Glycosyltransferase and its application to glycodiversification of natural products

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

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

A chemoinformatic analysis on natural glycosides with respect to biological origin and structural class

Covering: up to 202216.19% of reported natural products (NPs) in the Dictionary of Natural Products (DNP) are glycosides. As one of the most important NPs' structural modifications, glycosylation can change the NPs' polarity, making the aglycones more amphipathic. However, until now, little is known about the general distribution profile of the natural glycosides in different biological sources or structural types. The reason, structural or species preferences of the natural glycosylation remain unclear. In this highlight, chemoinformatic methods were employed to analyze the natural glycosides from DNP, the most comprehensively annotated NP database. We found that the glycosylation ratios of NPs from plants, bacteria, animals and fungi decrease successively, which are 24.99%, 20.84%, 8.40% and 4.48%, respectively. Echinoderm-derived NPs (56.11%) are the most frequently glycosylated, while those produced by molluscs (1.55%), vertebrates (2.19%) and Rhodophyta (3.00%) are the opposite. Among the diverse structural types, a large proportion of steroids (45.19%), tannins (44.78%) and flavonoids (39.21%) are glycosides, yet aminoacids and peptides (5.16%), alkaloids (5.66%) are comparatively less glycosylated. Even within the same biological source or structural type, their glycosylation rates fluctuate drastically between sub- or cross-categories. The substitute patterns of flavonoid and terpenoid glycosides and the most frequently glycosylated scaffolds were identified. NPs with different glycosylation levels occupy different chemical spaces of physicochemical property and scaffold. These findings could help us to interpret the preference of NPs' glycosylation and investigate how NP glycosylation could aid NP-based drug discovery.

The Analysis of the Glycosyltransferase Gene Function From a Novel Granaticin Producer, Streptomyces Vilmorinianum. YP1

Glycosylation is common among the synthesis of natural product and imparts the bioactivity for natural product. As for granaticin, a natural product with great bioactivity, glycosylation is an unusual sugar attachment and remains enigmatic. Orf14 in the gra cluster is the predicted glycosyltransferase but without being identified. Recently, we isolated and identified a novel granaticin producer Streptomyces vilmorinianum YP1. Orf14 gene in gra cluster of YP1 is knocked out and complemented. The instrumental analysis of the blue product synthesized by orf14-deficient mutant exhibits the none-granaticin detection and deglycosylated intermediates accumulation. The bioactivity and stability test suggests the weaker or none antibacterial activity and cytotoxicity of this blue product with greater ultraviolet stability and thermostability than granaticin and derivatives produced by YP1. All the result indicates that orf14 encodes glycosyltransferase and glycosylation played an important role in the bioactivity of granaticin. Meanwhile, the blue pigment, deglycosylated intermediates, has favorable processing characteristics. Our finding supplies the function of orf14 and glycosylation, but also indicates a promising candidate of edible blue pigment applicated in food industry.

Optimized enzymatic synthesis of digestive resistant anomalous isoquercitrin glucosides using amylosucrase and response surface methodology

Diverse flavonoid glycosides are present in the plant kingdom. Advanced technologies have been utilized to synthesize glycosyl flavonoids which exhibit good physicochemical characteristics. Previously, novel isoquercitrin (IQ) mono-, di-, and tri-glucosides (IQ-G1', IQ-G2', and IQ-G3'; atypical IQ-Gs (IQ-G_ap)) were synthesized through the reaction of amylosucrase. Here, the regio-selective transglycosylation yields were predicted using response surface methodology for three variables (glucose donor (sucrose; 100-1500 mM), glucose acceptor (IQ; 100-400 µM), and pH (5.0-8.8)) using 1 unit/mL of enzyme at 45 °C; then, the optima were verified according to the experimental responses. Acidity (pH 5.0) was a major contributor for IQ-G1' production (> 50%), and high sucrose concentration (1500 mM) limited IQ-G3' production (< 15%). Low sucrose concentration (100 mM) at pH 7.0 promoted higher glycosyl IQ production (> 30%). Time-course production of IQ-G_ap showed an exponential growth with different rates. IQ-G_ap was stable under the simulated intestinal conditions compared with typical IQ-Gs. Digestive stable IQ-G_ap can be effectively synthesized by modulating reaction conditions; thereby, atypical glycosyl products may contribute to the elucidation of nutraceutical potential of flavonoid glycosides. KEY POINTS: •Predictions of RSM were validated for the regio-selective IQ-G_ap production. • Time course changes of IQ-G_ap indicate non-processive glycosylation of DGAS. • IQ-G_ap exceed typical IQ-G in digestive stability at simulated intestinal condition.

