Efficient biosynthesis of d-allose from d-psicose by cross-linked recombinant l-rhamnose isomerase: Separation of product by ethanol crystallization

Enzyme cascades for high-yield conversion of d-xylose into d-ribose by overcoming equilibrium constraints and enhancing selectivity

d-Ribose is essential for critical cellular functions and the synthesis of antiviral nucleosides. However, traditional chemical synthesis and fermentation methods of d-ribose production suffer from low yields and inefficient resource utilization. Here, we present a highly efficient enzymatic cascade strategy that utilizes selective phosphorylation and dephosphorylation processes, coupled with ATP regeneration to convert d-xylose into d-ribose with high yield. By optimizing this enzyme cascade, we achieved a substantial increase in d-ribose yield from 23.4 % to 93.5 % mol/mol, effectively overcoming the equilibrium limitations of sugar conversion processes. Notably, our approach allows for the selective conversion of d-xylose to d-ribose in lignocellulosic hydrolysates, even in the presence of d-glucose. This work demonstrates the highly efficient enzymatic conversion of d-xylose into d-ribose offering a competitive alternative to existing chemical synthesis methods. Our findings provide a novel approach to cellulosic biomass valorization and represent a significant contribution to the field of biorefinery.

Enhanced Synthesis of Rare d-Allose from d-Glucose by Positively Pulling and Forcing Reversible Epimerization in Engineered Escherichia coli

d-Allose has great potential for application in the food and pharmaceutical industries owing to its remarkable physiological properties. Most studies on d-allose production have primarily focused on enzyme catalysis using the Izumoring strategy, which typically requires the use of expensive d-allulose as a substrate. Herein, a metabolically engineered strain of Escherichia coli was developed to synthesize d-allose directly from inexpensive d-glucose. The synthesis pathway was systematically optimized through a modular metabolic engineering. The functionality of the isomerases involved in the conversion of d-allulose to d-allose was confirmed in vivo, while the byproduct and transporter pathways were blocked to positively pull the reversible epimerization. Gene knockouts were employed to weaken glycolytic pathways, redirecting the carbon flux toward product synthesis. Additionally, the nonphosphorylated transport of d-glucose was introduced to enhance substrate utilization. In fed-batch fermentation, the engineered strain achieved a d-allose titer of 4.17 g/L, with a yield of 0.103 g/g from d-glucose. Our achievements are expected to advance the industrial production of d-allose, and this strategy is also applicable for producing other rare sugars.

Unleashing the innate ability of Escherichia coli to produce D-Allose

D-allose is a rare monosaccharide, found naturally in low abundances. Due to its low-calorie profile and similar taste to sucrose, D-allose has the potential to become an ideal sugar substitute. D-allose also displays unique properties and health benefits that can be applied to various fields, including food and medicine. D-allose can be produced using two enzymatic steps in vitro: the epimerization of D-fructose, then the isomerization of the resulting D-psicose. This method suffers from poor yield due to the reversible nature of both reactions. We found that Escherichia coli possesses all of the required enzymes to convert D-glucose to D-allose with a thermodynamically favorable pathway, through a series of phosphorylation-epimerization-isomerization-dephosphorylation steps. To increase carbon flux toward D-allose production, the pathway genes were additionally expressed, and the competing pathways were removed. The engineered strains achieved production of D-allose, at a titer of 56.4 g L-1, a productivity of 0.65 g L-1 hr-1, and a yield of 41.4% under test tube conditions.

Transforming monosaccharides: Recent advances in rare sugar production and future exploration

