Characterization of a UGT84 Family Glycosyltransferase Provides New Insights into Substrate Binding and Reactivity of Galloylglucose Ester-Forming UGTs

Comprehensive mutant chemotyping reveals embedding of a lineage-specific biosynthetic gene cluster in wider plant metabolism

Plants produce diverse specialized metabolites with important ecological functions. It has recently become apparent that the genes for many of these pathways are not dispersed in plant genomes, but rather are arranged like beads on a string in biosynthetic gene clusters (BGCs). Pathways encoded by BGCs are as a rule dedicated linear pathways that do not form parts of wider metabolic networks. In contrast, the genes for the biosynthesis of widely distributed more ancestral metabolites such as carotenoids and anthocyanins are not clustered. Little is known about how these more recently evolved clustered pathways interact with general plant metabolism. We recently characterized a 12-gene BGC for the biosynthesis of the antimicrobial defense compound avenacin A-1, a triterpene glycoside produced by oats. Avenacin A-1 is acylated with the fluorophore N-methyl anthranilate and confers bright blue fluorescence of oat root tips under ultraviolet light. Here, we exploit a suite of >100 avenacin-deficient mutants identified by screening for reduced root fluorescence to identify genes required for the function of this paradigm BGC. Using a combination of mutant chemotyping, biochemical and molecular analysis, and genome resequencing, we identify two nonclustered genes (Sad4 and Pal2) encoding enzymes that synthesize the donors required for avenacin glycosylation and acylation (recruited from the phenylpropanoid and tryptophan pathways). Our finding of these Cluster Auxiliary Enzymes (CAEs) provides insights into the interplay between general plant metabolism and a newly evolved lineage-specific BGC.

Multiprotein Complexes of Plant Glycosyltransferases Involved in Their Function and Trafficking

Plant cells utilize protein oligomerization for their functions in numerous important cellular processes. Protein-protein interactions are necessary to stabilize, optimize, and activate enzymes, as well as localize proteins to specific organelles and membranes. Glycosyltransferases-enzymes that attach sugars to polysaccharides, proteins, lipids, and RNA-across multiple plant biosynthetic processes have been demonstrated to interact with one another. The mechanisms behind these interactions are still unknown, but recent research has highlighted extensive examples of protein-protein interactions, specifically in the plant cell wall hemicellulose and pectin biosynthesis that takes place in the Golgi apparatus. In this review, we will discuss what is known so far about the interactions among Golgi-localized glycosyltransferases that are important for their functioning, trafficking, as well as structural aspects.

Discovery, characterization, and comparative analysis of new UGT72 and UGT84 family glycosyltransferases

Glycosylated derivatives of natural product polyphenols display a spectrum of biological activities, rendering them critical for both nutritional and pharmacological applications. Their enzymatic synthesis by glycosyltransferases is frequently constrained by the limited repertoire of characterized enzyme-catalyzed transformations. Here, we explore the glycosylation capabilities and substrate preferences of newly identified plant uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs) within the UGT72 and UGT84 families, with particular focus on natural polyphenol glycosylation from UDP-glucose. Four UGTs are classified according to their phylogenetic relationships and reaction products, identifying them as biocatalysts for either glucoside (UGT72 enzymes) or glucose ester (UGT84 members) formation from selected phenylpropanoid compounds. Detailed kinetic evaluations expose the unique attributes of these enzymes, including their specific activities and regio-selectivities towards diverse polyphenolic substrates, with product characterizations validating the capacity of UGT84 family members to perform di-O-glycosylation on flavones. Sequence analysis coupled with structural predictions through AlphaFold reveal an unexpected absence of a conserved threonine residue across all four enzymes, a trait previously linked to pentosyltransferases. This comparative analysis broadens the understood substrate specificity range for UGT72 and UGT84 enzymes, enhancing our understanding of their utility in the production of natural phenolic glycosides. The findings from this in-depth characterization provide valuable insights into the functional versatility of UGT-mediated reactions.

UDP-Glycosyltransferases in Edible Fungi: Function, Structure, and Catalytic Mechanism

UDP-glycosyltransferases (UGTs) are the most studied glycosyltransferases, and belong to large GT1 family performing the key roles in antibiotic synthesis, the development of bacterial glycosyltransferase inhibitors, and in animal inflammation. They transfer the glycosyl groups from nucleotide UDP-sugars (UDP-glucose, UDP-galactose, UDP-xylose, and UDP-rhamnose) to the acceptors including saccharides, proteins, lipids, and secondary metabolites. The present review summarized the recent of UDP-glycosyltransferases, including their structures, functions, and catalytic mechanism, especially in edible fungi. The future perspectives and new challenges were also summarized to understand of their structure–function relationships in the future. The outputs in this field could provide a reference to recognize function, structure, and catalytic mechanism of UDP-glycosyltransferases for understanding the biosynthesis pathways of secondary metabolites, such as hydrocarbons, monoterpenes, sesquiterpene, and polysaccharides in edible fungi.

