Biosynthesis of rare hexoses using microorganisms and related enzymes

Xylose isomerase: From fundamental research to applied enzyme technology

Xylose isomerases (XI, EC 5.3.1.5) are key enzymes for the metabolism of pentoses by microorganisms. The importance of XIs goes beyond academic biochemical research and the catalysis of aldo-ketose conversion by XIs is among the most successful examples of industrial enzyme technology in a market that generates multibillion dollar annual revenues. Here we present an in-depth review of how structural information has contributed to the current understanding of XI catalysis, and discuss topics related to the ongoing efforts to elucidate key aspects of the catalytic mechanism. An overview of XI immobilization is also provided that illustrates how the discoveries in basic enzyme technology research can generate opportunities for novel uses of XI, and we review not only historical aspects but also more recent applications in HFCS, biofuels and other applications. The systems biology revolution will impact all aspects of XI research and application, and we finalize by reviewing the contemporary efforts of metabolic and protein engineering using XI and the future roles of the enzyme in the expanding bioeconomy.

Eco-friendly production, separation and purification of D-tagatose and D-allulose from whey powder via one-pot whole-cells biotransformation, yeast fermentation and chromatography

Whey powder (WP), a dairy by-product with high biochemical oxygen demand (BOD) and chemical oxygen demand (COD), presents challenges due to its high production, low-value utilization, and environmental pollution. Based on the idea of turning waste into treasure, high-value use of WP was studied. Firstly, an engineered Bacillus subtilis co-expressing β-galactosidase (β-Gal) and L-arabinose isomerase (LAI) was constructed, which ultimately yielded 77.5 g/L D-tagatose from 500 g/L lactose. Subsequently, an engineered Escherichia coli co-expressing glucose isomerase (GI) and D-allulose 3-epimerase (DAE) was used together with above recombinant B. subtilis in a one-pot whole-cell biotransformation, and 29.11 g/L D-tagatose and 11.45 g/L D-allulose were derived from 200 g/L WP (equating to 140 g/L lactose) with yield of 0.29 g rare sugars/g lactose. In addition, the d-glucose, d-fructose and D-galactose in the reaction solution were removed by Saccharomyces cerevisiae S288C fermentation, and finally chromatography was used in separation of D-tagatose and D-allulose to obtain the purified products with 97.5 % and 95.0 % purities, respectively. This study showcases the eco-friendly production of D-tagatose and D-allulose from WP, with their separation and purification via yeast fermentation and chromatography successfully carried out for the first time.

Exploiting the biocatalytic potential of co-expressed l-fucose isomerase and d-tagatose 3-epimerase for the biosynthesis of 6-deoxy-l-sorbose

6-Deoxy-l-sorbose (6-DLS) is an imperative rare sugar employed in food, agriculture, pharmaceutical and cosmetic industeries. However, it is a synthetic and very expensive rare sugars, previously synthesized by chemo-enzymatic methods through a long chain of chemical processes. Recently, enzymatic synthesis of rare sugars has attracted a lot of attention due to its advantages over synthetic methods. In this work, a promising approach for the synthesis of 6-DLS from an inexpensive sugar l-fucose was identified. The genes for l-fucose isomerase from Paenibacillus rhizosphaerae (Pr-LFI) and genes for d-tagatose-3-epimerase from Caballeronia fortuita (Cf-DTE) have been used for cloning and co-expression in Escherichia coli, developed a recombinant plasmid harboring pANY1-Pr-LFI/Cf-DTE vector. The recombinant co-expression system exhibited an optimum activity at 50 °C of temperature and pH 6.5 in the presence of Co2+ metal ion which inflated the catalytic activity by 6.8 folds as compared to control group with no metal ions. The recombinant co-expressed system was stable up to more than 50 % relative activity after 12 h and revealed a melting temperature (Tm) of 63.38 °C exhibiting half-life of 13.17 h at 50 °C. The co-expression system exhibited, 4.93, 11.41 and 16.21 g/L of 6-DLS production from initial l-fucose concentration of 30, 70 and 100 g/L, which equates to conversion yield of 16.44 %, 16.30 % and 16.21 % respectively. Generally, this study offers a promising strategy for the biological production of 6-DLS from an inexpensive substrate l-fucose in slightly acidic conditions with the aid of co-expression system harboring Pr-LFI and CF-DTE genes.

Highly Efficient Synthesis of Rare Sugars from Glycerol in Endotoxin-Free ClearColi by Fermentation

Rare sugars possess potential applications as low-calorie sweeteners, especially for anti-obesity and anti-diabetes. In this study, a fermentation biosystem based on the "DHAP-dependent aldolases strategy" was established for D-allulose and D-sorbose production from glycerol in endotoxin-free ClearColi BL21 (DE3). Several engineering strategies were adopted to enhance rare sugar production. Firstly, the combination of different plasmids for aldO, rhaD, and yqaB expression was optimized. Then, the artificially constructed ribosomal binding site (RBS) libraries of aldO, rhaD, and yqaB genes were assembled individually and combinatorially. In addition, a peroxidase was overexpressed to eliminate the damage or toxicity from hydrogen peroxide generated by alditol oxidase (AldO). Finally, stepwise improvements in rare sugar synthesis were elevated to 15.01 g/L with a high yield of 0.75 g/g glycerol in a 3 L fermenter. This research enables the effective production of rare sugars from raw glycerol in high yields.

