Purification, cloning, and heterologous expression of a catalytically efficient flavonol 3-O-galactosyltransferase expressed in the male gametophyte of Petunia hybrida

Identification and characterisation of MdUGT78T2 as a galactosyltransferase with dual activity on flavonol and anthocyanidin substrates in red-skinned apple fruit (Malus domestica L.)

Anthocyanidin and flavonol glycosides have been linked to the health-promoting effects associated with apple consumption. However, very few enzymes involved in flavonoid glycosylation have been characterised to date. Here, we present the identification and phylogenetic analysis of 234 putative glycosyltransferases involved in flavonoid biosynthesis, and detail the biochemical and structural characterisation of MdUGT78T2 as a strict galactosyltransferase involved in the formation of quercetin-3-O-galactoside and cyanidin-3-O-galactoside, the major glycoconjugates of flavonoids in apple. The enzyme is also active on other flavonoids but with a lower catalytic efficiency. Our data, complemented with gene expression analysis suggest that MdUGT78T2 synthesises the glycoconjugates at both the early and late stages of fruit development. This newly discovered type of catalytic activity can potentially be exploited for in vitro modification of flavonoids to increase their stability in food products and to modify apple fruits and other commercial crops through breeding approaches to enhance their health benefits.

Two genes, ANS and UFGT2, from Vaccinium spp. are key steps for modulating anthocyanin production

Anthocyanins are a major group of red to blue spectrum plant pigments with many consumer health benefits. Anthocyanins are derived from the flavonoid pathway and diversified by glycosylation and methylation, involving the concerted action of specific enzymes. Blueberry and bilberry (Vaccinium spp.) are regarded as 'superfruits' owing to their high content of flavonoids, especially anthocyanins. While ripening-related anthocyanin production in bilberry (V. myrtillus) and blueberry (V. corymbosum) is regulated by the transcriptional activator MYBA1, the role of specific structural genes in determining the concentration and composition of anthocyanins has not been functionally elucidated. We isolated three candidate genes, CHALCONE SYNTHASE (VmCHS1), ANTHOCYANIDIN SYNTHASE (VmANS) and UDP-GLUCOSE : FLAVONOID-3-O-GLYCOSYLTRANSFERASE (VcUFGT2), from Vaccinium, which were predominantly expressed in pigmented fruit skin tissue and showed high homology between bilberry and blueberry. Agrobacterium-mediated transient expression of Nicotiana benthamiana showed that overexpression of VcMYBA1 in combination with VmANS significantly increased anthocyanin concentration (3-fold). Overexpression of VmCHS1 showed no effect above that induced by VcMYBA1, while VcUFGT2 modulated anthocyanin composition to produce delphinidin-3-galactosylrhamnoside, not naturally produced in tobacco. In strawberry (Fragaria × ananassa), combined transient overexpression of VcUFGT2 with a FLAVONOID 3´,5´-HYDROXYLASE from kiwifruit (Actinidia melanandra) modulated the anthocyanin profile to include galactosides and arabinosides of delphinidin and cyanidin, major anthocyanins in blueberry and bilberry. These findings provide insight into the role of the final steps of biosynthesis in modulating anthocyanin production in Vaccinium and may contribute to the targeted breeding of new cultivars with improved nutritional properties.

Development of an Escherichia coli whole cell biocatalyst for the production of hyperoside

OBJECTIVE

To produce high concentrations of hyperoside from quercetin using recombinant Escherichia coli with in situ regeneration of UDP-galactose.

RESULTS

Sucrose synthase from Glycine max (GmSUS) was co-expressed with UDP-glucose epimerase from E. coli (GalE) in E. coli for regenerating UDP-galactose from UDP and sucrose. Glycosyltransferase from Petunia hybrida (PhUGT) was introduced to synthesize hyperoside from quercetin through the regeneration system of UDP-galactose. Co-expressing with molecular chaperones GroEL/ES successfully enhanced the catalytic efficiency of the recombinant strain, which assisted the soluble expression of PhUGT. By using a fed-batch approach, the production of hyperoside reached 863.7 mg L^-1 with a corresponding molar conversion of 93.6% and a specific productivity of 72.5 mg L^-1 h^-1.

CONCLUSION

The method described herein for hyperoside production can be widely applied for the synthesis of isorhamnetin-3-O-galactoside, kaempferol-3-O-galactoside and other flavonoids.

Unravelling the consecutive glycosylation and methylation of flavonols in peach in response to UV-B irradiation

Flavonol glycosides are bioactive compounds important for plant defence and human nutrition. Glycosylation and methylation play an important role in enriching the diversity of flavonols in response to the environment. Peach flowers and fruit are rich in flavonol diglycosides such as isorhamnetin 3-O-rutinoside (I3Rut), kaempferol 3-O-rutinoside and quercetin 3-O-rutinoside, and flavonol monoglycosides such as I 3-O-glucoside and Q 3-O-galactoside. UV-B irradiation of fruit significantly induced accumulation of all these flavonol glycosides. Candidate biosynthetic genes induced by UV-B were identified by genome homology searches and the in vitro catalytic activities of purified recombinant proteins determined. PpUGT78T3 and PpUGT78A2 were identified as flavonol 3-O-glucosyltransferase and 3-O-galactosyltransferase, respectively. PpUGT91AK6 was identified as flavonol 1,6-rhamnosyl trasferase catalysing the formation of flavonol rutinosides and PpFOMT1 was identified as a flavonol O-methyltransferase that methylated Q at the 3'-OH-OH to form isorhamnetin derivatives. Transient expression in Nicotiana benthamiana confirmed the specificity of PpUGT78T3 as a flavonol 3-O-glucosyltransferase, PpUGT78A2 as a 3-O-galactosyltransferase, PpUGT91AK6 as a 1,6-rhamnosyltrasferase and PpFOMT1 as an O-methyltransferase. This study provides new insights into the mechanisms of glycosylation and methylation of flavonols, especially the formation of flavonol diglycosides such as I3Rut, and will also be useful for future potential metabolic engineering of complex flavonols.

Protein Engineering of PhUGT, a Donor Promiscuous Glycosyltransferase, for the Improved Enzymatic Synthesis of Antioxidant Quercetin 3-O-N-Acetylgalactosamine

Quercetin 3-O-N-acetylgalactosamine (Q3GalNAc), a derivative of dietary hyperoside, had never been enzymatically synthesized due to the lack of well-identified N-acetylgalactosamine-transferase (GalNAc-T). Herein, PhUGT, an identified flavonoid 3-O-galactosyltransferase from Petunia hybrida, was demonstrated to display quercetin GalNAc-T activity, transferring a N-acetylgalactosamine (GalNAc) from UDP-N-acetylgalactosamine (UDP-GalNAc) to the 3-OH of quercetin to form Q3GalNAc with a low conversion of 11.7% at 40 °C for 2 h. Protein engineering was thus performed, and the resultant PhUGT variant F368T got an enhanced conversion of 75.5% toward UDP-GalNAc. The enzymatically synthesized Q3GalNAc exhibited a comparable antioxidant activity with other quercetin 3-O-glycosides. Further studies revealed that PhUGT was a donor promiscuous glycosyltransferase (GT), recognizing seven sugar donors. This finding overturned a previous notion that PhUGT exclusively recognized UDP-galactose (UDP-Gal). The reason why PhUGT was mistaken for a UDP-Gal-specific GT was demonstrated to be a shorter reaction time, in which many quercetin 3-O-glycosides, except hyperoside, could not be effectively synthesized. The fact that the microbial cell factory expressing PhUGT could yield an array of Q3Gs further confirmed the donor promiscuity of PhUGT. This study laid a foundation for the scale production of Q3GalNAc and provided a potent biocatalyst capable of glycodiversifying quercetin as well.