Treponema primitia α1-2-fucosyltransferase-catalyzed one-pot multienzyme synthesis of fucosylated oligosaccharide lacto-N-fucopentaose I with antiviral activity against enterovirus 71

Fucosylated oligosaccharides have important biological functions as well as an excellent antiviral activity. A novel α 1-2-fucosyltransferase (α 2FT) from Treponema primitia (Tp2FT) was cloned and expressed in Escherichia coli BL21(DE3) and purified as an N-His6-tagged fusion protein (His6-Tp2FT). Mass spectrometry was carried out to identify the products of enzymatic reaction. The Tp2FT exhibited strict acceptor substrate specificity for type 1 structure (Galβ1-3GlcNAc)-containing glycans. It might be a promising emzyme for the chemo-enzymatic synthesis of lacto-N-fucopentaose I (LNFP I), which is one of the important fucosylated oligosaccharides. In this study, different in vitro experiments were used to study the biological activities of LNFP I. It could reduce the concentrations of inflammatory cytokines and effectively inhibit the synthesis of enterovirus 71 proliferation. LNFP I was an inhibitor of enterovirus 71 in the early stages of infection, it can used in infant nutrition and might provide a new drug for hand foot mouth disease.

Structure-function relationship of terpenoid glycosyltransferases from plants

Covering: up to 2021Terpenoids are physiologically active substances that are of great importance to humans. Their physicochemical properties are modified by glycosylation, in terms of polarity, volatility, solubility and reactivity, and their bioactivities are altered accordingly. Significant scientific progress has been made in the functional study of glycosylated terpenes and numerous plant enzymes involved in regio- and enantioselective glycosylation have been characterized, a reaction that remains chemically challenging. Crucial clues to the mechanism of terpenoid glycosylation were recently provided by the first crystal structures of a diterpene glycosyltransferase UGT76G1. Here, we review biochemically characterized terpenoid glycosyltransferases, compare their functions and primary structures, discuss their acceptor and donor substrate tolerance and product specificity, and elaborate features of the 3D structures of the first terpenoid glycosyltransferases from plants.

Structural modeling of two plant UDP-dependent sugar-sugar glycosyltransferases reveals a conserved glutamic acid residue that is a hallmark for sugar acceptor recognition

Glycosylation is one of the common modifications of plant metabolites, playing a major role in the chemical/biological diversity of a wide range of compounds. Plant metabolite glycosylation is catalyzed almost exclusively by glycosyltransferases, mainly by Uridine-diphosphate dependent Glycosyltransferases (UGTs). Several X-ray structures have been determined for primary glycosyltransferases, however, little is known regarding structure-function aspects of sugar-sugar/branch-forming O-linked UGTs (SBGTs) that catalyze the transfer of a sugar from the UDP-sugar donor to an acceptor sugar moiety of a previously glycosylated metabolite substrate. In this study we developed novel insights into the structural basis for SBGT catalytic activity by modelling the 3d-structures of two enzymes; a rhamnosyl-transferase Cs1,6RhaT - that catalyzes rhamnosylation of flavonoid-3-glucosides and flavonoid-7-glucosides and a UGT94D1 - that catalyzes glucosylation of (+)-Sesaminol 2-O-β-d-glucoside at the C6 of the primary sugar moiety. Based on these structural models and docking studies a glutamate (E290 or E268 in Cs1,6RhaT or UGT94D1, respectively) and a tryptophan (W28 or W15 in Cs1,6RhaT or UGT94D1, respectively) appear to interact with the sugar acceptor and are suggested to be important for the recognition of the sugar-moiety of the acceptor-substrate. Functional analysis of substitution mutants for the glutamate and tryptophan residues in Cs1,6RhaT further support their role in determining sugar-sugar/branch-forming GT specificity. Phylogenetic analysis of the UGT family in plants demonstrates that the glutamic-acid residue is a hallmark of SBGTs that is entirely absent from the corresponding position in primary UGTs.

