Improvement in the thermostability of D-psicose 3-epimerase from Agrobacterium tumefaciens by random and site-directed mutagenesis

Production, purification, characterization, and safety evaluation of constructed recombinant D-psicose 3-epimerase

BACKGROUND

D-psicose 3-epimerase (DPEase) is a potential catalytic enzyme for D-psicose production. D-psicose, also known as D-allulose, is a low-calorie sweetener that has gained considerable attention as a healthy alternative sweetener due to its notable physicochemical properties. This research focused on an in-depth investigation of the expression of the constructed DPEase gene from Agrobacterium tumefaciens in Escherichia coli for D-psicose synthesis. Experimentally, this research created the recombinant enzyme, explored the optimization of gene expression systems and protein purification strategies, investigated the enzymatic characterization, and then optimized the D-psicose production. Finally, the produced D-psicose syrup underwent acute toxicity evaluation to provide scientific evidence supporting its safety.

RESULTS

The optimization of DPEase expression involved the utilization of Mn2+ as a cofactor, fine-tuning isopropyl β-D-1-thiogalactopyranoside induction, and controlling the induction temperature. The purification process was strategically designed by a nickel column and an elution buffer containing 200 mM imidazole, resulting in purified DPEase with a notable 21.03-fold increase in specific activity compared to the crude extract. The optimum D-psicose conversion conditions were at pH 7.5 and 55 °C with a final concentration of 10 mM Mn2+ addition using purified DPEase to achieve the highest D-psicose concentration of 5.60% (w/v) using 25% (w/v) of fructose concentration with a conversion rate of 22.42%. Kinetic parameters of the purified DPEase were Vmax and Km values of 28.01 mM/min and 110 mM, respectively, which demonstrated the high substrate affinity and efficiency of DPEase conversion by the binding site of the fructose-DPEase-Mn2+ structure. Strategies for maintaining stability of DPEase activity were glycerol addition and storage at -20 °C. Based on the results from the acute toxicity study, there was no toxicity to rats, supporting the safety of the mixed D-fructose-D-psicose syrup produced using recombinant DPEase.

CONCLUSIONS

These findings have direct and practical implications for the industrial-scale production of D-psicose, a valuable rare sugar with a broad range of applications in the food and pharmaceutical industries. This research should advance the understanding of DPEase biocatalysis and offers a roadmap for the successful scale-up production of rare sugars, opening new avenues for their utilization in various industrial processes.

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.

Enhanced Biosynthesis of d-Allulose from a d-Xylose-Methanol Mixture and Its Self-Inductive Detoxification by Using Antisense RNAs in Escherichia coli

d-Allulose, a C-3 epimer of d-fructose, has great market potential in food, healthcare, and medicine due to its excellent biochemical and physiological properties. Microbial fermentation for d-allulose production is being developed, which contributes to cost savings and environmental protection. A novel metabolic pathway for the biosynthesis of d-allulose from a d-xylose-methanol mixture has shown potential for industrial application. In this study, an artificial antisense RNA (asRNA) was introduced into engineered Escherichia coli to diminish the flow of pentose phosphate (PP) pathway, while the UDP-glucose-4-epimerase (GalE) was knocked out to prevent the synthesis of byproducts. As a result, the d-allulose yield on d-xylose was increased by 35.1%. Then, we designed a d-xylose-sensitive translation control system to regulate the expression of the formaldehyde detoxification operon (FrmRAB), achieving self-inductive detoxification by cells. Finally, fed-batch fermentation was carried out to improve the productivity of the cell factory. The d-allulose titer reached 98.6 mM, with a yield of 0.615 mM/mM on d-xylose and a productivity of 0.969 mM/h.