The Amount of the Rare Sugar Tagatose on Tomato Leaves Decreases after Spray Application under Greenhouse Conditions

Tagatose is a rare sugar that suppresses plant diseases, such as late blight of tomato, caused by Phytophthora infestans. Tagatose can be metabolized by some microorganisms and no information is available on its persistence on tomato leaves. The aim of this study was to assess the persistence of tagatose on tomato leaves under commercial greenhouse conditions. The amount of tagatose on tomato leaves and the inhibitory activity against P. infestans decreased seven days after spray application in the absence of rain wash-off. Potential tagatose-degrading bacteria were isolated from tomato leaves, and they belonged to Acinetobacter sp., Bacillus sp., Comamonas sp., Enterobacter sp., Methylobacterium sp., Microbacterium sp., Pantoea sp., Plantibacter sp., Pseudomonas sp., Ralstonia sp., Rhodococcus sp., Sphingobium sp., and Sphingomonas sp. Thus, indigenous phyllosphere microorganisms could partially metabolize tagatose laid on plant leaves after spray application, reducing the persistence of this fungal inhibitor on tomato leaves.

Site-Selective Modification of (Oligo)Saccharides

Oligosaccharides, either as such or as part of glycolipids, glycopeptides, or glycoproteins, are ubiquitous in nature and fulfill important roles in the living cell. Also in medicine and to some extent in materials, oligosaccharides play an important role. In order to study their function, modifying naturally occurring oligosaccharides, and building in reactive groups and reporter groups in oligosaccharides, are key strategies. The development of oligosaccharides as drugs, or vaccines, requires the introduction of subtle modifications in the structure of oligosaccharides to optimize efficacy and, in the case of antibiotics, circumvent bacterial resistance. Provided the natural oligosaccharide is available, site-selective modification is an attractive approach as total synthesis of the target is often very laborious. Researchers in catalysis areas, such as transition-metal catalysis, enzyme catalysis, organocatalysis, and photoredox catalysis, have made considerable progress in the development of site-selective and late-stage modification methods for mono- and oligosaccharides. It is foreseen that the fields of enzymatic modification of glycans and the chemical modification of (oligo)saccharides will approach and potentially meet each other, but there is a lot to learn and discover before this will be the case.

A concise synthesis of l-gulose and its C-6 derivatives

A convenient route for the preparation of l-gulose and its C-6 derivatives starting from commercially available 2,3:5,6-diisopropylidene-d-mannofuranose via C-5 epimerization as the key step was developed. 1-O-Benzylation followed by regioselective hydrolysis of the 5,6-isopropylidene group furnished benzyl 2,3-isopropylidene-α-d-mannofuranoside, which was subjected upon regioselective one-pot 6-O-benzoylation and 5-O-mesylation, providing the corresponding 5-OMs-6-OBz derivative in excellent selectivity. Treatment of this mesylate compound with potassium t-butoxide to remove the benzoyl group followed by intramolecular S_N2 inversion led to benzyl 5,6-anhydro-2,3-isopropylidene-β-l-gulofuranoside, which could undergo not only nucleophilic substitutions to open the epoxide ring to give various C-6 derivatives, but also acidic hydrolysis to yield 1,6-anhydro-β-l-gulopyranose for further transformation into l-gulopyranosyl pentaacetate.

Metal-based micro-composite of L-arabinose isomerase and L-ribose isomerase for the sustainable synthesis of L-ribose and D-talose

The biocatalysts are broadly explored in the biological transformation processes. The enzyme cascade catalysis involves various catalytic activities in a sequential process to produce the desired product including the formation of reaction intermediates. Enzyme immobilization is a method in which enzymes are confined within a support or matrix either physically or chemically to enhance their relative stability and catalytic activity in the enzyme cascade catalysis. In view of this, L-arabinose isomerase (L-AI) and L-ribose isomerase (L-RI) were immobilized on zeolite based metal framework as a micro-composite construct (DEMC@L-AI+L-RI) using linker, and metal ions. Such immobilization could be of great significance and provide several advantages like mesoporous surface for enzyme adsorption, desirable functionality in the production of products in enzyme cascade reaction, high storage stability and enhanced recyclability. The developed DEMC@L-AI+L-RI was characterized using SEM, FTIR, CLSM and TGA. The immobilization yield was 32% and loading of enzyme was 22% on the surface of micro-composite. The DEMC@L-AI+L-RI showed relatively stable catalytic activity at pH 5-6 and temperature 40 °C. The catalytic efficiency (k_cat/K_m) of both the enzymes was increased by 1.5-fold after immobilization. With the immobilized biocatalyst, bioconversion of L-arabinose to L-ribose was 22.6% and D-galactose to D-talose was 15.2%. The reusability of developed biocatalyst for more than six cycles was observed for more than 50% yield of the sugars. The conversion of biomass sugars from beetroot and onion waste residues was 20% and 14% to produce ribose and talose, respectively.

