Metabolic engineering of the flavone-C-glycoside pathway using polyprotein technology

Recent advances in plant-based bioproduction

Unable to move on their own, plants have acquired the ability to produce a wide variety of low molecular weight compounds to survive against various stresses. It is estimated that there are as many as one million different kinds. Plants also have the ability to accumulate high levels of proteins. Although plant-based bioproduction has traditionally relied on classical tissue culture methods, the attraction of bioproduction by plants is increasing with the development of omics and bioinformatics and other various technologies, as well as synthetic biology. This review describes the current status and prospects of these plant-based bioproduction from five advanced research topics, (i) de novo production of plant-derived high value terpenoids in engineered yeast, (ii) biotransformation of plant-based materials, (iii) genome editing technology for plant-based bioproduction, (iv) environmental effect of metabolite production in plant factory, and (v) molecular pharming.

Molecular cloning and functional analysis of 4-coumarate: CoA ligases from Marchantia paleacea and their roles in lignin and flavanone biosynthesis

Phenylpropanoids play important roles in plant physiology and the enzyme 4-coumarate: coenzyme A ligase (4CL) catalyzes the formation of thioesters. Despite extensive characterization in various plants, the functions of 4CLs in the liverwort Marchantia paleacea remain unknown. Here, four 4CLs from M. paleacea were isolated and functionally analyzed. Heterologous expression in Escherichia coli indicated the presence of different enzymatic activities in the four enzymes. Mp4CL1 and Mp4CL2 were able to convert caffeic, p-coumaric, cinnamic, ferulic, dihydro-p-coumaric, and 5-hydroxyferulic acids to their corresponding CoA esters, while Mp4CL3 and Mp4CL4 catalyzed none. Mp4CL1 transcription was induced when M. paleacea thalli were treated with methyl jasmonate (MeJA). The overexpression of Mp4CL1 increased the levels of lignin in transgenic Arabidopsis. In addition, we reconstructed the flavanone biosynthetic pathway in E. coli. The pathway comprised Mp4CL1, co-expressed with chalcone synthase (CHS) from different plant species, and the efficiency of biosynthesis was optimal when both the 4CL and CHS were obtained from the same species M. paleacea.

A polycistronic system for multiplexed and precalibrated expression of multigene pathways in fungi

Synthetic biology requires efficient systems that support the well-coordinated co-expression of multiple genes. Here, we discover a 9-bp nucleotide sequence that enables efficient polycistronic gene expression in yeasts and filamentous fungi. Coupling polycistronic expression to multiplexed, markerless, CRISPR/Cas9-based genome editing, we develop a strategy termed HACKing (Highly efficient and Accessible system by CracKing genes into the genome) for the assembly of multigene pathways. HACKing allows the expression level of each enzyme to be precalibrated by linking their translation to those of host proteins with predetermined abundances under the desired fermentation conditions. We validate HACKing by rapidly constructing highly efficient Saccharomyces cerevisiae cell factories that express 13 biosynthetic genes, and produce model endogenous (1,090.41 ± 80.92 mg L^-1 squalene) or heterologous (1.04 ± 0.02 mg L^-1 mogrol) terpenoid products. Thus, HACKing addresses the need of synthetic biology for predictability, simplicity, scalability, and speed upon fungal pathway engineering for valuable metabolites.

Phenolic C-glycoside synthesis using microbial systems

Plants produce different types of phenolic compounds. The majority of these compounds are glycosylated. Phenolic O-glycosides are also common. Recently, C-glycosylation of phenolic compounds has received attention because of the biological importance of phenolic C-glycosides. To date, three classes of C-glycosyltransferases (CGTs) have been characterized based on the type of sugar acceptor: flavonoid CGT, coumarin CGT, and xanthone CGT. Phylogenetic analysis of glycosyltransferases has revealed that CGTs form a distinct class that is clearly different from that of O-glycosyltransferases. The characterized CGTs have been introduced into microbial systems to synthesize phenolic C-glycosides. Here, we review recent progress in the development of CGTs and their application in the synthesis of phenolic C-glycosides using microbial systems.

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.

Identification of a flavonoid C-glycosyltransferase from fern species Stenoloma chusanum and the application in synthesizing flavonoid C-glycosides in Escherichia coli

BACKGROUND

Flavonoid C-glycosides have many beneficial effects and are widely used in food and medicine. However, plants contain a limited number of flavonoid C-glycosides, and it is challenging to create these substances chemically.

