A highly selective synthesis of N-acetyllactosamine catalyzed by immobilised β-galactosidase from Bacillus circulans

Synthesis of N-Acetyllactosamine and N-Acetyllactosamine-Based Bioactives

N-Acetyllactosamine (LacNAc) or more specifically β-d-galactopyranosyl-1,4-N-acetyl-d-glucosamine is a unique acyl-amino sugar and a key structural unit in human milk oligosaccharides, an antigen component of many glycoproteins, and an antiviral active component for the development of effective drugs against viruses. LacNAc is useful itself and as a basic building block for producing various bioactive oligosaccharides, notably because this synthesis may be used to add value to dairy lactose. Despite a significant amount of information in the literature on the benefits, structures, and types of different LacNAc-derived oligosaccharides, knowledge about their effective synthesis for large-scale production is still in its infancy. This work provides a comprehensive analysis of existing production strategies for LacNAc and important LacNAc-based structures, including sialylated LacNAc as well as poly- and oligo-LacNAc. We conclude that direct extraction from milk is too complex, while chemical synthesis is also impractical at an industrial scale. Microbial routes have application when multiple step reactions are needed, but the major route to large-scale biochemical production will likely lie with enzymatic routes, particularly those using β-galactosidases (for LacNAc synthesis), sialidases (for sialylated LacNAc synthesis), and β-N-acetylhexosaminidases (for oligo-LacNAc synthesis). Glycosyltransferases, especially for the biosynthesis of extended complex LacNAc structures, could also play a major role in the future. In these cases, immobilization of the enzyme can increase stability and reduce cost. Processing parameters, such as substrate concentration and purity, acceptor/donor ratio, water activity, and temperature, can affect product selectivity and yield. More work is needed to optimize these reaction parameters and in the development of robust, thermally stable enzymes to facilitate commercial production of these important bioactive substances.

Synthesis of fucosylated oligosaccharides with α-L-fucosidase from Thermotoga maritima immobilized on Eupergit® CM

Fucosylated oligosaccharides present in human milk perform various biological functions that benefit infants' health. These compounds can be also obtained by enzymatic synthesis. In this work, the effect of the immobilization of α-L-fucosidase from Thermotoga maritima on the synthesis of fucosylated oligosaccharides was studied, using lactose and 4-nitrophenyl-α-L-fucopyranoside (pNP-Fuc) as acceptor and donor substrates, respectively, and Eupergit^® CM as an immobilization support. The enzyme was immobilized with 90% efficiency at pH 8 and ionic strength of 1.5 M. Immobilization decreased enzyme affinity for the donor substrate as shown by a 1.5-times higher K_M value and a 22-times decrease of the k_cat/K_M ratio in comparison to the unbound enzyme. In contrast, no effect was observed on the synthesis/hydrolysis ratio (r_s/r_h) when α-L-fucosidase was immobilized. Also, the effect of initial concentration of substrates was studied. An increase of the acceptor concentration improved the yields of fucosylated oligosaccharides regardless enzyme immobilization. The synthesis yields of 38.9 and 40.6% were obtained using Eupergit^® CM-bound or unbound enzyme, respectively, and 3.5 mM pNP-Fuc and 146 mM lactose. In conclusion, α-L-fucosidase from Thermotoga maritima was efficiently immobilized on Eupergit^® CM support without affecting the synthesis of fucosylated oligosaccharides.

Discovery of Novel High-Molecular Weight Oligosaccharides Containing N-Acetylhexosamine in Bovine Colostrum Whey Permeate Hydrolyzed with Aspergillus oryzae β-Galactosidase

Bovine milk oligosaccharides (BMOs) that resemble human milk oligosaccharides are found in whey permeate, indicating that dairy streams can be used as a potential source of bioactive oligosaccharides. Recovery of oligosaccharides from whey permeate is hindered by their low abundance and high concentration of lactose. In the present work, lactose in bovine colostrum whey permeate was hydrolyzed by Aspergillus oryzae β-galactosidase to facilitate subsequent monosaccharide removal by membrane separation. Chromatographic separation coupled with high-resolution mass spectrometry revealed β-galactosidase degradation of several β-linkage-containing BMOs and production of novel oligosaccharides that ranged in size from 5 to 11 monosaccharide units containing several galactose repeating units and N-acetylhexosamine at their reducing ends. Optimization of BMO hydrolysis and separation methodology could generate high amounts of hetero-oligosaccharides for improved recovery of potentially biotherapeutic oligosaccharides.

Enzymatic synthesis of N-acetyllactosamine from lactose enabled by recombinant β1,4-galactosyltransferases

Synthesis of LacNAc with reversible GalTs.

Sustainable synthesis of N-acetyllactosamine using an immobilized β-galactosidase on a tailor made porous polymer

An efficient enzymatic synthesis of N-acetyllactosamine has been developed in biosolvents, mediated by the action of an immobilized β-galactosidase on a tailor made porous polymer.

