Glucosylation of flavonols by Escherichia coli expressing glucosyltransferase from rice (Oryza sativa)

Evolution-aided engineering of plant specialized metabolism

The evolution of new traits in living organisms occurs via the processes of mutation, recombination, genetic drift, and selection. These processes that have resulted in the immense biological diversity on our planet are also being employed in metabolic engineering to optimize enzymes and pathways, create new-to-nature reactions, and synthesize complex natural products in heterologous systems. In this review, we discuss two evolution-aided strategies for metabolic engineering-directed evolution, which improves upon existing genetic templates using the evolutionary process, and combinatorial pathway reconstruction, which brings together genes evolved in different organisms into a single heterologous host. We discuss the general principles of these strategies, describe the technologies involved and the molecular traits they influence, provide examples of their use, and discuss the roadblocks that need to be addressed for their wider adoption. A better understanding of these strategies can provide an impetus to research on gene function discovery and biochemical evolution, which is foundational for improved metabolic engineering. These evolution-aided approaches thus have a substantial potential for improving our understanding of plant metabolism in general, for enhancing the production of plant metabolites, and in sustainable agriculture.

Genetic mapping identifies a rice naringenin O-glucosyltransferase that influences insect resistance

Naringenin, the biochemical precursor for predominant flavonoids in grasses, provides protection against UV damage, pathogen infection and insect feeding. To identify previously unknown loci influencing naringenin accumulation in rice (Oryza sativa), recombinant inbred lines derived from the Nipponbare and IR64 cultivars were used to map a quantitative trait locus (QTL) for naringenin abundance to a region of 50 genes on rice chromosome 7. Examination of candidate genes in the QTL confidence interval identified four predicted uridine diphosphate-dependent glucosyltransferases (Os07g31960, Os07g32010, Os07g32020 and Os07g32060). In vitro assays demonstrated that one of these genes, Os07g32020 (UGT707A3), encodes a glucosyltransferase that converts naringenin and uridine diphosphate-glucose to naringenin-7-O-β-d-glucoside. The function of Os07g32020 was verified with CRISPR/Cas9 mutant lines, which accumulated more naringenin and less naringenin-7-O-β-d-glucoside and apigenin-7-O-β-d-glucoside than wild-type Nipponbare. Expression of Os12g13800, which encodes a naringenin 7-O-methyltransferase that produces sakuranetin, was elevated in the mutant lines after treatment with methyl jasmonate and insect pests, Spodoptera litura (cotton leafworm), Oxya hyla intricata (rice grasshopper) and Nilaparvata lugens (brown planthopper), leading to a higher accumulation of sakuranetin. Feeding damage from O. hyla intricata and N. lugens was reduced on the Os07g32020 mutant lines relative to Nipponbare. Modification of the Os07g32020 gene could be used to increase the production of naringenin and sakuranetin rice flavonoids in a more targeted manner. These findings may open up new opportunities for selective breeding of this important rice metabolic trait.

A Review on Structure, Modifications and Structure-Activity Relation of Quercetin and Its Derivatives

Quercetin and its derivatives are important metabolites that belong to the flavonol class of flavonoids. Quercetin and some of the conjugates have been approved by the FDA for human use. They are widely distributed among plants and have various biological activities, such as being anticancer, antiviral, and antioxidant. Hence, the biosynthesis of novel derivatives is an important field of research. Glycosylation and methylation are two important modification strategies that have long been used and have resulted in many novel metabolites that are not present in natural sources. A strategy for modifying quercetin in E. coli by means of glycosylation, for example, involves overexpressing respective glycosyltransferases (GTs) in the host and metabolic engineering for increasing nucleoside diphosphate sugar (NDP sugar). Still others have used microorganisms other than E. coli, such as Streptomyces sp., for the biotransformation process. The overall study of the structural activity relationship has revealed that modification of some residues in quercetin decreased one activity but increased others. This review summarizes all of the information mentioned above.

