Enzymatic approaches to rare sugar production

Optimization of fermentation conditions for whole cell catalytic synthesis of D-allulose by engineering Escherichia coli

D-allulose/D-psicose is a significant rare sugar with broad applications in the pharmaceutical, food, and other industries. In this study, we cloned the D-allulose 3-epimerase (DPEase) gene from Arthrobacter globiformis M30, using pET22b as the vector. The recombinant E. coli strain pET22b(+) was successfully constructed and expressed, providing an efficient whole-cell catalyst for converting inexpensive D-fructose into D-allulose. Subsequently, we optimized the induction and incubation conditions step by step using the single-factor method and used Lactobacillus plantarum(LAB) 217-8 to enhance the purity of D-allulose in the system. Ultimately, the BL21/pET22b(+)-E. coli strain achieved a conversion rate of up to 33.91% under optimal conditions, converting D-fructose to D-allulose. After purification, the purity of D-allulose reached 64.73%. Efficient production of D-allulose is a significant achievement, paving the way for future probiotic applications in its conversion.

Tailored Enzymes for Difructose Anhydrides: From Biosynthesis to Degradation

Difructose anhydrides (DFAs), distinctive cyclic disaccharides mainly naturally produced by heating (caramelization), serve as potential candidates of functional sugars that modern humans consume on a daily basis due to their remarkable physiological effects. This review explores the complex domain of specialized enzymes implicated in the metabolism of DFAs, covering the entire process from biosynthesis to degradation. We provide a detailed examination of the enzymes responsible for DFA formation and degradation, specifically those classified within the GH91, GH32, and GH172 glycoside hydrolase families. Furthermore, the evolutionary relationships among the related enzymes were systematically analyzed. Subsequently, the underlying enzymatic mechanisms that drive DFAs' metabolism were elucidated, and key insights into the intricate interplay between enzyme structure and function were unveiled. Additionally, innovative strategies for enzyme engineering were discussed, aimed at improving thermostability, enhancing catalytic activity, and altering catalytic function. Finally, the applications of the related enzymes were comprehensively summarized with a focus on their product yields, conversion rates, and methods for product purification. Here, the review presents a comprehensive investigation into enzymatic degradation and biosynthesis pathways of DFAs.

Enzymatic Upgrading of Biomass-Derived Aldoses to Rare Deoxy Ketoses Catalyzed by Transketolase Variants

A sustainable, convenient, scalable, one-step method for the two-carbon chain elongation of cheap and biomass-derived pentoses (l-arabinose, and 2-deoxy-d-ribose) and hexose l-rhamnose was developed to produce Cn+2 deoxy ketoses (C-7 and C-8) using transketolase, an enzyme catalyzing the quasi-irreversible transfer of a ketol group from an α-keto acid to an aldehyde. Deoxygenated ketoses - commonly obtained by chemical synthesis - were afforded through a suitable combination of both nucleophile and electrophile substrates in the presence of rationally designed TK variants. Pyruvate as nucleophile with pentose l-arabinose (C-5) as electrophile gave 1-deoxy-L-gluco-heptulose (C-7), while ß-hydroxypyruvate (HPA) as nucleophile with acceptors 2-deoxy-d-ribose (C-5) and 6-deoxy-l-mannose (l-rhamnose) (C-6) led to formation of 4-deoxy-d-altro-heptulose (C-7) and 8-deoxy-l-glycero-l-galacto-octulose (C-8), respectively. These three deoxy ketoses were easily obtained with efficient TK variants under mild conditions with complete or high substrate conversions, good to excellent yields and high diastereoselectivities. This strategy offers interesting prospects to study the biological activities of these three rare and valuable deoxy ketoses on various cellular targets.

Multienzyme Cascade Synthesis of Rare Sugars From Glycerol in Bacillus subtilis

BACKGROUND

Rare sugars are valuable and unique monosaccharides extensively utilized in the food, cosmetics, and pharmaceutical industries. Considering the high purification costs and the complex processes of enzymatic synthesis, whole-cell conversion has emerged as a significantly important alternative. The Escherichia coli strain was initially used in whole-cell synthesis of rare sugars. However, its pathogenic nature poses limitations to its widespread applications. Consequently, there is an urgent need to explore biologically safe strains for the efficient production of rare sugars.

RESULTS

In this study, the generally regarded as safe (GRAS) strain Bacillus subtilis was employed as the chassis cells to produce rare sugars via whole-cell conversion. Three genes encoding alditol oxidase (AldO), L-rhamnulose-1-phosphate aldolase (RhaD), and fructose-1-phosphatase (YqaB) involved in rare sugars biosynthesis were heterogeneously expressed in B. subtilis to convert the only substrate glycerol into rare sugars. To enhance the expression levels of the relevant enzymes in B. subtilis, different promoters for aldO, rhaD, and yqaB were investigated and optimized in this system. Under the optimized reaction conditions, the maximum total production titer was 16.96 g/L of D-allulose and D-sorbose with a conversion yield of 33.9% from glycerol. Furthermore, the engineered strain produced 26.68 g/L of D-allulose and D-sorbose through fed-batch for the whole-cell conversion, representing the highest titer from glycerol reported to date.

CONCLUSION

This study demonstrated an efficient and cost-effective method for the synthesis of rare sugars, providing a food-grade platform with the potential to meet the growing demand for rare sugars in industries.

