential in industrial application. Therefore, it is necessary to find a proper method to improve the thermal stability of α-L-rhamnosidase.

Results

In this study, addition of sorbitol has been found to increase the thermostability of α-L-rhamnosidase from Aspergillus terreus at temperatures ranging from 65 °C to 80 °C. Half-life and activation free energy with addition of 2.0 M sorbitol at 70 °C were increased by 17.2-fold, 8.2 kJ/mol, respectively. The analyses of the results of fluorescence spectroscopy and CD have indicated that sorbitol helped to protect the tertiary and secondary structure of α-L-rhamnosidase. Moreover, the isoquercitrin yield increased from 60.01 to 96.43% with the addition of 1.5 M of sorbitol at 70 °C.

Conclusion

Our findings provide an effective approach for enhancing the thermostability of α-L-rhamnosidase from Aspergillus terreus, which makes it a good candidate for industrial processes of isoquercitrin preparation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0342-9) contains supplementary material, which is available to authorized users.

Quercetin and its analogues: optical and acido–basic properties

This study is focused on eight structurally analogous natural flavonoids that exhibit a wide range of biological activities, which are of interest in pharmacy, cosmetics and the food industry.

Protective effect of isoquercitrin against acute dextran sulfate sodium-induced rat colitis depends on the severity of tissue damage

BACKGROUND

Isoquercitrin (quercetin-3-O-β-d-glucopyranoside) is a flavonoid that exhibited antioxidant and anti-inflammatory activities in a number of in vitro and in vivo studies. Experimental evidence from rodent models of inflammatory bowel disease is, however, lacking. This study was designed to examine whether isoquercitrin effectively and dose-dependently attenuates acute dextran sulfate sodium (DSS)-induced rat colitis.

METHODS

Wistar rats were divided into negative control group (exposed to vehicle only), positive control group (DSS-induced colitis plus vehicle), low isoquercitrin group (DSS pretreated with isoquercitrin 1mg/kg/day) and high isoquercitrin group (DSS with isoquercitrin 10mg/kg/day). Isoquercitrin was administered daily for 14days, and during the last 7days rats drank DSS solution. The effect of isoquercitrin on DSS-induced colitis was assessed clinically (e.g. disease activity index), biochemically (tissue myeloperoxidase activity, local cyclooxygenase-2 expression), using histology (standard hematoxylin-eosin-based histomorphometry, immunohistochemical detection of inducible nitric oxide synthase) and hematology (blood count).

RESULTS

Isoquercitrin dose-dependently ameliorated whole colon shortening and mitigated DSS-induced expression of cyclooxygenase-2 and inducible nitric oxide synthase in the descending segment of the organ. However, when different parts of colon were assessed histomorphometrically, the results did not globally support the protective role of this flavonoid. Tissue healing trends observable in the descending colon were not apparent in the rectum, where histological damage was most severe.

CONCLUSIONS

We surmise that isoquercitrin may be effective in the prevention of acute colitis. Besides being dose-dependent, the potency of orally administered isoquercitrin may depend on the severity of tissue damage and/or on the site of its action.

Isoquercitrin Esters with Mono- or Dicarboxylic Acids: Enzymatic Preparation and Properties

A series of isoquercitrin (quercetin-3-O-β-d-glucopyranoside) esters with mono- or dicarboxylic acids was designed to modulate hydro- and lipophilicity and biological properties. Esterification of isoquercitrin was accomplished by direct chemoenzymatic reaction using Novozym 435 (lipase from Candida antarctica), which accepted C₅- to C12-dicarboxylic acids; the shorter ones, such as oxalic (C₂), malonic (C₃), succinic (C₄) and maleic (C₄) acids were not substrates of the lipase. Lipophilicity of monocarboxylic acid derivatives, measured as log P, increased with the chain length. Esters with glutaric and adipic acids exhibited hydrophilicity, and the dodecanedioic acid hemiester was more lipophilic. All derivatives were less able to reduce Folin-Ciocalteau reagent (FCR) and scavenge DPPH (1,1-diphenyl-2-picrylhydrazyl) than isoquercitrin; ABTS (2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid)) radical-scavenging activity was comparable. Dodecanoate and palmitate were the least active in FCR and ABTS scavenging; dodecanoate and hemiglutarate were the strongest DPPH scavengers. In contrast, most derivatives were much better inhibitors of microsomal lipoperoxidation than isoquercitrin; butyrate and hexanoate were the most efficient. Anti-lipoperoxidant activity of monocarboxylic derivatives, except acetates, decreased with increasing aliphatic chain. The opposite trend was noted for dicarboxylic acid hemiesters, isoquercitrin hemidodecanedioate being the most active. Overall, IQ butyrate, hexanoate and hemidodecanedioate are the most promising candidates for further studies.

