Orthogonal Deprotection of Photolabile Protecting Groups and Its Application in Oligosaccharide Synthesis

We herein report for the first time the inter- and intramolecular orthogonal cleavage of two ortho-nitrobenzyl (NB) analogues. It is shown that the nitroveratryl (NV) group can be photolyzed with high priority when NV and ortho-nitrobenzyl carbonate (oNBC) are used together as the protecting groups of glycans. Notably, the photolytic products could be used directly in the subsequent glycosylation without further purification. With the above-mentioned orthogonal photolabile protecting group strategy in hand, a Mycobacterium tuberculosis tetrasaccharide and a derivative of glucosyl glycerol were rapidly prepared.

Strategies for the synthesis of the osmolyte glucosylglycerate and its precursor glycerate

Glycosidic osmolytes are widespread natural compounds that protect microorganisms and their macromolecules from the deleterious effects of various environmental stresses. Their protective properties have attracted considerable interest for industrial applications, especially as active ingredients in cosmetics and healthcare products. In that regard, the osmolyte glucosylglycerate is somewhat overlooked. Glucosylglycerate is typically accumulated by certain organisms when they are exposed to high salinity and nitrogen starvation, and its potent stabilizing effects have been demonstrated in vitro. However, the applications of this osmolyte have not been thoroughly explored due to the lack of a cost-efficient production process. Here, we present an overview of the progress that has been made in developing promising strategies for the synthesis of glucosylglycerate and its precursor glycerate, and discuss the remaining challenges. KEY POINTS: • Bacterial milking could be explored for fermentative production of glucosylglycerate • Glycoside phosphorylases of GH13_18 represent attractive alternatives for biocatalytic production • Conversion of glycerol with alditol oxidase is a promising strategy for generating the precursor glycerate.

Halotolerance mechanisms in salt‑tolerant cyanobacteria

Cyanobacteria are ubiquitously distributed in nature and are the most abundant photoautotrophs on Earth. Their long evolutionary history reveals that cyanobacteria have a remarkable capacity and strong adaptive tendencies to thrive in a variety of conditions. Thus, they can survive successfully, especially in harsh environmental conditions such as salty environments, high radiation, or extreme temperatures. Among others, salt stress because of excessive salt accumulation in salty environments, is the most common abiotic stress in nature and hampers agricultural growth and productivity worldwide. These detrimental effects point to the importance of understanding the molecular mechanisms underlying the salt stress response. While it is generally accepted that the stress response mechanism is a complex network, fewer efforts have been made to represent it as a network. Substantial evidence revealed that salt-tolerant cyanobacteria have evolved genomic specific mechanisms and high adaptability in response to environmental changes. For example, extended gene families and/or clusters of genes encoding proteins involved in the adaptation to high salinity have been collectively reported. This chapter focuses on recent advances and provides an overview of the molecular basis of halotolerance mechanisms in salt‑tolerant cyanobacteria as well as multiple regulatory pathways. We elaborate on the major protective mechanisms, molecular mechanisms associated with halotolerance, and the global transcriptional landscape to provide a gateway to uncover gene regulation principles. Both knowledge and omics approaches are utilized in this chapter to decipher the mechanistic insights into halotolerance. Collectively, this chapter would have a profound impact on providing a comprehensive understanding of halotolerance in salt‑tolerant cyanobacteria.

Description of two cultivated and two uncultivated new Salinibacter species, one named following the rules of the bacteriological code: Salinibacter grassmerensis sp. nov.; and three named following the rules of the SeqCode: Salinibacter pepae sp. nov., Salinibacter abyssi sp. nov., and Salinibacter pampae sp. nov

