Enzymatic synthesis of novel phloretin glucosides

Synthesis, fungal biotransformation, and evaluation of the antimicrobial potential of chalcones with a chlorine atom

Chalcones are intermediate products in the biosynthesis of flavonoids, which possess a wide range of biological properties, including antimicrobial and anticancer activities. The introduction of a chlorine atom and the glucosyl moiety into their structure may increase their bioavailability, bioactivity, and pharmacological use. The combined chemical and biotechnological methods can be applied to obtain such compounds. Therefore, 2-chloro-2'-hydroxychalcone and 3-chloro-2'-hydroxychalcone were synthesized and biotransformed in cultures of two strains of filamentous fungi, i.e. Isaria fumosorosea KCH J2 and Beauveria bassiana KCH J1.5 to obtain their novel glycosylated derivatives. Pharmacokinetics, drug-likeness, and biological activity of them were predicted using cheminformatics tools. 2-Chloro-2'-hydroxychalcone, 3-chloro-2'-hydroxychalcone, their main glycosylation products, and 2'-hydrochychalcone were screened for antimicrobial activity against several microbial strains. The growth of Escherichia coli 10,536 was completely inhibited by chalcones with a chlorine atom and 3-chlorodihydrochalcone 2'-O-β-D-(4″-O-methyl)-glucopyranoside. The strain Pseudomonas aeruginosa DSM 939 was the most resistant to the action of the tested compounds. However, chalcone aglycones and glycosides with a chlorine atom almost completely inhibited the growth of bacteria Staphylococcus aureus DSM 799 and yeast Candida albicans DSM 1386. The tested compounds had different effects on lactic acid bacteria depending on the tested species. In general, chlorinated chalcones were more effective in the inhibition of the tested microbial strains than their unchlorinated counterparts and aglycones were a little more effective than their glycosides.

Mutability landscape guided engineering of a promiscuous microbial glycosyltransferase for regioselective synthesis of salidroside and icariside D2

Microbial glycosyltransferases efficiently synthesize glucosides and have garnered increasing interest. However, limited regioselectivity has impeded their broad application, particularly in the pharmaceutical industry. In this study, the UDP-glycosyltransferase YjiC from Bacillus licheniformis (BlYjiC) was engineered to achieve the bidirectional regioselective glycosylation of tyrosol and its derivatives. Initially, site-directed saturation mutagenesis was performed on two newly identified substrate-binding cavities in the acceptor pocket of BlYjiC to provide a comprehensive blueprint of the interplay between mutations and function (mutability landscape). Iterative saturation mutagenesis was performed, guided by the mutability landscape. Two highly regioselective mutants M6 (M112L/I325Y/L70R/Q136E/I67E/M77R) and M2' (M112D/I62L) were generated, exhibiting >99 % regioselectivity toward the alcoholic and phenolic hydroxyl of tyrosol, respectively, compared with the wild-type (product mixture: 51:49 %). Both mutants exhibited excellent regioselectivity toward several dihydroxy phenolic substrates, offering valuable biocatalysts for the regioselective synthesis of glucosides. Their application was confirmed in a short synthesis of salidroside (3.6 g/L) and icariside D2 (2.4 g/L), which exhibited near-perfect regioselectivity. This study provides valuable insights into future protein engineering of similar enzymes and opens new avenues for their practical applications.

A Rare Benzothiazole Glucoside as a Derivative of 'Albedo Bluing' Substance in Citrus Fruit and Its Antioxidant Activity

'Albedo bluing' of fruits occurs in many varieties of citrus, resulting in a significant reduction in their commercial value. We first presented a breakthrough method for successfully extracting and purifying the 'albedo bluing' substance (ABS) from citrus fruits, resulting in the attainment of highly purified ABS. Then, HPLC and UPLC-QTOF-MS were used to prove that ABS in the fruits of three citrus varieties (Citrus reticulate Blanco cv. 'Gonggan', 'Orah', and 'Mashuiju') are identical. However, the chemical structure of ABS remains elusive for many reasons. Fortunately, a more stable derivative of ABS (ABS-D) was successfully obtained. Through various analytical techniques such as HRESIMS, 1D and 2D NMR, and chemical shift calculation, ABS-D was identified as 2,4-dihydroxy-6-(β-D-glucopyranosyloxy)phenyl(5,6-dihydroxy-7-(β-D-glucopyranosyloxy)benzo[d]thiazol-2-yl)methanone, indicating that both ABS and its derivative belong to a rare category of benzothiazole glucosides. Furthermore, both ABS and ABS-D demonstrated potent antioxidant abilities. These findings lay the groundwork for further elucidating the chemical structure of ABS and the causative mechanism of the 'albedo bluing' phenomenon in citrus fruits.

Glycosylation of polyphenolic compounds: Design of a self-sufficient biocatalyst by co-immobilization of a glycosyltransferase, a sucrose synthase and the cofactor UDP

Glycosyltransferases catalyze the regioselective glycosylation of polyphenolic compounds, increasing their solubility without altering their antioxidant properties. Leloir-type glycosyltransferases require UDP-glucose as a cofactor to glycosylate a hydroxyl of the polyphenol, which is expensive and unstable. To simplify these processes for industrial implementation, the preparation of self-sufficient heterogeneous biocatalysts is needed. In this study, a glycosyltransferase and a sucrose synthase (as an UDP-regenerating enzyme) were co-immobilized onto porous agarose-based supports coated with polycationic polymers: polyethylenimine and polyallylamine. In addition, the UDP cofactor was strongly ionically adsorbed and co-immobilized with the enzymes, eliminating the need to add it separately. Thus, the optimal self-sufficient heterogeneous biocatalyst was able to catalyze the glycosylation of three polyphenolic compounds (piceid, phloretin and quercetin) with in situ regeneration of the UDP-glucose, allowing multiple consecutive reaction cycles without the addition of exogenous cofactor. A TTN value of 50 (theoretical maximum) was obtained in the reaction of piceid glycosylation, after 5 reaction cycles, using the self-sufficient biocatalyst based on an improved sucrose synthase variant. This result was 5-fold higher than the obtained using soluble cofactor and the co-immobilized enzymes, and much higher than those reported in the literature for similar processes.