Rare sugars are defined as monosaccharides and their derivatives that do not exist in nature at all or that exist in extremely limited amounts despite being theoretically possible. At present, no comprehensive dogma has been presented regarding how and why these rare sugars have deviated from the naturally selected monosaccharides. In this minireview, we adopt a hypothesis on the origin and evolution of elementary hexoses, previously presented by one of the authors (Hirabayashi, Q Rev Biol, 1996, 71:365-380). In this scenario, monosaccharides, which constitute various kinds of glycans in nature, are assumed to have been generated by formose reactions on the prebiotic Earth (chemical evolution era). Among them, the most stable hexoses, i.e., fructose, glucose, and mannose remained accumulated. After the birth of life, the "chemical origin" saccharides thus survived were transformed into a variety of "bricolage products", which include galactose and other recognition saccharides like fucose and sialic acid through the invention of diverse metabolic pathways (biological evolution era). The remaining monosaccharides that have deviated from this scenario are considered rare sugars. If we can produce rare sugars as we wish, it is expected that various more useful biomaterials will be created by using them as raw materials. Thanks to the pioneering research of the Izumori group in the 1990's, and to a few other investigations by other groups, almost all monosaccharides including l-sugars can now be produced by combining both chemical and enzymatic approaches. After briefly giving an overview of the origin of elementary hexoses and the current state of the rare sugar production, we will look ahead to the next generation of monosaccharide research which also targets glycosides including disaccharides.

Regioselective Copolymerization of Glucose-Derived Allopyranoside Epoxide with Cyclic Anhydrides: Developing Precise Sugar-Functionalized Polyesters with Unique Altrose Linkages

Uniform sugar-functionalized polyesters combine the benefits of sugar's structural diversity, biocompatibility, and biodegradability with precise postfunctionalization capabilities, making them a highly valuable class of materials with extensive application potential. However, the irregular placement of hydroxyl groups has limited the synthesis of these polyesters. Here, we present the first platform for uniform sugar-functionalized polyesters via regioselective ring-opening copolymerizations (ROCOPs) of allopyranoside anhydrosugar epoxide (1, derived from d-glucose) with cyclic anhydrides, followed by complete selective deprotection. This method yields polyesters with controlled molecular weights, narrow molecular weight distributions (D̵ < 1.19), high glass transition temperatures (up to 188 °C), and uniform hydroxyl functionality. Furthermore, the degradation of the polyesters offers an efficient route for producing the highly valuable d-altrose. Mechanistic insights, supported by DFT calculations, as well as NMR and HPLC analyses, confirm the regioselective nucleophilic attack at the C2 position of the pyranose ring. Kinetic studies reveal a first-order dependence on 1 and a zero-order dependence on the cyclic anhydrides. Additionally, these uniform sugar-functionalized polyesters can be incorporated into triblock terpolymers through one-pot/one-step or one-pot/two-step procedures, forming uniform sugar-functionalized multiblock copolymers.

Characterization of a novel ribose-5-phosphate isomerase B from Curtobacterium flaccumfaciens ZXL1 for D-allose production

Enzymatic preparation of rare sugars as an alternative to traditional sweeteners is an effective strategy to achieve a low-calorie healthy diet. Ribose-5-phosphate isomerase B (RpiB) is a key enzyme in the non-oxidative branch of the catalytic pentose phosphate pathway. Here, we investigated the potential of Curtobacterium flaccumfaciens ZXL1 (C. flaccumfaciens ZXL1) derived RpiB (CfRpiB) in D-allose preparation. The optimal reaction conditions for recombinant CfRpiB were found experimentally to be pH 7.0, 55 °C, and no metal ions. The kinetic parameters Km, kcat, and catalytic efficiency kcat/Km were 320 mM, 4769 s-1, and 14.9 mM-1 s-1 respectively. The conversion of D-allulose by purified enzyme (1 g L-1 ) to D-allose was 13% within 1 h. In addition, homology modeling and molecular docking were used to predict the active site residues: Asp13, Asp14, Cys72, Gly73, Thr74, Gly77, Asn106, and Lys144.

D-allose, a typical rare sugar: properties, applications, and biosynthetic advances and challenges

