Designing continuous flow reaction of xylan hydrolysis for xylooligosaccharides production in packed-bed reactors using xylanase immobilized on methacrylic polymer-based supports

Homologous expression, purification, and characterization of a recombinant acetylxylan esterase from Aspergillus nidulans

Acetylxylan esterases (AXEs) are essential enzymes that break down the acetyl groups in acetylated xylan found in plant cell walls polysaccharides. They work synergistically with backbone-depolymerizing xylanolytic enzymes to accelerate the degradation of complex polysaccharides. In this study, we cloned the gene axeA, which encodes the acetylxylan esterase from Aspergillus nidulans FGSC A4 (AxeAN), into the pEXPYR expression vector and introduced it into the high protein-producing strain A. nidulans A773. The purified AxeAN, with a molecular weight of 33.5 kDa as confirmed by SDS-PAGE, was found to be active on ρ-nitrophenyl acetate (ρNPA), exhibiting a remarkably high specific activity (170 U mg-1) at pH 7.0 and 55 °C. AxeAN demonstrated stability over a wide pH range (5.5-9.0), retaining >80% of its initial activity after 24 h. The KM and Vmax were 0.098 mmol L-1 and 320 U mg-1, respectively, using ρNPA as a substrate. We also evaluated the synergistic effect of AxeAN with an endo-1,4-β-xylanase from Malbranchea pulchella (MpXyn10) in the hydrolysis of four different xylans (Birchwood, Beechwood, Oat spelt, and Arabinoxylan) to produce xylooligosaccharides (XOS). The best results were obtained using Birchwood xylan as substrate and MpXyn10-AxeAN as biocatalysts after 24 h of reaction (50 °C), with a XOS-yield of 91%, value 41% higher when compared to MpXyn10 (XOS-yield of 63%). These findings showed the potential of the application of AxeAN, together with other xylanases, to produce xylooligosaccharides with high purity and other products with high added value in the field of lignocellulosic biorefinery.

Computer-Aided Reconstruction and Application of Bacillus halodurans S7 Xylanase with Heat and Alkali Resistance

β-1,4-Endoxylanase is the most critical hydrolase for xylan degradation during lignocellulosic biomass utilization. However, its poor stability and activity in hot and alkaline environments hinder its widespread application. In this study, BhS7Xyl from Bacillus halodurans S7 was improved using a computer-aided design through isothermal compressibility (βT) perturbation engineering and by combining three thermostability prediction algorithms (ICPE-TPA). The best variant with remarkable improvement in specific activity, heat resistance (70 °C), and alkaline resistance (both pH 9.0 and 70 °C), R69F/E137M/E145L, exhibited a 4.9-fold increase by wild-type in specific activity (1368.6 U/mg), a 39.4-fold increase in temperature half-life (458.1 min), and a 57.6-fold increase in pH half-life (383.1 min). Furthermore, R69F/E137M/E145L was applied to the hydrolysis of agricultural waste (corncob and hardwood pulp) to efficiently obtain a higher yield of high-value xylooligosaccharides. Overall, the ICPE-TPA strategy has the potential to improve the functional performance of enzymes under extreme conditions for the high-value utilization of lignocellulosic biomass.

Overcoming Biochemical Limitations of Galactose Oxidase through the Design of a Solid-Supported Self-Sufficient Biocatalyst

Galactose Oxidase (GalOx) has gained significant interest in biocatalysis due to its ability for selective oxidation beyond the natural oxidation of galactose, enabling the production of valuable derivatives. However, the practical application of GalOx has been hindered by the limited availability of active and stable biocatalysts, as well as the inherent biochemical limitations such as oxygen (O2 ) dependency and the need for activation. In this study, we addressed these challenges by immobilizing GalOx into agarose-based and Purolite supports to enhance its activity and stability. Additionally, we identified and quantified the oxygen supply limitation into solid catalysts by intraparticle oxygen sensing showing a trade-off between the amount of protein loaded onto the solid support and the catalytic effectiveness of the immobilized enzyme. Furthermore, we coimmobilized a heme-containing protein along with the enzyme to function as an activator. To evaluate the practical application of the immobilized GalOx, we conducted the oxidation of galactose in an instrumented aerated reactor. The results showcased the efficient performance of the immobilized enzyme in the 8 h reaction cycle. Notably, the GalOx immobilized into dextran sulfate-activated agarose exhibited improved stability, overcoming the need for a soluble activator supply, and demonstrated exceptional performance in galactose oxidation. These findings offer promising prospects for the utilization of GalOx in technical biocatalytic applications.