Highly selective and efficient biotransformation of linarin to produce tilianin by naringinase

OBJECTIVES

To develop a practical method to prepare tilianin by highly selective and efficient hydrolysis of the C-7 rhamnosyl group from linarin.

RESULTS

Naringinase was utilized to selectively catalyze the formation of tilianin using linarin as the starting material. The reaction conditions, including temperature, pH, metal ions, substrate concentration and enzyme concentration, were optimized. At 60 °C, naringinase showed enhanced α-L-rhamnosidase activity while the β-D-glucosidase activity was abrogated. The addition of Mg(2+), Fe(2+) and Co(2+) was also beneficial for selective biotransformation of linarin to tilianin. Under the optimized conditions (pH 7.0 at 60 °C), linarin could be nearly completely transformed to tilianin with excellent selectivity (>98.9 %), while that of the by-product acacetin was less than 1.1 %. In addition, the structure of target product tilianin was fully characterized by HR-MS and (1)H-NMR.

CONCLUSION

A highly selective and efficient biotransformation of linarin to tilianin was developed by the proper control of incubation temperature, which enhanced the α-L-rhamnosidase activity of naringinase and blocked its β-D-glucosidase activity.

(Anti)mutagenic and immunomodulatory properties of quercetin glycosides

BACKGROUND

Quercetin-3-O-β-D-glucopyranoside (isoquercitrin) and quercetin-3-O-rutinoside (rutin) are common components of a normal human diet and are increasingly used in food supplements. Here their effect on mutagenesis and immunity is shown.

RESULTS

The in vitro (anti)mutagenic potential was compared with that of quercetin using the Ames test in Salmonella typhimurium His(-) strains TA100, TA98 and TA102. Isoquercitrin only slightly increased the number of revertants, while rutin was totally non-mutagenic. On the other hand, all compounds displayed dose-dependent protective activity against H2O2 - and tert-butyl hydroperoxide-induced oxidative damage to the TA102 strain and at 75 µmol L(-1) inhibited H2O2/Fe(2+)-induced formation of the open circular and linear forms of the DNA plasmid pBSIISK(-). In mice, none of the flavonols (0.86 µmol day(-1), 34 days) induced harmful effects. In immunized animals, all compounds enhanced ex vivo B cell proliferation; quercetin stimulated lymphocyte basal proliferation and increased the number of IgM-producing lymphocytes. Rutin promoted NK cytotoxic activity, supported T cells and enhanced gut epithelium renewal. No effect on IgG-forming cells was found.

CONCLUSION

Isoquercitrin displayed negligible and rutin no mutagenicity, but both showed significant antimutagenic and DNA-protective effects against oxidative damage. In vivo, they supported the readiness of the immune system for specific humoral immune response.

Crystal structure of a novel two domain GH78 family α-rhamnosidase from Klebsiella oxytoca with rhamnose bound

The crystal structure of the GH78 family α-rhamnosidase from Klebsiella oxytoca (KoRha) has been determined at 2.7 Å resolution with rhamnose bound in the active site of the catalytic domain. Curiously, the putative catalytic acid, Asp 222, is preceded by an unusual non-proline cis-peptide bond which helps to project the carboxyl group into the active centre. This KoRha homodimeric structure is significantly smaller than those of the other previously determined GH78 structures. Nevertheless, the enzyme displays α-rhamnosidase activity when assayed in vitro, suggesting that the additional structural domains found in the related enzymes are dispensible for function. Proteins 2015; 83:1742–1749. © 2015 The Authors. Proteins: Structure, Function, and Bioinformatics Published by Wiley Periodicals, Inc.

