Microbial metabolism and biotechnological production of d-allose

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.

A novel thermotolerant L-rhamnose isomerase variant for biocatalytic conversion of D-allulose to D-allose

A novel L-rhamnose isomerase was identified and cloned from an extreme-temperature aquatic habitat metagenome. The deduced amino acid sequence homology suggested the possible source of this metagenomic sequence to be Chloroflexus islandicus. The gene expression was performed in a heterologous host, Escherichia coli, and the recombinant protein L-rhamnose isomerase (L-RIM) was extracted and purified. The catalytic function of L-RIM was characterized for D-allulose to D-allose bioconversion. D-Allose is a sweet, rare sugar molecule with anti-tumour, anti-hypertensive, cryoprotective, and antioxidative properties. The characterization experiments showed L-RIM to be a Co++- or Mn++-dependent metalloenzyme. L-RIM was remarkably active (~ 80%) in a broad spectrum of pH (6.0 to 9.0) and temperature (70 to 80 °C) ranges. Optimal L-RIM activity with D-allulose as the substrate occurred at pH 7.0 and 75 °C. The enzyme was found to be excessively heat stable, displaying a half-life of about 12 days and 5 days at 65 °C and 70 °C, respectively. L-RIM catalysis conducted at slightly acidic pH of 6.0 and 70 °C achieved biosynthesis of about 30 g L-1 from 100 g L-1 D-allulose in 3 h. KEY POINTS: • The present study explored an extreme temperature metagenome to identify a novel gene that encodes a thermostable l-rhamnose isomerase (L-RIM) • L-RIM exhibits substantial (80% or more) activity in a broad spectrum of pH (6.0 to 9.0) and temperature (70 to 80 °C) ranges • L-RIM is excessively heat stable, displaying a half-life of about 12 days and 5 days at 65 °C and 70 °C, respectively.

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.

Calorie restriction mimetic drugs could favorably influence gut microbiota leading to lifespan extension

Calorie restriction (CR) can prolong human lifespan, but enforcing long-term CR is difficult. Thus, a drug that reproduces the effects of CR without CR is required. More than 10 drugs have been listed as CR mimetics (CRM), and some of which are conventionally categorized as upstream-type CRMs showing glycolytic inhibition, whereas the others are categorized as downstream-type CRMs that regulate or genetically modulate intracellular signaling proteins. Intriguingly, recent reports have revealed the beneficial effects of CRMs on the body such as improving the host body condition via intestinal bacteria and their metabolites. This beneficial effect of gut microbiota may lead to lifespan extension. Thus, CRMs may have a dual effect on longevity. However, no reports have collectively discussed them as CRMs; hence, our knowledge about CRM and its physiological effects on the host remains fragmentary. This study is the first to present and collectively discuss the accumulative evidence of CRMs improving the gut environments for healthy lifespan extension, after enumerating the latest scientific findings related to the gut microbiome and CR. The conclusion drawn from this discussion is that CRM may partially extend the lifespan through its effect on the gut microbiota. CRMs increase beneficial bacteria abundance by decreasing harmful bacteria rather than increasing the diversity of the microbiome. Thus, the effect of CRMs on the gut could be different from that of conventional prebiotics and seemed similar to that of next-generation prebiotics.

Enhanced Thermostability of an l-Rhamnose Isomerase for d-Allose Synthesis by Computation-Based Rational Redesign of Flexible Regions

d-Allose is a low-calorie rare sugar with great application potential in the food and pharmaceutical industries. The production of d-allose has been accomplished using l-rhamnose isomerase (L-RI), but concomitantly increasing the enzyme's stability and activity remains challenging. Here, we rationally engineered an L-RI from Clostridium stercorarium to enhance its stability by comprehensive computation-aided redesign of its flexible regions, which were successively identified using molecular dynamics simulations. The resulting combinatorial mutant M2-4 exhibited a 5.7-fold increased half-life at 75 °C while also exhibiting improved catalytic efficiency. Especially, by combining structure modeling and multiple sequence alignment, we identified an α0 region that was universal in the L-RI family and likely acted as a "helix-breaker". Truncating this region is crucial for improving the thermostability of related enzymes. Our work provides a significantly stable biocatalyst with potential for the industrial production of d-allose.