D-allose, a C-3 epimer of D-glucose and an aldose-ketose isomer of D-allulose, exhibits 80% of sucrose's sweetness while being remarkably low in calories and nontoxic, making it an appealing sucrose substitute. Its diverse physiological functions, particularly potent anticancer and antitumor effects, render it a promising candidate for clinical treatment, garnering sustained attention. However, its limited availability in natural sources and the challenges associated with chemical synthesis necessitate exploring biosynthetic strategies to enhance production. This overview encapsulates recent advancements in D-allose's physicochemical properties, physiological functions, applications, and biosynthesis. It also briefly discusses the crucial role of understanding aldoketose isomerase structure and optimizing its performance in D-allose synthesis. Furthermore, it delves into the challenges and future perspectives in D-allose bioproduction. Early efforts focused on identifying and characterizing enzymes responsible for D-allose production, followed by detailed crystal structure analysis to improve performance through molecular modification. Strategies such as enzyme immobilization and implementing multi-enzyme cascade reactions, utilizing more cost-effective feedstocks, were explored. Despite progress, challenges remain, including the lack of efficient high-throughput screening methods for enzyme modification, the need for food-grade expression systems, the establishment of ordered substrate channels in multi-enzyme cascade reactions, and the development of downstream separation and purification processes.

Partial replacement of d-glucose with d-allose ameliorates peritoneal injury and hyperglycaemia induced by peritoneal dialysis fluid in rats

BACKGROUND

Peritoneal dialysis (PD) is a crucial dialysis method for treating end-stage kidney disease. However, its use is restricted due to high glucose-induced peritoneal injury and hyperglycaemia, particularly in patients with diabetes mellitus. In this study, we investigated whether partially replacing d-glucose with the rare sugar d-allose could ameliorate peritoneal injury and hyperglycaemia induced by peritoneal dialysis fluid (PDF).

METHODS

Rat peritoneal mesothelial cells (RPMCs) were exposed to a medium containing d-glucose or d-glucose partially replaced with different concentrations of d-allose. Cell viability, oxidative stress and cytokine production were evaluated. Sprague-Dawley (SD) rats were administrated saline, a PDF containing 4% d-glucose (PDF-G4.0%) or a PDF containing 3.6% d-glucose and 0.4% d-allose (PDF-G3.6%/A0.4%) once a day for 4 weeks. Peritoneal injury and PD efficiency were assessed using immuno-histological staining and peritoneal equilibration test, respectively. Blood glucose levels were measured over 120 min following a single injection of saline or PDFs to 24-h fasted SD rats.

RESULTS

In RPMCs, the partial replacement of d-glucose with d-allose increased cell viability and decreased oxidative stress and cytokine production compared to d-glucose alone. Despite the PDF-G3.6%/A0.4% having a lower d-glucose concentration compared to PDF-G4.0%, there were no significant changes in osmolality. When administered to SD rats, the PDF-G3.6%/A0.4% suppressed the elevation of peritoneal thickness and blood d-glucose levels induced by PDF-G4.0%, without impacting PD efficiency.

CONCLUSIONS

Partial replacement of d-glucose with d-allose ameliorated peritoneal injury and hyperglycaemia induced by high concentration of d-glucose in PDF, indicating that d-allose could be a potential treatment option in PD.

A comprehensive review of recent advances in the characterization of L-rhamnose isomerase for the biocatalytic production of D-allose from D-allulose

D-Allose and D-allulose are two important rare natural monosaccharides found in meager amounts. They are considered to be the ideal substitutes for table sugar (sucrose) for, their significantly lower calorie content with around 80 % and 70 % of the sweetness of sucrose, respectively. Additionally, both monosaccharides have gained much attention due to their remarkable physiological properties and excellent health benefits. Nevertheless, D-allose and D-allulose are rare in nature and difficult to produce by chemical methods. Consequently, scientists are exploring bioconversion methods to convert D-allulose into D-allose, with a key enzyme, L-rhamnose isomerase (L-RhIse), playing a remarkable role in this process. This review provides an in-depth analysis of the extractions, physiological functions and applications of D-allose from D-allulose. Specifically, it provides a detailed description of all documented L-RhIse, encompassing their biochemical properties including, pH, temperature, stabilities, half-lives, metal ion dependence, molecular weight, kinetic parameters, specific activities and specificities of the substrates, conversion ratio, crystal structure, catalytic mechanism as well as their wide-ranging applications across diverse fields. So far, L-RhIses have been discovered and characterized experimentally by numerous mesophilic and thermophilic bacteria. Furthermore, the crystal forms of L-RhIses from E. coli and Stutzerimonas/Pseudomonas stutzeri have been previously cracked, together with their catalytic mechanism. However, there is room for further exploration, particularly the molecular modification of L-RhIse for enhancing its catalytic performance and thermostability through the directed evolution or site-directed mutagenesis.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Immunomodulatory effects of D-allose on cytokine production by plasmacytoid dendritic cells

