The role of carbohydrates in biologically active natural products

Exploring the diverse biological significance and roles of fucosylated oligosaccharides

Long since, carbohydrates were thought to be used just as an energy source and structural material. However, in recent years, with the emergence of the field of glycobiology and advances in glycomics, much has been learned about the biological role of oligosaccharides, a carbohydrate polymer containing a small number of monosaccharides, in cell-cell interaction, signal transduction, immune response, pathogen adhesion processes, early embryogenesis, and apoptosis. The function of oligosaccharides in these processes is diversified by fucosylation, also known as modification of oligosaccharides. Fucosylation has allowed the identification of more than 100 different oligosaccharide structures that provide functional diversity. ABO blood group and Lewis antigens are among the best known fucosyl-linked oligosaccharides. In addition, the antigens in the ABO system are composed of various sugar molecules, including fucosylated oligosaccharides, and Lewis antigens are structurally similar to ABO antigens but differ in the linkage of sugars. Variation in blood group antigen expression affects the host's susceptibility to many infections. However, altered expression of ABO and Lewis antigens is related with prognosis in carcinoma types. In addition, many pathogens recognize and bind to human tissues using a protein receptor with high affinity for the fucose molecule in glycoconjugates, such as lectin. Fucosylated oligosaccharides also play vital roles during fertilization and early embryogenesis. Learning and memory-related processes such as neurite growth, neurite migration, and synapse formation seen during the development of the brain, which is among the first organs to develop in embryogenesis, are regulated by fucosylated oligosaccharides. In conclusion, this review mentions the vital roles of fucosylated oligosaccharides in biology, drawing attention to their importance in the development of chemical tools to be used in function analysis and the investigation of various therapeutic targets.

Molecular characterization and structure basis of a malonyltransferase with both substrate promiscuity and catalytic regiospecificity from Cistanche tubulosa

Enzymatic malonylation of natural glycosides provides a promising alternative method for drug-like malonylated glycosides supply. However, the catalytic potential and structural basis of plant malonyltransferase are far from being fully elucidated. This work identified a new malonyltransferase CtMaT1 from Cistanche tubulosa. It displayed unprecedented mono- and/or di-malonylation activity toward diverse glucosides with different aglycons. A "one-pot" system by CtMaT1 and a malonyl-CoA synthetase was established to biosynthesize nine new malonylated glucosides. Structural investigations revealed that CtMaT1 possesses an adequately spacious acyl-acceptor pocket capable of accommodating diverse glucosides. Additionally, it recognizes malonyl-CoA through strong electrotactic and hydrogen interactions. QM/MM calculation revealed the H167-mediated SN2 reaction mechanism of CtMaT1, while dynamic simulations detected the formation of stable hydrogen bonds between the glucose-6-OH group and H167, resulting in its high malonylation regiospecificity. Calculated energy profiles of two isomeric glycosides highlighted lower reaction energy barriers towards glucoside substrates, emphasizing CtMaT1's preference for glucosides. Furthermore, a mutant CtMaT1H36A with notably increased di-malonylation activity was obtained. The underlying molecular mechanism was illuminated through MM/GBSA binding free energy calculation. This study significantly advances the understanding of plant acyltransferases from both functional and protein structural perspectives, while also providing a versatile tool for enzymatic malonylation applications in pharmacology.

Fe(III)-catalyzed stereoselective synthesis of deoxyglycosides using stable bifunctional deoxy-phenylpropiolate glycoside donors

Herein, we report a mild and economical route for the stereoselective synthesis of 2-deoxy and 2,6-dideoxyglycosides via FeCl3-catalyzed activation of bench stable deoxy-phenylpropiolate glycosyl donors (D-PPGs). Optimized reaction conditions work well under additive-free conditions to afford the corresponding 2-deoxy and 2,6-dideoxyglycosides in good yields with high α-anomeric selectivity by reacting with sugar and non-sugar-based acceptors. The optimized conditions were also extended for disarmed D-PPG donors. In addition, the developed strategy is amenable to high-scale-up synthesis.

Production of hybrid macrolide antibiotics by exploiting the specific substrate recognition characteristics of multifunctional cytochrome P450 enzyme MycG

Macrolide antibiotics are biosynthesized via enzymatic modifications, including glycosylation, methylation, and oxidation, after the core macro-lactone ring is generated by a polyketide synthase system. This study explored the diversification of macrolides by combining biosynthetic enzymes and reports an approach to produce unnatural hybrid macrolide antibiotics. The cytochrome (CYP) P450 monooxygenase MycG exhibits bifunctional activity, catalyzing late-stage hydroxylation at C-14 followed by epoxidation at C-12/13 during mycinamicin biosynthesis. The mycinose sugar of mycinamicin serves as a key molecular recognition element for binding to MycG. Thus, we subjected the hybrid macrolide antibiotic 23-O-mycinosyl-20-deoxo-20-dihydro-12,13-deepoxyrosamicin (IZI) to MycG, and confirmed that MycG catalyzed hydroxylation at C-22 and epoxidation at C-12/13 in IZI. In addition, the introduction of mycinose biosynthesis-related genes and mycG into rosamicin-producing Micromonospora rosaria enabled the fermentative production of 22-hydroxylated and 12,13-epoxidized forms of IZI. Interestingly, MycG catalyzed the sequential oxidation of hydroxylation and epoxidation in mycinamicin biosynthesis, but only single reactions in IZI. These findings highlight the potential for expanding the application of the multifunctional P450 monooxygenase MycG for the production of unnatural compounds.

