Preparation of cross-linked enzyme aggregates by using bovine serum albumin as a proteic feeder

Addition of bovine serum albumin (BSA) as a proteic feeder facilitates obtaining cross-linked enzyme aggregates (CLEAs) in cases where the protein concentration in the enzyme preparation is low and/or the enzyme activity is vulnerable to the high concentration of glutaraldehyde required to obtain aggregates. CLEAs of Pseudomonas cepacia lipase and penicillin acylase were prepared. CLEA of lipase prepared in the presence of BSA retained 100% activity whereas CLEA prepared without BSA retained only 0.4% activity of the starting enzyme preparation. Lipase CLEA showed 12-fold increase in activity over free enzyme powder when the CLEA was used in transesterification of tributyrin. For the transesterification of Jatropha oil, while free enzyme powder required 8 h and 50 mg lipase to obtain 77% conversion, CLEA required only 6 h and 6.25 mg lipase to obtain 90% conversion. In the case of penicillin acylase, 86% activity could be retained in CLEA prepared with BSA whereas CLEA made without BSA retained only 50% activity. CLEA prepared without BSA lost 20% activity after 8 h at 45 degrees C whereas CLEA with BSA retained full activity. CLEA prepared with BSA showed Vmax/Km of 36.3 min-1 whereas CLEA prepared without BSA had Vmax/Km of 17.4 min-1 only. Scanning electron microscopy analysis showed that CLEAs prepared in the presence of BSA were less amorphous and closer in morphology to CLEAs of other enzymes described in the literature.     

Preparation of cross-linked tyrosinase aggregates

Tyrosinase from mushroom was immobilized as a cross-linked enzyme aggregate (CLEA) via precipitation with ammonium sulfate and cross-linking with glutaraldehyde. The effects of precipitation and cross-linking on CLEA activity were investigated and the immobilized tyrosinase was characterized. Sixty percent ammonium sulfate saturation and 2% glutaraldehyde were used; a 3-h cross-linking reaction at room temperature, at pH 7.0 was performed; particle sizes of the aggregates were reduced; consequently, 100% activity recovery was achieved in CLEAs with enhanced thermal and storage stabilities. Slight changes in optimum pH and temperature values of the enzyme were recorded after immobilization. Although immobilization did not affect V_max, substrate affinity of the enzyme increased. Highly stable CLEAs were also prepared from crude mushroom tyrosinase with 100% activity recovery.     

Designing cross-linked lipase aggregates for optimum performance as biocatalysts

Cross-linked enzyme aggregates (CLEAs) are prepared by precipitation of an enzyme and then chemical cross-linking the precipitate. Three CLEAs of lipase with glutaraldehyde concentrations of 10 mM (CLEA A), 40 mM (CLEA B) and 60 mM (CLEA C) were prepared. Studies show that there is a trade-off between thermal stability vs transesterification/hydrolysis rate vs enantioselectivity. The initial rates for transesterification of β-citronellol for the uncross-linked enzyme and CLEAs A, B and C were 243, 167, 102 and 40 µmol mg^-1 h^-1, respectively. Their thermal stabilities in aqueous media, as reflected by their half-life values at 55°C, were 6, 9, 13 and 16 h, respectively. The enantioselectivity, E values (for kinetic resolution of β-citronellol by transesterification) were 19, 74, 11 and 6, respectively. These results show that CLEA C was the most thermostable; the uncross-linked enzyme was best at obtaining the highest transesterification rate; and CLEA A was best suited for the enantioselective synthesis. Scanning electron microscopy (SEM) showed that the morphology of CLEA was dependent upon the extent of cross-linking.     

Designing cross-linked lipase aggregates for optimum performance as biocatalysts

Cross-linked enzyme aggregates (CLEAs) are prepared by precipitation of an enzyme and then chemical cross-linking the precipitate. Three CLEAs of lipase with glutaraldehyde concentrations of 10 mM (CLEA A), 40 mM (CLEA B) and 60 mM (CLEA C) were prepared. Studies show that there is a trade-off between thermal stability vs transesterification/hydrolysis rate vs enantioselectivity. The initial rates for transesterification of β-citronellol for the uncross-linked enzyme and CLEAs A, B and C were 243, 167, 102 and 40 µmol mg^?1 h^?1, respectively. Their thermal stabilities in aqueous media, as reflected by their half-life values at 55°C, were 6, 9, 13 and 16 h, respectively. The enantioselectivity, E values (for kinetic resolution of β-citronellol by transesterification) were 19, 74, 11 and 6, respectively. These results show that CLEA C was the most thermostable; the uncross-linked enzyme was best at obtaining the highest transesterification rate; and CLEA A was best suited for the enantioselective synthesis. Scanning electron microscopy (SEM) showed that the morphology of CLEA was dependent upon the extent of cross-linking.     