Using Amylosucrase for the Controlled Synthesis of Novel Isoquercitrin Glycosides with Different Glycosidic Linkages

Many attempts have been made to obtain natural products with certain glycosidic linkages for improvement of their chemo-physical characteristics. Amylosucrase from Deinococcus geothermalis (DGAS; EC.4.2.1.4) is able to transglycosylate natural products. A model compound, isoquercitrin (IQ; quercetin-3-O-glucoside), was employed for producing new IQ glucosides (IQ-Gs). Treatment of IQ with DGAS produced monoglucoside (IQ-G1'), diglucosides (IQ-G2' and IQ-G2″), and triglucoside (IQ-G3). Structural analysis by mass and nuclear magnetic resonance spectrometry revealed that three of the four IQ-Gs were unreported new compounds possessing α-1,2-, α-1,4-, and/or α-1,6-glucosidic linkages at the 3-O-glucosyl moiety of IQ. IQ-G2' and IQ-G3 were dominantly produced at pH 5.0 and 7.2 and 1500 and 100 mM sucrose, respectively (yields of total IQ-Gs: 50-97%). Kinetic studies indicated that the production rate was dependent on buffer/pH and sucrose concentration. The diverse transglycosylations were verified with a molecular docking simulation. This study sheds light on methods for simple glycodiversification of natural products using DGAS, which can synthesize diversely branched glycosides by modulating reaction conditions.

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.

Binding of the substrate UDP-glucuronic acid induces conformational changes in the xanthan gum glucuronosyltransferase

GumK is a membrane-associated glucuronosyltransferase of Xanthomonas campestris that is involved in xanthan gum biosynthesis. GumK belongs to the inverting GT-B superfamily and catalyzes the transfer of a glucuronic acid (GlcA) residue from uridine diphosphate (UDP)-GlcA (UDP-GlcA) to a lipid-PP-trisaccharide embedded in the membrane of the bacteria. The structure of GumK was previously described in its apo- and UDP-bound forms, with no significant conformational differences being observed. Here, we study the behavior of GumK toward its donor substrate UDP-GlcA. Turbidity measurements revealed that the interaction of GumK with UDP-GlcA produces aggregation of protein molecules under specific conditions. Moreover, limited proteolysis assays demonstrated protection of enzymatic digestion when UDP-GlcA is present, and this protection is promoted by substrate binding. Circular dichroism spectroscopy also revealed changes in the GumK tertiary structure after UDP-GlcA addition. According to the obtained emission fluorescence results, we suggest the possibility of exposure of hydrophobic residues upon UDP-GlcA binding. We present in silico-built models of GumK complexed with UDP-GlcA as well as its analogs UDP-glucose and UDP-galacturonic acid. Through molecular dynamics simulations, we also show that a relative movement between the domains appears to be specific and to be triggered by UDP-GlcA. The results presented here strongly suggest that GumK undergoes a conformational change upon donor substrate binding, likely bringing the two Rossmann fold domains closer together and triggering a change in the N-terminal domain, with consequent generation of the acceptor substrate binding site.

Development of an intergeneric conjugal transfer system for xinaomycins-producing Streptomyces noursei Xinao-4

To introduce DNA into Streptomyces noursei xinao-4, which produces xinaomycins, we explored an intergeneric conjugal transfer system. High efficiency of conjugation (8 × 10⁻³ exconjugants per recipient) was obtained when spores of S. noursei xinao-4 were heat-shocked at 50 °C for 10 min, mixed with Escherichia coli ET12567 (pUZ8002/pSET152) in the ratio of 1:100, plated on 2CMY medium containing 40 mmol/L MgCl₂, and incubated at 30 °C for 22 h. With this protocol, the plasmids pKC1139 and pSET152 were successfully transferred from E. coli ET12567 (pUZ8002) with different frequencies. Among all parameters, the ratio of donor to recipient cell number had the strongest effect on the transformation efficiency. In order to validate the above intergeneric conjugal transfer system, a glycosyltransferase gene was cloned and efficiently knocked out in S. noursei xinao-4 using pSG5-based plasmid pKC1139.

Purification and characterization of a betanidin glucosyltransferase from Amaranthus tricolor L catalyzing non-specific biotransformation of flavonoids

Betacyanins are the major pigments present in Amaranthus tricolor, a leafy vegetable consumed globally. The terminal glycosylation of the aglycone betanidin is an important step in the biosynthesis of this natural red antioxidant pigment. A betanidin 5-O-glucosyltransferase (BGT) was fully purified to 134 folds (specific activity, 265.2 nkat mg(-1)) from the red amaranth by ammonium sulfate precipitation followed by hydrophobic interaction, anion exchange and size exclusion chromatography. Homogeneity of the purified protein was confirmed by 2-dimensional polyacrylamide gel electrophoresis (2D PAGE). The molecular weight of the enzyme determined by liquid chromatography-mass spectrometry (LC-MS) was found to be 62.8 kDa. Furthermore, the enzyme glycosylated flavonoids (kaempferol and quercetin) but not anthocyanidins, presence of which is mutually exclusive to betacyanin accumulating plants. The apparent Km (344±2.34 μM) and Vmax (17.24 μM min(-1)) of the enzyme were determined by LC-MS/MS. Peptide mass fingerprinting of the purified protein showed 38.4% coverage of peptide masses with anthocyanidin 3-O-glucosyltransferase from Zea mays. Study on this purified enzyme, for the first time, revealed its role of glycosylation in biosynthesis of betacyanin in A. tricolor and indicates promiscuous substrate-specificity.