Metabolic Engineering of Escherichia coli for Hyperoside Biosynthesis

Hyperoside (quercetin 3-O-galactoside) exhibits many biological functions, along with higher bioactivities than quercetin. In this study, three UDP-dependent glycosyltransferases (UGTs) were screened for efficient hyperoside synthesis from quercetin. The highest hyperoside production of 58.5 mg·L⁻¹ was obtained in a recombinant Escherichia coli co-expressing UGT from Petunia hybrida (PhUGT) and UDP-glucose epimerase (GalE, a key enzyme catalyzing the conversion of UDP-glucose to UDP-galactose) from E. coli. When additional enzymes (phosphoglucomutase (Pgm) and UDP-glucose pyrophosphorylase (GalU)) were introduced into the recombinant E. coli, the increased flux toward UDP-glucose synthesis led to enhanced UDP-galactose-derived hyperoside synthesis. The efficiency of the recombinant strain was further improved by increasing the copy number of the PhUGT, which is a limiting step in the bioconversion. Through the optimization of the fermentation conditions, the production of hyperoside increased from 245.6 to 411.2 mg·L⁻¹. The production was also conducted using a substrate-fed batch fermentation, and the maximal hyperoside production was 831.6 mg·L⁻¹, with a molar conversion ratio of 90.2% and a specific productivity of 27.7 mg·L⁻¹·h⁻¹ after 30 h of fermentation. The efficient hyperoside synthesis pathway described here can be used widely for the glycosylation of other flavonoids and bioactive substances.

Identification of UDP-rhamnosyltransferases and UDP-galactosyltransferase involved in flavonol glycosylation in Morella rubra

Flavonol glycosides are health-promoting phytochemicals important for human nutrition and plant defense against environmental stresses. Glycosylation modification greatly enriches the diversity of flavonols. Morella rubra, a member of the Myricaceae, contains high amounts of myricetin 3-O-rhamnoside (M3Rha), quercetin 3-O-rhamnoside (Q3Rha), and quercetin 3-O-galactoside (Q3Gal). In the present study, MrUGT78R1 and MrUGT78R2 were identified as two functional UDP-rhamnosyltransferases, while MrUGT78W1 was identified as a UDP-galactosyltransferase. Site-directed mutagenesis identified Pro143 and Asn386 as important residues for rhamnosyl transfer activity of MrUGT78R1, while the two corresponding positions in MrUGT78W1 (i.e. Ser147 and Asn370) also play important roles in galactosyl transfer activity. Transient expression data for these three MrUGTs in Nicotiana benthamiana tested the function of MrUGT78R1 and MrUGT78R2 as rhamnosyltransferases and MrUGT78W1 as a galactosyltransferase in glycosylation of flavonols. This work enriches knowledge of the diversity of UDP-rhamnosyltransferase in planta and identifies two amino acid positions important for both rhamnosyltransferase and galactosyltransferase.

Phenolic profiles and their responses to pre- and post-harvest factors in small fruits: a review

The consumption of small fruits has increased in recent years. Besides their appealing flavor, the commercial success of small fruits has been partially attributed to their high contents of phenolic compounds with multiple health benefits. The phenolic profiles and contents in small fruits vary based on the genetic background, climate, growing conditions, and post-harvest handling techniques. In this review, we critically compare the profiles and contents of phenolics such as anthocyanins, flavonols, flavan-3-ols, and phenolic acids that have been reported in bilberries, blackberries, blueberries, cranberries, black and red currants, raspberries, and strawberries during fruit development and post-harvest storage. This review offers researchers and breeders a general guideline for the improvement of phenolic composition in small fruits while considering the critical factors that affect berry phenolics from cultivation to harvest and to final consumption.

A thermostable glycosyltransferase from Paenibacillus polymyxa NJPI29: recombinant expression, characterization, and application in synthesis of glycosides

Glycosylation is a prominent biological mechanism, affecting the structural and functional diversity of many natural products. In this study, a novel thermostable uridine diphosphate-dependent glycosyltransferase gene PpGT1 was cloned from Paenibacillus polymyxa NJPI29 and recombinantly expressed in B. subtilis WB600. The purified PpGT1 had a molecular weight of 45 kDa, as estimated using SDS-PAGE. The PpGT1 could catalyze the glycosylation of vanillic acid, methyl vanillate, caffeic acid, cinnamic alcohol, and ferulic acid. Moreover, PpGT1 possessed good thermostability and retained 80% of its original activity even after 12 h of incubation at 45 °C. In addition, PpGT1 remained stable within a neutral to alkaline pH range as well as in the presence of metal ions. The synthesis of methyl vanillate 4-O-β-D-glucoside by purified PpGT1 reached a yield 3.58 mM in a system with pH 8.0, 45 °C, 12 mM UDP-Glc, and 4 mM methyl vanillate. 3D-structure-based amino acid sequence alignments revealed that the catalytic residues and C-terminated PSPG motif were conserved. These unusual properties indicated that PpGT1 is a candidate UGT for valuable natural product industrial applications.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02855-z.

Cyanidin 3-O-galactoside: A Natural Compound with Multiple Health Benefits

Cyanidin 3-O-galactoside (Cy3Gal) is one of the most widespread anthocyanins that positively impacts the health of animals and humans. Since it is available from a wide range of natural sources, such as fruits (apples and berries in particular), substantial studies were performed to investigate its biosynthesis, chemical stability, natural occurrences and content, extraction methods, physiological functions, as well as potential applications. In this review, we focus on presenting the previous studies on the abovementioned aspects of Cy3Gal. As a conclusion, Cy3Gal shares a common biosynthesis pathway and analogous stability with other anthocyanins. Galactosyltransferase utilizing uridine diphosphate galactose (UDP-galactose) and cyanidin as substrates is unique for Cy3Gal biosynthesis. Extraction employing different methods reveals chokeberry as the most practical natural source for mass-production of this compound. The antioxidant properties and other health effects, including anti-inflammatory, anticancer, antidiabetic, anti-toxicity, cardiovascular, and nervous protective capacities, are highlighted in purified Cy3Gal and in its combination with other polyphenols. These unique properties of Cy3Gal are discussed and compared with other anthocyanins with related structure for an in-depth evaluation of its potential value as food additives or health supplement. Emphasis is laid on the description of its physiological functions confirmed via various approaches.

Enzymatic synthesis of myricetin 3-O-galactoside through a whole-cell biocatalyst

Objective

Myricetin 3-O-galactoside is an active compound with pharmaceutical potential. The insufficient supply of this compound becomes a bottleneck in the druggability study of myricetin 3-O-galactoside. Thus, it is necessary to develop a biosynthetic process for myricetin 3-O-galactoside through metabolic engineering.

Methods

Two genes OcSUS1 and OcUGE1 encoding sucrose synthase and UDP-glucose 4-epimerase were introduced into BL21(DE3) to reconstruct a UDP-D-galactose (UDP-Gal) biosynthetic pathway in Escherichia coli. The resultant chassis strain was able to produce UDP-Gal. Subsequently, a flavonol 3-O-galactosyltransferase DkFGT gene was transformed into the chassis strain producing UDP-Gal. An artificial pathway for myricetin 3-O-galactoside biosynthesis was thus constructed in E. coli.

Results

The obtained engineered strain was demonstrated to be capable of producing myricetin 3-O-galactoside, reaching 29.7 mg/L.

Conclusion

Biosynthesis of myricetin 3-O-galactoside through engineered E. coli could be achieved. This result lays the foundation for the large-scale preparation of myricetin 3-O-galactoside.