Reconstruction of a Cofactor Self-Sufficient Whole-Cell Biocatalyst System for Efficient Biosynthesis of Allitol from d-Glucose

The combined catalysis of glucose isomerase (GI), d-psicose 3-epimerase (DPEase), ribitol dehydrogenase (RDH), and formate dehydrogenase (FDH) provides a convenient route for the biosynthesis of allitol from d-glucose; however, the low catalytic efficiency restricts its industrial applications. Here, the supplementation of 0.32 g/L NAD⁺ significantly promoted the cell catalytic activity by 1.18-fold, suggesting that the insufficient intracellular NAD(H) content was a limiting factor in allitol production. Glucose dehydrogenase (GDH) with 18.13-fold higher activity than FDH was used for reconstructing a cofactor self-sufficient system, which was combined with the overexpression of the rate-limiting genes involved in NAD⁺ salvage metabolic flow to expand the available intracellular NAD(H) pool. Then, the multienzyme self-assembly system with SpyTag and SpyCatcher effectively channeled intermediates, leading to an 81.1% increase in allitol titer to 15.03 g/L from 25 g/L d-glucose. This study provided a facilitated strategy for large-scale and efficient biosynthesis of allitol from a low-cost substrate.

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.

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.

The Differential Growth Inhibition of Phytophthora spp. Caused by the Rare Sugar Tagatose Is Associated With Species-Specific Metabolic and Transcriptional Changes

Tagatose is a rare sugar with no negative impacts on human health and selective inhibitory effects on plant-associated microorganisms. Tagatose inhibited mycelial growth and negatively affected mitochondrial processes in Phytophthora infestans, but not in Phytophthora cinnamomi. The aim of this study was to elucidate metabolic changes and transcriptional reprogramming activated by P. infestans and P. cinnamomi in response to tagatose, in order to clarify the differential inhibitory mechanisms of tagatose and the species-specific reactions to this rare sugar. Phytophthora infestans and P. cinnamomi activated distinct metabolic and transcriptional changes in response to the rare sugar. Tagatose negatively affected mycelial growth, sugar content and amino acid content in P. infestans with a severe transcriptional reprogramming that included the downregulation of genes involved in transport, sugar metabolism, signal transduction, and growth-related process. Conversely, tagatose incubation upregulated genes related to transport, energy metabolism, sugar metabolism and oxidative stress in P. cinnamomi with no negative effects on mycelial growth, sugar content and amino acid content. Differential inhibitory effects of tagatose on Phytophthora spp. were associated with an attempted reaction of P. infestans, which was not sufficient to attenuate the negative impacts of the rare sugar and with an efficient response of P. cinnamomi with the reprogramming of multiple metabolic processes, such as genes related to glucose transport, pentose metabolism, tricarboxylic acid cycle, reactive oxygen species detoxification, mitochondrial and alternative respiration processes. Knowledge on the differential response of Phytophthora spp. to tagatose represent a step forward in the understanding functional roles of rare sugars.

Efficient whole-cell biosynthesis of l-gulose by coupling mannitol-1-dehydrogenase with NADH oxidase

L-Gulose is a rare aldohexose to serve as a building block for anticancer drug bleomycin and nucleoside-based antivirals. However, preparative inaccessibility and high cost have hindered its pharmaceutical application. Despite a regio- and stereo-selective enzymatic synthesis of l-gulose from d-sorbitol using a variant of NAD⁺-dependent mannitol-1-dehydrogenase from Apium graveolens (mMDH) was explored, low efficiency and productivity caused by NADH accumulation or insufficient amount of NAD⁺ limited the practical utility of this process. In this study, a stable and efficient NADH oxidase from Bacillus cereus (bcNOX) was found to be more compatible with mMDH to recycle NAD⁺ in E. coli cells for l-gulose biosynthesis. After a systematic optimization of the whole-cell system, efficient biosynthesis of l-gulose was achieved. Starting with 70 g/L of readily available and cheap d-sorbitol resulted in a volumetric productivity of 5.5 g/L/d. This whole-cell approach enables practical, efficient and environmentally friendly biosynthesis of l-gulose and exhibits the potential of becoming a biocatalytic strategy for various enzymatic oxidative transformations.

Rare Sugars: Recent Advances and Their Potential Role in Sustainable Crop Protection

Rare sugars are monosaccharides with a limited availability in the nature and almost unknown biological functions. The use of industrial enzymatic and microbial processes greatly reduced their production costs, making research on these molecules more accessible. Since then, the number of studies on their medical/clinical applications grew and rare sugars emerged as potential candidates to replace conventional sugars in human nutrition thanks to their beneficial health effects. More recently, the potential use of rare sugars in agriculture was also highlighted. However, overviews and critical evaluations on this topic are missing. This review aims to provide the current knowledge about the effects of rare sugars on the organisms of the farming ecosystem, with an emphasis on their mode of action and practical use as an innovative tool for sustainable agriculture. Some rare sugars can impact the plant growth and immune responses by affecting metabolic homeostasis and the hormonal signaling pathways. These properties could be used for the development of new herbicides, plant growth regulators and resistance inducers. Other rare sugars also showed antinutritional properties on some phytopathogens and biocidal activity against some plant pests, highlighting their promising potential for the development of new sustainable pesticides. Their low risk for human health also makes them safe and ecofriendly alternatives to agrochemicals.

