Characterization of a novel metal-dependent D-psicose 3-epimerase from Clostridium scindens 35704

D-allulose 3-epimerase for low-calorie D-allulose synthesis: microbial production, characterization, and applications

D-allulose, an epimer of D-fructose at C-3 position, is a low-calorie rare sugar with favorable physiochemical properties and special physiological functions, which displays promising perspectives in the food and pharmaceutical industries. Currently, D-allulose is extremely sparse in nature and is predominantly biosynthesized through the isomerization of D-fructose by D-allulose 3-epimerase (DAEase). In recent years, D-allulose 3-epimerase as the key biocatalyst for D-allulose production has received increasing interest. The current review begins by providing a summary of D-allulose regarding its characteristics and applications, as well as different synthesis pathways dominated by biotransformation. Then, the research advances of D-allulose 3-epimerase are systematically reviewed, focusing on heterologous expression and biochemical characterization, crystal structure and molecular modification, and application in D-allulose production. Concerning the constraint of low yield of DAEase for industrial application, this review addresses the various attempts made to promote the production of DAEase in different expression systems. Also, various strategies have been adopted to improve its thermotolerance and catalytic activity, which is mainly based on the structure-function relationship of DAEase. The application of DAEase in D-allulose biosynthesis from D-fructose or low-cost feedstocks through single- or multi-enzymatic cascade reaction has been discussed. Finally, the prospects for related research of D-allulose 3-epimerase are also proposed, facilitating the industrialization of DAEase and more efficient and economical bioproduction of D-allulose.

Simultaneous Enhancement of the Thermostability and Catalytic Activity of D-Allulose 3-Epimerase from Clostridium bolteae ATTC BAA-613 Based on the "Back to Consensus Mutations" Hypothesis

D-Allulose is a high value rare sugar with multiple physiological functions and commercial potential that can be enzymatically synthesized from D-fructose by D-allulose 3-epimerase (DAEase). Poor catalytic activity and thermostability of DAEase prevent the industrial production of D-allulose. In this work, rational design was applied to a previously identified DAEase from Clostridium bolteae ATCC BAA-613 based on the "back to consensus mutations" hypothesis, and the catalytic activity of the Cb-I265 V variant was enhanced 2.5-fold. Furthermore, the Cb-I265 V/E268D double-site variant displayed 2.0-fold higher specific catalytic activity and 1.4-fold higher thermostability than the wild-type enzyme. Molecular docking and kinetic simulation results indicated increased hydrogen bonds between the active pocket and substrate, possibly contributing to the improved thermal stability and catalytic activity of the double-site mutant. The findings outlined a feasible approach for the rational design of multiple preset functions of target enzymes simultaneously.

Enhancement of the d-Allulose 3-Epimerase Expression in Bacillus subtilis through Both Transcriptional and Translational Regulations

d-Allulose, a functional bulk sweetener, has recently attracted increasing attention because of its low-caloric-ness properties and diverse health effects. d-Allulose is industrially produced by the enzymatic epimerization of d-fructose, which is catalyzed by ketose 3-epimerase (KEase). In this study, the food-grade expression of KEase was studied using Bacillus subtills as the host. Clostridium sp. d-allulose 3-epimerase (Clsp-DAEase) was screened from nine d-allulose-producing KEases, showing better potential for expression in B. subtills WB600. Promoter-based transcriptional regulation and N-terminal coding sequence (NCS)-based translational regulation were studied to enhance the DAEase expression level. In addition, the synergistic effect of promoter and NCS on the Clsp-DAEase expression was studied. Finally, the strain with the combination of a PHapII promoter and gln A-Up NCS was selected as the best Clsp-DAEase-producing strain. It efficiently produced Clsp-DAEase with a total activity of 333.2 and 1860.6 U/mL by shake-flask and fed-batch cultivations, respectively.

Sequence- and Structure-Based Mining of Thermostable D-Allulose 3-Epimerase and Computer-Guided Protein Engineering To Improve Enzyme Activity

D-Allulose, a functional sweetener, can be synthesized from fructose using D-allulose 3-epimerase (DAEase). Nevertheless, a majority of the reported DAEases have inadequate stability under harsh industrial reaction conditions, which greatly limits their practical applications. In this study, big data mining combined with a computer-guided free energy calculation strategy was employed to discover a novel DAEase with excellent thermostability. Consensus sequence analysis of flexible regions and comparison of binding energies after substrate docking were performed using phylogeny-guided big data analyses. TtDAE from Thermogutta terrifontis was the most thermostable among 358 candidate enzymes, with a half-life of 32 h at 70 °C. Subsequently, structure-guided virtual screening and a customized strategy based on a combinatorial active-site saturation test/iterative saturation mutagenesis were utilized to engineer TtDAE. Finally, the catalytic activity of the M4 variant (P105A/L14C/T63G/I65A) was increased by 5.12-fold. Steered molecular dynamics simulations indicated that M4 had an enlarged substrate-binding pocket, which enhanced the fit between the enzyme and the substrate. The approach presented here, combining DAEases mining with further rational modification, provides guidance for obtaining promising catalysts for industrial-scale production.

Irreversible biosynthesis of D-allulose from D-glucose in Escherichia coli through fine-tuning of carbon flux and cofactor regeneration engineering

BACKGROUND

As a rare hexose with low calories and various physiological functions, d-allulose has drawn increasing attention. The current industrial production of d-allulose from d-fructose or d-glucose is achieved via epimerization based on the Izumoring strategy; however, the inherent reaction equilibrium during reversible reaction limits its high conversion yield. Although the conversion of d-fructose to d-allulose could be enhanced via phosphorylation-dephosphorylation mediated by metabolic engineering, biomass reduction and byproduct accumulation remain a largely unresolved issue.

RESULTS

After modifying the glycolytic pathway of Escherichia coli and optimizing the whole-cell reaction condition, the engineered strain produced 7.57 ± 0.61 g L^-1 d-allulose from 30 g L^-1 d-glucose after 24 h of catalysis. By developing an ATP regeneration system for enhanced substrate phosphorylation, the cell growth inhibition was alleviated and d-allulose production increased by 55.3% to 11.76 ± 0.58 g L^-1 (0.53 g g^-1 ). Fine-tuning of carbon flux caused a 48% reduction in d-fructose accumulation to 1.47 ± 0.15 g L^-1 . After implementing fed-batch co-substrate strategy, the d-allulose titer reached 15.80 ± 0.31 g L^-1 (0.62 g g^-1 ) with a d-glucose conversion rate of 84.8%.

CONCLUSION

The present study reports a novel strategy for high-yield d-allulose production from low-cost substrate. © 2023 Society of Chemical Industry.