Recent progress in the synthesis of natural product inspired bioactive glycohybrids

Carbohydrates are a basic structural component that are indispensable to all cellular processes. In addition to being employed as chiral starting materials in the synthesis of a variety of natural products, carbohydrates are recognized as naturally occurring molecules having an enormous variety of functional, stereochemical, and structural properties. The understanding and biological roles of carbohydrate derived molecules can be greatly improved by selectively synthesizing functional carbohydrates through incorporating them with privileged scaffolds. For a deeper understanding of their roles and the development of functional materials based on sugar, it is crucial to develop new techniques for efficiently synthesizing, functionalizing, and modifying carbohydrates. Glycohybrids have a wide range of structural and functional characteristics along with protein-carbohydrate interactions that are crucial to mammalian biology and a number of disease states. This review, consisting the literature from January 2017 to July 2023 and provide an overview of recent developments in the chemical synthesis of glycohybrids based on natural product scaffolds of coumarin, quinolone, naphthalene diimide, indole, isatin, naphthoquinone, imidazole and pyrimidine. The biological activity of active glycohybrids are discussed in this review.

De novo artificial synthesis of hexoses from carbon dioxide

Developing artificial "CO2-sugar" platforms is meaningful for addressing challenges posed by land scarcity and climate change to the supply of dietary sugar. However, upcycling CO2 into complex polyoxygenated carbohydrates involves several major challenges, including achieving enantioselective and thermodynamically driven transformation and expanding product repertoires while reducing energy consumption. We present a versatile chemoenzymatic roadmap based on aldol condensation, iso/epimerization, and dephosphorylation reactions for asymmetric CO2 and H2 assembly into sugars with perfect stereocontrol. In particular, we developed a minimum ATP consumption and the shortest pathway for bottom-up biosynthesis of the fundamental precursor, fructose-6-phosphate, which is valuable for synthesizing structure-diverse sugars and derivatives. Engineering bottleneck-associated enzyme catalysts aided in the thermodynamically driven synthesis of several energy-dense and functional hexoses, such as glucose and D-allulose, featuring higher titer (63 mmol L-1) and CO2-product conversion rates (25 mmol C L-1 h-1) compared to established in vitro CO2-fixing pathways. This chemical-biological platform demonstrated greater carbon conversion yield than the conventional "CO2-bioresource-sugar" process and could be easily extended to precisely synthesize other high-order sugars from CO2.

Mechanistic Studies on the 1,2-Spin-Center Shift in Carbohydrate Systems with a Fluorenylcyclopropyl Radical Clock

The mechanism of the 1,2-spin-center shift in carbohydrate systems was studied with a fluorenylcyclopropyl radical clock. The 1,2-rearrangement of the acyl fluorenylcyclopropane group without opening of the cyclopropane ring provides the strongest evidence that the 1,2-spin-center shift in carbohydrate systems occurs through a concerted transition state without the intermediacy of a 1,3-dioxolanyl radical.

All-in-One Synthesis of 3,6-Dideoxysugars: An Olefin Metathesis-Isomerization Approach

A collective synthesis of 3,6-dideoxysugars, including seven naturally known congeners, has been reported using commercially available methyl lactates in five steps. The essential tandem process involving the olefin cross-metathesis and isomerization steps was enabled by the dual function of Grubbs-II catalyst, affording the products in good yields and providing concise and practical access to a class of biologically important deoxysugars.

Switchable reactivity of 2-benzoyl glycals towards stereoselective access of 1-3 and 1-1 S/O linked disaccharides

We have developed a synthesis of 1-3 and 1-1 disaccharides from 2-benzoyl glycal and anomeric thiol and/or hydroxy sugar acceptors under mild conditions at room temperature. The regio and stereo-selectivity of the newly formed inter-glycosidic linkages are dependent on the nature of the glycal donor (D or L) and anomeric acceptor.

Efficient and stereoselective synthesis of sugar fused pyrano[3,2-c]pyranones as anticancer agents

A highly stereoselective, efficient and facile route was achieved for the synthesis of novel and biochemically potent sugar fused pyrano[3,2-c]pyranone derivatives starting from inexpensive, naturally occurring d-galactose and d-glucose. First, β-C-glycopyranosyl aldehydes were synthesized from these d-hexose sugars in six steps, with overall yields 41-55%. Next, two different 1-C-formyl glycals were synthesized from these β-C-glycopyranosyl aldehydes by treatment in basic conditions. The optimization of reaction conditions was carried out following reactions between 1-C-formyl galactal and 4-hydroxycoumarin. Next, 1-C-formyl galactal and 1-C-formyl glucal were treated with nine substituted 4-hydroxy coumarins at room temperature (25 °C) in ethyl acetate for ∼1-2 h in the presence of l-proline to obtain exclusively single diastereomers of pyrano[3,2-c]pyranone derivatives in excellent yields. Four compounds were found to be active for the MCF-7 cancer cell line. The MTT assay, apoptosis assay and migration analysis showed significant death of the cancer cells induced by the synthesized compounds.

Glucosylation of Isoeugenol and Monoterpenes in Corynebacterium glutamicum by YdhE from Bacillus lichenformis

Corynebacterium glutamicum has been regarded as a food-grade microorganism. In recent years, the research to improve the activities of beneficial therapeutics and pharmaceutical substances has resulted in the engineering of the therapeutically favorable cell factory system of C. glutamicum. In this study, we successfully glucosylated isoeugenol and other monoterpene derivatives in C. glutamicum using a promiscuous YdhE, which is a glycosyltransferase from Bacillus lichenformis. For efficient glucosylation, cultivation conditions such as the production time, substrate concentration, carbon source, and culture medium were optimized. Our system successfully converted about 93% of the isoeugenol to glucosylated compounds in the culture. The glucoside compounds were then purified, analyzed, and identified as isoeugenol-1-O-β-d-glucoside and isoeugenol-1-O-β-d-(2″-acetyl)-glucoside.