Topochemical Design of Cellulose-Based Carriers for Immobilization of Endoxylanase

Xylooligosaccharides (XOSs) gained much attention for their use in food and animal feed, attributed to their prebiotic function. These short-chained carbohydrates can be enzymatically produced from xylan, one of the most prevalent forms of hemicellulose. In this work, endo-1,4-β-xylanase from Thermotoga maritima was immobilized on cellulose-based beads with the goal of producing xylooligosaccharides with degrees of polymerization (DPs) in the range of 4-6 monomeric units. More specifically, the impact of different spacer arms, tethers connecting the enzyme with the particle, on the expressed enzymatic activity and oligosaccharide yield was investigated. After surface functionalization of the cellulose beads, the presence of amines was confirmed with time of flight secondary ion mass spectrometry (TOF-SIMS), and the influence of different spacer arms on xylanase activity was established. Furthermore, XOSs (DPs 2-6) with up to 58.27 mg/g xylan were obtained, which were greatly enriched in longer oligosaccharides. Approximately 80% of these XOSs displayed DPs between 4 and 6. These findings highlight the importance of topochemical engineering of carriers to influence enzyme activity, and the work puts forward an enzymatic system focusing on the production of longer xylooligosaccharides.

Immobilization and Application of the Recombinant Xylanase GH10 of Malbranchea pulchella in the Production of Xylooligosaccharides from Hydrothermal Liquor of the Eucalyptus (Eucalyptus grandis) Wood Chips

Xylooligosaccharides (XOS) are widely used in the food industry as prebiotic components. XOS with high purity are required for practical prebiotic function and other biological benefits, such as antioxidant and inflammatory properties. In this work, we immobilized the recombinant endo-1,4-β-xylanase of Malbranchea pulchella (MpXyn10) in various chemical supports and evaluated its potential to produce xylooligosaccharides (XOS) from hydrothermal liquor of eucalyptus wood chips. Values >90% of immobilization yields were achieved from amino-activated supports for 120 min. The highest recovery values were found on Purolite (142%) and MANAE-MpXyn10 (137%) derivatives, which maintained more than 90% residual activity for 24 h at 70 °C, while the free-MpXyn10 maintained only 11%. In addition, active MpXyn10 derivatives were stable in the range of pH 4.0−6.0 and the presence of the furfural and HMF compounds. MpXyn10 derivatives were tested to produce XOS from xylan of various sources. Maximum values were observed for birchwood xylan at 8.6 mg mL−1 and wheat arabinoxylan at 8.9 mg mL−1, using Purolite-MpXyn10. Its derivative was also successfully applied in the hydrolysis of soluble xylan present in hydrothermal liquor, with 0.9 mg mL−1 of XOS after 3 h at 50 °C. This derivative maintained more than 80% XOS yield after six cycles of the assay. The results obtained provide a basis for the application of immobilized MpXyn10 to produce XOS with high purity and other high-value-added products in the lignocellulosic biorefinery field.

