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

Overexpression of AtMYB12 transcription factor simultaneously enhances quercetin-dependent metabolites in radish callus

The study aimed to enhance quercetin production in radish by optimizing Agrobacterium tumefaciens-mediated in-planta transformation. This protocol involved infecting radish seed embryo axis with A. tumefaciens EHA105 strain carrying the 35S::AtMYB12. Radish seeds were infected with the Agrobacterium suspension (0.8 OD600) for 30 min, followed by sonication for 60 s and vacuum infiltration for 90 s at 100 mm Hg. A 3-day co-cultivation in Murashige and Skoog medium with 150 μM acetosyringone yielded a transformation efficiency of 59.6% and a transgenic callus induction rate of 32.3%. Transgenic plant and callus lines were confirmed by GUS histochemical assay, PCR, and qRT-PCR. The transgenic lines showed an increased expression of flavonoid pathway genes (AtMYB12, CHS, F3H, and FLS) and antioxidant genes (GPX, APX, CAT, and SOD) compared to WT plants. Overexpression of AtMYB12 in transgenic callus increased enzyme activity of phenylalanine ammonia lyase, catalase, and ascorbate peroxidase. In half-strength MS medium with 116.8 mM sucrose, the highest growth index (7.63) was achieved after 20 days. In AtMYB12 overexpressed callus lines, phenolic content (357.31 mg g-1 dry weight), flavonoid content (463 mg g-1 dry weight), and quercetin content (48.24 mg g-1 dry weight) increased significantly by 9.41-fold. Micro-wounding, sonication, and vacuum infiltration improved in-planta transformation in radishes. These high-quercetin-content transgenic callus lines hold promise as valuable sources of flavonoids.

Efficient and Economic Utilization of Cellobiose for Glycosylation Modification by Regulating Carbon Source Supply and Metabolic Pathway In Vivo

Glycosylation, one of the most common and significant modifications in nature, has prompted the development of a cellobiose phosphorolysis route for glycosylation in vivo. However, the process of glycosylation is hampered by the notably low conversion rate of cellobiose. In this work, regulation of the carbon source supply by changing the ratio of glucose to cellobiose improved the conversion rate of cellobiose, resulting in enhancing the efficiency of glycosylation and the production of vitexin. Moreover, three genes (pgm, agp, and ushA) involved in the degradation of UDP-glucose were knocked out to relieve the degradation and diversion of the cellobiose phosphorolysis route. Finally, through the optimization of conversion conditions, we observed a continuous enhancement in cellobiose conversion rate and vitexin production in BL21ΔushAΔagp-TcCGT-CepA, corresponding to an increased concentration of added glucose. The maximum production of vitexin reached 2228 mg/L with the addition of 2 g/L cellobiose and 6 g/L glucose, which was 312% of that in BL21-TcCGT-CepA with the addition of 2 g/L cellobiose. The conversion rate of cellobiose in BL21ΔushAΔagp-TcCGT-CepA reached 88%, which was the highest conversion rate of cellobiose to date. Therefore, this study presents a cost-effective and efficient method to enhance the conversion rate of cellobiose during the glycosylation process.

Functional characterization and structural basis of a reversible glycosyltransferase involves in plant chemical defence

Plants experience numerous biotic stresses throughout their lifespan, such as pathogens and pests, which can substantially affect crop production. In response, plants have evolved various metabolites that help them withstand these stresses. Here, we show that two specialized metabolites in the herbaceous perennial Belamcanda chinensis, tectorigenin and its glycoside tectoridin, have diverse defensive effects against phytopathogenic microorganisms and antifeeding effects against insect pest. We further functionally characterized a 7-O-uridine diphosphate glycosyltransferase Bc7OUGT, which catalyses a novel reversible glycosylation of tectorigenin and tectoridin. To elucidate the catalytic mechanisms of Bc7OUGT, we solved its crystal structure in complex with UDP and UDP/tectorigenin respectively. Structural analysis revealed the Bc7OUGT possesses a narrow but novel substrate-binding pocket made up by plentiful aromatic residues. Further structure-guided mutagenesis of these residues increased both glycosylation and deglycosylation activities. The catalytic reversibility of Bc7OUGT was also successfully applied in an one-pot aglycon exchange reaction. Our findings demonstrated the promising biopesticide activity of tectorigenin and its glycosides, and the characterization and mechanistic study of Bc7OUGT could facilitate the design of novel reversible UGTs to produce valuable glycosides with health benefits for both plants and humans.

Enhancing Glycosylation of Flavonoids by Engineering the Uridine Diphosphate Glucose Supply in Escherichia coli

Glycosylation can enhance the solubility and stability of flavonoids. The main limitation of the glycosylation process is low intracellular uridine diphosphate glucose (UDPG) availability. This study aimed to create a glycosylation platform strain in Escherichia coli BL21(DE3) by multiple metabolic engineering of the UDPG supply. Glycosyltransferase TcCGT1 was introduced to synthesize vitexin and orientin from apigenin and luteolin, respectively. To further expand this glycosylation platform strain, not only were UDP rhamnose and UDP galactose synthesis pathways constructed, but rhamnosyltransferase (GtfC) and galactosyltransferase (PhUGT) were also introduced, respectively. In a 5 L bioreactor with apigenin, luteolin, kaempferol, and quercetin as glycosyl acceptors, vitexin, orientin, afzelin, quercitrin, hyperoside, and trifolin glycosylation products reached 17.2, 36.5, 5.2, 14.1, 6.4, and 11.4 g/L, respectively, the highest titers reported to date for all. The platform strain has great potential for large-scale production of glycosylated flavonoids.