Engineered production of bioactive polyphenolic O-glycosides

Polyphenolic compounds (such as quercetin and resveratrol) possess potential medicinal values due to their various bioactivities, but poor water solubility hinders their health benefits to humankind. Glycosylation is a well-known post-modification method to biosynthesize natural product glycosides with improved hydrophilicity. Glycosylation has profound effects on decreasing toxicity, increasing bioavailability and stability, together with changing bioactivity of polyphenolic compounds. Therefore, polyphenolic glycosides can be used as food additives, therapeutics, and nutraceuticals. Engineered biosynthesis provides an environmentally friendly and cost-effective approach to generate polyphenolic glycosides through the use of various glycosyltransferases (GTs) and sugar biosynthetic enzymes. GTs transfer the sugar moieties from nucleotide-activated diphosphate sugar (NDP-sugar) donors to sugar acceptors such as polyphenolic compounds. In this review, we systematically review and summarize the representative polyphenolic O-glycosides with various bioactivities and their engineered biosynthesis in microbes with different biotechnological strategies. We also review the major routes towards NDP-sugar formation in microbes, which is significant for producing unusual or novel glycosides. Finally, we discuss the trends in NDP-sugar based glycosylation research to promote the development of prodrugs that positively impact human health and wellness.

Nitroso-Ene-type Cyclization Toward Diversified Synthesis of Amino Deoxysugars: A Proof of Concept

Amino deoxysugars are abundant in nature and play an important role in various biological functions, promoting numerous efforts to synthesize their structurally unique motifs. In this report, a de novo approach from a readily available lactic acid derivative is devised to construct several amino deoxysugars embedded in natural products, featuring a novel nitroso-ene-type cyclization to introduce a nitrogen atom into the carbon framework. This efficient synthesis provides an unprecedented synthetic route to explore the nitroso-ene cyclization to accumulate intriguing amino deoxysugars.

A "biphasic glycosyltransferase high-throughput screen" identifies novel anthraquinone glycosides in the diversification of phenolic natural products

The sugar moieties of many glycosylated small molecule natural products are essential for their biological activity. Glycosyltransferases (GTs) are enzymes responsible for installing these sugar moieties on a variety of biomolecules. Many GTs active on natural products are inherently substrate promiscuous and thus serve as useful tools in manipulating natural product glycosylation to generate new combinations of sugar units (glycones) and scaffold molecules (aglycones) in a process called glycodiversification. It is important to have an effective screening tool to detect the activity of promiscuous enzymes and their resulting glycoside products. Toward this aim, we developed a strategy for screening natural product GTs in a high-throughput fashion enabled by rapid isolation and detection of chromophoric or fluorescent glycosylated natural products. This involves a solvent extraction step to isolate the resulting polar glycoside product from the unreacted aglycone acceptor substrate and the detection of the formed glycoside by the innate absorbance or fluorescence of the aglycone moiety. Using our approach, we screened a collection of natural product GTs against a panel of precursors to therapeutically important molecules. Three GTs showed previously unreported promiscuity toward anthraquinones resulting in novel ε-rhodomycinone glycosides. Considering the pharmaceutical value of clinically used anthraquinone glycosides that are biosynthesized from an ε-rhodomycinone precursor, and the significance that the sugar moiety has on the biological activity of these drugs, our results are of particular importance toward the glycodiversification of therapeutics in this class. The GTs identified and the novel compounds they produce show promise toward new biocatalytic tools and therapeutics.

Protecting group enabled stereocontrolled approach for rare-sugars talose/gulose via dual-ruthenium catalysis

We herein report a convenient and highly stereocontrolled approach for rare and vital ᴅ-talo and ᴅ-gulo sugars directly from economical ᴅ-galactal through dual ruthenium-catalysis. The stereo-divergent strategy involves Ru(III)Cl₃-catalyzed Ferrier glycosylation of ᴅ-galactal to give 2,3-unsaturated ᴅ-galactopyranoside, further selective functionalization of C-4 and C-6 position with diverse protecting groups and dihydroxylation with Ru(VIII)O₄ generated in situ providing access to talo/gulo isomers. The α-anomeric stereoselectivity and syn-diastereoselectivity in glycosylation-dihydroxylation steps have been predominantly achieved by judicious selection of stereoelectronically diverse protecting groups. The synthetic utility of the dual-ruthenium catalysis was demonstrated for efficiently assembling the ᴅ-talose and/or ᴅ-gulose sugars in natural products and bioactive scaffolds.

New metal-free one-pot synthesis of α-2-deoxy-Ulosides by microwave-assisted double Michael Addition of β-enamino ketones

New metal-free one-pot synthesis of α-2-deoxy-ulosides in moderate to good yields by microwave-assisted double Michael addition of various O-nucleophiles to β-enamino ketones in the presence of 12 N HCl. These glycosyl additions occurred with high α-stereoselectivity and were complete in 10-25 min in 51-93% yield. In addition, high α-stereoselectivity was also observed when S-nucleophiles were examined.

Kukhtin-Ramirez-Reaction-Inspired Deprotection of Sulfamidates for the Synthesis of Amino Sugars

Herein, we present a mild strategy for deprotecting cyclic sulfamidates via the Kukhtin-Ramirez reaction to access amino sugars. The method features the removal of the sulfonic group of cyclic sulfamidates, which occurs through an N-H insertion reaction that implicates the Kukhtin-Ramirez adducts, followed by a base-promoted reductive N-S bond cleavage. The mild reaction conditions of the protocol enable the formation of amino alcohols including analogs that bear multiple functional groups.

In Vitro Reconstitution of the dTDP-l-Daunosamine Biosynthetic Pathway Provides Insights into Anthracycline Glycosylation

Many small molecule natural products are decorated with sugar moieties that are essential for their biological activity. A considerable number of natural product glycosides and their derivatives are clinically important therapeutics. Anthracyclines like daunorubicin and doxorubicin are examples of valuable glycosylated natural products used in medicine as potent anticancer agents. The sugar moiety, l-daunosamine (a highly modified deoxyhexose), plays a key role in the bioactivity of these molecules as evidenced by semisynthetic anthracycline derivatives such as epirubicin, wherein alteration in the configuration of a single stereocenter of the sugar unit generates a chemotherapeutic drug with lower cardiotoxicity. The nucleotide activated sugar donor that provides the l-daunosamine group for attachment to the natural product scaffold in the biosynthesis of these anthracyclines is dTDP-l-daunosamine. In an in vitro system, we have reconstituted the enzymes in the daunorubicin/doxorubicin pathway involved in the biosynthesis of dTDP-l-daunosamine. Through the study of the enzymatic steps in this reconstituted pathway, we have gained several insights into the assembly of this precursor including the identification of a major bottleneck and competing reactions. We carried out kinetic analysis of the aminotransferase that catalyzes a limiting step of the pathway. Our in vitro reconstituted pathway also provided a platform to test the combinatorial enzymatic synthesis of other dTDP-activated deoxyhexoses as potential tools for "glycodiversification" of natural products. To this end, we replaced the stereospecific ketoreductase that acts in the last step of dTDP-l-daunosamine biosynthesis with an enzyme from a heterologous pathway with opposite stereospecificity and found that it is active in the in vitro pathway, demonstrating the potential for the enzymatic synthesis of nucleotide-activated sugars with regio- and stereospecific tailoring.

