High-throughput mass-spectrometry monitoring for multisubstrate enzymes: determining the kinetic parameters and catalytic activities of glycosyltransferases

Structure-based enzyme engineering improves donor-substrate recognition of Arabidopsis thaliana glycosyltransferases

Glycosylation of secondary metabolites involves plant UDP-dependent glycosyltransferases (UGTs). UGTs have shown promise as catalysts in the synthesis of glycosides for medical treatment. However, limited understanding at the molecular level due to insufficient biochemical and structural information has hindered potential applications of most of these UGTs. In the absence of experimental crystal structures, we employed advanced molecular modeling and simulations in conjunction with biochemical characterization to design a workflow to study five Group H Arabidopsis thaliana (76E1, 76E2, 76E4, 76E5, 76D1) UGTs. Based on our rational structural manipulation and analysis, we identified key amino acids (P129 in 76D1; D374 in 76E2; K275 in 76E4), which when mutated improved donor substrate recognition than wildtype UGTs. Molecular dynamics simulations and deep learning analysis identified structural differences, which drive substrate preferences. The design of these UGTs with broader substrate specificity may play important role in biotechnological and industrial applications. These findings can also serve as basis to study other plant UGTs and thereby advancing UGT enzyme engineering.

A versatile strategy to synthesize N-methyl-anthranilic acid-labelled glycoprobes for fluorescence-based screening assays

Here we propose a general strategy to label carbohydrates with N-methyl-anthranilic acid to generate glycotools for fluorescence-based screening and carbohydrate–protein interaction studies.

Rapid screening of sugar-nucleotide donor specificities of putative glycosyltransferases

Determining the correct enzymatic activity of putative glycosyltransferases (GTs) can be challenging as these enzymes can utilize multiple donor and acceptor substrates. Upon initial determination of the donor-sugar nucleotide(s), a GT utilizes various acceptor molecules that can then be tested. Here, we describe a quick method to screen sugar-nucleotide donor specificities of GTs utilizing a sensitive, nonradioactive, commercially available bioluminescent uridine diphosphate detection kit. This in vitro method allowed us to validate the sugar-nucleotide donor-substrate specificities of recombinantly expressed human, bovine, bacterial and protozoan GTs. Our approach, which is less time consuming than many traditional assays that utilize radiolabeled sugars and chromatographic separations, should facilitate discovery of novel GTs that participate in diverse biological processes.

Screening Glycosyltransferases for Polyphenol Modifications

Glycosyltransferases offer the opportunity to glycosylate a variety of substrates including health beneficial molecules like flavonoids in a regiospecific manner. Flavonoids are plant secondary metabolites that have antimicrobial, antioxidative, and health beneficial effects. Glycosylation often has impact on these properties and furthermore enhances the water solubility, the stability, and the bioavailability of the molecules. To detect flavonoid glycosylating enzymes we established a metagenome screen for the discovery of modifying clones. This function based screening technique can furthermore detect other modifications like methylations. The method relies on analysis of the culture supernatant extracts from biotransformation reactions in a thin layer chromatography (TLC) approach.

High throughput mass spectrometry-based characterisation of Arabidopsis thaliana group H glycosyltransferases

In this report, we cloned and characterised four members of group H glycosyltransferases (GTs) by studying their substrate specificities and kinetics. The formation of products and possible glycosylation position was confirmed using MS/MS. The results revealed that 76E1 and 76E5 have broader donor specificity, including UDP-glucose (UDPGlc), UDP-galactose (UDPGal) and UDP-N-acetylglucosamine (UDPGlcNAc) with various flavonoids as acceptor substrates. Pseudo-single substrate kinetics data showed a relatively low K_M, indicating a high affinity for substrate UDPGlc and also supported that 76E5 is more of a galactosyl and N-acetylglucosamine transferase. Sequence alignment and site-directed mutagenesis studies indeed suggested that serine is a crucial residue in the UDPGlcNAc and UDPGal activity.

