Magnetic cross-linked enzyme aggregates (CLEAs): A novel concept towards carrier free immobilization of lignocellulolytic enzymes

Enzyme immobilization technology as a tool to innovate in the production of biofuels: A special review of the Cross-Linked Enzyme Aggregates (CLEAs) strategy

This review emphasizes the crucial role of enzyme immobilization technology in advancing the production of two main biofuels, ethanol and biodiesel, with a specific focus on the Cross-linked Enzyme Aggregates (CLEAs) strategy. This method of immobilization has gained attention due to its simplicity and affordability, as it does not initially require a solid support. CLEAs synthesis protocol includes two steps: enzyme precipitation and cross-linking of aggregates using bifunctional agents. We conducted a thorough search for papers detailing the synthesis of CLEAs utilizing amylases, cellulases, and hemicellulases. These key enzymes are involved in breaking down starch or lignocellulosic materials to produce ethanol, both in first and second-generation processes. CLEAs of lipases were included as these enzymes play a crucial role in the enzymatic process of biodiesel production. However, when dealing with large or diverse substrates such as lignocellulosic materials for ethanol production and oils/fats for biodiesel production, the use of individual enzymes may not be the most efficient method. Instead, a system that utilizes a blend of enzymes may prove to be more effective. To innovate in the production of biofuels (ethanol and biodiesel), enzyme co-immobilization using different enzyme species to produce Combi-CLEAs is a promising trend.

Immobilized enzyme for screening and identification of anti-diabetic components from natural products by ligand fishing

Diabetes is a chronic metabolic disease caused by insufficient insulin secretion and insulin resistance. Natural product is one of the most important resources for anti-diabetic drug. However, due to the extremely complex composition, this research is facing great challenges. After the advent of ligand fishing technology based on enzyme immobilization, the efficiency of screening anti-diabetic components has been greatly improved. In order to provide critical knowledge for future research in this field, the application progress of immobilized enzyme in screening anti-diabetic components from complex natural extracts in recent years was reviewed comprehensively, including novel preparation technologies and strategies of immobilized enzyme and its outstanding application prospect in many aspects. The basic principles and preparation steps of immobilized enzyme were briefly described, including entrapment, physical adsorption, covalent binding, affinity immobilization, multienzyme system and carrier-free immobilization. New formatted immobilized enzymes with different carriers, hollow fibers, magnetic materials, microreactors, metal organic frameworks, etc., were widely used to screen anti-diabetic compositions from various natural products, such as Ginkgo biloba, Morus alba, lotus leaves, Pueraria lobata, Prunella vulgaris, and Magnolia cortex. Furthermore, the challenges and future prospects in this field were put forward in this review.

(Magnetic) Cross-Linked Enzyme Aggregates of Cellulase from T. reesei: A Stable and Efficient Biocatalyst

Cross-linked enzyme aggregates (CLEAs) represent an effective tool for carrier-free immobilization of enzymes. The present study promotes a successful application of functionalized magnetic nanoparticles (MNPs) for stabilization of cellulase CLEAs. Catalytically active CLEAs and magnetic cross-linked enzyme aggregates (mCLEAs) of cellulase from Trichoderma reesei were prepared using glutaraldehyde (GA) as a cross-linking agent and the catalytic activity and stability of the CLEAs/mCLEAs were investigated. The influence of precipitation agents, cross-linker concentration, concentration of enzyme, addition of bovine serum albumin (BSA), and addition of sodium cyanoborohydride (NaBH₃CN) on expressed activity and immobilization yield of CLEAs/mCLEAs was studied. Particularly, reducing the unsaturated Schiff's base to form irreversible linkages is important and improved the activity of CLEAs (86%) and mCLEAs (91%). For increased applicability of CLEAs/mCLEAs, we enhanced the activity and stability at mild biochemical process conditions. The reusability after 10 cycles of both CLEAs and mCLEAs was investigated, which retained 72% and 65% of the initial activity, respectively. The thermal stability of CLEAs and mCLEAs in comparison with the non-immobilized enzyme was obtained at 30 °C (145.65% and 188.7%, respectively) and 50 °C (185.1% and 141.4%, respectively). Kinetic parameters were determined for CLEAs and mCLEAs, and the K_M constant was found at 0.055 ± 0.0102 mM and 0.037 ± 0.0012 mM, respectively. The maximum velocity rate (V_max) was calculated as 1.12 ± 0.0012 µmol/min for CLEA and 1.17 ± 0.0023 µmol/min for mCLEA. Structural characterization was studied using XRD, SEM, and FT-IR. Catalytical properties of immobilized enzyme were improved with the addition of reducent NaBH₃CN by enhancing the activity of CLEAs and with addition of functionalized aminosilane MNPs by enhancing the activity of mCLEAs.