Nano-fibrillated cellulose-based scaffolds for enzyme (co)-immobilization: Application to natural product glycosylation by Leloir glycosyltransferases

Polysaccharide-based scaffolds are promising carriers for enzyme immobilization. Here, we demonstrate a porous scaffold prepared by direct-ink-writing 3D printing of an ink consisting of nanofibrillated cellulose, carboxymethyl cellulose and citric acid for immobilization application. Negative surface charge introduced by the components made the scaffold amenable for an affinity-like immobilization via the cationic protein module Z_basic2. Z_basic2 fusions of two sugar nucleotide-dependent glycosyltransferases (C-glycosyltransferase, Z-CGT; sucrose synthase, Z-SuSy) were immobilized individually, or co-immobilized, and applied to synthesize the natural C-glycoside nothofagin. The cascade reaction involved β-C-glycosylation of phloretin (10 mM, ~90 % conversion) from UDP-glucose, provided from sucrose and catalytic amounts of UDP (1.0 mM). Enzymes were co-immobilized at ~65 mg protein/g carrier to receive activities of 9.5 U/g (Z-CGT) and 4.5 U/g (Z-SuSy) in 22-33 % yield (protein) and an effectiveness of 23 % (Z-CGT) and 13 % (Z-SuSy). The scaffold-bound enzymes were recyclable for 5 consecutive reactions.

Biocatalytic Production of a Nylon 6 Precursor from Caprolactone in Continuous Flow

6-Aminocaproic acid (6ACA) is a key building block and an attractive precursor of caprolactam, which is used to synthesize nylon 6, one of the most common polymers manufactured nowadays. (Bio)-production of platform chemicals from renewable feedstocks is instrumental to tackle climate change and decrease fossil fuel dependence. Here, the cell-free biosynthesis of 6ACA from 6-hydroxycaproic acid was achieved using a co-immobilized multienzyme system based on horse liver alcohol dehydrogenase, Halomonas elongata transaminase, and Lactobacillus pentosus NADH oxidase for in-situ cofactor recycling, with >90 % molar conversion (m.c.) The integration of a step to synthesize hydroxy-acid from lactone by immobilized Candida antarctica lipase B resulted in >80 % m.c. of ϵ-caprolactone to 6ACA, >20 % of δ-valerolactone to 5-aminovaleric acid, and 30 % of γ-butyrolactone to γ-aminobutyric acid in one-pot batch reactions. Two serial packed-bed reactors were set up using these biocatalysts and applied to the continuous-flow synthesis of 6ACA from ϵ-caprolactone, achieving a space-time yield of up to 3.31 g_6ACA  h^-1  L^-1 with a segmented liquid/air flow for constant oxygen supply.

Efficacious bioconversion of waste walnut shells to xylotetrose and xylopentose by free xylanase (Xy) and MOF immobilized xylanase (Xy-Cu-BTC)

This study uses a cost effective and efficient method for production of higher DP (degree of polymerization) Xylooligosaccharides (XOS) from xylan extracted from the waste walnut shells. Copper based metal organic framework (Cu-BTC MOF) was prepared for immobilization of free xylanase (Xy) enzyme by green synthesis method. Both free and immobilized xylanase (Xy-Cu-BTC) were able to cause the bioconversion of xylan (87.4% yield) into XOS. Predominant production of xylotetrose (X4) and xylopentose (X5) was observed for both the methods. Percentage XOS conversion for free enzyme (Xy) was found to be 4.1% X4 and 60.57% X5 whereas these values increased in case of immobilized system where 11.8% X4 and 64.2% X5 were produced. Xylose production was minute in case of immobilized xylanase 0.88% which makes it a better method for XOS production free from xylose interference. Xy-Cu-BTC MOF can hence be used as an attractive alternative for pure XOS production.

Cross-Linked Enzyme Aggregates and Their Application in Enzymatic Pretreatment of Microalgae: Comparison Between CLEAs and Combi-CLEAs