Recent Applications and Prospects of Enzymes in Quality and Safety Control of Fermented Foods

Fermented foods have gained global attention for their unique flavor and immense health benefits. These flavor compounds and nutrients result from the metabolic activities of microorganism during fermentation. However, some unpleasant sensory characteristics and biohazard substances could also be generated in fermentation process. These quality and safety issues in fermented foods could be addressed by endogenous enzymes. In this review, the applications of enzymes in quality control of fermented foods, including texture improvement, appearance stability, aroma enhancement, and debittering, are discussed. Furthermore, the enzymes employed in eliminating biohazard compounds such as ethyl carbamate, biogenic amines, and nitrites, formed during fermentation, are reviewed. Advanced biological methods used for enhancing the enzymatic activity and stability are also summarized. This review focused on the applications and future prospects of enzymes in the improvement quality and safety qualities of fermented foods.

Tailoring the natural rare sugars D-tagatose and L-sorbose to produce novel functional carbohydrates

This multidisciplinary study details the biosynthesis of novel non-digestible oligosaccharides derived from rare sugars, achieved through transfructosylation of D-tagatose and L-sorbose by levansucrase from Bacillus subtilis CECT 39 (SacB). The characterization of these carbohydrates using NMR and molecular docking was instrumental in elucidating the catalytic mechanism and substrate preference of SacB. Tagatose-based oligosaccharides were higher in abundance than L-sorbose-based oligosaccharides, with the most representative structures being: β-D-Fru-(2→6)-β-D-Fru-(2→1)-D-Tag and β-D-Fru-(2→1)-D-Tag. In vitro studies demonstrated the resistance of tagatose-based oligosaccharides to intestinal digestion and their prebiotic properties, providing insights into their structure-function relationship. β-D-Fru-(2→1)-D-Tag was the most resistant structure to small-intestinal digestion after three hours (99.8% remained unaltered). This disaccharide and the commercial FOS clustered in similar branches, indicating comparable modulatory properties on human fecal microbiota, and exerted a higher bifidogenic effect than unmodified tagatose. The bioconversion of selected rare sugars into β-fructosylated species with a higher degree of polymerization emerges as an efficient strategy to enhance the bioavailability of these carbohydrates and promote their interaction with the gut microbiota. These findings open up new opportunities for tailoring natural rare sugars, like D-tagatose and L-sorbose, to produce novel biosynthesized carbohydrates with functional and structural properties desirable for use as emerging prebiotics and low-calorie sweeteners.

Microbial synthesis of sedoheptulose from glucose by metabolically engineered Corynebacterium glutamicum

BACKGROUND

Seven-carbon sugars, which rarely exist in nature, are the key constitutional unit of septacidin and hygromycin B in bacteria. These sugars exhibit a potential therapeutic effect for hypoglycaemia and cancer and serve as building blocks for the synthesis of C-glycosides and novel antibiotics. However, chemical and enzymatic approaches for the synthesis of seven-carbon sugars have faced challenges, such as complex reaction steps, low overall yields and high-cost feedstock, limiting their industrial-scale production.

RESULTS

In this work, we propose a strain engineering approach for synthesising sedoheptulose using glucose as sole feedstock. The gene pfkA encoding 6-phosphofructokinase in Corynebacterium glutamicum was inactivated to direct the carbon flux towards the pentose phosphate pathway in the cellular metabolic network. This genetic modification successfully enabled the synthesis of sedoheptulose from glucose. Additionally, we identified key enzymes responsible for product formation through transcriptome analysis, and their corresponding genes were overexpressed, resulting in a further 20% increase in sedoheptulose production.

CONCLUSION

We achieved a sedoheptulose concentration of 24 g/L with a yield of 0.4 g/g glucose in a 1 L fermenter, marking the highest value up to date. The produced sedoheptulose could further function as feedstock for synthesising structural seven-carbon sugars through coupling with enzymatic isomerisation, epimerisation and reduction reactions.

Discovery of a Thermostable Tagatose 4-Epimerase Powered by Structure- and Sequence-Based Protein Clustering

d-Tagatose is a highly promising functional sweetener known for its various physiological functions. In this study, a novel tagatose 4-epimerase from Thermoprotei archaeon (Thar-T4Ease), with the ability to convert d-fructose to d-tagatose, was discovered through a combination of structure similarity search and sequence-based protein clustering. The recombinant Thar-T4Ease exhibited optimal activity at pH 8.5 and 85 °C, in the presence of 1 mM Ni2+. Its kcat and kcat/Km values toward d-fructose were measured to be 248.5 min-1 and 2.117 mM-1·min-1, respectively. Notably, Thar-T4Ease exhibited remarkable thermostability, with a t1/2 value of 198 h at 80 °C. Moreover, it achieved a conversion ratio of 18.9% using 100 g/L d-fructose as the substrate. Finally, based on sequence and structure analysis, crucial residues for the catalytic activity of Thar-T4Ease were identified by molecular docking and site-directed mutagenesis. This research expands the repertoire of enzymes with C4-epimerization activity and opens up new possibilities for the cost-effective production of d-tagatose from d-fructose.