A study of targeted mutation of l-rhamnose isomerase to improve the conversion efficiency of D-allose

D-Allose is a rare cis-caprose with a wide range of physiological functions, which has a wide range of applications in medicine, food, and other industries. L-Rhamnose isomerase (L-Rhi) is the earliest enzyme found to catalyze the production of D-allose from D-psicose. This catalyst has a high conversion rate, but its specificity for substrates is limited; thus, it cannot fulfill the requirements of industrial production of D-allose. In this study, L-Rhi derived from Bacillus subtilis was employed as the research subject, and D-psicose as the conversion substrate. Two mutant libraries were constructed through alanine scanning, saturation mutation, and rational design based on the analysis of the secondary structure, tertiary structure, and interactions with ligands of the enzyme. The yield of D-allose produced by these mutants was assessed; it was found that the conversion rate of mutant D325M to D-allose was increased by 55.73 %, and the D325S improved by 15.34 %, while mutant W184H increased by 10.37 % at 55 °C, respectively. According to modeling analysis, manganese (Mn²⁺) had no significant effect on the production of D-psicose from D-psicose by L-Rhi. The results of molecular dynamics simulation demonstrated that the mutants W184H, D325M, and D325S had more stable protein structures while binding with the substrate D-psicose, as evidenced by its root mean square deviation (RMSD), root mean square fluctuation (RMSF), and binding free energy values. It was more conducive to binding D-psicose and facilitating its conversion to D-allose, providing the basis for the production of D-allose.

Horizontal Gene Transfer and Loss of Serotype-Specific Genes in Listeria monocytogenes Can Lead to Incorrect Serotype Designations with a Commonly-Employed Molecular Serotyping Scheme

Listeria monocytogenes is a Gram-positive, facultative intracellular foodborne pathogen capable of causing severe, invasive illness (listeriosis). Three serotypes, 1/2a, 1/2b, and 4b, are leading contributors to human listeriosis, with 4b including the major hypervirulent clones. The multiplex PCR scheme developed by Doumith and collaborators employs primers targeting specific lineages (e.g., lineage II-specific lmo0737, lineage I-specific LMOf2365_2059) or serotypes (e.g., serotype 4b-specific LMOf2365_1900). The Doumith scheme (DS) is extensively employed for molecular serotyping of L. monocytogenes due to its high accuracy, relative ease, and affordability. However, for certain strains, the DS serotype designations are in conflict with those relying on antibody-based schemes or whole-genome sequence (WGS) analysis. In the current study, all 27 tested serotype 4b strains with sequence type 782 (ST782) within the hypervirulent clonal complex 2 (CC2) were designated 1/2b/3b using the DS. These strains lacked the serotype 4b-specific gene LMOf2365_1900, while retaining LMOf2365_2059, which, together with prs, yields the DS 1/2b/3b profile. Furthermore, 15 serotype 1/2a strains of four STs, mostly from water, were designated 1/2b/3b using the DS. These strains lacked the lmo0737 cassette but harbored genomic islands with LMOf2365_2059, thus yielding the DS 1/2b/3b profile. Lastly, we investigated a novel, dual 1/2a-1/2b profile obtained using the DS with 21 serotype 1/2a strains of four STs harboring both the lmo0737 cassette and genomic islands with LMOf2365_2059. The findings suggest that for certain strains and clones of L. monocytogenes the DS designations should be viewed with caution and complemented with alternative tools, e.g., traditional serotyping or WGS analysis. IMPORTANCE Listeria monocytogenes is a foodborne pathogen responsible for severe illness (listeriosis), especially in pregnant women and their fetuses, immunocompromised individuals, and the elderly. Three serotypes, 1/2a, 1/2b, and 4b, account for most human listeriosis, with certain serotype 4b clonal complexes (CCs) overrepresented in human disease. Serotyping remains extensively employed in Listeria epidemiologic investigations, and a multiplex PCR-based serotyping scheme is widely used. However, the PCR gene targets can be lost or gained via horizontal gene transfer, leading to novel PCR profiles without known serotype designations or to incorrect serotype assignments. Thus, an entire serotype 4b clone of the hypervirulent CC2 would be misidentified as serotype 1/2b, and several strains of serotype 1/2a would be identified as serotype 1/2b. Such challenges are especially common in novel clones from underexplored habitats, e.g., wildlife and surface water. The findings suggest caution in application of molecular serotyping, while highlighting Listeria's diversity and potential for horizontal gene transfer.