Systematic Engineering of Saccharomyces cerevisiae Chassis for Efficient Flavonoid-7-O-Disaccharide Biosynthesis

Flavonoids are an essential class of secondary metabolites found in plants and possess various nutritional, medicinal, and agricultural properties. However, the poor water solubility of flavonoid aglycones limits their potential applications. To overcome this issue, glycosylation is a promising approach for improving water solubility and bioavailability. In this study, we constructed a flavonoid-7-O-disaccharide biosynthetic pathway with flavonoid aglycones as substrates in Saccharomyces cerevisiae. Subsequently, through metabolic engineering and promoter strategies, we constructed a UDP-rhamnose regeneration system and optimized the UDP-glucose (UDPG) synthetic pathway. The optimized strain produced up to 131.3 mg/L eriocitrin. After this, the chassis cells were applied to other flavonoids, with substrates such as (2S)-naringenin, (2S)-hesperetin, diosmetin, and (2S)-eriodictyol, which resulted in the synthesis of 179.9 mg/L naringin, 276.6 mg/L hesperidin, 249.0 mg/L neohesperidin, 30.4 mg/L diosmin, and 100.7 mg/L neoeriocitrin. To the best of our knowledge, this is the first report on the biosynthesis of flavonoid-7-O-disaccharide.

The parallel biosynthesis routes of hyperoside from naringenin in Hypericum monogynum

Hyperoside is a bioactive flavonoid galactoside in both medicinal and edible plants. It plays an important physiological role in the growth of flower buds. However, the hyperoside biosynthesis pathway has not been systematically elucidated in plants, including its original source, Hypericaceae. Our group found abundant hyperoside in the flower buds of Hypericum monogynum, and we sequenced its transcriptome to study the biosynthetic mechanism of hyperoside. After gene screening and functional verification, four kinds of key enzymes were identified. Specifically, HmF3Hs (flavanone 3-hydroxylases) and HmFLSs (flavonol synthases) could catalyze flavanones into dihydroflavonols, as well as catalyzing dihydroflavonols into flavonols. HmFLSs could also convert flavanones into flavonols and flavones with varying efficiencies. HmF3'H (flavonoid 3'-hydroxylase) was found to act broadly on 4'-hydroxyl flavonoids to produce 3',4'-diydroxylated flavanones, dihydroflavonols, flavonols, and flavones. HmGAT (flavonoid 3-O-galactosyltransferase) would transform flavonols into the corresponding 3-O-galactosides, including hyperoside. The parallel hyperoside biosynthesis routes were thus depicted, one of which was successfully reconstructed in Escherichia coli BL21(DE3) by feeding naringenin, resulting in a hyperoside yield of 25 mg/l. Overall, this research not only helped us understand the interior catalytic mechanism of hyperoside in H. monogynum concerning flower development and bioactivity, but also provided valuable insights into these enzyme families.

Microbial production of nutraceuticals: Metabolic engineering interventions in phenolic compounds, poly unsaturated fatty acids and carotenoids synthesis

Nutraceuticals have attained substantial attention due to their health-boosting or disease-prevention characteristics. Growing awareness about the potential of nutraceuticals for the prevention and management of diseases affecting human has led to an increase in the market value of nutraceuticals in several billion dollars. Nevertheless, limitations in supply and isolation complications from plants, animals or fungi, limit the large-scale production of nutraceuticals. Microbial engineering at metabolic level has been proved as an environment friendly substitute for the chemical synthesis of nutraceuticals. Extensively used microbial systems such as E. coli and S. cerevisiae have been modified as versatile cell factories for the synthesis of diverse nutraceuticals. This review describes current interventions in metabolic engineering for synthesising some of the therapeutically important nutraceuticals (phenolic compounds, polyunsaturated fatty acids and carotenoids). We focus on the interventions in enhancing product yield through engineering at gene level or pathway level.

Engineered production of bioactive polyphenolic O-glycosides

Polyphenolic compounds (such as quercetin and resveratrol) possess potential medicinal values due to their various bioactivities, but poor water solubility hinders their health benefits to humankind. Glycosylation is a well-known post-modification method to biosynthesize natural product glycosides with improved hydrophilicity. Glycosylation has profound effects on decreasing toxicity, increasing bioavailability and stability, together with changing bioactivity of polyphenolic compounds. Therefore, polyphenolic glycosides can be used as food additives, therapeutics, and nutraceuticals. Engineered biosynthesis provides an environmentally friendly and cost-effective approach to generate polyphenolic glycosides through the use of various glycosyltransferases (GTs) and sugar biosynthetic enzymes. GTs transfer the sugar moieties from nucleotide-activated diphosphate sugar (NDP-sugar) donors to sugar acceptors such as polyphenolic compounds. In this review, we systematically review and summarize the representative polyphenolic O-glycosides with various bioactivities and their engineered biosynthesis in microbes with different biotechnological strategies. We also review the major routes towards NDP-sugar formation in microbes, which is significant for producing unusual or novel glycosides. Finally, we discuss the trends in NDP-sugar based glycosylation research to promote the development of prodrugs that positively impact human health and wellness.

Neurological damages in COVID-19 patients: Mechanisms and preventive interventions

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel coronavirus, causes coronavirus disease 2019 (COVID-19) which led to neurological damage and increased mortality worldwide in its second and third waves. It is associated with systemic inflammation, myocardial infarction, neurological illness including ischemic strokes (e.g., cardiac and cerebral ischemia), and even death through multi-organ failure. At the early stage, the virus infects the lung epithelial cells and is slowly transmitted to the other organs including the gastrointestinal tract, blood vessels, kidneys, heart, and brain. The neurological effect of the virus is mainly due to hypoxia-driven reactive oxygen species (ROS) and generated cytokine storm. Internalization of SARS-CoV-2 triggers ROS production and modulation of the immunological cascade which ultimately initiates the hypercoagulable state and vascular thrombosis. Suppression of immunological machinery and inhibition of ROS play an important role in neurological disturbances. So, COVID-19 associated damage to the central nervous system, patients need special care to prevent multi-organ failure at later stages of disease progression. Here in this review, we are selectively discussing these issues and possible antioxidant-based prevention therapies for COVID-19-associated neurological damage that leads to multi-organ failure.