Activity and stability of cross-linked tyrosinase aggregates in aqueous and nonaqueous media

Cross-linked tyrosinase aggregates were prepared by precipitating the enzyme with ammonium sulfate and subsequent cross-linking with glutaraldehyde. Both activity and stability of these cross-linked enzyme aggregates (CLEAs) in aqueous solution, organic solvents, and ionic liquids have been investigated. Immobilization effectively improved the stability of the enzyme in aqueous solution against various deactivating conditions such as pH, temperature, denaturants, inhibitors, and organic solvents. The stability of the CLEAs in various organic solvents such as tert-butanol (t(1/2)=326.7h at 40°C) was significantly enhanced relative to that in aqueous solution (t(1/2)=5.5h). The effect of thermodynamic water activity (a(w)) on the CLEA activity in organic media was examined, demonstrating that the enzyme incorporated into CLEAs required an extensive hydration (with an a(w) approaching 1.0) for optimizing its activity. The impact of ionic liquids on the CLEA activity in aqueous solution was also assessed.     

Immobilization of formate dehydrogenase from Candida boidinii through cross-linked enzyme aggregates

We employed a cross-linked enzyme aggregate (CLEA) method to immobilize formate dehydrogenase (FDH) from Candida boidinii. The optimal conditions for the preparation of CLEAs were determined by examining effects of various parameters: the nature and amount of cross-linking reagent, additive concentration, cross-linking time, and pH during CLEA preparation. The recovered activities of CLEAs were significantly dependent on the concentration of glutaraldehyde; however, the recovered activity was not severely influenced by the content of dextran polyaldehyde as a mild cross-linker. Bovine serum albumin (BSA) was also used as a proteic feeder and enhanced the activity recovery by 130%. The highest recovered activity of CLEA was 18% for formate oxidation reaction and 25% for CO₂ reduction reaction. The residual activity of CLEA prepared with dextran polyaldehyde (Dex-CLEA) was over 95% after 10 cycles of reuse. The thermal stability of Dex-CLEA was increased by a factor of 3.6 more than that of the free enzyme. CLEAs of FDH could be utilized efficiently for both NADH regeneration and CO₂ reduction.     

Preparation of nano-enzyme aggregates by crosslinking lipase with sodium tripolyphosphate

Cross-linked enzyme aggregates (CLEAs) are considered as an effective tool for the immobilization of enzyme. The ionic cross-linking agent-sodium tripolyphosphate (TPP) was first used in preparing CLEAs. Aspergillus niger lipase was precipitated with ammonium sulfate and further cross-linked by TPP. The factors including enzyme concentration, pH of cross-linking medium, TPP dosage and cross-linking time were optimized. Maximum recovery activity (99.5 ± 0.634 %) and cross-linking yield (88.4 ± 0.46 %) can be obtained under the optimal process conditions, which can illustrate TPP had little effect on enzyme activity. CLEAs showed improved activity over broad pH and temperature range compared to the free enzyme. The thermal stability was obviously improved compared to free enzyme under the optimal temperature (40℃) and the half-life was 7.5-fold higher than that of free enzyme. Moreover, scanning electron microscopy (SEM) revealed that CLEAs had a cavity with porous structure and the particle size was 249 ± 3.98 nm. X-ray diffraction (XRD) showed the crystallinity of the CLEAs decreased. The changes in secondary structures of CLEAs revealed the increment in conformational rigidity. Such results suggested that the CLEAs has ideal application prospects.     