The Effect of Allulose on the Attenuation of Glucose Release from Rice in a Static In Vitro Digestion Model

Allulose is a rare sugar that provides <10% of the energy but 70% of the sweetness of sucrose. Allulose has been shown to attenuate glycemic responses to carbohydrate-containing foods in vivo. This study aimed to determine the optimal allulose dose for minimizing in vitro glucose release from rice compared to a rice control and fructose. A triphasic static in vitro digestion method was used to evaluate the in vitro digestion of a rice control compared to the co-digestion of rice with allulose (10 g, 20 g, and 40 g) and fructose (40 g). In vitro glucose release was affected by treatment (p < 0.001), time (p < 0.001), and treatment-by-time interaction (p = 0.002). Allulose (40 g) resulted in a reduction in in vitro glucose release from rice alone and rice digested with allulose (10 g), allulose (20 g), and fructose. The incremental area under the curve (iAUC) for in vitro glucose release was lower after allulose (40 g) (p = 0.005) compared to rice control and allulose (10 g) but did not differ from allulose (20 g) or fructose. This study demonstrates that allulose reduces glucose release from carbohydrates, particularly at higher doses, underscoring its potential as a food ingredient with functional benefits.

Impact of artificial sweeteners and rare sugars on the gut microbiome

Alternative sugars are often used as sugar substitutes because of their low calories and glycemic index. Recently, consumption of these sweeteners in diet foods and beverages has increased dramatically, raising concerns about their health effects. This review examines the types and characteristics of artificial sweeteners and rare sugars and analyzes their impact on the gut microbiome. In the section on artificial sweeteners, we have described the chemical structures of different sweeteners, their digestion and absorption processes, and their effects on the gut microbiota. We have also discussed the biochemical properties and production methods of rare sugars and their positive and negative effects on gut microbial communities. Finally, we have described how artificial sweeteners and rare sugars alter the gut microbiome and how these changes affect the gut environment. Our observations aim to improve our understanding regarding the potential health implications of the consumption of artificial sweeteners and low-calorie sugars.

Co-immobilization of whole cells and enzymes by covalent organic framework for biocatalysis process intensification

Co-immobilization of cells and enzymes is often essential for the cascade biocatalytic processes of industrial-scale feasibility but remains a vast challenge. Herein, we create a facile co-immobilization platform integrating enzymes and cells in covalent organic frameworks (COFs) to realize the highly efficient cascade of inulinase and E. coli for bioconversion of natural products. Enzymes can be uniformly immobilized in the COF armor, which coats on the cell surface to produce cascade biocatalysts with high efficiency, stability and recyclability. Furthermore, this one-pot in situ synthesis process facilitates a gram-scale fabrication of enzyme-cell biocatalysts, which can generate a continuous-flow device conversing inulin to D-allulose, achieving space-time yield of 161.28 g L-1 d-1 and high stability (remaining >90% initial catalytic efficiency after 7 days of continuous reaction). The created platform is applied for various cells (e.g., E. coli, Yeast) and enzymes, demonstrating excellent universality. This study paves a pathway to break the bottleneck of extra- and intracellular catalysis, creates a high-performance and customizable platform for enzyme-cell cascade biomanufacturing, and expands the scope of biocatalysis process intensification.

Identification of hyperthermophilic D-allulose 3-epimerase from Thermotoga sp. and its application as a high-performance biocatalyst for D-allulose synthesis

D-Allulose 3-epimerase (DAE) is a vital biocatalyst for the industrial synthesis of D-allulose, an ultra-low calorie rare sugar. However, limited thermostability of DAEs hinders their use at high-temperature production. In this research, hyperthermophilic TI-DAE (Tm = 98.4 ± 0.7 ℃) from Thermotoga sp. was identified via in silico screening. A comparative study of the structure and function of site-directed saturation mutagenesis mutants pinpointed the residue I100 as pivotal in maintaining the high-temperature activity and thermostability of TI-DAE. Employing TI-DAE as a biocatalyst, D-allulose was produced from D-fructose with a conversion rate of 32.5%. Moreover, TI-DAE demonstrated excellent catalytic synergy with glucose isomerase CAGI, enabling the one-step conversion of D-glucose to D-allulose with a conversion rate of 21.6%. This study offers a promising resource for the enzyme engineering of DAEs and a high-performance biocatalyst for industrial D-allulose production.

Enzymatic biosynthesis of D-galactose derivatives: Advances and perspectives

D-Galactose derivatives, including galactosyl-conjugates and galactose-upgrading compounds, provide various physiological benefits and find applications in industries such as food, cosmetics, feed, pharmaceuticals. Many research on galactose derivatives focuses on identification, characterization, development, and mechanistic aspects of their physiological function, providing opportunities and challenges for the development of practical approaches for synthesizing galactose derivatives. This study focuses on recent advancements in enzymatic biosynthesis of galactose derivatives. Various strategies including isomerization, epimerization, transgalactosylation, and phosphorylation-dephosphorylation were extensively discussed under the perspectives of thermodynamic feasibility, theoretical yield, cost-effectiveness, and by-product elimination. Specifically, the enzymatic phosphorylation-dephosphorylation cascade is a promising enzymatic synthesis route for galactose derivatives because it can overcome the thermodynamic equilibrium of isomerization and utilize cost-effective raw materials. The study also elucidates the existing challenges and future trends in enzymatic biosynthesis of galactose derivatives. Collectively, this review provides a real-time summary aimed at promoting the practical biosynthesis of galactose derivatives through enzymatic catalysis.