D-Allose is classified as a 'rare sugar,' i.e., part of the group of monosaccharides that are present in low quantities in the natural world. D-Allose has been demonstrated to exert many physiological functions. The effects of the rare sugars on immune responses are largely unexplored. Here, we investigated the physiological effects of D-allose on murine dendritic cells' cytokine production. When plasmacytoid dendritic cells (pDCs) were stimulated with a Toll-like receptor 7 (TLR7) ligand, a single-stranded RNA (ssRNA), or a TLR9 ligand, CpG DNA, in the medium containing D-allose, the productions of both interferon-alpha (IFN-α) and interleukin (IL)-12p40 were severely decreased. In contrast, a normal production of these cytokines was observed when pDCs were stimulated with other TLR7 ligands, an imidazoquinoline, or a guanosine analog. In contrast to the pDCs, conventional dendritic cells (cDCs) produced IL-12p40 and tumor necrosis factor-alpha (TNF-α) in response to an imidazoquinoline or CpG DNA even though D-allose was present in the medium. D-Allose did not induce pDC death, and not inhibit the endocytic uptake of fluorophore-labeled CpG DNA into pDCs. These results suggested that D-allose exerts its inhibitory effects after CpG DNA is internalized. We analyzed the TLR7/9 signal-induced activation of downstream signaling molecules in pDCs and observed that when pDCs were stimulated with a ssRNA or CpG DNA, the phosphorylation status of the MAPK family, which includes Erk1/2, JNK/SAPK, and p38 MAPK, was attenuated in the presence of D-allose compared to D-glucose controls. The stimulation of pDCs with an imidazoquinoline induced a strong phosphorylation of these MAPK family members even in the presence of D-allose. These findings reveal that D-allose can inhibit the cytokine production by pDCs stimulated with ssRNA or CpG DNA via an attenuation of the phosphorylation of MAPK family members.

Glucose Isomerase: Functions, Structures, and Applications

Glucose isomerase (GI, also known as xylose isomerase) reversibly isomerizes D-glucose and D-xylose to D-fructose and D-xylulose, respectively. GI plays an important role in sugar metabolism, fulfilling nutritional requirements in bacteria. In addition, GI is an important industrial enzyme for the production of high-fructose corn syrup and bioethanol. This review introduces the functions, structure, and applications of GI, in addition to presenting updated information on the characteristics of newly discovered GIs and structural information regarding the metal-binding active site of GI and its interaction with the inhibitor xylitol. This review provides an overview of recent advancements in the characterization and engineering of GI, as well as its industrial applications, and will help to guide future research in this field.

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.

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.

Characteristics of rice starch film blended with sugar (trehalose/allose) and oil (canola oil/coconut oil): Part I - Filmogenic solution behavior and mechanical properties

The concentrations effects of sugars (trehalose and allose) and oils (canola and coconut oil) on the characteristics of rice starch suspension and mechanical properties of rice starch film were studied. The samples were prepared using 3% (w/w) rice starch, with 10% or 30% (w/w) sugar (trehalose or allose) added and 10% or 30% (w/w) oil (canola or coconut). The droplet size of the film suspension increased with increasing oil concentration both in trehalose and allose, which blended with oil. The flow behavior of the film suspensions showed shear-thinning behavior as calculated by the Power Law model. The apparent viscosity tended to increase with the addition of sugar and oil. The breaking stress of the films blended with sugar and oil was less than that of control. On preparation day and after 7 days' storage, the breaking strain tended to increase more with the addition of coconut oil than with that of canola oil. However, breaking stress and breaking strain decreased after 28 days' storage. Adding sugar had correlation with mechanical properties whereas adding oil had correlation with film suspension characteristics, allowed the sugar and oil to interact and inhibited starch chain mobility due to concentration, sugar type, and oil type. PRACTICAL APPLICATION: Trehalose, allose, canola oil, and coconut oil could be used as a plasticizer in a starch edible/biodegradable film system. The preparation process of filmogenic solution was depended on the combination of sugar and oil that could change the flow behavior and affected the mechanical properties of the edible film. The sugar and oil might improve the mechanical properties of the film by a hydroxyl group of sugar and lubricating properties of the oil.