d-allose protects brain microvascular endothelial cells from hypoxic/reoxygenated injury by inhibiting endoplasmic reticulum stress

Ischemic stroke is an acute brain disease with a high mortality rate. Currently, the only effective method is to restore the blood supply. But the inflammation and oxidative stress induced by this approach can damage the integrity of the endothelial system, which hampers the patient's outcome. d-allose has the biological activity to protect against ischemia-reperfusion injury, however, the underlying mechanism remains unclear. Here, brain microvascular endothelial cells (RBMECs) were used as the study material to establish an IR-injury model. Cell viability of RBMECs was suppressed after hypoxia/reoxygenation (H/R) treatment and significantly increased after d-allose supplementation. RNAseq results showed 180 differentially expressed genes (DEGs) between the therapy group (H/R + Dal) and the model group (H/R), of which 151 DEGs were restored to control levels by d-allose. Enrichment analysis revealed that DEGs were mainly involved in protein processing in endoplasmic reticulum. 6 DEGs in the unfolded protein response (UPR) pathway were verified by qRT-PCR. All of them were significantly down-regulated by d-allose, indicating that endoplasmic reticulum stress (ERS) was relieved. In addition, d-allose significantly inhibited the phosphorylation level of eIF2α, a marker of ERS. The downstream molecules of Phosphorylation of eIF2α, Gadd45a and Chac1, which trigger cycle arrest and apoptosis, respectively, were also significantly inhibited by d-allose. Thus, we conclude that d-allose inhibits the UPR pathway, attenuates eIF2α phosphorylation and ERS, restores the cell cycle, inhibits apoptosis, and thus enhances endothelial cell tolerance to H/R injury.

Engineering of Escherichia coli for D-allose fermentative synthesis from D-glucose through izumoring cascade epimerization

D-Allose is a potential alternative to sucrose in the food industries and a useful additive for the healthcare products in the future. At present, the methods for large-scale production of D-allose are still under investigation, most of which are based on in vitro enzyme-catalyzed Izumoring epimerization. In contrast, fermentative synthesis of D-allose has never been reported, probably due to the absence of available natural microorganisms. In this work, we co-expressed D-galactose: H+ symporter (GalP), D-glucose isomerase (DGI), D-allulose 3-epimerase (DAE), and ribose-5-phosphate isomerase (RPI) in Escherichia coli, thereby constructing an in vivo Izumoring pathway for yielding D-allose from D-glucose. The carbon fluxes and carbon catabolite repression (CCR) were rationally regulated by knockout of FruA, PtsG, Glk, Mak, PfkA, and PfkB involved in the pathways capable of phosphorylating D-fructose, D-glucose, and fructose-6-phosphate. Moreover, the native D-allose transporter was damaged by inactivation of AlsB, thus driving the reversible Izumoring reactions towards the target product. Fermentation was performed in the M9 medium supplemented with glycerol as a carbon source and D-glucose as a substrate. The results show that the engineered E. coli cell factory was able to produce approximately 127.35 mg/L of D-allose after 84 h. Our achievements in the fermentative production of D-allose in this work may further promote the green manufacturing of rare sugars.

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.