RESULTS

To screen more robust C-glycosyltransferases (CGTs) for the biosynthesis of flavonoid C-glycosides, one CGT enzyme from Stenoloma chusanum (ScCGT1) was characterized. Biochemical analyses revealed that ScCGT1 showed the C-glycosylation activity for phloretin, 2-hydroxynaringenin, and 2-hydroxyeriodictyol. Structure modeling and mutagenesis experiments indicated that the glycosylation of ScCGT1 may be initiated by the synergistic action of conserved residue His26 and Asp14. The P164T mutation increased C-glycosylation activity by forming a hydrogen bond with the sugar donor. Furthermore, when using phloretin as a substrate, the extracellular nothofagin production obtained from the Escherichia coli strain ScCGT1-P164T reached 38 mg/L, which was 2.3-fold higher than that of the wild-type strain. Finally, it is proved that the coupling catalysis of CjFNS I/F2H and ScCGT1-P164T could convert naringenin into vitexin and isovitexin.

CONCLUSION

This is the first time that C-glycosyltransferase has been characterized from fern species and provides a candidate gene and strategy for the efficient production of bioactive C-glycosides using enzyme catalysis and metabolic engineering.

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.

Plant Flavonoid Production in Bacteria and Yeasts

Flavonoids, a major group of secondary metabolites in plants, are promising for use as pharmaceuticals and food supplements due to their health-promoting biological activities. Industrial flavonoid production primarily depends on isolation from plants or organic synthesis, but neither is a cost-effective or sustainable process. In contrast, recombinant microorganisms have significant potential for the cost-effective, sustainable, environmentally friendly, and selective industrial production of flavonoids, making this an attractive alternative to plant-based production or chemical synthesis. Structurally and functionally diverse flavonoids are derived from flavanones such as naringenin, pinocembrin and eriodictyol, the major basic skeletons for flavonoids, by various modifications. The establishment of flavanone-producing microorganisms can therefore be used as a platform for producing various flavonoids. This review summarizes metabolic engineering and synthetic biology strategies for the microbial production of flavanones. In addition, we describe directed evolution strategies based on recently-developed high-throughput screening technologies for the further improvement of flavanone production. We also describe recent progress in the microbial production of structurally and functionally complicated flavonoids via the flavanone modifications. Strategies based on synthetic biology will aid more sophisticated and controlled microbial production of various flavonoids.

Tricin Biosynthesis and Bioengineering

Tricin (3',5'-dimethoxyflavone) is a specialized metabolite which not only confers stress tolerance and involves in defense responses in plants but also represents a promising nutraceutical. Tricin-type metabolites are widely present as soluble tricin O-glycosides and tricin-oligolignols in all grass species examined, but only show patchy occurrences in unrelated lineages in dicots. More strikingly, tricin is a lignin monomer in grasses and several other angiosperm species, representing one of the "non-monolignol" lignin monomers identified in nature. The unique biological functions of tricin especially as a lignin monomer have driven the identification and characterization of tricin biosynthetic enzymes in the past decade. This review summarizes the current understanding of tricin biosynthetic pathway in grasses and tricin-accumulating dicots. The characterized and potential enzymes involved in tricin biosynthesis are highlighted along with discussion on the debatable and uncharacterized steps. Finally, current developments of bioengineering on manipulating tricin biosynthesis toward the generation of functional food as well as modifications of lignin for improving biorefinery applications are summarized.

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.

De novo biosynthesis of C-arabinosylated flavones by utilization of indica rice C-glycosyltransferases

Flavone C-arabinosides/xylosides are plant-originated glycoconjugates with various bioactivities. However, the potential utility of these molecules is hindered by their low abundance in nature. Engineering biosynthesis pathway in heterologous bacterial chassis provides a sustainable source of these C-glycosides. We previously reported bifunctional C-glucosyl/C-arabinosyltransferases in Oryza sativa japonica and O. sativa indica, which influence the C-glycoside spectrum in different rice varieties. In this study, we proved the C-arabinosyl-transferring activity of rice C-glycosyltransferases (CGTs) on the mono-C-glucoside substrate nothofagin, followed by taking advantage of specific CGTs and introducing heterologous UDP-pentose supply, to realize the production of eight different C-arabinosides/xylosides in recombinant E. coli. Fed-batch fermentation and precursor supplement maximized the titer of rice-originated C-arabinosides to 20–110 mg/L in an E. coli chassis. The optimized final titer of schaftoside and apigenin di-C-arabinoside reached 19.87 and 113.16 mg/L, respectively. We demonstrate here the success of de novo bio-production of C-arabinosylated and C-xylosylated flavones by heterologous pathway reconstitution. These results lay a foundation for further optimal manufacture of complex flavonoid compounds in microbial cell factories.