Regioselective Deacetylation of Peracetylated Deoxy-C-glycopyranosides by Boron Trichloride (BCl3)

A general approach for regioselective deacetylation at sugar 3-OH of peracetylated 6-deoxy-C-glucopyranosides mediated by BCl3 was developed. The approach could be extended to other sugar-derived 6-deoxy-C-glycopyranosides, such as those derived from mannose, galactose, and rhamnose, with deacetylation occurring at varied sugar hydroxyl groups, and further extended to 4-deoxy-C-glucopyranosides with deacetylation occurring at sugar 3-OH. The approach would enable access to synthetically challenging carbohydrate derivatives. A possible mechanism of the regioselectivity was proposed.

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

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effect of phosphate buffer concentration on the isomerization of galactose to rare sugars under subcritical water conditions

Galactose was treated in sodium phosphate buffer at various concentrations (0.1-500 mmol/L) under subcritical water conditions (140 °C), and the effects of the buffer concentration and reaction time (0-300 s) on the reaction behavior were evaluated. The reaction proceeded rapidly at higher buffer concentrations. Rare sugars (tagatose, talose, and sorbose) were formed from galactose by isomerization. The highest yield of the main product, tagatose, was approximately 14 % in 50 mmol/L buffer. However, the tagatose yield did not increase further with increasing buffer concentration. On the other hand, the formation of talose and sorbose was accelerated at higher buffer concentrations, with the highest yields of approximately 5 % and 12 %, respectively, in 500 mmol/L buffer. At the same time, the formation of byproducts (organic acids and colored substances) was also accelerated in high-concentration buffers. These results suggest that phosphate buffer promoted all reactions occurring under subcritical water conditions.

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

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

Highly efficient production and simultaneous purification of d-tagatose through one-pot extraction-assisted isomerization of d-galactose

A one-pot extraction-assisted d-galactose-to-d-tagatose isomerization strategy was proposed based on the selective extraction of d-tagatose by phenylborate anions. 4-Vinylphenylboronic acid was selected with high extraction efficiency and selectivity towards d-tagatose. The extracted sugars could be desorbed through a two-staged stripping process with the purity of d-tagatose significantly increased. In-situ extraction-assisted d-galactose-to-d-tagatose isomerization was implemented for the first time ever reported, and the effect of boron-to-sugar ratio (boron: sugar) was investigated. The conversion yield of d-tagatose at 60 °C increased from ∼ 39 % (boron: sugar = 0.5) to ∼ 56 % (boron: sugar = 1) but then decreased to ∼ 44 % (boron: sugar = 1.5). With temperature increased to 70 °C, the conversion yield of d-tagatose was further improved to ∼ 61 % (boron: sugar = 1.5), with the minimized formation of byproducts. Moreover, high purity (∼83 %) and concentrated d-tagatose solution (∼40 g/L) was obtained after sequential desorption. The proposed extraction-assisted isomerization strategy achieved improving the yield and purity of d-tagatose, proving its feasibility in industrial applications.

De novo artificial synthesis of hexoses from carbon dioxide

Developing artificial "CO2-sugar" platforms is meaningful for addressing challenges posed by land scarcity and climate change to the supply of dietary sugar. However, upcycling CO2 into complex polyoxygenated carbohydrates involves several major challenges, including achieving enantioselective and thermodynamically driven transformation and expanding product repertoires while reducing energy consumption. We present a versatile chemoenzymatic roadmap based on aldol condensation, iso/epimerization, and dephosphorylation reactions for asymmetric CO2 and H2 assembly into sugars with perfect stereocontrol. In particular, we developed a minimum ATP consumption and the shortest pathway for bottom-up biosynthesis of the fundamental precursor, fructose-6-phosphate, which is valuable for synthesizing structure-diverse sugars and derivatives. Engineering bottleneck-associated enzyme catalysts aided in the thermodynamically driven synthesis of several energy-dense and functional hexoses, such as glucose and D-allulose, featuring higher titer (63 mmol L-1) and CO2-product conversion rates (25 mmol C L-1 h-1) compared to established in vitro CO2-fixing pathways. This chemical-biological platform demonstrated greater carbon conversion yield than the conventional "CO2-bioresource-sugar" process and could be easily extended to precisely synthesize other high-order sugars from CO2.

Enzymatic production of rare sugars with a new mutant of cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus

Cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus (CsCE) can epimerize and isomerize lactose into epilactose and lactulose respectively. Competition between these reactions reactions has prompted the search for new enzymes to drive the reaction in one direction or the other. The isomerization and epimerization capacity of a novel mutant CsCE (CsCE H356N) was evaluated, obtaining a maximum lactulose yield of 64.3 % and a lactulose selectivity of 9.9. A Michaelis-Menten constant of 551.93 mM and a catalytic efficiency of 0,058 s-1 mM-1 were obtained for lactose epimerization. The ability of CsCE H356N to recognize other substrates was evaluated using lactulose, glucose, mannose, fructose, galactose, talose and tagatose as substrates, assessing the reversibility of such reactions. Yields of 14.8 % mannose and 4.8 % of fructose were obtained from glucose, while talose and tagatose yields of 9.2 % and 5.2 % were obtained from galactose respectively. No significant reaction occurred with lactulose, fructose or tagatose as substrates.