Carrier-free immobilization is a key process to develop efficient biocatalysts able to catalyze the cell wall degradation in microalgae where the traditional solid supports cannot penetrate. Thus, the insolubilization of commercial Celluclast®, Alcalase®, and Viscozyme® enzymes by carrier-free immobilization and their application in microalgae pretreatment was investigated. In this study, different precipitants at different ratios (ethanol, acetone, and polyethylene glycol 4000) were tested in the first part of the method, to establish the precipitation conditions. The screening of the best precipitant is needed as it depends on the nature of the enzyme. The best results were studied in terms of immobilization yield, thermal stability, and residual activity and were analyzed using scanning electron microscopy. Moreover, a novel strategy was intended including the three enzymes (combi-CLEAs) to catalyze the enzymatic degradation of Nannochloropsis gaditana microalgal cell wall in one pot. The carrier-free immobilized derivatives were 10 times more stable compared to soluble enzymes under the same. At the best conditions showed its usefulness in the pretreatment of microalgae combined with ultrasounds, facilitating the cell disruption and lipid recovery. The results obtained suggested the powerful application of these robust biocatalysts with great catalytic properties on novel and sustainable biomass such as microalgae to achieve cost-effective and green process to extract valuable bioactive compounds.

A multifaceted enzyme conspicuous in fruit juice clarification: An elaborate review on xylanase

Xylanase enzyme has been classified as an enzyme belonging to the glycoside hydrolase family. The catalytic action of xylanase is focused on the degradation of xylan, a substrate for this enzyme comprising of a complex arrangement of monosaccharides interlinked with the help of ester and glycosidic bonds. Xylan represents the second most profuse renewable polysaccharide present on earth. Breakage of the β- 1, 4-glycoside linkage in the xylan polymer is what makes xylanase enzyme an important biocatalyst favoring various applications including treatment of pulp for improving paper quality, improvement of bread quality, treatment of lignocelluloses waste, production of xylose sugar and production of biological fuels. Most recently, xylanase has been exploited in the food industry for the purpose of fruit juice clarification. Turbidity caused by the colloidal polysaccharides present in the freshly squeezed fruit juice poses a setback to the fruit juice industry since the commercial product must be clear and free of excess polysaccharides to improve juice quality and storage life. This review gives an overview of the recent advancements made in regards to xylanase enzyme being used commercially with main focus on its role in fruit juice clarification.

Stabilization of enzymes via immobilization: Multipoint covalent attachment and other stabilization strategies

The use of enzymes in industrial processes requires the improvement of their features in many instances. Enzyme immobilization, a requirement to facilitate the recovery and reuse of these water-soluble catalysts, is one of the tools that researchers may utilize to improve many of their properties. This review is focused on how enzyme immobilization may improve enzyme stability. Starting from the stabilization effects that an enzyme may experience by the mere fact of being inside a solid particle, we detail other possibilities to stabilize enzymes: generation of favorable enzyme environments, prevention of enzyme subunit dissociation in multimeric enzymes, generation of more stable enzyme conformations, or enzyme rigidification via multipoint covalent attachment. In this last point, we will discuss the features of an "ideal" immobilization protocol to maximize the intensity of the enzyme-support interactions. The most interesting active groups in the support (glutaraldehyde, epoxide, glyoxyl and vinyl sulfone) will be also presented, discussing their main properties and uses. Some instances in which the number of enzyme-support bonds is not directly related to a higher stabilization will be also presented. Finally, the possibility of coupling site-directed mutagenesis or chemical modification to get a more intense multipoint covalent immobilization will be discussed.

Biocatalytic access to betazole using a one-pot multienzymatic system in continuous flow

As an alternative to classical synthetic approaches for the production of betazole drug, a one-pot biocatalytic system for this pharmaceutical molecule from its alcohol precursor has been developed. An ω-transaminase, an alcohol dehydrogenase and a water-forming NADH oxidase for in situ cofactor recycling have been combined to catalyse this reaction, yielding 75% molar conversion in batch reactions with soluble enzymes. This multienzyme system was then co-immobilised through a newly established protocol for sequential functionalization of a methacrylate-based porous carrier to enable tailored immobilisation chemistries for each enzyme. This pluri-catalytic system has been set up in a continuous flow packed-bed reactor, generating a space-time yield of up to 2.59 g L-1 h-1 with 15 min residence and a constant supply of oxygen for in situ cofactor recycling through a segmented air-liquid flow. The addition of an in-line catch-and-release column afforded >80% product recovery.