Phosphate sugar isomerases and their potential for rare sugar bioconversion

Phosphate sugar isomerases, catalyzing the isomerization between ketopentose/ketohexose phosphate and aldopentose/aldohexose phosphate, play an important role in microbial sugar metabolism. They are present in a wide range of microorganisms. They have attracted increasing research interest because of their broad substrate specificity and great potential in the enzymatic production of various rare sugars. Here, the enzymatic properties of various phosphate sugar isomerases are reviewed in terms of their substrate specificities and their applications in the production of valuable rare sugars because of their functions such as low-calorie sweeteners, bulking agents, and pharmaceutical precursor. Specifically, we focused on the industrial applications of D-ribose-5-phosphate isomerase and D-mannose-6-phosphate isomerase to produce D-allose and L-ribose, respectively.

Production of D-allose from D-fructose using immobilized L-rhamnose isomerase and D-psicose 3-epimerase

D-Allose is a rare sugar, can be used as an ingredient in a range of foods and dietary supplements, has alimentary activities, especially excellent anti-cancer effects and used in assisting cancer chemotherapy and radiotherapy, etc. To develop a simple and low-cost process for D-allose production, a one-pot enzymatic process using the substrate of D-fructose, and the recombinant enzymes of D-psicose 3-epimerase (DPE) and L-rhamnose isomerase (L-RhI) was developed. These enzymes were cloned from Ruminococcus sp. and B. subtilis, respectively, successfully expressed in E. coli, extracted and immobilized using anion exchange resin and amino resin, respectively. The mass ratio of D-fructose, D-psicose and D-allose was 6.6:2.4:1.0 when the reaction reached equilibrium after 5 h of reaction. Using the low-cost substrate of D-fructose, the reusable immobilized enzymes and the one-pot reaction, the production process is simplified and the production cost is decreased. In addition, to simplify the enzyme extraction and immobilization processes, new methods for enzyme capture and immobilization were developed especially for DPE immobilization. This is the first report for one-pot D-allose production using immobilized L-RhI and DPE.

Recent research on the physiological functions, applications, and biotechnological production of D-allose

D-Allose is a rare monosaccharide, which rarely appears in the natural environment. D-Allose has an 80% sweetness relative to table sugar but is ultra-low calorie and non-toxic and is thus an ideal candidate to take the place of table sugar in food products. It displays unique health benefits and physiological functions in various fields, including food systems, clinical treatment, and the health care fields. However, it is difficult to produce chemically. The biotechnological production of D-allose has become a research hotspot in recent years. Therefore, an overview of recent studies on the physiological functions, applications, and biotechnological production of D-allose is presented. In this review, the physiological functions of D-allose are introduced in detail. In addition, the different types of D-allose-producing enzymes are compared for their enzymatic properties and for the biotechnological production of D-allose. To date, very little information is available on the molecular modification and food-grade expression of D-allose-producing enzymes, representing a very large research space yet to be explored.

Characterization of L-rhamnose isomerase from Clostridium stercorarium and its application to the production of D-allose from D-allulose (D-psicose)

OBJECTIVE

To characterize L-rhamnose isomerase (L-RI) from the thermophilic bacterium Clostridium stercorarium and apply it to produce D-allose from D-allulose.

RESULTS

A recombinant L-RI from C. stercorarium exhibited the highest specific activity and catalytic efficiency (k _cat/K _m) for L-rhamnose among the reported L-RIs. The L-RI was applied to the high-level production of D-allose from D-allulose. The isomerization activity for D-allulose was maximal at pH 7, 75 °C, and 1 mM Mn²⁺ over 10 min reaction time. The half-lives of the L-RI at 65, 70, 75, and 80 °C were 22.8, 9.5, 1.9, and 0.2 h, respectively. To ensure full stability during 2.5 h incubation, the optimal temperature was set at 70 °C. Under the optimized conditions of pH 7, 70 °C, 1 mM Mn²⁺, 27 U L-RI l^-1, and 600 g D-allulose l^-1, L-RI from C. stercorarium produced 199 g D-allose l^-1 without by-products over 2.5 h, with a conversion yield of 33% and a productivity of 79.6 g l^-1 h^-1.