Halogen Atom Participation in Guiding the Stereochemical Outcomes of Acetal Substitution Reactions

Acetal substitution reactions of α-halogenated five- and six-membered rings can be highly stereoselective. Erosion of stereoselectivity occurs as nucleophilicity increases, which is consistent with additions to a halogen-stabilized oxocarbenium ion, not a three-membered-ring halonium ion. Computational investigations confirmed that the open-form oxocarbenium ions are the reactive intermediates involved. Kinetic studies suggest that hyperconjugative effects and through-space electrostatic interactions can both contribute to the stabilization of halogen-substituted oxocarbenium ions.

Elucidation of the di-c-glycosylation steps during biosynthesis of the antitumor antibiotic, kidamycin

Kidamycins belong to the pluramycin family of antitumor antibiotics that contain di-C-glycosylated angucycline. Owing to its interesting biological activity, several synthetic derivatives of kidamycins are currently being developed. However, the synthesis of these complex structural compounds with unusual C-glycosylated residues is difficult. In the kidamycin-producing Streptomyces sp. W2061 strain, the genes encoding the biosynthetic enzymes responsible for the structural features of kidamycin were identified. Two glycosyltransferase-coding genes, kid7 and kid21, were found in the kidamycin biosynthetic gene cluster (BGC). Gene inactivation studies revealed that the subsequent glycosylation steps occurred in a sequential manner, in which Kid7 first attached N,N-dimethylvancosamine to the C10 position of angucycline aglycone, following which Kid21 transferred an anglosamine moiety to C8 of the C10-glycosylated angucycline. Therefore, this is the first report to reveal the sequential biosynthetic steps of the unique C-glycosylated amino-deoxyhexoses of kidamycin. Additionally, we confirmed that all three methyltransferases (Kid4, Kid9, and Kid24) present in this BGC were involved in the biosynthesis of these amino-deoxyhexoses, N,N-dimethylvancosamine and anglosamine. Aglycone compounds and the mono-C-glycosylated compound obtained in this process will be used as substrates for the development of synthetic derivatives in the future.

Small Carbohydrate Derivatives as Potent Antibiofilm Agents

Biofilm formation by most pathogenic bacteria is considered as one of the key mechanisms associated with virulence and antibiotic resistance. Biofilm-forming bacteria adhere to the surfaces of biological or implant medical devices and create communities within their self-produced extracellular matrix that are difficult to treat by existing antibiotics. There is an urgent need to synthesize and screen structurally diverse molecules for their antibiofilm activity that can remove or minimize the bacterial biofilm. The development of carbohydrate-based small molecules as antibiofilm agents holds a great promise in addressing the problem of the eradication of biofilm-related infections. Owing to their structural diversity and specificity, the sugar scaffolds are valuable entities for developing antibiofilm agents. In this perspective, we discuss the literature pertaining to carbohydrate-based natural antibiofilm agents and provide an overview of the design, activity, and mode of action of potent synthetic carbohydrate-based molecules.

Selective Axial-to-Equatorial Epimerization of Carbohydrates

Radical-mediated transformations have emerged as powerful methods for the synthesis of rare and unnatural branched, deoxygenated, and isomeric sugars. Here, we describe a radical-mediated axial-to-equatorial alcohol epimerization method to transform abundant glycans into rare isomers. The method delivers highly predictable and selective reaction outcomes that are complementary to other sugar isomerization methods. The synthetic utility of isomer interconversion is showcased through expedient glycan synthesis, including one-step glycodiversification. Mechanistic studies reveal that both site- and diastereoselectivities are achieved by highly selective H atom abstraction of equatorially disposed α-hydroxy C-H bonds.

Multi-strategy engineering unusual sugar TDP-l-mycarose biosynthesis to improve the production of 3-O-α-mycarosylerythronolide B in Escherichia coli

The insufficient supply of sugar units is the key limitation for the biosynthesis of glycosylated products. The unusual sugar TDP-l-mycarose is initially attached to the C3 of the polyketide erythronolide B, resulting in 3-O-α-mycarosylerythronolide B (MEB). Here, we present the de novo biosynthesis of MEB in Escherichia coli and improve its production using multi-strategy metabolic engineering. Firstly, by blocking precursor glucose-1-phosphate competing pathways, the MEB titer of triple knockout strain QC13 was significantly enhanced to 41.2 mg/L, 9.8-fold to that produced by parental strain BAP230. Subsequently, the MEB production was further increased to 48.3 mg/L through overexpression of rfbA and rfbB. Moreover, the CRISPRi was implemented to promote the TDP-l-mycarose biosynthesis via repressing the glycolysis and TDP-l-rhamnose pathway. Our study paves the way for efficient production of erythromycins in E. coli and provides a promising platform that can be applied for biosynthesis of other glycosylated products with unusual sugars.

Application of the Spin-Center Shift in Organic Synthesis

Spin-center shift (SCS) is a radical process involving 1,2-radical translocation along with a two-electron ionic movement, such as elimination of an adjacent leaving group. Such a process was initially observed in some important biochemical transformations, and the unique property has also attracted considerable interest in synthetic chemistry. Experimental, kinetic, as well as computational studies have been performed, and a series of useful radical transformations have been developed and applied in organic synthesis based on SCS processes in the last 20 years. This Perspective is an overview of radical transformations involving the SCS mechanism.