Process intensification using immobilized enzymes for the development of white biotechnology

Process intensification of biocatalysed reactions using different techniques such as microwaves, ultrasound, hydrodynamic cavitation, ionic liquids, microreactors and flow chemistry in various industries is critically analysed and future directions provided.

High Stabilization of Enzymes Immobilized on Rigid Hydrophobic Glyoxyl-Supports: Generation of Hydrophilic Environments on Support Surfaces

Very rigid supports are useful for enzyme immobilization to design continuous flow reactors and/or to work in non-conventional media. Among them, epoxy-methacrylic supports are easily functionalized with glyoxyl groups, which makes them ideal candidates for enzyme stabilization via multipoint covalent immobilization. However, these supports present highly hydrophobic surfaces, which might promote very undesirable effects on enzyme activity and/or stability. The hydrophilization of the support surface after multipoint enzyme immobilization is proposed here as an alternative to reduce these undesirable effects. The remaining aldehyde groups on the support are modified with aminated hydrophilic small molecules (glycine, lysine or aspartic acid) in the presence of 2-picoline borane. The penicillin G acylase from Escherichia coli (PGA) and alcohol dehydrogenase from Thermus thermophilus HB27 (ADH2) were immobilized on glyoxyl-functionalized agarose, Relizyme and Relisorb. Despite the similar density of aldehyde groups displayed by functionalized supports, their stabilization effects on immobilized enzymes were quite different: up to 300-fold lower by hydrophobic supports than by highly hydrophilic glyoxyl-agarose. A dramatic increase in the protein stabilities was shown when a hydrophilization treatment of the hydrophobic support surface was done. The PGA immobilized on the glyoxyl-Relisorb hydrophilized with aspartic acid becomes 280-fold more stable than without any treatment, and it is even more stable than the PGA immobilized on the glyoxyl agarose.

Production of xylooligosaccharides and monosaccharides from hydrogen peroxide-acetic acid-pretreated poplar by two-step enzymatic hydrolysis

The severe pretreatment of poplar makes xylan difficult to utilize efficiently. In this work, poplar was pretreated by hydrogen peroxide-acetic acid (HPAC) with H₂SO₄ as catalyst to remove lignin, and the solid residues were used to produce xylooligosaccharides (XOS) and monosaccharides by two-step xylanase and cellulase hydrolysis. The results indicated that higher H₂SO₄ concentrations in the HPAC pretreatment of poplar afforded stronger lignin removal ability. An increased XOS yield of 19.8% was obtained from 200 mM H₂SO₄-catalyzed poplar by xylanase and the XOS purity was high, with a very low xylose/XOS ratio of 0.14. Higher glucose (75.2%) and xylose (61.4%) yields were obtained from the HPAC-pretreated poplar using 50 mM H₂SO₄ as catalyst. Finally, 16.9 g XOS and 296.4 g glucose were produced from 1 kg poplar by xylanase and cellulase. This study provides a method for producing functional XOS and monosaccharides from poplar using a simple reduced-pollution strategy.

Protein immobilization technology for flow biocatalysis

Enzymatic immobilization has been at the forefront of applied biocatalysis as it enables convenient isolation and reuse of the catalyst if the target reaction is conducted in batch, and it has opened up significant opportunities to conduct biocatalysis in continuous mode. Over the last few years, an array of techniques to immobilize enzymes have been developed, spanning from covalent multipoint attachment to noncovalent electrostatic strategies to rational architecture to suitably orient the enzyme(s). In addition, new materials have been adapted to support biological catalysts. Here, we discuss the advances of the last two years in enzyme immobilization for continuous flow applications.

Efficient Enzymatic Hydrolysis of Biomass Hemicellulose in the Absence of Bulk Water

Current enzymatic methods for hemicellulosic biomass depolymerization are solution-based, typically require a harsh chemical pre-treatment of the material and large volumes of water, yet lack in efficiency. In our study, xylanase (E.C. 3.2.1.8) from Thermomyces lanuginosus is used to hydrolyze xylans from different sources. We report an innovative enzymatic process which avoids the use of bulk aqueous, organic or inorganic solvent, and enables hydrolysis of hemicellulose directly from chemically untreated biomass, to low-weight, soluble oligoxylosaccharides in >70% yields.