Enhancing the stability of a novel D-allulose 3-epimerase from Ruminococcus sp. CAG55 by interface interaction engineering and terminally attached a self-assembling peptide

D-allulose, a highly desirable sugar substitute, is primarily produced using the D-allulose 3-epimerase (DAE). However, the availability of usable DAE enzymes is limited. In this study, we discovered and engineered a novel DAE Rum55, derived from a human gut bacterium Ruminococcus sp. CAG55. The activity of Rum55 was strictly dependent on the presence of Co2+, and it exhibited an equilibrium conversion rate of 30.6 % and a half-life of 4.5 h at 50 °C. To enhance its performance, we engineered the interface interaction of Rum55 to stabilize its tetramer structure, and the best variant E268R was then attached with a self-assembling peptide to form active enzyme aggregates as carrier-free immobilization. The half-life of the best variant E268R-EKL16 at 50 °C was dramatically increased 30-fold to 135.3 h, and it maintained 90 % of its activity after 13 consecutive reaction cycles. Additionally, we identified that metal ions played a key role in stabilizing the tetramer structure of Rum55, and the dependence on metal ions for E268R-EKL16 was significantly reduced. This study provides a useful route for improving the thermostability of DAEs, opening up new possibilities for the industrial production 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.

Growth-Coupled Evolutionary Pressure Improving Epimerases for D-Allulose Biosynthesis Using a Biosensor-Assisted In Vivo Selection Platform

Fast screening strategies that enable high-throughput evaluation and identification of desired variants from diversified enzyme libraries are crucial to tailoring biocatalysts for the synthesis of D-allulose, which is currently limited by the poor catalytic performance of ketose 3-epimerases (KEases). Here, the study designs a minimally equipment-dependent, high-throughput, and growth-coupled in vivo screening platform founded on a redesigned D-allulose-dependent biosensor system. The genetic elements modulating regulator PsiR expression levels undergo systematic optimization to improve the growth-responsive dynamic range of the biosensor, which presents ≈30-fold facilitated growth optical density with a high signal-to-noise ratio (1.52 to 0.05) toward D-allulose concentrations from 0 to 100 mm. Structural analysis and evolutionary conservation analysis of Agrobacterium sp. SUL3 D-allulose 3-epimerase (ADAE) reveal a highly conserved catalytic active site and variable hydrophobic pocket, which together regulate substrate recognition. Structure-guided rational design and directed evolution are implemented using the growth-coupled in vivo screening platform to reprogram ADAE, in which a mutant M42 (P38N/V102A/Y201L/S207N/I251R) is identified with a 6.28-fold enhancement of catalytic activity and significantly improved thermostability with a 2.5-fold increase of the half-life at 60 °C. The research demonstrates that biosensor-assisted growth-coupled evolutionary pressure combined with structure-guided rational design provides a universal route for engineering KEases.

Turning waste into treasure: Biosynthesis of value-added 2-O-α-glucosyl glycerol and d-allulose from waste cane molasses through an in vitro synthetic biology platform

The efficient and economical conversion of agricultural waste into glycosides and rare sugars is challenging. Herein, an in vitro synthetic bienzyme system consisting of sucrose phosphorylase and d-allulose 3-epimerase was constructed to produce 2-O-α-glucosyl glycerol and d-allulose from cane molasses. Lactic acid in the cane molasses significantly induced sucrose phosphorylase to hydrolyze sucrose instead of glycosylation. Notably, lactic acid significantly inhibited the catalytic performance of d-allulose 3-epimerase only in the presence of Na+ and K+, with an inhibition rate of 75%. After removing lactic acid and metal ions, 116 g/L 2-O-α-glucosyl glycerol and 51 g/L d-allulose were synthesized from 500 mM sucrose in the treated cane molasses with a sucrose consumption rate of 97%. Our findings offer an economically efficient and environmentally friendly pathway for the industrial production of glycosides and rare sugars from food industry waste.

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.

The Characterization of a Novel D-allulose 3-Epimerase from Blautia produca and Its Application in D-allulose Production

D-allulose is a natural rare sugar with important physiological properties that is used in food, health care items, and even the pharmaceutical industry. In the current study, a novel D-allulose 3-epimerase gene (Bp-DAE) from the probiotic strain Blautia produca was discovered for the production and characterization of an enzyme known as Bp-DAE that can epimerize D-fructose into D-allulose. Bp-DAE was strictly dependent on metals (Mn²⁺ and Co²⁺), and the addition of 1 mM of Mn²⁺ could enhance the half-life of Bp-DAE at 55 °C from 60 to 180 min. It exhibited optimal activity in a pH of 8 and 55 °C, and the K_m values of Bp-DAE for the different substrates D-fructose and D-allulose were 235.7 and 150.7 mM, respectively. Bp-DAE was used for the transformation from 500 g/L D-fructose to 150 g/L D-allulose and exhibited a 30% of conversion yield during biotransformation. Furthermore, it was possible to employ the food-grade microbial species Bacillus subtilis for the production of D-allulose using a technique of whole-cell catalysis to circumvent the laborious process of enzyme purification and to obtain a more stable biocatalyst. This method also yields a 30% conversion yield.