Involvement of MdUGT75B1 and MdUGT71B1 in flavonol galactoside/glucoside biosynthesis in apple fruit

Apple is rich in flavonol glycosides, which are believed to contribute to putative health benefits associated with apple consumption. Glycosylation, catalyzed by uridine diphospho-glycosyltransferases (UGTs), is the last step in flavonol biosynthesis, which confers molecular stability and solubility to the flavonol. In the present study, the involvement of two UGTs, MdUGT75B1 and MdUGT71B1, in flavonol biosynthesis in apple was investigated. The major flavonols are quercetin 3-O-glycosides, and UV-B and blue light treatment significantly enhanced the accumulation of quercetin 3-O-galactoside, quercetin 3-O-glucoside, and kaempferol 3-O-galactoside. Transcript levels of MdUGT75B1 and MdUGT71B1 in fruit subjected to different treatments were correlated well with flavonol accumulation. MdUGT75B1 showed flavonol-specific activity with a preference for UDP-galactose as the sugar donor, while MdUGT71B1 using UDP-glucose exhibited a wider substrate acceptance. Thus, MdUGT75B1 and MdUGT71B1 are key UGTs involved in flavonol biosynthesis and may have important roles in regulating accumulation of these health-promoting bioactive compounds in apple.

TMT-Based Quantitative Proteomic Analysis Reveals the Crucial Biological Pathways Involved in Self-Incompatibility Responses in Camellia oleifera

Camellia oleifera is a valuable woody oil plant belonging to the Theaceae, Camellia oil extracted from the seed is an excellent edible oil source. Self-incompatibility (SI) in C. oleifera results in low fruit set, and our knowledge about the mechanism remains limited. In the present study, the Tandem mass tag (TMT) based quantitative proteomics was employed to analyze the dynamic change of proteins response to self- and cross-pollinated in C. oleifera. A total of 6,616 quantified proteins were detected, and differentially abundant proteins (DAPs) analysis identified a large number of proteins. Combined analysis of differentially expressed genes (DEGs) and DAPs of self- and cross-pollinated pistils based on transcriptome and proteome data revealed that several candidate genes or proteins involved in SI of C. oleifera, including polygalacturonase inhibitor, UDP-glycosyltransferase 92A1-like, beta-D-galactosidase, S-adenosylmethionine synthetase, xyloglucan endotransglucosylase/hydrolase, ABC transporter G family member 36-like, and flavonol synthase. Venn diagram analysis identified 11 proteins that may participate in pollen tube growth in C. oleifera. Our data also revealed that the abundance of proteins related to peroxisome was altered in responses to SI in C. oleifera. Moreover, the pathway of lipid metabolism-related, flavonoid biosynthesis and splicesome were reduced in self-pollinated pistils by the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. In summary, the results of the present study lay the foundation for learning the regulatory mechanism underlying SI responses as well as provides valuable protein resources for the construction of self-compatibility C. oleifera through genetic engineering in the future.

The Sweet Side of Plant-Specialized Metabolism

Glycosylation plays a major role in the structural diversification of plant natural products. It influences the properties of molecules by modifying the reactivity and solubility of the corresponding aglycones, so influencing cellular localization and bioactivity. Glycosylation of plant natural products is usually carried out by uridine diphosphate(UDP)-dependent glycosyltransferases (UGTs) belonging to the carbohydrate-active enzyme glycosyltransferase 1 (GT1) family. These enzymes transfer sugars from UDP-activated sugar moieties to small hydrophobic acceptor molecules. Plant GT1s generally show high specificity for their sugar donors and recognize a single UDP sugar as their substrate. In contrast, they are generally promiscuous with regard to acceptors, making them attractive biotechnological tools for small molecule glycodiversification. Although microbial hosts have traditionally been used for heterologous engineering of plant-derived glycosides, transient plant expression technology offers a potentially disruptive platform for rapid characterization of new plant glycosyltransferases and biosynthesis of complex glycosides.

Identification of a natural ligand of the hazel allergen Cor a 1

Hazelnut is one of the most frequent causes of food allergy. The major hazel allergen in Northern Europe is Cor a 1, which is homologous to the major birch pollen allergen Bet v 1. Both allergens belong to the pathogenesis related class PR-10. We determined the solution structure of Cor a 1.0401 from hazelnut and identified a natural ligand of the protein. The structure reveals the protein fold characteristic for PR-10 family members, which consists of a seven-stranded antiparallel β-sheet, two short α-helices arranged in V-shape and a long C-terminal α-helix encompassing a hydrophobic pocket. However, despite the structural similarities between Cor a 1 and Bet v 1, they bind different ligands. We have shown previously that Bet v 1 binds to quercetin-3-O-sophoroside. Here, we isolated Cor a 1 from hazel pollen and identified the bound ligand, quercetin-3-O-(2"-O-β-D-glucopyranosyl)-β-D-galactopyranoside, by mass spectrometry and nuclear magnetic resonance spectroscopy (NMR). NMR experiments were performed to confirm binding. Remarkably, although it has been shown that PR-10 allergens show promiscuous binding behaviour in vitro, we can demonstrate that Cor a 1.0401 and Bet v 1.0101 exhibit highly selective binding for their specific ligand but not for the respective ligand of the other allergen.

Highly Promiscuous Flavonoid 3- O-Glycosyltransferase from Scutellaria baicalensis

A highly regio-specific and donor-promiscuous 3- O-glycosyltransferase, Sb3GT1 (UGT78B4), was discovered from Scutellaria baicalensis. Sb3GT1 could accept five sugar donors (UDP-Glc/-Gal/-GlcNAc/-Xyl/-Ara) to catalyze 3- O-glycosylation of 17 flavonols, and the conversion rates could be >98%. Five new glycosides were obtained by scaled-up enzymatic catalysis. Molecular modeling and site-directed mutagenesis revealed that G15 and P187 were critical catalytic residues for the donor promiscuity. Sb3GT1 could be a promising catalyst to increase structural diversity of flavonoid 3- O-glycosides.

Genome-wide identification and biochemical characterization of the UGT88F subfamily in Malus x domestica Borkh

The UDP-glycosyltransferase UGT88F subfamily has been described first in Malus x domestica with the characterization of UGT88F1. Up to now UGT88F1 was one of the most active UGT glycosylating dihydrochalcones in vitro. The involvement of UGT88F1 in phloridzin (phloretin 2'-O-glucoside) synthesis, the main apple tree dihydrochalcone, was further confirmed in planta. Since the characterization of UGT88F1, this new UGT subfamily has been poorly studied probably because it seemed restricted to Maloideae. In the present study, we investigate the apple tree genome to identify and biochemically characterize the whole UGT88F subfamily. The apple tree genome contains five full-length UGT88F genes out of which three newly identified members (UGT88F6, UGT88F7 and UGT88F8) and a pseudogene. These genes are organized into two genomic clusters resulting from the recent global genomic duplication event in the apple tree. We show that recombinant UGT88F8 protein specifically glycosylates phloretin in the 2'OH position to synthetize phloridzin in vitro and was therefore named UDP-glucose: phloretin 2'-O-glycosyltransferase. The Km values of UGT88F8 are 7.72 μM and 10.84 μM for phloretin and UDP-glucose respectively and are in the same range as UGT88F1 catalytic parameters thus constituting two isoforms. Co-expression patterns of both UGT88F1 and UGT88F8 argue for a redundant function in phloridzin biosynthesis in planta. Contrastingly, recombinant UGT88F6 protein is able to glycosylate in vitro a wide range of flavonoids including flavonols, flavones, flavanones, chalcones and dihydrochalcones, although flavonols are the preferred substrates, e.g. Km value for kaempferol is 2.1 μM. Depending on the flavonoid, glycosylation occurs at least on the 3-OH and 7-OH positions. Therefore UGT88F6 corresponds to an UDP-glucose: flavonoid 3/7-O-glycosyltransferase. Finally, a molecular modeling study highlights a very high substitution rate of residues in the acceptor binding pocket between UGT88F8 and UGT88F6 which is responsible for the enzymes divergence in substrate and regiospecificity, despite an overall high protein homology.