Production of D-Allose From D-Allulose Using Commercial Immobilized Glucose Isomerase

Rare sugars are regarded as functional biological materials due to their potential applications as low-calorie sweeteners, antioxidants, nucleoside analogs, and immunosuppressants. D-Allose is a rare sugar that has attracted substantial attention in recent years, owing to its pharmaceutical activities, but it is still not widely available. To address this limitation, we continuously produced D-allose from D-allulose using a packed bed reactor with commercial glucose isomerase (Sweetzyme IT). The optimal conditions for D-allose production were determined to be pH 8.0 and 60°C, with 500 g/L D-allulose as a substrate at a dilution rate of 0.24/h. Using these optimum conditions, the commercial glucose isomerase produced an average of 150 g/L D-allose over 20 days, with a productivity of 36 g/L/h and a conversion yield of 30%. This is the first report of the successful continuous production of D-allose from D-allulose by commercial glucose isomerase using a packed bed reactor, which can potentially provide a continuous production system for industrial applications of D-allose.

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 tagatose and talose through isomerization of galactose in a buffer solution under subcritical water conditions

Galactose was isomerized in pure water or in 10 mmol/L sodium phosphate buffer at 160 °C under pressurized conditions. The isomerization of galactose to tagatose and talose in phosphate buffer resulted in 14% and 1.4% yields, respectively, which were significantly higher than those obtained in subcritical pure water (0.6% and <0.1%, respectively). The effect of the temperature on isomerization was examined between 100 and 160 °C. The most remarkable isomerization was observed at 120 °C or higher. The effect of the buffer solution type was also examined. The pH drop of the treated solution was lesser in MOPS and PIPES buffers than in the phosphate buffer; however, the isomerization was less likely to occur in MOPS and PIPES buffers. The relationship between the pH drop during the reaction in phosphate buffer and the yields of tagatose and talose revealed that the isomerization proceeded only when the pH was >6.3. Our results indicate that the galactose isomerization by Lobry de Bruyn-Alberda-van Ekenstein transformation rarely occurred at low pH due to the formation of organic acids.

Biochemical and structural insights into an Ochrobactrum sp. CSL1 ribose-5-phosphate isomerase A and its roles in isomerization of rare sugars

Rare sugars have received increasing attention due to their important applications as sweeteners and building blocks. The substrate specificity and catalytic properties of ribose-5-phosphate isomerase A (RpiA) in isomerization of rare sugars have not been extensively explored. In this study, an RpiA from Ochrobactrum sp. CSL1 was cloned and expressed in Escherichia coli. The biochemical and reaction features were explored and its broad substrate specificity was identified. A higher reaction rate in isomerizing l-rhamnose to l-rhamnulose by OsRpiA, compared with OsRpiB found in the same strain indicated higher efficiency in preparing rare sugars, which was verified by kinetics study. The 2.8 Å resolution structure of OsRpiA was then solved and used in subsequent molecular dynamics experiments, providing a possible explanation for its distinct substrate specificity. The present study highlighted the unique role of microbial RpiA in preparing rare sugars, and its structural information provided a reliable reference for further reaction mechanism research and enzyme engineering work.

Microbial and enzymatic strategies for the production of L-ribose

L-Ribose is a non-naturally occurring pentose that recently has become known for its potential application in the pharmaceutical industry, as it is an ideal starting material for use in synthesizing L-nucleosides analogues, an important class of antiviral drugs. In the past few decades, the synthesis of L-ribose has been mainly undertaken through the chemical route. However, chemical synthesis of L-ribose is difficult to achieve on an industrial scale. Therefore, the biotechnological production of L-ribose has gained considerable attention, as it exhibits many merits over the chemical approaches. The present review focuses on various biotechnological strategies for the production of L-ribose through microbial biotransformation and enzymatic catalysis, and in particular on an analysis and comparison of the synthetic methods and different enzymes. The physiological functions and applications of L-ribose are also elucidated. In addition, different sugar isomerases involved in the production of L-ribose from a number of sources are discussed in detail with regard to their biochemical properties. Furthermore, analysis of the separation issues of L-ribose from the reaction solution and different purification methods is presented.Key points • l -Arabinose, l -ribulose and ribitol can be used to produce l -ribose by enzymes. • Five enzymes are systematically introduced for production of l -ribose. • Microbial transformation and enzymatic methods are promising for yielding l -ribose.

