Lactulose production from lactose isomerization by chemo-catalysts and enzymes: Current status and future perspectives

Engineering the substrate-binding pocket of Cellobiose 2-epimerase for enhancing lactulose biosynthesis

Cellobiose 2-epimerase (CE) converts lactose into lactulose and epilactose with high added value and prebiotic benefits. However, the low conversion of lactulose by CsCE limits its industrial application. Inspired by previous work introducing engineered disulfide bonds on the loop of CsCE, we performed saturation mutagenesis and high-throughput screening of all residues within 4 Å of the engineered disulfide bond. The best mutant, CsCE-S173C/F231C/L172S/I178V(M4), showed a 1.3-fold increase in isomerization activity over CsCE-S173C/F231C. Molecular dynamics simulations and binding pocket analysis revealed that mutations reshaped the shape and size of the substrate-binding pocket and increased non-covalent interactions of the protein with the ligand. Binding free energy calculations showed a higher binding affinity of M4 for lactose and lactulose. Therefore, we demonstrated that the catalytic characteristics of CE can be modulated by further manipulating the loop region near the pocket, which is important for improving the catalytic efficiency and promiscuity of the enzyme.

Development of boronate affinity adsorbent bearing -B(OH)2 and F groups with low pKa value for enhanced production and purification of lactulose through adsorption-assisted bio-isomerization of lactose

In this study, a boronate affinity material (FA-5) with a low pKa value was prepared to facilitate lactulose production. At physiological pH, 58.23 % of the B atoms in FA-5 existed in an sp3 hybridization state, and FA-5 demonstrated excellent adsorption capacity (94.78 mg/g) and selectivity (91.82 %) for lactulose from a 1:1 lactulose-lactose solution. Additionally, FA-5 demonstrated an exceptional capability for separating lactulose. The purity of lactulose increased by up to 6.82-fold in lactulose-lactose solutions with varying concentration ratios. A cascade system was formed by an immobilized enzyme-packed column with an FA-5-packed column to enhance lactulose production and purification. By adopting the adsorption-assisted isomerization strategy, the lactulose yield was improved from 52.50 % to 71.14 % with lactulose purity reaching 83.57 %. Moreover, the cascade system demonstrated excellent catalytic stability (86.28 %) and adsorption stability (93.68 %) over six consecutive cycles. The results highlight the potential of the cascade system for enzymatic production of lactulose.

Enhancement of isomerization activity and thermostability of cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus using semi-rational design

Cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus (CsCE) is crucial for lactulose production; however, its low isomerization activity and poor thermostability limit its industrial application. Herein, four single-point mutants (S70D, F171W, S173L, and L354F) with significantly improved isomerization activity were obtained using a computer-aided semi-rational design, and combined mutants were introduced. The isomerization specific activities of the mutants CsCE-F171W/S173L/L354F and CsCE-S70D/F171W/S173L/L354F were 2.10 and 2.05 times higher than that of wild-type CsCE, respectively. Enzymatic properties revealed that the mutant CsCE-F171W/S173L/L354F demonstrated a 2.50-fold increase in t1/2 at 80 °C and a 2.70-fold improvement in kcat, while the mutant CsCE-S70D/F171W/S173L/L354F exhibited a 2.91-fold increase in t1/2 at 80 °C and a 3.19-fold improvement in kcat. Three-dimensional structural analysis suggested that conformational changes in the flexible ring induced by mutation could enhance isomerization activity, while increased structural rigidity, hydrophobic contacts, van der Waals forces, and favorable electrostatic potentials may promote thermostability. These findings provide novel insights into the molecular modification of CsCE and significantly enhance its potential for industrial applications.

High-efficiency secretion expression of cellobiose 2-epimerase in Escherichia coli and its applications

Cellobiose 2-epimerase (CE) plays a crucial role in catalyzing the conversion of lactose. In this study, the N-terminal 20 amino acids of Lactobacillus amylovorus feruloyl esterase (N20) were employed as a signal peptide and fused with the CE gene from Caldicellulosiruptor bescii for recombinant expression. Following ligation with the pET-22b(+) vector, Escherichia coli BL21 (DE3) was transformed. SDS-PAGE analysis confirmed the extracellular secretion of the CE following fusion with the signal peptide. Following fermentation optimization to maximize extracellular protein secretion, the optimal conditions were identified as a 2 × YT medium, supplemented with 0.8 mM IPTG, 0.1 mM ferrous ion (Fe2+), and 25 mM glycine after a 2.5 h induction, with incubation at 37 °C and 200 rpm for 36 h. The CE was purified using ammonium sulfate precipitation at 60 % saturation, yielding 1529.61 mg of enzyme protein per liter of fermentation broth, with a specific activity of 19.25 U/mg. A lactose substrate at 40 % concentration was employed, with varying enzyme concentrations (0.3825 g/L, 0.765 g/L, 1.1475 g/L, and 1.53 g/L) and reaction times (3 h, 6 h, 9 h, 12 h, and 24 h). After reaction, high-performance liquid chromatography (HPLC) was used for analysis, determining that an enzyme concentration of 1.53 g/L reacting with the lactose substrate for 24 h achieved the highest lactulose conversion rate at 56 %. This constitutes the first study on the direct extracellular secretion of CE, laying the groundwork for its production and application.