Interactions of tagatose with the sugar metabolism are responsible for Phytophthora infestans growth inhibition

Tagatose is a rare sugar metabolised by a limited number of microorganisms that inhibits a large spectrum of phytopathogens. In particular, tagatose inhibited Phytophthora infestans growth and negatively affected mitochondrial processes. However, the possible effects of tagatose on P. infestans metabolism have not yet been investigated. The aim of this study was to analyse the impact of this rare sugar on the sugar metabolism in P. infestans, in order to better understand its mode of action. Tagatose inhibited the growth of P. infestans with a precise reprogramming of the carbohydrate metabolism that involved a decrease of glucose, glucose-1-phosphate and mannose content and β-glucosidase activity. The combination of tagatose with common sugars led to three different responses and highlighted antagonistic interactions. In particular, glucose partially attenuated the inhibitory effects of tagatose, while fructose fully impaired tagatose-mediated growth inhibition and metabolite changes. Moreover, sucrose did not attenuate tagatose effects, suggesting that the inhibition of sucrose catabolism and the alteration of glucose-related pathways contributed to the growth inhibition caused by tagatose to P. infestans. The interactions of tagatose with the common sugar metabolism were found to be a key mode of action against P. infestans growth, which may represent the basis for the further development of tagatose as an eco-friendly fungicide.

Expanding the Enzyme Repertoire for Sugar Nucleotide Epimerization: The CDP-Tyvelose 2-Epimerase from Thermodesulfatator atlanticus for Glucose/Mannose Interconversion

Epimerization of sugar nucleotides is central to the structural diversification of monosaccharide building blocks for cellular biosynthesis. Epimerase applicability to carbohydrate synthesis can be limited, however, by the high degree of substrate specificity exhibited by most sugar nucleotide epimerases. Here, we discovered a promiscuous type of CDP-tyvelose 2-epimerase (TyvE)-like enzyme that promotes C2-epimerization in all nucleotide (CDP, UDP, GDP, ADP, TDP)-activated forms of d-glucose. This new epimerase, originating from Thermodesulfatator atlanticus, is a functional homodimer that contains one tightly bound NAD⁺/subunit and shows optimum activity at 70°C and pH 9.5. The enzyme exhibits a k _cat with CDP-dglucose of ∼1.0 min^-1 (pH 7.5, 60°C). To characterize the epimerase kinetically and probe its substrate specificity, we developed chemo-enzymatic syntheses for CDP-dmannose, CDP-6-deoxy-dglucose, CDP-3-deoxy-dglucose and CDP-6-deoxy-dxylo-hexopyranos-4-ulose. Attempts to obtain CDP-dparatose and CDP-dtyvelose were not successful. Using high-resolution carbohydrate analytics and in situ NMR to monitor the enzymatic conversions (60°C, pH 7.5), we show that the CDP-dmannose/CDP-dglucose ratio at equilibrium is 0.67 (± 0.1), determined from the kinetic Haldane relationship and directly from the reaction. We further show that deoxygenation at sugar C6 enhances the enzyme activity 5-fold compared to CDP-dglucose whereas deoxygenation at C3 renders the substrate inactive. Phylogenetic analysis places the T. atlanticus epimerase into a distinct subgroup within the sugar nucleotide epimerase family of SDR (short-chain dehydrogenases/reductases), for which the current study now provides the functional context. Collectively, our results expand an emerging toolbox of epimerase-catalyzed reactions for sugar nucleotide synthesis.IMPORTANCE Epimerases of the sugar nucleotide-modifying class of enzymes have attracted considerable interest in carbohydrate (bio)chemistry, for the mechanistic challenges and the opportunities for synthesis involved in the reactions catalyzed. Discovery of new epimerases with expanded scope of sugar nucleotide substrates used is important to promote the mechanistic inquiry and can facilitate the development of new enzyme applications. Here, a CDP-tyvelose 2-epimerase-like enzyme from Thermodesulfatator atlanticus is shown to catalyze sugar C2 epimerization in CDP-glucose and other nucleotide-activated forms of dglucose. The reactions are new to nature in the context of enzymatic sugar nucleotide modification. The current study explores the substrate scope of the discovered C2-epimerase and, based on modeling, suggests structure-function relationships that may be important for specificity and catalysis.