An efficient preparation and biocatalytic synthesis of novel C-glycosylflavonols kaempferol 8-C-glucoside and quercetin 8-C-glucoside through using resting cells and macroporous resins

BACKGROUND

C-glycosylated flavonoids are a main type of structural modification and can endow flavonoids with greater stability, bioactivity, and bioavailability. Although some C-glycosylated flavonoids have been biosynthesized in vivo or vitro, only a few C-glycosylflavonols have been prepared by these methods.

RESULTS

In this study, several uridine 5'-diphosphate (UDP)-glucose biosynthesis pathways and Escherichia coli hosts were screened to reconstruct recombinant strains for producing the novel C-glycosylflavonols kaempferol 8-C-glucoside and quercetin 8-C-glucoside. To increase C-glycosylflavonol production, the timing of flavonol addition was adjusted, and glycerol was added to avoid degradation of C-glycosylflavonols. By using resting cell bioconversion, the highest kaempferol 8-C-glucoside and quercetin 8-C-glucoside production reached 16.6 g/L and 12.5 g/L, respectively. Then, ultrasound-assisted adsorption/desorption was used to prepare C-glycosylflavonols by using macroporous resins. Through screening macroporous resins and optimizing the adsorption/desorption conditions, the highest adsorption capacity and desorption capacity for kaempferol 8-C-glucoside on HPD100 reached 28.57 mg/g and 24.15 mg/g, respectively. Finally, kaempferol 8-C-glucoside (15.4 g) with a yield of 93% and quercetin 8-C-glucoside (11.3 g) with a yield of 91% were obtained from 1 L of fermentation broth.

CONCLUSIONS

Kaempferol 8-C-glucoside and quercetin 8-C-glucoside are novel C-glycosylflavonols, which have not been extracted from plants. This study provides an efficient method for the preparation and biocatalytic synthesis of kaempferol 8-C-glucoside and quercetin 8-C-glucoside by metabolic engineering of Escherichia coli.

May phytophenolics alleviate aflatoxins-induced health challenges? A holistic insight on current landscape and future prospects

The future GCC-connected environmental risk factors expedited the progression of nCDs. Indeed, the emergence of AFs is becoming a global food security concern. AFs are lethal carcinogenic mycotoxins, causing damage to the liver, kidney, and gastrointestinal organs. Long-term exposure to AFs leads to liver cancer. Almost a variety of food commodities, crops, spices, herbaceous materials, nuts, and processed foods can be contaminated with AFs. In this regard, the primary sections of this review aim to cover influencing factors in the occurrence of AFs, the role of AFs in progression of nCDs, links between GCC/nCDs and exposure to AFs, frequency of AFs-based academic investigations, and world distribution of AFs. Next, the current trends in the application of PPs to alleviate AFs toxicity are discussed. Nearly, more than 20,000 published records indexed in scientific databases have been screened to find recent trends on AFs and application of PPs in AFs therapy. Accordingly, shifts in world climate, improper infrastructures for production/storage of food commodities, inconsistency of global polices on AFs permissible concentration in food/feed, and lack of the public awareness are accounting for a considerable proportion of AFs damages. AFs exhibited their toxic effects by triggering the progression of inflammation and oxidative/nitrosative stress, in turn, leading to the onset of nCDs. PPs could decrease AFs-associated oxidative stress, genotoxic, mutagenic, and carcinogenic effects by improving cellular antioxidant balance, regulation of signaling pathways, alleviating inflammatory responses, and modification of gene expression profile in a dose/time-reliant fashion. The administration of PPs alone displayed lower biological properties compared to co-treatment of these metabolites with AFs. This issue might highlight the therapeutic application of PPs than their preventative content. Flavonoids such as quercetin and oxidized tea phenolics, curcumin and resveratrol were the most studied anti-AFs PPs. Our literature review clearly disclosed that considering PPs in antioxidant therapies to alleviate complications of AFs requires improvement in their bioavailability, pharmacokinetics, tissue clearance, and off-target mode of action. Due to the emergencies in the elimination of AFs in food/feedstuffs, further large-scale clinical assessment of PPs to decrease the consequences of AFs is highly required.

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.

Hyperoside as a Potential Natural Product Targeting Oxidative Stress in Liver Diseases

Hyperoside (Hyp), also known as quercetin-3-O-galactoside or 3-O-β-D-galactopyranosyl, is a well-known flavonol glycoside that is abundant in various fruits, vegetables, and medicinal plants. Hyp has been suggested to exhibit a wide range of biological actions, including cardiovascular, renal, neuroprotective, antifungal, antifibrotic, and anticancer effects. Accumulating evidence supports the pharmacological activities of Hyp in improving liver pathophysiology. Hence, the present literature review aims to summarize preclinical data suggesting the beneficial effects and underlying mechanisms of Hyp. In addition, our study focuses on hepatic antioxidant defense signaling to assess the underlying mechanisms of the biological actions of Hyp that are closely associated with liver diseases. Experimental findings from an up-to-date search showed that Hyp possesses hepatoprotective, antiviral, antisteatotic, anti-inflammatory, antifibrotic, and anticancer activities in cellular and animal models related to liver dysfunction by enhancing antioxidant responses. In particular, hepatocellular antioxidant defense via activation of erythroid-related nuclear factor 2 by Hyp chiefly explains how this compound acts as a therapeutic agent in liver diseases. Thus, this review emphasizes the therapeutic potential of Hyp as a strong antioxidative substance that plays a crucial role in the regulation of various liver disorders during their pathogenesis.