Substantial Improvement of an Epimerase for the Synthesis of D-Allulose by Biosensor-Based High-Throughput Microdroplet Screening

Biosynthesis of D-allulose has been achieved using ketose 3-epimerases (KEases), but its application is limited by poor catalytic performance. In this study, we redesigned a genetically encoded biosensor based on a D-allulose-responsive transcriptional regulator for real-time monitoring of D-allulose. An ultrahigh-throughput droplet-based microfluidic screening platform was further constructed by coupling with this D-allulose-detecting biosensor for the directed evolution of the KEases. Structural analysis of Sinorhizobium fredii D-allulose 3-epimerase (SfDAE) revealed that a highly flexible helix/loop region exposes or occludes the catalytic center as an essential lid conformation regulating substrate recognition. We reprogrammed SfDAE using structure-guided rational design and directed evolution, in which a mutant M3-2 was identified with 17-fold enhanced catalytic efficiency. Our research offers a paradigm for the design and optimization of a biosensor-based microdroplet screening platform.

Produce D-allulose from non-food biomass by integrating corn stalk hydrolysis with whole-cell catalysis

D-allulose is a high-value rare sugar with many health benefits. D-allulose market demand increased dramatically after approved as generally recognized as safe (GRAS). The current studies are predominantly focusing on producing D-allulose from either D-glucose or D-fructose, which may compete foods against human. The corn stalk (CS) is one of the main agricultural waste biomass in the worldwide. Bioconversion is one of the promising approach to CS valorization, which is of significance for both food safety and reducing carbon emission. In this study, we tried to explore a non-food based route by integrating CS hydrolysis with D-allulose production. Firstly we developed an efficient Escherichia coli whole-cell catalyst to produce D-allulose from D-glucose. Next we hydrolyzed CS and achieved D-allulose production from the CS hydrolysate. Finally we immobilized the whole-cell catalyst by designing a microfluidic device. Process optimization improved D-allulose titer by 8.61 times, reaching 8.78 g/L from CS hydrolysate. With this method, 1 kg CS was finally converted to 48.87 g D-allulose. This study validated the feasibility of valorizing corn stalk by converting it to D-allulose.

Engineering of Acid-Resistant d-Allulose 3-Epimerase for Functional Juice Production

d-Allulose, a rare sugar and functional sweetener, can be biosynthesized by d-allulose 3-isomerase (DAE). However, most of the reported DAEs exhibit poor resistance under acidic conditions, which severely limited their application. Here, surface charge engineering and random mutagenesis were used to construct a mutant library of CcDAE from Clostridium cellulolyticum H10, combined with high-throughput screening to identify mutants with high activity and resistance under acidic conditions. The mutant M3 (I114R/K123E/H209R) exhibited high activity (3.36-fold of wild-type) and acid resistance (10.6-fold of wild-type) at pH 4.5. The structure-function relationship was further analyzed by molecular dynamics (MD) simulations, which indicated that M3 had a higher number of hydrogen bonds and negative surface charges than the wild type. A multienzyme cascade system including M3 was used to convert high-calorie sugars in acidic juices, and functional juices containing 7.8-15.4 g/L d-allulose were obtained. Our study broadens the manufacture of functional foods containing d-allulose.

Optimization of Ultrahigh-Throughput Screening Assay for Protein Engineering of d-Allulose 3-Epimerase

d-Allulose is the corresponding epimer of d-fructose at the C-3 position, which exhibits a similar taste and sweetness to sucrose. As a low-calorie sweetener, d-allulose has broad application prospects in the fields of medicine, food, and so on. Currently, the production method of d-allulose is mainly the enzymatic conversion of d-fructose by d-allulose 3-epimerase (DAEase). However, the limited specific activity and thermal stability of DAEase restrict its industrial application. Herein, an ultrahigh-throughput screening assay based on the transcription factor PsiR was extensively optimized from the aspects of culture medium components, screening plasmid, and expression host, which enhanced the correction between the fluorescent readout and the enzyme activity. Then, the error-prone PCR (epPCR) library of Clostridium cellulolyticum H10 DAEase (CcDAEase) was screened through the above optimized method, and the variant I228V with improved specific activity and thermal stability was obtained. Moreover, after combining two beneficial substitutions, D281G and C289R, which were previously obtained by this optimized assay, the specific activity of the triple-mutation variant I228V/D281G/C289R reached up to 1.42-fold of the wild type (WT), while its half-life (T1/2) at 60 °C was prolonged by 62.97-fold. The results confirmed the feasibility of the optimized screening assay as a powerful tool for the directed evolution of DAEase.

Immobilization of D-allulose 3-epimerase into magnetic metal-organic framework nanoparticles for efficient biocatalysis

D-allulose is a rare low-calorie sugar that has many fundamental biological functions. D-allulose 3-epimerase from Agrobacterium tumefaciens (AT-DAEase) catalyzes the conversion of D-fructose to D-allulose. The enzyme has attracted considerable attention because of its mild catalytic properties. However, the bioconversion efficiency and reusability of AT-DAEase limit its industrial application. Magnetic metal-organic frameworks (MOFs) have uniform pore sizes and large surface areas and can facilitate mass transport and enhance the capacity for enzyme immobilization. Here, we successfully encapsulated cobalt-type AT-DAEase into the cobalt-based magnetic MOF ZIF-67@Fe₃O₄ using a self-assembly strategy. We confirmed the immobilization of enzyme AT-DAEase and characterized the enzymatic properties of the MOF-immobilized AT-DAEase@ZIF-67@Fe₃O₄. The AT-DAEase@ZIF-67@Fe₃O₄ nanoparticles had higher catalytic activity (65.1 U mg^-1) and bioconversion ratio (38.1%) than the free AT-DAEase. The optimal conditions for maximum enzyme activity of the AT-DAEase@ZIF-67@Fe₃O₄ nanoparticles were 55 °C and pH 8.0, which were significantly higher than those of the free AT-DAEase (50 °C and pH 7.5). The AT-DAEase@ZIF-67@Fe₃O₄ nanoparticles displayed significantly improved thermal stability and excellent recycling performance, with 80% retention of enzyme activity at a temperature range of 45-70 °C and > 45% of its initial activity after eight cycles of enzyme use. The AT-DAEase@ZIF-67@Fe₃O₄ nanoparticles have great potential for large-scale industrial preparation of D-allulose by immobilizing cobalt-type AT-DAEase into magnetic MOF ZIF-67@Fe₃O₄.

