Immobilization of cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus on commercial resin Duolite A568

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

Lactulose, a semisynthetic nondigestive disaccharide with versatile applications in the food and pharmaceutical industries, has received increasing interest due to its significant health-promoting effects. Currently, industrial lactulose production is exclusively carried out by chemical isomerization of lactose via the Lobry de Bruyn-Alberda van Ekenstein (LA) rearrangement, and much work has been directed toward improving the conversion efficiency in terms of lactulose yield and purity by using new chemo-catalysts and integrated catalytic-purification systems. Lactulose can also be produced by an enzymatic route offering a potentially greener alternative to chemo-catalysis with fewer side products. Compared to the controlled trans-galactosylation by β-galactosidase, directed isomerization of lactose with high isomerization efficiency catalyzed by the most efficient lactulose-producing enzyme, cellobiose 2-epimerase (CE), has gained much attention in recent decades. To further facilitate the industrial translation of CE-based lactulose biotransformation, numerous studies have been reported on improving biocatalytic performance through enzyme mediated molecular modification. This review summarizes recent developments in the chemical and enzymatic production of lactulose. Related catalytic mechanisms are also highlighted and described in detail. Emerging techniques that aimed at advancing lactulose production, such as the boronate affinity-based technique and molecular biological techniques, are reviewed. Finally, perspectives on challenges and opportunities in lactulose production and purification are also discussed.

Molecular Characterization of a Mesophilic Cellobiose 2-Epimerase That Maintains a High Catalytic Efficiency at Low Temperatures

Cellobiose 2-epimerase (CE) can catalyze bioconversion of lactose to its prebiotic derivative epilactose. The catalytic property of a novel CE from Treponema brennaborense (Trbr-CE) was investigated. Trbr-CE showed the highest catalytic efficiency of epimerization toward lactose among all of the previously reported CEs. This enzyme's specific activity could reach as high as 208.5 ± 5.3 U/mg at its optimum temperature, which is 45 °C. More importantly, this enzyme demonstrated a considerably high activity at low temperatures, suggesting Trbr-CE as a promising enzyme for industrial low-temperature production of epilactose. This structurally flexible enzyme exhibited a comparatively high binding affinity toward substrates, which was confirmed by both experimental verification and computational analysis. Molecular dynamics (MD) simulations and binding free energy calculations were applied to provide insights into molecular recognition upon temperature changes. Compared with thermophilic CEs, Trbr-CE presents a more negative enthalpy change and a higher entropy change when the temperature drops.

A review on l-ribose isomerases for the biocatalytic production of l-ribose and l-ribulose

Presently, because of the extraordinary roles and potential applications, rare sugars turn into a focus point for countless researchers in the field of carbohydrates. l-ribose and l-ribulose are rare sugars and isomers of each other. This aldo and ketopentose are expensive but can be utilized as an antecedent for the manufacturing of various rare sugars and l-nucleoside analogue. The bioconversion approach turns into an excellent alternative method to l-ribulose and l-ribose production, as compared to the complex and lengthy chemical methods. The basic purpose of this research was to describe the importance of rare sugars in various fields and their easy production by using enzymatic methods. l-Ribose isomerase (L-RI) is an enzyme discovered by Tsuyoshi Shimonishi and Ken Izumori in 1996 from Acinetobacter sp. strain DL-28. L-RI structure was cupin-type-β-barrel shaped with a catalytic site between two β-sheets surrounded by metal ions. The crystal structures of the L-RI showed that it contains a homotetramer structure. Current review have concentrated on the sources, characteristics, applications, conclusions and future prospects including the potentials of l-ribose isomerase for the commercial production of l-ribose and l-ribulose. The MmL-RIse and CrL-RIse have the potential to produce the l-ribulose up to 32% and 31%, respectively. The CrL-RIse is highly stable as compared to other L-RIs. The results explained that the L-RIs have great potential in the production of rare sugars especially, l-ribose and l-ribulose, while the immobilization technique can enhance its functionality and properties. The present study precises the applications of L-RIs acquired from various sources for l-ribose and l-ribulose production.