Current -omics methods allow the collection of a large amount of information that helps in describing the microbial diversity in nature. Here, and as a result of a culturomic approach that rendered the collection of thousands of isolates from 5 different hypersaline sites (in Spain, USA and New Zealand), we obtained 21 strains that represent two new Salinibacter species. For these species we propose the names Salinibacter pepae sp. nov. and Salinibacter grassmerensis sp. nov. (showing average nucleotide identity (ANI) values < 95.09% and 87.08% with Sal. ruber M31^T, respectively). Metabolomics revealed species-specific discriminative profiles. Sal. ruber strains were distinguished by a higher percentage of polyunsaturated fatty acids and specific N-functionalized fatty acids; and Sal. altiplanensis was distinguished by an increased number of glycosylated molecules. Based on sequence characteristics and inferred phenotype of metagenome-assembled genomes (MAGs), we describe two new members of the genus Salinibacter. These species dominated in different sites and always coexisted with Sal. ruber and Sal. pepae. Based on the MAGs from three Argentinian lakes in the Pampa region of Argentina and the MAG of the Romanian lake Fără Fund, we describe the species Salinibacter pampae sp. nov. and Salinibacter abyssi sp. nov. respectively (showing ANI values 90.94% and 91.48% with Sal. ruber M31^T, respectively). Sal. grassmerensis sp. nov. name was formed according to the rules of the International Code for Nomenclature of Prokaryotes (ICNP), and Sal. pepae, Sal. pampae sp. nov. and Sal. abyssi sp. nov. are proposed following the rules of the newly published Code of Nomenclature of Prokaryotes Described from Sequence Data (SeqCode). This work constitutes an example on how classification under ICNP and SeqCode can coexist, and how the official naming a cultivated organism for which the deposit in public repositories is difficult finds an intermediate solution.

Glucosylglycerol Extends Chronological Lifespan of the Budding Yeast via an Increased Osmolarity Response

Glucosylglycerol (GG) is an osmolyte that protects cells from extreme conditions. It is produced by sucrose phosphorylase, an enzyme that uses sucrose and glycerol as substrate. GG protects tissue integrity in desert plants during harsh conditions and guards cyanobacteria against high salinity (halotolerant). However, no extensive research has been conducted on the lifespan application of this compound on the yeast Saccharomyces cerevisiae. We designed this study to (1) characterize GG's effect on yeast chronological lifespan (CLS) and (2) to determine the mechanisms underlying its lifespan promotion on strain DBY746. The results obtained in our study confirm that GG causes increased longevity when administered at moderate doses (48 mM and 120 mM). In addition, we discovered that GG promotes yeast cell longevity by increasing the osmolarity of the culture medium. The maximum lifespan increased by approximately 15.38% and 34.6%, (i.e., 115.38 and 134.61) respectively, upon administration of GG at 48 mM and 120 mM concentrations. Elucidation of the mechanisms underlying this positive response suggests that GG promotes CLS by activities that modulate reactive oxygen species (ROS) generation, as evident in its increased ROS generation (mitohormesis). An increase in medium osmolarity caused by GG supplementation triggers ROS production and promotes longevity in the yeast (S. cerevisiae). An in-depth study on the potential application of this molecule in aging research is crucial; this will aid in expounding the mechanisms of this geroprotector and its longevity supportive tendencies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-023-01055-y.

Ni(II)-Catalyzed Regio- and Stereoselective O-Alkylation for the Construction of 1,2-cis-Glycosidic Linkages

A transition-metal-catalyzed O-alkylation for the regio- and stereoselective construction of 1,2-cis-glycosidic linkages is presented. With nonprecious and readily available Ni(II) as a catalyst, 1,2-cis-glycosides were obtained via O-alkylation of 1,2-carbohydrate diols that can be accessed in a small number of steps. The tedious design of protecting groups or anomeric leaving groups could be avoided with this method. The strategy was applied for the efficient preparation of an important commercialized glycosidic compatible solute GG, its derivative MGG, and a branched α-glucan.

An excited state intramolecular proton transfer induced phosphate ion targeted ratiometric fluorescent switch to monitor phosphate ions in human peripheral blood mononuclear cells

Detection of biological phosphate is very important for environmental and health care applications. In this study, a new ratiometric fluorescent probe (E)-N'-(3-(benzo[d]thiazol-2-yl)-2-hydroxybenzylidene) picolinohydrazide (BTP) is developed and exhibits a prominent excited-state intramolecular proton-transfer (ESIPT) mechanism. The probe BTP undergoes a unique phosphate induced hydrolytic reaction in mixed aqueous solution which produces a colorimetric change associated with a huge red-shift of ∼130 nm in the UV-visible absorption spectrum. Initially, BTP exhibits a strong fluorescence emission as the ESIPT process is 'on' and the tautomeric hydrogen remains flexible and is free to give two tautomeric forms. Eventually, after the addition of PO₄^3-, the two tautomeric forms break and thereby shift the equilibrium towards the 'enol' form. The phosphate ion binds with BTP which is associated with a ratiometric change and accounts for an enhancement in the fluorescence intensity with a large blue shift and the limit of detection value of 8.33 × 10^-8 M in a mixed aqueous medium. The binding constant (1.92 × 10⁵ M^-1) proportionally reflects the stability of the complexation between the binding sites of BTP with the guest PO₄^3- anion. The probable mechanism is supported by the NMR spectroscopy studies. The sensing phenomenon is found to be reversible towards Zn²⁺ and thus the sensor beautifully mimics the INHIBIT logic gate. Observations have been made in fluorescence imaging studies with human peripheral blood mononuclear cells (PBMCs) which indicates that BTP can be employed to successfully monitor the phosphate ion in human PBMCs.