The Rare Sugar Tagatose Differentially Inhibits the Growth of Phytophthora infestans and Phytophthora cinnamomi by Interfering With Mitochondrial Processes

Rare sugars are monosaccharides with limited availability in nature and their biological functions are largely unknown. Among them, tagatose was developed as a low-calorie sweetener and showed beneficial effects on human health. Tagatose is metabolized by only certain microbial taxa and inhibits the growth of important crop pathogens (e.g., Phytophthora infestans), but its mode of action and the microbial responses are unknown. The aim of this study was to understand the tagatose mode of action against Phytophthora spp., with the final aim of developing new plant protection products. Tagatose inhibited P. infestans growth in vitro and caused severe ultrastructural alterations, with the formation of circular and concentric mitochondrial cristae. Decreased ATP content and reduced oxygen consumption rate (OCR) were found in tagatose-incubated P. infestans as compared to the control, with the consequent accumulation of reactive oxygen species (ROS) and induction of genes related to apoptosis and oxidative stress response. On the other hand, tagatose did not, or only slightly, affect the growth, cellular ultrastructure and mitochondrial processes in Phytophthora cinnamomi, indicating a species-specific response to this rare sugar. The mode of action of tagatose against P. infestans was mainly based on the inhibition of mitochondrial processes and this rare sugar seems to be a promising active substance for the further development of eco-friendly fungicides, thanks to its anti-nutritional properties on some phytopathogens and low risk for human health.

One-Pot Multienzyme Synthesis of Rare Ketoses from Glycerol

A facile approach is introduced here for the synthesis of rare ketoses from glycerol and d-/l-glyceraldehyde (d-/l-GA). The reactions were carried out in a one-pot multienzyme fashion in which the only carbon source is glycerol. In the enzymatic cascade, glycerol is phosphorylated and then oxidized at C2 to afford dihydroxyacetone phosphate (DHAP), the key donor for enzymatic aldol reaction. Meanwhile, the primary alcohol of glycerol is also oxidized to give the acceptor molecule GA in situ (d- or l-isomer could be formed stereospecifically with either alditol oxidase or horse liver alcohol dehydrogenase). Different DHAP-dependent aldolases were used to generate the aldol adducts (rare ketohexose phosphates) with various stereoconfigurations and diastereomeric ratios. It is worth noting that the enzyme that catalyzes the phosphorylation reaction in the first step could also help recycle the phosphate in the last step to provide free rare sugar molecules. This study provides a useful method for rare ketose synthesis on a 100 mg to g scale, starting from relatively inexpensive materials which solved the problem of supplying both glycerol 3-phosphate and GA in our previous work. It also demonstrates an example of green synthesis due to highly efficient carbon usage and recycling of cofactors.

Access to d- and l-Psicose Derivatives via Hydroxy Methylation of Ribono Lactone

2,3,5-Tri-O-benzyl- and 2,3,5-tri-O-methyl-d-ribono-γ-lactone were converted with (methoxyethoxymethoxy)methyl and benzyloxy tributylstannane into the corresponding protected d-psicoses as mixtures of anomers in 31%–72% yield. Treatment of 2,3,5-tri-O-methyl-l-ribono-γ-lactone with benzyloxy tributylstannane afforded the corresponding l-psicose derivative as an anomeric mixture in 72% yield. Both methylated psicoses were further converted into 1,2-O-isopropylidene-3,4,6-tri-O-methyl-d- and l-psicofuranosides, the respective α- and β-anomers of which could be separated and characterized.

Ecological impact of a rare sugar on grapevine phyllosphere microbial communities

Plants host a complex microbiota inside or outside their tissues, and phyllosphere microorganisms can be influenced by environmental, nutritional and agronomic factors. Rare sugars are defined as monosaccharides with limited availability in nature and they are metabolised by only few certain microbial taxa. Among rare sugars, tagatose (TAG) is a low-calories sweetener that stimulates and inhibits beneficial and pathogenic bacteria in the human gut microbiota, respectively. Based on this differential effect on human-associated microorganisms, we investigated the effect of TAG treatments on the grapevine phyllosphere microorganisms to evaluate whether it can engineer the microbiota and modify the ratio between beneficial and pathogenic plant-associated microorganisms. TAG treatments changed the structure of the leaf microbiota and they successfully reduced leaf infections of downy mildew (caused by Plasmopara viticola) and powdery mildew (caused by Erysiphe necator) under field conditions. TAG increased the relative abundance of indigenous beneficial microorganisms, such as some potential biocontrol agents, which could partially contribute to disease control. The taxonomic composition of fungal and bacterial leaf populations differed according to grapevine locations, therefore TAG effects on the microbial structure were influenced by the composition of the originally residing microbiota. TAG is a promising biopesticide that could shift the balance of pathogenic and beneficial plant-associated microorganisms, suggesting selective nutritional/anti-nutritional properties for some specific taxa. More specifically, TAG displayed possible plant prebiotic effects on the phyllosphere microbiota and this mechanism of action could represent a novel strategy that can be further developed for sustainable plant protection.