Production of Carminic Acid by Metabolically Engineered Escherichia coli

Carminic acid is an aromatic polyketide found in scale insects (i.e., Dactylopius coccus) and is a widely used natural red colorant. It has long been produced by the cumbersome farming of insects followed by multistep purification processes. Thus, there has been much interest in producing carminic acid by the fermentation of engineered bacteria. Here we report the complete biosynthesis of carminic acid from glucose in engineered Escherichia coli. We first optimized the type II polyketide synthase machinery from Photorhabdus luminescens, enabling a high-level production of flavokermesic acid upon coexpression of the cyclases ZhuI and ZhuJ from Streptomyces sp. R1128. To discover the enzymes responsible for the remaining two reactions (hydroxylation and C-glucosylation), biochemical reaction analyses were performed by testing enzyme candidates reported to perform similar reactions. The two identified enzymes, aklavinone 12-hydroxylase (DnrF) from Streptomyces peucetius and C-glucosyltransferase (GtCGT) from Gentiana triflora, could successfully perform hydroxylation and C-glucosylation of flavokermesic acid, respectively. Then, homology modeling and docking simulations were performed to enhance the activities of these two enzymes, leading to the generation of beneficial mutants with 2-5-fold enhanced conversion efficiencies. In addition, the GtCGT mutant was found to be a generally applicable C-glucosyltransferase in E. coli, as was showcased by the successful production of aloesin found in Aloe vera. Simple metabolic engineering followed by fed-batch fermentation resulted in 0.63 ± 0.02 mg/L of carminic acid production from glucose. The strategies described here will be useful for the design and construction of biosynthetic pathways involving unknown enzymes and consequently the production of diverse industrially important natural products.

Natural product C-glycosyltransferases - a scarcely characterised enzymatic activity with biotechnological potential

Covering: up to 2020C-Glycosyltransferases are enzymes that catalyse the transfer of sugar molecules to carbon atoms in substituted aromatic rings of a variety of natural products. The resulting β-C-glycosidic bond is more stable in vivo than most O-glycosidic bonds, hence offering an attractive modulation of a variety of compounds with multiple biological activities. While C-glycosylated natural products have been known for centuries, our knowledge of corresponding C-glycosyltransferases is scarce. Here, we discuss commonalities and differences in the known C-glycosyltransferases, review attempts to leverage them as synthetic biocatalysts, and discuss current challenges and limitations in their research and application.

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.

Flavonoids are indispensable for complete male fertility in rice

Flavonoids are essential for male fertility in some but not all plant species. In rice (Oryza sativa), the chalcone synthase mutant oschs1 produces flavonoid-depleted pollen and is male sterile. The mutant pollen grains are viable with normal structure, but they display reduced germination rate and pollen-tube length. Analysis of oschs1/+ heterozygous lines shows that pollen flavonoid deposition is a paternal effect and fertility is independent of the haploid genotypes (OsCHS1 or oschs1). To understand which classes of flavonoids are involved in male fertility, we conducted detailed analysis of rice mutants for branch-point enzymes of the downstream flavonoid pathways, including flavanone 3-hydroxylase (OsF3H; flavonol pathway entry enzyme), flavone synthase II (CYP93G1; flavone pathway entry enzyme), and flavanone 2-hydroxylase (CYP93G2; flavone C-glycoside pathway entry enzyme). Rice osf3h and cyp93g1 cyp93g2 CRISPR/Cas9 mutants, and cyp93g1 and cyp93g2 T-DNA insertion mutants showed altered flavonoid profiles in anthers, but only the osf3h and cyp93g1 cyp93g2 mutants displayed reduction in seed yield. Our findings indicate that flavonoids are essential for complete male fertility in rice and a combination of different classes (flavanones, flavonols, flavones, and flavone C-glycosides) appears to be important, as opposed to the essential role played primarily by flavonols that has been previously reported in several plant species.

Advances on the in vivo and in vitro glycosylations of flavonoids

Flavonoids possess diverse bioactivity and potential medicinal values. Glycosylation of flavonoids, coupling flavonoid aglycones and glycosyl groups in conjugated form, can change the biological activity of flavonoids, increase water solubility, reduce toxic and side effects, and improve specific targeting. Therefore, it is desirable to synthesize various flavonoid glycosides for further investigation on their medicinal values. Compared with chemical glycosylations, biotransformations catalyzed by uridine diphospho-glycosyltransferases provide an environmentally friendly way to construct glycosidic bonds without repetitive chemical synthetic steps of protection, activation, coupling, and deprotection. In this review, we will summarize the existing knowledge on the biotechnological glycosylation reactions either in vitro or in vivo for the synthesis of flavonoid O- and C-glycosides and other rare analogs.Key points• Flavonoid glycosides usually show improved properties compared with their flavonoid aglycones.• Chemical glycosylation requires repetitive synthetic steps and purifications.• Biotechnological glycosylation reactions either in vitro or in vivo were discussed.• Provides representative synthetic examples in detail.