CONCLUSION

To the best of our knowledge, this is the highest concentration and productivity of D-allose reported thus far.

The anti-tumor efficacy of 2-deoxyglucose and D-allose are enhanced with p38 inhibition in pancreatic and ovarian cell lines

PURPOSE

The anti-tumor activity of glucose analogs 2-deoxy-glucose (2-DG) and D-allose was investigated alone or in combination with p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190 or platinum analogs as a strategy to pharmacologically target glycolytic tumor phenotypes.

METHODS

Hypoxia inducible factor-1 alpha (HIF-1α) protein accumulation in pancreatic cell lines treated with SB202190 alone and in combination with glucose analogs was analyzed by Western blot. HIF-1α transcriptional activity was measured in MIA PaCa-2 cells stably transfected with a hypoxia response element luciferase reporter following treatment with glucose analogs alone, and in combination with SB202190. Induction of cleaved poly(ADP-ribose) polymerase (PARP) was measured by Western blot in the MIA PaCa-2 cells. In vitro anti-proliferative activity of 2-DG and D-allose alone, or in combination with oxaliplatin (pancreatic cell lines), cisplatin (ovarian cell lines), or with SB202190 were investigated using the MTT assay.

RESULTS

SB202190 decreased HIF-1α protein accumulation and transcriptional activity. 2-DG demonstrated greater anti-proliferative activity than D-allose. Pre-treatment with SB202190 enhanced activity of both 2-DG and D-allose in MIA PaCa-2, BxPC-3, ASPC-1, and SK-OV-3 cells. The combination of D-allose and platinum agents was additive to moderately synergistic in all but the OVCAR-3 and HEY cells. SB202190 pre-treatment further enhanced activity of D-allose and 2-DG with platinum agents in most cell lines investigated.

CONCLUSIONS

SB202190 induced sensitization of tumor cells to 2-DG and D-allose may be partially mediated by inhibition of HIF-1α activity. Combining glucose analogs and p38 MAPK inhibitors with chemotherapy may be an effective approach to target glycolytic tumor phenotypes.

Crystal structure of β-d,l-allose

The title compound, C6H12O6, a C-3 position epimer of glucose, was crystallized from an equimolar mixture of d- and l-allose. It was confirmed that d-allose (l-allose) formed β-pyran-ose with a (4) C 1 ((1) C 4) conformation in the crystal. In the crystal, molecules are linked by O-H⋯O hydrogen bond, forming a three-dimensional framework. The cell volume of the racemic β-d,l-allose is 739.36 (3) Å(3), which is about 10 Å(3) smaller than that of chiral β-d-allose [V = 751.0 (2) Å(3)].

Artificial sweeteners - a review

Now a days sugar free food are very much popular because of their less calorie content. So food industry uses various artificial sweeteners which are low in calorie content instead of high calorie sugar. U.S. Food and Drug Administration has approved aspartame, acesulfame-k, neotame, cyclamate and alitame for use as per acceptable daily intake (ADI) value. But till date, breakdown products of these sweeteners have controversial health and metabolic effects. On the other hand, rare sugars are monosaccharides and have no known health effects because it does not metabolize in our body, but shows same sweet taste and bulk property as sugar. Rare sugars have no such ADI value and are mainly produced by using bioreactor and so inspite of high demand, rare sugars cannot be produced in the desired quantities.

Biosynthesis of rare hexoses using microorganisms and related enzymes

Rare sugars, referred to as monosaccharides and their derivatives that rarely exist in nature, can be applied in many areas ranging from foodstuffs to pharmaceutical and nutrition industry, or as starting materials for various natural products and drug candidates. Unfortunately, an important factor restricting the utilization of rare sugars is their limited availability, resulting from limited synthetic methods. Nowadays, microbial and enzymatic transformations have become a very powerful tool in this field. This article reviews the biosynthesis and enzymatic production of rare ketohexoses, aldohexoses and sugar alcohols (hexitols), including D-tagatose, D-psicose, D-sorbose, L-tagatose, L-fructose, 1-deoxy-L-fructose, D-allose, L-glucose, L-talose, D-gulose, L-galactose, L-fucose, allitol, D-talitol, and L-sorbitol. New systems and robust catalysts resulting from advancements in genomics and bioengineering are also discussed.