General Strategy for the Synthesis of Rare Sugars via Ru(II)-Catalyzed and Boron-Mediated Selective Epimerization of 1,2-trans-Diols to 1,2-cis-Diols

Human glycans are primarily composed of nine common sugar building blocks. On the other hand, several hundred monosaccharides have been discovered in bacteria and most of them are not readily available. The ability to access these rare sugars and the corresponding glycoconjugates can facilitate the studies of various fundamentally important biological processes in bacteria, including interactions between microbiota and the human host. Many rare sugars also exist in a variety of natural products and pharmaceutical reagents with significant biological activities. Although several methods have been developed for the synthesis of rare monosaccharides, most of them involve lengthy steps. Herein, we report an efficient and general strategy that can provide access to rare sugars from commercially available common monosaccharides via a one-step Ru(II)-catalyzed and boron-mediated selective epimerization of 1,2-trans-diols to 1,2-cis-diols. The formation of boronate esters drives the equilibrium toward 1,2-cis-diol products, which can be immediately used for further selective functionalization and glycosylation. The utility of this strategy was demonstrated by the efficient construction of glycoside skeletons in natural products or bioactive compounds.

Identification of a novel glucuronyltransferase from Streptomyces chromofuscus ATCC 49982 for natural product glucuronidation

Glycosylation is an effective way to increase the polarity of natural products. UDP-glucuronyltransferases (UGTs) are commonly observed and extensively studied in phase II drug metabolism. However, UGTs in microorganisms are not well studied, which hampered the utilization of this type of enzyme in microbial glucuronidation of natural products. Screening of five actinomycete strains showed that Streptomyces chromofuscus ATCC 49982 can convert diverse plant polyphenols into more polar products, which were characterized as various glucuronides based on their spectral data. Analysis of the genome of this strain revealed a putative glucuronidation gene cluster that contains a UGT gene (gcaC) and two UDP-glucuronic acid biosynthetic genes (gcaB and gcaD). The gcaC gene was cloned and heterologously expressed in Escherichia coli BL21(DE3). Incubation of the purified enzyme with resveratrol and UDP-glucuronic acid led to the production of resveratrol-4'-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide, allowing GcaC to be characterized as a flexible UGT. The optimal in vitro reaction pH and temperature for GcaC are 7.5 and 30 °C, respectively. Its activity can be stimulated by Ca²⁺, Mg²⁺, and Mn²⁺, whereas Zn²⁺, Cu²⁺, and Fe²⁺ showed inhibitory effects. Furthermore, GcaC has a broad substrate specificity, which can glucuronidate various substrates besides resveratrol, including quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone, and apigenin. The titers of resveratrol-4'-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide in E. coli-GcaC were 78.381 ± 0.366 mg/L and 14.991 ± 0.248 mg/L from 114.125 mg/L resveratrol within 3 h. Therefore, this work provides an effective way to produce glucuronides of resveratrol and other health-benefitting natural products. KEY POINTS: • A novel versatile microbial UDP-glucuronyltransferase was discovered and characterized from Streptomyces chromofuscus ATCC 49982. • The UDP-glucuronyltransferase was expressed in Escherichia coli and can convert resveratrol into two glucuronides both in vitro and in vivo. • The UDP-glucuronyltransferase has a highly flexible substrate specificity and is an effective tool to prepare mono- or diglucuronides of bioactive molecules.

Glycosides for Peripheral Neuropathic Pain: A Potential Medicinal Components

Neuropathic pain is a refractory disease that occurs across the world and pharmacotherapy has limited efficacy and/or safety. This disease imposes a significant burden on both the somatic and mental health of patients; indeed, some patients have referred to neuropathic pain as being 'worse than death'. The pharmacological agents that are used to treat neuropathic pain at present can produce mild effects in certain patients, and induce many adverse reactions, such as sedation, dizziness, vomiting, and peripheral oedema. Therefore, there is an urgent need to discover novel drugs that are safer and more effective. Natural compounds from medical plants have become potential sources of analgesics, and evidence has shown that glycosides alleviated neuropathic pain via regulating oxidative stress, transcriptional regulation, ion channels, membrane receptors and so on. In this review, we summarize the epidemiology of neuropathic pain and the existing therapeutic drugs used for disease prevention and treatment. We also demonstrate how glycosides exhibit an antinociceptive effect on neuropathic pain in laboratory research and describe the antinociceptive mechanisms involved to facilitate the discovery of new drugs to improve the quality of life of patients experiencing neuropathic pain.

Chemical Synthesis of Saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Selective Transformations of Carbohydrates Inspired by Radical-Based Enzymatic Mechanisms

Enzymes are a longstanding source of inspiration for synthetic reaction development. However, enzymatic reactivity and selectivity are frequently untenable in a synthetic context, as the principles that govern control in an enzymatic setting often do not translate to small molecule catalysis. Recent synthetic methods have revealed the viability of using small molecule catalysts to promote highly selective radical-mediated transformations of minimally protected sugar substrates. These transformations share conceptual similarities with radical SAM enzymes found in microbial carbohydrate biosynthesis and present opportunities for synthetic chemists to access microbial and unnatural carbohydrate building blocks without the need for protecting groups or lengthy synthetic sequences. Here, we highlight strategies through which radical reaction pathways can enable the site-, regio-, and diastereoselective transformation of minimally protected carbohydrates in both synthetic and enzymatic systems.