Effect of different parameters on the hydrolytic activity of cross-linked enzyme aggregates (CLEAs) of lipase from Thermomyces lanuginosa

Cross-linked enzyme aggregates (CLEAs) of lipase from Thermomyces lanuginosa (TLL) were synthesized using (NH₄)₂SO₄ as precipitant and glutaraldehyde as cross-linking agent. CLEAs were assayed for their hydrolytic activity in a reaction performed in an emulsioned medium. The effects of the amount of precipitant, cross-linker, and different additives such as protein cofeeder, oleic acid, n-heptane, sodium dodecyl sulfate (SDS), polyethylenglicol (PEG) and ethylendiamine were studied at selected ratios with respect to TLL mass. Traditional non-layered CLEAs of TLL showed recovered activities between 3 and 31% when compared with native lipase. Novel TLL layered CLEAs consisting of a protein cofeeder core and successive layers of target lipase showed an important increase in their retained activity. The highest recovered activity was found for the one-layered non-additivated CLEAs of TLL which showed a recovered activity of 75%.     

交联酶聚集体制备中钙离子的结构调控作用

以葡萄糖氧化酶(GOx)为研究对象,系统地研究了钙离子对交联酶聚集体(CLEA)粒子尺寸和微观结构的调控作用以及酶催化性能和实用性的影响。研究结果表明,GOx酶沉淀过程中引入钙离子可显著降低CLEA粒子尺寸并导致粒子内纳米孔道结构逐步消失。在0.1 mmol/L钙离子浓度下,GOx酶的CLEA仍保有清晰的纳米孔道结构。以葡萄糖为底物的GOx酶CLEA催化结果显示,该CLEA粒子的酶活性为对照CLEA粒子的2.69倍。即便1.0 mmol/L钙离子浓度下制备的CLEA粒子的GOx酶活性仍高出对照CLEA粒子约42%。此外,0.1 mmol/L钙离子浓度下制备的CLEA不仅具有更高的底物转化速率和很好的操作稳定性,而且CLEA中GOx酶的最大反应速度显著提高。这些实验结果明确了钙离子对CLEA粒子尺寸和微观结构的调控作用,为制备具有高效生物催化活性的CLEA粒子奠定了基础。     

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.     

Chemical amination of lipase B from Candida antarctica is an efficient solution for the preparation of crosslinked enzyme aggregates

Lipase B from Candida antarctica (CALB) is not very adequate to prepare crosslinked enzyme aggregates (CLEAs). Although the precipitation step is easy using different precipitants, the crosslinking step becomes a problem due to the low amount of Lys residues in this enzyme. In this paper, we have enriched the enzyme in amino groups by chemical amination of the enzyme using ethylenediamine and carbodiimide. The modification was performed using a solid phase strategy modifying the enzyme adsorbed on octyl-Sepharose. After desorption from the support, the enzyme was more active at pH 7.0 than the unmodified enzyme. This modified enzyme showed to be suitable to produce CLEAs. Using this modified enzyme, precipitation is also effective but the crosslinking step did not fail in giving an intense intermolecular crosslinking. This way, the CLEA did not release enzyme molecules even if boiled in SDS. Stability of this CLEA was higher in both thermal and cosolvent inactivation experiments than that of the coCLEA produced by coagregation of BSA and CALB; another alternative to produce a CLEA of this interesting enzyme.The strategy may be of high interest for many other enzymes as a way to both permit the production of CLEAs and to improve enzyme stability during CLEA production.     

Preparation and use of cross-linked enzyme aggregates (CLEAs) of laccases

Cross-linked enzyme aggregates (CLEA^®) were prepared from laccases from three different sources: Trametes versicolor, Trametes villosa and Agaricus bisporus. The effect of the various parameters – nature of the precipitant, pH, temperature, glutaraldehyde concentration and cross-linking time – on the activity recovery and storage and operational stability of the resulting CLEAs was different. The laccase CLEAs exhibited the expected increased stability compared to the free enzyme but there was no direct correlation with the number of surface lysine residues in the latter. It is clearly not the only parameter influencing the properties of the CLEA. Co-aggregation with albumin did not improve the stability. The laccase CLEAs, in combination with the stable N-oxy radical, TEMPO, were shown to be active and stable catalysts for the aerobic oxidation of linear C₅–C₁₀ aliphatic alcohols, to the corresponding aldehydes, in aqueous buffer (pH 4). Rates were an order of magnitude higher than those observed with the corresponding free enzyme and the CLEAs could be recycled several times without appreciable loss of activity. The addition of water immiscible or water miscible solvents showed no further improvement in rate compared with reactions in aqueous buffer alone.     