Characterization of a recombinant L-rhamnose isomerase from Dictyoglomus turgidum and its application for L-rhamnulose production

A putative recombinant enzyme from Dictyoglomus turgidum was characterized and immobilized on Duolite A568 beads. The native enzyme was a 46 kDa tetramer. Its activity was highest for L-rhamnose, indicating that it is an L-rhamnose isomerase. The maximum activities of both the free and immobilized enzymes for L-rhamnose isomerization were at pH 8.0 and 75 °C in the presence of Mn(2+). Under these conditions, the half-lives of the free and immobilized enzymes were 28 and 112 h, respectively. In a packed-bed bioreactor, the immobilized enzyme produced an average of 130 g L-rhamnulose l(-1) from 300 g L-rhamnose l(-1) after 240 h at pH 8.0, 70 °C, and 0.6 h(-1), with a productivity of 78 g l(-1) h(-1) and a conversion yield of 43 %. To the best of our knowledge, this is the first report describing the enzymatic production of L-rhamnulose.

Quantitative analyses of individual sugars in mixture using FRET-based biosensors

Molecular biosensors were developed and applied to measure individual sugars in biological mixtures such as bacterial culture broths. As the sensing units, four sugar-binding proteins (SBPs for allose, arabinose, ribose, and glucose) were selected from the Escherichia coli genome and connected to a cyan fluorescent protein and yellow fluorescent protein via dipeptide linkers (CFP-L-SBP-YFP). The putative sensors were randomized in the linker region (L) and then investigated with regard to the intensity of fluorescence resonance energy transfer on the binding of the respective sugars. As a result, four representatives were selected from each library and examined for their specificity using 16 available sugars. The apparent dissociation constants of the allose, arabinose, ribose, and glucose sensors were estimated to be 0.35, 0.36, 0.17, and 0.18 μM. Finally, the sugar sensors were applied to monitor the consumption rate of individual sugars in an E. coli culture broth. The individual sugar profiles exhibited a good correlation with those obtained using an HPLC method, confirming that the biosensors offer a rapid and easy-to-use method for monitoring individual sugars in mixed compositions.

Characterization of a thermophilic L-rhamnose isomerase from Caldicellulosiruptor saccharolyticus ATCC 43494

L-Rhamnose isomerase (EC 5.3.1.14, l-RhI) catalyzes the reversible aldose-ketose isomerization between L-rhamnose and L-rhamnulose. In this study, the L-rhi gene encoding L-RhI was PCR-cloned from Caldicellulosiruptor saccharolyticus ATCC 43494 and then expressed in Escherichia coli. A high yield of active L-RhI, 3010 U/g of wet cells, was obtained after 20 °C induction for 20 h. The enzyme was purified sequentially using heat treatment, nucleic acid precipitation, and ion-exchange chromatography. The purified L-RhI showed an apparent optimal pH of 7 and an optimal temperature at 90 °C. The enzyme was stable at pH values ranging from 4 to 11 and retained >90% activity after a 6 h incubation at 80 °C and pH 7-8. Compared with other previously characterized L-RhIs, the L-RhI from C. saccharolyticus ATCC 43494 has a good thermostability, the widest pH-stable range, and the highest catalytic efficiencies (k(cat)/K(M)) against L-rhamnose, L-lyxose, L-mannose, D-allose, and D-ribose, suggesting that this enzyme has the potential to be applied in rare sugar production.