Screening and characterizing flavone synthases and its application in biosynthesizing vitexin from naringenin by a one-pot enzymatic cascade

C-glycosylated flavonoids are important structural derivatives of flavonoids and have a variety of physiological activities. Flavone synthase is a key enzyme for producing C-glycosylated flavonoids. In this study, three flavone synthase genes were cloned, overexpressed and characterized in E. coli. By analyzing the enzymatic properties of the enzymes, Aethusa cynapium flavone synthase (AcFNS) was better than Apium graveolens flavone synthase (AgFNS) and Petroselinum crispum flavone synthase (PcFNS) in terms of catalytic ability, organic solvent tolerance and stability. Then, a one-pot enzymatic cascade was developed to synthesize vitexin from naringenin by using AcFNS, C-glycosyltransferase (TcCGT) from Trollius chinensis, and sucrose synthase (GmSUS) from Glycine max. The effects of enzyme ratios, substrate concentrations, cofactors, and reaction conditions on vitexin production were determined. The highest vitexin production reached 935.6 mg/L with a corresponding molar conversion of 78.7 % for (2 S)-naringenin. Thus, this is the first report of a one-pot enzymatic cascade for vitexin production from naringenin in vitro.

Production of Flavonoid 7-O-glucosides by Bioconversion Using Escherichia coli Expressing a 7-O-glucosyltransferase from Tobacco (Nicotiana tabacum)

Flavonoid 7-O-glucosides exhibit various biological activities; however, some are not abundant in nature. Therefore, a method to produce flavonoid 7-O-glucosides was investigated. Escherichia coli expressing tobacco-derived glucosyltransferase (Ec-NtGT2) converted several flavonoids (apigenin, luteolin, quercetin, kaempferol, and naringenin) to their 7-O-glucosides with conversion rates of 67-98%. In scaled-up production, Ec-NtGT2 yielded 24 mg/L of apigenin 7-O-glucoside, 41 mg/L of luteolin 7-O-glucoside, 118 mg/L of quercetin 7-O-glucoside, 40 mg/L of kaempferol 7-O-glucoside, and 75 mg/L of naringenin 7-O-glucoside through sequential administration of substrates in 4-9 h. The conversion rates of apigenin, luteolin, quercetin, kaempferol, and naringenin were 97%, 72%, 77%, 98%, and 96%, respectively. These results indicated that Ec-NtGT2 is a simple and efficient bioconversion system for the production of flavonoid 7-O-glucosides.

Multi-strategy engineering unusual sugar TDP-l-mycarose biosynthesis to improve the production of 3-O-α-mycarosylerythronolide B in Escherichia coli

The insufficient supply of sugar units is the key limitation for the biosynthesis of glycosylated products. The unusual sugar TDP-l-mycarose is initially attached to the C3 of the polyketide erythronolide B, resulting in 3-O-α-mycarosylerythronolide B (MEB). Here, we present the de novo biosynthesis of MEB in Escherichia coli and improve its production using multi-strategy metabolic engineering. Firstly, by blocking precursor glucose-1-phosphate competing pathways, the MEB titer of triple knockout strain QC13 was significantly enhanced to 41.2 mg/L, 9.8-fold to that produced by parental strain BAP230. Subsequently, the MEB production was further increased to 48.3 mg/L through overexpression of rfbA and rfbB. Moreover, the CRISPRi was implemented to promote the TDP-l-mycarose biosynthesis via repressing the glycolysis and TDP-l-rhamnose pathway. Our study paves the way for efficient production of erythromycins in E. coli and provides a promising platform that can be applied for biosynthesis of other glycosylated products with unusual sugars.

Comprehensive utilization of sucrose resources via chemical and biotechnological processes: A review

Sucrose, one of the most widespread disaccharides in nature, has been available in daily human life for many centuries. As an abundant and cheap sweetener, sucrose plays an essential role in our diet and the food industry. However, it has been determined that many diseases, such as obesity, diabetes, hyperlipidemia, etc., directly relate to the overconsumption of sucrose. It arouses many explorations for the conversion of sucrose to high-value chemicals. Production of valuable substances from sucrose by chemical methods has been studied since a half-century ago. Compared to chemical processes, biotechnological conversion approaches of sucrose are more environmentally friendly. Many enzymes can use sucrose as the substrate to generate functional sugars, especially those from GH68, GH70, GH13, and GH32 families. In this review, enzymatic catalysis and whole-cell fermentation of sucrose for the production of valuable chemicals were reviewed. The multienzyme cascade catalysis and metabolic engineering strategies were addressed.

Hyperoside: A review on its sources, biological activities, and molecular mechanisms

Hyperoside is a natural flavonol glycoside in various plants, such as Crataegus pinnatifida Bge, Forsythia suspensa, and Cuscuta chinensis Lam. Medical research has found that hyperoside possesses a broad spectrum of biological activities, including anticancer, anti-inflammatory, antibacterial, antiviral, antidepressant, and organ protective effects. These pharmacological properties lay the foundation for its use in treating multiple diseases, such as sepsis, arthritis, colitis, diabetic nephropathy, myocardial ischemia-reperfusion, pulmonary fibrosis, and cancers. Hyperoside is obtained from the plants and chemical synthesis. This study aims to provide a comprehensive overview of hyperoside on its sources and biological activities to provide insights into its therapeutic potential, and to provide a basis for high-quality studies to determine the clinical efficacy of this compound.

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.