Cytotoxic saponins and other natural products from flowering tops of Narthecium ossifragum L

For more than four centuries, the intake of Narthecium ossifragum has been associated with poisoning in domesticated animals. Saponins occurring in flowering tops of the plant are considered to cause kidney damage in calves. At present, there are more than 30 papers on the saponins of N. ossifragum in the literature, although the structures of these compounds have hitherto not been determined. Here, we identify the saponins of N. ossifragum as sarsasapogenin, sarsasapogenin-3-O-β-galactopyranoside, sarsasapogenin-3-O-(2'-O-β-glucopyranosyl-β-galactopyranoside) and sarsasapogenin-3-O-(2'-O-β-glucopyranosyl-3'-O-α-arabinopyranosyl-β-galactopyranoside). Moreover, six aromatic natural products were isolated and characterized from the methanolic extract from flowers of N. ossifragum. Five of these aromatic compounds, chrysoeriol 6-C-β-arabinofuranoside-8-C-β-glucopyranoside, chrysoeriol 6-C-β-arabinopyranosyl-8-C-β-glucopyranoside, chrysoeriol 6-C-β-xylopyranosyl-8-C-β-galactopyranoside, chrysoeriol 6-C-β-galactopyranosyl-8-C-β-glucopyranoside and chrysoeriol 6-C-β-glucopyranosyl-8-C-β-galactopyranoside are undescribed. All compounds were tested for cytotoxicity in mammalian cell lines derived from the heart, kidney, and haematological tissues. The saponins exhibited cytotoxicity in the micromolar range, with proportionally increasing cytotoxicity with increasing number of glycosyl substituents. The most potent compound was the main saponin sarsasapogenin-3-O-(2'-O-β-glucopyranosyl-3'-O-α-arabinopyranosyl-β-galactopyranoside), which produced cell death at concentrations below 3-4 μM in all three cell lines tested. This indicates that the saponins are the toxicants mainly responsible for kidney damage observed in cattle after ingestion of N. ossifragum. Our findings also pave the way for analysis of individual compounds isolated during the biopsies of intoxicated animals.

A sensitive and high throughput method for the analysis of d-psicose by capillary electrophoresis

d-Psicose/allulose is a rare sugar and it has high potential benefits for pharmaceutical and food industry. The existed analytical methods have its own limitations to quantify fructose and d-psicose mixtures. Hence there is a need for the development of an effective, efficient and sensitive analytical method for quantification of d-psicose in presence of other sugars. Quantification of sugars by capillary electrophoresis (CE) have been previously reported. However, the list does not include d-psicose. In this study, d-psicose is successfully quantified for the first time in the presence of d-fructose and glucose with a good resolution. Standard curves for all the sugars are established in a concentration range of 0.1 mM (0.0018% w/v) to 3.0 mM (0.0540% w/v) with a coefficient of determination of >0.99. The scope of this method can be extended to quantify d-psicose and their processed impurities in food products with minor modifications in sample preparation.

Production of the potential sweetener 5-ketofructose from fructose in fed-batch cultivation with Gluconobacter oxydans

Sweeteners improve the dietary properties of many foods. A candidate for a new natural sweetener is 5-ketofructose. In this study a fed-batch process for the production of 5-ketofructose was developed. A Gluconobacter oxydans strain overexpressing a fructose dehydrogenase from G. japonicus was used and the sensory properties of 5-ketofructose were analyzed. The compound showed an identical sweet taste quality as fructose and a similar intrinsic sweet threshold concentration of 16.4 mmol/L. The production of 5-ketofructose was characterized online by monitoring of the respiration activity in shake flasks. Pulsed and continuous fructose feeding was realized in 2 L stirred tank reactors and maximum fructose consumption rates were determined. 5-Ketofructose concentrations of up to 489 g/L, product yields up to 0.98 g_5-KF/g_fructose and space time yields up to 8.2 g/L/h were reached highlighting the potential of the presented process.

Target highlights from the first post-PSI CASP experiment (CASP12, May-August 2016)

The functional and biological significance of the selected CASP12 targets are described by the authors of the structures. The crystallographers discuss the most interesting structural features of the target proteins and assess whether these features were correctly reproduced in the predictions submitted to the CASP12 experiment.

Influence of Low Glycaemic Index Sweeteners on Antioxidant, Sensory, Mechanical, and Physicochemical Properties of a Watermelon Jelly

The replacement of sucrose by new noncariogenic and low glycaemic index sweeteners (isomaltulose and tagatose) and the addition of natural watermelon juice in jelly have been assessed in terms of composition, texture, colour, antioxidant activity, microbiology, and sensory properties. These analyses were performed initially and after 15 days of storage. Furthermore, the values were compared with those obtained in the analyses of a commercial watermelon jelly. The results showed that the antioxidant activity increased with the storage time in the control sample and in samples combining isomaltulose and tagatose. In addition, noncariogenic and low glycaemic index sweeteners did not affect the instrumental texture. However, the colour changed, especially in the sample containing tagatose only. Finally, the dessert containing tagatose and isomaltulose in equal proportion achieved a similar score in the sensory evaluation as the commercial one, showing the feasibility of using these sweeteners to reformulate watermelon jelly.