Metabolically Engineered Escherichia coli for Conversion of D-Fructose to D-Allulose via Phosphorylation-Dephosphorylation

D-Allulose is an ultra-low calorie sweetener with broad market prospects. As an alternative to Izumoring, phosphorylation-dephosphorylation is a promising method for D-allulose synthesis due to its high conversion of substrate, which has been preliminarily attempted in enzymatic systems. However, in vitro phosphorylation-dephosphorylation requires polyphosphate as a phosphate donor and cannot completely deplete the substrate, which may limit its application in industry. Here, we designed and constructed a metabolic pathway in Escherichia coli for producing D-allulose from D-fructose via in vivo phosphorylation-dephosphorylation. PtsG-F and Mak were used to replace the fructose phosphotransferase systems (PTS) for uptake and phosphorylation of D-fructose to fructose-6-phosphate, which was then converted to D-allulose by AlsE and A6PP. The D-allulose titer reached 0.35 g/L and the yield was 0.16 g/g. Further block of the carbon flux into the Embden-Meyerhof-Parnas (EMP) pathway and introduction of an ATP regeneration system obviously improved fermentation performance, increasing the titer and yield of D-allulose to 1.23 g/L and 0.68 g/g, respectively. The E. coli cell factory cultured in M9 medium with glycerol as a carbon source achieved a D-allulose titer of ≈1.59 g/L and a yield of ≈0.72 g/g on D-fructose.

Efficient D-allulose synthesis under acidic conditions by auto-inducing expression of the tandem D-allulose 3-epimerase genes in Bacillus subtilis

BACKGROUND

D-allulose, a hexulose monosaccharide with low calorie content and high sweetness, is commonly used as a functional sugar in food and nutrition. However, enzyme preparation of D-allulose from D-frutose was severely hindered by the non-enzymatic browning under alkaline and high-temperature, and the unnecessary by-products further increased the difficulties in separation and extraction for industrial applications. Here, to address the above issue during the production process, a tandem D-allulose 3-epimerase (DPEases) isomerase synergistic expression strategy and an auto-inducible promoter engineering were levered in Bacillus subtilis 168 (Bs168) for efficient synthesis of D-allulose under the acidic conditions without browning.

RESULTS

First, based on the dicistron expression system, two DPEases with complementary functional characteristics from Dorea sp. CAG:317 (DSdpe) and Clostridium cellulolyticum H10 (RCdpe) were expressed in tandem under the promoter HpaII in one cell. A better potential strain Bs168/pMA5-DSdpe-RCdpe increases enzyme activity to 18.9 U/mL at acidic conditions (pH 6.5), much higher than 17.2 and 16.7 U/mL of Bs168/pMA5-DSdpe and Bs168/pMA5-RCdpe, respectively. Subsequently, six recombinant strains based on four constitutive promoters were constructed in variable expression cassettes for improving the expression level of protein. Among those engineered strains, Bs168/pMA5-P_spoVG-DSdpe-P_srfA-RCdpe exhibited the highest enzyme activity with 480.1 U/mL on fed-batch fermentation process in a 5 L fermenter at pH 6.5, about 2.1-times higher than the 228.5 U/mL of flask fermentation. Finally, the maximum yield of D-allulose reached as high as 163.5 g/L at the fructose concentration (50% w/v) by whole-cell biocatalyst.

CONCLUSION

In this work, the engineered recombinant strain Bs168/pMA5-P_spoVG-DSdpe-P_srfA-RCdpe was demonstrated as an effective microbial cell factory for the high-efficient synthesis of D-allulose without browning under acidic conditions. Based on the perspectives from this research, this strategy presented here also made it possible to meet the requirements of the industrial hyper-production of other rare sugars under more acidic conditions in theory.

D-allulose, a versatile rare sugar: recent biotechnological advances and challenges

D-Allulose is the C-3 epimer of D-fructose, and widely regarded as a promising substitute for sucrose. It's an excellent low-calorie sweetener, with 70% sweetness of sucrose, 0.4 kcal/g dietary energy, and special physiological functions. It has been approved as GRAS by the U.S. Food and Drug Administration, and is allowed to be excluded from total and added sugar counts on the food labels. Therefore, D-allulose gradually attracts more public attention. Owing to scarcity in nature, the bioproduction of D-allulose by using ketose 3-epimerase (KEase) has become the research hotspot. Herein, we give a summary of the physicochemical properties, physiological function, applications, and the chemical and biochemical synthesis methods of D-allulose. In addition, the recent progress in the D-allulose bioproduction using KEases, and the possible solutions for existing challenges in the D-allulose industrial production are comprehensively discussed, focusing on the molecular modification, immobilization, food-grade expression, utilizing low-cost biomass as feedstock, overcoming thermodynamic limitation, as well as the downstream separation and purification. Finally, Prospects for further development are also proposed.

Engineering Escherichia coli for d-Allulose Production from d-Fructose by Fermentation

d-Allulose is considered an ideal alternative to sucrose and has shown tremendous application potential in many fields. Recently, most efforts on production of d-allulose have focused on in vitro enzyme-catalyzed epimerization of cheap hexoses. Here, we proposed an approach to efficiently produce d-allulose through fermentation using metabolically engineered Escherichia coli JM109 (DE3), in which a SecY (ΔP) channel and a d-allulose 3-epimerase (DPEase) were co-expressed, ensuring that d-fructose could be transported in its nonphosphorylated form and then converted into d-allulose by cells. Further deletion of fruA, manXYZ, mak, galE, and fruK and the use of Ni²⁺ in a medium limited the carbon flux flowing into the byproduct-generating pathways and the Embden-Meyerhof-Parnas (EMP) pathway, achieving a ≈ 0.95 g/g yield of d-allulose on d-fructose using E. coli (DPEase, SecY [ΔP], ΔFruA, ΔManXYZ, ΔMak, ΔGalE, ΔFruK) and 8 μM Ni²⁺. In fed-batch fermentation, the titer of d-allulose reached ≈23.3 g/L.

Biochemical identification of a hyperthermostable l-ribulose 3-epimerase from Labedella endophytica and its application for d-allulose bioconversion

Currently, low sugar and low energy have become an important trend in the food industries. Therefore, the bioconversion of the functional low-calorie rare sugars attracts more and more attention. l-Ribulose 3-epimerase (LREase) belongs to the ketose 3-epimerase (KEase) family, which could not only efficiently catalyze the reversible C-3 epimerization between l-ribulose and l-xylulose but also between d-fructose and d-allulose. In this paper, a hyperthermostable LREase from Labedella endophytica was identified and characterized. It exhibited maximum catalytic activity at pH 6.0 and 80 °C with 1 mM Ni²⁺. In the presence of Co²⁺, the t_1/2 values at 60, 65, and 70 °C were 37.7, 9.0, and 4.6 h, respectively, and T_m value was 80.9 °C. From 500 g/L d-fructose, it could produce 154.2 g/L d-allulose with a conversion rate of 30.8% in 10 h. In view of its strong thermostability and high catalytic efficiency, L. endophytica LREase might be a good potential alternative for d-allulose industrial production.