Glycosyltransferases: Mining, engineering and applications in biosynthesis of glycosylated plant natural products

UDP-Glycosyltransferases (UGTs) catalyze the transfer of nucleotide-activated sugars to specific acceptors, among which the GT1 family enzymes are well-known for their function in biosynthesis of natural product glycosides. Elucidating GT function represents necessary step in metabolic engineering of aglycone glycosylation to produce drug leads, cosmetics, nutrients and sweeteners. In this review, we systematically summarize the phylogenetic distribution and catalytic diversity of plant GTs. We also discuss recent progress in the identification of novel GT candidates for synthesis of plant natural products (PNPs) using multi-omics technology and deep learning predicted models. We also highlight recent advances in rational design and directed evolution engineering strategies for new or improved GT functions. Finally, we cover recent breakthroughs in the application of GTs for microbial biosynthesis of some representative glycosylated PNPs, including flavonoid glycosides (fisetin 3-O-glycosides, astragalin, scutellarein 7-O-glucoside), terpenoid glycosides (rebaudioside A, ginsenosides) and polyketide glycosides (salidroside, polydatin).

Quercetin for managing type 2 diabetes and its complications, an insight into multitarget therapy

BACKGROUND

Quercetin, a bioflavonoid abundant in grapefruit, onion, berries, etc., has vast therapeutic potential, especially against Type 2 diabetes and its complications. Quercetin showed similar effects as that of metformin, (widely prescribed antidiabetic drug) in cell lines models (Sajan et al., 2010; Dhanya et al., 2017). In vivo findings also showcase it as a promising agent against diabetes and its pathophysiological complications.

SCOPE AND APPROACH

Quercetin can be produced on a large scale through a novel fermentation-based glycosylation strategy from cheap substrates and can be utilized as a dietary supplement. The review focuses on the mounting evidence pointing to Quercetin as a promising candidate for managing type 2 diabetes and its oxidative stress mediated pathophysiological complications.

CONCLUSION

Quercetin acts on multiple targets of diabetes and regulates key signalling pathways which improve the symptoms as well as the complications of Type 2 diabetes. However further studies are needed to improve the bioavailability and to establish a dosing regimen for Quercetin.

Undifferentiated In Vitro Cultured Actinidia deliciosa as Cell Factory for the Production of Quercetin Glycosides

Land plants produce a vast arsenal of specialized metabolites and many of them display interesting bioactivities in humans. Recently, flavonol quercetin gained great attention in the light of the COVID-19 pandemic because, in addition to the anti-inflammatory, antiviral and anti-cancer activity already described, it emerged as possible inhibitor of 3CLpro, the major protease of SARS-CoV-2 virus. Plant cell and tissue culture (PCTC) is an attractive platform for the biotechnological production of plant metabolites. This technology allows a large amount of water and agricultural land to be saved and, being free of contaminants in the process, it is suitable for scaling up the production in bioreactors. In a project aimed to generate and screen in vitro plant cells for the production of valuable specialized metabolites for commercial production, we generated various cell lines from Actinidia deliciosa (kiwi fruit tree) and Actinidia chinensis (gold kiwi fruit tree), that were able to produce relevant amounts of quercetin derivatives, mainly quercetin glycosides. Three cell lines from A. deliciosa were characterized by targeted and untargeted metabolomics. In standard growing conditions, they produce and accumulate up to 13.26 mg/100 g fresh weight (419.76 mg/100 g dry weight) of quercetin derivatives. To address future industrial applications, these cell lines should be entered into an acceleration program to further increase the amount of these metabolites by optimizing the culture conditions and elicitation.

Whole-Cell Biocatalyst for Rubusoside Production in Saccharomyces cerevisiae

Rubusoside (Rub) is a highly sweet diterpene glycoside mainly isolated from the leaves of Rubus suavissimus (Rosaceae). It has been used as a low-calorie natural sweetener for decades and was recently found to be a potential drug lead. In this study, we designed a whole-cell biocatalyst to achieve the glycosylation of steviol to Rub in Saccharomyces cerevisiae. The sucrose synthases were applied to construct a uridine diphosphate glucose regeneration system, which were coupled with optimal combinations of different uridine diphosphate (UDP) glycosyltransferases from multiple plant species. After optimization of reaction conditions, the residues in SrUGT74G1 probably influencing glycosylation efficiency were subjected to site-directed mutagenesis. Double mutations of S84A/E87A reduced the accumulation of intermediates, finally glucosylating 1.27 g/L steviol to 0.45 ± 0.06 g/L steviolmonoside (conversion rate = 23.3%) and 1.92 ± 0.17 g/L Rub (conversion rate = 74.9%). A high efficiency of Rub biosynthesis could be achieved without supply of additional UDPG. This work provided the first example of multi-step glycosylation reactions in whole-cell biocatalysis, which laid a foundation of scalable production of the value-added diterpene sweetener in the future.

Microbial Transformation of Galangin Derivatives and Cytotoxicity Evaluation of Their Metabolites

Galangin (1), 3-O-methylgalangin (2), and galangin flavanone (3), the major bioactive flavonoids isolated from Alpinia officinarum, were biotransformed into one novel and four known metabolites (4–8) by application of the fungal strains Mucor hiemalis and Absidia coerulea as biocatalysts. Their structures were characterized by extensive spectroscopic analyses including one- and two-dimensional nuclear magnetic resonance spectroscopy and mass spectrometry. Compounds 1–7 were evaluated for their cytotoxic activities against cancer cell lines using the MTT assay. The new compound 3-O-methylgalangin-7-O-β-D-glucopyranoside (6) exhibited the most potent cytotoxic activity against MCF-7, A375P, B16F10, B16F1, and A549 cancer cell lines with the IC50 values at 3.55–6.23 μM.

Production of flavonol and flavone 6-C-glucosides by bioconversion in Escherichia coli expressing a C-glucosyltransferase from wasabi (Eutrema japonicum)

OBJECTIVES

To produce flavonol and flavone 6-C-glucosides by bioconversion using recombinant Escherichia coli expressing a C-glucosyltransferase from wasabi (WjGT1).