Boosting the Heterologous Expression of d-Allulose 3-Epimerase in Bacillus subtilis through Protein Engineering and Catabolite-Responsive Element Box Engineering

As a natural sweetener with low calories and various physiological activities, d-allulose has drawn worldwide attention. Currently, d-allulose 3-epimerase (DAEase) is mainly used to catalyze the epimerization of d-fructose to d-allulose. Therefore, it is quite necessary to enhance the food-grade expression of DAEase to meet the surging market demand for d-allulose. In this study, initially, the promising variant H207L/D281G/C289R of Clostridium cellulolyticum H10 DAEase (CcDAEase) was generated by protein engineering, the specific activity and the T_1/2 of which were 2.24-fold and 13.45-fold those of the CcDAEase wild type at 60 °C, respectively. After that, PamyE was determined as the optimal promoter for the recombinant expression of CcDAEase in Bacillus subtilis, and catabolite-responsive element (CRE) box engineering was further performed to eliminate the carbon catabolite repression (CCR) effect. Lastly, high-density fermentation was carried out and the final activity peaked at 4971.5 U mL^-1, which is the highest expression level and could effectively promote the industrial production of DAEase. This research provides a theoretical basis and technical support for the molecular modification of DAEase and its efficient fermentation preparation.

Structural Analysis and Construction of a Thermostable Antifungal Chitinase

Chitin is a biopolymer of N-acetyl-d-glucosamine with β-1,4-bond and is the main component of arthropod exoskeletons and the cell walls of many fungi. Chitinase (EC 3.2.1.14) is an enzyme that hydrolyzes the β-1,4-bond in chitin and degrades chitin into oligomers. It has been found in a wide range of organisms. Chitinase from Gazyumaru (Ficus microcarpa) latex exhibits antifungal activity by degrading chitin in the cell wall of fungi and is expected to be used in medical and agricultural fields. However, the enzyme's thermostability is an important factor; chitinase is not thermostable enough to maintain its activity under the actual application conditions. In addition to the fact that thermostable chitinases exhibiting antifungal activity can be used under various conditions, they have some advantages for the production process and long-term preservation, which are highly demanded in industrial use. We solved the crystal structure of chitinase to explore the target sites to improve its thermostability. We rationally introduced proline residues, a disulfide bond, and salt bridges in the chitinase using protein-engineering methods based on the crystal structure and sequence alignment among other chitinases. As a result, we successfully constructed the thermostable mutant chitinases rationally with high antifungal and specific activities. The results provide a useful strategy to enhance the thermostability of this enzyme family. IMPORTANCE We solved the crystal structure of the chitinase from Gazyumaru (Ficus microcarpa) latex exhibiting antifungal activity. Furthermore, we demonstrated that the thermostable mutant enzyme with a melting temperature (T_m) 6.9°C higher than wild type (WT) and a half-life at 60°C that is 15 times longer than WT was constructed through 10 amino acid substitutions, including 5 proline residues substitutions, making disulfide bonding, and building a salt bridge network in the enzyme. These mutations do not affect its high antifungal activity and chitinase activity, and the principle for the construction of the thermostable chitinase was well explained by its crystal structure. Our results provide a useful strategy to enhance the thermostability of this enzyme family and to use the thermostable mutant as a seed for antifungal agents for practical use.

Computer-Aided Targeted Mutagenesis of Thermoclostridium caenicola d-Allulose 3-Epimerase for Improved Thermostability

d-Allulose 3-epimerase (DAEase) is a key enzyme in d-allulose bioproduction. DAEase from Thermoclostridium caenicola suffers from poor thermostability, hampering its large-scale applications in industry. In this study, mutants A70P, G107P, F155Y, and D162T with increased melting point temperature (T_m) were obtained by targeted mutagenesis based on the calculation of the free energy of folding. The optimal single-point mutant G107P showed 11.08 h, 5, and 5.70 °C increases in the values of half-life (t_1/2) at 60 °C, the optimum temperature (T_opt), and T_m, respectively. Beneficial mutations were combined by ordered recombination mutagenesis, and the combinational mutant Var3 (G107P/F155Y/D162T/A70P) was generated with ΔT_opt of 10 °C and ΔT_m of 12.25 °C. Its t_1/2 value at 65 °C was more than 140 times higher than that of the wild-type enzyme. Molecular dynamics simulations and homology modeling analysis indicated that the enhanced overall rigidity, increased hydrogen bonds between subunits, and redistributed surface electrostatic charges might be responsible for the improved thermostability of the mutant Var3.