Toward quantifying the adaptive role of bacterial pangenomes during environmental perturbations

Metagenomic surveys have revealed that natural microbial communities are predominantly composed of sequence-discrete, species-like populations but the genetic and/or ecological processes that maintain such populations remain speculative, limiting our understanding of population speciation and adaptation to perturbations. To address this knowledge gap, we sequenced 112 Salinibacter ruber isolates and 12 companion metagenomes from four adjacent saltern ponds in Mallorca, Spain that were experimentally manipulated to dramatically alter salinity and light intensity, the two major drivers of this ecosystem. Our analyses showed that the pangenome of the local Sal. ruber population is open and similar in size (~15,000 genes) to that of randomly sampled Escherichia coli genomes. While most of the accessory (noncore) genes were isolate-specific and showed low in situ abundances based on the metagenomes compared to the core genes, indicating that they were functionally unimportant and/or transient, 3.5% of them became abundant when salinity (but not light) conditions changed and encoded for functions related to osmoregulation. Nonetheless, the ecological advantage of these genes, while significant, was apparently not strong enough to purge diversity within the population. Collectively, our results provide an explanation for how this immense intrapopulation gene diversity is maintained, which has implications for the prokaryotic species concept.

Salt-Tolerant Synechococcus elongatus UTEX 2973 Obtained via Engineering of Heterologous Synthesis of Compatible Solute Glucosylglycerol

The recently isolated cyanobacterium Synechococcus elongatus UTEX 2973 (Syn2973) is characterized by a faster growth rate and greater tolerance to high temperature and high light, making it a good candidate chassis for autotrophic photosynthetic microbial cell factories. However, Syn2973 is sensitive to salt stress, making it urgently important to improve the salt tolerance of Syn2973 for future biotechnological applications. Glucosylglycerol, a compatible solute, plays an important role in resisting salt stress in moderate and marine halotolerant cyanobacteria. In this study, the salt tolerance of Syn2973 was successfully improved by introducing the glucosylglycerol (GG) biosynthetic pathway (OD₇₅₀ improved by 24% at 60 h). In addition, the salt tolerance of Syn2973 was further enhanced by overexpressing the rate-limiting step of glycerol-3-phosphate dehydrogenase and downregulating the gene rfbA, which encodes UDP glucose pyrophosphorylase. Taken together, these results indicate that the growth of the end-point strain M-2522-GgpPS-drfbA was improved by 62% compared with the control strain M-pSI-pSII at 60 h under treatment with 0.5 M NaCl. Finally, a comparative metabolomic analysis between strains M-pSI-pSII and M-2522-GgpPS-drfbA was performed to characterize the carbon flux in the engineered M-2522-GgpPS-drfbA strain, and the results showed that more carbon flux was redirected from ADP-GLC to GG synthesis. This study provides important engineering strategies to improve salt tolerance and GG production in Syn2973 in the future.

Whole cell-based catalyst for enzymatic production of the osmolyte 2-O-α-glucosylglycerol

Background

Glucosylglycerol (2-O-α-d-glucosyl-sn-glycerol; GG) is a natural osmolyte from bacteria and plants. It has promising applications as cosmetic and food-and-feed ingredient. Due to its natural scarcity, GG must be prepared through dedicated synthesis, and an industrial bioprocess for GG production has been implemented. This process uses sucrose phosphorylase (SucP)-catalyzed glycosylation of glycerol from sucrose, applying the isolated enzyme in immobilized form. A whole cell-based enzyme formulation might constitute an advanced catalyst for GG production. Here, recombinant production in Escherichia coli BL21(DE3) was compared systematically for the SucPs from Leuconostoc mesenteroides (LmSucP) and Bifidobacterium adolescentis (BaSucP) with the purpose of whole cell catalyst development.