Pathway-specific enzymes from bamboo and crop leaves biosynthesize anti-nociceptive C-glycosylated flavones

C-glycosylated flavones (CGFs) are promising candidates as anti-nociceptive compounds. The leaves of bamboo and related crops in the grass family are a largely unexploited bioresource with a wide array of CGFs. We report here pathway-specific enzymes including C-glycosyltransferases (CGTs) and P450 hydroxylases from cereal crops and bamboo species accumulating abundant CGFs. Mining of CGTs and engineering of P450s that decorate the flavonoid skeleton allowed the production of desired CGFs (with yield of 20–40 mg/L) in an Escherichia coli cell factory. We further explored the antinociceptive activity of major CGFs in mice models and identified isoorientin as the most potent, with both neuroanalgesic and anti-inflammatory effects superior to clinical drugs such as rotundine and aspirin. Our discovery of the pain-alleviating flavonoids elicited from bamboo and crop leaves establishes this previously underutilized source, and sheds light on the pathway and pharmacological mechanisms of the compounds.

Yuwei Sun, Zhuo Chen, Jingya Yang et al. identify bamboo as a rich source of C-glycosylated flavonoids that reduces pain and inflammation. They identify isoorientin as the most potent C-glycosylated flavonoid, superior to aspirin, and report new enzymes that synthesize pain-alleviating C-glycosylated flavonoids.

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.

Engineered Pichia pastoris production of fusaruside, a selective immunomodulator

BACKGROUD

Fusaruside is an immunomodulatory fungal sphingolipid which has medical potentials for treating colitis and liver injury, but its poor natural abundance limits its further study.

RESULTS

In this study, we described a synthetic biology approach for fusaruside production by engineered Pichia pastoris that was based on polycistronic expression. Two fusaruside biosynthesis genes (Δ3(E)-sd and Δ10(E)-sd), were introduced into P. pastoris to obtain fusaruside producing strain FUS2. To further enhance the yield of fusaruside, three relevant biosynthetic genes (Δ3(E)-sd, Δ10(E)-sd and gcs) were subsequently introduced into P. pastoris to obtain FUS3. All of the biosynthetic genes were successfully co-expressed in FUS2 and FUS3. Compared to that produced by FUS2, fusaruside achieved from FUS3 were slightly increased. In addition, the culture conditions including pH, temperature and methanol concentration were optimized to improve the fusaruside production level.

CONCLUSIONS

Here a novel P. pastoris fusaruside production system was developed by introducing the biosynthetic genes linked by 2A peptide gene sequences into a polycistronic expression construct, laying a foundation for further development and application of fusaruside.

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.

Indirect and direct routes to C-glycosylated flavones in Saccharomyces cerevisiae

BACKGROUND

C-glycosylated flavones have recently attracted increased attention due to their possible benefits in human health. These biologically active compounds are part of the human diet, and the C-linkage makes them more resistant to hydrolysis and degradation than O-glycosides. In contrast to O-glycosyltransferases, few C-glycosyltransferases (CGTs) have so far been characterized. Two different biosynthetic routes for C-glycosylated flavones have been identified in plants. Depending on the type of C-glycosyltransferase, flavones can be glycosylated either directly or indirectly via C-glycosylation of a 2-hydroxyflavanone intermediate formed by a flavanone 2-hydroxylase (F2H).

RESULTS

In this study, we reconstructed the pathways in the yeast Saccharomyces cerevisiae, to produce some relevant CGT substrates, either the flavanones naringenin and eriodictyol or the flavones apigenin and luteolin. We then demonstrated two-step indirect glycosylation using combinations of F2H and CGT, to convert 2-hydroxyflavanone intermediates into the 6C-glucoside flavones isovitexin and isoorientin, and the 8C-glucoside flavones vitexin and orientin. Furthermore, we established direct glycosylation of flavones using the recently identified GtUF6CGT1 from Gentiana triflora. The ratio between 6C and 8C glycosylation depended on the CGT used. The indirect route resulted in mixtures, similar to what has been reported for in vitro experiments. In this case, hydroxylation at the flavonoid 3'-position shifted the ratio towards the 8C-glucosylated orientin. The direct flavone glycosylation by GtUF6CGT1, on the other hand, resulted exclusively in 6C-glucosides.