UGT79B31 is responsible for the final modification step of pollen-specific flavonoid biosynthesis in Petunia hybrida

UGT79B31 encodes flavonol 3- O -glycoside: 2″- O -glucosyltransferase, an enzyme responsible for the terminal modification of pollen-specific flavonols in Petunia hybrida. Flavonoids are known to be involved in pollen fertility in petunia (P. hybrida) and maize (Zea mays). As a first step toward elucidating the role of flavonoids in pollen, we have identified a glycosyltransferase that is responsible for the terminal modification of petunia pollen-specific flavonoids. An in silico search of the petunia transcriptome database revealed four candidate UDP-glycosyltransferase (UGT) genes. UGT79B31 was selected for further analyses based on a correlation between the accumulation pattern of flavonol glycosides in various tissues and organs and the expression profiles of the candidate genes. Arabidopsis ugt79b6 mutants that lacked kaempferol/quercetin 3-O-glucosyl(1 → 2)glucosides, were complemented by transformation with UGT79B31 cDNA under the control of Arabidopsis UGT79B6 promoter, showing that UGT79B31 functions as a flavonol 3-O-glucoside: 2″-O-glucosyltransferase in planta. Recombinant UGT79B31 protein can convert kaempferol 3-O-galactoside/glucoside to kaempferol 3-O-glucosyl(1 → 2)galactoside/glucoside. UGT79B31 prefers flavonol 3-O-galactosides to the 3-O-glucosides and rarely accepted the 3-O-diglycosides as sugar acceptors. UDP-glucose was the preferred sugar donor for UGT79B31. These results indicated that UGT79B31 encodes a flavonoid 3-O-glycoside: 2″-O-glucosyltransferase. Transient expression of UGT79B31 fused to green fluorescent protein (GFP) in Nicotiana benthamiana showed that UGT79B31 protein was localized in the cytosol.

One-Pot Synthesis of Hyperoside by a Three-Enzyme Cascade Using a UDP-Galactose Regeneration System

Hyperoside exhibits many biological properties and is more soluble in water than quercetin. A uridine 5'-diphosphate (UDP) galactose regeneration system and one-pot synthesis of hyperoside was described herein. Glycine max sucrose synthase (GmSUS) was coupled with Escherichia coli UDP-galactose 4-epimerase (GalE) to regenerate UDP-galactose from sucrose and UDP. Petunia hybrida glycosyltransferase (PhUGT) with high activity toward quercetin was used to synthesize hyperoside via the UDP-galactose regeneration system. The important factors for optimal synergistic catalysis were determined. Through the use of a fed-batch operation, the final titer of hyperoside increased to 2134 mg/L, with a corresponding molar conversion of 92% and maximum number of UDP-galactose regeneration cycles (RC_max) of 18.4 under optimal conditions. Therefore, the method described herein for the regeneration of UDP-galactose from UDP and sucrose can be widely used for the glycosylation of flavonoids and other bioactive substances.

Trifolin induces apoptosis via extrinsic and intrinsic pathways in the NCI-H460 human non-small cell lung-cancer cell line

BACKGROUND

Trifolin (kaempferol-3-O-galactoside), which is a galactose-conjugated flavonol, exhibits antifungal and anticancer effects. However, the mechanisms underlying its anticancer activities have not yet been examined.

PURPOSE

In this study, the anticancer effects of trifolin were examined in human lung cancer cells.

METHODS

Cytotoxicity was determined by evaluating cell viability. Apoptosis was analyzed through flow cytometry and western blotting analysis. Death receptors and inhibitors of apoptosis were evaluated through RT-PCR.

RESULTS

Trifolin induced apoptosis in NCI-H460 human non-small cell lung cancer (NSCLC) cells by inhibiting the survival pathway and inducing the intrinsic and extrinsic apoptosis pathways. Trifolin decreased levels of Akt/p-Akt, whereas levels of expression of phosphatidylinositide 3-kinase (PI3K), cyclin D1, cyclin E, and cyclin A were not altered. Trifolin initiated cytochrome c release by inducing mitochondrial outer membrane permeabilization (MOMP). Trifolin increased Bcl-2-associated X protein (Bax) levels and decreased b-cell lymphoma 2 (Bcl-2) levels, while the levels of Bcl-xL were not altered. In addition, trifolin increased the levels of the death receptor involving the Fas/Fas ligand (FasL) and Fas-associated protein with the death domain (FADD), which consequently activated caspase-8, caspase-9, caspase-3, and the proteolytic cleavage of poly (ADP-ribose) polymerase (PARP).

CONCLUSION

These results suggested that trifolin induced apoptosis via death receptor-dependent and mitochondria-dependent pathways and that trifolin can be used as a therapeutic agent in human lung cancer.

Identification and characterization of DcUCGalT1, a galactosyltransferase responsible for anthocyanin galactosylation in purple carrot (Daucus carota L.) taproots

Purple carrots (Daucus carota ssp. sativus var. atrorubens Alef.) accumulate large amounts of cyanidin-based anthocyanins in their taproots. Cyanidin can be glycosylated with galactose, xylose, and glucose in sequence by glycosyltransferases resulting in cyanidin 3-xylosyl (glucosyl) galactosides in purple carrots. The first step in the glycosylation of cyanidin is catalysis by UDP-galactose: cyanidin galactosyltransferase (UCGalT) transferring the galactosyl moiety from UDP-galactose to cyanidin. In the present study, a gene from 'Deep purple' carrot, DcUCGalT1, was cloned and heterologously expressed in E. coli BL21 (DE3). The recombinant DcUCGalT1 galactosylated cyanidin to produce cyanidin-3-O-galactoside and showed optimal activity for cyanidin at 30 °C and pH 8.6. It showed lower galactosylation activity for peonidin, pelargonidin, kaempferol and quercetin. It accepted only UDP-galactose as a glycosyl donor when cyanidin was used as an aglycone. The expression level of DcUCGalT1 was positively correlated with anthocyanin biosynthesis in carrots. The enzyme extractions from 'Deep purple' exhibited galactosylation activity for cyanidin, peonidin and pelargonidin, while those from 'Kuroda' (a non-purple cultivar) did not.

Gene Expression Changes during the Gummosis Development of Peach Shoots in Response to Lasiodiplodia theobromae Infection Using RNA-Seq

Lasiodiplodia theobromae is a causal agent of peach (Prunus persica L.) tree gummosis, a serious disease affecting peach cultivation and production. However, the molecular mechanism underlying the pathogenesis remains unclear. RNA-Seq was performed to investigate gene expression in peach shoots inoculated or mock-inoculated with L. theobromae. A total of 20772 genes were detected in eight samples; 4231, 3750, 3453, and 3612 differentially expressed genes were identified at 12, 24, 48, and 60 h after inoculation, respectively. Furthermore, 920 differentially co-expressed genes (515 upregulated and 405 downregulated) were found, respectively. Gene ontology annotation revealed that phenylpropanoid biosynthesis and metabolism, uridine diphosphate-glucosyltransferase activity, and photosynthesis were the most differentially regulated processes during gummosis development. Significant differences were also found in the expression of genes involved in glycometabolism and in ethylene and jasmonic acid biosynthesis and signaling. These data illustrate the dynamic changes in gene expression in the inoculated peach shoots at the transcriptome level. Overall, gene expression in defense response and glycometabolism might result in the gummosis of peach trees induced by L. theobromae.