The possible presence of natural β-D-glucosidase inhibitors in jujube leaf extract

Isoquercitrin is a phenolic compound well-known for having greater health benefits than quercitin, its aglycone derivative, and other related glycosides. However, isoquercitrin is rarely found in nature. Here, we optimized the conditions for the enzymatic transformation of isoquercitrin from rutin that was extracted from jujube leaf using the hesperidinase, enzyme complex containing β-D-glucosidase and α-L-rhamnosidase. The maximum productivity (2.57±0.16mg/mL) was experimentally found under the following conditions: 47.3°C, 52.16h, and pH 5.31, which agreed well with the predicted value (2.65mg/mL). However, the achievement of this maximum yield was due to the absence of β-D-glucosidase activity. Further investigations using a β-D-glucosidase assay and reaction measurements under various conditions revealed that the β-D-glucosidase activity was not blocked by denaturation or known inhibitory factors. Currently, there are no recognized β-D-glucosidase inhibitors present in the jujube leaf; however, our observations strongly suggest that an unidentified β-D-glucosidase inhibitor exists in jujube leaf extract.

Sulfation modulates the cell uptake, antiradical activity and biological effects of flavonoids in vitro: An examination of quercetin, isoquercitrin and taxifolin

Quercetin 3'-O-sulfate is one of the main metabolites of the natural flavonoid quercetin in humans. This study was designed to prepare quercetin 3'-O-sulfate (1), isoquercitrin 4'-O-sulfate (2) and taxifolin 4'-O-sulfate (3) by the sulfation of quercetin, isoquercitrin (quercetin 3-O-glucoside) and taxifolin (2,3-dihydroquercetin) using the arylsulfate sulfotransferase from Desulfitobacterium hafniense, and to examine the effect of sulfation on selected biological properties of the flavonoids tested. We found that flavonoid sulfates 1-3 were weaker DPPH radical scavengers than the corresponding nonsulfated flavonoids, and that 1-3, unlike quercetin, did not induce the expression of either heme oxygenase-1 in RAW264.7 cells or cytochrome P450 1A1 in HepG2 cells. In both cell types, the cell uptake of compounds 1-3 was much lower than that of quercetin, but comparable to that of the glycoside isoquercitrin. Moreover, HPLC/MS metabolic profiling in HepG2 cells showed that flavonoid sulfates 1-3 were metabolized to a limited extent compared to the nonsulfated compounds. We conclude that sulfation of the tested flavonoids reduces their antiradical activity, and affects their cell uptake and biological activity in vitro.

An effective biphase system accelerates hesperidinase-catalyzed conversion of rutin to isoquercitrin

Isoquercitrin is a rare, natural ingredient with several biological activities that is a key precursor for the synthesis of enzymatically modified isoquercitrin (EMIQ). The enzymatic production of isoquercitrin from rutin catalyzed by hesperidinase is feasible; however, the bioprocess is hindered by low substrate concentration and a long reaction time. Thus, a novel biphase system consisting of [Bmim][BF4]:glycine-sodium hydroxide (pH 9) (10:90, v/v) and glyceryl triacetate (1:1, v/v) was initially established for isoquercitrin production. The biotransformation product was identified using liquid chromatography-mass spectrometry, and the bonding mechanism of the enzyme and substrate was inferred using circular dichroism spectra and kinetic parameters. The highest rutin conversion of 99.5% and isoquercitrin yield of 93.9% were obtained after 3 h. The reaction route is environmentally benign and mild, and the biphase system could be reused. The substrate concentration was increased 2.6-fold, the reaction time was reduced to three tenths the original time. The three-dimensional structure of hesperidinase was changed in the biphase system, which α-helix and random content were reduced and β-sheet content was increased. Thus, the developed biphase system can effectively strengthen the hesperidinase-catalyzed synthesis of isoquercitrin with high yield.