Enantioselective Reductive Oligomerization of Carbon Dioxide into l-Erythrulose via a Chemoenzymatic Catalysis

A cell-free enantioselective transformation of the carbon atom of CO₂ has never been reported. In the urgent context of transforming CO₂ into products of high value, the enantiocontrolled synthesis of chiral compounds from CO₂ would be highly desirable. Using an original hybrid chemoenzymatic catalytic process, we report herein the reductive oligomerization of CO₂ into C₃ (dihydroxyacetone, DHA) and C₄ (l-erythrulose) carbohydrates, with perfect enantioselectivity of the latter chiral product. This was achieved with the key intermediacy of formaldehyde. CO₂ is first reduced selectively by 4e^- by an iron-catalyzed hydroboration reaction, leading to the isolation and complete characterization of a new bis(boryl)acetal compound derived from dimesitylborane. In an aqueous buffer solution at 30 °C, this compound readily releases formaldehyde, which is then involved in selective enzymatic transformations, giving rise either (i) to DHA using a formolase (FLS) catalysis or (ii) to l-erythrulose with a cascade reaction combining FLS and d-fructose-6-phosphate aldolase (FSA) A129S variant. Finally, the nature of the synthesized products is noteworthy, since carbohydrates are of high interest for the chemical and pharmaceutical industries. The present results prove that the cell-free de novo synthesis of carbohydrates from CO₂ as a sustainable carbon source is a possible alternative pathway in addition to the intensely studied biomass extraction and de novo syntheses from fossil resources.

A Unified Strategy to Access 2- and 4-Deoxygenated Sugars Enabled by Manganese-Promoted 1,2-Radical Migration

The selective manipulation of carbohydrate scaffolds is challenging due to the presence of multiple, nearly chemically indistinguishable O-H and C-H bonds. As a result, protecting-group-based synthetic strategies are typically necessary for carbohydrate modification. Here we report a concise semisynthetic strategy to access diverse 2- and 4-deoxygenated carbohydrates without relying on the exhaustive use of protecting groups to achieve site-selective reaction outcomes. Our approach leverages a Mn²⁺-promoted redox isomerization step, which proceeds via sugar radical intermediates accessed by neutral hydrogen atom abstraction under visible light-mediated photoredox conditions. The resulting deoxyketopyranosides feature chemically distinguishable functional groups and are readily transformed into diverse carbohydrate structures. To showcase the versatility of this method, we report expedient syntheses of the rare sugars l-ristosamine, l-olivose, l-mycarose, and l-digitoxose from commercial l-rhamnose. The findings presented here validate the potential for radical intermediates to facilitate the selective transformation of carbohydrates and showcase the step and efficiency advantages attendant to synthetic strategies that minimize a reliance upon protecting groups.

The inverting mechanism of the metal ion-independent LanGT2: the first step to understand the glycosylation of natural product antibiotic precursors through QM/MM simulations

Glycosyltransferases (GTs) from the GT1 family are responsible for the glycosylation of various important organic structures such as terpenes, steroids and peptide antibiotics, making it one of the most intensely studied families of GTs. The target of our study, LanGT2, is a member of the GT1 family that uses an inverting mechanism for transferring olivose from TDP-olivose, the donor substrate, to the natural product tetrangulol (Tet), the precursor of the antibiotic landomycin A. X-ray crystallography in conjunction with mutagenesis experiments has revealed the catalytic significance of 3 amino acids (Ser10, Ser219 and Asp137), suggesting Asp137 as the base catalyst. In the absence of X-ray structures that include the acceptor substrate Tet, in silico experiments and MD simulations that have modeled ternary complexes propose that Asp137 could recruit a water molecule to facilitate the nucleophilic activation of Tet, since the distance between Asp137 and the nucleophile is too long to directly deprotonate the nucleophilic moiety. So far, there is no computational evidence regarding the precise mechanism by which LanGT2 catalyzes the transfer of olivose, which raises questions such as: is a water-assisted mechanism possible? and how does this metal ion-independent GT stabilize the growing negative charge of the diphosphate leaving group? In this work, the QM/MM approach was used to unravel the catalytic mechanism of LanGT2, and to identify the role of crucial catalytic amino acids at a molecular level. Our calculations show that the minimum energy path (MEP) describes an SN2-like mechanism, identifying an oxocarbenium ion-like TS in which the olivosyl moiety adopts a 4H3 conformation. Interactions established between the diphosphate group of TDP and Ser10, Ser219, Arg220 and His283 are key to stabilize the development of charge on the leaving group. Our work also suggests that a water-mediated proton transfer mechanism is feasible, in which the water molecule is key to stabilize the phenolate ion-like nucleophile in the TS. This is the first computational insight into the inverting mechanism of an antibiotic natural product GT, and its implications may serve to guide the design of new biocatalysts for natural product glycodiversification.

Convergent Synthesis of Tetrasaccharide Fragment of Cervimycin K

A new synthetic approach toward oligosaccharides consisting only of 2,3,6-trideoxypyranoglycosides is reported. The key feature is highlighted by the convergent approach that allows the introduction of the aglycon moiety in the late stage of the synthesis. As an illustrative example, the tetrasaccharide portion of cervimycin K was prepared as cyclohexyl glycoside.

Directed evolution of glycosyltransferase for enhanced efficiency of avermectin glucosylation

Avermectin, produced by Streptomyces avermitilis, is an active compound protective against nematodes, insects, and mites. However, its potential usage is limited by its low aqueous solubility. The uridine diphosphate (UDP)-glycosyltransferase (BLC) from Bacillus licheniformis synthesizes avermectin glycosides with improved water solubility and in vitro antinematodal activity. However, enzymatic glycosylation of avermectin by BLC is limited due to the low conversion rate of this reaction. Thus, improving BLC enzyme activity is necessary for mass production of avermectin glycosides for field application. In this study, the catalytic activity of BLC toward avermectin was enhanced via directed evolution. Three mutants from the BLC mutant library (R57H, V227A, and D252V) had specific glucosylation activity for avermectin 2.0-, 1.8-, and 1.5-fold higher, respectively, than wild-type BLC. Generation of combined mutations via site-directed mutagenesis led to even further enhancement of activity. The triple mutant, R57H/V227A/D252V, had the highest activity, 2.8-fold higher than that of wild-type BLC. The catalytic efficiencies (K_cat/K_m) of the best mutant (R57H/V227A/D252V) toward the substrates avermectin and UDP-glucose were improved by 2.71- and 2.29-fold, respectively, compared to those of wild-type BLC. Structural modeling analysis revealed that the free energy of the mutants was - 1.1 to - 7.1 kcal/mol lower than that of wild-type BLC, which was correlated with their improved activity. KEY POINTS: • Directed evolution improved the glucosylation activity of BLC toward avermectin. • Combinatorial site-directed mutagenesis led to further enhanced activity. • The mutants exhibited lower free energy values than wild-type BLC.