Functional characterization of a glucosyltransferase gene, LcUFGT1, involved in the formation of cyanidin glucoside in the pericarp of Litchi chinensis

Anthocyanins generate the red color in the pericarp of Litchi chinensis. UDP-glucose: flavonoid 3-O-glycosyltransferase (UFGT, EC. 2.4.1.91) stabilizes anthocyanidin by attaching sugar moieties to the anthocyanin aglycone. In this study, the function of an UFGT gene involved in the biosynthesis of anthocyanin was verified through heterologous expression and virus-induced gene silencing assays. A strong positive correlation between UFGT activity and anthocyanin accumulation capacity was observed in the pericarp of 15 cultivars. Four putative flavonoid 3-O-glycosyltransferase-like genes, designated as LcUFGT1 to LcUFGT4, were identified in the pericarp of litchi. Among the four UFGT gene members, only LcUFGT1 can use cyanidin as its substrate. The expression of LcUFGT1 was parallel with developmental anthocyanin accumulation, and the heterologously expressed protein of LcUFGT1 displayed catalytic activities in the formation of anthocyanin. The LcUFGT1 over-expression tobacco had darker petals and pigmented filaments and calyxes resulting from higher anthocyanin accumulations compared with non-transformed tobacco. In the pericarp with LcUFGT1 suppressed by virus-induced gene silencing, pigmentation was retarded, which was well correlated with the reduced-LcUFGT1 transcriptional activity. These results suggested that the glycosylation-related gene LcUFGT1 plays a critical role in red color formation in the pericarp of litchi.

Transcriptome-wide identification of Rauvolfia serpentina microRNAs and prediction of their potential targets

MicroRNAs (miRNAs) are small non-coding RNAs of ∼ 19-24 nucleotides (nt) in length and considered as potent regulators of gene expression at transcriptional and post-transcriptional levels. Here we report the identification and characterization of 15 conserved miRNAs belonging to 13 families from Rauvolfia serpentina through in silico analysis of available nucleotide dataset. The identified mature R. serpentina miRNAs (rse-miRNAs) ranged between 20 and 22nt in length, and the average minimal folding free energy index (MFEI) value of rse-miRNA precursor sequences was found to be -0.815 kcal/mol. Using the identified rse-miRNAs as query, their potential targets were predicted in R. serpentina and other plant species. Gene Ontology (GO) annotation showed that predicted targets of rse-miRNAs include transcription factors as well as genes involved in diverse biological processes such as primary and secondary metabolism, stress response, disease resistance, growth, and development. Few rse-miRNAs were predicted to target genes of pharmaceutically important secondary metabolic pathways such as alkaloids and anthocyanin biosynthesis. Phylogenetic analysis showed the evolutionary relationship of rse-miRNAs and their precursor sequences to homologous pre-miRNA sequences from other plant species. The findings under present study besides giving first hand information about R. serpentina miRNAs and their targets, also contributes towards the better understanding of miRNA-mediated gene regulatory processes in plants.

Metabolic engineering of Escherichia coli into a versatile glycosylation platform: production of bio-active quercetin glycosides

Background

Flavonoids are bio-active specialized plant metabolites which mainly occur as different glycosides. Due to the increasing market demand, various biotechnological approaches have been developed which use Escherichia coli as a microbial catalyst for the stereospecific glycosylation of flavonoids. Despite these efforts, most processes still display low production rates and titers, which render them unsuitable for large-scale applications.

Results

In this contribution, we expanded a previously developed in vivo glucosylation platform in E. coli W, into an efficient system for selective galactosylation and rhamnosylation. The rational of the novel metabolic engineering strategy constitutes of the introduction of an alternative sucrose metabolism in the form of a sucrose phosphorylase, which cleaves sucrose into fructose and glucose 1-phosphate as precursor for UDP-glucose. To preserve these intermediates for glycosylation purposes, metabolization reactions were knocked-out. Due to the pivotal role of UDP-glucose, overexpression of the interconverting enzymes galE and MUM4 ensured the formation of both UDP-galactose and UDP-rhamnose, respectively. By additionally supplying exogenously fed quercetin and overexpressing a flavonol galactosyltransferase (F3GT) or a rhamnosyltransferase (RhaGT), 0.94 g/L hyperoside (quercetin 3-O-galactoside) and 1.12 g/L quercitrin (quercetin 3-O-rhamnoside) could be produced, respectively. In addition, both strains showed activity towards other promising dietary flavonols like kaempferol, fisetin, morin and myricetin.

Conclusions

Two E. coli W mutants were engineered that could effectively produce the bio-active flavonol glycosides hyperoside and quercitrin starting from the cheap substrates sucrose and quercetin. This novel fermentation-based glycosylation strategy will allow the economically viable production of various glycosides.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0326-1) contains supplementary material, which is available to authorized users.

Biosynthesis and production of glycosylated flavonoids in Escherichia coli: current state and perspectives

Flavonoids are plant secondary metabolites containing several hydroxyl groups that are targets for modification reactions such as methylation and glycosylation. In plants, flavonoids are present as glycones. Although glucose is the most common sugar attached to flavonoids, arabinose, galactose, glucuronic acid, rhamnose, and xylose are also linked to flavonoids. Depending on the kind and the position of the attached sugar, flavonoid glycones show different biological properties. Flavonoid-O-glycosides are synthesized by uridine diphosphate-dependent glycosyltransferases (UGTs), which use nucleotide sugar as a sugar donor and a diverse compound as a sugar acceptor. Recently, diverse flavonoid-O-glycosides have been synthesized in Escherichia coli by introducing UGTs from plants and bacteria and modifying endogenous pathways. The nucleotide sugar biosynthesis pathway in E. coli has been engineered to provide the proper nucleotide sugar for flavonoid-O-glycoside biosynthesis. In this review, we will discuss recent advances in flavonoid-O-glycoside biosynthesis using engineered E. coli.

Metabolic engineering of Escherichia coli for the biosynthesis of flavonoid-O-glucuronides and flavonoid-O-galactoside

Most flavonoids are glycosylated and the nature of the attached sugar can strongly affect their physiological properties. Although many flavonoid glycosides have been synthesized in Escherichia coli, most of them are glucosylated. In order to synthesize flavonoids attached to alternate sugars such as glucuronic acid and galactoside, E. coli was genetically modified to express a uridine diphosphate (UDP)-dependent glycosyltransferase (UGT) specific for UDP-glucuronic acid (AmUGT10 from Antirrhinum majus or VvUGT from Vitis vinifera) and UDP-galactoside (PhUGT from Petunia hybrid) along with the appropriate nucleotide biosynthetic genes to enable simultaneous production of their substrates, UDP-glucuronic acid and UDP-galactose. To engineer UDP-glucuronic acid biosynthesis, the araA gene encoding UDP-4-deoxy-4-formamido-L-arabinose formyltransferase/UDP-glucuronic acid C-4″ decarboxylase, which also used UDP-glucuronic acid as a substrate, was deleted in E. coli, and UDP-glucose dehydrogenase (ugd) gene was overexpressed to increase biosynthesis of UDP-glucuronic acid. Using these strategies, luteolin-7-O-glucuronide and quercetin-3-O-glucuronide were biosynthesized to levels of 300 and 687 mg/L, respectively. For the synthesis of quercetin 3-O-galactoside, UGE (encoding UDP-glucose epimerase from Oryza sativa) was overexpressed along with a glycosyltransferase specific for quercetin and UDP-galactose. Using this approach, quercetin 3-O-galactoside was successfully synthesized to a level of 280 mg/L.