Results

Expression from pQE30 and pET21 plasmids in E. coli BL21(DE3) gave recombinant protein at 40–50% share of total intracellular protein, with the monomeric LmSucP mostly soluble (≥ 80%) and the homodimeric BaSucP more prominently insoluble (~ 40%). The cell lysate specific activity of LmSucP was 2.8-fold (pET21; 70 ± 24 U/mg; N = 5) and 1.4-fold (pQE30; 54 ± 9 U/mg, N = 5) higher than that of BaSucP. Synthesis reactions revealed LmSucP was more regio-selective for glycerol glycosylation (~ 88%; position O2 compared to O1) than BaSucP (~ 66%), thus identifying LmSucP as the enzyme of choice for GG production. Fed-batch bioreactor cultivations at controlled low specific growth rate (µ = 0.05 h⁻¹; 28 °C) for LmSucP production (pET21) yielded ~ 40 g cell dry mass (CDM)/L with an activity of 2.0 × 10⁴ U/g CDM, corresponding to 39 U/mg protein. The same production from the pQE30 plasmid gave a lower yield of 6.5 × 10³ U/g CDM, equivalent to 13 U/mg. A single freeze–thaw cycle exposed ~ 70% of the intracellular enzyme activity for GG production (~ 65 g/L, ~ 90% yield from sucrose), without releasing it from the cells during the reaction.

Conclusions

Compared to BaSucP, LmSucP is preferred for regio-selective GG production. Expression from pET21 and pQE30 plasmids enables high-yield bioreactor production of the enzyme as a whole cell catalyst. The freeze–thaw treated cells represent a highly active, solid formulation of the LmSucP for practical synthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01569-4.

Putative trehalose biosynthesis proteins function as differential floridoside-6-phosphate synthases to participate in the abiotic stress response in the red alga Pyropia haitanensis

BACKGROUND

The heteroside floridoside is a primary photosynthetic product that is known to contribute to osmotic acclimation in almost all orders of Rhodophyta. However, the encoding genes and enzymes responsible for the synthesis of floridoside and its isomeric form, L- or D-isofloridoside, are poorly studied.

RESULTS

Here, four putative trehalose-6-phosphate synthase (TPS) genes, designated as PhTPS1, PhTPS2, PhTPS3, and PhTPS4, were cloned and characterized from the red alga Pyropia haitanensis (Bangiophyceae). The deduced amino acid sequence is similar to the annotated TPS proteins of other organisms, especially the UDP-galactose substrate binding sites of PhTPS1, 2, which are highly conserved. Of these, PhTPS1, 4 are involved in the biosynthesis of floridoside and isofloridoside, with isofloridoside being the main product. PhTPS3 is an isofloridoside phosphate synthase, while PhTPS2 exhibits no activity. When challenged by desiccation, high temperature, and salt stress, PhTPS members were expressed to different degrees, but the responses to thermal stress and desiccation were stronger.

CONCLUSIONS

Thus, in P. haitanensis, PhTPSs encode the enzymatical activity of floridoside and isofloridoside phosphate synthase and are crucial for the abiotic stress defense response.

Production of the compatible solute α-D-glucosylglycerol by metabolically engineered Corynebacterium glutamicum

Background

α-d-Glucosylglycerol (αGG) has beneficial functions as a moisturizing agent in cosmetics and potential as a health food material, and therapeutic agent. αGG serves as compatible solute in various halotolerant cyanobacteria such as Synechocystis sp. PCC 6803, which synthesizes αGG in a two-step reaction: The enzymatic condensation of ADP-glucose and glycerol 3-phosphate by GG-phosphate synthase (GGPS) is followed by the dephosphorylation of the intermediate by the GG-phosphate phosphatase (GGPP). The Gram-positive Corynebacterium glutamicum, an industrial workhorse for amino acid production, does not utilize αGG as a substrate and was therefore chosen for the development of a heterologous microbial production platform for αGG.