Yellow pigments produced by oxidative oligomerization of dihydrochalcone glucoside and the reaction mechanism

From the dried leaves of Lithocarpus polystachyus, yellow pigments, lithocarputins B (11) and C (12), were isolated with a colorless dihydrochalcone dimer, lithocarputin A (10). The pigments 11 and 12 are dimeric dihydrochalcone glycosides with bicyclo[3.2.1]octane structures. Each pigment is a diastereomeric mixture with enantiomeric aglycones that could not be separated. The production mechanisms of the pigments were proposed based on the in vitro enzymatic preparation from trilobatin (1), the major dihydrochalcone glucoside of L. polystachyus. The majority of the pigments in the dried leaves were the oligomers of the dihydrochalcone glycosides generated by a mechanism similar to dimerization. The pigments are probably artifacts produced in the drying process. This is the first report disclosing a detailed chemical mechanism for pigment formation from dihydrochalcone.

Highly efficient synthesis of mono-β-1,6-Glucosylated Rebaudioside A derivative catalyzed by glycosyltransferase YjiC

Steviol glycosides have attracted great interest because of their high levels of sweetness and safety, and absence of calories. Improvement of their sensory qualities via glycosylation modification by glycosyltransferase is a research hotspot. In this study, YjiC, a uridine diphosphate-dependent glycosyltransferase from Bacillus subtilis 168, was found with the ability to glycosylate rebaudioside A (Reb A) to produce a novel mono β-1, 6-glycosylated Reb A derivative rebaudioside L2 (Reb L2). It has an improved sweetness compared with Reb A. Next, a cascade reaction was established by combining YjiC with sucrose synthase AtSuSy from Arabidopsis thaliana for scale-up preparation of Reb L2. It shows that Reb L2 (30.94 mg/mL) could be efficiently synthesized with an excellent yield of 91.34% within 12 h. Therefore, this study provides a potential approach for the production and application of new steviol glycoside Reb L2, expanding the scope of steviol glycosides.

Therapeutic Potential and Pharmaceutical Development of a Multitargeted Flavonoid Phloretin

Phloretin is a flavonoid of the dihydrogen chalcone class, present abundantly in apples and strawberries. The beneficial effects of phloretin are mainly associated with its potent antioxidant properties. Phloretin modulates several signaling pathways and molecular mechanisms to exhibit therapeutic benefits against various diseases including cancers, diabetes, liver injury, kidney injury, encephalomyelitis, ulcerative colitis, asthma, arthritis, and cognitive impairment. It ameliorates the complications associated with diabetes such as cardiomyopathy, hypertension, depression, memory impairment, delayed wound healing, and peripheral neuropathy. It is effective against various microbial infections including Salmonella typhimurium, Listeria monocytogenes, Mycobacterium tuberculosis, Escherichia coli, Candida albicans and methicillin-resistant Staphylococcus aureus. Considering the therapeutic benefits, it generated interest for the pharmaceutical development. However, poor oral bioavailability is the major drawback. Therefore, efforts have been undertaken to enhance its bioavailability by modifying physicochemical properties and molecular structure, and developing nanoformulations. In the present review, we discussed the pharmacological actions, underlying mechanisms and molecular targets of phloretin. Moreover, the review provides insights into physicochemical and pharmacokinetic characteristics, and approaches to promote the pharmaceutical development of phloretin for its therapeutic applications in the future. Although convincing experimental data are reported, human studies are not available. In order to ascertain its safety, further preclinical studies are needed to encourage its pharmaceutical and clinical development.

Insight into glucocorticoids glucosylation by glucosyltransferase: A combined experimental and in-silico approach

Glucosyltransferases catalyze the glucosidic bond formation by transferring a glucose molecule from an activated sugar donor to an acceptor substrate. Glucocorticoids (GCs) are adrenal-derived steroid hormones most widely used for anti-inflammatory treatments. In this study, we biotransformed two selected GCs, cortisone and prednisone, into their O-glucoside derivatives using a versatile UDP-glycosyltransferase UGT-1. Complete structural assignment of glucosylated products revealed that the bioconversion by regio-selective glucosylation of cortisone and prednisone produced cortisone 21-glucoside and prednisone 21-glucoside, respectively. We also combined molecular dynamics (MD) simulation to study the binding feature and mechanism of glucosylation. MD simulation studies showed the formation of a stable complex between protein, glucose donor, and substrate, stabilized by hydrogen bonds. Overall, we were able to provide explanations for the experimentally observed selectivity for glucosylation by integrating experimental and computational techniques.