Bioproduction of D-allulose: Properties, applications, purification, and future perspectives

D-allulose is the C-3 epimer of D-fructose, which rarely exists in nature, and can be biosynthesized from D-fructose by the catalysis of D-psicose 3-epimerase. D-allulose is safe for human consumption and was recently approved by the United States Food and Drug Administration for food applications. It is not only able be used in food and dietary supplements as a low-calorie sweetener, but also modulates a variety of physiological functions. D-allulose has gained increasing attention owing to its excellent properties. This article presents a review of recent progress on the properties, applications, and bioproduction progress of D-allulose.

A Possible Mechanism of Graphene Oxide to Enhance Thermostability of D-Psicose 3-Epimerase Revealed by Molecular Dynamics Simulations

Thermal stability is a limiting factor for effective application of D-psicose 3-epimerase (DPEase) enzyme. Recently, it was reported that the thermal stability of DPEase was improved by immobilizing enzymes on graphene oxide (GO) nanoparticles. However, the detailed mechanism is not known. In this study, we investigated interaction details between GO and DPEase by performing molecular dynamics (MD) simulations. The results indicated that the domain (K248 to D268) of DPEase was an important anchor for immobilizing DPEase on GO surface. Moreover, the strong interactions between DPEase and GO can prevent loop α1′-α1 and β4-α4 of DPEase from the drastic fluctuation. Since these two loops contained active site residues, the geometry of the active pocket of the enzyme remained stable at high temperature after the DPEase was immobilized by GO, which facilitated efficient catalytic activity of the enzyme. Our research provided a detailed mechanism for the interaction between GO and DPEase at the nano–biology interface.

Continuous Spectrophotometric Assay for High-Throughput Screening of Predominant d-Allulose 3-Epimerases

d-Allulose is an attractive noncaloric sugar substitute with numerous health benefits, which can be biosynthesized by d-allulose 3-epimerases (DAEases). However, enzyme instability under harsh industrial reaction conditions hampered its practical applications. Here, we developed a continuous spectrophotometric assay (CSA) for the efficient analysis of d-allulose in a mixture. Furthermore, a high-throughput screening strategy for DAEases was developed using CSA by coupling DAEase with a NADH-dependent ribitol dehydrogenase, enabling high-throughput screening of DAEase variants with desired properties. The variant M15S/P40N/S209N exhibited a half-life of 22 h at 60 °C and an 8.7 °C increase of the T₅₀⁶⁰ value, with a 1.2-fold increase of activity. Structural modeling and molecular dynamics simulations indicated that the improvement of thermostability and activity was due to some new hydrogen bonds between chains at the dimer interface and between the residue and the substrate d-fructose. This work offers a robust tool and theoretical basis for the improvement of DAEases, which will benefit the enzymatic biosynthesis of d-allulose and promote its industrial application.

Enhanced Thermostability of D-Psicose 3-Epimerase from Clostridium bolteae through Rational Design and Engineering of New Disulfide Bridges

D-psicose 3-epimerase (DPEase) catalyzes the isomerization of D-fructose to D-psicose (aka D-allulose, a low-calorie sweetener), but its industrial application has been restricted by the poor thermostability of the naturally available enzymes. Computational rational design of disulfide bridges was used to select potential sites in the protein structure of DPEase from Clostridium bolteae to engineer new disulfide bridges. Three mutants were engineered successfully with new disulfide bridges in different locations, increasing their optimum catalytic temperature from 55 to 65 °C, greatly improving their thermal stability and extending their half-lives (t_1/2) at 55 °C from 0.37 h to 4−4.5 h, thereby greatly enhancing their potential for industrial application. Molecular dynamics simulation and spatial configuration analysis revealed that introduction of a disulfide bridge modified the protein hydrogen–bond network, rigidified both the local and overall structures of the mutants and decreased the entropy of unfolded protein, thereby enhancing the thermostability of DPEase.

Research Advances of d-allulose: An Overview of Physiological Functions, Enzymatic Biotransformation Technologies, and Production Processes

d-allulose has a significant application value as a sugar substitute, not only as a food ingredient and dietary supplement, but also with various physiological functions, such as improving insulin resistance, anti-obesity, and regulating glucolipid metabolism. Over the decades, the physiological functions of d-allulose and the corresponding mechanisms have been studied deeply, and this product has been applied to various foods to enhance food quality and prolong shelf life. In recent years, biotransformation technologies for the production of d-allulose using enzymatic approaches have gained more attention. However, there are few comprehensive reviews on this topic. This review focuses on the recent research advances of d-allulose, including (1) the physiological functions of d-allulose; (2) the major enzyme families used for the biotransformation of d-allulose and their microbial origins; (3) phylogenetic and structural characterization of d-allulose 3-epimerases, and the directed evolution methods for the enzymes; (4) heterologous expression of d-allulose ketose 3-epimerases and biotransformation techniques for d-allulose; and (5) production processes for biotransformation of d-allulose based on the characterized enzymes. Furthermore, the future trends on biosynthesis and applications of d-allulose in food and health industries are discussed and evaluated in this review.

Crystal structure of a novel homodimeric l-ribulose 3-epimerase from Methylomonus sp

d-Allulose has potential as a low-calorie sweetener which can suppress fat accumulation. Several enzymes capable of d-allulose production have been isolated, including d-tagatose 3-epimerases. Here, we report the isolation of a novel protein from Methylomonas sp. expected to be a putative enzyme based on sequence similarity to ketose 3-epimerase. The synthesized gene encoding the deduced ketose 3-epimerase was expressed as a recombinant enzyme in Escherichia coli, and it exhibited the highest enzymatic activity toward l-ribulose, followed by d-ribulose and d-allulose. The X-ray structure analysis of l-ribulose 3-epimerase from Methylomonas sp. (MetLRE) revealed a homodimeric enzyme, the first reported structure of dimeric l-ribulose 3-epimerase. The monomeric structure of MetLRE is similar to that of homotetrameric l-ribulose 3-epimerases, but the short C-terminal α-helix of MetLRE is unique and different from those of known l-ribulose 3 epimerases. The length of the C-terminal α-helix was thought to be involved in tetramerization and increasing stability; however, the addition of residues to MetLRE at the C terminus did not lead to tetramer formation. MetLRE is the first dimeric l-ribulose 3-epimerase identified to exhibit high relative activity toward d-allulose.