Genome-wide identification and tissue-specific expression analysis of UDP-glycosyltransferases genes confirm their abundance in Cicer arietinum (Chickpea) genome

UDP-glycosyltransferases (EC 2.4.1.x; UGTs) are enzymes coded by an important gene family of higher plants. They are involved in the modification of secondary metabolites, phytohormones, and xenobiotics by transfer of sugar moieties from an activated nucleotide molecule to a wide range of acceptors. This modification regulates various functions like detoxification of xenobiotics, hormone homeostasis, and biosynthesis of secondary metabolites. Here, we describe the identification of 96 UGT genes in Cicer arietinum (CaUGT) and report their tissue-specific differential expression based on publically available RNA-seq and expressed sequence tag data. This analysis has established medium to high expression of 84 CaUGTs and low expression of 12 CaUGTs. We identified several closely related orthologs of CaUGTs in other genomes and compared their exon-intron arrangement. An attempt was made to assign functional specificity to chickpea UGTs by comparing substrate binding sites with experimentally determined specificity. These findings will assist in precise selection of candidate genes for various applications and understanding functional genomics of chickpea.

Transcriptional evidence for inferred pattern of pollen tube-stigma metabolic coupling during pollination

It is difficult to derive all qualitative proteomic and metabolomic experimental data in male (pollen tube) and female (pistil) reproductive tissues during pollination because of the limited sensitivity of current technology. In this study, genome-scale enzyme correlation network models for plants (Arabidopsis/maize) were constructed by analyzing the enzymes and metabolic routes from a global perspective. Then, we developed a data-driven computational pipeline using the "guilt by association" principle to analyze the transcriptional coexpression profiles of enzymatic genes in the consecutive steps for metabolic routes in the fast-growing pollen tube and stigma during pollination. The analysis identified an inferred pattern of pollen tube-stigma ethanol coupling. When the pollen tube elongates in the transmitting tissue (TT) of the pistil, this elongation triggers the mobilization of energy from glycolysis in the TT cells of the pistil. Energy-rich metabolites (ethanol) are secreted that can be taken up by the pollen tube, where these metabolites are incorporated into the pollen tube's tricarboxylic acid (TCA) cycle, which leads to enhanced ATP production for facilitating pollen tube growth. In addition, our analysis also provided evidence for the cooperation of kaempferol, dTDP-alpha-L-rhamnose and cell-wall-related proteins; phosphatidic-acid-mediated Ca2+ oscillations and cytoskeleton; and glutamate degradation IV for γ-aminobutyric acid (GABA) signaling activation in Arabidopsis and maize stigmas to provide the signals and materials required for pollen tube tip growth. In particular, the "guilt by association" computational pipeline and the genome-scale enzyme correlation network models (GECN) developed in this study was initiated with experimental "omics" data, followed by data analysis and data integration to determine correlations, and could provide a new platform to assist inachieving a deeper understanding of the co-regulation and inter-regulation model in plant research.

In-silico analysis of binding site features and substrate selectivity in plant flavonoid-3-O glycosyltransferases (F3GT) through molecular modeling, docking and dynamics simulation studies

Flavonoids are a class of plant secondary metabolites that act as storage molecules, chemical messengers, as well as participate in homeostasis and defense processes. They possess pharmaceutical properties important for cancer treatment such as antioxidant and anti-tumor activities. The drug-related properties of flavonoids can be improved by glycosylation. The enzymes glycosyltransferases (GTs) glycosylate acceptor molecules in a regiospecific manner with the help of nucleotide sugar donor molecules. Several plant GTs have been characterized and their amino acid sequences determined. However, three-dimensional structures of only a few are reported. Here, phylogenetic analysis using amino acid sequences have identified a group of GTs with the same regiospecific activity. The structures of these closely related GTs were modeled using homologous GT structures. Their substrate binding sites were elaborated by docking flavonoid acceptor and UDP-sugar donor molecules in the modeled structures. Eight regions near the acceptor binding site in the N- and C- terminal domain of GTs have been identified that bind and specifically glycosylate the 3-OH group of acceptor flavonoids. Similarly, a conserved motif in the C-terminal domain is known to bind a sugar donor substrate. In certain GTs, the substitution of a specific glutamine by histidine in this domain changes the preference of sugar from glucose to galactose as a result of changed pattern of interactions. The molecular modeling, docking, and molecular dynamics simulation studies have revealed the chemical and topological features of the binding site and thus provided insights into the basis of acceptor and donor recognition by GTs.

Secret of the major birch pollen allergen Bet v 1: identification of the physiological ligand

The major birch pollen allergen Bet v 1 is the main elicitor of airborne type I allergies and belongs to the PR-10 family (pathogenesis-related proteins 10). Bet v 1 is the most extensively studied allergen, and is well characterized at a biochemical and immunological level; however, its physiological function remains elusive. In the present study, we identify Q3OS (quercetin-3-O-sophoroside) as the natural ligand of Bet v 1. We isolated Q3OS bound to Bet v 1 from mature birch pollen and confirmed its binding by reconstitution of the Bet v 1-Q3OS complex. Fluorescence and UV-visible spectroscopy experiments, as well as HSQC (heteronuclear single-quantum coherence) titration, and the comparison with model compounds, such as quercetin, indicated the specificity of Q3OS binding. Elucidation of the binding site by NMR combined with a computational model resulted in a more detailed understanding and shed light on the physiological function of Bet v 1. We postulate that the binding of Q3OS to Bet v 1 plays an important, but as yet unclear, role during the inflammation response and Bet v 1 recognition by IgE.

Engineering flavonoid glycosyltransferases for enhanced catalytic efficiency and extended sugar-donor selectivity

Flavonoids are predominantly found as glycosides in plants. The glycosylation of flavonoids is mediated by uridine diphosphate-dependent glycosyltransferases (UGT). UGTs attach various sugars, including arabinose, glucose, galactose, xylose, and glucuronic acid, to flavonoid aglycones. Two UGTs isolated from Arabidopsis thaliana, AtUGT78D2 and AtUGT78D3, showed 89 % amino acid sequence similarity (75 % amino acid sequence identity) and both attached a sugar to the 3-hydroxyl group of flavonols using a UDP-sugar. The two enzymes used UDP-glucose and UDP-arabinose, respectively, and AtUGT78D2 was approximately 90-fold more efficient than AtUGT78D3 when judged by the k(cat)/K(m) value. Domain exchanges between AtUGT78D2 and AtUGT78D3 were carried out to find UGTs with better catalytic efficiency for UDP-arabinose and exhibiting dual sugar selectivity. Among 19 fusion proteins examined, three showed dual sugar selectivity, and one fusion protein had better catalytic efficiency for UDP-arabinose compared with AtUGT78D3. Using molecular modeling, the changes in enzymatic properties in the chimeric proteins were elucidated. To the best of our knowledge, this is the first report on the construction of fusion proteins with expanded sugar-donor range and enhanced catalytic efficiencies for sugar donors.

Molecular cloning and biochemical characterization of two UDP-glycosyltransferases from poplar

Two pathogen-induced uridine diphosphate glycosyltransferases (UGTs) identified previously via co-expression with induced proanthocyanidin (PA) synthesis in poplar were cloned and characterized. Phylogenetic analysis grouped both genes with other known flavonoid UGTs that act on flavonols and anthocyanins. Recombinant enzymes were produced in order to test if they could glycoslate flavonoids. PtUGT78L1 accepted the flavonols quercetin and kaempferol as well as cyanidin, and used UDP-galactose as a sugar donor. PtUGT78M1 did not accept any of the flavonoids tested as a substrate, but did transfer glucose from UDP-glucose to the universal substrate 2,4,6-trichlorophenol. However, neither enzyme acted on the flavan-3-ols catechin or epicatechin, intermediates in the PA biosynthetic pathway.