RESULTS

Escherichia coli expressing WjGT1 (Ec-WjGT1) converted flavones (apigenin and luteolin) and flavonols (quercetin and kaempferol) into their 6-C-glucosides in M9 minimal media supplemented with glucose, and released these products into the culture media. Ec-WjGT1 system also converts a flavanone (naringenin) into its C-glucoside at a conversion rate of 60% in 6 h. For scale-up production, apigenin, kaempferol, and quercetin were sequentially fed into the Ec-WjGT1 system at concentrations of 20-50 µM every 15-60 min, and the system was then able to produce isovitexin, kaempferol 6-C-glucoside, and quercetin 6-C-glucoside at an 89-99% conversion rate.

CONCLUSIONS

The Ec-WjGT1 system quickly and easily produces flavone and flavonol 6-C-glucosides at high conversion rates when using sequential administration to avoid precipitation of substrates.

Efficient Production of Orientin and Vitexin from Luteolin and Apigenin Using Coupled Catalysis of Glycosyltransferase and Sucrose Synthase

Orientin and vitexin are flavone 8-C-glycosides that exhibit many biological characteristics. This study aimed to establish a two-enzyme-coupled catalytic strategy to enhance the biosynthesis of orientin and vitexin from apigenin and luteolin, respectively. The C-glucosyltransferase (TcCGT1) gene from Trollius chinensis was cloned and expressed in Escherichia coli BL21(DE3). The optimal activity of TcCGT1 was achieved at pH 9.0 and 37 °C. TcCGT1 was relatively stable over the pH range of 7.0-10.0 at a temperature lower than 45 °C. The coupled catalytic strategy of TcCGT1 and different sucrose synthases was adopted to enhance the production of orientin and vitexin. By optimizing the coupling reaction conditions, orientin and vitexin production successfully achieved 2324.4 and 5524.1 mg/L with a yield of 91.4 and 89.3% (mol/mol), respectively. The coupled catalytic strategy proposed in this study might serve as a promising candidate for the large-scale production of orientin and vitexin in the future.

Rhamnose in plants - from biosynthesis to diverse functions

In plants, the deoxy sugar l-rhamnose is widely present as rhamnose-containing polymers in cell walls and as part of the decoration of various specialized metabolites. Here, we review the current knowledge on the distribution of rhamnose, highlighting the differences between what is known in dicotyledoneuos compared to commelinid monocotyledoneous (grasses) plants. We discuss the biosynthesis and transport of UDP-rhamnose, as well as the transfer of rhamnose from UDP-rhamnose to various primary and specialized metabolites. This is carried out by rhamnosyltransferases, enzymes that can use a large variety of substrates. Some unique characteristics of rhamnose synthases, the multifunctional enzymes responsible for the conversion of UDP-glucose into UDP-rhamnose, are considered, particularly from the perspective of their ability to convert glucose present in flavonoids. Finally, we discuss how little is still known with regards to how plants rescue rhamnose from the many compounds to which it is linked, or how rhamnose is catabolized.

Functional characterization of UDP-glycosyltransferases from the liverwort Plagiochasma appendiculatum and their potential for biosynthesizing flavonoid 7-O-glucosides

Flavonoid glucosides, typically generated from aglycones via the action of uridine diphosphate-dependent glycosyltransferases (UGTs), both contribute to plant viability and are pharmacologically active. The properties of UGTs produced by liverworts, one of the basal groups of non-vascular land plants, have not been systematically explored. Here, two UGTs potentially involved in flavonoids synthesis were identified from the transcriptome of Plagiochasma appendiculatum. Enzymatic analysis showed that PaUGT1 and PaUGT2 accepted various flavones, flavonols, flavanones and dihydrochalcones as substrates. A mutated form PaUGT1-Q19A exhibited a higher catalytic efficiency than did the wild type enzyme. When expressed in Escherichia coli, the yield of flavonol 7-O-glucosides reached to over 70 %. Co-expression of PaUGT1-Q19A with the upstream flavone synthase I PaFNS I-1 proved able to convert the flavanone aglycones naringenin and eriodictyol into the higher-yield apigenin 7-O-glucoside (A7G) and luteolin 7-O-glucoside (L7G). The maximum concentration of 81.0 μM A7G and 88.6 μM L7G was achieved upon supplementation with 100 μM naringenin and 100 μM eriodictyol under optimized conditions. This is the first time that flavonoids UGTs have been characterized from liverworts and co-expression of UGTs and FNS Is from the same species serves as an effective strategy to synthesize flavone 7-O-glucosides in E. coli.

Enhancing UDP-Rhamnose Supply for Rhamnosylation of Flavonoids in Escherichia coli by Regulating the Modular Pathway and Improving NADPH Availability

UDP-rhamnose is the main type of sugar donor and endows flavonoids with special activity, selectivity, and pharmacological properties by glycosylation. In this study, several UDP-glucose synthesis pathways and UDP-rhamnose synthases were screened to develop an efficient UDP-rhamnose biosynthesis pathway in Escherichia coli. Maximal UDP-rhamnose production reached 82.2 mg/L in the recombinant strain by introducing the cellobiose phosphorolysis pathway and Arabidopsis thaliana UDP-rhamnose synthase (AtRHM). Quercitrin production of 3522 mg/L was achieved in the recombinant strain by coupling the UDP-rhamnose generation system with A. thaliana rhamnosyltransferase (AtUGT78D1) to recycle UDP-rhamnose. To further increase UDP-rhamnose supply, an NADPH-independent fusion enzyme was constructed, the UTP supply was improved, and NADPH regenerators were overexpressed in vivo. Finally, by optimizing the bioconversion conditions, the highest quercitrin production reached 7627 mg/L with the average productivity of 141 mg/(L h), which is the highest yield of quercitrin and efficiency of UDP-rhamnose supply reported to date in E. coli. Therefore, the method described herein for the regeneration of UDP-rhamnose from cellobiose may be widely used for the rhamnosylation of flavonoids and other bioactive substances.