Expression of d-psicose-3-epimerase from Clostridium bolteae and Dorea sp. and whole-cell production of d-psicose in Bacillus subtilis

Purpose

d-psicose-3-epimerase (DPEase) catalyses the isomerisation of d-fructose to d-psicose, a rare sugar in nature with unique nutritional and biological functions. An effective industrial-scale method is needed for d-psicose production. Herein, the expression of a neutral and a slightly acidic pH DPEase in Bacillus subtilis was evaluated.

Methods

Two DPEase genes from Clostridium bolteae and Dorea sp. were separately expressed in B. subtilis via plasmid pSTOP1622, and an extra P43 promoter was employed to the expression cassette. The fermentation conditions of the engineered B. subtilis strains were also optimised, to facilitate both cell growth and enzyme production.

Result

The introduction of P43 promoter to the two DPEase genes increased enzyme production by about 20%. Optimisation of fermentation conditions increased DPEase production to 21.90 U/g at 55 °C and 24.01 U/g at 70 °C in B. subtilis expressing C. bolteae or Dorea sp. DPEase, equating to a 94.67% and 369.94% increase, respectively, relative to controls.

Conclusion

Enhanced DPEase production was achieved in B. subtilis expressing C. bolteae or Dorea sp. DPEase genes.

Cascade synthesis of rare ketoses by whole cells based on L-rhamnulose-1-phosphate aldolase

Dihydroxyacetone phosphate (DHAP)-dependent aldolases demonstrate important values in the production of rare ketoses due to their unique stereoselectivities. As a specific example, we developed an efficient Escherichia coli whole-cell biocatalytic cascade system in which rare ketoses were produced from abundant glycerol and catalyzed by four enzymes based on L-rhamnulose-1-phosphate aldolase (RhaD). For the semicontinuous bioconversion in which D-glyceraldehyde was continuously added, once D-glyceraldehyde was consumed, the final yields of D-sorbose and D-psicose were 15.30 g/L and 6.35 g/L, respectively. Moreover, the maximum conversion rate and productivity of D-sorbose and D-psicose were 99% and 1.11 g/L/h at 8 h, respectively. When L-glyceraldehyde was used instead of the D-isomer, the final yield of L-fructose was 16.80 g/L. Furthermore, the maximum conversion rate and productivity of L-fructose were 95% and 1.08 g/L/h at 8 h, respectively. This synthetic platform was also compatible with other various aldehydes, which allowed the production of many other high-value chemicals from glycerol.

Release of sugars and fatty acids from heavy oil biodegradation by common hydrolytic enzymes

In response to recent advances in understanding relating to the remarkable persistence of soil organic matter during burial and diagenesis, we examine the extent to which bitumen compositionally reflects the soil organic matter from which it was derived. Through a simple set of experiments, exposure of bitumen to lipase and cellulase, two enzymes effective in the biodegradation of soil organic matter, resulted in the release of glycerin, palmitic and oleic fatty acids from lipase digestion in addition to the release of glucose, alkylphenols and acyclic polyols from fermentation with cellulase, consistent with the products expected these enzymes. These results are significant in that they suggest that heavy oils are more similar to their soil precursor than previously thought, that biodegradation of bitumen can be accelerated using common over the counter enzymes in aerobic conditions and that heavy oils, which are 1000 times more abundant than coal, can release similar biomolecules as those generated in bioreactor culture or biomass harvest, using two of the most abundantly produced enzymes presently available.

Metabolic engineering pathways for rare sugars biosynthesis, physiological functionalities, and applications-a review

Biomolecules like rare sugars and their derivatives are referred to as monosaccharides particularly uncommon in nature. Remarkably, many of them have various known physiological functions and biotechnological applications in cosmetics, nutrition, and pharmaceutical industries. Also, they can be exploited as starting materials for synthesizing fascinating natural bioproducts with significant biological activities. Regrettably, most of the rare sugars are quite expensive, and their synthetic chemical routes are both limited and economically unfeasible due to expensive raw materials. On the other hand, their production by enzymatic means often suffers from low space-time yields and high catalyst costs due to hasty enzyme denaturation/degradation. In this context, biosynthesis of rare sugars with industrial importance is receiving renowned scientific attention, across the globe. Moreover, the utilization of renewable resources as energy sources via microbial fermentation or microbial metabolic engineering has appeared a new tool. This article presents a comprehensive review of physiological functions and biotechnological applications of rare ketohexoses and aldohexoses, including D-psicose, D-tagatose, L-tagatose, D-sorbose, L-fructose, D-allose, L-glucose, D-gulose, L-talose, L-galactose, and L-fucose. Novel in-vivo recombination pathways based on aldolase and phosphatase for the biosynthesis of rare sugars, particularly D-psicose and D-sorbose using robust microbial strains are also deliberated.