The Synthesis of (Magnetic) Crosslinked Enzyme Aggregates With Laccase, Cellulase, β-Galactosidase and Transglutaminase

Immobilized enzymes have important aspects due to the fact that they possess higher stability, have the possibility to be easily removed from the reaction mixture, and are much easier to use when compared to free enzymes. In this research, the enzymes laccase, cellulase, β-galactosidase (β-gal), and transglutaminase (TGM) were immobilized by two different methods: crosslinked enzyme aggregates (CLEAs) and magnetic crosslinked enzyme aggregates (mCLEAs). The processes for CLEAs and mCLEAs preparation with different enzymes have been optimized, where the aim was to achieve the highest possible relative activity of the immobilized enzyme. The optimal conditions of the synthesis of CLEAs in mCLEAs are described, thus emphasizing the difference between the two types of immobilization based on different enzymes. This comparative study, which represents the synthesis of crosslinked enzyme aggregates using different enzymes, has not been performed so far. Moreover, the obtained activity of CLEAs and mCLEAs is presented, which is important for further use in different biocatalytic processes. Specifically, of a higher importance is the selection of enzymes involved in immobilization, as they belong to the three different most applicable enzymes (oxidoreductases, hydrolases, and transferases). The study confirmed that the resulting activity of the immobilized enzyme and the optimization of enzyme immobilization depended on the type of the enzyme. Moreover, the prepared CLEAs and mCLEAs were exposed to the supercritical carbon dioxide (scCO₂) at different pressures to determine the effect of scCO₂ on enzyme activity in immobilized form. Additionally, to demonstrate the reuse and stability of the immobilized enzyme, the stability and reusability tests of CLEAs and mCLEAs were performed. The catalytic performance of immobilized enzyme was tested, where the catalytic efficiency and long-term operational stability of mCLEAs were obviously superior to those of CLEAs. However, the higher activity observed for CLEAs compared to mCLEAs suggests a significant effect of magnetic nanoparticles in the stabilization of an enzyme crosslinked aggregate structure.

Magnetic Nanomaterials as Biocatalyst Carriers for Biomass Processing: Immobilization Strategies, Reusability, and Applications

Environmental concerns, along with oil shortages, have increased industrial interest in biomass conversion to produce biofuels and other valuable chemicals. A green option in biomass processing is the use of enzymes, such as cellulases, hemicellulases, and ligninolytic (laccase and peroxidases), which have outstanding specificity toward their substrates and can be reused if immobilized onto magnetic nanocarriers. Numerous studies report the biocatalysts’ performance after covalent binding or adsorption on differently functionalized magnetic nanoparticles (MNPs). Functionalization strategies of MNPs include silica-based surfaces obtained through a sol–gel process, graphene oxide-based nanocomposites, polymer-coated surfaces, grafting polymer brushes, and others, which have been emphasized in this review of the immobilization and co-immobilization of enzymes used for biomass conversion. Careful analysis of the parameters affecting the performance of enzyme immobilization for new hybrid matrices has enabled us to achieve wider tolerance to thermal or chemical stress by these biosystems during saccharification. Additionally, it has enabled the application of immobilized laccase to remove toxic organic compounds from lignin, among other recent advances addressed here related to the use of reusable magnetic carriers for bioderived chemical manufacturing.

Multifaceted Therapy of Nanocatalysts in Neurological Diseases

With the development of enzymes immobilization technology and the discover of nanozymes, catalytic therapy exhibited tremendous potential for neurological diseases therapy. In especial, since the discovery of Fe₃O₄ nanoparticles possessing intrinsic peroxidase-like activity, various nanozymes have been developed and recently started to explore for neurological diseases therapy, such as Alzheimer's disease, Parkinson's disease and stroke. By combining the catalytic activities with other properties (such as optical, thermal, electrical, and magnetic properties) of nanomaterials, the multifunctional nanozymes would not only alleviate oxidative and nitrosative stress on the basis of multienzymes-mimicking activity, but also exert positive effects on immunization, inflammation, autophagy, protein aggregation, which provides the foundation for multifaceted treatments. This review will summarize various types of nanocatalysts and further provides a valuable discussion on multifaceted treatment by nanozymes for neurological diseases, which is anticipated to provide an easily accessible guide to the key opportunities and current challenges of the nanozymes-mediated treatments for neurological diseases.

Various type immobilizations of Isocitrate dehydrogenases enzyme on hyaluronic acid modified magnetic nanoparticles as stable biocatalysts

Magnetic nanoparticles (MNPs) were modified by hyaluronic acid (HA). After the process of functionalization, two different strategies have been used to immobilize isocitrate dehydrogenases (IDH) on MNPs. In the first strategy, cross-linked enzyme aggregates were prepared. For this, firstly hyaluronic acid modified magnetic nanoparticles cross-linked enzyme fine aggregates of isocitrate dehydrogenases (IDH/HA/MNPs-CLEAs) were synthesized, and secondly bovine serum albumin (BSA) as co-feeder was used to synthesize the IDH/BSA/HA/MNPs-CLEAs. In the second strategy, the IDH was effectively immobilized on the HA/MNPs surface. The features of MNPs and its derivatives have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), vibrating sample magnetometer (VSM), and zeta potential measurements. The activity and stability of IDH in IDH/HA/MNPs, IDH/HA/MNPs-CLEAs, and IDH/BSA/HA/MNPs-CLEAs were enhanced. Besides, the enzyme immobilized was readily separated via external magnet from the reaction medium and reused many times. The acquired findings indicate that HA/MNPs are a novel binder/support system to IDH, and IDH immobilized on this system can become a very important biocatalyst working with high accuracy and sensitivity for the determination of magnesium in drinking water and other biological solutions.