Synthesis of flavonol 3-O-glycoside by UGT78D1

Glycosylation is an important method for the structural modification of various flavonols, resulting in the glycosides with increased solubility, stability and bioavailability compared with the corresponding aglycone. From the physiological point of view, glycosylation of plant flavonoids is of importance and interest. However, it is notoriously complicated that flavonols such as quercetin, kaempferol and myricetin, are glucosylated regioselectively at the specific position by chemical method. Compared to the chemical method, enzymatic synthesis present several advantages, such as mild reaction condition, high stereo or region selectivity, no protection/deprotection and high yield. UGT78D1 is a flavonol-specific glycosyltransferase, responsible for transferring rhamnose or glucose to the 3-OH position in vitro. In this study, the activity of UGT78D1 was tested against 28 flavonoids acceptors using UDP-glucose as donor nucleoside in vitro, and 5 acceptors, quercetin, myricetin, kaempferol, fisetin and isorhamnetin, were discovered to be glucosylated at 3-OH position. Herein, the small-scale 3-O-glucosylated quercetin, kaempferol and myricetin were synthesized by UGT78D1 and their chemical structures were confirmed by (1)H and (13)C nuclear magnetic resonance (NMR) and high resolution mass spectrometry (HRMS).

A genome-wide phylogenetic reconstruction of family 1 UDP-glycosyltransferases revealed the expansion of the family during the adaptation of plants to life on land

For almost a decade, our knowledge on the organisation of the family 1 UDP-glycosyltransferases (UGTs) has been limited to the model plant A. thaliana. The availability of other plant genomes represents an opportunity to obtain a broader view of the family in terms of evolution and organisation. Family 1 UGTs are known to glycosylate several classes of plant secondary metabolites. A phylogeny reconstruction study was performed to get an insight into the evolution of this multigene family during the adaptation of plants to life on land. The organisation of the UGTs in the different organisms was also investigated. More than 1500 putative UGTs were identified in 12 fully sequenced and assembled plant genomes based on the highly conserved PSPG motif. Analyses by maximum likelihood (ML) method were performed to reconstruct the phylogenetic relationships existing between the sequences. The results of this study clearly show that the UGT family expanded during the transition from algae to vascular plants and that in higher plants the clustering of UGTs into phylogenetic groups appears to be conserved, although gene loss and gene gain events seem to have occurred in certain lineages. Interestingly, two new phylogenetic groups, named O and P, that are not present in A. thaliana were discovered.

Molecular cloning and biochemical characterization of the UDP-glucose: flavonoid 3-O-glucosyltransferase from Concord grape (Vitis labrusca)

Glucosylation of anthocyanidin substrates at the 3-O-position is crucial for the red pigmentation of grape berries and wine. The gene that encodes the enzyme involved in this reaction has been cloned from Vitis labrusca cv. Concord, heterologously expressed, and the recombinant enzyme (rVL3GT) was characterized. VL3GT has 96% amino acid sequence identity with Vitis vinifera VV3GT and groups phylogenetically with several other flavonoid 3-O-glycosyltransferases. In vitro substrate specificity studies and kinetic analyses of rVL3GT indicate that this enzyme preferentially glucosylates cyanidin as compared with quercetin. Crude protein extracts from several Concord grape tissues were assayed for glucosyltransferase activity with cyanidin and quercetin as acceptor substrates. A comparison of the VL3GT activities toward with these substrates showed that the 3GT enzyme activity is consistent with the expression of VL3GT in these tissues and is coincident with the biosynthesis of anthocyanins in both location and developmental stages. Enzyme activities in grape mesocarp, pre-veraison exocarp, leaf, flower bud, and flower tissues glucosylated quercetin but not cyanidin at high rates, suggesting the presence of additional enzymes which are able to glucosylate the 3-O-position of flavonols with higher specificity than anthocyanidins.

Identification and characterisation of F3GT1 and F3GGT1, two glycosyltransferases responsible for anthocyanin biosynthesis in red-fleshed kiwifruit (Actinidia chinensis)

Much of the diversity of anthocyanins is due to the action of glycosyltransferases, which add sugar moieties to anthocyanidins. We identified two glycosyltransferases, F3GT1 and F3GGT1, from red-fleshed kiwifruit (Actinidia chinensis) that perform sequential glycosylation steps. Red-fleshed genotypes of kiwifruit accumulate anthocyanins mainly in the form of cyanidin 3-O-xylo-galactoside. Genes in the anthocyanin and flavonoid biosynthetic pathway were identified and shown to be expressed in fruit tissue. However, only the expression of the glycosyltransferase F3GT1 was correlated with anthocyanin accumulation in red tissues. Recombinant enzyme assays in vitro and in vivo RNA interference (RNAi) demonstrated the role of F3GT1 in the production of cyanidin 3-O-galactoside. F3GGT1 was shown to further glycosylate the sugar moiety of the anthocyanins. This second glycosylation can affect the solubility and stability of the pigments and modify their colour. We show that recombinant F3GGT1 can catalyse the addition of UDP-xylose to cyanidin 3-galactoside. While F3GGT1 is responsible for the end-product of the pathway, F3GT1 is likely to be the key enzyme regulating the accumulation of anthocyanin in red-fleshed kiwifruit varieties.

Functional differentiation of the glycosyltransferases that contribute to the chemical diversity of bioactive flavonol glycosides in grapevines (Vitis vinifera)

We identified two glycosyltransferases that contribute to the structural diversification of flavonol glycosides in grapevine (Vitis vinifera): glycosyltransferase 5 (Vv GT5) and Vv GT6. Biochemical analyses showed that Vv GT5 is a UDP-glucuronic acid:flavonol-3-O-glucuronosyltransferase (GAT), and Vv GT6 is a bifunctional UDP-glucose/UDP-galactose:flavonol-3-O-glucosyltransferase/galactosyltransferase. The Vv GT5 and Vv GT6 genes have very high sequence similarity (91%) and are located in tandem on chromosome 11, suggesting that one of these genes arose from the other by gene duplication. Both of these enzymes were expressed in accordance with flavonol synthase gene expression and flavonoid distribution patterns in this plant, corroborating their significance in flavonol glycoside biosynthesis. The determinant of the specificity of Vv GT5 for UDP-glucuronic acid was found to be Arg-140, which corresponded to none of the determinants previously identified for other plant GATs in primary structures, providing another example of convergent evolution of plant GAT. We also analyzed the determinants of the sugar donor specificity of Vv GT6. Gln-373 and Pro-19 were found to play important roles in the bifunctional specificity of the enzyme. The results presented here suggest that the sugar donor specificities of these Vv GTs could be determined by a limited number of amino acid substitutions in the primary structures of protein duplicates, illustrating the plasticity of plant glycosyltransferases in acquiring new sugar donor specificities.

Two novel disaccharides, rutinose and methylrutinose, are involved in carbon metabolism in Datisca glomerata

Datisca glomerata forms nitrogen-fixing root nodules in symbiosis with soil actinomycetes from the genus Frankia. Analysis of sugars in roots, nodules and leaves of D. glomerata revealed the presence of two novel compounds that were identified as alpha-L-rhamnopyranoside-(1 --> 6)-D-glucose (rutinose) and alpha-L-rhamnopyranoside-(1 --> 6)-1-O-beta-D-methylglucose (methylrutinose). Rutinose has been found previously as a/the glycoside part of several flavonoid glycosides, e.g. rutin, also of datiscin, the main flavonoid of Datisca cannabina, but had not been reported as free sugar. Time course analyses suggest that both rutinose and methylrutinose might play a role in transient carbon storage in sink organs and, to a lesser extent, in source leaves. Their concentrations show that they can accumulate in the vacuole. Rutinose, but not methylrutinose, was accepted as a substrate by the tonoplast disaccharide transporter SUT4 from Arabidopsis. In vivo (14)C-labeling and the study of uptake of exogenous sucrose and rutinose from the leaf apoplast showed that neither rutinose nor methylrutinose appreciably participate in phloem translocation of carbon from source to sink organs, despite rutinose being found in the apoplast at significant levels. A model for sugar metabolism in D. glomerata is presented.