Enzymatic synthesis of phlorizin fructosides

Phlorizin is a low soluble dihydrochalcone with relevant pharmacological properties. In this study, enzymatic fructosylation was approached to enhance the water solubility of phlorizin, and consequently its bioavailability. Three enzymes were assayed for phlorizin fructosylation in aqueous reactions using sucrose as fructosyl donor. Levansucrase (EC 2.4.1.10) from Gluconacetobacter diazotrophicus (Gd_LsdA) was 6.5-fold more efficient than invertase (EC 3.2.1.26) from Rhodotorula mucilaginosa (Rh_Inv), while sucrose:sucrose 1-fructosyltransferase (EC 2.4.1.99) from Schedonorus arundinaceus (Sa_1-SST) failed to modify the non-sugar acceptor. Gd_LsdA synthesized series of phlorizin mono- di- and tri-fructosides with maximal conversion efficiency of 73 %. The three most abundant products were identified by ESI-MS and NMR analysis as β-D-fructofuranosyl-(2→6)-phlorizin (P1a), phlorizin-4'-O-β-D-fructofuranosyl-(2→6)-D-fructofuranoside (P2c) and phlorizin-4-O-monofructofuranoside (P1b), respectively. Purified P1a was 16 times (30.57 g L^-1 at 25 °C) more soluble in water than natural phlorizin (1.93 g L^-1 at 25 °C) and exhibited 44.56 % free radical scavenging activity. Gd_LsdA is an attractive candidate enzyme for the scaled synthesis of phlorizin fructosides in the absence of co-solvent.

Structural and biochemical studies of the glycosyltransferase Bs-YjiC from Bacillus subtilis

Glycosylation possess prominent biological and pharmacological significance in natural product and drug candidate synthesis. The glycosyltransferase YjiC, discovered from Bacillus subtilis (Bs-YjiC), shows potential applications in drug development due to its wide substrate spectrums. In order to elucidate its catalytic mechanism, we solved the crystal structure of Bs-YjiC, demonstrating that Bs-YjiC adopts a typical GT-B fold consisting of a flexible N-domain and a relatively rigid C-domain. Structural analysis coupled with site-directed mutagenesis studies revealed that site Ser277 was critical for Nucleoside Diphosphate (NDP) recognition, while Glu317, Gln318, Ser128 and Ser129 were crucial for glycosyl moiety recognition. Our results illustrate the structural basis for acceptor promiscuity in Bs-YjiC and provide a starting point for further protein engineering of Bs-YjiC in industrial and pharmaceutical applications.

Natural products as inspiration for the development of bacterial antibiofilm agents

Natural products have historically been a rich source of diverse chemical matter with numerous biological activities, and have played an important role in drug discovery in many areas including infectious disease. Synthetic and medicinal chemistry have been, and continue to be, important tools to realize the potential of natural products as therapeutics and as chemical probes. The formation of biofilms by bacteria in an infection setting is a significant factor in the recalcitrance of many bacterial infections, conferring increased tolerance to many antibiotics and to the host immune response, and as yet there are no approved therapeutics for combatting biofilm-based bacterial infections. Small molecules that interfere with the ability of bacteria to form and maintain biofilms can overcome antibiotic tolerance conferred by the biofilm phenotype, and have the potential to form combination therapies with conventional antibiotics. Many natural products with anti-biofilm activity have been identified from plants, microbes, and marine life, including: elligic acid glycosides, hamamelitannin, carolacton, skyllamycins, promysalin, phenazines, bromoageliferin, flustramine C, meridianin D, and brominated furanones. Total synthesis and medicinal chemistry programs have facilitated structure confirmation, identification of critical structural motifs, better understanding of mechanistic pathways, and the development of more potent, more accessible, or more pharmacologically favorable derivatives of anti-biofilm natural products.

Optimization of Culture Conditions for the Efficient Biosynthesis of Trilobatin from Phloretin by Engineered Escherichia coli Harboring the Apple Phloretin-4'-O-glycosyltransferase

Trilobatin, a dihydrochalcone glucoside and natural sweetener, has diverse biological and therapeutic properties. In the present study, we developed a microbial system to produce trilobatin from phloretin using Escherichia coli (E. coli) overexpressing the phloretin-4'-O-glycosyltransferase from Malus x domestica Borkh. Various optimization strategies were employed for the efficient production of trilobatin using a one-factor-at-a-time method. The effect of UDP-glucose supplementation, substrate, and inducer concentrations, time of substrate feeding as well as protein induction, and different culture media combinations were evaluated and optimized to enhance the production of trilobatin. As a result, the highest trilobatin production, 246.83 μM (107.64 mg L-1), was obtained with an LB-TB medium combination, 22 h of induction with 0.1 mM IPTG followed by 4 h of feeding with 250 μM phloretin and without extracellular UDP-glucose supplementation. These results demonstrate the efficient production of trilobatin and constitute a promising foundation for large-scale production of the dihydrochalcone glycosides in engineered E. coli.

In Vivo Production of Five Crocins in the Engineered Escherichia coli

Crocins are highly valuable medicinal compounds for treating human disorders, and they also serve as spices and coloring agents. However, the supply of crocins from plant extractions is insufficient for current demands, and using synthetic biology to produce crocins remains a big challenge. Here, we report the in vivo production of five types of crocins in E. coli with GjUGT94E13 and GjUGT74F8, which are responsible for the glycosylation of crocetin, from the crocin-producing plant Gardenia jasminoides. Subsequently, native UDP-glucose biosynthesis in E. coli is strengthened by the overexpression of pgm and galU. The optimization of catalytic reactions has demonstrated that 50 mM NaH₂PO₄-Na₂HPO₄ buffer (pH 8.0) plus 5% glucose is the best medium to use for the efficient glycosylation of crocetin. In engineered E. coli, the conversion rate of crocin III and crocin V from crocetin (50 mg/L) by the catalysis of GjUGT74F8 was increased to 66.1%, and the conversion rate of five types of crocins from crocetin (50 mg/L) via GjUGT94E13 and GjUGT74F8 was 59.6%, much higher than the catalytic activity of the reported microbial UGTs. This study not only sheds light on the in vivo biosynthesis of crocins in E. coli, but also provides important genetic tools for the de novo synthesis of crocins.