CONCLUSIONS

The current study features yeast as a promising host for production of flavone C-glycosides, and it provides a set of tools and strains for identifying and studying CGTs and their mechanisms of C-glycosylation.

Antinutritional factors in pearl millet grains: Phytate and goitrogens content variability and molecular characterization of genes involved in their pathways

Pearl millet [Pennisetum glaucum (L.) R. Br.] is an important "orphan" cereal and the most widely grown of all the millet species worldwide. It is also the sixth most important cereal in the world after wheat, rice, maize, barley, and sorghum, being largely grown and used in West Africa as well as in India and Pakistan. The present study was carried out in the frame of a program designed to increase benefits and reduce potential health problems deriving from the consumption of pearl millet. The specific goal was to provide a database of information on the variability existing in pearl millet germplasm as to the amounts of phytate, the most relevant antinutrient compound, and the goitrogenic compounds C-glycosylflavones (C-GFs) accumulated in the grain.Results we obtained clearly show that, as indicated by the range in values, a substantial variability subsists across the investigated pearl millet inbred lines as regards the grain level of phytic acid phosphate, while the amount of C-GFs shows a very high variation. Suitable potential parents to be used in breeding programs can be therefore chosen from the surveyed material in order to create new germplasm with increased nutritional quality and food safety. Moreover, we report novel molecular data showing which genes are more relevant for phytic acid biosynthesis in the seeds as well as a preliminary analysis of a pearl millet orthologous gene for C-GFs biosynthesis. These results open the way to dissect the genetic determinants controlling key seed nutritional phenotypes and to the characterization of their impact on grain nutritional value in pearl millet.

Biocatalytic C-Glucosylation of Coumarins Using an Engineered C-Glycosyltransferase

The enzymatic synthesis of coumarin C-glucosides by an engineered C-glycosyltransferase, MiCGTb-GAGM, was explored in vitro and in vivo. MiCGTb-GAGM exhibited a robust C-glucosylation capability toward structurally diverse coumarin derivatives. The whole-cell bioconversion of MiCGTb-GAGM was exploited for large-scale production of coumarin C-glucosides. Two C-glucosides exhibited potent SGLT2 inhibitory activities with IC₅₀ values at 10^-6 M. These findings provide cost-effective and practical synthetic strategies to generate structurally diverse and novel bioactive coumarin C-glycosides for drug discovery.

Engineering co-culture system for production of apigetrin in Escherichia coli

Microbial cells have extensively been utilized to produce value-added bioactive compounds. Based on advancement in protein engineering, DNA recombinant technology, genome engineering, and metabolic remodeling, the microbes can be re-engineered to produce industrially and medicinally important platform chemicals. The emergence of co-culture system which reduces the metabolic burden and allows parallel optimization of the engineered pathway in a modular fashion restricting the formation of undesired byproducts has become an alternative way to synthesize and produce bioactive compounds. In this study, we present genetically engineered E. coli-based co-culture system to the de novo synthesis of apigetrin (APG), an apigenin-7-O-β-D-glucopyranoside of apigenin. The culture system consists of an upstream module including 4-coumarate: CoA ligase (4CL), chalcone synthase, chalcone flavanone isomerase (CHS, CHI), and flavone synthase I (FNSI) to synthesize apigenin (API) from p-coumaric acid (PCA). Whereas, the downstream system contains a metabolizing module to enhance the production of UDP-glucose and expression of glycosyltransferase (PaGT3) to convert API into APG. To accomplish this improvement in titer, the initial inoculum ratio of strains for making the co-culture system, temperature, and media component was optimized. Following large-scale production, a yield of 38.5 µM (16.6 mg/L) of APG was achieved. In overall, this study provided an efficient tool to synthesize bioactive compounds in microbial cells.

C-Glycosyltransferases catalyzing the formation of di-C-glucosyl flavonoids in citrus plants