TM0416, a Hyperthermophilic Promiscuous Nonphosphorylated Sugar Isomerase, Catalyzes Various C5 and C6 Epimerization Reactions

There is currently little information on nonphosphorylated sugar epimerases, which are of potential interest for producing rare sugars. We found a gene (the TM0416 gene) encoding a putative d-tagatose-3-epimerase-related protein from the hyperthermophilic bacterium Thermotoga maritima We overexpressed the TM0416 gene in Escherichia coli and purified the resulting recombinant protein for detailed characterization. Amino acid sequence alignment and a structural similarity search revealed that TM0416 is a putative nonphosphorylated sugar epimerase. The recombinant enzyme exhibited maximal C-3 epimerization of l-ribulose to l-xylulose at ∼80°C and pH 7 in the presence of 1 mM Mn²⁺ In addition, this enzyme showed unusually high activity for the epimerization of d-tagatose to d-sorbose, with a conversion yield of 20% after 6 h at 80°C. Remarkably, the enzyme catalyzed the isomerization of d-erythrose or d-threose to d-erythrulose significantly, with conversion yields of 71% and 54.5%, respectively, after 6 h at 80°C at pH 7. To further investigate the substrate specificity of TM0416, we determined its crystal structures in complex with divalent metal ions and l-erythrulose at resolutions of 1.5 and 1.6 Å. Detailed inspection of the structural features and biochemical data clearly demonstrated that this metalloenzyme, with a freely accessible substrate-binding site and neighboring hydrophobic residues, exhibits different and promiscuous substrate preferences, compared with its mesophilic counterparts. Therefore, this study suggests that TM0416 can be functionally classified as a novel type of l-ribulose 3-epimerase (R3E) with d-erythrose isomerase activity.IMPORTANCE Rare sugars, which occur naturally in small amounts, have attracted considerable attention in the food and drug industries. However, there is little information on nonphosphorylated sugar epimerases, which might potentially be applied for the production of rare sugars. This study describes the characterization and functional annotation of a putative nonphosphorylated sugar 3-epimerase from a hyperthermophilic bacterium. Furthermore, we determined its crystal structures in complex with divalent metal ions and l-erythrulose, highlighting its metal-dependent, bifunctional, sugar-isomerizing activity. This hyperthermophilic R3E exhibited d-erythrose/d-threose isomerase activity, with structural features near the substrate-binding site distinct from those of its mesophilic counterparts. Moreover, this metalloenzyme showed unusually high activity for the epimerization of d-tagatose to d-sorbose at 70°C. Therefore, TM0416 can be functionally classified as a novel type of promiscuous R3E with a potential for the production of rare sugars for the food and pharmaceutical industries.

The role of artificial and natural sweeteners in reducing the consumption of table sugar: A narrative review

The rapid increase in the prevalence of obesity worldwide has been partially attributed to the overconsumption of added sugars. Recent guidelines call for limiting the consumption of simple sugars to less than 10% of daily caloric consumption. High intensity sweeteners are regulated as food additives and include aspartame, acesulfame-k, neotame, saccharin, sucralose, cyclamate and alitame. Steviol glycosides and Luo Han Guo fruit extracts are high intensity sweeteners that are designated as generally recognized as safe (GRAS). Commonly used non-caloric artificial sweeteners may have unfavorable effect on health including glucose intolerance and failure to cause weight reduction. The nutritive sweeteners include sugar alcohols such as sorbitol, xylitol, lactitol, mannitol, erythritol, trehalose and maltitol. Naturally occurring rare sugars have recently emerged as an alternative category of sweeteners. These monosaccharides and their derivatives are found in nature in small quantities and lack significant calories. This category includes d-allulose (d-psicose), d-tagatose, d-sorbose and d-allose. Limiting consumption of any sweetener may well be the best health advice. Identifying natural sweeteners that have favorable effects on body weight and metabolism may help achieving the current recommendations of restricting simple sugar consumption.

Pathway Construction in Corynebacterium glutamicum and Strain Engineering To Produce Rare Sugars from Glycerol

Rare sugars are valuable natural products widely used in pharmaceutical and food industries. In this study, we expected to synthesize rare ketoses from abundant glycerol using dihydroxyacetone phosphate (DHAP)-dependent aldolases. First, a new glycerol assimilation pathway was constructed to synthesize DHAP. The enzymes which convert glycerol to 3-hydroxypropionaldehyde and l-glyceraldehyde were selected, and their corresponding aldehyde synthesis pathways were constructed in vivo. Four aldol pathways based on different aldolases and phosphorylase were gathered. Next, three pathways were assembled and the resulting strains synthesized 5-deoxypsicose, 5-deoxysorbose, and 5-deoxyfructose from glucose and glycerol and produce l-fructose, l-tagatose, l-sorbose, and l-psicose with glycerol as the only carbon source. To achieve higher product titer and yield, the recombinant strains were further engineered and fermentation conditions were optimized. Fed-batch culture of engineered strains obtained 38.1 g/L 5-deoxypsicose with a yield of 0.91 ± 0.04 mol product per mol of glycerol and synthesized 20.8 g/L l-fructose, 10.3 g/L l-tagatose, 1.2 g/L l-sorbose, and 0.95 g/L l-psicose.