Results

Plasmid-bound expression of ggpS and ggpP from Synechocystis sp. PCC 6803 enabled αGG synthesis exclusively in osmotically stressed cells of C. glutamicum (pEKEx2-ggpSP), which is probably due to the unique intrinsic control mechanism of GGPS activity in response to intracellular ion concentrations. C. glutamicum was then engineered to optimize precursor supply for αGG production: The precursor for αGG synthesis ADP-glucose gets metabolized by both the glgA encoded glycogen synthase and the otsA encoded trehalose-6-phosphate synthase. Upon deletion of both genes the αGG concentration in culture supernatants was increased from 0.5 mM in C. glutamicum (pEKEx3-ggpSP) to 2.9 mM in C. glutamicum ΔotsA IMglgA (pEKEx3-ggpSP). Upon nitrogen limitation, which inhibits synthesis of amino acids as compatible solutes, C. glutamicum ΔotsA IMglgA (pEKEx3-ggpSP) produced more than 10 mM αGG (about 2 g L⁻¹).

Conclusions

Corynebacterium glutamicum can be engineered as efficient platform for the production of the compatible solute αGG. Redirection of carbon flux towards αGG synthesis by elimination of the competing pathways for glycogen and trehalose synthesis as well as optimization of nitrogen supply is an efficient strategy to further optimize production of αGG.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0939-2) contains supplementary material, which is available to authorized users.

Convergent engineering of syntrophic Escherichia coli coculture for efficient production of glycosides

Synthetic microbial coculture to express heterologous biosynthetic pathway for de novo production of medicinal ingredients is an emerging strategy for metabolic engineering and synthetic biology. Here, taking efficient production of salidroside as an example of glycosides, we design and construct a syntrophic Escherichia coli-E. coli coculture composed of the aglycone (AG) strain and the glycoside (GD) strain, which convergently accommodate biosynthetic pathways of tyrosol and salidroside, respectively. To accomplish this the phenylalanine-deficient AG strain was engineered to utilize xylose preferentially and to overproduce precursor tyrosol, while the tyrosine-deficient GD strain was constructed to consume glucose exclusively and to enhance another precursor UDP-glucose availability for synthesis of salidroside. The AG and GD strains in the synthetic consortium are obligatory cooperators through crossfeeding of tyrosine and phenylalanine and compatible in glucose and xylose mixture. Through balancing the metabolic pathway strength, we show that the syntrophic coculture was robust and stable, and produced 6.03 g/L of salidroside. It was the de novo production of salidroside for the first time in E. coli coculture system, which would be applicable for production of other important glycosides and natural products.

Production of glucosyl glycerol by immobilized sucrose phosphorylase: Options for enzyme fixation on a solid support and application in microscale flow format

2-O-(α-d-Glucopyranosyl)-sn-glycerol (αGG) is a natural osmolyte. αGG is produced industrially for application as an active cosmetic ingredient. The biocatalytic process involves a selective transglucosylation from sucrose to glycerol catalyzed by sucrose phosphorylase (SPase). Here we examined immobilization of SPase (from Leuconostoc mesenteroides) on solid support with the aim of enabling continuous production of αGG. By fusing SPase to the polycationic binding module Z_basic2 we demonstrated single-step noncovalent immobilization of the enzyme chimera to different porous supports offering an anionic surface. We showed that immobilization facilitated by Z_basic2 was similarly efficient as immobilization by multipoint covalent attachment on epoxy-activated supports in terms of production of αGG. Enzyme loadings of up to 90mg enzyme g^-1 support were obtained and the immobilized SPase was about half as effective as the enzyme in solution. The high regio- and chemo-selectivity of soluble SPase in αGG synthesis was retained in the immobilized enzyme and product yields of >85% were obtained at titers of ∼800mM. The Z_basic2-SPase immobilizates were fully recyclable: besides reuse of the enzyme activity, easy recovery of the solid support for fresh immobilizations was facilitated by the reversible nature of the enzyme attachment. Application of immobilized Z_basic2-SPase for continuous production of αGG in a microstructured flow reactor was demonstrated. Space-time yields of 500mmol αGG L^-1h^-1 were obtained at product titers of ∼200mM. The continuous microreactor was operated for 16days and an operational half-life of about 10days was determined.