Identification and Characterization of Glucosyltransferase That Forms 1-Galloyl-β-d-Glucogallin in Canarium album L., a Functional Fruit Rich in Hydrolysable Tannins

Hydrolysable tannins (HTs) are useful secondary metabolites that are responsible for pharmacological activities and astringent taste, flavor, and quality in fruits. They are also the main polyphenols in Canarium album L. (Chinese olive) fruit, an interesting and functional fruit that has been cultivated for over 2000 years. The HT content of C. album fruit was 2.3-13 times higher than that of berries with a higher content of HT. 1-galloyl-β-d-glucose (βG) is the first intermediate and the key metabolite in the HT biosynthesis pathway. It is catalyzed by UDP-glucosyltransferases (UGTs), which are responsible for the glycosylation of gallic acid (GA) to form βG. Here, we first reported 140 UGTs in C. album. Phylogenetic analysis clustered them into 14 phylogenetic groups (A, B, D-M, P, and Q), which are different from the 14 typical major groups (A~N) of Arabidopsis thaliana. Expression pattern and correlation analysis showed that UGT84A77 (Isoform0117852) was highly expressed and had a positive correlation with GA and βG content. Prokaryotic expression showed that UGT84A77 could catalyze GA to form βG. These results provide a theoretical basis on UGTs in C. album, which will be helpful for further functional research and availability on HTs and polyphenols.

The Sweet Side of Plant-Specialized Metabolism

Glycosylation plays a major role in the structural diversification of plant natural products. It influences the properties of molecules by modifying the reactivity and solubility of the corresponding aglycones, so influencing cellular localization and bioactivity. Glycosylation of plant natural products is usually carried out by uridine diphosphate(UDP)-dependent glycosyltransferases (UGTs) belonging to the carbohydrate-active enzyme glycosyltransferase 1 (GT1) family. These enzymes transfer sugars from UDP-activated sugar moieties to small hydrophobic acceptor molecules. Plant GT1s generally show high specificity for their sugar donors and recognize a single UDP sugar as their substrate. In contrast, they are generally promiscuous with regard to acceptors, making them attractive biotechnological tools for small molecule glycodiversification. Although microbial hosts have traditionally been used for heterologous engineering of plant-derived glycosides, transient plant expression technology offers a potentially disruptive platform for rapid characterization of new plant glycosyltransferases and biosynthesis of complex glycosides.

PgUGT95B2 preferentially metabolizes flavones/flavonols and has evolved independently from flavone/flavonol UGTs identified in Arabidopsis thaliana

UDP-dependent glycosyltransferases (UGTs) convert aglycones into more stable, bioactive, and structurally diverse glycosylated derivatives. Pomegranate (Punica granatum L.) produces various glycosylated phenolic metabolites, e.g. hydrolyzable tannins (HTs), anthocyanins, and flavonoids, and constitutes an excellent system for investigating the corresponding UGT activities. Here we report the cloning and functional characterization of a pomegranate UGT, PgUGT95B2, which is highly active towards flavones and flavonols and can glycosylate at more than one position in the substrate molecule. Particularly, PgUGT95B2 has the strongest activity towards tricetin (flavone with a tri-hydroxylated B-ring) and can act at the 4'-O position of its B-ring. In addition, PgUGT95B2 was able to glycosylate flavones present in pomegranate metabolite extracts. Conversely, PgUGT95B2 did not produce a galloylglucose ester (precursor for HT biosynthesis) or anthocyanins in enzyme assays. Our phylogenetic analysis suggested an independent evolution of PgUGT95B2 and flavone/flavonol UGTs identified in the model plant Arabidopsis thaliana through convergent evolution or gene loss.

Effective genome editing and identification of a regiospecific gallic acid 4-O-glycosyltransferase in pomegranate (Punica granatum L.)

Pomegranate (Punica granatum L.) trees are woody perennials that bear colorful and nutritious fruits rich in phenolic metabolites, e.g., hydrolyzable tannins (HTs) and flavonoids. We here report genome editing and gene discovery in pomegranate hairy roots using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) (CRISPR/Cas9), coupled with transcriptome and biochemical analyses. Single guide RNAs (sgRNAs) were designed to target two UDP-dependent glycosyltransferases (UGTs), PgUGT84A23 and PgUGT84A24, which possess overlapping activities in β-glucogallin (a galloylglucose ester; biosynthetic precursor of HTs) biosynthesis. A unique accumulation of gallic acid 3-O- and 4-O-glucosides (galloylglucose ethers) was observed in the PgUGT84A23 and PgUGT84A24 dual CRISPR/Cas9-edited lines (i.e., ugt84a23 ugt84a24) but not the control (empty vector) or PgUGT84A23/PgUGT84A24 single edited lines (ugt84a23 or ugt84a24). Transcriptome and real-time qPCR analyses identified 11 UGTs with increased expression in the ugt84a23 ugt84a24 hairy roots compared to the controls. Of the 11 candidate UGTs, only PgUGT72BD1 used gallic acid as substrate and produced a regiospecific product gallic acid 4-O-glucoside. This work demonstrates that the CRISPR/Cas9 method can facilitate functional genomics studies in pomegranate and shows promise for capitalizing on the metabolic potential of pomegranate for germplasm improvement.