Immobilization of Aspergillus oryzae β-galactosidase in cation functionalized agarose matrix and its application in the synthesis of lactulose

Aspergillus oryzae β-galactosidase was immobilized in in-house quaternary ammonium agarose (QAA) and used for the first time in the synthesis of lactulose. A biocatalyst was obtained with a specific activity of 24,690 IU_H∙g^-1; protein immobilization yield of 97% and enzyme immobilization yield of 76% were obtained at 30 °C in 10 mM phosphate buffer pH 7 for standard size agarose at 100 mg_protein∙g_support^-1 which the maximum protein load of QAA. Highest yield and specific productivity of lactulose were 0.24 g∙g^-1 and 9.78 g∙g^-1 h^-1 respectively, obtained at pH 6, 100 IU_H∙g lactose^-1 enzyme/lactose ratio and 12 lactose/fructose molar ratio. In repeated-batch operation with the immobilized enzyme, the cumulative mass of lactulose per unit mass of contacted protein and cumulative specific productivity were higher than obtained with the soluble enzyme since the first batch. After enzyme activity exhaustion, the enzyme was desorbed and QAA support was reused without alteration in its maximum enzyme load capacity and without detriment in yield, productivity and selectivity in the batch synthesis of lactulose with the resulting biocatalyst. This significantly decreases the economic impact of the support, presenting itself as a distinctive advantage of immobilization by ionic interaction.

Recent advances in β-galactosidase and fructosyltransferase immobilization technology

The highly demanding conditions of industrial processes may lower the stability and affect the activity of enzymes used as biocatalysts. Enzyme immobilization emerged as an approach to promote stabilization and easy removal of enzymes for their reusability. The aim of this review is to go through the principal immobilization strategies addressed to achieve optimal industrial processes with special care on those reported for two types of enzymes: β-galactosidases and fructosyltransferases. The main methods used to immobilize these two enzymes are adsorption, entrapment, covalent coupling and cross-linking or aggregation (no support is used), all of them having pros and cons. Regarding the support, it should be cost-effective, assure the reusability and an easy recovery of the enzyme, increasing its stability and durability. The discussion provided showed that the type of enzyme, its origin, its purity, together with the type of immobilization method and the support will affect the performance during the enzymatic synthesis. Enzymes' immobilization involves interdisciplinary knowledge including enzymology, nanotechnology, molecular dynamics, cellular physiology and process design. The increasing availability of facilities has opened a variety of possibilities to define strategies to optimize the activity and re-usability of β-galactosidases and fructosyltransferases, but there is still great place for innovative developments.

Simulation-guided enzyme discovery: A new microbial source of cellobiose 2-epimerase

Cellobiose 2-epimerase (CE) is a promising industrial enzyme that can be utilized in the dairy industry. More thermostable CEs from different microorganisms are still needed for a higher lactulose productivity. This study demonstrated the feasibility to use molecular dynamics (MD) simulation as the preliminary computational filter for thermostable enzymes screening. Sequence information of eleven uncharacterized CEs were chosen to be analyzed by MD simulations. The CE from Dictyoglomus thermophilum (Dith-CE) was determined experimentally to be one of the most thermostable CEs with the highest epimerization (160 ± 6.5 U mg^-1) and isomerization activities (3.52 ± 0.23 U mg^-1) among all the reported CEs. This enzyme shows the highest isomerization activity at 85 °C and pH 7.0. The kinetic parameters (k_cat and K_m) of isomerization activity of this CE are 3.98 ± 0.3 s^-1 and 235.2 ± 11.2 mM, respectively. These results suggest that the CE from Dith-CE is a promising lactulose-producing enzyme.

Characterisation of a novel cellobiose 2-epimerase from thermophilic Caldicellulosiruptor obsidiansis for lactulose production

BACKGROUND

Lactulose, a bioactive lactose derivative, has been widely used in food and pharmaceutical industries. Isomerisation of lactose to lactulose by cellobiose 2-epimerase (CE) has recently attracted increasing attention, since CE produces lactulose with high yield from lactose as a single substrate. In this study, a new lactulose-producing CE from Caldicellulosiruptor obsidiansis was extensively characterised.

RESULTS

The recombinant enzyme exhibited maximal activity at pH 7.5 and 70 °C. It displayed high thermostability with T_m of 86.7 °C. The half-life was calculated to be 8.1, 2.8 and 0.6 h at 75, 80, and 85 °C, respectively. When lactose was used as substrate, epilactose was rapidly produced in a short period, and afterwards both epilactose and lactose were steadily isomerised to lactulose, with a final ratio of 35:11:54 for lactose:epilactose:lactulose. When the reverse reaction was investigated using lactulose as substrate, both lactose and epilactose appeared to be steadily produced from the start.

CONCLUSION

The recombinant CE showed both epimerisation and isomerisation activities against lactose, making it an alternative promising biocatalyst candidate for lactulose production. © 2016 Society of Chemical Industry.