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.

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.

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.

Improving the enzyme property of D-allulose 3-epimerase from a thermophilic organism of Halanaerobium congolense through rational design

The rare sugar d-allulose is an attractive sucrose substitute due to its sweetness and ultra-low caloric value. It can be produced from D-fructose using d-allulose 3-epimerase (DAE) as the biocatalyst. However, most of the reported DAEs show low catalytic efficiency and poor thermostability, which limited their further use in food industrial. Here, a putative d-allulose 3-epimerase from a thermophilic organism of Halanaerobium congolense (HcDAE) was characterized, showing optimal activity at pH 8.0 and 70 °C in the presence of Mg²⁺. Saturation mutagenesis of Y7, C66, and I108, the putative residues responsible for substrate recognition at the O-4, -5, and -6 atoms of D-fructose was performed, and it yielded the triple mutant Y7H/C66L/I108A with improved activity toward D-fructose (345 % of wild-type enzyme). The combined mutant Y7H/C66L/I108A/R156C/K260C exhibited a half-half (t_1/2) of 5.2 h at 70 °C and an increase of the T_m value by 6.5 °C due to the introduction of disulfide bridges between intersubunit with increased interface interactions. The results indicate that mutants could be used as industrial biocatalysts for d-allulose production.

An integrative approach to improving the biocatalytic reactions of whole cells expressing recombinant enzymes

Biotransformation is a selective, stereospecific, efficient, and environment friendly method, compared to chemical synthesis, and a feasible tool for industrial and pharmaceutical applications. The design of biocatalysts using enzyme engineering and metabolic engineering tools has been widely reviewed. However, less importance has been given to the biocatalytic reaction of whole cells expressing recombinant enzymes. Along with the remarkable development of biotechnology tools, a variety of techniques have been applied to improve the biocatalytic reaction of whole cell biotransformation. In this review, techniques related to the biocatalytic reaction are examined, reorganized, and summarized via an integrative approach. Moreover, equilibrium-shifted biotransformation is reviewed for the first time.

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.

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.

Fusion and secretory expression of an exo-inulinase and a d-allulose 3-epimerase to produce d-allulose syrup from inulin

BACKGROUND

This study developed a feasible catalytic method for d-allulose syrup production using a fusion enzyme, either in free or immobilized form, through hydrolysis of inulin extracted from Jerusalem artichoke tubers.

RESULTS

d-Allulose 3-epimerase (DAE) was actively expressed in secretory form by fusing with the extracellular exo-inulinase CSCA in Escherichia coli BL21 (DE3). The best linker ligating the two enzymes was a flexible peptide containing 12 residues (GSAGSAAGSGEF). At 55 °C and pH 8.0, and as with the addition of 1 mmol L^-1 Mn²⁺ , the CSCA-linkerE-DAE fusion enzyme obtained through high cell-density cultivation displayed a maximal exo-inulinase activity of 21.8 U mg^-1 and resulted in a yield of 6.3 g L^-1 d-allulose and 39.2 g L^-1 d-fructose using 60 g L^-1 inulin as the raw material. Catechol-modified alginate with titanium ions (Alg(Ti)PDA) was found to be a promising immobilization material for the fusion enzyme. After conversion for 8 days, the Alg(Ti)PDA-immobilized CSCA-linkerE-DAE (8 U g^-1 ) completed 24 reaction cycles and retained over 80% of its original activity. Each reaction obtained an average of 19.8 g L^-1 d-allulose and 32.7 g L^-1 D-fructose from 60 g L^-1 inulin.

CONCLUSION

This study shed light on a feasible and cost-effective approach for the production of syrup containing d-allulose and D-fructose with inulin as the raw material via the use of a CSCA and DAE fusion enzyme. This syrup is of added value as a functional sweetener. © 2020 Society of Chemical Industry.