Sucrose synthase: A unique glycosyltransferase for biocatalytic glycosylation process development

Sucrose synthase (SuSy, EC 2.4.1.13) is a glycosyltransferase (GT) long known from plants and more recently discovered in bacteria. The enzyme catalyzes the reversible transfer of a glucosyl moiety between fructose and a nucleoside diphosphate (NDP) (sucrose+NDP↔NDP-glucose+fructose). The equilibrium for sucrose conversion is pH dependent, and pH values between 5.5 and 7.5 promote NDP-glucose formation. The conversion of a bulk chemical to high-priced NDP-glucose in a one-step reaction provides the key aspect for industrial interest. NDP-sugars are important as such and as key intermediates for glycosylation reactions by highly selective Leloir GTs. SuSy has gained renewed interest as industrially attractive biocatalyst, due to substantial scientific progresses achieved in the last few years. These include biochemical characterization of bacterial SuSys, overproduction of recombinant SuSys, structural information useful for design of tailor-made catalysts, and development of one-pot SuSy-GT cascade reactions for production of several relevant glycosides. These advances could pave the way for the application of Leloir GTs to be used in cost-effective processes. This review provides a framework for application requirements, focusing on catalytic properties, heterologous enzyme production and reaction engineering. The potential of SuSy biocatalysis will be presented based on various biotechnological applications: NDP-sugar synthesis; sucrose analog synthesis; glycoside synthesis by SuSy-GT cascade reactions.

Salt acclimation of cyanobacteria and their application in biotechnology

The long evolutionary history and photo-autotrophic lifestyle of cyanobacteria has allowed them to colonize almost all photic habitats on Earth, including environments with high or fluctuating salinity. Their basal salt acclimation strategy includes two principal reactions, the active export of ions and the accumulation of compatible solutes. Cyanobacterial salt acclimation has been characterized in much detail using selected model cyanobacteria, but their salt sensing and regulatory mechanisms are less well understood. Here, we briefly review recent advances in the identification of salt acclimation processes and the essential genes/proteins involved in acclimation to high salt. This knowledge is of increasing importance because the necessary mass cultivation of cyanobacteria for future use in biotechnology will be performed in sea water. In addition, cyanobacterial salt resistance genes also can be applied to improve the salt tolerance of salt sensitive organisms, such as crop plants.

Mannosylglycerate: structural analysis of biosynthesis and evolutionary history

Halophilic and halotolerant microorganisms adapted to thrive in hot environments accumulate compatible solutes that usually have a negative charge either associated with a carboxylic group or a phosphodiester unit. Mannosylglycerate (MG) has been detected in several members of (hyper)thermophilic bacteria and archaea, in which it responds primarily to osmotic stress. The outstanding ability of MG to stabilize protein structure in vitro as well as in vivo has been convincingly demonstrated. These findings led to an increasingly supported link between MG and microbial adaptation to high temperature. However, the accumulation of MG in many red algae has been known for a long time, and the peculiar distribution of MG in such distant lineages was intriguing. Knowledge on the biosynthetic machinery together with the rapid expansion of genome databases allowed for structural and phylogenetic analyses and provided insight into the distribution of MG. The two pathways for MG synthesis have distinct evolutionary histories and physiological roles: in red algae MG is synthesised exclusively via the single-step pathway and most probably is unrelated with stress protection. In contrast, the two-step pathway is strongly associated with osmoadaptation in (hyper)thermophilic prokaryotes. The phylogenetic analysis of the two-step pathway also reveals a second cluster composed of fungi and mesophilic bacteria, but MG has not been demonstrated in members of this cluster; we propose that the synthase is part of a more complex pathway directed at the synthesis of yet unknown molecules containing the mannosyl-glyceryl unit.

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.

Functional genomics of novel secondary metabolites from diverse cyanobacteria using untargeted metabolomics

Mass spectrometry-based metabolomics has become a powerful tool for the detection of metabolites in complex biological systems and for the identification of novel metabolites. We previously identified a number of unexpected metabolites in the cyanobacterium Synechococcus sp. PCC 7002, such as histidine betaine, its derivatives and several unusual oligosaccharides. To test for the presence of these compounds and to assess the diversity of small polar metabolites in other cyanobacteria, we profiled cell extracts of nine strains representing much of the morphological and evolutionary diversification of this phylum. Spectral features in raw metabolite profiles obtained by normal phase liquid chromatography coupled to mass spectrometry (MS) were manually curated so that chemical formulae of metabolites could be assigned. For putative identification, retention times and MS/MS spectra were cross-referenced with those of standards or available sprectral library records. Overall, we detected 264 distinct metabolites. These included indeed different betaines, oligosaccharides as well as additional unidentified metabolites with chemical formulae not present in databases of metabolism. Some of these metabolites were detected only in a single strain, but some were present in more than one. Genomic interrogation of the strains revealed that generally, presence of a given metabolite corresponded well with the presence of its biosynthetic genes, if known. Our results show the potential of combining metabolite profiling and genomics for the identification of novel biosynthetic genes.