Two-step biosynthesis of d-allulose via a multienzyme cascade for the bioconversion of fruit juices

d-Allulose, a low-calorie rare sugar with potential as sucrose substitute for diabetics, can be produced using d-allulose 3-epimerase (DAE). Here, we characterized a putative thermostable DAE from Pirellula sp. SH-Sr6A (PsDAE), with a half-life of 6 h at 60 °C. Bioconversion of 500 g/L d-fructose using immobilized PsDAE on epoxy support yielded 152.7 g/L d-allulose, which maintained 80% of the initial activity after 11 reuse cycles. A multienzyme cascade system was developed to convert sucrose to d-allulose comprising sucrose invertase, d-glucose isomerase and PsDAE. Fruit juices were treated using this system to convert the high-calorie sugars, such as sucrose, d-glucose, and d-fructose, into d-allulose. The content of d-allulose among total monosaccharides in the treated fruit juice remained between 16 and 19% during 15 reaction cycles. This study provides an efficient strategy for the development of functional fruit juices containing d-allulose for diabetics and other special consumer categories.

Characterization of a Recombinant D-Allulose 3-epimerase from Thermoclostridium caenicola with Potential Application in D-Allulose Production

In recent years, with the increasing public health awareness, low-calorie rare sugars have received more attention on a global scale. D-Allulose, the C-3 epimer of D-fructose, is a representative rare sugar. It displays high sweetness and excellent physiological functions, but only provides a caloric value of 0.4 kcal/g. D-Allulose 3-epimerase (DAEase) is indispensable in D-allulose production. In this study, a putative DAEase from Thermoclostridium caenicola was identified and characterized. The novel T. caenicola DAEase displayed maximum activity at pH 7.5 and 65 °C in the presence of 1 mM Co²⁺. The half-life (t_1/2) at 50 °C was 13.6 h, and the melting temperature (T_m) was 62.4 °C. It was strictly metal-dependent, and the addition of Co²⁺ remarkably enhanced its thermostability, with a 5.4-fold increase in t_1/2 value at 55 °C and 4.8 °C increase in T_m. Furthermore, DAEase displayed high relative activity (89.0%) at a weakly acidic pH 6.5 and produced 139.8 g/L D-allulose from 500 g/L D-fructose, achieving a conversion ratio of 28.0%. These findings suggest that T. caenicola DAEase is a promising biocatalyst for the production of D-allulose.

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.

D-Allulose 3-epimerase of Bacillus sp. origin manifests profuse heat-stability and noteworthy potential of D-fructose epimerization

BACKGROUND

D-Allulose is an ultra-low calorie sugar of multifarious health benefits, including anti-diabetic and anti-obesity potential. D-Allulose 3-epimerase family enzymes catalyze biosynthesis of D-allulose via epimerization of D-fructose.

RESULTS

A novel D-allulose 3-epimerase (DaeB) was cloned from a plant probiotic strain, Bacillus sp. KCTC 13219, and expressed in Bacillus subtilis cells. The purified protein exhibited substantial epimerization activity in a broad pH spectrum, 6.0-11.0. DaeB was able to catalyze D-fructose to D-allulose bioconversion at the temperature range of 35 °C to 70 °C, exhibiting at least 50 % activity. It displaced excessive heat stability, with the half-life of 25 days at 50 °C, and high turnover number (kcat 367 s- 1). The coupling of DaeB treatment and yeast fermentation of 700 g L- 1 D-fructose solution yielded approximately 200 g L- 1 D-allulose, and 214 g L- 1 ethanol.

CONCLUSIONS

The novel D-allulose 3-epimerase of Bacillus sp. origin discerned a high magnitude of heat stability along with exorbitant epimerization ability. This biocatalyst has enormous potential for the large-scale production of D-allulose.

Biocatalytic Synthesis of D-Allulose Using Novel D-Tagatose 3-Epimerase From Christensenella minuta

D-allulose, which is one of the important rare sugars, has gained significant attention in the food and pharmaceutical industries as a potential alternative to sucrose and fructose. Enzymes belonging to the D-tagatose 3-epimerase (DTEase) family can reversibly catalyze the epimerization of D-fructose at the C3 position and convert it into D-allulose by a good number of naturally occurring microorganisms. However, microbial synthesis of D-allulose is still at its immature stage in the industrial arena, mostly due to the preference of slightly acidic conditions for Izumoring reactions. Discovery of novel DTEase that works at acidic conditions is highly preferred for industrial applications. In this study, a novel DTEase, DTE-CM, capable of catalyzing D-fructose into D-allulose was applications. In this study, a novel DTEase, DTE-CM, capable of catalyzing D-fructose into D-allulose was DTE-CM on D-fructose was found to be remarkably influenced and modulated by the type of metal ions (co-factors). The DTE-CM on D-fructose was found to be remarkably influenced and modulated by the type of metal ions (co-factors). The 50°C from 0.5 to 3.5 h at a concentration of 0.1 mM. The enzyme exhibited its maximum catalytic activity on D-fructose at pH 6.0 and 50°C from 0.5 to 3.5 h at a concentration of 0.1 mM. The enzyme exhibited its maximum catalytic activity on -fructose at pH 6.0 and 50°C with a K cat /K m value of 45 mM-1min-1. The 500 g/L D-fructose, which corresponded to 30% conversion rate. With these interesting catalytic properties, this enzyme could be a promising candidate for industrial biocatalytic applications.

Identification of a Novel Cobamide Remodeling Enzyme in the Beneficial Human Gut Bacterium Akkermansia muciniphila

Cobamides, comprising the vitamin B₁₂ family of cobalt-containing cofactors, are required for metabolism in all domains of life, including most bacteria. Cobamides have structural variability in the lower ligand, and selectivity for particular cobamides has been observed in most organisms studied to date.