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.

Purification and Characterization of D-psicose 3 Epimerase (DPEase) From <i>Escherichia coli</i> BL21 (DE3) pET21b <i>dpe</i>

BACKGROUND AND OBJECTIVE

The DPEase enzyme from Agrobacterium tumefaciens is more efficient and has a high activity in D-fructose. The dpe gene has been successfully cloned to Escherichia coli BL21 (DE3) pET-21b dpe but the enzyme has not been purified and its character is unknown. The intent of this study was to purify and assign of DPEase enzyme by recombinant E. coli.

MATERIALS AND METHODS

The enzyme was clarified by affinity chromatography and then characterized by following pH, temperature, co-factor parameters. Analysis of molecular weight proteins was done by SDS-PAGE.

RESULTS

Through purification, the purified DPEase activity was increased 1,01 times than crude and with 84.2% of yield. The DPEase had an the maximum temperature is 40°C and pH was 8.5. The presence of Mg2+, Mo2+, Cu2+, Ca2+ and Zn2+ inhibited the activity of the enzyme while of Co2+, Mn2+, Fe2+, Ni2+ enhanced the activity. Estimation of molecular weight through SDS-PAGE revealed that weight of DPEase was 32 kDa.

CONCLUSION

Purified DPease enzymes shows clear bands that demonstrate successful purification using affinity chromatography. It is expected that after pure enzymes are obtained the character of the enzymes working will be maximized.

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.

Structure-guided protein engineering increases enzymatic activities of the SGNH family esterases

BACKGROUND

Esterases and lipases hydrolyze short-chain esters and long-chain triglycerides, respectively, and therefore play essential roles in the synthesis and decomposition of ester bonds in the pharmaceutical and food industries. Many SGNH family esterases share high similarity in sequences. However, they have distinct enzymatic activities toward the same substrates. Due to a lack of structural information, the detailed catalytic mechanisms of these esterases remain barely investigated.

RESULTS

In this study, we identified two SGNH family esterases, CrmE10 and AlinE4, from marine bacteria with significantly different preferences for pH, temperature, metal ion, and organic solvent tolerance despite high sequence similarity. The crystal structures of these two esterases, including wild type and mutants, were determined to high resolutions ranging from 1.18 Å to 2.24 Å. Both CrmE10 and AlinE4 were composed of five β-strands and nine α-helices, which formed one compact N-terminal α/β globular domain and one extended C-terminal domain. The aspartic residues (D178 in CrmE10/D162 in AlinE4) destabilized the conformations of the catalytic triad (Ser-Asp-His) in both esterases, and the metal ion Cd²⁺ might reduce enzymatic activity by blocking proton transfer or substrate binding. CrmE10 and AlinE4 showed distinctly different electrostatic surface potentials, despite the similar atomic architectures and a similar swap catalytic mechanism. When five negatively charged residues (Asp or Glu) were mutated to residue Lys, CrmE10 obtained elevated alkaline adaptability and significantly increased the enzymatic activity from 0 to 20% at pH 10.5. Also, CrmE10 mutants exhibited dramatic change for enzymatic properties when compared with the wide-type enzyme.

CONCLUSIONS

These findings offer a perspective for understanding the catalytic mechanism of different esterases and might facilitate the industrial biocatalytic applications.

Biosensor-based enzyme engineering approach applied to psicose biosynthesis

Bioproduction of chemical compounds is of great interest for modern industries, as it reduces their production costs and ecological impact. With the use of synthetic biology, metabolic engineering and enzyme engineering tools, the yield of production can be improved to reach mass production and cost-effectiveness expectations. In this study, we explore the bioproduction of D-psicose, also known as D-allulose, a rare non-toxic sugar and a sweetener present in nature in low amounts. D-psicose has interesting properties and seemingly the ability to fight against obesity and type 2 diabetes. We developed a biosensor-based enzyme screening approach as a tool for enzyme selection that we benchmarked with the Clostridium cellulolyticum D-psicose 3-epimerase for the production of D-psicose from D-fructose. For this purpose, we constructed and characterized seven psicose responsive biosensors based on previously uncharacterized transcription factors and either their predicted promoters or an engineered promoter. In order to standardize our system, we created the Universal Biosensor Chassis, a construct with a highly modular architecture that allows rapid engineering of any transcription factor-based biosensor. Among the seven biosensors, we chose the one displaying the most linear behavior and the highest increase in fluorescence fold change. Next, we generated a library of D-psicose 3-epimerase mutants by error-prone PCR and screened it using the biosensor to select gain of function enzyme mutants, thus demonstrating the framework's efficiency.