Enzymatic C-glycosylation: Insights from the study of a complementary pair of plant O- and C-glucosyltransferases

C-Glycosylation presents a rare mode of sugar attachment to the core structure of natural products and is catalyzed by a special type of LeloirC-glycosyltransferases (C-GTs). Elucidation of mechanistic principles for these glycosyltransferases (GTs) is of fundamental interest, and it could also contribute to the development of new biocatalysts for the synthesis of valuableC-glycosides, potentially serving as analogues of the highly hydrolysis-sensitiveO‑glycosides. Enzymatic glucosylation of the natural dihydrochalcone phloretin from UDP‑D-glucose was applied as a model reaction in the study of a structurally and functionally homologous pair of plant glucosyltransferases, where the enzyme from rice (Oryza sativa) was specific forC-glycosylation and the enzyme from pear (Pyrus communis) was specific forO-glycosylation. We show that distinct active-site motifs are used by the two enzymes to differentiate betweenC- andO-glucosylation of the phloretin acceptor. An enzyme design concept is therefore developed where exchange of active-site motifs results in a reversible switch betweenC/O-glycosyltransferase (C/O-GT) activity. Mechanistic proposal for enzymaticC-glycosylation involves a single nucleophilic displacement at the glucosyl anomeric carbon, proceeding through an oxocarbenium ion-like transition state. Alternatively, the reaction could be described as Friedel&ndash;Crafts-like direct alkylation of the phenolic acceptor.

Perturbing the metabolic dynamics of myo-inositol in developing Brassica napus seeds through in vivo methylation impacts its utilization as phytate precursor and affects downstream metabolic pathways

BACKGROUND

myo-Inositol (Ins) metabolism during early stages of seed development plays an important role in determining the distributional relationships of some seed storage components such as the antinutritional factors, sucrose galactosides (also known as raffinose oligosaccharides) and phytic acid (PhA) (myo-inositol 1,2,3,4,5,6-hexakisphosphate). The former is a group of oligosaccharides, which plays a role in desiccation at seed maturation. They are not easily digested by monogastric animals, hence their flatulence-causing properties. Phytic acid is highly negatively charged, which chelates positive ions of essential minerals and decreases their bioavailability. It is also a major cause of phosphate-related water pollution. Our aim was to investigate the influence of competitive diversion of Ins as common substrate on the biosynthesis of phytate and sucrose galactosides.

RESULTS

We have studied the initial metabolic patterns of Ins in developing seeds of Brassica napus and determined that early stages of seed development are marked by rapid deployment of Ins into a variety of pathways, dominated by interconversion of polar (Ins phosphates) and non-polar (phospholipids) species. In a time course experiment at early stages of seed development, we show Ins to be a highly significant constituent of the endosperm and seed coat, but with no phytate biosynthesis occurring in either tissue. Phytate accumulation appears to be confined mainly within the embryo throughout seed development and maturation. In our approach, the gene for myo-inositol methyltransferase (IMT), isolated from Mesembryanthemum crystallinum (ice plant), was transferred to B. napus under the control of the seed-specific promoters, napin and phaseolin. Introduction of this new metabolic step during seed development prompted Ins conversion to the corresponding monomethyl ether, ononitol, and affected phytate accumulation. We were able to produce homozygous transgenic lines with 19%-35% average phytate reduction. Additionally, changes in the raffinose content and related sugars occurred along with enhanced sucrose levels. Germination rates, viability and other seed parameters were unaffected by the IMT transgene over-expression.

CONCLUSIONS

Competitive methylation of Ins during seed development reduces seed antinutritional components and enhances its nutritional characteristics while maintaining adequate phosphate reserves. Such approach should potentially raise the canola market value and likely, that of other crops.