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

Characterization of Runella zeae D-mannose 2-epimerase and its expression in Bacillus subtilis for D-mannose production from D-glucose

D-Mannose 2-epimerase (MEase) catalyzes the bioconversion between D-glucose and D-mannose. It is an important potential biocatalyst for large-scale production of D-mannose, a functional monosaccharide used in pharmaceutical and food industries. In this study, a new microbial MEase was characterized from Runella zeae DSM 19591. The enzyme was purified by one-step nickel-affinity chromatography and determined to be a dimeric protein with two identical subunits of approximately 86.1 kDa by gel filtration. The enzyme showed the highest activity at pH 8.0 and 40 °C, with a specific activity of 2.99 U/mg on D-glucose and 3.71 U/mg on D-mannose. The melting temperature (Tm) was 49.4 °C and the half-life was 115.14 and 3.23 h at 35 and 40 °C, respectively. The purified enzyme (1 U/mL) produced 115.7 g/L of D-mannose from 500 g/L of D-glucose for 48 h, with a conversion ratio of 23.14 %. It was successfully expressed in Bacillus subtilis WB600 via pP43NMK as the vector. The highest fermentation activity was 10.58 U/mL after fed-batch cultivation for 28 h, and the whole cells of recombinant B. subtilis produced 114.0 g/L of D-mannose from 500 g/L of D-glucose, with a conversion ratio of 22.8 %.

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.

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

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

Biochemical characterization, structure-guided mutagenesis, and application of a recombinant D-allulose 3-epimerase from Christensenellaceae bacterium for the biocatalytic production of D-allulose

D-Allulose has become a promising alternative sweetener due to its unique properties of low caloric content, moderate sweetness, and physiological effects. D-Allulose 3-epimerase (DAEase) is a promising enzyme for D-Allulose production. However, the low catalytic efficiency limited its large-scale industrial applications. To obtain a more effective biocatalyst, a putative DAEase from Christensenellaceae bacterium (CbDAE) was identified and characterized. The recombinant CbDAE exhibited optimum activity at pH 7.5°C and 55°C, retaining more than 60% relative activity from 40°C to 70°C, and the catalytic activity could be significantly increased by Co2+ supplementation. These enzymatic properties of purified CbDAE were compared with other DAEases. CbDAE was also found to possess desirable thermal stability at 55°C with a half-life of 12.4 h. CbDAE performed the highest relative activity towards D-allulose and strong affinity for D-fructose but relatively low catalytic efficiency towards D-fructose. Based on the structure-guided design, the best double-mutation variant G36N/W112E was obtained which reached up to 4.21-fold enhancement of catalytic activity compared with wild-type (WT) CbDAE. The catalytic production of G36N/W112E with 500 g/L D-fructose was at a medium to a higher level among the DAEases in 3.5 h, reducing 40% catalytic reaction time compared to the WT CbDAE. In addition, the G36N/W112E variant was also applied in honey and apple juice for D-allulose conversion. Our research offers an extra biocatalyst for D-allulose production, and the comprehensive report of this enzyme makes it potentially interesting for industrial applications and will aid the development of industrial biocatalysts for D-allulose.

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

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

Fine-Tuning of Carbon Flux and Artificial Promoters in Bacillus subtilis Enables High-Level Biosynthesis of d-Allulose

d-Allulose is an attractive rare sugar that can be used as a low-calorie sweetener with significant health benefits. To meet the increasing market demands, it is necessary to develop an efficient and extensive microbial fermentation platform for the synthesis of d-allulose. Here, we applied a comprehensive systematic engineering strategy in Bacillus subtilis WB600 by introducing d-allulose 3-epimerase (DAEase), combined with the deactivation of fruA, levDEFG, and gmuE, to balance the metabolic network for the efficient production of d-allulose. This resulting strain initially produced 3.24 g/L of d-allulose with a yield of 0.93 g of d-allulose/g d-fructose. We further screened and obtained a suitable dual promoter combination and performed fine-tuning of its spacer region. After 64 h of fed-batch fermentation, the optimized engineered B. subtilis produced d-allulose at titers of 74.2 g/L with a yield of 0.93 g/g and a conversion rate of 27.6%. This d-allulose production strain is a promising platform for the industrial production of rare sugar.