Increased D-allose production by the R132E mutant of ribose-5-phosphate isomerase from Clostridium thermocellum

Ribose-5-phosphate isomerase from Clostridium thermocellum converted D-psicose to D-allose, which may be useful as a pharmaceutical compound, with no by-product. The 12 active-site residues, which were obtained by molecular modeling on the basis of the solved three-dimensional structure of the enzyme, were substituted individually with Ala. Among the 12 Ala-substituted mutants, only the R132A mutant exhibited an increase in D-psicose isomerization activity. The R132E mutant showed the highest activity when the residue at position 132 was substituted with Ala, Gln, Ile, Lys, Glu, or Asp. The maximal activity of the wild-type and R132E mutant enzymes for D-psicose was observed at pH 7.5 and 80°C. The half-lives of the wild-type enzyme at 60°C, 65°C, 70°C, 75°C, and 80°C were 11, 7.0, 4.2, 1.5, and 0.6 h, respectively, whereas those of the R132E mutant enzymes were 13, 8.2, 5.1, 3.1, and 0.9 h, respectively. The specific activity and catalytic efficiency (k(cat)/K(m)) of the R132E mutant for D-psicose were 1.4- and 1.5-fold higher than those of the wild-type enzyme, respectively. When the same amount of enzyme was used, the conversion yield of D-psicose to D-allose was 32% for the R132E mutant enzyme and 25% for the wild-type enzyme after 80 min.

Characterization of a thermophilic L-rhamnose isomerase from Thermoanaerobacterium saccharolyticum NTOU1

L-rhamnose isomerase (EC 5.3.1.14, L-RhI) catalyzes the reversible aldose-ketose isomerization between L-rhamnose and L-rhamnulose. In this study, the L-Rhi gene encoding L-Rhi was PCR-cloned from Thermoanaerobacterium saccharolyticum NTOU1 and then expressed in Escherichia coli. A high yield of the active L-RhI, 9780 U/g of wet cells, was obtained in the presence of 0.2 mM IPTG induction. L-RhI was purified sequentially using heat treatment, nucleic acid precipitation, and anion-exchange chromatography. The purified L-RhI showed an apparent optimal pH of 7 and an optimal temperature at 75 °C. The enzyme was stable at pH values ranging from 5 to 9, and the activity was fully retained after a 2 h incubation at 40-70 °C. L-RhI from T. saccharolyticum NTOU1 is the most thermostable L-RhI to date, and it has a high specific activity (163 U/mg) and an acceptable purity after heat treatment, suggesting that this enzyme has the potential to be used in rare sugar production.

Features and applications of microbial sugar epimerases

Sugar (carbohydrate) epimerases catalyze the reversible conversion of a sugar epimer into its counterpart form. More than 20 types of sugar epimerase have been reported to date, and their biological properties, catalytic mechanisms, and 3D structures are very diverse among them. Recently, microbial sugar epimerases have been characterized in detail. This review surveys the catalytic aspects of microbial epimerases, which are relevant for production of bioactive mono- and oligosaccharides.

D-ribose-5-phosphate isomerase B from Escherichia coli is also a functional D-allose-6-phosphate isomerase, while the Mycobacterium tuberculosis enzyme is not

Interconversion of D-ribose-5-phosphate (R5P) and D-ribulose-5-phosphate is an important step in the pentose phosphate pathway. Two unrelated enzymes with R5P isomerase activity were first identified in Escherichia coli, RpiA and RpiB. In this organism, the essential 5-carbon sugars were thought to be processed by RpiA, while the primary role of RpiB was suggested to instead be interconversion of the rare 6-carbon sugars D-allose-6-phosphate (All6P) and D-allulose-6-phosphate. In Mycobacterium tuberculosis, where only an RpiB is found, the 5-carbon sugars are believed to be the enzyme's primary substrates. Here, we present kinetic studies examining the All6P isomerase activity of the RpiBs from these two organisms and show that only the E. coli enzyme can catalyze the reaction efficiently. All6P instead acts as an inhibitor of the M. tuberculosis enzyme in its action on R5P. X-ray studies of the M. tuberculosis enzyme co-crystallized with All6P and 5-deoxy-5-phospho-D-ribonohydroxamate (an inhibitor designed to mimic the 6-carbon sugar) and comparison with the E. coli enzyme's structure allowed us to identify differences in the active sites that explain the kinetic results. Two other structures, that of a mutant E. coli RpiB in which histidine 99 was changed to asparagine and that of wild-type M. tuberculosis enzyme, both co-crystallized with the substrate ribose-5-phosphate, shed additional light on the reaction mechanism of RpiBs generally.