Citrus plants accumulate many kinds of flavonoids, including di-C-glucosyl flavonoids, which have attracted considerable attention due to their health benefits. However, the biosynthesis of di-C-glucosyl flavonoids has not been elucidated at the molecular level. Here, we identified the C-glycosyltransferases (CGTs) FcCGT (UGT708G1) and CuCGT (UGT708G2) as the primary enzymes involved in the biosynthesis of di-C-glucosyl flavonoids in the citrus plants kumquat (Fortunella crassifolia) and satsuma mandarin (Citrus unshiu), respectively. The amino acid sequences of these CGTs were 98% identical, indicating that CGT genes are highly conserved in the citrus family. The recombinant enzymes FcCGT and CuCGT utilized 2-hydroxyflavanones, dihydrochalcone, and their mono-C-glucosides as sugar acceptors and produced corresponding di-C-glucosides. The K_m and k_cat values of FcCGT toward phloretin were <0.5 μm and 12.0 sec^-1 , and those toward nothofagin (3'-C-glucosylphloretin) were 14.4 μm and 5.3 sec^-1 , respectively; these values are comparable with those of other glycosyltransferases reported to date. Transcripts of both CGT genes were found to concentrate in various plant organs, and particularly in leaves. Our results suggest that di-C-glucosyl flavonoid biosynthesis proceeds via a single enzyme using either 2-hydroxyflavanones or phloretin as a substrate in citrus plants. In addition, Escherichia coli cells expressing CGT genes were found to be capable of producing di-C-glucosyl flavonoids, which is promising for commercial production of these valuable compounds.

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.

Industrial systems biology and its impact on synthetic biology of yeast cell factories

Engineering industrial cell factories to effectively yield a desired product while dealing with industrially relevant stresses is usually the most challenging step in the development of industrial production of chemicals using microbial fermentation processes. Using synthetic biology tools, microbial cell factories such as Saccharomyces cerevisiae can be engineered to express synthetic pathways for the production of fuels, biopharmaceuticals, fragrances, and food flavors. However, directing fluxes through these synthetic pathways towards the desired product can be demanding due to complex regulation or poor gene expression. Systems biology, which applies computational tools and mathematical modeling to understand complex biological networks, can be used to guide synthetic biology design. Here, we present our perspective on how systems biology can impact synthetic biology towards the goal of developing improved yeast cell factories. Biotechnol. Bioeng. 2016;113: 1164-1170. © 2015 Wiley Periodicals, Inc.

A dual-intein autoprocessing domain that directs synchronized protein co-expression in both prokaryotes and eukaryotes

Being able to coordinate co-expression of multiple proteins is necessary for a variety of important applications such as assembly of protein complexes, trait stacking, and metabolic engineering. Currently only few options are available for multiple recombinant protein co-expression, and most of them are not applicable to both prokaryotic and eukaryotic hosts. Here, we report a new polyprotein vector system that is based on a pair of self-excising mini-inteins fused in tandem, termed the dual-intein (DI) domain, to achieve synchronized co-expression of multiple proteins. The DI domain comprises an Ssp DnaE mini-intein N159A mutant and an Ssp DnaB mini-intein C1A mutant connected in tandem by a peptide linker to mediate efficient release of the flanking proteins via autocatalytic cleavage. Essentially complete release of constituent proteins, GFP and RFP (mCherry), from a polyprotein precursor, in bacterial, mammalian, and plant hosts was demonstrated. In addition, successful co-expression of GFP with chloramphenicol acetyltransferase, and thioredoxin with RFP, respectively, further substantiates the general applicability of the DI polyprotein system. Collectively, our results demonstrate the DI-based polyprotein technology as a highly valuable addition to the molecular toolbox for multi-protein co-expression which finds vast applications in biotechnology, biosciences, and biomedicine.

Biotechnological advances in UDP-sugar based glycosylation of small molecules

Glycosylation of small molecules like specialized (secondary) metabolites has a profound impact on their solubility, stability or bioactivity, making glycosides attractive compounds as food additives, therapeutics or nutraceuticals. The subsequently growing market demand has fuelled the development of various biotechnological processes, which can be divided in the in vitro (using enzymes) or in vivo (using whole cells) production of glycosides. In this context, uridine glycosyltransferases (UGTs) have emerged as promising catalysts for the regio- and stereoselective glycosylation of various small molecules, hereby using uridine diphosphate (UDP) sugars as activated glycosyldonors. This review gives an extensive overview of the recently developed in vivo production processes using UGTs and discusses the major routes towards UDP-sugar formation. Furthermore, the use of interconverting enzymes and glycorandomization is highlighted for the production of unusual or new-to-nature glycosides. Finally, the technological challenges and future trends in UDP-sugar based glycosylation are critically evaluated and summarized.

Microbial biosynthesis of medicinally important plant secondary metabolites

Secondary metabolites derived from plants are a valuable source of pharmaceuticals, nutraceuticals, and cosmetics. To harness the potential of these natural products, reliable methods must be developed for their rapid and sustainable resupply. Microbial production of plant secondary metabolites through the heterologous expression of plant biosynthetic genes represents one such solution. This highlight focuses on recent advances in the microbial biosynthesis of plant secondary metabolites including terpenoids, flavonoids, and alkaloids as well as providing a brief insight into the current limitations and future prospects.