Advances and prospects on production of lactulose and epilactose by cellobiose 2-epimerases: A review

Lactulose and epilactose are nondigestible disaccharides with a wide range of applications in clinical medicine, nutrition, and the food industry due to their health-benefiting properties. Their chemical synthesis typically involves stringent catalytic conditions and intricate reaction procedures, resulting in elevated production costs and challenges in product separation. Cellobiose 2-epimerases (CEs) facilitate the isomerization and epimerization of lactose to produce lactulose and epilactose directly, without the need for co-substrates. This enzymatic process offers advantages such as mild reaction conditions, straightforward operation, high conversion efficiency, and reduced by-product formation. Recently, numerous CE genes have been identified and characterized, with their enzymatic properties undergoing extensive analysis. This review consolidates information on the properties of CEs from various sources and examines their catalytic mechanisms based on crystal structure data. Additionally, the current research progress in the enzymatic synthesis of lactulose and epilactose is comprehensively reviewed. The future direction of CE research is discussed, highlighting the potential for large-scale production of lactulose and epilactose through environmentally sustainable enzymatic methods.

Loop engineering of cellobiose 2-epimerase from Dictyoglomus thermophilum for efficient synthesis of lactulose

Lactulose holds broad application due to its unique pharmaceutical and prebiotic properties. Cellobiose 2-epimerase (CE) efficiently catalyzes the production of lactulose, offering an eco-friendly biosynthetic alternative. However, its relatively low isomerization activity hampers widespread application in the manufacture of lactulose. In this study, we selected DtCE sourced from Dictyoglomus thermophilum and successfully generated mutants M4 and M5 through an innovative loop engineering process that integrates computer-aided design with directed evolution. Remarkably, the isomerization activities of the mutants M4 and M5 increased by 46.5 % and 81.8 %, respectively, marking a significant improvement compared to the wild-type. Conversely, their epimerization activities underwent a dramatic decline, dropping by 80.0 % and 50.0 %, respectively. Therefore, these mutants demonstrated considerable superiority in the synthesis of lactulose. Moreover, molecular dynamics simulations showed that modifications of the flexible loop affected protonation, enhancing isomerization selectivity. This study underscores the precision and efficacy of our engineering approach in customizing DtCE's properties to meet specific needs, while concurrently establishing a technical foundation for industrial-scale biosynthetic production of lactulose.

Heterologous Production, Purification and Characterization of Two Cold-Active β-d-Galactosidases with Transglycosylation Activity from the Psychrotolerant Arctic Bacterium Arthrobacter sp. S3* Isolated from Spitsbergen Island Soil

Cold-adapted microorganisms possess cold-active enzymes with potential applications in different industries and research areas. In this study, two genes encoding β-d-galactosidases belonging to Glycoside Hydrolase families 2 and 42 from the psychrotolerant Arctic bacterium Arthrobacter sp. S3* were cloned, expressed in Escherichia coli and Komagataella phaffii, purified and characterized. The GH2 β-d-galactosidase is a tetramer with a molecular weight of 450 kDa, while the GH42 β-d-galactosidase is a 233 kDa trimer. The Bgal2 was optimally active at pH 7.5 and 22 °C and maintained 57% of maximum activity at 10 °C, whereas the Bgal42 was optimally active at pH 7.0 and 40 °C and exhibited 44% of maximum activity at 10 °C. Both enzymes hydrolyzed lactose and showed transglycosylation activity. We also found that 2 U/mL of the Bgal2 hydrolyzed 85% of lactose in milk within 10 h at 10 °C. The enzyme synthesized galactooligosaccharides, heterooligosaccharides, alkyl galactopyranosides and glycosylated salicin. The Bgal42 synthesized galactooligosaccharides and 20 U/mL of the enzyme hydrolyzed 72% of milk lactose within 24 h at 10 °C. The properties of Arthrobacter sp. S3* Bgal2 make it a candidate for lactose hydrolysis in the dairy industry and a promising tool for the glycosylation of various acceptors in the biomedical sector.