OleD Loki as a Catalyst for Hydroxamate Glycosylation

Herein we describe the ability of the permissive glycosyltransferase (GT) OleD Loki to convert a diverse set of >15 histone deacetylase (HDAC) inhibitors (HDACis) into their corresponding hydroxamate glycosyl esters. Representative glycosyl esters were subsequently evaluated in assays for cancer cell line cytotoxicity, chemical and enzymatic stability, and axolotl embryo tail regeneration. Computational substrate docking models were predictive of enzyme-catalyzed turnover and suggest certain HDACis may form unproductive, potentially inhibitory, complexes with GTs.

Biocatalytic Synthesis of a Novel Bioactive Ginsenoside Using UDP-Glycosyltransferase from Bacillus subtilis 168

Ginsenoside Rg3 is a bioactive compound from Panax ginseng and exhibits diverse notable biological properties. Glycosylation catalyzed by uridine diphosphate-dependent glycosyltransferase (UGT) is the final biosynthetic step of ginsenoside Rg3 and determines its diverse pharmacological activities. In the present study, promiscuous UGT Bs-YjiC from Bacillus subtilis 168 was expressed in Escherichia coli and purified via one-step nickel chelate affinity chromatography. The in vitro glycosylation reaction demonstrated Bs-Yjic could selectively glycosylate the C12 hydroxyl group of ginsenoside Rg3 to synthesize an unnatural ginsenoside Rd12. Ginsenoside Rd12 was about 40-fold more water-soluble than that of ginsenoside Rg3 (90 μM). Furthermore, in vitro cytotoxicity of ginsenoside Rd12 against diverse cancer cells was much stronger than that of ginsenoside Rg3. Our studies report the UGT-catalyzed synthesis of unnatural ginsenoside Rd12 for the first time. Ginsenoside Rd12 with antiproliferative activity might be further exploited as a potential anticancer drug.

Efficient modification of the Pseudomonas aeruginosa toxin 2-heptyl-1-hydroxyquinolin-4-one by three Bacillus glycosyltransferases with broad substrate ranges

Glycosylation of natural and synthetic products can alter the physical, chemical and pharmacological properties of the aglycon. Conversion of 2-heptyl-1-hydroxyquinolin-4-one (HQNO), a potent respiratory inhibitor produced by Pseudomonas aeruginosa, to the less toxic 2-heptyl-1-(β-D-glucopyranosydyl)-quinolin-4-one, was recently demonstrated for Bacillus subtilis strain 168. In this study, we compared the genomes of several Bacillus spp. to identify candidate enzymes for HQNO glucosylation. All three (putative) UDP-glycosyltransferases (GT) of B. subtilis 168 tested, YjiC, YdhE and YojK, were capable of HQNO glucosylation, with YjiC showing the highest turnover rate (k_cat) of 4.6 s^-1, and YdhE exhibiting the lowest K_m value for HQNO of 9.1 μM. All three GT predominantly utilized UDP-glucose, but YdhE was similarly active with TDP-glucose. Among the aglycons tested, HQNO was the preferred substrate of all three GT, but they also showed activities toward the P. aeruginosa exoproducts pyocyanin, 2-heptyl-3-hydroxyquinolin-4(1H)-one (the Pseudomonas quinolone signal) and 2,4-dihydroxyquinoline, the plant derived antimicrobials vanillin and quercetin, and the macrolide antibiotic tylosin A. Our results underline the promiscuity and substrate flexibility of YjiC, YdhE and YojK, and suggest a physiological role in natural toxin resistance of B. subtilis. Especially YdhE appears to be an attractive biocatalyst for the glycoengineering of natural products.

Glycosyltransformation of ginsenoside Rh2 into two novel ginsenosides using recombinant glycosyltransferase from Lactobacillus rhamnosus and its in vitro applications

Background

Ginsenoside Rh2 is well known for many pharmacological activities, such as anticancer, antidiabetes, antiinflammatory, and antiobesity properties. Glycosyltransferases (GTs) are ubiquitous enzymes present in nature and are widely used for the synthesis of oligosaccharides, polysaccharides, glycoconjugates, and novel derivatives. We aimed to synthesize new ginsenosides from Rh2 using the recombinant GT enzyme and investigate its cytotoxicity with diverse cell lines.

Methods

We have used a GT gene with 1,224-bp gene sequence cloned from Lactobacillus rhamnosus (LRGT) and then expressed in Escherichia coli BL21 (DE3). The recombinant GT protein was purified and demonstrated to transform Rh2 into two novel ginsenosides, and they were characterized by nuclear magnetic resonance (NMR) techniques and evaluated by 3-(4, 5-dimethylthiazol-2-yl)-2-5-diphenyltetrazolium bromide assay.

Results

Two novel ginsenosides with an additional glucopyranosyl (6→1) and two additional glucopyranosyl (6→1) linked with the C-3 position of the substrate Rh2 were synthesized, respectively. Cell viability assay in the lung cancer (A549) cell line showed that glucosyl ginsenoside Rh2 inhibited cell viability more potently than ginsenoside Rg3 and Rh2 at a concentration of 10 μM. Furthermore, glucosyl ginsenoside Rh2 did not exhibit any cytotoxic effect in murine macrophage cells (RAW264.7), mouse embryo fibroblasts cells (3T3-L1), and skin cells (B16BL6) at a concentration of 10 μM compared with ginsenoside Rh2 and Rg3.