Isomerase and epimerase: overview and practical application in production of functional sugars

The biosynthesis of functional sugars has gained significant attention due to their potential health benefits and increasing demand in the food industry. Enzymatic synthesis has emerged as a promising approach, offering high catalytic efficiency, chemoselectivity, and stereoselectivity. However, challenges such as poor thermostability, low catalytic efficiency, and food safety concerns have limited the commercial production of functional sugars. Protein engineering, including directed evolution and rational design, has shown promise in overcoming these barriers and improving biocatalysts for large-scale production. Furthermore, enzyme immobilization has proven effective in reducing costs and facilitating the production of functional sugars. To ensure food safety, the use of food-grade expression systems has been explored. However, downstream technologies, including separation, purification, and crystallization, still pose challenges in terms of efficiency and cost-effectiveness. Addressing these challenges is crucial to optimize the overall production process. Despite the obstacles, the future outlook for functional sugars is promising, driven by increasing awareness of their health benefits and continuous technological advancements. With further research and technological breakthroughs, industrial-scale production of functional sugars through biosynthesis will become a reality, leading to their widespread incorporation in various industries and products.

The Engineering, Expression, and Immobilization of Epimerases for D-allulose Production

The rare sugar D-allulose is a potential replacement for sucrose with a wide range of health benefits. Conventional production involves the employment of the Izumoring strategy, which utilises D-allulose 3-epimerase (DAEase) or D-psicose 3-epimerase (DPEase) to convert D-fructose into D-allulose. Additionally, the process can also utilise D-tagatose 3-epimerase (DTEase). However, the process is not efficient due to the poor thermotolerance of the enzymes and low conversion rates between the sugars. This review describes three newly identified DAEases that possess desirable properties for the industrial-scale manufacturing of D-allulose. Other methods used to enhance process efficiency include the engineering of DAEases for improved thermotolerance or acid resistance, the utilization of Bacillus subtilis for the biosynthesis of D-allulose, and the immobilization of DAEases to enhance its activity, half-life, and stability. All these research advancements improve the yield of D-allulose, hence closing the gap between the small-scale production and industrial-scale manufacturing of D-allulose.

Engineering the thermostability of d-lyxose isomerase from Caldanaerobius polysaccharolyticus via multiple computer-aided rational design for efficient synthesis of d-mannose

d-Mannose is an attractive functional sugar that exhibits many physiological benefits on human health. The demand for low-calorie sugars and sweeteners in foods are increasingly available on the market. Some sugar isomerases, such as d-lyxose isomerase (d-LIase), can achieve an isomerization reaction between d-mannose and d-fructose. However, the weak thermostability of d-LIase limits its efficient conversion from d-fructose to d-mannose. Nonetheless, few studies are available that have investigated the molecular modification of d-LIase to improve its thermal stability. In this study, computer-aided tools including FireProt, PROSS, and Consensus Finder were employed to jointly design d-LIase mutants with improved thermostability for the first time. Finally, the obtained five-point mutant M5 (N21G/E78P/V58Y/C119Y/K170P) showed high thermal stability and catalytic activity. The half-life of M5 at 65 °C was 10.22 fold, and the catalytic efficiency towards 600 g/L of d-fructose was 2.6 times to that of the wild type enzyme, respectively. Molecular dynamics simulation and intramolecular forces analysis revealed a thermostability mechanism of highly rigidity conformation, newly formed hydrogen bonds and π-cation interaction between and within protein domains, and redistributed surface electrostatic charges for the mutant M5. This research provided a promising d-LIase mutant for the industrial production of d-mannose from d-fructose.

Mass Spectrometry Approach for Differentiation of Positional Isomers of Saccharides: Toward Direct Analysis of Rare Sugars

Rare sugars have gained popularity in recent years due to their use in antiaging treatments, their ability to sweeten with few calories, and their ability to heal infections. Rare sugars are found in small quantities in nature, and they exist typically as isomeric forms of traditional sugars, rendering some challenges in their isolation, synthesis, and characterization. In this work, we present the first direct mass spectrometric approach for differentiating structural isomers of sucrose that differ only by their glycosidic linkages. The method employed a noncontact nanoelectrospray (nESI) platform capable of analyzing minuscule volumes (5 μL) of saccharides via the formation of halide adducts ([M+X]-; X = Cl and Br). Tandem mass spectrometry analysis of the five structural isomers of sucrose afforded diagnostic fragment ions that can be used to distinguish each isomer. Detailed mechanisms showcasing the distinct fragmentation pattern for each isomer are discussed. The method was applied to characterize and confirm the presence of all five selected rare sugars in raw honey complex samples. Aside from the five natural α isomers of sucrose, the method was also suitable for differentiating some β isomers of the same glycosidic linkages, provided the monomeric sugar units are different. The halide adduct formation via the noncontact nESI source was also proven to be effective for oligosaccharides such as raffinose, β-cyclodextrin, and maltoheptaose. The results from this study encourage the future development of methods that function with simple operation to enable straightforward characterization of small quantities of rare sugars.