Magnetic Cross-Linked Enzyme Aggregates of a Transpeptidase-Specialized Variant (N450D) of Bacillus licheniformis γ-Glutamyl Transpeptidase: An Efficient and Stable Biocatalyst for l-Theanine Synthesis

γ-Glutamyl transpeptidase (GGT) catalyzes the transfer of glutathione’s γ-glutamyl group and related γ-glutamyl amides to water, amino acids or peptides, and utilizes a conserved Thr residue to process its own polypeptide chain into a large and a small subunit that then assemble to produce a catalytically competent enzyme. In this study, the magnetic cross-linked enzyme aggregates (mCLEAs) of a transpeptidase-specialized variant (N450D) of Bacillus licheniformis GGT were successfully prepared with optimized process parameters viz.1.25:1 (v/v) of isopropanol to N450D (0.3 mg/mL) ratio/0.02:1 (w/w) of enzyme to 3-aminopropyl triethoxysilane (APTES)-coated magnetic nanoparticle ratio/20 mM of glutaraldehyde. The prepared magnetic nanoparticles and immobilized enzyme (N450D-mCLEAs) were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscope integrated with energy dispersive X-ray spectroscopy (FESEM/EDS), and superparamagnetic analysis. As compared with free enzyme, N450D-mCLEAs displayed significantly higher heat resistance at temperatures of 55 and 60 °C, and had a greater stability over a storage period of one month. The immobilized enzyme could also be reused for 10 consecutive biocatalytic cycles with no significant reduction in the percent yield of l-theanine. Conclusively, this immobilization strategy surely provides a meaningful glance of developing N450D-mediated biocatalysis for the production of physiologically important γ-glutamyl compounds.

Characterization of Cross-Linked Enzyme Aggregates of the Y509E Mutant of a Glycoside Hydrolase Family 52 β-xylosidase from G. stearothermophilus

Cross-linked enzyme aggregates (CLEAs) of the Y509E mutant of glycoside hydrolase family 52 β-xylosidase from Geobacillus stearothermophilus with dual activity of β-xylosidase and xylanase (XynB2^Y509E) were prepared. Ammonium sulfate was used as the precipitant agent, and glutaraldehyde as cross-linking agent. The optimum conditions were found to be 90% ammonium sulfate, 12.5 mM glutaraldehyde, 3 h of cross-linking reaction at 25 °C, and pH 8.5. Under these (most effective) conditions, XynB2^Y509E-CLEAs retained 92.3% of their original β-xylosidase activity. Biochemical characterization of both crude and immobilized enzymes demonstrated that the maximum pH and temperature after immobilization remained unchanged (pH 6.5 and 65 °C). Moreover, an improvement in pH stability and thermostability was also found after immobilization. Analysis of kinetic parameters shows that the K _m value of XynB2^Y509E-CLEAs obtained was slightly higher than that of free XynB2^Y509E (1.2 versus 0.9 mM). Interestingly, the xylanase activity developed by the mutation was also conserved after the immobilization process.

Preparation, characterization and stability studies of cross-linked α-amylase aggregates (CLAAs) for continuous liquefaction of starch

In current study, α-amylase of fungal origin was immobilized using cross-linking strategy. The influence of precipitant (ammonium sulphate) and cross-linker (glutaraldehyde) concentration revealed that 60% (w/v) precipitant and 1.5% (v/v) cross-linker saturation was required to attain optimum activity. Cross-linked amylase aggregates (CLAAs) were characterized and 10-degree shift in optimum temperature (soluble enzyme: 50 °C; cross-linked: 60 °C) and 1-unit shift in pH (soluble enzyme: pH -6; cross-linked: pH -7) was observed after immobilization. The V_max for soluble α-amylase and its cross-linked form was 1225 U ml^-1 and 3629 U ml^-1, respectively. The CLAAs was more thermostable than its soluble form and retained its 30% activity even after 60 min of incubation at 70 °C. Moreover, cross-linked amylase retained its activity after two months while its soluble counterpart lost its complete activity after 10 and 20 days at 30 °C and 4 °C storage, respectively. Reusability test showed that cross-linked amylase could retain 13% of its residual activity after 10 repeated cycles. Therefore, 10 times more glucose was produced after cross-linking than soluble amylase when it was utilized multiple times. This study indicates that amylase aggregates are highly effective for continuous liquefaction of starch, hence have strong potential to be used for different industrial processes.

Immobilization of Eversa® Transform via CLEA Technology Converts It in a Suitable Biocatalyst for Biolubricant Production Using Waste Cooking Oil

The performance of the previously optimized magnetic cross-linked enzyme aggregate of Eversa (Eversa-mCLEA) in the enzymatic synthesis of biolubricants by transesterification of waste cooking oil (WCO) with different alcohols has been evaluated. Eversa-mCLEA showed good activities using these alcohols, reaching a transesterification activity with isoamyl alcohol around 10-fold higher than with methanol. Yields of isoamyl fatty acid ester synthesis were similar using WCO or refined oil, confirming that this biocatalyst could be utilized to transform this residue into a valuable product. The effects of WCO/isoamyl alcohol molar ratio and enzyme load on the synthesis of biolubricant were also investigated. A maximum yield of around 90 wt.% was reached after 72 h of reaction using an enzyme load of 12 esterification units/g oil and a WCO/alcohol molar ratio of 1:6 in a solvent-free system. At the same conditions, the liquid Eversa yielded a maximum ester yield of only 34%. This study demonstrated the great changes in the enzyme properties that can be derived from a proper immobilization system. Moreover, it also shows the potential of WCO as a feedstock for the production of isoamyl fatty acid esters, which are potential candidates as biolubricants.