Improvement of cellobiose 2-epimerase expression in Bacillus subtilis for efficient bioconversion of lactose to epilactose

Epilactose, a lactose derivative known for its prebiotic properties and potential health benefits, has garnered significant interest. Cellulose 2-epimerase (CEase) is responsible for catalyzing the conversion of lactose to epilactose. In this study, the enhancement of food-grade CEase expression in Bacillus subtilis WB600 was systematically investigated. Among seven selected epilactose-producing CEases, Rhodothermus marinus CEase (RmCE) exhibited the highest epimerization activity when expressed in B. subtilis. Translational and transcriptional regulations were employed to enhance CEase expression by screening effective N-terminal coding sequences (NCSs) and promoters. The final strain demonstrated efficient production of CEase, with epimerization activity reaching 273.6 ± 6.5 U/mL and 1255 ± 26.4 U/mL in shake-flask and fed-batch cultivation, respectively. Utilizing only 0.25 % (V/V) of the fed-batch cultivation broth for lactose biotransformation, epilactose was efficiently produced from 300 g/L of lactose within 4 h, achieving a yield of 29.5 %. These findings provide significant support for the potential industrialization of enzymatic epilactose production.

Multiple Regulatory Mechanisms Synergistically Control the Soluble Expression of CsCE for Enhanced Enzymatic Productivity of Lactulose in E. coli

The limited expression of cellobiose 2-epimerase poses a significant constraint on the industrial enzymatic production of lactulose. Extensive modifications to the expression cassette offer a means to enhance the yield of recombinant proteins. In this study, an integrated strategy, combining individual and collaborative approaches, is proposed to fine-tune each stage of the CsCE overexpression program. This strategy involves the multidimensional integration of standardized genetic elements at various levels, including transcription, translation, folding, and three-dimensional structure. The volumetric activity of the final recombinant strain was markedly increased by 12-fold compared to the wild-type strain, reaching 2260.62 U/L. The protein expression in the newly developed high-yield recombinant strain exhibited a significant enhancement, with a higher proportion of soluble protein compared to that of inclusion bodies. Our findings offer insights into the multifaceted synergistic regulation of protein expression processes, holding promising implications for the production of heterologous recombinant proteins.

Rational Design of Loop Dynamics for a Barrel-Shaped Enzyme by Introducing Disulfide Bonds

Loop dynamics redesign is an important strategy to manipulate protein function. Cellobiose 2-epimerase (CE) and other members of its superfamily are widely used for diverse industrial applications. The structural feature of the loops connecting barrel helices contributes greatly to the differences in their functional characteristics. Inspired by the in-silico mutation with molecular dynamics (MD) simulation analysis, we propose a strategy for identifying disulfide bond mutation candidates based on the prediction of protein flexibility and residue-residue interaction. The most beneficial mutant with the newly introduced disulfide bond would simultaneously improve both its thermostability and its reaction propensity to the targeting isomerization product. The ratio of the isomerization/epimerization catalytic rate was improved from 4:103 to 9:22. MD simulation and binding free energy calculations were applied to provide insights into molecular recognition upon mutations. The comparative analysis of enzyme/substrate binding modes indicates that the altered catalytic reaction pathway is due to less efficient binding of the native product. The key residue responsible for the observed phenotype was identified by energy decomposition and was further confirmed by the mutation experiment. The rational design of the key loop region might be a promising strategy to alter the catalytic behavior of all (α/α)6-barrel-like proteins.

Engineering Bacillus subtilis for highly efficient production of functional disaccharide lactulose from lactose

Bioconversion of lactose to functional lactose derivatives attracts increasing attention. Lactulose is an important high-value lactose derivative, which has been widely used in pharmaceutical, nutraceutical, and food industries. Lactulose can be enzymatically produced from lactose by cellobiose 2-epimerase (CEase). Several studies have already focused on the food-grade expression of CEase, but they are all aimed at the biosynthesis of epilactose. Herein, we reported for the first time the biosynthesis of lactulose using the recombinant food-grade Bacillus subtilis. Lactulose biosynthesis was optimized by varying lactulose-producing CEases and expression vectors. Caldicellulosiruptor saccharolyticus CEase and pP43NMK were determined to be the optimal CEase and expression vector. Fine-tuning of CEase expression was investigated by screening a beneficial N-terminal coding sequence. After fed-batch cultivation, the highest fermentation isomerization activity reached 11.6 U/mL. Lactulose was successfully produced by the broth of the engineered B. subtilis with a yield of 52.1 %.

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.