Conclusion

This is the first report on the synthesis of two novel ginsenosides, namely, glucosyl ginsenoside Rh2 and diglucosyl ginsenoside Rh2 from Rh2 by using recombinant GT isolated from L. rhamnosus. Moreover, diglucosyl ginsenoside Rh2 might be a new candidate for treatment of inflammation, obesity, and skin whiting, and especially for anticancer.

Enzyme-Catalyzed Glycosylation of Curcumin and Its Analogues by Glycosyltransferases from Bacillus subtilis ATCC 6633

Curcumin is a naturally occurring polyphenolic compound that is commonly used in both medicine and food additives, but its low aqueous solubility and poor bioavailability hinder further clinical applications. For assessing the effect of the glycosylation of curcumin on its aqueous solubility, two glycosyltransferase genes (BsGT1 and BsGT2) were cloned from the genome of the strain Bacillus subtilis ATCC 6633 and over-expressed in Escherichia coli. Then, the two glycosyltransferases were purified, and their glycosylation capacity toward curcumin and its two analogues was verified. The results showed that both BsGT1 and BsGT2 could convert curcumin and its two analogues into their glucosidic derivatives. Then, the structures of the derivatives were characterized as curcumin 4′-O-β-D-glucoside and two new curcumin analogue monoglucosides namely, curcumoid-O-α-D-glucoside (2a) and 3-pentadienone-O-α-D-glucoside (3a) by nuclear magnetic resonance (NMR) spectroscopy. Subsequently, the dissolvability of curcumin 4′-O-β-D-glucoside was measured to be 18.78 mg/L, while its aglycone could not be determined. Furthermore, the optimal catalyzing conditions and kinetic parameters of BsGT1 and BsGT2 toward curcumin were determined, which showed that the Kcat value of BsGT1 was about 2.6-fold higher than that of BsGT2, indicating that curcumin is more favored for BsGT2. Our findings effectively apply the enzymatic approach to obtain glucoside derivatives with enhanced solubility.

Biocatalytic Synthesis of Non-Natural Monoterpene O-Glycosides Exhibiting Superior Antibacterial and Antinematodal Properties

A promiscuous Bacillus glycosyltransferase (YjiC) was explored for the enzymatic synthesis of monoterpene O-glycosides in vitro and in vivo. YjiC converted seven monoterpenes into 41 different sugar-conjugated novel glycoside derivatives. The whole-cell biotransformation of the same set of monoterpenes exhibited robust enzyme activity to synthesize O-glucosyl derivatives from Escherichia coli. These newly synthesized selected monoterpene-O-glucosyl derivatives exhibited enhanced antibacterial activities against human pathogenic bacteria and antinematodal activities against pine wood nematode Bursaphelenchus xylophilus.

Microbial Synthesis of Non-Natural Anthraquinone Glucosides Displaying Superior Antiproliferative Properties

Anthraquinones, naturally occurring bioactive compounds, have been reported to exhibit various biological activities, including anti-inflammatory, antiviral, antimicrobial, and anticancer effects. In this study, we biotransformed three selected anthraquinones into their novel O-glucoside derivatives, expressing a versatile glycosyltransferase (YjiC) from Bacillus licheniformis DSM 13 in Escherichia coli. Anthraflavic acid, alizarin, and 2-amino-3-hydroxyanthraquinone were exogenously fed to recombinant E. coli as substrate for biotransformation. The products anthraflavic acid-O-glucoside, alizarin 2-O-β-d-glucoside, and 2-amino-3-O-glucosyl anthraquinone produced in the culture broths were characterized by various chromatographic and spectroscopic analyses. The comparative anti-proliferative assay against various cancer cells (gastric cancer-AGS, uterine cervical cancer-HeLa, and liver cancer-HepG2) were remarkable, since the synthesized glucoside compounds showed more than 60% of cell growth inhibition at concentrations ranging from ~50 μM to 100 μM. Importantly, one of the synthesized glucoside derivatives, alizarin 2-O-glucoside inhibited more than 90% of cell growth in all the cancer cell lines tested.

Metabolic engineering of glycosylated polyketide biosynthesis

Microbial cell factories are extensively used for the biosynthesis of value-added chemicals, biopharmaceuticals, and biofuels. Microbial biosynthesis is also realistic for the production of heterologous molecules including complex natural products of plant and microbial origin. Glycosylation is a well-known post-modification method to engineer sugar-functionalized natural products. It is of particular interest to chemical biologists to increase chemical diversity of molecules. Employing the state-of-the-art systems and synthetic biology tools, a range of small to complex glycosylated natural products have been produced from microbes using a simple and sustainable fermentation approach. In this context, this review covers recent notable metabolic engineering approaches used for the biosynthesis of glycosylated plant and microbial polyketides in different microorganisms. This review article is broadly divided into two major parts. The first part is focused on the biosynthesis of glycosylated plant polyketides in prokaryotes and yeast cells, while the second part is focused on the generation of glycosylated microbial polyketides in actinomycetes.

Antimicrobial activity-guided identification of compounds from the deciduous leaves of Malus doumeri by HPLC-ESI-QTOF-MS/MS

This paper intends to identify the antimicrobial activity compounds from the deciduous leaves of Malus doumeri (Dong Li Tea) by HPLC-ESI-QTOF-MS/MS. The ethanol extracts of Malus doumeri were partitioned into petroleum ether, dichloromethane, ethyl acetate, n-butanol and water fraction, respectively. The antimicrobial screening experiments showed that ethyl acetate fraction has a certain antibacterial activity by inhibition zone method in vitro. And then we used the HPLC-ESI-QTOF-MS/MS method to verify the identities of bioactive compounds. Finally, 41 compounds were determined and 11 of which were firstly reported in this plant. Notably, compounds (32, 34, 38) are new dihydrochalcones, and three chlorogenic acid analogues (10, 13, 17) may be potential antimicrobial active ingredient. Which is of great significance to the isolation of novel compounds and the discovery of new natural preservative candidates from the deciduous leaves of Malus doumeri.