Methods for improving enzymatic trans-glycosylation for synthesis of human milk oligosaccharide biomimetics

Recently, significant progress has been made within enzymatic synthesis of biomimetic, functional glycans, including, for example, human milk oligosaccharides. These compounds are mainly composed of N-acetylglucosamine, fucose, sialic acid, galactose, and glucose, and their controlled enzymatic synthesis is a novel field of research in advanced food ingredient chemistry, involving the use of rare enzymes, which have until now mainly been studied for their biochemical significance, not for targeted biosynthesis applications. For the enzymatic synthesis of biofunctional glycans reaction parameter optimization to promote "reverse" catalysis with glycosidases is currently preferred over the use of glycosyl transferases. Numerous methods exist for minimizing the undesirable glycosidase-catalyzed hydrolysis and for improving the trans-glycosylation yields. This review provides an overview of the approaches and data available concerning optimization of enzymatic trans-glycosylation for novel synthesis of complex bioactive carbohydrates using sialidases, α-l-fucosidases, and β-galactosidases as examples. The use of an adequately high acceptor/donor ratio, reaction time control, continuous product removal, enzyme recycling, and/or the use of cosolvents may significantly improve trans-glycosylation and biocatalytic productivity of the enzymatic reactions. Protein engineering is also a promising technique for obtaining high trans-glycosylation yields, and proof-of-concept for reversing sialidase activity to trans-sialidase action has been established. However, the protein engineering route currently requires significant research efforts in each case because the structure-function relationship of the enzymes is presently poorly understood.

From protein engineering to immobilization: promising strategies for the upgrade of industrial enzymes

Enzymes found in nature have been exploited in industry due to their inherent catalytic properties in complex chemical processes under mild experimental and environmental conditions. The desired industrial goal is often difficult to achieve using the native form of the enzyme. Recent developments in protein engineering have revolutionized the development of commercially available enzymes into better industrial catalysts. Protein engineering aims at modifying the sequence of a protein, and hence its structure, to create enzymes with improved functional properties such as stability, specific activity, inhibition by reaction products, and selectivity towards non-natural substrates. Soluble enzymes are often immobilized onto solid insoluble supports to be reused in continuous processes and to facilitate the economical recovery of the enzyme after the reaction without any significant loss to its biochemical properties. Immobilization confers considerable stability towards temperature variations and organic solvents. Multipoint and multisubunit covalent attachments of enzymes on appropriately functionalized supports via linkers provide rigidity to the immobilized enzyme structure, ultimately resulting in improved enzyme stability. Protein engineering and immobilization techniques are sequential and compatible approaches for the improvement of enzyme properties. The present review highlights and summarizes various studies that have aimed to improve the biochemical properties of industrially significant enzymes.

A comparative study of the regioselectivity of the β-galactosidases from Kluyveromyces lactis and Bacillus circulans in the enzymatic synthesis of N-Acetyl-lactosamine in aqueous media

The enzymatic synthesis of N-acetyl-lactosamine (LacNAc) was studied in aqueous media with high substrate concentrations using the transgalactosylation of N-acetyl-D-glucosamine (GlcNAc), starting from lactose as a galactosyl donor. The efficiency and regioselectivity of the β-galactosidases from Kluyveromyces lactis (KlβGal) and Bacillus circulans (BcβGal) were compared. The reaction was optimized by varying the experimental conditions (pH, catalytic activity concentration, and mass concentration ratio of the substrates), which enhanced the synthesis yields with both enzymes and especially with BcβGal. BcβGal catalyzed the formation of the maximal LacNAc concentration obtained (101 mM or 39 g L(-1), corresponding to a yield of 11% on the basis of GlcNAc conversion), after 5 h at pH 6.5 and for a substrate mass concentration ratio of 1. This enzyme also gave an optimal synthesis yield of about 17.5%. No change in regioselectivity was observed when using KlβGal, whereas the regioselectivity of BcβGal proved to be subject to variations, the 1-4 and 1-6 linkages being favored under kinetic and thermodynamic control conditions, respectively. Finally, it was demonstrated that the N-acetyl-allolactosamine synthesized during the GlcNAc transgalactosylation catalyzed by BcβGal was a thermodynamic product and did not result from a chemical and/or enzymatic isomerization of LacNAc.

The effects of organic solvents on the efficiency and regioselectivity of N-acetyl-lactosamine synthesis, using the β-galactosidase from Bacillus circulans in hydro-organic media

The enzymatic synthesis of N-acetyl-lactosamine (LacNAc) by the transgalactosylation of N-acetyl-D-glucosamine (GlcNAc), catalyzed by the β-galactosidase from Bacillus circulans (BcβGal), was studied in hydro-organic media, starting from o-nitrophenyl-β-D-galactopyranoside (oNPG) as a galactosyl donor. Thermal stability and synthesis activity of BcβGal were shown to depend on the organic solvent polarity, characterized by its Log P value. BcβGal was thus most stable in 10% (v/v) t-BuOH, an organic solvent found to have a stabilizing and/or weakly denaturing property, which was confirmed for high t-BuOH concentrations. In the same manner, the optimal synthesis yield increased as the Log P value of the organic solvent increased. The best results were obtained for reactions carried out in 10% (v/v) pyridine or 2-methyl-2-butanol, which gave 47% GlcNAc transgalactosylation yield based on starting oNPG, of which 23% (11 mM; 4.3 g/L) consisted in LacNAc synthesis. Furthermore, it was also established that both the GlcNAc transgalactosylation yield and the enzyme regioselectivity depended on the percentage of organic solvent used, the optimal percentage varying from 10 to 40% (v/v), depending on the solvent. This phenomenon was found to correlate mainly with the thermodynamic activity of water (a(w)) in the aqueous organic solvent mixture, which was found to be optimal when close to 0.96, whatever the organic solvent used. Finally, this study highlighted the fact that the regioselectivity of BcβGal for 1-4 linkage formation could be advantageously managed by controlling the a(w) parameter.