Synthesis of a Healthy Sweetener d-Tagatose from Starch Catalyzed by Semiartificial Cell Factories

d-Tagatose is one of the several healthy sweeteners that can be a substitute for sucrose and fructose in our daily life. Whole cell-catalyzed phosphorylation and dephosphorylation previously reported by our group afford a thermodynamic-driven strategy to achieve tagatose production directly from starch with high product yields. Nonetheless, the poor structural stability of cells and difficulty in biocatalyst recycling restrict its practical application. Herein, an efficient and stable semiartificial cell factory (SACF) was developed by constructing an organosilica network (OSN) artificial shell on the cells bearing five thermophilic enzymes to produce tagatose. The OSN artificial shell, the thickness of which can be regulated by changing the tetraethyl silicate concentration, exhibited tunable permeability and superior mechanical strength. In contrast with cells, SACFs showed a relative activity of 99.5% and an extended half-life from 33.3 to 57.8 h. Over 50% of initial activity was retained after 20 reuses. The SACFs can catalyze seven consecutive reactions with tagatose yields of over 40.7% in field applications.

Efficient enzymatic synthesis of d-allulose using a novel d-allulose-3-epimerase from Caballeronia insecticola

BACKGROUND

Rare sugars have become promising 'sugar alternatives' because of their low calories and unique physiological functions. Among the family of rare sugars, d-allulose is one of the sugars attracting interest. Ketose 3-epimerases (KEase), including d-tagatose 3-epimerase (DTEase) and d-allulose 3-epimerase (DAEase), are mainly used for d-allulose production.

RESULTS

In this study, a putative xylose isomerase from Caballeronia insecticola was characterized and identified as a novel DAEase. Caballeronia insecticola DAEase displayed prominent enzymatic properties, and 150 g L^-1 d-allulose was produced from 500 g L^-1 d-fructose in 45 min with a conversion rate of 30% and high productivity of 200 g L^-1  h^-1 . Furthermore, DAEase was employed in a phosphorylation-dephosphorylation cascade reaction, which significantly increased the conversion rate of d-allulose. Under optimized conditions, the conversion rate of d-allulose was approximately 100% when the concentration of d-fructose was 50 mmol L^-1 .

CONCLUSION

This research described a very beneficial and facile approach for d-allulose production based on C. insecticola DAEase. © 2022 Society of Chemical Industry.

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.

Protecting group enabled stereocontrolled approach for rare-sugars talose/gulose via dual-ruthenium catalysis

We herein report a convenient and highly stereocontrolled approach for rare and vital ᴅ-talo and ᴅ-gulo sugars directly from economical ᴅ-galactal through dual ruthenium-catalysis. The stereo-divergent strategy involves Ru(III)Cl₃-catalyzed Ferrier glycosylation of ᴅ-galactal to give 2,3-unsaturated ᴅ-galactopyranoside, further selective functionalization of C-4 and C-6 position with diverse protecting groups and dihydroxylation with Ru(VIII)O₄ generated in situ providing access to talo/gulo isomers. The α-anomeric stereoselectivity and syn-diastereoselectivity in glycosylation-dihydroxylation steps have been predominantly achieved by judicious selection of stereoelectronically diverse protecting groups. The synthetic utility of the dual-ruthenium catalysis was demonstrated for efficiently assembling the ᴅ-talose and/or ᴅ-gulose sugars in natural products and bioactive scaffolds.

Directional immobilization of D-allulose 3-epimerase using SpyTag/SpyCatcher strategy as a robust biocatalyst for synthesizing D-allulose

D-Allulose, as low-calorie rare sugar, possessed several notable biological activities and was biosynthesized by D-allulose 3-epimerase (DAEase). Here, CcDAE from Clostridium cellulolyticum was successfully immobilization via covalent attachment (RI-CcDAE), and Resin-SpyCatcher/SpyTag-CcDAE modular (DI-CcDAE). Both immobilized CcDAEs exhibited higher thermal and pH stabilities than the free form, and they maintained 80.0 % of relative activity after 7 consecutive cycles and 25 days of storage. Predominantly, DI-CcDAE represented superior catalytic efficiency with a 2.4-fold increase of k_cat/K_m, compared with RI-CcDAE (0.75 s^-1 mM^-1 vs 0.31 s^-1 mM^-1). The RI-CcDAE and DI-CcDAE were then applied in mixed fruit Jiaosu to convert D-fructose into D-allulose, which exhibited the productivity of D-allulose 1.08 g/Lh^-1 and 1.57 g/Lh^-1, respectively. This research provided a promising directional immobilization strategy for DAEase, and robust biocatalyst for production of functional foodstuff containing D-allulose.

Sustainable isomaltulose production in Corynebacterium glutamicum by engineering the thermostability of sucrose isomerase coupled with one-step simplified cell immobilization

Sucrose isomerase (SI), catalyzing sucrose to isomaltulose, has been widely used in isomaltulose production, but its poor thermostability is still resisted in sustainable batches production. Here, protein engineering and one-step immobilized cell strategy were simultaneously coupled to maintain steady state for long-term operational stabilities. First, rational design of Pantoea dispersa SI (PdSI) for improving its thermostability by predicting and substituting the unstable amino acid residues was investigated using computational analysis. After screening mutagenesis library, two single mutants (PdSIV280L and PdSIS499F) displayed favorable characteristics on thermostability, and further study found that the double mutant PdSIV280L/S499F could stabilize PdSIWT better. Compared with PdSIWT, PdSIV280L/S499F displayed a 3.2°C-higher T m , and showed a ninefold prolonged half-life at 45°C. Subsequently, a one-step simplified immobilization method was developed for encapsulation of PdSIV280L/S499F in food-grade Corynebacterium glutamicum cells to further enhance the recyclability of isomaltulose production. Recombinant cells expressing combinatorial mutant (RCSI2) were successfully immobilized in 2.5% sodium alginate without prior permeabilization. The immobilized RCSI2 showed that the maximum yield of isomaltulose by batch conversion reached to 453.0 g/L isomaltulose with a productivity of 41.2 g/l/h from 500.0 g/L sucrose solution, and the conversion rate remained 83.2% after 26 repeated batches.