Enzymatic glycosylation of bioactive acceptors catalyzed by an immobilized fungal β-xylosidase and its multi-glycoligase variant

A recombinant β-xylosidase (rBxTW1) from the ascomycete Talaromyces amestolkiae and a mutant derived from it, with mostly synthetic activity, have been immobilized as magnetic cross-linked enzyme aggregates (mCLEAs). The mCLEAs of rBxTW1 kept the excellent hydrolytic and O-transxylosylating activities of the free enzyme and had improved thermal and pH stability. The mCLEAs of the mutant also maintained or improved the catalytic properties of the soluble enzyme, synthetizing the O-xylosides of vanillin and (-)-epigallocatechin gallate, and the N- and S-xyloside of 3,5-dibromo-1,2,4-triazole and thiophenol, respectively. The mCLEAs were recyclable across 4 cycles of synthesis of the O-xylosides through a green and highly selective process. The magnetic properties of the scaffold used for immobilization may allow the easy recovery and reuse of the biocatalyst even from reactions containing insoluble lignocellulosic biomass.

Soluble and Cross-Linked Aggregated Forms of α-Galactosidase from Vigna mungo Immobilized on Magnetic Nanocomposites: Improved Stability and Reusability

α-Galactosidases hold immense potential due to their biotechnological applications in various industrial and functional food sectors. In the present study, soluble and covalently cross-linked aggregated forms of a low molecular weight, thermo-labile α-galactosidase from Vigna mungo (VM-αGal) seeds were immobilized onto chitosan-coated magnetic nanoparticles for improved stability and repeated usage by magnetic separation. Parameters like precipitants (type, amount, and ratio), glutaraldehyde concentration, and enzyme load were optimized for the preparation of chitosan-coated magnetic nanocomposites of cross-linked VM-αGal (VM-αGal-MC) and VM-αGal (VM-αGal-M) resulted in 100% immobilization efficiency. Size and morphology of VM-αGal-M were studied through dynamic light scattering (DLS) and scanning electron microscopy (SEM), while Fourier transform infrared spectroscopy (FTIR) was used to study the chemical composition of VM-αGal-MC and VM-αGal-M. VM-αGal-MC and VM-αGal-M were found more active in a broad range of pH (3-8) and displayed optimal temperatures up to 25 °C higher than VM-αGal. Addition of non-ionic detergents (except Tween-40) improved VM-αGal-MC activity by up to 44% but negatively affected VM-αGal-M activity. Both VM-αGal-MC (15% residual activity after 21 min at 85 °C, E_d 92.42 kcal/mol) and VM-αGal-M (69.0% residual activity after 10 min at 75 °C, E_d 39.87 kcal/mol) showed remarkable thermal stability and repeatedly hydrolyzed the substrate for 10 cycles.

Biocatalysis and biomass conversion: enabling a circular economy

This paper is based on a lecture presented to the Royal Society in London on 24 June 2019. Two of the grand societal and technological challenges of the twenty-first century are the 'greening' of chemicals manufacture and the ongoing transition to a sustainable, carbon neutral economy based on renewable biomass as the raw material, a so-called bio-based economy. These challenges are motivated by the need to eliminate environmental degradation and mitigate climate change. In a bio-based economy, ideally waste biomass, particularly agricultural and forestry residues and food supply chain waste, are converted to liquid fuels, commodity chemicals and biopolymers using clean, catalytic processes. Biocatalysis has the right credentials to achieve this goal. Enzymes are biocompatible, biodegradable and essentially non-hazardous. Additionally, they are derived from inexpensive renewable resources which are readily available and not subject to the large price fluctuations which undermine the long-term commercial viability of scarce precious metal catalysts. Thanks to spectacular advances in molecular biology the landscape of biocatalysis has dramatically changed in the last two decades. Developments in (meta)genomics in combination with 'big data' analysis have revolutionized new enzyme discovery and developments in protein engineering by directed evolution have enabled dramatic improvements in their performance. These developments have their confluence in the bio-based circular economy. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Crosslinked Enzyme Aggregates (CLEAs) of Laccases from Pleurotus citrinopileatus Induced in Olive Oil Mill Wastewater (OOMW)

The enzymatic factory of ligninolytic fungi has proven to be a powerful tool in applications regarding the degradation of various types of pollutants. The degradative potential of fungi is mainly due to the production of different types of oxidases, of which laccases is one of the most prominent enzymatic activities. In the present work, crude laccases from the supernatant of Pleurotus citrinopileatus cultures grown in olive oil mill wastewater (OOMW) were immobilized in crosslinked enzyme aggregates (CLEAs), aiming at the development of biocatalysts suitable for the enzymatic treatment of OOMW. The preparation of laccase CLEAs was optimized, resulting in a maximum of 72% residual activity. The resulting CLEAs were shown to be more stable in the presence of solvents and at elevated temperatures compared to the soluble laccase preparation. The removal of the phenolic component of OOMW catalyzed by laccase-CLEAs exceeded 35%, while they were found to retain their activity for at least three cycles of repetitive use. The described CLEAs can be applied for the pretreatment of OOMW, prior to its use for valorization processes, and thus, facilitate its complete biodegradation towards a consolidated process in the context of circular economy.

Extraction and crosslinking of bromelain aggregates for improved stability and reusability from pineapple processing waste

The present study is first of its kind that focuses upon the extraction of bromelain from pineapple core waste and stabilising it as insoluble cross-linked aggregates. The influence of process variables such as the choice of precipitant, type of cross-linker, concentration of cross-linker and the reaction time for cross-linking step was investigated upon the activity recovery of bromelain cross-linked aggregates. The optimization of this biocatalyst preparation specifically recovered 87% of the enzymatic activity available in pineapple core waste by ammonium sulphate (60%, w/v) precipitation followed by cross-linking for 4 h with 80 mM glutaraldehyde. Cross-linked bromelain aggregates were thermally more stable and exhibited higher pH stability in comparison to free bromelain. The cross-linked bromelain aggregates exhibited higher operational stability in different organic solvents at 4 °C. The highest operational stability (% stability given in parenthesis) was observed in acetone (100%) followed by hexane (53.6%), ethyl acetate (39.6%), ethanol (32.5%) and chloroform (14.9%). The kinetic studies revealed higher Km value (5.45 mM) after the formation of cross-linked bromelain aggregates as compared to free bromelain (5.04 mM) with almost similar Vmax values. Cross-linked bromelain aggregates also showed significant reusability characteristics with an activity retention of >85% after 5-time cycles. Such recyclability of bromelain cross-linked aggregates could lead to potential industrial applications in both food and non-food sector. In addition, the present extraction method avoids costs related to purification and expensive immobilization carriers.