We cloned and characterised four group H glycosyltransferases by studying their substrate specificities and kinetics. Sequence alignment and site-directed mutagenesis studies showed that serine is a crucial residue for UDPGlcNAc and UDPGal activity.

Integration of an In Situ MALDI-Based High-Throughput Screening Process: A Case Study with Receptor Tyrosine Kinase c-MET

Mass spectrometry (MS) is known for its label-free detection of substrates and products from a variety of enzyme reactions. Recent hardware improvements have increased interest in the use of matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS for high-throughput drug discovery. Despite interest in this technology, several challenges remain and must be overcome before MALDI-MS can be integrated as an automated “in-line reader” for high-throughput drug discovery. Two such hurdles include in situ sample processing and deposition, as well as integration of MALDI-MS for enzymatic screening assays that usually contain high levels of MS-incompatible components. Here we adapt our c-MET kinase assay to optimize for MALDI-MS compatibility and test its feasibility for compound screening. The pros and cons of the Echo (Labcyte) as a transfer system for in situ MALDI-MS sample preparation are discussed. We demonstrate that this method generates robust data in a 1536-grid format. We use the MALDI-MS to directly measure the ratio of c-MET substrate and phosphorylated product to acquire IC50 curves and demonstrate that the pharmacology is unaffected. The resulting IC50 values correlate well between the common label-based capillary electrophoresis and the label-free MALDI-MS detection method. We predict that label-free MALDI-MS-based high-throughput screening will become increasingly important and more widely used for drug discovery.

What can mathematical modelling say about CHO metabolism and protein glycosylation?

Chinese hamster ovary cells have been in the spotlight for process optimization in recent years, due to being the major, long established cell factory for the production of recombinant proteins. A deep, quantitative understanding of CHO metabolism and mechanisms involved in protein glycosylation has proven to be attainable through the development of high throughput technologies. Here we review the most notable accomplishments in the field of modelling CHO metabolism and protein glycosylation.

Chemical intervention in plant sugar signalling increases yield and resilience

The pressing global issue of food insecurity due to population growth, diminishing land and variable climate can only be addressed in agriculture by improving both maximum crop yield potential and resilience. Genetic modification is one potential solution, but has yet to achieve worldwide acceptance, particularly for crops such as wheat. Trehalose-6-phosphate (T6P), a central sugar signal in plants, regulates sucrose use and allocation, underpinning crop growth and development. Here we show that application of a chemical intervention strategy directly modulates T6P levels in planta. Plant-permeable analogues of T6P were designed and constructed based on a 'signalling-precursor' concept for permeability, ready uptake and sunlight-triggered release of T6P in planta. We show that chemical intervention in a potent sugar signal increases grain yield, whereas application to vegetative tissue improves recovery and resurrection from drought. This technology offers a means to combine increases in yield with crop stress resilience. Given the generality of the T6P pathway in plants and other small-molecule signals in biology, these studies suggest that suitable synthetic exogenous small-molecule signal precursors can be used to directly enhance plant performance and perhaps other organism function.

A Systematic Investigation of the Best Buffers for Use in Screening by MALDI-Mass Spectrometry

Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) offers a label-free alternative for the screening of biochemical targets in both 1536- and 6144-assay formats, as well as potentially providing increased sensitivity, reproducibility, and the simultaneous detection of multiple assay components within a specified m/z range. Ion suppression effects are one of the principal limitations reported for MS analysis. Within MALDI-MS screening, it has been identified that certain biochemical components incorporated into the assay (e.g., the buffers used to preserve the physiological conditions of the enzyme, salts, and other additives) induce suppression of the analyte ion signals monitored. This poorly understood phenomenon of ion suppression is a key reason the screening community has been reluctant to shift their investigations toward MS methods with reduced sample cleanup. Using acetylcholine as an assay substrate mimic, we have generated robust data to quantify the degree to which the most highly used components (base buffers, additional components, detergents, cell culture media, and other additives) within current screening assays are compatible with MALDI-MS. Here, the most suitable buffers and components, along with their identified optimal concentrations in terms of limiting ion suppression effects, are proposed for use in screening assays measured by MALDI-MS.