One-Pot Synthesis of Ginsenoside Rh2 and Bioactive Unnatural Ginsenoside by Coupling Promiscuous Glycosyltransferase from Bacillus subtilis 168 to Sucrose Synthase

Ginsenosides, the major effective ingredients of Panax ginseng, exhibit various biological properties. UDP-glycosyltransferase (UGT)-mediated glycosylation is the last biosynthetic step of ginsenosides and contributes to their immense structural and functional diversity. In this study, UGT Bs-YjiC from Bacillus subtilis 168 was demonstrated to transfer a glucosyl moiety to the free C3-OH and C12-OH of protopanaxadiol (PPD) and PPD-type ginsenosides to synthesize natural and unnatural ginsenosides. In vitro assays showed that unnatural ginsenoside F12 (3- O-β-d-glucopyranosyl-12- O-β-d-glucopyranosyl-20( S)-protopanaxadiol) exhibited remarkable activity against diverse human cancer cell lines. A one-pot reaction by coupling Bs-YjiC to sucrose synthase (SuSy) was performed to regenerate UDP-glucose from sucrose and UDP. With PPD as the aglycon, an unprecedented high yield of ginsenosides F12 (3.98 g L^-1) and Rh2 (0.20 g L^-1) was obtained by optimizing the conversion conditions. This study provides an efficient approach for the biosynthesis of ginsenosides using a UGT-SuSy cascade reaction.

Biosynthetic Pathway and Metabolic Engineering of Plant Dihydrochalcones

Dihydrochalcones are plant natural products containing the phenylpropanoid backbone and derived from the plant-specific phenylpropanoid pathway. Dihydrochalcone compounds are important in plant growth and response to stresses and, thus, can have large impacts on agricultural activity. In recent years, these compounds have also received increased attention from the biomedical community for their potential as anticancer treatments and other benefits for human health. However, they are typically produced at relatively low levels in plants. Therefore, an attractive alternative is to express the plant biosynthetic pathway genes in microbial hosts and to engineer the metabolic pathway/host to improve the production of these metabolites. In the present review, we discuss in detail the functions of genes and enzymes involved in the biosynthetic pathway of the dihydrochalcones and the recent strategies and achievements used in the reconstruction of multi-enzyme pathways in microorganisms in efforts to be able to attain higher amounts of desired dihydrochalcones.

Use of a Promiscuous Glycosyltransferase from Bacillus subtilis 168 for the Enzymatic Synthesis of Novel Protopanaxatriol-Type Ginsenosides

Ginsenosides are the principal bioactive ingredients of Panax ginseng and possess diverse notable pharmacological activities. UDP-glycosyltransferase (UGT)-mediated glycosylation of the C6-OH and C20-OH of protopanaxatriol (PPT) is the prominent biological modification that contributes to the immense structural and functional diversity of PPT-type ginsenosides. In this study, the glycosylation of PPT and PPT-type ginsenosides was achieved using a promiscuous glycosyltransferase (Bs-YjiC) from Bacillus subtilis 168. PPT was selected as the probe for the in vitro glycodiversification of PPT-type ginsenosides using diverse UDP-sugars as sugar donors. Structural analysis of the newly biosynthesized products demonstrated that Bs-YjiC can transfer a glucosyl moiety to the free C3-OH, C6-OH, and C12-OH of PPT. Five PPT-type ginsenosides were biosynthesized, including ginsenoside Rh1 and four unnatural ginsenosides. The present study suggests flexible microbial UGTs play an important role in the enzymatic synthesis of novel ginsenosides.

Enzymatic synthesis of avermectin B1a glycosides for the effective prevention of the pine wood nematode Bursaphelenchus xylophilus

Avermectin produced by Streptomyces avermitilis is an anti-nematodal agent against the pine wood nematode Bursaphelenchus xylophilus. However, its potential usage is limited by its poor water solubility. For this reason, continuous efforts are underway to produce new derivatives that are more water soluble. Here, the enzymatic glycosylation of avermectin was catalyzed by uridine diphosphate (UDP)-glycosyltransferase from Bacillus licheniformis with various UDP sugars. As a result, the following four avermectin B_1a glycosides were produced: avermectin B_1a 4″-β-D-glucoside, avermectin B_1a 4″-β-D-galactoside, avermectin B_1a 4″-β-L-fucoside, and avermectin B_1a 4″-β-2-deoxy-D-glucoside. The avermectin B_1a glycosides were structurally analyzed based on HR-ESI MS and 1D and 2D nuclear magnetic resonance spectra, and the anti-nematodal effect of avermectin B_1a 4″-β-D-glucoside was found to exhibit the highest activity (IC₅₀ = 0.23 μM), which was approximately 32 times greater than that of avermectin B_1a (IC₅₀ = 7.30 μM), followed by avermectin B_1a 4″-β-2-deoxy-D-glucoside (IC₅₀ = 0.69 μM), avermectin B_1a 4″-β-L-fucoside (IC₅₀ = 0.89 μM), and avermectin B_1a 4″-β-D-galactoside (IC₅₀ = 1.07 μM). These results show that glycosylation of avermectin B_1a effectively enhances its in vitro anti-nematodal activity and that avermectin glycosides can be further applied for treating infestations of the pine wood nematode B. xylophilus.