The beneficial human gut bacterium Akkermansia muciniphila provides metabolites to other members of the gut microbiota by breaking down host mucin, but most of its other metabolic functions have not been investigated. A. muciniphila strain Muc^T is known to use cobamides, the vitamin B₁₂ family of cofactors with structural diversity in the lower ligand. However, A. muciniphila Muc^T is unable to synthesize cobamides de novo, and the specific forms that can be used by A. muciniphila have not been examined. We found that the levels of growth of A. muciniphila Muc^T were nearly identical with each of seven cobamides tested, in contrast to nearly all bacteria that had been studied previously. Unexpectedly, this promiscuity is due to cobamide remodeling—the removal and replacement of the lower ligand—despite the absence of the canonical remodeling enzyme CbiZ in A. muciniphila. We identified a novel enzyme, CbiR, that is capable of initiating the remodeling process by hydrolyzing the phosphoribosyl bond in the nucleotide loop of cobamides. CbiR does not share similarity with other cobamide remodeling enzymes or B₁₂-binding domains and is instead a member of the apurinic/apyrimidinic (AP) endonuclease 2 enzyme superfamily. We speculate that CbiR enables bacteria to repurpose cobamides that they cannot otherwise use in order to grow under cobamide-requiring conditions; this function was confirmed by heterologous expression of cbiR in Escherichia coli. Homologs of CbiR are found in over 200 microbial taxa across 22 phyla, suggesting that many bacteria may use CbiR to gain access to the diverse cobamides present in their environment.

Ribose-5-phosphate isomerases: characteristics, structural features, and applications

Ribose-5-phosphate isomerase (Rpi, EC 5.3.1.6) is widespread in microorganisms, animals, and plants. It has a pivotal role in the pentose phosphate pathway and responsible for catalyzing the isomerization between D-ribulose 5-phosphate and D-ribose 5-phosphate. In recent years, Rpi has received considerable attention as a multipurpose biocatalyst for production of rare sugars, including D-allose, L-rhamnulose, L-lyxose, and L-tagatose. Besides, it has been thought of as a potential drug target in the treatment of trypanosomatid-caused diseases such as Chagas' disease, leishmaniasis, and human African trypanosomiasis. Despite increased research activities, up to now, no systematic review of Rpi has been published. To fill this gap, this paper provides detailed information about the enzymatic properties of various Rpis. Furthermore, structural features, catalytic mechanism, and molecular modifications of Rpis are summarized based on extensive crystal structure research. Additionally, the applications of Rpi in rare sugar production and the role of Rpi in trypanocidal drug design are reviewed. Key points • Fundamental properties of various ribose-5-phosphate isomerases (Rpis). • Differences in crystal structure and catalytic mechanism between RpiA and RpiB. • Application of Rpi as a rare sugar producer and a potential drug target.

Efficient Allitol Bioproduction from D-Fructose Catalyzed by Recombinant E. coli Whole Cells, and the Condition Optimization, Product Purification

Allitol is a kind of rare sugar alcohol with potential application value. An engineered strain, which simultaneously expressed D-psicose-3-epimerase (DPE), ribitol dehydrogenase (RDH), and formate dehydrogenase (FDH) three enzymes, was constructed by cloning above three genes into one plasmid and transformed into the host E. coli strain, and used as the whole-cell catalysts for biotransformation of allitol from the low-cost substrate of D-fructose. The whole cell allitol biotransformation conditions were optimized. The medium, recombinant gene induction conditions, and the substrate feeding rate for cultivation of the catalytic cells were optimized. Then, the fed-batch culture was made and scaled up to 10 L fermentor. Finally, 63.44 g/L allitol was obtained from 100 g/L D-fructose after 3 h of biotransformation, and the allitol crystals of 99.9% purity were obtained by using cooling recrystallization. The allitol production method developed in this research has high product purity, and is highly efficient, easily scaled up, and suitable for large-scale production of highly purified allitol.

A Novel d-Allulose 3-Epimerase Gene from the Metagenome of a Thermal Aquatic Habitat and d-Allulose Production by Bacillus subtilis Whole-Cell Catalysis

A novel d-allulose 3-epimerase gene (daeM) has been identified from the metagenomic resource of a hot-water reservoir. The enzyme epimerizes d-fructose into d-allulose, a functional sugar of rare abundance in nature. The metagenomic DNA fragment was cloned and expressed in Escherichia coli The purified recombinant protein (DaeM) was found to be metal dependent (Co2+ or Mn2+). It displayed the maximal levels of catalytic activity in a pH range of 6 to 11 and a temperature range of 75°C to 80°C. The enzyme exhibited remarkably high thermal stability at 60°C and 70°C, with half-life values of 9,900 and 3,240 min, respectively. To the best of our knowledge, this is the highest thermal stability demonstrated by a d-allulose 3-epimerase that has been characterized to date. The enzymatic treatment of 700 mg·ml-1 d-fructose yielded about 217 mg·ml-1 d-allulose, under optimal condition. The catalytic product was purified, and its nuclear magnetic resonance (NMR) spectra were found to be indistinguishable from those of standard d-allulose. For biomolecule production, the whole-cell catalysis procedure avoids the tedious process of extraction and purification of enzyme and also offers better biocatalyst stability. Further, it is desirable to employ safe-grade microorganisms for the biosynthesis of a product. The daeM gene was expressed intracellularly in Bacillus subtilis A whole-cell catalysis reaction performed with a reaction volume of 1 liter at 60°C yielded approximately 196 g·liter-1 d-allulose from 700 g·liter-1 d-fructose. Further, the whole recombinant cells were able to biosynthesize d-allulose in apple juice, mixed fruit juice, and honey.IMPORTANCE d-Allulose is a noncaloric sugar substitute with antidiabetes and antiobesity potential. With several characteristics of physiological significance, d-allulose has wide-ranging applications in the food and pharmacology industries. The development of a thermostable biocatalyst is an objective of mainstream research aimed at achieving industrial acceptability of the enzyme. Aquatic habitats of extreme temperatures are considered a potential metagenomic resource of heat-tolerant biocatalysts of industrial importance. The present study explored the thermal-spring metagenome of the Tattapani geothermal region, Chhattisgarh, India, discovering a novel d-allulose 3-epimerase gene, daeM, encoding an enzyme of high-level heat stability. The daeM gene was expressed in the microbial cells of a nonpathogenic and safe-grade species, B. subtilis, which was found to be capable of performing d-fructose to d-allulose interconversion via a whole-cell catalysis reaction. The results indicate that DaeM is a potential biocatalyst for commercial production of the rare sugar d-allulose. The study established that extreme environmental niches represent a genomic resource of functional sugar-related biocatalysts.

Review on D-Allulose: In vivo Metabolism, Catalytic Mechanism, Engineering Strain Construction, Bio-Production Technology

Rare sugar D-allulose as a substitute sweetener is produced through the isomerization of D-fructose by D-tagatose 3-epimerases (DTEases) or D-allulose 3-epimerases (DAEases). D-Allulose is a kind of low energy monosaccharide sugar naturally existing in some fruits in very small quantities. D-Allulose not only possesses high value as a food ingredient and dietary supplement, but also exhibits a variety of physiological functions serving as improving insulin resistance, antioxidant enhancement, and hypoglycemic controls, and so forth. Thus, D-allulose has an important development value as an alternative to high-energy sugars. This review provided a systematic analysis of D-allulose characters, application, enzymatic characteristics and molecular modification, engineered strain construction, and processing technologies. The existing problems and its proposed solutions for D-allulose production are also discussed. More importantly, a green and recycling process technology for D-allulose production is proposed for low waste formation, low energy consumption, and high sugar yield.