Engineering a thermostable version of D-allulose 3-epimerase from Rhodopirellula baltica via site-directed mutagenesis based on B-factors analysis

D-allulose has received increasing attention due to its excellent physiological properties and commercial potential. The D-allulose 3-epimerase from Rhodopirellula baltica (RbDAEase) catalyzes the conversion of D-fructose to D-allulose. However, its poor thermostability has hampered its industrial application. Site-directed mutagenesis based on homologous structures in which the residuals on high flexible regions were substituted according to B-factors analysis, is an effective way to improve the thermostability and robustness of an enzyme. RbDAEase showed substrate specificity toward D-allulose with a K_m of 58.57 mM and k_cat of 1849.43 min^-1. It showed a melting temperature (T_m) of 45.7 °C and half-life (t_1/2) of 52.3 min at pH 8.0, 60 °C with 1 mM Mn²⁺. The Site-directed mutation L144 F strengthened the thermostability to a Δt_1/2 of 50.4 min, ΔTm of 12.6 °C, and ΔT₅₀⁶⁰ of 22 °C. It also improved the conversion rate to 28.6%. Structural analysis reveals that a new hydrophobic interaction was formed by the mutation. Thus, site-directed mutagenesis based on B-factors analysis would be an efficient strategy to enhance the thermostability of designed ketose 3-epimerases.

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.

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.

Production of d-allulose from d-glucose by Escherichia coli transformant cells co-expressing d-glucose isomerase and d-psicose 3-epimerase genes

BACKGROUND

d-Allulose is a novel and low-calorie rare monosaccharide that is a C-3 epimer of d-fructose. Because of its excellent physiological properties and commercial potential, d-allulose has attracted researchers' interests. Based on the Izumoring strategy, d-allulose is converted from d-fructose by d-psicose 3-epimerase (DPEase), while d-fructose is converted from d-glucose by d-glucose isomerase (GIase). In this study, we created a cellular system capable of converting d-glucose to d-allulose in a one-step process that co-expressed the GIase from Acidothermus cellulolyticus and the DPEase from Dorea sp. CAG.

RESULTS

The co-expression plasmid pETDuet-Dosp-DPE/Acce-GI was generated and transformed into Escherichia coli BL21(DE3) cells. The recombinant co-expression cells exhibited maximum catalytic activity at pH 6.5 and 75 °C. These cells were thermostable at less than 60 °C. The addition of Co²⁺ significantly increased the catalytic activity by 10.8-fold. When the reaction equilibrium was reached, the ratio of d-glucose, d-fructose and d-allulose was approximately 6.5:7:3, respectively.

CONCLUSION

A recombinant co-expression strain that catalysed the bioconversion of d-allulose from d-glucose in a one-step process was created and characterised. When adding 500 g L^-1 d-glucose as a substrate, 204.3 g L^-1 d-fructose and 89.1 g L^-1 d-allulose were produced. © 2016 Society of Chemical Industry.

Improving the Thermostability and Catalytic Efficiency of the d-Psicose 3-Epimerase from Clostridium bolteae ATCC BAA-613 Using Site-Directed Mutagenesis

d-Psicose is a highly valuable rare sugar because of its excellent physiological properties and commercial potential. d-Psicose 3-epimerase (DPEase) is the key enzyme catalyzing the isomerization of d-fructose to d-psicose. However, the poor thermostability and low catalytic efficiency are serious constraints on industrial application. To address these issues, site-directed mutagenesis of Tyr68 and Gly109 of the Clostridium bolteae DPEase was performed. Compared with the wild-type enzyme, the Y68I variant displayed the highest substrate-binding affinity and catalytic efficiency, and the G109P variant showed the highest thermostability. Furthermore, the double-site Y68I/G109P variant was generated and exhibited excellent enzyme characteristics. The Km value decreased by 17.9%; the kcat/Km increased by 1.2-fold; the t1/2 increased from 156 to 260 min; and the melting temperature (Tm) increased by 2.4 °C. Moreover, Co(2+) enhanced the thermostability significantly, including the t1/2 and Tm values. All of these indicated that the Y68I/G109P variant would be appropriate for the industrial production of d-psicose.