Analyses of Plant UDP-Dependent Glycosyltransferases to Identify Their Volatile Substrates Using Recombinant Proteins

Glycosylation is one of major modifications for plant secondary metabolites. In the case of volatile compounds, glycosylation makes them nonvolatile and odorless. Identification of UDP-dependent glycosyltransferases responsible for volatile glycosylation is essential to understand the regulatory mechanism of volatile release from plant tissues. Here, we describe an efficient protocol to find possible combinations of volatiles/glycosyltransferases using tomato (Solanum lycopersicum) enzymes expressed in Escherichia coli. The presented method requires a basic gas chromatography system and conventional laboratory tools.

High-Throughput Mass Spectrometry Screening for Inhibitors of Phosphatidylserine Decarboxylase

A high-throughput mass spectrometry assay to measure the catalytic activity of phosphatidylserine decarboxylase (PISD) is described. PISD converts phosphatidylserine to phosphatidylethanolamine during lipid synthesis. Traditional methods of measuring PISD activity are low throughput and unsuitable for the high-throughput screening of large compound libraries. The high-throughput mass spectrometry assay directly measures phosphatidylserine and phosphatidylethanolamine using the RapidFire™ platform at a rate of 1 sample every 7.5 s. The assay is robust, with an average Z′ value of 0.79 from a screen of 9920 compounds. Of 60 compounds selected for confirmation, 54 are active in dose-response studies. The application of high-throughput mass spectrometry permitted a high-quality screen to be performed for an otherwise intractable target. ( Journal of Biomolecular Screening 2007:628-634)

Bacterial Glycosyltransferases: Challenges and Opportunities of a Highly Diverse Enzyme Class Toward Tailoring Natural Products

The enzyme subclass of glycosyltransferases (GTs; EC 2.4) currently comprises 97 families as specified by CAZy classification. One of their important roles is in the biosynthesis of disaccharides, oligosaccharides, and polysaccharides by catalyzing the transfer of sugar moieties from activated donor molecules to other sugar molecules. In addition GTs also catalyze the transfer of sugar moieties onto aglycons, which is of great relevance for the synthesis of many high value natural products. Bacterial GTs show a higher sequence similarity in comparison to mammalian ones. Even when most GTs are poorly explored, state of the art technologies, such as protein engineering, domain swapping or computational analysis strongly enhance our understanding and utilization of these very promising classes of proteins. This perspective article will focus on bacterial GTs, especially on classification, screening and engineering strategies to alter substrate specificity. The future development in these fields as well as obstacles and challenges will be highlighted and discussed.

High-Throughput Screening Using Mass Spectrometry within Drug Discovery

In order to detect a biochemical analyte with a mass spectrometer (MS) it is necessary to ionize the analyte of interest. The analyte can be ionized by a number of different mechanisms, however, one common method is electrospray ionization (ESI). Droplets of analyte are sprayed through a highly charged field, the droplets pick up charge, and this is transferred to the analyte. High levels of salt in the assay buffer will potentially steal charge from the analyte and suppress the MS signal. In order to avoid this suppression of signal, salt is often removed from the sample prior to injection into the MS. Traditional ESI MS relies on liquid chromatography (LC) to remove the salt and reduce matrix effects, however, this is a lengthy process. Here we describe the use of RapidFire™ coupled to a triple-quadrupole MS for high-throughput screening. This system uses solid-phase extraction to de-salt samples prior to injection, reducing processing time such that a sample is injected into the MS ~every 10 s.