Synthesis of MeON-Glycoside Derivatives of Oleanolic Acid by Neoglycosylation and Evaluation of Their Cytotoxicity Against Selected Cancer Cell Lines

A series of C-3 and C-28 MeON-neoglycosides of oleanolic acid were designed and synthesized by neoglycosylation as potential antiproliferative agents. Their cytotoxicity was evaluated in vitro against five human cancer cell lines: human non-small cell lung cancer cell line (A549), human melanoma cell line (A375), human colon cancer cell line (HCT116), human liver carcinoma cell line (HepG2), human breast adenocarcinoma cell line (MCF-7) by the Cell Counting Kit-8 (CCK-8) assay. Most of C-3 and C-28 MeON-neoglycosides of oleanolic acid exhibited notably inhibitory effects against the tested cancer cells and more sensitive to HepG2 cells than 5-Fluorouracil (5-FU). Structure-activities relationship (SAR) analysis revealed that sugar types and the d/l configuration of sugars would significantly affect their antiproliferative activities of neoglycosides. Among them, compound 8a (28-N-methoxyaminooleanane-β-d-glucoside) exhibited the most potent antiproliferative activities against HepG2 cells with IC₅₀ values of 2.1 µM. Further pharmacological experiments revealed that compound 8a could cause morphological changes and cell cycle arrest at G0/G1 phase and induce apoptosis in HepG2 cells. These results suggested that neoglycosylation could provide a rapid strategy for the discovery of potential antiproliferative agents and their possible pharmacological mechanisms need more further research.

Glycorandomization: A promising diversification strategy for the drug development

Glycorandomization is a natural product derivatization strategy in which different sugar moieties are linked to the aglycone part of the naturally existing glycosides to create glycorandomized libraries. Sugars attached to the natural products are responsible for affecting their solubility, mechanism of action, target recognition, and toxicity and thus, by changing the sugar part, these properties could be modified. Glycorandomization can be done via two approaches (i) a synthetic approach known as neoglycorandomization, and (ii) chemoenzymatic approach including in-vitro and in-vivo glycorandomization. Glycorandomization can be a promising technology for the drug discovery that has proved its potential to improve pharmacokinetic (solubility) and pharmacodynamic profile (mechanism of action, toxicity, and target recognition) of the parent compounds. The substrate flexibility of glycosyltransferases and other enzymes towards sugars and/or aglycone substrates has made this technique versatile. Further, the enzymes can be altered by genetic engineering to generate glycorandomized libraries of diverse natural product scaffolds. This technique has the potential to produce new compounds that can be helpful to the mankind by treating the threatening disease states. This review covers the different strategies for glycorandomization as a tool in drug discovery and development. The fundamentals of glycorandomization, different types, and further development of differentially glycorandomized libraries of natural products and small molecule based drugs have been discussed.

The mechanism of conversion of substituted glycals to chiral acenes via Diels-Alder reaction: a computational study

Synthesis of linearly fused aromatic systems using a glycal-based diene with an aryne is a long-standing topic of interest in glycal chemistry. We have examined the mechanistic pathways for the transformation of substituted glycals to chiral fused aromatic cores via Diels-Alder (DA) reaction using the SMDACN-M06-2X/6-31G(d) level of theory. The DA reactions of E (1a) and Z (1a') forms of C-2 alkenyl glycal and an aryl glycal (1b) as a diene were examined with a benzyne intermediate generated as a dienophile. The computational results reveal that 1a and 1b can only be transformed into the fused aromatic cores by the base-catalyzed reaction because a [1,5] sigmatropic hydrogen shift is not feasible. The activation free energy barrier for the base-catalyzed proton abstraction process is 4.2 kcal mol-1 and there is almost no barrier for stereoisomeric 1a DA-complexes. The activation free energy barrier values for stereoisomeric 1b DA-complexes for the base-catalyzed proton abstraction process are 10.8 and 12.4 kcal mol-1. The appropriate orientation of glycal-ring-oxygen and hydrogen at the 5th position of Z (1a') forms of C-2 alkenyl glycal facilitates the [1,5] sigmatropic hydrogen shift; however, the base-catalyzed reaction is energetically more favored than the former case. The rate-determining step for 1a and 1a' is the ring-opening step (18.2 and 19.5 kcal mol-1 for the S-stereoisomer), whereas the DA adduct formation step is the rate-determining step for 1b (16.1 kcal mol-1 for the S-stereoisomer). The structural analysis reveals the formation of the preferred S-stereoisomer over the R-stereoisomer with the respective dienes.

Synthesis and evaluation of the antibiotic-adjuvant activity of carbohydrate-based phosphoramidate derivatives

Phosphoramidates are becoming increasingly recognized as molecular targets for therapeutic development. Their biological functions are significantly influenced by their inherent properties such as reactivity, as well as the P-N backbone which allows for structural diversity. In this study we report the synthesis of novel carbohydrate-based phosphoramidate derivatives via the Staudinger-phosphite reaction; along with an evaluation of their adjuvant activity in combination with popular antibiotics. Our targets involved variation in both the sugar residue as well as the identity of the phosphoramidate. Moderate to excellent yields of these derivatives were obtained. Notable adjuvant activity was observed with the halogenated phosphoramidates. For the fluorinated glucose derivative in particular, a remarkable 32-fold decrease in the MIC of Ampicillin was obtained against Methicillin-resistant S. aureus.

First total syntheses of two natural glycosides

Isosyringinoside (1) and 3-(O-β-_d-glucopyranosyl)-α-(O-β-_d-glucopyranosyl)-4-hydroxy phenylethanol (2), the natural bioactive compounds contained unique structures, were first totally synthesized using easily available materials in short convenient routes with overall yields of 20.2% and 27.0%, respectively. An efficient total synthesis of 1 was developed in six steps, which contained two key steps of highly regioselective glycosylation without any selective protection steps. The seven-step synthesis of 2 involved two steps of regioselective glycosylations using BF₃-O(C₂H₅)₂ and TMSOTf as catalysts, respectively.