Natural Product Glycosylation: Biocatalytic Synthesis of Quercetin-3,4'-O-diglucoside

Flavonoids have gained much attention for their proposed positive effects for human health. Glycosylation is a significant method for the structural modification of various flavanols, resulting in glycosides with increased solubility, stability, and bioavailability compared with the corresponding aglycone. Natural product glycosylation by using enzymes has emerged as a topic of interest as it offers a sustainable and economical alternative source so as to address supply scalability limitations associated with plant-based production. Quercetin-3,4'-O-diglucoside, as one of the major but trace bioactive flavonoids in onion (Allium cepa), is superior or at least equal to quercetin aglycone in its bioavailability. In the present study, the onion-derived enzyme, UGT73G1, coupled with sucrose synthase, StSUS1, from Solanum tuberosum formed a circulatory system to produce quercetin-3,4'-O-diglucoside from quercetin, which preferred sucrose as a sugar donor and quercetin as a sugar acceptor. The optimal conditions were determined in order to increase the production of quercetin-3,4'-O-diglucoside. The maximum concentration of quercetin-3,4'-O-diglucoside achieved in a 10-mL reaction was 427.11 mg/L, from the conversion of 1 g/L of quercetin for 16 h at 40 °C and pH 7.2.

Microbial Transformation of Flavonoids by Isaria fumosorosea ACCC 37814

Glycosylation is an efficient strategy to modulate the solubility, stability, bioavailability and bioactivity of drug-like natural products. Biological methods, such as whole-cell biocatalyst, promise a simple but highly effective approach to glycosylate biologically active small molecules with remarkable regio- and stereo-selectivity. Herein, we use the entomopathogenic filamentous fungus Isaria fumosorosea ACCC 37814 to biotransform a panel of phenolic natural products, including flavonoids and anthraquinone, into their glycosides. Six new flavonoid (4-O-methyl)glucopyranosides are obtained and structurally characterized using high resolution mass and nuclear magnetic resonance spectroscopic techniques. These compounds further expand the structural diversity of flavonoid glycosides and may be used in biological study.

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

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

When plants produce not enough or at all: metabolic engineering of flavonoids in microbial hosts

As a result of the discovery that flavonoids are directly or indirectly connected to health, flavonoid metabolism and its fascinating molecules that are natural products in plants, have attracted the attention of both the industry and researchers involved in plant science, nutrition, bio/chemistry, chemical bioengineering, pharmacy, medicine, etc. Subsequently, in the past few years, flavonoids became a top story in the pharmaceutical industry, which is continually seeking novel ways to produce safe and efficient drugs. Microbial cell cultures can act as workhorse bio-factories by offering their metabolic machinery for the purpose of optimizing the conditions and increasing the productivity of a selective flavonoid. Furthermore, metabolic engineering methodology is used to reinforce what nature does best by correcting the inadequacies and dead-ends of a metabolic pathway. Combinatorial biosynthesis techniques led to the discovery of novel ways of producing natural and even unnatural plant flavonoids, while, in addition, metabolic engineering provided the industry with the opportunity to invest in synthetic biology in order to overcome the currently existing restricted diversification and productivity issues in synthetic chemistry protocols. In this review, is presented an update on the rationalized approaches to the production of natural or unnatural flavonoids through biotechnology, analyzing the significance of combinatorial biosynthesis of agricultural/pharmaceutical compounds produced in heterologous organisms. Also mentioned are strategies and achievements that have so far thrived in the area of synthetic biology, with an emphasis on metabolic engineering targeting the cellular optimization of microorganisms and plants that produce flavonoids, while stressing the advances in flux dynamic control and optimization. Finally, the involvement of the rapidly increasing numbers of assembled genomes that contribute to the gene- or pathway-mining in order to identify the gene(s) responsible for producing species-specific secondary metabolites is also considered herein.

Regiospecific modifications of naringenin for astragalin production in Escherichia coli

We report the production of astragalin (AST) from regiospecific modifications of naringenin (NRN) in Escherichia coli BL21(DE3). The exogenously supplied NRN was converted into dihydrokaempferol (DHK) and then kaempferol (KMF) in the presence of flavanone-3-hydroxylase (f3h) and flavonone synthase (fls1) from Arabidopsis thaliana, respectively. KMF was further modified to produce AST by 3-O-glucosylation utilizing the endogeneous UDP-glucose in presence of UGT78K1 from Glycine max. To increase the intracellular UDP-glucose concentration by channeling the carbon flux toward UDP-glucose at the branch point of glucose-6-phosphate (G6P), the chromosomal glucose phosphate isomerase (pgi) and D-glucose-6-phosphate dehydrogenase (zwf) were knocked-out in E. coli BL21(DE3). The two enzymes directly involved in the synthesis of UDP-glucose from G6P, phosphoglucomutase (nfa44530) from Nocardia farcinia and glucose-1-phosphate uridylyltransferase (galU) from E. coli K12 were overexpressed, which successfully diverted the carbon flow from glycolysis to the synthesis of UDP-glucose. Furthermore, to prevent the dissociation of UDP-glucose into UDP and glucose, the UDP-glucose hydrolase (ushA) was deleted. The E. coli ΔpgiΔzwfΔushA mutant harboring the UDP-glucose biosynthetic pathway and the aforementioned genes for the regiospecific glucosylation produced 109.3 mg/L (244 µM) of AST representing 48.8% conversion from 500 µM of NRN in 60 h without any supplementation of extracellular UDP-glucose.