Comparison of broad-scope assays of nucleotide sugar-dependent glycosyltransferases

Glycosyltransferases (GTs) are abundant in nature and diverse in their range of substrates. Application of GTs is, however, often complicated by their narrow substrate specificity. GTs with tailored specificities are highly demanded for targeted glycosylation reactions. Engineering of such GTs is, however, restricted by lack of practical and broad-scope assays currently available. Here we present an improvement of an inexpensive and simple assay that relies on the enzymatic detection of inorganic phosphate cleaved from nucleoside phosphate products released in GT reactions. This phosphatase-coupled assay (PCA) is compared with other GT assays: a pH shift assay and a commercially available immunoassay in Escherichia coli cell-free extract (CE). Furthermore, we probe PCA with three GTs with different specificities. Our results demonstrate that PCA is a versatile and apparently general GT assay with a detection limit as low as 1 mU. The detection limit of the pH shift assay is roughly 4 times higher. The immunoassay, by contrast, detected only nucleoside diphosphates (NDPs) but had the lowest detection limit. Compared with these assays, PCA showed superior robustness and, therefore, appears to be a suitable general screening assay for nucleotide sugar-dependent GTs.

Tracking down biotransformation to the genetic level: identification of a highly flexible glycosyltransferase from Saccharothrix espanaensis

Saccharothrix espanaensis is a member of the order Actinomycetales. The genome of the strain has been sequenced recently, revealing 106 glycosyltransferase genes. In this paper, we report the detection of a glycosyltransferase from Saccharothrix espanaensis which is able to rhamnosylate different phenolic compounds targeting different positions of the molecules. The gene encoding the flexible glycosyltransferase is not located close to a natural product biosynthetic gene cluster. Therefore, the native function of this enzyme might be not the biosynthesis of a secondary metabolite but the glycosylation of internal and external natural products as part of a defense mechanism.

Understanding substrate selectivity of human UDP-glucuronosyltransferases through QSAR modeling and analysis of homologous enzymes

The UDP-glucuronosyltransferase (UGT) enzyme catalyzes the glucuronidation reaction which is a major metabolic and detoxification pathway in humans. Understanding the mechanisms for substrate recognition by UGT assumes great importance in an attempt to predict its contribution to xenobiotic/drug disposition in vivo. Spurred on by this interest, 2D/3D-quantitative structure activity relationships and pharmacophore models have been established in the absence of a complete mammalian UGT crystal structure. This review discusses the recent progress in modeling human UGT substrates including those with multiple sites of glucuronidation. A better understanding of UGT active site contributing to substrate selectivity (and regioselectivity) from the homologous enzymes (i.e. plant and bacterial UGTs, all belong to family 1 of glycosyltransferase (GT1)) is also highlighted, as these enzymes share a common catalytic mechanism and/or overlapping substrate selectivity.

High throughput discovery of heteroaromatic-modifying enzymes allows enhancement of novobiocin selectivity

Glycosylated analogues of novobiocin, discovered using a broad library of enzymes, have 100-fold improved activity against breast, brain, pancreatic, lung and ovarian cancers and ablated associated off-target activity leading to an up to 2.7 × 10(4) fold increase in selectivity.

ESI-MS assay of M. tuberculosis cell wall antigen 85 enzymes permits substrate profiling and design of a mechanism-based inhibitor

Mycobacterium tuberculosis Antigen 85 enzymes are vital to the integrity of the highly impermeable cell envelope and are potential therapeutic targets. Kinetic analysis using a label-free assay revealed both mechanistic details and a substrate profile that allowed the design and construction of a selective in vitro mechanism-based inhibitor.

Substrate and metal ion promiscuity in mannosylglycerate synthase

The enzymatic transfer of the sugar mannose from activated sugar donors is central to the synthesis of a wide range of biologically significant polysaccharides and glycoconjugates. In addition to their importance in cellular biology, mannosyltransferases also provide model systems with which to study catalytic mechanisms of glycosyl transfer. Mannosylglycerate synthase (MGS) catalyzes the synthesis of α-mannosyl-D-glycerate using GDP-mannose as the preferred donor species, a reaction that occurs with a net retention of anomeric configuration. Past work has shown that the Rhodothermus marinus MGS, classified as a GT78 glycosyltransferase, displays a GT-A fold and performs catalysis in a metal ion-dependent manner. MGS shows very unusual metal ion dependences with Mg(2+) and Ca(2+) and, to a lesser extent, Mn(2+), Ni(2+), and Co(2+), thus facilitating catalysis. Here, we probe these dependences through kinetic and calorimetric analyses of wild-type and site-directed variants of the enzyme. Mutation of residues that interact with the guanine base of GDP are correlated with a higher k(cat) value, whereas substitution of His-217, a key component of the metal coordination site, results in a change in metal specificity to Mn(2+). Structural analyses of MGS complexes not only provide insight into metal coordination but also how lactate can function as an alternative acceptor to glycerate. These studies highlight the role of flexible loops in the active center and the subsequent coordination of the divalent metal ion as key factors in MGS catalysis and metal ion dependence. Furthermore, Tyr-220, located on a flexible loop whose conformation is likely influenced by metal binding, also plays a critical role in substrate binding.