Enhancement of L-ribulose Production from L-ribose Through Modification of Ochrobactrum sp. CSL1 Ribose-5-phosphate Isomerase A

L-ribulose, a kind of high-value rare sugar, could be utilized to manufacture L-form sugars and antiviral drugs, generally produced from L-arabinose as a substrate. However, the production of L-ribulose from L-arabinose is limited by the equilibrium ratio of the catalytic reaction, hence, it is necessary to explore a new biological enzymatic method to produce L-ribulose. Ribose-5-phosphate isomerase (Rpi) is an enzyme that can catalyze the reversible isomerization between L-ribose and L-ribulose, which is of great significance for the preparation of L-ribulose. In order to obtain highly active ribose-5-phosphate isomerase to manufacture L-ribulose, ribose-5-phosphate isomerase A (OsRpiA) from Ochrobactrum sp. CSL1 was engineered based on structural and sequence analyses. Through a rational design strategy, a triple-mutant strain A10T/T32S/G101N with 160% activity was acquired. The enzymatic properties of the mutant were systematically investigated, and the optimum conditions were characterized to achieve the maximum yield of L-ribulose. Kinetic analysis clarified that the A10T/T32S/G101N mutant had a stronger affinity for the substrate and increased catalytic efficiency. Furthermore, molecular dynamics simulations indicated that the binding of the substrate to A10T/T32S/G101N was more stable than that of wild type. The shorter distance between the catalytic residues of A10T/T32S/G101N and L-ribose illuminated the increased activity. Overall, the present study provided a solid basis for demonstrating the complex functions of crucial residues in RpiAs as well as in rare sugar preparation.

Recent advances in biotransformation, extraction and green production of D-mannose

D-mannose is a natural and biologically active monosaccharide. It is the C-2 epimer of glucose and a component of a variety of polysaccharides in plants. In addition, D-mannose also naturally exists in some cells of the human body and participates in the immune regulation of cells as a prebiotic. Its good physiological benefits to human health and wide application in the food and pharmaceutical industries have attracted widespread attention. Therefore, in-depth research on preparation methods of D-mannose has been widely developed. This article summarizes the main production methods of D-mannose in recent years, especially the in-depth excavation from biomass raw materials such as coffee grounds, konjac flour, acai berry, etc., to provide new ideas for the green manufacture of D-mannose.

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Various methods of recent mannose production were comprehensively summarized.

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The new technical progress of obtaining mannose from biomass as emphatically discussed.

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Discuss various preparation methods including different pretreatments, enzymatic hydrolysis, etc.

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.

Selective Transformations of Carbohydrates Inspired by Radical-Based Enzymatic Mechanisms

Enzymes are a longstanding source of inspiration for synthetic reaction development. However, enzymatic reactivity and selectivity are frequently untenable in a synthetic context, as the principles that govern control in an enzymatic setting often do not translate to small molecule catalysis. Recent synthetic methods have revealed the viability of using small molecule catalysts to promote highly selective radical-mediated transformations of minimally protected sugar substrates. These transformations share conceptual similarities with radical SAM enzymes found in microbial carbohydrate biosynthesis and present opportunities for synthetic chemists to access microbial and unnatural carbohydrate building blocks without the need for protecting groups or lengthy synthetic sequences. Here, we highlight strategies through which radical reaction pathways can enable the site-, regio-, and diastereoselective transformation of minimally protected carbohydrates in both synthetic and enzymatic systems.

Recent advances in (chemo)enzymatic cascades for upgrading bio-based resources

Developing (chemo)enzymatic cascades is very attractive for green synthesis, because they streamline multistep synthetic processes. In this Feature Article, we have summarized the recent advances in in vitro or whole-cell cascade reactions with a focus on the use of renewable bio-based resources as starting materials. This includes the synthesis of rare sugars (such as ketoses, L-ribulose, D-tagatose, myo-inositol or aminosugars) from readily available carbohydrate sources (cellulose, hemi-cellulose, starch), in vitro enzyme pathways to convert glucose to various biochemicals, cascades to convert 5-hydroxymethylfurfural and furfural obtained from lignin or xylose into novel precursors for polymer synthesis, the syntheses of phenolic compounds, cascade syntheses of aliphatic and highly reduced chemicals from plant oils and fatty acids, upgrading of glycerol or ethanol as well as cascades to transform natural L-amino acids into high-value (chiral) compounds. In several examples these processes have demonstrated their efficiency with respect to high space-time yields and low E-factors enabling mature green chemistry processes. Also, the strengths and limitations are discussed and an outlook is provided for improving the existing and developing new cascades.