Kinetic resolution of racemic naproxen methyl ester by magnetic and non-magnetic cross-linked lipase aggregates

In this study, the non-magnetic and the magnetic cross-linked enzyme aggregates (CLEAs) from Candida rugosa lipase were synthesized to catalyze the kinetic resolution reaction of naproxen methyl ester (NME). Magnetic iron oxide nanoparticles (MIONPs) were produced through co-precipitation method and their surfaces were modified by silanization reaction. The MIONPs were used as a platform to synthesize the magnetic CLEAs (M-CLEAs). The biocatalysts and MIONPs synthesized were characterized by FTIR spectroscopy and SEM analysis. The kinetic resolution of racemic NME was studied in aqueous buffer solution/isooctane biphasic system to compare the performance of M-CLEAs and CLEAs. The effects of reaction parameters such as temperature, pH, stirring rate on the enantiomeric excess of the substrate (ee_s%) were investigated in a batch reactor system. The activity recovery of CRL enzyme in CLEAs was higher than M-CLEAs. Compared with M-CLEAs, CLEAs biocatalysts had previously reached ee_s% values. Although both biocatalysts showed similar cavity structure from SEM analysis, the lower performance of M-CLEAs may be due to the different microenvironments of M-CLEAs from CLEAs. However, the reusability performance of M-CLEAs was higher than that of CLEAs. The optimal reaction conditions for M-CLEAs and CLEAs were found to be 37 °C, pH 7.5, and 300 rpm.

CLEAs, Combi-CLEAs and ‘Smart’ Magnetic CLEAs: Biocatalysis in a Bio-Based Economy

Biocatalysis has emerged in the last decade as a pre-eminent technology for enabling the envisaged transition to a more sustainable bio-based economy. For industrial viability it is essential that enzymes can be readily recovered and recycled by immobilization as solid, recyclable catalysts. One method to achieve this is via carrier-free immobilization as cross-linked enzyme aggregates (CLEAs). This methodology proved to be very effective with a broad selection of enzymes, in particular carbohydrate-converting enzymes. Methods for optimizing CLEA preparations by, for example, adding proteic feeders to promote cross-linking, and strategies for making the pores accessible for macromolecular substrates are critically reviewed and compared. Co-immobilization of two or more enzymes in combi-CLEAs enables the cost-effective use of multiple enzymes in biocatalytic cascade processes and the use of “smart” magnetic CLEAs to separate the immobilized enzyme from other solids has raised the CLEA technology to a new level of industrial and environmental relevance. Magnetic-CLEAs of polysaccharide-converting enzymes, for example, are eminently suitable for use in the conversion of first and second generation biomass.

Effective one-step saccharification of lignocellulosic biomass using magnetite-biocatalysts containing saccharifying enzymes

Lignocellulosic biomass, packed with sugars, is one of the most available renewable resources for biofuels and bioproducts production. To release the sugars for the production, enzymatic hydrolysis (saccharification) of pretreated lignocellulosic biomass are required. However, the saccharification process is costly, inefficient, and requires multi-step operations. This is in part due to the high cost and the limited selection of commercial enzymes which commonly have different optimal pH and temperatures. Here we reported a one-step saccharification of pretreated lignocellulosic biomass using immobilized biocatalysts containing five different saccharifying enzymes (SEs) with a similar optimum pH and temperature. The five SEs - endo-1,4-β-d-glucanase (an endoglucanase, eglS), cellobiohydrolase (an exoglucanase, cbhA), and β-glucosidase (bglH), endo-1,4-β-xylanase (an endoxylanase, xynC) and β-xylosidase (bxlB) - were successfully expressed and produced by E. coli BL21. Better saccharification of pretreated corn husks was observed when using the five crude SE enzymes than those using two commonly used SEs, endo-1,4-β-d-glucanase and β-glucosidase. The five SEs were cross-linked in the absence or the presence of magnetic nanoparticles (hereafter referred as SE-CLEAs and M-SE-CLEAs, respectively). By using SE-CLEAs, the highest amount of reduced sugar (250 mg/g biomass) was measured. The activity of immobilized SEs is better than free crude SEs. The M-SE-CLEAs can be reused at least 3 times for effective saccharification of pretreated lignocellulosic biomass.