An easy colorimetric assay for glycosyltransferases

Glycosyltransferases are involved in biosynthesis of both protein-bound and non-bound glycans that have multiple and important biological functions in all species. A variety of methods for assaying glycosyltransferase activity have been developed driven by the specific interests and type of information required by researchers. In this work, a novel colorimetric assay for the glycosyltransferase-catalyzed reaction was established. Compared with measuring the newly formed product, which might not exhibit visible absorption, the unreacted acceptor could be readily detected by measuring the visible absorption of the hydrolysis product. In the assay, 4-nitrophenyl-beta-D-glycoside (glycosyl-beta-pNP) is used as the glycosyl acceptor, which can be hydrolyzed by a special exoglycosidase to release the p-nitrophenol before glycosylation reactions. Absorbance change of the p-nitrophenolate corresponds to unreacted glycosyl acceptor that accompanied the glycosyl transfer. The assay is demonstrated to be useful in the initial characterization of recombinant glycosyltransferases for their kinetic parameters, optimal metal cofactor, and pH value. It provides a simple, sensitive, and quantitative method for assessing glycosyltransferase activity and is thus expected to have broad applications including automated high-throughput screening.

A novel fluorescent probe for retaining galactosyltransferases

Glycosyltransferases (GTs) are a large class of carbohydrate-active enzymes that are involved, in both pro- and eukaryotic organisms, in numerous important biological processes, from cellular adhesion to carcinogenesis. GTs have enormous potential as molecular targets for chemical biology and drug discovery. For the full realisation of this potential, operationally simple and generally applicable GT bioassays, especially for inhibitor screening, are indispensable tools. In order to facilitate the development of GT high-throughput screening assays for the identification of GT inhibitors, we have developed novel, fluorescent derivatives of UDP-galactose (UDP-Gal) that are recognised as donor analogues by several different retaining galactosyltransferases (GalTs). We demonstrate for one of these derivatives that fluorescence emission is quenched upon specific binding to individual GalTs, and that this effect can be used as the read-out in ligand-displacement experiments. The novel fluorophore acts as an excellent sensor for several different enzymes and is suitable for the development of a new type of GalT bioassay, whose modular nature and operational simplicity will significantly facilitate inhibitor screening. Importantly, the structural differences between the natural donor UDP-Gal and the new fluorescent derivatives are minimal, and the general assay principle described herein may therefore also be applicable to other GalTs and/or proteins that use nucleotides or nucleotide conjugates as their cofactor.

Chimeric glycosyltransferases for the generation of hybrid glycopeptides

Glycodiversification, an invaluable tool for generating biochemical diversity, can be catalyzed by glycosyltransferases, which attach activated sugar "donors" onto "acceptor" molecules. However, many glycosyltransferases can tolerate only minor modifications to their native substrates, thus making them unsuitable tools for current glycodiversification strategies. Here we report the production of functional chimeric glycosyltransferases by mixing and matching the N- and C-terminal domains of glycopeptide glycosyltransferases. Using this method we have generated hybrid glycopeptides and have demonstrated that domain swapping can result in a predictable switch of substrate specificity, illustrating that N- and C-terminal domains predominantly dictate acceptor and donor specificity, respectively. The determination of the structure of a chimera in complex with a sugar donor analog shows that almost all sugar-glycosyltransferase binding interactions occur in the C-terminal domain.