Biochemical characterization of a D-psicose 3-epimerase from Treponema primitia ZAS-1 and its application on enzymatic production of D-psicose

BACKGROUND

The rare sugar D-psicose is a hexoketose monosaccharide and a C-3 epimer of D-fructose. D-Psicose is a novel functional sweetener with 70% of the sweetness but only 0.3% of the energy content of sucrose. Generally, the industrial production of D-psicose involves a bioconversion from D-fructose induced by ketose 3-epimerases.

RESULTS

The D-psicose 3-epimerase (DPEase) gene from Treponema primitia ZAS-1 (Trpr-DPEase) was cloned and overexpressed in Escherichia coli BL21 (DE3). The recombinant enzyme was purified with a molecular mass of 33 kDa. Trpr-DPEase exhibited optimal activity at pH 8.0 and 70 °C and was sensitive to temperature, with relative thermal stability below 50 °C. It was strictly metal-dependent and displayed maximum catalytic activity with 450 µmol L(-1) Co(2+). The Km values of the enzyme for D-psicose and D-fructose were 209 and 279 mmol L(-1) respectively. The D-psicose/D-fructose equilibrium ratio of Trpr-DPEase was 28:72.

CONCLUSION

A novel DPEase from T. primitia ZAS-1 was characterized that could catalyze the formation of D-psicose from D-fructose. D-Psicose was produced at a yield of 137.5 g L(-1) from 500 g L(-1) D-fructose, suggesting that Trpr-DPEase might be appropriate for the industrial production of D-psicose.

Production of d-psicose from d-fructose by whole recombinant cells with high-level expression of d-psicose 3-epimerase from Agrobacterium tumefaciens

The specific activity of recombinant Escherichia coli cells expressing the double-site variant (I33L-S213C) d-psicose 3-epimerase (DPEase) from Agrobacterium tumefaciens was highest at 24 h of cultivation time in Terrific Broth (TB) medium among the media tested. The contents of crude protein and DPEase in recombinant cells at 24 h were 37.0 and 8.6% (w/w), respectively, indicating that the enzyme was highly expressed. The reaction conditions for the production of d-psicose from d-fructose by whole recombinant cells with the highest specific activity were optimal at 60°C, pH 8.5, 4 g/l cells, and 700 g/l d-fructose. Under these conditions, whole recombinant cells produced 230 g/l d-psicose after 40 min, with a conversion yield of 33% (w/w), a volumetric productivity of 345 g/l/h, and a specific productivity of 86.2 g/g/h. These are the highest conversion yield and volumetric and specific productivities of d-psicose from d-fructose by cells reported thus far.

Structure-based engineering of alkaline α-amylase from alkaliphilic Alkalimonas amylolytica for improved thermostability

This study aimed to improve the thermostability of alkaline α-amylase from Alkalimonas amylolytica through structure-based rational design and systems engineering of its catalytic domain. Separate engineering strategies were used to increase alkaline α-amylase thermostability: (1) replace histidine residues with leucine to stabilize the least similar region in domain B, (2) change residues (glycine, proline, and glutamine) to stabilize the highly conserved α-helices in domain A, and (3) decrease the free energy of folding predicted by the PoPMuSiC program to stabilize the overall protein structure. A total of 15 single-site mutants were obtained, and four mutants - H209L, Q226V, N302W, and P477V - showed enhanced thermostability. Combinational mutations were subsequently introduced, and the best mutant was triple mutant H209L/Q226V/P477V. Its half-life at 60 °C was 3.8-fold of that of the wild type and displayed a 3.2 °C increase in melting temperature compared with that of the wild type. Interestingly, other biochemical properties of this mutant also improved: the optimum temperature increased from 50 °C to 55 °C, the optimum pH shifted from 9.5 to 10.0, the stable pH range expanded from 7.0-11.0 to 6.0-12.0, the specific activity increased by 24 %, and the catalytic efficiency (k cat/K m) increased from 1.8×10(4) to 3.5 × 10(4) l/(g min). Finally, the mechanisms responsible for the increased thermostability were analyzed through comparative analysis of structure models. The structure-based rational design and systems engineering strategies in this study may also improve the thermostability of other industrial enzymes.