A mutation in the rice chalcone isomerase gene causes the golden hull and internode 1 phenotype

The biosynthesis of flavonoids, important secondary plant metabolites, has been investigated extensively, but few mutants of genes in this pathway have been identified in rice (Oryza sativa). The rice gold hull and internode (gh) mutants exhibit a reddish-brown pigmentation in the hull and internode and their phenotype has long been used as a morphological marker trait for breeding and genetic study. Here, we characterized that the gh1 mutant was a mutant of the rice chalcone isomerase gene (OsCHI). The result showed that gh1 had a Dasheng retrotransposon inserted in the 5′ UTR of the OsCHI gene, which resulted in the complete loss of OsCHI expression. gh1 exhibited golden pigmentation in hulls and internodes once the panicles were exposed to light. The total flavonoid content in gh1 hulls was increased threefold compared to wild type. Consistent with the gh1 phenotype, OsCHI transcripts were expressed in most tissues of rice and most abundantly in internodes. It was also expressed at high levels in panicles before heading, distributed mainly in lemmas and paleae, but its expression decreased substantially after the panicles emerged from the sheath. OsCHI encodes a protein functionally and structurally conserved to chalcone isomerases in other species. Our findings demonstrated that the OsCHI gene was indispensable for flux of the flavonoid pathway in rice.

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

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

Cloning and characterization of a glucosyltransferase from Crocus sativus stigmas involved in flavonoid glucosylation

BACKGROUND

Flavonol glucosides constitute the second group of secondary metabolites that accumulate in Crocus sativus stigmas. To date there are no reports of functionally characterized flavonoid glucosyltransferases in C. sativus, despite the importance of these compounds as antioxidant agents. Moreover, their bitter taste makes them excellent candidates for consideration as potential organoleptic agents of saffron spice, the dry stigmas of C. sativus.

RESULTS

Using degenerate primers designed to match the plant secondary product glucosyltransferase (PSPG) box we cloned a full length cDNA encoding CsGT45 from C. sativus stigmas. This protein showed homology with flavonoid glucosyltransferases. In vitro reactions showed that CsGT45 catalyses the transfer of glucose from UDP_glucose to kaempferol and quercetin. Kaempferol is the unique flavonol present in C. sativus stigmas and the levels of its glucosides changed during stigma development, and these changes, are correlated with the expression levels of CsGT45 during these developmental stages.

CONCLUSION

Findings presented here suggest that CsGT45 is an active enzyme that plays a role in the formation of flavonoid glucosides in C. sativus.

Four glucosyltransferases from rice: cDNA cloning, expression, and characterization

Four UDP-dependent glucosyltransferase (UGT) genes, UGT706C1, UGT706D1, UGT707A3, and UGT709A4 were cloned from rice, expressed in Escherichia coli, and purified to homogeneity. In order to find out whether these enzymes could use flavonoids as glucose acceptors, apigenin, daidzein, genistein, kaempferol, luteolin, naringenin, and quercetin were used as potential glucose acceptors. UGT706C1 and UGT707A3 could use kaempferol and quercetin as glucose acceptors and the major glycosylation position was the hydroxyl group of carbon 3 based on the comparison of HPLC retention times, UV spectra, and NMR spectra with those of corresponding authentic flavonoid 3-O-glucosides. On the other hand, UGT709A4 only used the isoflavonoids genistein and daidzein and transferred glucose onto 7-hydroxyl group. In addition, UGT706D1 used a broad range of flavonoids including flavone, flavanone, flavonol, and isoflavone, and produced at least two products with glycosylation at different hydroxyl groups. Based on their substrate preferences and the flavonoids present in rice, the in vivo function of UGT706C1, UGT706D1, and UGT707A3 is most likely the biosynthesis of kaempferol and quercetin glucosides.