Magnetic Combined Cross-Linked Enzyme Aggregates of Ketoreductase and Alcohol Dehydrogenase: An Efficient and Stable Biocatalyst for Asymmetric Synthesis of (R)-3-Quinuclidinol with Regeneration of Coenzymes In Situ

Enzymes are biocatalysts. In this study, a novel biocatalyst consisting of magnetic combined cross-linked enzyme aggregates (combi-CLEAs) of 3-quinuclidinone reductase (QNR) and glucose dehydrogenase (GDH) for enantioselective synthesis of (R)-3-quinuclidinolwith regeneration of cofactors in situ was developed. The magnetic combi-CLEAs were fabricated with the use of ammonium sulfate as a precipitant and glutaraldehyde as a cross-linker for direct immobilization of QNR and GDH from E. coli BL(21) cell lysates onto amino-functionalized Fe3O4 nanoparticles. The physicochemical properties of the magnetic combi-CLEAs were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and magnetic measurements. Field emission scanning electron microscope (FE-SEM) images revealed a spherical structure with numerous pores which facilitate the movement of the substrates and coenzymes. Moreover, the magnetic combi-CLEAs exhibited improved operational and thermal stability, enhanced catalytic performance for transformation of 3-quinuclidinone (33 g/L) into (R)-3-quinuclidinol in 100% conversion yield and 100% enantiomeric excess (ee) after 3 h of reaction. The activity of the biocatalysts was preserved about 80% after 70 days storage and retained more than 40% of its initial activity after ten cycles. These results demonstrated that the magnetic combi-CLEAs, as cost-effective and environmentally friendly biocatalysts, were suitable for application in synthesis of (R)-3-quinuclidinol essential for the production of solifenacin and aclidinium with better performance than those currently available.

Immobilization of ligninolytic enzymes from white-rot fungi in cross-linked aggregates

Ligninolytic enzymes from white-rot fungi are widely used in biotechnological processes. However, the application of these enzymes as free enzymes is limited due to their instability and lack of reusability. Enzyme stabilization is therefore a major challenge in biocatalytic process research, and immobilization methods are desirable. Using cross-linked enzyme aggregates (CLEAs) such as magnetic CLEAs, porous-CLEAs and combi-CLEAs is a promising technique for overcoming these issues. Cross-linking methods can stabilize and immobilize enzymes by interconnecting enzyme molecules via multiple bonds using cross-linking agents such as glutaraldehyde. The high catalyst density and microporous assembly of CLEAs guarantee high catalyst activity, which, together with their long shelf life, operational stability, and reusability, provide a cost-efficient alternative to matrix-assisted immobilization approaches. Here, we review current progress in ligninolytic enzyme immobilization and provide a comprehensive review of CLEAs. Moreover, we summarize the use of these CLEAs for biocatalysis processes, bioremediation such as dye decolourization, wastewater treatment or pharmaceutically active compound elimination.

Techniques for Preparation of Cross-Linked Enzyme Aggregates and Their Applications in Bioconversions

Enzymes are biocatalysts. They are useful in environmentally friendly production processes and have high potential for industrial applications. However, because of problems with operational stability, cost, and catalytic efficiency, many enzymatic processes have limited applications. The use of cross-linked enzyme aggregates (CLEAs) has been introduced as an effective carrier-free immobilization method. This immobilization method is attractive because it is simple and robust, and unpurified enzymes can be used. Coimmobilization of different enzymes can be achieved. CLEAs generally show high catalytic activities, good storage and operational stabilities, and good reusability. In this review, we summarize techniques for the preparation of CLEAs for use as biocatalysts. Some important applications of these techniques in chemical synthesis and environmental applications are also included. CLEAs provide feasible and efficient techniques for improving the properties of immobilized enzymes for use in industrial applications.

Designing cross-linked xylanase aggregates for bioconversion of agroindustrial waste biomass towards potential production of nutraceuticals

Immobilized biocatalysts design has the potential to efficiently produce valuable bioproducts from lignocellulosic biomass. Among them, the carrier-free immobilization through the cross-linked enzyme aggregates technology is a simple and low-cost alternative. A two steps statistical approach was utilized to evaluate the synthesis of a cross-linked enzyme aggregate from a xylanolytic preparation, which was produced by Cohnella sp. AR92 grown in a peptone-based culture medium. The resulting immobilized biocatalyst, Xyl-CLEA, was significate more stable (25 to 45%) towards temperatures up to 50°C with respect to the free enzyme, and retained over 50% of its initial activity after 5 consecutive cycles of reuse. By means of infrared spectroscopy and electron microscopy, the Xyl-CLEA showed architectural features described as signature of type I and type II of protein aggregates. These, were the result of the simultaneous aggregation of a multiplicity of proteins from the crude enzymatic extract. The enzymatic activity was assessed using alkali pretreated sugar cane bagasse as substrate. Whereas the free enzymatic preparation released xylose as the main product, the immobilized xylanase produced xylooligosaccharides, thus showing that the immobilization procedure modified the potential application of the extracellular xylanase from Conhella sp. AR92.

Preparation and Optimisation of Cross-Linked Enzyme Aggregates Using Native Isolate White Rot Fungi Trametes versicolor and Fomes fomentarius for the Decolourisation of Synthetic Dyes

The key to obtaining an optimum performance of an enzyme is often a question of devising a suitable enzyme and optimisation of conditions for its immobilization. In this study, laccases from the native isolates of white rot fungi Fomes fomentarius and/or Trametes versicolor, obtained from Czech forests, were used. From these, cross-linked enzyme aggregates (CLEA) were prepared and characterised when the experimental conditions were optimized. Based on the optimization steps, saturated ammonium sulphate solution (75 wt.%) was used as the precipitating agent, and different concentrations of glutaraldehyde as a cross-linking agent were investigated. CLEA aggregates formed under the optimal conditions showed higher catalytic efficiency and stabilities (thermal, pH, and storage, against denaturation) as well as high reusability compared to free laccase for both fungal strains. The best concentration of glutaraldehyde seemed to be 50 mM and higher efficiency of cross-linking was observed at a low temperature 4 °C. An insignificant increase in optimum pH for CLEA laccases with respect to free laccases for both fungi was observed. The results show that the optimum temperature for both free laccase and CLEA laccase was 35 °C for T. versicolor and 30 °C for F. fomentarius. The CLEAs retained 80% of their initial activity for Trametes and 74% for Fomes after 70 days of cultivation. Prepared cross-linked enzyme aggregates were also investigated for their decolourisation activity on malachite green, bromothymol blue, and methyl red dyes. Immobilised CLEA laccase from Trametes versicolor showed 95% decolourisation potential and CLEA from Fomes fomentarius demonstrated 90% decolourisation efficiency within 10 h for all dyes used. These results suggest that these CLEAs have promising potential in dye decolourisation.