Enantioselective Reductive Oligomerization of Carbon Dioxide into l-Erythrulose via a Chemoenzymatic Catalysis

A cell-free enantioselective transformation of the carbon atom of CO₂ has never been reported. In the urgent context of transforming CO₂ into products of high value, the enantiocontrolled synthesis of chiral compounds from CO₂ would be highly desirable. Using an original hybrid chemoenzymatic catalytic process, we report herein the reductive oligomerization of CO₂ into C₃ (dihydroxyacetone, DHA) and C₄ (l-erythrulose) carbohydrates, with perfect enantioselectivity of the latter chiral product. This was achieved with the key intermediacy of formaldehyde. CO₂ is first reduced selectively by 4e^- by an iron-catalyzed hydroboration reaction, leading to the isolation and complete characterization of a new bis(boryl)acetal compound derived from dimesitylborane. In an aqueous buffer solution at 30 °C, this compound readily releases formaldehyde, which is then involved in selective enzymatic transformations, giving rise either (i) to DHA using a formolase (FLS) catalysis or (ii) to l-erythrulose with a cascade reaction combining FLS and d-fructose-6-phosphate aldolase (FSA) A129S variant. Finally, the nature of the synthesized products is noteworthy, since carbohydrates are of high interest for the chemical and pharmaceutical industries. The present results prove that the cell-free de novo synthesis of carbohydrates from CO₂ as a sustainable carbon source is a possible alternative pathway in addition to the intensely studied biomass extraction and de novo syntheses from fossil resources.

Investigation of d-allulose effects on high-sucrose diet-induced insulin resistance via hyperinsulinemic-euglycemic clamps in rats

d-Allulose, a C-3 epimer of d-fructose, is a rare sugar that has no calories. Although d-allulose has been reported to have several health benefits, such as anti-obesity and anti-diabetic effects, there have been no reports evaluating the effects of d-allulose on insulin resistance using a hyperinsulinemic-euglycemic clamp (HE-clamp). Therefore, we investigated the effects of d-allulose on a high-sucrose diet (HSD)-induced insulin resistance model. Wistar rats were randomly divided into three dietary groups: HSD containing 5% cellulose (HSC), 5% d-allulose (HSA), and a commercial diet. The insulin tolerance test (ITT) and HE-clamp were performed after administration of the diets for 4 and 7 weeks. After 7 weeks, the muscle and adipose tissues of rats were obtained to analyze Akt signaling via western blotting, and plasma adipocytokine levels were measured. ITT revealed that d-allulose ameliorated systemic insulin resistance. Furthermore, the results of the 2-step HE-clamp procedure indicated that d-allulose reversed systemic and muscular insulin resistance. d-Allulose reversed the insulin-induced suppression of Akt phosphorylation in the soleus muscle and epididymal fat tissues and reduced plasma TNF-α levels. This study is the first to show that d-allulose improves systemic and muscle insulin sensitivity in conscious rats.

Cloning, expression, and characterization of an arabitol dehydrogenase and coupled with NADH oxidase for effective production of L-xylulose

A novel arabitol dehydrogenase (ArDH) gene was cloned from a bacterium named Aspergillus nidulans and expressed heterologously in Escherichia coli. The purified ArDH exhibited the maximal activity in pH 9.5 Tris-HCl buffer at 40 °C, showed K_m and V_max of 1.2 mg/mL and 9.1 U/mg, respectively. The ArDH was used to produce the L-xylulose and coupled with the NADH oxidase (Nox) for the regeneration of NAD⁺. In further optimization, a high conversion of 84.6% in 8 hours was achieved under the optimal conditions: 20 mM of xylitol, 100 µM NAD⁺ in pH 9.0 Tris-HCl buffer at 30 °C. The results indicated the coupling system with cofactor regeneration provides a promising approach for L-xylulose production from xylitol.

Enzymes, In Vivo Biocatalysis, and Metabolic Engineering for Enabling a Circular Economy and Sustainability

Since the industrial revolution, the rapid growth and development of global industries have depended largely upon the utilization of coal-derived chemicals, and more recently, the utilization of petroleum-based chemicals. These developments have followed a linear economy model (produce, consume, and dispose). As the world is facing a serious threat from the climate change crisis, a more sustainable solution for manufacturing, i.e., circular economy in which waste from the same or different industries can be used as feedstocks or resources for production offers an attractive industrial/business model. In nature, biological systems, i.e., microorganisms routinely use their enzymes and metabolic pathways to convert organic and inorganic wastes to synthesize biochemicals and energy required for their growth. Therefore, an understanding of how selected enzymes convert biobased feedstocks into special (bio)chemicals serves as an important basis from which to build on for applications in biocatalysis, metabolic engineering, and synthetic biology to enable biobased processes that are greener and cleaner for the environment. This review article highlights the current state of knowledge regarding the enzymatic reactions used in converting biobased wastes (lignocellulosic biomass, sugar, phenolic acid, triglyceride, fatty acid, and glycerol) and greenhouse gases (CO₂ and CH₄) into value-added products and discusses the current progress made in their metabolic engineering. The commercial aspects and life cycle assessment of products from enzymatic and metabolic engineering are also discussed. Continued development in the field of metabolic engineering would offer diversified solutions which are sustainable and renewable for manufacturing valuable chemicals.