Chapter 12. The power of glycosyltransferases to generate bioactive natural compounds

Glycosyltransferases (GTs), which catalyze the attachment of a sugar moiety to an aglycone are key enzymes for the biosynthesis of many valuable natural products. Their use in pharmaceutical biotechnology is becoming more and more visible. The promiscuity of GTs has prompted efforts to modify sugar structures and alter the glycosylation patterns of natural products. Here, we present the state of the art in this field. After describing the importance of GTs in determining the functions of natural products, a general survey of glycosyltransferase-catalyzed reactions is documented. This is followed by an overview of crystallized GT-B superfamily members and a discussion of the amino acids of these GTs involved in substrate binding. The main chapter is concerned with emphasizing the application of GTs in metabolic pathway engineering leading to novel unnatural bioactive compounds. A strategy to explore new GTs is presented as well as strategies to generate artificial GTs either randomly or in a rational design.

Recent Developments in Glycoside Synthesis with Glycosynthases and Thioglycoligases

Glycosynthases are hydrolytically incompetent engineered glycosidases that catalyze the high-yielding synthesis of glycoconjugates from glycosyl fluoride donor substrates and appropriate acceptors. Glycosynthases from more than 10 glycoside hydrolase families have now been generated, allowing the synthesis of a wide range of oligosaccharides. Recent examples include glycosynthase-mediated syntheses of xylo-oligosaccharides, xyloglucans, glycolipids, and aryl glycosides. Glycosynthases have also now been generated from inverting glycosidases, increasing the range of enzyme scaffolds. Improvement of glycosynthase activity and broadening of specificity has been achieved through directed evolution approaches, and several novel high-throughput screens have been developed to allow this. Finally, metabolically stable glycoside analogues have been generated using another class of mutant glycosidases: thioglycoligases. Recent developments in all these aspects are discussed.

Enzyme assay and activity fingerprinting of hydrolases with the red-chromogenic adrenaline test

The adrenaline test for enzymes is a colorimetric enzyme assay based on the quantification of periodate-sensitive reaction products such as 1,2-diols and 1,2-aminoalcohols by back-titration of the oxidant with adrenaline to produce adrenochrome as an easily detectable red product. The test uses commercial reagents and is suitable for screening the activity of various hydrolases. It is demonstrated here for testing epoxide hydrolases, lipases and esterases, and for activity fingerprinting of these enzymes across substrate series. The complete assay requires 2-3 h.

Enzyme assays

Enzyme assays are analytical tools to visualize enzyme activities. In recent years a large variety of enzyme assays have been developed to assist the discovery and optimization of industrial enzymes, in particular for "white biotechnology" where selective enzymes are used with great success for economically viable, mild and environmentally benign production processes. The present article highlights the aspects of fluorogenic and chromogenic substrates, sensors, and enzyme fingerprinting, which are our particular areas of interest.

Enhancing the latent nucleotide triphosphate flexibility of the glucose-1-phosphate thymidylyltransferase RmlA

Nucleotidyltransferases are central to nearly all glycosylation-dependent processes and have been used extensively for the chemoenzymatic synthesis of sugar nucleotides. The determination of the NTP specificity of the model thymidylyltransferase RmlA revealed RmlA to utilize all eight naturally occurring NTPs with varying levels of catalytic efficiency, even in the presence of nonnative sugar-1-phosphates. Guided by structural models, active site engineering of RmlA led to alterations of the inherent pyrimidine/purine bias by up to three orders of magnitude. This study sets the stage for engineering single universal nucleotidyltransferases and also provides new catalysts for the synthesis of novel nucleotide diphosphosugars.