Increase in Lactulose Content in a Hot-Alkaline-Based System through Fermentation with a Selected Lactic Acid Bacteria Strain Followed by the β-Galactosidase Catalysis Process

In this study, lactic acid bacteria (LAB) fermentation and β-galactosidase catalysis methods were combined to increase the lactulose concentration and reduce the galactose and lactose content in a hot-alkaline-based system. The optimal conditions for chemical isomerization were 70 °C for 50 min for lactulose production, in which the concentration of lactulose was 31.3 ± 1.2%. Then, the selection and identification of LAB, which can utilize lactose and cannot affect lactulose content, were determined from 451 strains in the laboratory. It was found that Lactobacillus salivarius TM-2-8 had weak lactulose utilization and more robust lactose utilization. Lactobacillus rhamnosus grx.21 was weak in terms of lactulose utilization and strong in terms of galactose utilization. These two strains fermented the chemical isomerization system of lactulose to reduce the content of lactose and galactose. The results showed that the lactose concentration was 48.96 ± 2.92 g/L and the lactulose concentration was 59.73 ± 1. 8 g/L for fermentation lasting 18 h. The β-galactosidase was used to increase the content of lactulose in the fermented system at this time. The highest concentration of 74.89 ± 1.68 g/L lactulose was obtained at an enzymatic concentration of 3 U/mL and catalyzed at 50 °C for 3 h by β-galactosidase.

Regulatory effect of lactulose on intestinal flora and serum metabolites in colitis mice: In vitro and in vivo evaluation

Lactulose is a common component in foods. However, the effect of lactulose on intestinal flora and overall metabolic levels remains unclear. Therefore, this study aims to explore the regulative role of lactulose on intestinal flora and serum metabolites via in vitro simulated colonic fermentation model and in vivo colitis mouse model. The results showed that lactulose significantly enriched beneficial bacteria including Dubosiella and Bifidobacterium, and reduced pathogenic bacteria such as Fusobacterium. Moreover, lactulose significantly inhibited dextran sodium sulfate-induced body weight loss, colon shortening, colonic inflammatory infiltration, and pro-inflammatory cytokines IL-6, TNF-α, IL-17, and IL-1β. Lactulose significantly affected serum metabolome in colitis mice and total 24 metabolites representing a high inter-group difference were obtained. Correlation analysis revealed that the changes in serum metabolites were closely associated with the role of intestinal flora, and thus affected phenotypic indicators. Our study provides a reference for nutritional characteristics and application scenarios of dietary lactulose.

Computer-aided design of novel cellobiose 2-epimerase for efficient synthesis of lactulose using lactose

Cellobiose 2-epimerase (CE) is ideally suited to synthesize lactulose from lactose, but the poor thermostability and catalytic efficiency restrict enzymatic application. Herein, a non-characterized CE originating from Caldicellulosiruptor morganii (CmCE) was discovered in the NCBI database. Then, a smart mutation library was constructed based on FoldX ΔΔG calculation and modeling structure analysis, from which a positive mutant D226G located within the α₈/α₉ loop exhibited longer half-lives at 65-75 °C as well as lower K_m and higher k_cat/K_m values compared with CmCE. Molecular modeling demonstrated that the improvement of D226G was largely attributed to the rigidification of the flexible loop, the compactness of the catalysis pocket and the increment of substrate-binding capability. Finally, the yield of synthesizing lactulose catalyzed by D226G reached 45.5%, higher than the 35.9% achieved with CmCE. The disclosed effect of the flexible loop on enzymatic stability and catalysis provides insight to redesign efficient CEs to biosynthesize lactulose.

Co-Immobilization of Lactase and Glucose Isomerase on the Novel g-C3N4/CF Composite Carrier for Lactulose Production

The g-C₃N₄/CF composite carrier was prepared by ultrasound-assisted maceration and high-temperature calcination. The enzyme immobilization using the g-C₃N₄/CF as the novel carrier to immobilize lactase and glucose isomerase was enhanced for lactulose production. The carbon fiber (CF) was mixed with melamine powder in the mass ratio of 1:8. The g-C₃N₄/CF composite carrier was obtained by calcination at 550 °C for 3 h. After the analysis of characteristics, the g-C₃N₄/CF was successfully composited with the carbon nitride and CF, displaying the improvement of co-immobilization efficiency with the positive effects on the stability of the enzyme. The immobilization efficiency of the co-immobilized enzyme was 37% by the novel carrier of g-C₃N₄/CF, with the enzyme activity of 13.89 U g^-1 at 60 °C. The relative activities of co-immobilized enzymes maintained much more steadily at the wider pH and higher temperature than those of the free dual enzymes, respectively. In the multi-batches of lactulose production, the relative conversion rates in enzymes co-immobilized by the composite carrier were higher than that of the free enzymes during the first four batches, as well as maintaining about a 90% relative conversation rate after the sixth batch. This study provides a novel method for the application of g-C₃N₄/CF in the field of immobilizing enzymes for the production of lactulose.