Xylanase from Acinetobacter pittii MASK 25 and developed magnetic cross-linked xylanase aggregate produce predominantly xylopentose and xylohexose from agro biomass

Most of the chemical and biochemical processes used for the de-polymerization of structural polymers of lignocellulosic biomass are environment unfriendly and costly. Here an efficient process based on xylanase, produced by Acinetobacter pittii MASK25 (MTCC 25132), hydrolysis of only physically treated rice straw and corn cob has been developed for the production of xylooligosaccharides. Bacterial strain isolated from soil was found to produce maximum xylanase at 30°C and pH 7. While the optimum temperature and pH of xylanase were characterized as 40°C and 5. Process was further improved by developing magnetic-xylanase CLEA. Crude xylanase and magnetic-xylanase CLEA could convert respectively more than 45% and 60% xylan of the powdered rice straw and corn cob into xylooligosaccharides. Interestingly, hydrolysis by both types of enzymatic forms was found to produce predominantly xylopentose and xylohexose. Hence, the process is environment friendly and the predominant production of xylopentose and xylohexose could find unique prebiotic applications.

A strategic approach for direct recovery and stabilization of Fusarium sp. ICT SAC1 cutinase from solid state fermented broth by carrier free cross-linked enzyme aggregates

The major hurdles in commercial exploitation of cutinase (having both esterolytic and lipolytic activities) with potent industrial applications are its high production cost, operational instability and reusability. Although commercially available in immobilized form, its immobilization process (synthesis of support/carrier) makes it expensive. Herein we tried to address multiple issues of production cost, stability, and reusability, associated with cutinase. Waste watermelon rinds, an agroindustrial waste was considered as a cheap support for solid state fermentation (SSF) for cutinase production by newly isolated Fusarium sp. ICT SAC1. Subsequently, carrier free cross-linked enzyme aggregates of cutinase (cut-CLEA) directly from the SSF crude broth were developed. All the process variables affecting CLEA formation along with the different additives were evaluated. It was found that 50% (w/v) of ammonium sulphate, 125μmol of glutaraldehyde, cross-linking for 1h at 30°C and broth pH of 7.0, yielded 58.12% activity recovery. All other additives (hexane, butyric acid, sodium dodecyl sulphate, Trition-X 100, Tween-20, BSA) evaluated presented negative results to our hypothesis. Kinetics and morphology studies confirmed the diffusive nature of cut-CLEA and BSA cut-CLEA. Developed CLEA showed better thermal, solvent, detergent and storage stability, making it more elegant and efficient for industrial biocatalytic process.

Solid-binding peptides for immobilisation of thermostable enzymes to hydrolyse biomass polysaccharides

BACKGROUND

Solid-binding peptides (SBPs) bind strongly to a diverse range of solid materials without the need for any chemical reactions. They have been used mainly for the functionalisation of nanomaterials but little is known about their use for the immobilisation of thermostable enzymes and their feasibility in industrial-scale biocatalysis.

RESULTS

A silica-binding SBP sequence was fused genetically to three thermostable hemicellulases. The resulting enzymes were active after fusion and exhibited identical pH and temperature optima but differing thermostabilities when compared to their corresponding unmodified enzymes. The silica-binding peptide mediated the efficient immobilisation of each enzyme onto zeolite, demonstrating the construction of single enzyme biocatalytic modules. Cross-linked enzyme aggregates (CLEAs) of enzyme preparations either with or without zeolite immobilisation displayed greater activity retention during enzyme recycling than those of free enzymes (without silica-binding peptide) or zeolite-bound enzymes without any crosslinking. CLEA preparations comprising all three enzymes simultaneously immobilised onto zeolite enabled the formation of multiple enzyme biocatalytic modules which were shown to degrade several hemicellulosic substrates.

CONCLUSIONS

The current work introduced the construction of functional biocatalytic modules for the hydrolysis of simple and complex polysaccharides. This technology exploited a silica-binding SBP to mediate effectively the rapid and simple immobilisation of thermostable enzymes onto readily-available and inexpensive silica-based matrices. A conceptual application of biocatalytic modules consisting of single or multiple enzymes was validated by hydrolysing various hemicellulosic polysaccharides.

Lignocellulose-Biorefinery: Ethanol-Focused

The development prospects of the world markets for petroleum and other liquid fuels are diverse and partly contradictory. However, comprehensive changes for the energy supply of the future are essential. Notwithstanding the fact that there are still very large deposits of energy resources from a geological point of view, the finite nature of conventional oil reserves is indisputable. To reduce our dependence on oil, the EU, the USA, and other major economic zones rely on energy diversification. For this purpose, alternative materials and technologies are being sought, and is most obvious in the transport sector. The objective is to progressively replace fossil fuels with renewable and more sustainable fuels. In this respect, biofuels have a pre-eminent position in terms of their capability of blending with fossil fuels and being usable in existing cars without substantial modification. Ethanol can be considered as the primary renewable liquid fuel. In this chapter enzymes, micro-organisms, and processes for ethanol production based on renewable resources are described.