Genome-wide identification of glycosyltransferases converting phloretin to phloridzin in Malus species

Phloridzin (phloretin 2'-O-glucoside) is the most abundant phenolic compound in Malus species, accounting for up to 18% of the dry weight in leaves. Glycosylation of phloretin at the 2' position is the last and key step in phloridzin biosynthesis. It is catalyzed by a uridine diphosphate (UDP)-glucose:phloretin 2'-O-glucosyltransferase (P2'GT), which directly determines the concentration of phloridzin. However, this process is poorly understood. We conducted a large-scale investigation of phloridzin accumulations in leaves from 64 Malus species and cultivars. To identify the responsible P2'GT, we performed a genome-wide analysis of the expression patterns of UDP-dependent glycosyltransferase genes (UGTs). Two candidates were screened preliminarily in Malus spp. cv. Adams (North American Begonia). Results from further qRT-PCR analyses of the genotypes showed a divergence in phloridzin production. Our assays of enzyme activity also suggested that MdUGT88F4 and MdUGT88F1 regulate the conversion of phloretin to phloridzin in Malus plants. Finally, when they were silenced in 'GL-3' ('Royal Gala'), the concentrations of phloridzin and phloretin (and trilobatin) were significantly reduced and increased, respectively.

Chemical Modification and Detoxification of the Pseudomonas aeruginosa Toxin 2-Heptyl-4-hydroxyquinoline N-Oxide by Environmental and Pathogenic Bacteria

2-Heptyl-4-hydroxyquinoline N-oxide (HQNO), a major secondary metabolite and virulence factor produced by the opportunistic pathogen Pseudomonas aeruginosa, acts as a potent inhibitor of respiratory electron transfer and thereby affects host cells as well as microorganisms. In this study, we demonstrate the previously unknown capability of environmental and pathogenic bacteria to transform and detoxify this compound. Strains of Arthrobacter and Rhodococcus spp. as well as Staphylococcus aureus introduced a hydroxyl group at C-3 of HQNO, whereas Mycobacterium abscessus, M. fortuitum, and M. smegmatis performed an O-methylation, forming 2-heptyl-1-methoxy-4-oxoquinoline as the initial metabolite. Bacillus spp. produced the glycosylated derivative 2-heptyl-1-(β-d-glucopyranosydyl)-4-oxoquinoline. Assaying the effects of these metabolites on cellular respiration and on quinol oxidase activity of membrane fractions revealed that their EC₅₀ values were up to 2 orders of magnitude higher than that of HQNO. Furthermore, cellular levels of reactive oxygen species were significantly lower in the presence of the metabolites than under the influence of HQNO. Therefore, the capacity to transform HQNO should lead to a competitive advantage against P. aeruginosa. Our findings contribute new insight into the metabolic diversity of bacteria and add another layer of complexity to the metabolic interactions which likely contribute to shaping polymicrobial communities comprising P. aeruginosa.

Improvement of the pharmacological activity of menthol via enzymatic β-anomer-selective glycosylation

Menthol has a considerable cooling effect, but the use range of menthol is limited because of its extremely low solubility in water and inherent flavor. (−)-Menthol β-glucoside was determined to be more soluble in water (>27 times) than (−)-menthol α-glucoside; hence, β-anomer-selective glucosylation of menthol is necessary. The in vitro glycosylation of (−)-menthol by uridine diphosphate glycosyltransferase (BLC) from Bacillus licheniformis generated (−)-menthol β-glucoside and new (−)-menthol β-galactoside and (−)-menthol N-acetylglucosamine. The maximum conversion rate of menthol to (−)-menthol β-d-glucoside by BLC was found to be 58.9%. Importantly, (−)-menthol β-d-glucoside had a higher cooling effect and no flavor compared with menthol. In addition, (−)-menthol β-d-glucoside was determined to be a non-sensitizer in a skin allergy test in the human cell line activation test, whereas menthol was a sensitizer.

Enzymatic biosynthesis of novel neobavaisoflavone glucosides via Bacillus UDP-glycosyltransferase

The present study was designed to perform structural modifications of of neobavaisoflavone (NBIF), using an in vitro enzymatic glycosylation reaction, in order to improve its water-solubility. Two novel glucosides of NBIF were obtained from an enzymatic glycosylation by UDP-glycosyltransferase. The glycosylated products were elucidated by LC-MS, HR-ESI-MS, and NMR analysis. The HPLC peaks were integrated and the concentrations in sample solutions were calculated. The MTT assay was used to detect the cytotoxic activity of compounds in cancer cell lines. Based on the spectroscopic analyses, the two novel glucosides were identified as neobavaisoflavone-4'-O-β-D-glucopyranoside (1) and neobavaisoflavone-4', 7-di-O-β-D-glucopyranoside (2). Additionally, the water-solubilities of compounds 1 and 2 were approximately 175.1- and 4 031.9-fold higher than that of the substrate, respectively. Among the test compounds, only NBIF exhibited weak cytotoxicity against four human cancer cell lines, with IC₅₀ values ranging from 63.47 to 72.81 µmol·L^-1. These results suggest that in vitro enzymatic glycosylation is a powerful approach to structural modification, improving water-solubility.

Synthesis of umbelliferone derivatives in Escherichia coli and their biological activities

BACKGROUND

Umbelliferone, also known as 7-hydroxycoumarin, is a phenolic metabolite found in many familiar plants. Its derivatives have been shown to have various pharmacological and chemo-preventive effects on human health. A uridine diphosphate glycosyltransferase YjiC from Bacillus licheniformis DSM 13, a cytochrome P450BM3 (CYP450 BM3) variant namely mutant 13 (M13) from Bacillus megaterium, and an O-methyltransferase from Streptomyces avermitilis (SaOMT2) were used for modifications of umbelliferone.