Molecular identification of 1-Cys peroxiredoxin and anthocyanidin/flavonol 3-O-galactosyltransferase from proanthocyanidin-rich young fruits of persimmon (Diospyros kaki Thunb.)

Fruits of persimmon (Diospyros kaki Thunb.) accumulate large amounts of proanthocyanidins (PAs) in the early stages of development. Astringent (A)-type fruits remain rich in soluble PAs even after they reach full-mature stage, whereas non-astringent (NA)-type fruits lose these compounds before full maturation. As a first step to elucidate the mechanism of PA accumulation in this non-model species, we used suppression subtractive hybridization to identify transcripts accumulating differently in young fruits of A- and NA-type. Interestingly, only a few clones involved in PA biosynthesis were identified in A-NA libraries. Represented by multiple clones were those encoding a novel 1-Cys peroxiredoxin and a new member of family 1 glycosyltransferases. Quantitative RT-PCR analyses confirmed correlation of the amount of PAs and accumulation of transcripts encoding these proteins in young persimmon fruits. Furthermore, the new family 1 glycosyltransferase was produced in Escherichia coli and shown to efficiently catalyze galactosylation at 3-hydroxyl groups of several anthocyanidins and flavonols. These findings suggest a complex mechanism of PA accumulation in persimmon fruits.

Local differentiation of sugar donor specificity of flavonoid glycosyltransferase in Lamiales

Flavonoids are most commonly conjugated with various sugar moieties by UDP-sugar:glycosyltransferases (UGTs) in a lineage-specific manner. Generally, the phylogenetics and regiospecificity of flavonoid UGTs are correlated, indicating that the regiospecificity of UGT differentiated prior to speciation. By contrast, it is unclear how the sugar donor specificity of UGTs evolved. Here, we report the biochemical, homology-modeled, and phylogenetic characterization of flavonoid 7-O-glucuronosyltransferases (F7GAT), which is responsible for producing specialized metabolites in Lamiales plants. All of the Lamiales F7GATs were found to be members of the UGT88-related cluster and specifically used UDP-glucuronic acid (UDPGA). We identified an Arg residue that is specifically conserved in the PSPG box in the Lamiales F7GATs. Substitution of this Arg with Trp was sufficient to convert the sugar donor specificity of the Lamiales F7GATs from UDPGA to UDP-glucose. Homology modeling of the Lamiales F7GAT suggested that the Arg residue plays a critical role in the specific recognition of anionic carboxylate of the glucuronic acid moiety of UDPGA with its cationic guanidinium moiety. These results support the hypothesis that differentiation of sugar donor specificity of UGTs occurred locally, in specific plant lineages, after establishment of general regiospecificity for the sugar acceptor. Thus, the plasticity of sugar donor specificity explains, in part, the extraordinary structural diversification of phytochemicals.

Substrate specificity of plant UDP-dependent glycosyltransferases predicted from crystal structures and homology modeling

Plant family 1 UDP-dependent glycosyltransferases (UGTs) catalyze the glycosylation of a plethora of bioactive natural products. In Arabidopsis thaliana, 120 UGT encoding genes have been identified. The crystal-based 3D structures of four plant UGTs have recently been published. Despite low sequence conservation, the UGTs show a highly conserved secondary and tertiary structure. The sugar acceptor and sugar donor substrates of UGTs are accommodated in the cleft formed between the N- and C-terminal domains. Several regions of the primary sequence contribute to the formation of the substrate binding pocket including structurally conserved domains as well as loop regions differing both with respect to their amino acid sequence and sequence length. In this review we provide a detailed analysis of the available plant UGT crystal structures to reveal structural features determining substrate specificity. The high 3D structural conservation of the plant UGTs render homology modeling an attractive tool for structure elucidation. The accuracy and utility of UGT structures obtained by homology modeling are discussed and quantitative assessments of model quality are performed by modeling of a plant UGT for which the 3D crystal structure is known. We conclude that homology modeling offers a high degree of accuracy. Shortcomings in homology modeling are also apparent with modeling of loop regions remaining as a particularly difficult task.

Bidirectional N-acetylglucosamine transfer mediated by beta-1,4-N-acetylglucosaminyltransferase III

beta-1,4-N-Acetylglucosaminyltransferase III (GnT-III) catalyzes the formation of the bisecting GlcNAc and plays a regulatory role in the biosynthesis of the N-linked oligosaccharide. In this study, we examined whether the glycosyl transfer catalyzed by GnT-III is reversible, and, in addition, investigated the equilibrium of the GnT-III-catalyzed reaction. Incubation of the agalactosyl-bisected biantennary oligosaccharide with GnT-III in the presence of the sufficiently high concentration of uridine diphosphate (UDP) resulted in conversion of the bisected oligosaccharide into the nonbisected one. This reaction was accompanied by the stoichiometric formation of UDP-GlcNAc, which appeared to result from the transfer of GlcNAc from the oligosaccharide to UDP. Thus, these results indicate that GnT-III is capable of perceivably catalyzing the reverse reaction in vitro, as found in some glycosyltransferases. When the equilibrium of the reaction was kinetically analyzed, it was found that the state of the equilibrium is greatly displaced toward the formation of the bisecting GlcNAc. In terms of free energy change, as estimated, the reaction by GnT-III can be comparable to the hydrolysis of ATP. Although GnT-III catalyzes bidirectional transfer of GlcNAc between the oligosaccharide and UDP, the removal of the bisecting GlcNAc is unlikely in vivo, due to the displacement of the equilibrium. It is known that equilibria of certain glycosyltransferase reactions are not biased as greatly as the case of GnT-III, and thus it seems likely that there are a variety of equilibrium states in glycosyltransferase reactions. In living cells, the assembly of oligosaccharides could be regulated by not only rate control but also equilibrium control.

Enzyme-catalyzed synthesis of isosteric phosphono-analogues of sugar nucleotides

Efficient enzymatic syntheses of isosteric phosphono analogues of sugar nucleotides have been accomplished using a thymidylyltransferase.

Non-targeted analysis of spatial metabolite composition in strawberry (Fragariaxananassa) flowers

Formation of flower organs and the subsequent pollination process require a coordinated spatial and temporal regulation of particular metabolic pathways. In this study a comparison has been made between the metabolite composition of individual flower organs of strawberry (Fragariaxananassa) including the petal, sepal, stamen, pistil and the receptacle that gives rise to the strawberry fruit. Non-targeted metabolomics analysis of the semi-polar secondary metabolites by the use of UPLC-qTOF-MS was utilized in order to localize metabolites belonging to various chemical classes (e.g. ellagitannins, proanthocyanidins, flavonols, terpenoids, and spermidine derivatives) to the different flower organs. The vast majority of the tentatively identified metabolites were ellagitannins that accumulated in all five parts of the flower. Several metabolite classes were detected predominantly in certain flower organs, as for example spermidine derivatives were present uniquely in the stamen and pistil, and the proanthocyanidins were almost exclusively detected in the receptacle and sepals. The latter organ was also rich in terpenoids (i.e. triterpenoid and sesquiterpenoid derivatives) whereas phenolic acids and flavonols were the predominant classes of compounds detected in the petals. Furthermore, we observed extensive variation in the accumulation of metabolites from the same class in a single organ, particularly in the case of ellagitannins, and the flavonols quercetin, kaempferol and isorhamnetin. These results allude to spatially-restricted production of secondary metabolite classes and specialized derivatives in flowers that take part in implementing the unique program of individual organs in the floral life cycle.