Purification, molecular cloning and functional characterization of flavonoid C-glucosyltransferases from Fagopyrum esculentum M. (buckwheat) cotyledon

C-Glycosides are characterized by their C-C bonds in which the anomeric carbon of the sugar moieties is directly bound to the carbon atom of aglycon. C-Glycosides are remarkably stable, as their C-C bonds are resistant to glycosidase or acid hydrolysis. A variety of plant species are known to accumulate C-glycosylflavonoids; however, the genes encoding for enzymes that catalyze C-glycosylation of flavonoids have been identified only from Oryza sativa (rice) and Zea mays (maize), and have not been identified from dicot plants. In this study, we identified the C-glucosyltransferase gene from the dicot plant Fagopyrum esculentum M. (buckwheat). We purified two isozymes from buckwheat seedlings that catalyze C-glucosylation of 2-hydroxyflavanones, which are expressed specifically in the cotyledon during seed germination. Following purification we isolated the cDNA corresponding to each isozyme [FeCGTa (UGT708C1) and FeCGTb (UGT708C2)]. When expressed in Escherichia coli, both proteins demonstrated C-glucosylation activity towards 2-hydroxyflavanones, dihydrochalcone, trihydroxyacetophenones and other related compounds with chemical structures similar to 2',4',6'-trihydroxyacetophenone. Molecular phylogenetic analysis of plant glycosyltransferases shows that flavonoid C-glycosyltransferases form a different clade with other functionally analyzed plant glycosyltransferases.

Cytochrome P450 93G1 Is a Flavone Synthase II That Channels Flavanones to the Biosynthesis of Tricin O-Linked Conjugates in Rice

Flavones are a major class of flavonoids with a wide range of physiological functions in plants. They are constitutively accumulated as C-glycosides and O-linked conjugates in vegetative tissues of grasses. It has long been presumed that the two structural modifications of flavones occur through independent metabolic routes. Previously, we reported that cytochrome P450 93G2 (CYP93G2) functions as a flavanone 2-hydroxylase (F2H) that provides 2-hydroxyflavanones for C-glycosylation in rice (Oryza sativa). Flavone C-glycosides are subsequently formed by dehydratase activity on 2-hydroxyflavanone C-glycosides. On the other hand, O-linked modifications were proposed to proceed after the flavone nucleus is generated. In this study, we demonstrate that CYP93G1, the closest homolog of CYP93G2 in rice, is a bona fide flavone synthase II (FNSII) that catalyzes the direct conversion of flavanones to flavones. In recombinant enzyme assays, CYP93G1 desaturated naringenin and eriodictyol to apigenin and luteolin, respectively. Consistently, transgenic expression of CYP93G1 in Arabidopsis (Arabidopsis thaliana) resulted in the accumulation of different flavone O-glycosides, which are not naturally present in cruciferous plants. Metabolite analysis of a rice CYP93G1 insertion mutant further demonstrated the preferential depletion of tricin O-linked flavanolignans and glycosides. By contrast, redirection of metabolic flow to the biosynthesis of flavone C-glycosides was observed. Our findings established that CYP93G1 is a key branch point enzyme channeling flavanones to the biosynthesis of tricin O-linked conjugates in rice. Functional diversification of F2H and FNSII in the cytochrome P450 CYP93G subfamily may represent a lineage-specific event leading to the prevalent cooccurrence of flavone C- and O-linked derivatives in grasses today.

A two-step O- to C-glycosidic bond rearrangement using complementary glycosyltransferase activities

An efficient 2'-O- to 3'-C-β-d-glucosidic bond rearrangement on the dihydrochalcone phloretin to convert phlorizin into nothofagin was achieved by combining complementary O-glycosyltransferase (OGT) and C-glycosyltransferase (CGT) activities in a one-pot transformation containing catalytic amounts of uridine 5'-diphosphate (UDP). Two separate enzymes or a single engineered dual-specific O/CGT were applied. Overall (quantitative) conversion occurred in two steps via intermediary UDP-glucose and phloretin.

Biochemical strategies for enhancing the in vivo production of natural products with pharmaceutical potential

Natural products have been associated with significant health benefits in preventing and treating various chronic human diseases such as cancer, cardiovascular diseases, diabetes, Alzheimer's disease, and pathogenic infections. However, the isolation, characterization and evaluation of natural products remain a challenge, mainly due to their limited bioavailability. Metabolic engineering and fermentation technology have emerged as alternative approaches for generating natural products under controlled conditions that can be optimized to maximize yields. Optimization of these processes includes the evaluation of factors such as host selection, product biosynthesis interaction with the cell's central metabolism, product degradation, and byproduct formation. This review summarizes the most recent biochemical strategies and advances in expanding and diversifying natural compounds as well as maximizing their production in microbial and plants cells.