Characterization of a D-psicose-producing enzyme, D-psicose 3-epimerase, from Clostridium sp

The gene coding for D-psicose 3-epimerase (DPEase) from Clostridium sp. BNL1100 was cloned and expressed in Escherichia coli. The recombinant enzyme was purified by Ni-affinity chromatography. It was a metal-dependent enzyme and required Co(2+) as optimum cofactor. It displayed catalytic activity maximally at pH 8.0 and 65 °C (as measured over 5 min). The optimum substrate was D-psicose, and the K m, turnover number (k cat), and catalytic efficiency (k cat/K m) for D-psicose were 227 mM, 32,185 min(-1), and 141 min(-1 )mM(-1), respectively. At pH 8.0 and 55 °C, 120 g D-psicose l(-1) was produced from 500 g D-fructose l(-1) after 5 h.

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

The noncharacterized protein CLOSCI_02528 from Clostridium scindens ATCC 35704 was characterized as D-psicose 3-epimerase. The enzyme showed maximum activity at pH 7.5 and 60°C. The half-life of the enzyme at 50°C was 108 min, suggesting the enzyme was relatively thermostable. It was strictly metal-dependent and required Mn²⁺ as optimum cofactor for activity. In addition, Mn²⁺ improved the structural stability during both heat- and urea-induced unfolding. Using circular dichroism measurements, the apparent melting temperature (T_m) and the urea midtransition concentration (C_m) of metal-free enzyme were 64.4°C and 2.68 M. By comparison, the Mn²⁺-bound enzyme showed higher T_m and C_m with 67.3°C and 5.09 M. The Michaelis-Menten constant (K_m), turnover number (k_cat), and catalytic efficiency (k_cat/K_m) values for substrate D-psicose were estimated to be 28.3 mM, 1826.8 s⁻¹, and 64.5 mM⁻¹ s⁻¹, respectively. The enzyme could effectively produce D-psicose from D-fructose with the turnover ratio of 28%.

From protein engineering to immobilization: promising strategies for the upgrade of industrial enzymes

Enzymes found in nature have been exploited in industry due to their inherent catalytic properties in complex chemical processes under mild experimental and environmental conditions. The desired industrial goal is often difficult to achieve using the native form of the enzyme. Recent developments in protein engineering have revolutionized the development of commercially available enzymes into better industrial catalysts. Protein engineering aims at modifying the sequence of a protein, and hence its structure, to create enzymes with improved functional properties such as stability, specific activity, inhibition by reaction products, and selectivity towards non-natural substrates. Soluble enzymes are often immobilized onto solid insoluble supports to be reused in continuous processes and to facilitate the economical recovery of the enzyme after the reaction without any significant loss to its biochemical properties. Immobilization confers considerable stability towards temperature variations and organic solvents. Multipoint and multisubunit covalent attachments of enzymes on appropriately functionalized supports via linkers provide rigidity to the immobilized enzyme structure, ultimately resulting in improved enzyme stability. Protein engineering and immobilization techniques are sequential and compatible approaches for the improvement of enzyme properties. The present review highlights and summarizes various studies that have aimed to improve the biochemical properties of industrially significant enzymes.

Reduction of abdominal fat accumulation in rats by 8-week ingestion of a newly developed sweetener made from high fructose corn syrup

Many studies have shown that ingestion of high-fructose corn syrup (HFCS) may cause an increase in body weight and abdominal fat. We recently developed a new sweetener containing rare sugars (rare sugar syrup; RSS) by slight isomerization of HFCS. Here, the functional effects of RSS on body weight and abdominal fat, and biochemical parameters in Wistar rats were examined. Rats (n=30) were randomly divided into three groups and maintained for 8-weeks on starch, starch+HFCS (50:50), and starch+RSS (50:50) diets. Rats in the Starch and HFCS groups gained significantly more body weight and abdominal fat than the RSS group. Fasting serum insulin in the RSS group was significantly lower than in the Starch and HFCS groups, although serum glucose in the HFCS and RSS groups was significantly lower than that in the Starch group. Thus, the substitution of HFCS with RSS prevents obesity induced by the consumption of HFCS.