RESULTS

Three umbelliferone derivatives (esculetin, skimmin, and herniarin) were generated through enzymatic and whole cell catalysis. To improve the efficiencies of biotransformation, different media, incubation time and concentration of substrate were optimized and the production was scaled up using a 3-L fermentor. The maximum yields of esculetin, skimmin, and herniarin were 337.10 μM (67.62%), 995.43 μM (99.54%), and 37.13 μM (37.13%), respectively. The water solubility of esculetin and skimmin were 1.28-folds and 3.98-folds as high as umbelliferone, respectively, whereas herniarin was 1.89-folds less soluble than umbelliferone. Moreover, the antibacterial and anticancer activities of herniarin showed higher than umbelliferone, esculetin and skimmin.

CONCLUSIONS

This study proves that both native and engineered enzymes could be employed for the production of precious compounds via whole cell biocatalysis. We successfully produced three molecules herniarin, esculetin and skimmin in practical amounts and their antibacterial and anticancer properties were accessed. One of the newly synthesized molecules the present research suggests that the combinatorial biosynthesis of different biosynthetic enzymes could rapidly promote to a novel secondary metabolite.

Biosynthesis of natural and non-natural genistein glycosides

Biosynthesis of various genistein glycopyranoside scaffolds using versatile GTs and SOMTs. Each compound was structurally characterized and biological activity assay was carried out.

Purification and characterization of a novel phloretin-2'-O-glycosyltransferase favoring phloridzin biosynthesis

Phloretin-2'-O-glycosyltransferase (P2'GT) catalyzes the last glycosylation step in the biosynthesis of phloridzin that contributes to the flavor, color and health benefits of apples and processed apple products. In this work, a novel P2'GT of Malus x domestica (MdP2'GT) with a specific activity of 46.82 μkat/Kg protein toward phloretin and uridine diphosphate glucose (UDPG) at an optimal temperature of 30 °C and pH 8.0 was purified from the engineered Pichia pastoris broth to homogeneity by anion exchange chromatography, His-Trap affinity chromatography and gel filtration. The purified MdP2'GT was low N-glycosylated and secreted as a stable dimer with a molecular mass of 70.7 kDa in its native form. Importantly, MdP2'GT also exhibited activity towards quercetin and adenosine diphosphate glucose (ADPG), kaempferol and UDPG, quercetin and UDP-galactose, isoliquiritigenin and UDPG, and luteolin and UDPG, producing only one isoquercitrin, astragalin, hyperoside, isoliquiritin, or cynaroside, respectively. This broad spectrum of activities make MdP2'GT a promising biocatalyst for the industrial preparation of the corresponding polyphenol glycosides, preferably for their subsequent isolation and purification. Besides, MdP2'GT displayed the lowest K_m and the highest k_cat/K_m for phloretin and UDPG compared to all previously reported P2'GTs, making MdP2'GT favor phloridzin synthesis the most.

Identification and characterization of UDP-glucose:Phloretin 4'-O-glycosyltransferase from Malus x domestica Borkh

Apples (Malus x domestica Brokh.) are among the world's most important food crops with nutritive and medicinal importance. Many of the health beneficial properties of apple fruit are suggested to be due to (poly)phenolic metabolites, including various dihydrochalcones. Although many of the genes and enzymes involved in polyphenol biosynthesis are known in many plant species, the specific reactions that lead to the biosynthesis of the sweet tasting dihydrochalcones, such as trilobatin, are unknown. To identify candidate genes for involvement in the glycosylation of dihydrochalcones, existing genome databases of the Rosaceae were screened for apple genes with significant sequence similarity to Bacillus subtilis phloretin glycosyltransferase. Herein reported is the identification and functional characterization of a Malus x domestica gene encoding phloretin-4'-O-glycosyltransferase designated MdPh-4'-OGT. Recombinant MdPh-4'-OGT protein glycosylates phloretin in the presence of UDP-glucose into trilobatin in vitro. Its apparent Km values for phloretin and UDP-glucose were 26.1 μM and 1.2 mM, respectively. Expression analysis of the MdPh-4'-OGT gene indicated that its transcript levels showed significant variation in apple tissues of different developmental stages.

Genome-wide identification and characterization of macrolide glycosyltransferases from a marine-derived Bacillus strain and their phylogenetic distribution

Clarifying glycosyltrasferases (GTs) function is of significance for the development of GT inhibitors as drugs, and the use of GTs to glycodiversify small molecules in the search of drug leads. While many Actinomyces natural-product GTs had been functionally characterized, our understanding towards Bacillus natural-product GTs is so far very limited. Herein, genome-wide identification of macrolide GT genes from marine-derived Bacillus methylotrophicus B-9987 revealed the presence of three macrolide GT genes bmmGT1-3. While bmmGT1 was previously revealed to be involved in the biosynthesis of trans-acyltransferase (AT) polyketides compounds macrolactins (MLNs) and bacillaenes (BAEs), the functions of bmmGT2 and bmmGT3 were probed, demonstrating that they are capable to biochemically catalyze glycosylation of MLNs and BAEs as well but interestingly with different regioselectivity, affording four new MLNs analogs. Notably, further genome mining revealed that the orthologs of these three macrolide GT genes showed a regular distribution in the subtilis- and the cereus-clade Bacillus strains; interestingly, bmmGT1 orthologs only occurred in the subtilis-clade Bacillus, and they were also found in the genomes of Streptomyces strains, suggesting their close phylogenetic relationship. These results provide the first significant insight into the important roles of Bacillus macrolide GTs in the biology of the species.