A detailed overview of xylanases: an emerging biomolecule for current and future prospective

Xylan is the second most abundant naturally occurring renewable polysaccharide available on earth. It is a complex heteropolysaccharide consisting of different monosaccharides such as l-arabinose, d-galactose, d-mannoses and organic acids such as acetic acid, ferulic acid, glucuronic acid interwoven together with help of glycosidic and ester bonds. The breakdown of xylan is restricted due to its heterogeneous nature and it can be overcome by xylanases which are capable of cleaving the heterogeneous β-1,4-glycoside linkage. Xylanases are abundantly present in nature (e.g., molluscs, insects and microorganisms) and several microorganisms such as bacteria, fungi, yeast, and algae are used extensively for its production. Microbial xylanases show varying substrate specificities and biochemical properties which makes it suitable for various applications in industrial and biotechnological sectors. The suitability of xylanases for its application in food and feed, paper and pulp, textile, pharmaceuticals, and lignocellulosic biorefinery has led to an increase in demand of xylanases globally. The present review gives an insight of using microbial xylanases as an “Emerging Green Tool” along with its current status and future prospective.

Design of stable magnetic hybrid nanoparticles of Si-entrapped HRP

Hybrid and composite nanoparticles represent an attractive material for enzyme integration due to possible synergic advantages of the structural builders in the properties of the nanobiocatalyst. In this study, we report the synthesis of a new stable hybrid nanobiocatalyst formed by biomimetic silica (Si) nanoparticles entrapping both Horseradish Peroxidase (HRP) (EC 1.11.1.7) and magnetic nanoparticles (MNPs). We have demonstrated that tailoring of the synthetic reagents and post immobilization treatments greatly impacted physical and biocatalytic properties such as an unprecedented ~280 times increase in the half-life time in thermal stability experiments. The optimized nanohybrid biocatalyst that showed superparamagnetic behaviour, was effective in the batch conversion of indole-3-acetic acid, a prodrug used in Direct Enzyme Prodrug Therapy (DEPT). Our system, that was not cytotoxic per se, showed enhanced cytotoxic activity in the presence of the prodrug towards HCT-116, a colorectal cancer cell line. The strategy developed proved to be effective in obtaining a stabilized nanobiocatalyst combining three different organic/inorganic materials with potential in DEPT and other biotechnological applications.

Stabilization of Enzymes by Multipoint Covalent Attachment on Aldehyde-Supports: 2-Picoline Borane as an Alternative Reducing Agent

Enzyme immobilization by multipoint covalent attachment on supports activated with aliphatic aldehyde groups (e.g., glyoxyl agarose) has proven to be an excellent immobilization technique for enzyme stabilization. Borohydride reduction of immobilized enzymes is necessary to convert enzyme–support linkages into stable secondary amino groups and to convert the remaining aldehyde groups on the support into hydroxy groups. However, the use of borohydride can adversely affect the structure–activity of some immobilized enzymes. For this reason, 2-picoline borane is proposed here as an alternative milder reducing agent, especially, for those enzymes sensitive to borohydride reduction. The immobilization-stabilization parameters of five enzymes from different sources and nature (from monomeric to multimeric enzymes) were compared with those obtained by conventional methodology. The most interesting results were obtained for bacterial (R)-mandelate dehydrogenase (ManDH). Immobilized ManDH reduced with borohydride almost completely lost its catalytic activity (1.5% of expressed activity). In contrast, using 2-picoline borane and blocking the remaining aldehyde groups on the support with glycine allowed for a conjugate with a significant activity of 19.5%. This improved biocatalyst was 357-fold more stable than the soluble enzyme at 50 °C and pH 7. The results show that this alternative methodology can lead to more stable and active biocatalysts.