Engineering a more sustainable world through catalysis and green chemistry

The grand challenge facing the chemical and allied industries in the twenty-first century is the transition to greener, more sustainable manufacturing processes that efficiently use raw materials, eliminate waste and avoid the use of toxic and hazardous materials. It requires a paradigm shift from traditional concepts of process efficiency, focusing on chemical yield, to one that assigns economic value to replacing fossil resources with renewable raw materials, eliminating waste and avoiding the use of toxic and/or hazardous substances. The need for a greening of chemicals manufacture is readily apparent from a consideration of the amounts of waste generated per kilogram of product (the E factors) in various segments of the chemical industry. A primary source of this waste is the use of antiquated 'stoichiometric' technologies and a major challenge is to develop green, catalytic alternatives. Another grand challenge for the twenty-first century, driven by the pressing need for climate change mitigation, is the transition from an unsustainable economy based on fossil resources--oil, coal and natural gas--to a sustainable one based on renewable biomass. In this context, the valorization of waste biomass, which is currently incinerated or goes to landfill, is particularly attractive. The bio-based economy involves cross-disciplinary research at the interface of biotechnology and chemical engineering, focusing on the development of green, chemo- and biocatalytic technologies for waste biomass conversion to biofuels, chemicals and bio-based materials. Biocatalysis has many benefits to offer in this respect. The catalyst is derived from renewable biomass and is biodegradable. Processes are performed under mild conditions and generally produce less waste and are more energy efficient than conventional ones. Thanks to modern advances in biotechnology 'tailor-made' enzymes can be economically produced on a large scale. However, for economic viability it is generally necessary to recover and re-use the enzyme and this can be achieved by immobilization, e.g. as solid cross-linked enzyme aggregates (CLEAs), enabling separation by filtration or centrifugation. A recent advance is the use of 'smart', magnetic CLEAs, which can be separated magnetically from reaction mixtures containing suspensions of solids; truly an example of cross-disciplinary research at the interface of physical and life sciences, which is particularly relevant to biomass conversion processes.

Hybrid Cross-Linked Lipase Aggregates with Magnetic Nanoparticles: A Robust and Recyclable Biocatalysis for the Epoxidation of Oleic Acid

Highly stable and easily recyclable hybrid magnetic cross-linked lipase aggregates (HM-CSL-CLEAs) were prepared by coaggregation of lipase aggregates with nonfunctionalized magnetic nanoparticles and subsequent chemical cross-linking with glutaraldehyde. Analysis by SEM and CLSM indicated that the CLEAs were embedded in nanoparticle aggregates instead of covalently immobilized. The resulting HM-CSL-CLEAs exhibited higher thermostability, storage stability, and reusability than standard CLEAs. For example, HM-CSL-CLEAs maintained >60% of their initial activity after 40 min of incubation at 60 °C, whereas standard CLEAs lost most of their activities. The HM-CSL-CLEAs can be easily recovered from the reaction mixture by an external magnetic field. Moreover, the H2O2 tolerance of the lipase in HM-CSL-CLEAs was also enhanced, which could relieve the inhibitory effect on lipase activity. A high conversion yield (55%) for the epoxidation of oleic acid using H2O2 as oxidizing agent was achieved by HM-CSL-CLEAs.

Immobilization of Glycoside Hydrolase Families GH1, GH13, and GH70: State of the Art and Perspectives

Glycoside hydrolases (GH) are enzymes capable to hydrolyze the glycosidic bond between two carbohydrates or even between a carbohydrate and a non-carbohydrate moiety. Because of the increasing interest for industrial applications of these enzymes, the immobilization of GH has become an important development in order to improve its activity, stability, as well as the possibility of its reuse in batch reactions and in continuous processes. In this review, we focus on the broad aspects of immobilization of enzymes from the specific GH families. A brief introduction on methods of enzyme immobilization is presented, discussing some advantages and drawbacks of this technology. We then review the state of the art of enzyme immobilization of families GH1, GH13, and GH70, with special attention on the enzymes β-glucosidase, α-amylase, cyclodextrin glycosyltransferase, and dextransucrase. In each case, the immobilization protocols are evaluated considering their positive and negative aspects. Finally, the perspectives on new immobilization methods are briefly presented.

Adding an appropriate amino acid during crosslinking results in more stable crosslinked enzyme aggregates

Carrier free immobilization, especially crosslinked enzyme aggregates (CLEAs), has become an important design for biocatalysis in several areas. Adding amino acids during formation of CLEAs was found to give biocatalysts more stable at 55 °C and in the presence of 60% acetonitrile. The half-lives of CLEAs prepared with and without Arg addition were 21 and 15 h (subtilisin) and 4 and 1.6 h (α-chymotrypsin) at 55 °C, respectively. The corresponding half-lives during acetonitrile presence were 4.1 and 3.0 h (subtilisin) and 39 and 22 min (α-chymotrypsin), respectively. CLEAs made with Arg had higher percentages of alpha helix. CLEAs made by adding Lys, Ala, or Asp also were more stable. In the case of Thermomyces lanuginosus lipase (TLL), CLEA with Ala was even more stable than CLEA with Arg. The addition of a suitable amino acid, thus, enhances CLEA stabilities. The results are discussed in the light of earlier results on chemical modification of proteins and the observation that the Arg/Lys ratio is invariably high in the case of enzymes from thermophiles.