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.

Structural characterization of the thermostable Bradyrhizobium japonicumD-sorbitol dehydrogenase

Bradyrhizobium japonicum sorbitol dehydrogenase is NADH-dependent and is active at elevated temperatures. The best substrate is D-glucitol (a synonym for D-sorbitol), although L-glucitol is also accepted, giving it particular potential in industrial applications. Crystallization led to a hexagonal crystal form, with crystals diffracting to 2.9 Å resolution. In attempts to phase the data, a molecular-replacement solution based upon PDB entry 4nbu (33% identical in sequence to the target) was found. The solution contained one molecule in the asymmetric unit, but a tetramer similar to that found in other short-chain dehydrogenases, including the search model, could be reconstructed by applying crystallographic symmetry operations. The active site contains electron density consistent with D-glucitol and phosphate, but there was not clear evidence for the binding of NADH. In a search for the features that determine the thermostability of the enzyme, the Tm for the orthologue from Rhodobacter sphaeroides, for which the structure was already known, was also determined, and this enzyme proved to be considerably less thermostable. A continuous β-sheet is formed between two monomers in the tetramer of the B. japonicum enzyme, a feature not generally shared by short-chain dehydrogenases, and which may contribute to thermostability, as may an increased Pro/Gly ratio.

Two-step enzymatic synthesis of 6-deoxy-L-psicose

Rare sugars offer a plethora of applications in the pharmaceutical, medicinal, and industries, as well as in synthetic chemistry. However, studies of rare sugars have been hampered by their relative scarcity. In this work, we describe a two-step strategy to efficiently and conveniently prepare 6-deoxy-L-psicose from L-rhamnose. In the first reaction step, the isomerization of L-rhamnose (6-deoxy-L-mannose) to L-rhamnulose (6-deoxy-L-fructose) catalyzed by L-rhamnose isomerase (RhaI), and the epimerization of L-rhamnulose to 6-deoxy-L-psicose catalyzed by D-tagatose 3-epimerase (DTE) were coupled with selective phosphorylation reaction by fructose kinase from human (HK), which selectively phosphorylate 6-deoxy-L-psicose at C-1 position. 6-deoxy-L-psicose 1-phosphate was purified by a silver nitrate precipitation method. In the second step, the phosphate group of the 6-deoxy-L-sorbose 1-phosphate was hydrolyzed with acid phosphatase (AphA) to produce 6-deoxy-L-psicose in 81% yield with respect to L-rhamnose. This method allows that the 6-deoxy-L-psicose to be obtained from readily available starting materials with high purity and without having to undergo isomer separation.

D-allose protects the blood brain barrier through PPARγ-mediated anti-inflammatory pathway in the mice model of ischemia reperfusion injury

Our early experiments confirmed that D-allose was closely involved in the blood brain barrier (BBB) protection from ischemia reperfusion (IR) injury, but the regulatory mechanism is not fully defined. In this study, we aimed to investigate the role of D-allose in the protection of BBB integrity and the relevant mechanisms involved in the mice model of middle cerebral artery occlusion and reperfusion (MCAO/Rep). D-allose was intravenously injected via a tail vein (0.2mg/g and 0.4mg/g, 1h before ischemia), GW9662 was intraperitoneal injected to the mice (4mg/kg) before inducing ischemia 24h. Pretreatment with D-allose ameliorated the neurological deficits, infarct volume and brain edema in brains of MCAO/Rep mice. D-allose inhibited cell apoptosis in the mice model of MCAO/Rep. We observed that D-allose remarkably decreased BBB permeability and prevented the reduction of ZO-1, Occludin and Claudin-5 in mice brains with MCAO/Rep injury. D-allose also repressed the levels of TNF-α, NF-κB, interleukin (IL)-1β and IL-8 in inflammatory responses. The increases of intercellular adhesion molecular-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1) and CD11b/CD18 were significantly inhibited by D-allose during the MCAO/Rep injury. And D-allose decreased the L-selectin and P-selectin levels after MCAO/Rep. Moreover, D-allose induced up-regulation of peroxisome proliferator-activated receptor γ (PPARγ), and down-regulation of TNF-α and NF-κB after MCAO/Rep, which were abolished by utilization of GW9662. In conclusion, we provided evidences that D-allose may has therapeutic potential against brain IR injury through attenuating BBB disruption and the inflammatory response via PPARγ-dependent regulation of NF-κB.