A two-step strategy for the preparation of 6-deoxy-l-sorbose

A two-step enzymatic strategy for the efficient and convenient synthesis of 6-deoxy-l-sorbose was reported herein. In the first reaction step, the isomerization of l-fucose (6-deoxy-l-galactose) to l-fuculose (6-deoxy-l-tagatose) catalyzed by l-fucose isomerase (FucI), and the epimerization of l-fuculose to 6-deoxy-l-sorbose catalyzed by d-tagatose 3-epimerase (DTE) were coupled with the targeted phosphorylation of 6-deoxy-l-sorbose by fructose kinase from human (HK) in a one-pot reaction. The resultant 6-deoxy-l-sorbose 1-phosphate was purified by silver nitrate precipitation method. In the second reaction step, the phosphate group of the 6-deoxy-l-sorbose 1-phosphate was hydrolyzed with acid phosphatase (AphA) to produce 6-deoxy-l-sorbose in 81% yield with regard to l-fucose.

Triacetonide of Glucoheptonic Acid in the Scalable Syntheses of d-Gulose, 6-Deoxy-d-gulose, l-Glucose, 6-Deoxy-l-glucose, and Related Sugars

Ease of separation of petrol-soluble acetonides derived from the triacetonide of methyl glucoheptonate allows scalable syntheses of rare sugars containing the l-gluco or d-gulo structural motif with any oxidation level at the C6 or C1 position of the hexose, usually without chromatography: meso-d-glycero-d-guloheptitol available in two steps is an ideal entry point for the study of the biotechnological production of heptoses.

The Ecology of Acidobacteria: Moving beyond Genes and Genomes

The phylum Acidobacteria is one of the most widespread and abundant on the planet, yet remarkably our knowledge of the role of these diverse organisms in the functioning of terrestrial ecosystems remains surprisingly rudimentary. This blatant knowledge gap stems to a large degree from the difficulties associated with the cultivation of these bacteria by classical means. Given the phylogenetic breadth of the Acidobacteria, which is similar to the metabolically diverse Proteobacteria, it is clear that detailed and functional descriptions of acidobacterial assemblages are necessary. Fortunately, recent advances are providing a glimpse into the ecology of members of the phylum Acidobacteria. These include novel cultivation and enrichment strategies, genomic characterization and analyses of metagenomic DNA from environmental samples. Here, we couple the data from these complementary approaches for a better understanding of their role in the environment, thereby providing some initial insights into the ecology of this important phylum. All cultured acidobacterial type species are heterotrophic, and members of subdivisions 1, 3, and 4 appear to be more versatile in carbohydrate utilization. Genomic and metagenomic data predict a number of ecologically relevant capabilities for some acidobacteria, including the ability to: use of nitrite as N source, respond to soil macro-, micro nutrients and soil acidity, express multiple active transporters, degrade gellan gum and produce exopolysaccharide (EPS). Although these predicted properties allude to a competitive life style in soil, only very few of these prediction shave been confirmed via physiological studies. The increased availability of genomic and physiological information, coupled to distribution data in field surveys and experiments, should direct future progress in unraveling the ecology of this important but still enigmatic phylum.

ATP-binding Cassette (ABC) Transport System Solute-binding Protein-guided Identification of Novel d-Altritol and Galactitol Catabolic Pathways in Agrobacterium tumefaciens C58

Innovations in the discovery of the functions of uncharacterized proteins/enzymes have become increasingly important as advances in sequencing technology flood protein databases with an exponentially growing number of open reading frames. This study documents one such innovation developed by the Enzyme Function Initiative (EFI; U54GM093342), the use of solute-binding proteins for transport systems to identify novel metabolic pathways. In a previous study, this strategy was applied to the tripartite ATP-independent periplasmic transporters. Here, we apply this strategy to the ATP-binding cassette transporters and report the discovery of novel catabolic pathways for d-altritol and galactitol in Agrobacterium tumefaciens C58. These efforts resulted in the description of three novel enzymatic reactions as follows: 1) oxidation of d-altritol to d-tagatose via a dehydrogenase in Pfam family PF00107, a previously unknown reaction; 2) phosphorylation of d-tagatose to d-tagatose 6-phosphate via a kinase in Pfam family PF00294, a previously orphan EC number; and 3) epimerization of d-tagatose 6-phosphate C-4 to d-fructose 6-phosphate via a member of Pfam family PF08013, another previously unknown reaction. The epimerization reaction catalyzed by a member of PF08013 is especially noteworthy, because the functions of members of PF08013 have been unknown. These discoveries were assisted by the following two synergistic bioinformatics web tools made available by the Enzyme Function Initiative: the EFI-Enzyme Similarity Tool and the EFI-Genome Neighborhood Tool.

Facile Enzymatic Synthesis of Ketoses

Studies of rare ketoses have been hampered by a lack of efficient preparation methods. A convenient, efficient, and cost-effective platform for the facile synthesis of ketoses is described. This method enables the preparation of difficult-to-access ketopentoses and ketohexoses from common and inexpensive starting materials with high yield and purity and without the need for a tedious isomer separation step.