Neurotrophic and anti-neuroinflammatory constituents from the aerial parts of Coriandrum sativum

In the course of our continuing search for biologically active compounds from medicinal sources, we investigated the MeOH extract of the aerial parts of Coriandrum sativum Linn. An extended phytochemical investigation of the aerial parts of C. sativum led to the isolation and identification of seven compounds (1-7) including two new isocoumarin glycosides (1-2) and a new phenolic glycoside (5). The chemical structures of the new compounds (1, 2, and 5) were elucidated by analysis of 1D and 2D NMR (¹H and ¹³C NMR, COSY, HSQC, and HMBC) and HRESIMS data as well as by using chemical methods. All the isolates were evaluated not only for their potential neurotrophic activity by means of induction of nerve growth factor (NGF) in C6 glioma cells but also for production of nitric oxide (NO) levels in lipopolysaccharide (LPS)-activated murine microglia BV-2 cells to assess their anti-neuroinflammatory activity. Compounds 1-3 and 7 were stimulants of NGF release, with levels of NGF stimulated at 127.23 ± 1.89%, 128.22 ± 5.45%, 121.23 ± 6.66%, and 120.94 ± 3.97%, respectively. Furthermore, the aglycones of 1 and 2 (1a and 2a) showed more potent NGF secretion activity and anti-neuroinflammatory effect than did their glycosides (1a : 130.81 ± 5.45% and 2a : 134.44 ± 5.45%).

Too sweet: cheminformatics for deglycosylation in natural products

Sugar units in natural products are pharmacokinetically important but often redundant and therefore obstructing the study of the structure and function of the aglycon. Therefore, it is recommended to remove the sugars before a theoretical or experimental study of a molecule. Deglycogenases, enzymes that specialized in sugar removal from small molecules, are often used in laboratories to perform this task. However, there is no standardized computational procedure to perform this task in silico. In this work, we present a systematic approach for in silico removal of ring and linear sugars from molecular structures. Particular attention is given to molecules of biological origin and to their structural specificities. This approach is made available in two forms, through a free and open web application and as standalone open-source software.

Emodin 8-O-glucoside primes macrophages more strongly than emodin aglycone via activation of phagocytic activity and TLR-2/MAPK/NF-κB signalling pathway

Emodin (Emo) is a natural plant anthraquinone derivative with a wide spectrum of pharmacological properties, including anticancer, antioxidant, and hepatoprotective activities. Glycosylation of natural anthraquinones with various sugar moieties can affect their physical, chemical, and biological functions. In this study, the potential immunomodulatory activities of Emo and its glycosylated derivative, emodin 8-O-glucoside (E8G), were evaluated and compared using murine macrophage RAW264.7 cells and human monocytic THP-1 cells. The results showed that E8G (20 μM) induced the secretion of TNF-α and IL-6 from RAW264.7 cells more effectively than unglycosylated Emo aglycone, by 4.9- and 1.6-fold, respectively, with no significant cytotoxicity in the concentration range tested (up to 20 μM). E8G (2.5-20 μM) significantly and dose-dependently induced inducible nitric oxide synthase (iNOS) expression by up to 3.2-fold compared to that of untreated control following a remarkable increase in nitric oxide (NO) production. E8G also significantly increased the expression of TLR-2 mRNA and the phosphorylation of MAPKs (JNK and p38). The activation and subsequent nuclear translocation of NF-κB was substantially enhanced upon treatment with E8G (2.5-20 μM). Moreover, E8G markedly induced macrophage-mediated phagocytosis of apoptotic Jurkat T cells. These results demonstrated that E8G far more strongly stimulates the secretion of proinflammatory cytokines, such as TNF-α and IL-6, and NO production from macrophages through upregulation of the TLR-2/MAPK/NF-κB signalling pathway than its nonglycosylated form, Emo aglycone. These results suggest for the first time that E8G may represent a novel immunomodulator, enhancing the early innate immunity.

Exploring the broad nucleotide triphosphate and sugar-1-phosphate specificity of thymidylyltransferase Cps23FL from Streptococcus pneumonia serotype 23F

Glucose-1-phosphate thymidylyltransferase (Cps23FL) from Streptococcus pneumonia serotype 23F is the initial enzyme that catalyses the thymidylyl transfer reaction in prokaryotic deoxythymidine diphosphate-l-rhamnose (dTDP-Rha) biosynthetic pathway. In this study, the broad substrate specificity of Cps23FL towards six glucose-1-phosphates and nine nucleoside triphosphates as substrates was systematically explored, eventually providing access to nineteen sugar nucleotide analogs.

The broad substrate specificities of thymidylyltransferase Cps23FL towards nucleotide triphosphates and sugar-1-phosphates were systemically investigated.

Exploring the Nucleophilic N- and S-Glycosylation Capacity of Bacillus licheniformis YjiC Enzyme

YjiC, a glycosyltransferase from Bacillus licheniformis, is a well-known versatile enzyme for glycosylation of diverse substrates. Although a number of O-glycosylated products have been produced using YjiC, no report has been updated for nucleophilic N-, S-, and C- glycosylation. Here, we report the additional functional capacity of YjiC for nucleophilic N- and S- glycosylation using a broad substrate spectrum including UDP-α-D-glucose, UDP-N-acetyl glucosamine, UDP-N-acetylgalactosamine, UDP-α-D-glucuronic acid, TDP-α-L-rhamnose, TDP-α-D-viosamine, and GDP-α-Lfucose as donor and various amine and thiol groups containing natural products as acceptor substrates. The results revealed YjiC as a promiscuous enzyme for conjugating diverse sugars at amine and thiol functional groups of small molecules applicable for generating glycofunctionalized chemical diversity libraries. The glycosylated products were analyzed using HPLC and LC/MS and compared with previous reports.