Microwave Assisted Pyrolysis of Waste Biomass Resources for Bio-Oil Production

The agro-industrial sector of many countries generates considerable quantity of waste biomass and potential exploitation of this reside is necessary for economic and environmental reasons. Pakistan is an agricultural based country with widespread amount of crop residue generated annually. This study utilized rice husk, sawdust and bagasse residues to investigate the effects of microwave absorber loading on process temperature, pyrolysis products, and bio-oil composition using multimode microwave pyrolysis system operated at 300W and 2.54GHz. The results indicated that pyrolysis process temperature depends on the type of waste residue and microwave absorber loading. The maximum bio-oil yield of 22.41wt%, 33.61wt% and 19.1wt% were produced at 75wt% microwave absorber loading from rice husk, sawdust and bagasse, respectively. The D-Allose of 21.95 %area, dodecanoic acid of 71.22 %area and octasiloxane of 74.50 %area under GC-MS in rice husk, sawdust and bagasse bio-oils, respectively suggests potential use as chemical feedstock. Keywords: Waste biomass; microwave absorber; microwave assisted pyrolysis; process temperature; product distribution; bio-oil composition

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 ribose-5-phosphate isomerase converting D-psicose to D-allose from Thermotoga lettingae TMO

The gene coding for ribose-5-phosphate isomerase (Rpi) from Thermotoga lettingae TMO was cloned and expressed in E. coli. The recombinant enzyme was purified by Ni-affinity chromatography. It converted D-psicose to D-allose maximally at 75 °C and pH 8.0 with a 32 % conversion yield. The k m, turnover number (k cat), and catalytic efficiency (k cat k m (-1) ) for substrate D-psicose were 64 mM, 6.98 min(-1) and 0.11 mM(-1) min(-1) respectively.

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.

Enzymes for the biocatalytic production of rare sugars

Carbohydrates are much more than just a source of energy as they also mediate a variety of recognition processes that are central to human health. As such, saccharides can be applied in the food and pharmaceutical industries to stimulate our immune system (e.g., prebiotics), to control diabetes (e.g., low-calorie sweeteners), or as building blocks for anticancer and antiviral drugs (e.g., l-nucleosides). Unfortunately, only a small number of all possible monosaccharides are found in nature in sufficient amounts to allow their commercial exploitation. Consequently, so-called rare sugars have to be produced by (bio)chemical processes starting from cheap and widely available substrates. Three enzyme classes that can be used for rare sugar production are keto–aldol isomerases, epimerases, and oxidoreductases. In this review, the recent developments in rare sugar production with these biocatalysts are discussed.

Recent advances on applications and biotechnological production of D-psicose

D-Psicose is a hexoketose monosaccharide sweetener, which is a C-3 epimer of D-fructose and is rarely found in nature. It has 70 % relative sweetness but 0.3 % energy of sucrose, and is suggested as an ideal sucrose substitute for food products. It shows important physiological functions, such as blood glucose suppressive effect, reactive oxygen species scavenging activity, and neuroprotective effect. It also improves the gelling behavior and produces good flavor during food process. This article presents a review of recent studies on the properties, physiological functions, and food application of D-psicose. In addition, the biochemical properties of D-tagatose 3-epimerase family enzymes and the D-psicose-producing enzyme are compared, and the biotechnological production of D-psicose from D-fructose is reviewed.