Fed-batch operation for the synthesis of lactulose with β-galactosidase of Aspergillus oryzae

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.

β-Galactosidase: a traditional enzyme given multiple roles through protein engineering

β-Galactosidases are crucial carbohydrate-active enzymes that naturally catalyze the hydrolysis of galactoside bonds in oligo- and disaccharides. These enzymes are commonly used to degrade lactose and produce low-lactose and lactose-free dairy products that are beneficial for lactose-intolerant people. β-galactosidases exhibit transgalactosylation activity, and they have been employed in the synthesis of galactose-containing compounds such as galactooligosaccharides. However, most β-galactosidases have intrinsic limitations, such as low transglycosylation efficiency, significant product inhibition effects, weak thermal stability, and a narrow substrate spectrum, which greatly hinder their applications. Enzyme engineering offers a solution for optimizing their catalytic performance. The study of the enzyme's structure paves the way toward explaining catalytic mechanisms and increasing the efficiency of enzyme engineering. In this review, the structure features of β-galactosidases from different glycosyl hydrolase families and the catalytic mechanisms are summarized in detail to offer guidance for protein engineering. The properties and applications of β-galactosidases are discussed. Additionally, the latest progress in β-galactosidase engineering and the strategies employed are highlighted. Based on the combined analysis of structure information and catalytic mechanisms, the ultimate goal of this review is to furnish a thorough direction for β-galactosidases engineering and promote their application in the food and dairy industries.

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.

Characterization of a halotolerant GH2 family β-galactosidase GalM from Microvirga sp. strain MC18

A new glycoside hydrolase family 2 (GH2) β-galactosidase encoding gene galM was cloned from Microvirga sp. strain MC18 and overexpressed in Escherichia coli. The recombinant β-galactosidase GalM showed optimal activity at pH 7.0 and 50 °C, with a stability at pH 6.0-9.0 and 20-40 °C, which are conditions suitable for the diary environment. The K_m and V_max values for o-nitrophenyl-β-d-galactopyranoside (oNPG) were 1.30 mmol/L and 15.974 μmol/(min·mg), respectively. GalM showed low product inhibition by galactose with a K_i of 73.18 mM and high tolerance for glucose that 86.5% activity retained in the presence of 500 mM glucose. It was also stable and active in 20% of methanol, ethanol and isopropanol. In addition, the enzyme activity of GalM was activated significantly over 0-2 mol/L NaCl (1.6-4.3 fold). These favorable properties make GalM a potential candidate for the industrial application.

Enzymatic production of lactulose by fed-batch and repeated fed-batch reactor

The effects of the most significant operational variables on reactor performance of fed-batch and repeated fed-batch were evaluated in the lactulose production by enzymatic transgalactosylation. Feed flowrate in the fed stage (F) and fructose to lactose molar ratio (Fr/L) were the variables that mostly affected the values ​​of lactulose yield (Y_Lu), lactulose productivity (π_Lu) and selectivity of transgalactosylation (S_Lu/_TOS). Maximum Y_Lu of 0.21 g lactulose per g lactose was obtained at 50% w/w inlet carbohydrates concentration (IC) of, 50 °C, Fr/L 8, F 1 mL⋅min^-1, 200 IU∙g_Lactose^-1 reactor enzyme load and pH 4.5. At these conditions the selectivity was 7.4, productivity was 0.71 g_Lu∙g^-1∙h^-1and lactose conversion was 0.66. The operation by repeated fed batch increases the efficiency of use of the biocatalysts (E_B) and the accumulated productivity compared to batch and fed batch operation with the same biocatalyst. E_B obtained was 4.13 g_Lu∙mg_{biocatalyst protein}^-1, 10.6 times higher than in fed-batch.

Continuous enzymatic synthesis of lactulose in packed-bed reactor with immobilized Aspergillus oryzae β-galactosidase

Lactulose synthesis from fructose and lactose in continuous packed-bed reactor operation with glyoxyl-agarose immobilized Aspergillus oryzae β-galactosidase is reported for the first time. Alternative strategies to conventional batch synthesis have been scarcely explored for lactulose synthesis. The effect of flow rate, substrates ratio and biocatalyst-inert packing material mass ratio (M_B/M_IM) were studied on reactor performance. Increase in any of these variables produced an increase in lactulose yield (Y_Lu) being higher than obtained in batch synthesis at comparable conditions. Maximum Y_Lu of 0.6 g·g^-1 was obtained at 50 °C, pH 4.5, 50% w/w total sugars, 15 mL·min^-1, fructose/lactose molar ratio of 12 and M_B/M_IM of 1/8 g·g^-1; at such conditions yield of transgalactosylated oligosaccharides (Y_TOS) was 0.16 g·g^-1, selectivity (lactulose/TOS molar ratio) was 5.4 and lactose conversion (X_Lactose) was 28%. Reactor operation with recycle had no significant effect on yield, producing only some decrease in productivity.

New β-galactosidase producers with potential for prebiotic synthesis

β-Galactosidases (EC 3.2.1.23) are interesting enzymes with potential application in the pharmaceutical and food industry. In this work, a screening study was carried out to identify new fungal sources of β-galactosidase. A total of 50 fungi were evaluated using a chromogenic test performed in agar plates. The most promising microorganisms were validated as effective β-galactosidase producers under submerged fermentation conditions. The crude β-galactosidases were characterized regarding their optimal pH (3.0-5.5) and temperature (45-65 °C). All enzymes showed ability to synthesize lactose-based prebiotics, namely lactulose (maximal yield 3.3%) and a galacto-oligosaccharide (GOS) (maximal yield 20%). Additionally, some enzymatic extracts with fructosyltransferase activity allowed to produce other type of prebiotics, namely fructo-oligosaccharides (FOS). This work, reports for the first time the simultaneous synthesis of different mixtures of GOS (2-15% yield and 0.07-0.5 g/L·h^-1 productivity) and FOS (4-30% yield and 0.1-1 g/L·h^-1 productivity) by crude extracts exhibiting dual enzymatic activity.