Galactosylation of Monosaccharide Derivatives of Glycyrrhetinic Acid by UDP-Glycosyltransferase GmSGT2 from Glycine max

Glycyrrhetinic acid (GA), a pentacyclic triterpenoid aglycone, is the major functional component in licorice which mainly exists in the form of functional glycosides in licorice. The introduction of a sugar moiety to the C-3 OH of GA to yield glycosylated derivatives has been reported, but the late-stage glycosylation of GA-3-O-sugar to form rare GA glycosides with more complexed glycosyl decoration has been rarely reported. In this study, a unique UDP-galactosyltransferase GmSGT2 from Glycine max was found to transfer a galactose to the C2 position of the sugar moiety of GA-3-O-monoglucuronide (GAMG) and GA-3-O-monoglucose. In addition to UDP-galactose, GmSGT2 also recognizes UDP-glucose, UDP-xylose, and UDP-arabinose with relative activities of 32.1-89.2%. Based on a test of 12 typical natural products, GmSGT2 showed high specificity toward the pentacyclic triterpenoid skeleton as the sugar acceptor. Molecular docking was performed to elucidate the substrate recognition mechanism of GmSGT2 toward GAMG.

Advances in engineering UDP-sugar supply for recombinant biosynthesis of glycosides in microbes

Plant glycosides are of great interest for industries. Glycosylation of plant secondary metabolites can greatly improve their solubility, biological activity, or stability. This allows some plant glycosides to be used as food additives, cosmetic products, health products, antisepsis and anti-cancer drugs. With the continuous expansion of market demand, a variety of biological fermentation technologies has emerged. This review focuses on recombinant microbial biosynthesis of plant glycosides, which uses UDP-sugars as precursors, and summarizes various strategies to increase the yield of glycosides with a key concentration on UDP-sugar supply based on four aspects, i.e., gene overexpression, UDP-sugar recycling, mixed fermentation, and carbon co-utilization. Meanwhile, the application potential and advantages of various techniques are introduced, which provide guidance to the development of high-yield strains for recombinant microbial production of plant glycosides. Finally, the technical challenges of glycoside biosynthesis are pointed out with discussions on future directions of improving the yield of recombinantly synthesized glycosides.

Production of isoorientin and isovitexin from luteolin and apigenin using coupled catalysis of glycosyltransferase and sucrose synthase

Isoorientin and isovitexin, kinds of flavone C-glycosides, exhibit a number of biological properties. In this work, The C-glucosyltransferase (Gt6CGT) gene from Gentiana triflora was cloned and expressed in Escherichia coli BL21(DE3). The optimal activity of Gt6CGT was at pH 7.5 and 50° C. The enzyme was stable over pH range of 6.5-9.0, and had a 1-h half-life at 50° C. The V_max for luteolin and apigenin was 21.1 nmol/min/mg and 31.7 nmol/min/mg, while the K_m was 0.21 mM and 0.22 mM, respectively. Then, we developed an environmentally safe and efficient method for isoorientin and isovitexin production using the coupled catalysis of Gt6CGT and Glycine max sucrose synthase (GmSUS). By optimizing coupled reaction conditions, the titer of isoorientin and isovitexin reached 3820 mg/L with a corresponding molar conversion of 94.7% and 3772 mg/L with a corresponding molar conversion of 97.1%, respectively. The maximum number of UDP-glucose regeneration cycles (RC_max) reached 28.4 for isoorientin and 29.1 for isovitexin. The coupled catalysis reported herein represents a promising method to meet industrial requirements for large-scale isoorientin and isovitexin production in the future. Graphical Abstract.

Sucrose 6F-phosphate phosphorylase: a novel insight in the human gut microbiome

The human gut microbiome plays an essential role in maintaining human health including in degradation of dietary fibres and carbohydrates further used as nutrients by both the host and the gut bacteria. Previously, we identified a polysaccharide utilization loci (PUL) involved in sucrose and raffinose family oligosaccharide (RFO) metabolism from one of the most common Firmicutes present in individuals, Ruminococcus gnavus E1. One of the enzymes encoded by this PUL was annotated as a putative sucrose phosphate phosphorylase (RgSPP). In the present study, we have in-depth characterized the heterologously expressed RgSPP as sucrose 6^F-phosphate phosphorylase (SPP), expanding our knowledge of the glycoside hydrolase GH13_18 subfamily. Specifically, the enzymatic characterization showed a selective activity on sucrose 6^F-phosphate (S6^FP) acting both in phosphorolysis releasing alpha-d-glucose-1-phosphate (G1P) and alpha-d-fructose-6-phosphate (F6P), and in reverse phosphorolysis from G1P and F6P to S6^FP. Interestingly, such a SPP activity had never been observed in gut bacteria before. In addition, a phylogenetic and synteny analysis showed a clustering and a strictly conserved PUL organization specific to gut bacteria. However, a wide prevalence and abundance study with a human metagenomic library showed a correlation between SPP activity and the geographical origin of the individuals and, thus, most likely linked to diet. Rgspp gene overexpression has been observed in mice fed with a high-fat diet suggesting, as observed for humans, that intestine lipid and carbohydrate microbial metabolisms are intertwined. Finally, based on the genomic environment analysis, in vitro and in vivo studies, results provide new insights into the gut microbiota catabolism of sucrose, RFOs and S6^FP.