Leloir Glycosyltransferases and Natural Product Glycosylation: Biocatalytic Synthesis of the C-Glucoside Nothofagin, a Major Antioxidant of Redbush Herbal Tea

Nothofagin is a major antioxidant of redbush herbal tea and represents a class of bioactive flavonoid-like C-glycosidic natural products. We developed an efficient enzymatic synthesis of nothofagin based on a one-pot coupled glycosyltransferase-catalyzed transformation that involves perfectly selective 3'-C-β-d-glucosylation of naturally abundant phloretin and applies sucrose as expedient glucosyl donor. C-Glucosyltransferase from Oryza sativa (rice) was used for phloretin C-glucosylation from uridine 5'-diphosphate (UDP)-glucose, which was supplied continuously in situ through conversion of sucrose and UDP catalyzed by sucrose synthase from Glycine max (soybean). In an evaluation of thermodynamic, kinetic, and stability parameters of the coupled enzymatic reactions, poor water solubility of the phloretin acceptor substrate was revealed as a major bottleneck of conversion efficiency. Using periodic feed of phloretin controlled by reaction progress, nothofagin concentrations (45 mM; 20 g l-1) were obtained that vastly exceed the phloretin solubility limit (5-10 mM). The intermediate UDP-glucose was produced from catalytic amounts of UDP (1.0 mM) and was thus recycled 45 times in the process. Benchmarked against comparable glycosyltransferase-catalyzed transformations (e.g., on quercetin), the synthesis of nothofagin has achieved intensification in glycosidic product formation by up to three orders of magnitude (μM→mM range). It thus makes a strong case for the application of Leloir glycosyltransferases in biocatalytic syntheses of glycosylated natural products as fine chemicals.

Identification of a bifunctional maize C- and O-glucosyltransferase

Flavonoids accumulate in plant vacuoles usually as O-glycosylated derivatives, but several species can also synthesize flavonoid C-glycosides. Recently, we demonstrated that a flavanone 2-hydroxylase (ZmF2H1, CYP93G5) converts flavanones to the corresponding 2-hydroxy derivatives, which are expected to serve as substrates for C-glycosylation. Here, we isolated a cDNA encoding a UDP-dependent glycosyltransferase (UGT708A6), and its activity was characterized by in vitro and in vivo bioconversion assays. In vitro assays using 2-hydroxyflavanones as substrates and in vivo activity assays in yeast co-expressing ZmF2H1 and UGT708A6 show the formation of the flavones C-glycosides. UGT708A6 can also O-glycosylate flavanones in bioconversion assays in Escherichia coli as well as by in vitro assays with the purified recombinant protein. Thus, UGT708A6 is a bifunctional glycosyltransferase that can produce both C- and O-glycosidated flavonoids, a property not previously described for any other glycosyltransferase.

Challenges and pitfalls of P450-dependent (+)-valencene bioconversion by Saccharomyces cerevisiae

Natural nootkatone is a high value ingredient for the flavor and fragrance industry because of its grapefruit flavor/odor, low sensorial threshold and low availability. Valencene conversion into nootkatol and nootkatone is known to be catalyzed by cytochrome P450 enzymes from both prokaryotic and eukaryotic organisms, but so far development of a viable bioconversion process using either native microorganisms or recombinant enzymes was not successful. Using an in silico gene-mining approach, we selected 4 potential candidate P450 enzymes from higher plants and identified two of them that selectively converted (+)-valencene into β-nootkatol with high efficiency when tested using recombinant yeast microsomes in vitro. Recombinant yeast expressing CYP71D51v2 from tobacco and a P450 reductase from arabidopsis was used for optimization of a bioconversion process. Bioconversion assays led to production of β-nootkatol and nootkatone, but with low yields that decreased upon increase of the substrate concentration. The reasons for this low bioconversion efficiency were further investigated and several factors potentially hampering industry-compatible valencene bioconversion were identified. One is the toxicity of the products for yeast at concentrations exceeding 100 mg L⁻¹. The second is the accumulation of β-nootkatol in yeast endomembranes. The third is the inhibition of the CYP71D51v2 hydroxylation reaction by the products. Furthermore, we observed that the formation of nootkatone from β-nootkatol is not P450-dependent but catalyzed by a yeast component. Based on these data, we propose new strategies for implementation of a viable P450-based bioconversion process.