High-Throughput Screening by Mass Spectrometry: Comparison with the Scintillation Proximity Assay with a Focused-File Screen of AKT1/PKBα

Mass spectrometry is an emerging format for label-free high-throughput screening. The main limitation of mass spectrometry is throughput, due to the requirement to purify samples prior to ionization. Here the authors compare an automated high-throughput mass spectrometry (HTMS) system (RapidFire™) with the scintillation proximity assay (SPA). The cancer therapy target AKT1/PKBα was screened against a focused library of kinase inhibitors and IC50 values determined for all compounds that exhibit > 50% inhibition. A selection of additional compounds that exhibited ≤ 50% inhibition in the primary screen was chosen as controls to confirm inactives. The selection of compounds is expected to identify common actives, common inactives, false positives, and false negatives. Agreement is found between HTMS and SPA in terms of primary hit identification and hit confirmation. ( Journal of Biomolecular Screening 2007:473-480)

Cloning, functional expression and characterization of Mesorhizobium loti arylamine N-acetyltransferases: rhizobial symbiosis supplies leguminous plants with the xenobiotic N-acetylation pathway

Arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes involved in the detoxification of numerous aromatic chemicals. The NAT-dependent N-acetylation pathway has not previously been detected in plants. We demonstrate here the occurrence of the NAT-dependent pathway in leguminous plants, due to symbiosis with Mesorhizobium loti. We cloned two NAT enzymes from M. loti and showed that these two recombinant enzymes catalysed the N-acetylation of several known NAT substrates, including aniline-derived pesticide residues. We also demonstrate the existence of a functional NAT-dependent acetylation pathway in the root nodules of Lotus japonicus inoculated with M. loti. M. loti is the first non-eukaryotic organism shown to express two catalytically active NAT isoforms. This work also provides the first evidence for acquisition of a xenobiotic detoxification pathway by a plant through symbiosis with a soil microbe.

Structure of a flavonoid glucosyltransferase reveals the basis for plant natural product modification

Glycosylation is a key mechanism for orchestrating the bioactivity, metabolism and location of small molecules in living cells. In plants, a large multigene family of glycosyltransferases is involved in these processes, conjugating hormones, secondary metabolites, biotic and abiotic environmental toxins, to impact directly on cellular homeostasis. The red grape enzyme UDP-glucose:flavonoid 3-O-glycosyltransferase (VvGT1) is responsible for the formation of anthocyanins, the health-promoting compounds which, in planta, function as colourants determining flower and fruit colour and are precursors for the formation of pigmented polymers in red wine. We show that VvGT1 is active, in vitro, on a range of flavonoids. VvGT1 is somewhat promiscuous with respect to donor sugar specificity as dissected through full kinetics on a panel of nine sugar donors. The three-dimensional structure of VvGT1 has also been determined, both in its 'Michaelis' complex with a UDP-glucose-derived donor and the acceptor kaempferol and in complex with UDP and quercetin. These structures, in tandem with kinetic dissection of activity, provide the foundation for understanding the mechanism of these enzymes in small molecule homeostasis.

Substrate flexibility of a 2,6-dideoxyglycosyltransferase

We report the first 2,6-dideoxysugar-O-glycosyltransferase with substrate flexibility at the 2 position, confirm the function of a putative NDP-hexose 2,3-dehydratase in the jadomycin B biosynthetic gene cluster and deduce the substrate flexibility of downstream enzymes in l-digitoxose assembly, enabling reprogramming of biosynthetic gene clusters to modify sugar substituents.

Structural dissection and high-throughput screening of mannosylglycerate synthase

The enzymatic transfer of activated mannose yields mannosides in glycoconjugates and oligo- and polysaccharides. Yet, despite its biological necessity, the mechanism by which glycosyltransferases recognize mannose and catalyze its transfer to acceptor molecules is poorly understood. Here, we report broad high-throughput screening and kinetic analyses of both natural and synthetic substrates of Rhodothermus marinus mannosylglycerate synthase (MGS), which catalyzes the formation of the stress protectant 2-O-alpha-D-mannosyl glycerate. The sequence of MGS indicates that it is at the cusp of inverting and retaining transferases. The structures of apo MGS and complexes with donor and acceptor molecules, including GDP-mannose, combined with mutagenesis of the binding and catalytic sites, unveil the mannosyl transfer center. Nucleotide specificity is as important in GDP-D-mannose recognition as the nature of the donor sugar.