Modulating heterologous pathways and optimizing fermentation conditions for biosynthesis of kaempferol and astragalin from naringenin in Escherichia coli

Kaempferol and astragalin are used as standards to assess the quality of Ginkgo biloba extract and Radix astragali, respectively, and possess numerous biological properties. In this study, we constructed a recombinant strain with a highly efficient biosynthetic pathway of kaempferol by screening key enzyme genes, designing a synthetic fusion enzyme and increasing the gene copy number. By optimizing conversion and fed-batch fermentation conditions, maximal kaempferol production reached 1184.2 ± 16.5 mg/L, which represents the highest yield of kaempferol from naringenin reported to date. Based on this result, glycosyltransferase (AtUGT78D2) and an efficient UDP-glucose synthesis pathway were introduced into the recombinant strain to produce astragalin, resulting in maximal astragalin production at 1738.5 ± 24.8 mg/L without kaempferol accumulation. The efficient synthesis pathway described in this study for kaempferol and astragalin biosynthesis can be widely used for flavonoid biosynthesis in Escherichia coli.

Efficient Biotransformation of Luteolin to Isoorientin through Adjusting Induction Strategy, Controlling Acetic Acid, and Increasing UDP-Glucose Supply in Escherichia coli

Isoorientin is a C-glycosylated derivative of luteolin and exhibits a number of biological properties. In this study, multiple strategies were adopted to improve isoorientin production from luteolin in Escherichia coli. Isoorientin production was improved substantially by adjusting induction strategies and controlling acetic acid accumulation, with maximum isoorientin production reaching 826 mg/L. Additionally, a novel UDP-glucose synthesis pathway was reconstructed in E. coli through cellobiose phosphorylase-catalyzed phosphorolysis of cellobiose for the production of glucose 1-phosphate, which serves as a precursor in UDP-glucose formation. The results from two mechanisms of UDP-glucose formation in E. coli, cellobiose phosphorolysis and sucrose phosphorolysis, were compared. Increasing the UDP-glucose supply resulted in maximal isoorientin production reaching 1371 mg/L. Finally, isoorientin (1059 mg) was obtained from 1 L of fermentation broth by simple purification steps with a yield of 81.5%. Therefore, this study provides an efficient method for isoorientin production and a novel UDP-glucose synthesis pathway.

Chemical and molecular characterization of metabolites from Flavobacterium sp

In this study, a Flavobacterium sp. is isolated from natural spring, and identified using molecular techniques. Extracellular and intracellular secondary metabolites are identified using solid phase microextraction gas chromatography-mass spectrometry and ultra performance liquid chromatography. Cytotoxic activity of the extracellular compounds produced by the Flavobacterium sp. and quercetin as the standard are measured using ECV304 human endothelial cells in vitro. Our results show that Flavobacterim sp. isolate has the highest percentage of similarity with Flavobacterium cheonhonense strain ARSA-15 (99%). Quercetin is detected as the major extracellular compound produced by the Flavobacterium sp. Methanol extract of Flavobacterium sp. resulted in a higher cell viability results when compared to DMSO extracts. Computational chemistry approach was used and it has been found that polar solvent (methanol) contributed to higher antioxidant activity. In conclusion, Flavobacterium sp. can be used to produce quercetin for industrial purposes.

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.

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

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

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.

Glycan Phosphorylases in Multi-Enzyme Synthetic Processes

Glycoside phosphorylases catalyse the reversible synthesis of glycosidic bonds by glycosylation with concomitant release of inorganic phosphate. The equilibrium position of such reactions can render them of limited synthetic utility, unless coupled with a secondary enzymatic step where the reaction lies heavily in favour of product. This article surveys recent works on the combined use of glycan phosphorylases with other enzymes to achieve synthetically useful processes.

Functional analyses of OcRhS1 and OcUER1 involved in UDP-L-rhamnose biosynthesis in Ornithogalum caudatum

UDP-L-rhamnose (UDP-Rha) is an important sugar donor for the synthesis of rhamnose-containing compounds in plants. However, only a few enzymes and their encoding genes involved in UDP-Rha biosynthesis are available in plants. Here, two genes encoding rhamnose synthase (RhS) and bi-functional UDP-4-keto-6-deoxy-D-glucose (UDP-4K6DG) 3, 5-epimerase/UDP-4-keto-L-rhamnose (UDP-4KR) 4-keto-reductase (UER) were isolated from Ornithogalum caudatum based on the RNA-Seq data. The OcRhS1 gene has an ORF (open reading frame) of 2019 bp encoding a tri-functional RhS enzyme. In vitro enzymatic assays revealed OcRhS1 can really convert UDP-D-glucose (UDP-Glc) into UDP-Rha via three consecutive reactions. Biochemical evidences indicated that the recombinant OcRhS1 was active in the pH range of 5-11 and over the temperature range of 0-60 °C. The K_m value of OcRhS1 for UDP-Glc was determined to be 1.52 × 10^-4 M. OcRhS1 is a multi-domain protein with two sets of cofactor-binding motifs. The cofactors dependent properties of OcRhS1 were thus characterized in this research. Moreover, the N-terminal portion of OcRhS1 (OcRhS1-N) was observed to metabolize UDP-Glc to form intermediate UDP-4K6DG. OcUER1 contains an ORF of 906 bp encoding a polypeptide of 301 aa. OcUER1 shared high similarity with the carboxy-terminal domain of OcRhS1 (OcRhS1-C), suggesting its intrinsic ability of converting UDP-4K6DG into UDP-Rha. It was thus reasonably inferred that UDP-Glc could be bio-transformed into UDP-Rha under the collaborating action of OcRhS1-N and OcUER1. The subsequently biochemical assay verified this notion. Importantly, expression profiles of OcRhS1 and OcUER1 revealed their possible involvement in the biosynthesis of rhamnose-containing polysaccharides in O. caudatum.