Galactose to tagatose isomerization at moderate temperatures with high conversion and productivity

There are many industrially-relevant enzymes that while active, are severely limited by thermodynamic, kinetic, or stability issues (isomerases, lyases, transglycosidases). In this work, we study Lactobacillus sakeil-arabinose isomerase (LsLAI) for d-galactose to d-tagatose isomerization—that is limited by all three reaction parameters. The enzyme demonstrates low catalytic efficiency, low thermostability at temperatures > 40 °C, and equilibrium conversion < 50%. After exploring several strategies to overcome these limitations, we show that encapsulating LsLAI in gram-positive Lactobacillus plantarum that is chemically permeabilized enables reactions at high rates, high conversions, and elevated temperatures. In a batch process, this system enables ~ 50% conversion in 4 h starting with 300 mM galactose (an average productivity of 37 mM h⁻¹), and 85% conversion in 48 h. We suggest that such an approach may be invaluable for other enzymatic processes that are similarly kinetically-, thermodynamically-, and/or stability-limited.

Production of tagatose, a sugar substitute, by isomerization of galactose suffers from unfavorable enzymatic kinetics, low enzyme stability, and low equilibrium constant. Here, the authors simultaneously overcome these limitations by encapsulating l-arabinose isomerase in permeabilized Lactobacillus plantarum.

High Conversion of D-Fructose into D-Allulose by Enzymes Coupling with an ATP Regeneration System

D-Allulose is a rare monosaccharide that exists in extremely small quantities in nature, and it is also hard to prepare at a large scale via chemical or enzyme synthetic route due to low conversion and downstream separation complexity. Using D-psicose epimerase and L-rhamnulose kinase, a method enabling high conversion of D-allulose from D-fructose without the need for a tedious isomer separation step was established recently. However, this method requires expensive ATP to facilitate the reaction. In the present study, an ATP regenerate system was developed coupling with polyphosphate kinase. In our optimized reaction with purified enzymes, the conversion rate of 99% D-fructose was achieved at the concentrations of 2 mM ATP, 5 mM polyphosphate, 20 mM D-fructose, and 20 mM Mg²⁺ when incubated at 50 °C and at pH 7.5. ATP usage can be reduced to 10% of the theoretical amount compared to that without the ATP regeneration system. A fed-batch mode was also studied to minimize the inhibitory effect of polyphosphate. The biosynthetic system reported here offers a potential and promising platform for the conversion of D-fructose into D-allulose at reduced ATP cost.

Redesign of a novel D-allulose 3-epimerase from Staphylococcus aureus for thermostability and efficient biocatalytic production of D-allulose

BACKGROUND

A novel D-allulose 3-epimerase from Staphylococcus aureus (SaDAE) has been screened as a D-allulose 3-epimerase family enzyme based on its high specificity for D-allulose. It usually converts both D-fructose and D-tagatose to respectively D-allulose and D-sorbose. We targeted potential biocatalysts for the large-scale industrial production of rare sugars.

RESULTS

SaDAE showed a high activity on D-allulose with an affinity of 41.5 mM and catalytic efficiency of 1.1 s-1 mM-1. Four residues, Glu146, Asp179, Gln205, and Glu240, constitute the catalytic tetrad of SaDAE. Glu146 and Glu240 formed unique interactions with substrates based on the structural model analysis. The redesigned SaDAE_V105A showed an improvement of relative activity toward D-fructose of 68%. The conversion rate of SaDAE_V105A reached 38.9% after 6 h. The triple mutant S191D/M193E/S213C showed higher thermostability than the wild-type enzyme, exhibiting a 50% loss of activity after incubation for 60 min at 74.2 °C compared with 67 °C for the wild type.

CONCLUSIONS

We redesigned SaDAE for thermostability and biocatalytic production of D-allulose. The research will aid the development of industrial biocatalysts for D-allulose.

Overcoming the thermodynamic equilibrium of an isomerization reaction through oxidoreductive reactions for biotransformation

Isomerases perform biotransformations without cofactors but often cause an undesirable mixture of substrate and product due to unfavorable thermodynamic equilibria. We demonstrate the feasibility of using an engineered yeast strain harboring oxidoreductase reactions to overcome the thermodynamic limit of an isomerization reaction. Specifically, a yeast strain capable of consuming lactose intracellularly is engineered to produce tagatose from lactose through three layers of manipulations. First, GAL1 coding for galactose kinase is deleted to eliminate galactose utilization. Second, heterologous xylose reductase (XR) and galactitol dehydrogenase (GDH) are introduced into the ∆gal1 strain. Third, the expression levels of XR and GDH are adjusted to maximize tagatose production. The resulting engineered yeast produces 37.69 g/L of tagatose from lactose with a tagatose and galactose ratio of 9:1 in the reaction broth. These results suggest that in vivo oxidoreaductase reactions can be employed to replace isomerases in vitro for biotransformation.

A desired product cannot be obtained at higher concentration than its equilibrium concentration when isomerases are used for biotransformation. Here, the authors engineer in vivo oxidoreductive reactions in yeast to overcome the equilibrium limitation of in vitro isomerases-based tagatose production.

Bioprocessing and technoeconomic feasibility analysis of simultaneous production of d-psicose and ethanol using engineered yeast strain KAM-2GD

The objective of this study was to analyze the processing and technoeconomic feasibility of coproduction of d-psicose and ethanol in a modified dry grind ethanol process. The yeast strain was constructed by expressing d-psicose 3-epimerases (DPE) in Sachharomyces cerevisiae. The strain was capable of converting d-fructose to d-psicose at 55 °C with a conversion efficiency of 26.6%. A comprehensive process model for modified dry grind ethanol plant with 396,000 MT/yr corn processing capacity was developed using SuperPro Designer. Predicted ethanol and d-psicose yields were 390.4 L and 75.3 kg per MT of corn, with total annual production of 154.6 million L and 29,835 MT respectively. The capital investment for the plant was estimated as 150.3 million USD with total operating cost of 85.2 million USD/yr. The unit production cost and minimum selling price of d-psicose with an internal rate of return of 15% were calculated as $0.43/kg and $1.29/kg respectively.

Biochemical characterization and biocatalytic application of a novel d-tagatose 3-epimerase from Sinorhizobium sp

Sinorhizobium sp. d-tagatose 3-epimerase (sDTE) catalyzes the conversion of d-tagatose to d-sorbose. It also recognizes d-fructose as a substrate for d-allulose production. The optimal temperature and pH of the purified sDTE was 50 °C and 8.0, respectively. Based on the sDTE homologous model, Glu154, Asp187, Gln213, and Glu248, form a hydrogen bond network with the active-site Mn²⁺ and constitute the catalytic tetrad. The amino acid residues around O-1, -2, and -3 atoms of the substrates (d-tagatose/d-fructose) are strictly conserved and thus likely regulate the catalytic reaction. However, the residues at O-4, -5, and -6, being responsible for the substrate-binding, are different. In particular, Arg65 and Met9 were found to form a unique interaction with O-4 of d-fructose and d-tagatose. The whole cells with recombinant sDTE showed a higher bioconversion rate of 42.5% in a fed-batch bioconversion using d-fructose as a substrate, corresponding to a production of 476 g L⁻¹d-allulose. These results suggest that sDTE is a potential industrial biocatalyst for the production of d-allulose in fed-batch mode.

Sinorhizobium sp. d-tagatose 3-epimerase (sDTE) catalyzes the conversion of d-tagatose to d-sorbose.

Isomerases and epimerases for biotransformation of pentoses

Pentoses represent monosaccharides with five carbon atoms. They are organized into two main groups, aldopentoses and ketopentoses. There are eight aldopentoses and four ketopentoses and each ketopentose corresponds to two aldopentoses. Only D-xylose, D-ribose, and L-arabinose are natural sugars, but others belong to rare sugars that occur in very small quantities in nature. Recently, rare pentoses attract much attention because of their great potentials for commercial applications, especially as precursors of many important medical drugs. Pentoses Izumoring strategy provides a complete enzymatic approach to link all pentoses using four types of enzymes, including ketose 3-epimerases, aldose-ketose isomerases, polyol dehydrogenases, and aldose reductases. At least 10 types of epimerases and isomerases have been used for biotransformation of all aldopentoses and ketopentoses, and these enzymes are reviewed in detail in this article.

Thermostability Improvement of the d-Allulose 3-Epimerase from Dorea sp. CAG317 by Site-Directed Mutagenesis at the Interface Regions

d-Allulose is a low-calorie sweetener and has broad applications in the food, cosmetics, and pharmaceutical industries. Recently, most studies focus on d-allulose production from d-fructose by d-allulose 3-epimerase (DAEase). However, the major blocker of industrial production of d-allulose is the poor thermostability. In this study, site-directed mutagenesis at the interface regions of Dorea sp. DAEase was carried out, and the F154Y/E191D/I193F mutation was obtained. The mutant protein displayed much higher thermostability, with a t_1/2 value of 20.47 h (50 °C) and a T_m value of 74.18 °C. Compared with the wild-type DAEase, the t_1/2 value at 50 °C increased by 5.4-fold, and the T_m value increased by 17.54 °C. In the d-allulose production from 500 g/L d-fructose, 148.2 g/L d-allulose could be obtained by F154Y/E191D/I193F mutant protein. The results suggest that site-directed mutagenesis at the interface regions is an efficient approach for improving the thermostability of DAEase.

Development of a thermo-stable and recyclable magnetic nanobiocatalyst for bioprocessing of fruit processing residues and D-allulose synthesis

The aim of the study was to covalently immobilize Smt3-D-psicose 3-epimerase onto functionalized iron oxide magnetic nanoparticles. After immobilization, K_m of the immobilized enzyme increased, however, V_max was nearly the same as that of its free form, indicating that immobilization has no detrimental effects on its catalytic output. The covalent immobilization caused a reduction in the deactivation rate constant (k_d) values leading to 4-5 fold enhancement in its half-life at 50-65°C, indicating significant thermal stability of the iron-enzyme nanobioconjugate. The immobilized enzyme showed excellent storage stability by losing only 20% activity even after 60days of storage at 4°C. The immobilized enzyme retained up to 90% of its initial activity even after 10 consecutive cycles of catalyzing D-fructose epimerization reactions. Thus, after immobilization the enzyme exhibited remarkable improvements in thermal tolerance, storage stability and recycling efficiency, useful for development of industrially exploitable process for D-allulose production.

High-level intra- and extra-cellular production of d-psicose 3-epimerase via a modified xylose-inducible expression system in Bacillus subtilis

d-Psicose 3-epimerase (DPEase) converts d-fructose into d-psicose which exists in nature in limited quantities and has key physiological functions. In this study, RDPE (DPEase from Ruminococcus sp. 5_1_39BFAA) was successfully constitutively expressed in Bacillus subtilis, which is the first report of its kind. Three sugar-inducible promoters were compared, and the xylose-inducible promoter PxylA was proved to be the most efficient for RDPE production. Based on the analysis of the inducer concentration and RDPE expression, we surmised that there was an extremely close correlation between the intracellular RDPE expression and xylose accumulation level. Subsequently, after the metabolic pathway of xylose was blocked by deletion of xylAB, the intra- and extra-cellular RDPE expression was significantly enhanced. Meanwhile, the optimal xylose induction concentration was reduced from 4.0 to 0.5 %. Eventually, the secretion level of RDPE reached 95 U/mL and 2.6 g/L in a 7.5-L fermentor with the fed-batch fermentation, which is the highest production of DPEase by a microbe to date.

Purification and characterization of d-allulose 3-epimerase derived from Arthrobacter globiformis M30, a GRAS microorganism

An enzyme that catalyzes C-3 epimerization between d-fructose and d-allulose was found in Arthrobacter globiformis strain M30. Arthrobacter species have long been used in the food industry and are well-known for their high degree of safety. The enzyme was purified by ion exchange and hydrophobic interaction chromatographies and characterized as a d-allulose 3-epimerase (d-AE). The molecular weight of the purified enzyme was estimated to be 128 kDa with four identical subunits. The enzyme showed maximal activity and thermostability in the presence of Mg²⁺. The optimal pH and temperature for enzymatic activity were 7.0-8.0 and 70°C, respectively. The enzyme was immobilized to ion exchange resin whereupon it was stable for longer periods than the free enzyme when stored at below 10°C. In the column reaction, the enzyme activity also maintained stability for more than 4 months. Under these conditions, 215 kg of d-allulose produced per liter immobilized enzyme, and this was the highest production yield of d-allulose reported so far. These highly stable properties suggest that this enzyme represents an ideal candidate for the industrial production of d-allulose.