Crosslinked aggregates of Rhizopus oryzae lipase as industrial biocatalysts: Preparation, optimization, characterization, and application for enantioselective resolution reactions

Lipase from Rhizopus oryzae (ROL) was immobilized as crosslinked enzyme aggregate (CLEA) via precipitation with ammonium sulfate and simultaneous crosslinking with glutaraldehyde. The optimum conditions of the immobilization process were determined. Lipase CLEAs showed a twofold increase in activity when Tween 80‐pretreated lipase was used for CLEA preparation. CLEAs were shown to have several advantages compared to free lipase. CLEAs were more stable at 50°C and 60°C as well as for a wide range of pH. After incubation at 50°C, CLEA showed 74% of initial activity whereas free enzyme was totally inactivated. Reduction of Schiff bases has been performed for the first time in the CLEA preparation process significantly improving the chemically modified CLEAs' reusability, thus providing an enzyme with high potential for recycling even under aqueous reaction conditions where enzyme leakage is, in general, one of the major problems. The CLEA retained 91% activity after 10 cycles in aqueous medium. The immobilized enzyme was used for kinetic resolution reactions. Results showed that immobilization had an enhancing effect on the conversion (c) as well as on the enantiomeric ratio (E). ROL CLEA displayed five times higher enantioselectivity for the hydrolysis of (R,S)‐1‐phenylethyl acetate and likewise 1.5 times higher enantioselectivity for the transesterification of racemic (R–S)‐1‐phenylethanol with vinylacetate. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 28: 937–945, 2012 This article was published online on June 26, 2012. An edit was subsequently requested. This notice is included in the online and print versions to indicate that both have been corrected [27 June 2012].     

Crosslinked enzyme aggregates in hierarchically-ordered mesoporous silica: a simple and effective method for enzyme stabilization

alpha-chymotrypsin (CT) and lipase (LP) were immobilized in hierarchically-ordered mesocellular mesoporous silica (HMMS) in a simple but effective way for the enzyme stabilization, which was achieved by the enzyme adsorption followed by glutaraldehyde (GA) crosslinking. This resulted in the formation of nanometer scale crosslinked enzyme aggregates (CLEAs) entrapped in the mesocellular pores of HMMS (37 nm), which did not leach out of HMMS through narrow mesoporous channels (13 nm). CLEA of alpha-chymotrypsin (CLEA-CT) in HMMS showed a high enzyme loading capacity and significantly increased enzyme stability. No activity decrease of CLEA-CT was observed for 2 weeks under even rigorously shaking condition, while adsorbed CT in HMMS and free CT showed a rapid inactivation due to the enzyme leaching and presumably autolysis, respectively. With the CLEA-CT in HMMS, however, there was no tryptic digestion observed suggesting that the CLEA-CT is not susceptible to autolysis. Moreover, CLEA of lipase (CLEA-LP) in HMMS retained 30% specific activity of free lipase with greatly enhanced stability. This work demonstrates that HMMS can be efficiently employed as host materials for enzyme immobilization leading to highly enhanced stability of the immobilized enzymes with high enzyme loading and activity.     

Highly efficient biosynthesis of sucrose-6-acetate with cross-linked aggregates of Lipozyme TL 100 L

As a short chain monoester, sucrose-6-acetate (S-6-a) is a key intermediate in the preparation of an eminent sweetener (sucralose). To replace the traditional multi-step chemical route for sucralose biosynthesis, enzymatic synthesis of S-6-a was investigated, using cross-linked enzyme aggregates (CLEAs) of Lipozyme TL 100 L. The optimal CLEA preparation conditions was obtained as follows: using 33.3% (v/v) PEG600 co-precipitated with additive of D-sorbierite, then cross-linking with 1.5% (v/v) glutaraldehyde at 0 °C for 4 h. As a result, the immobilized Lipozyme had high specific bioactivity (34.64 U/g) of transesterification in non-aqueous media. With these immobilized enzymes, the optimum transesterification conditions were investigated systematically, including CLEA loading, the mole ratio of vinyl acetate versus sucrose, temperature and reaction time, etc. The results showed that the highest concentration and yield of S-6-a was 49.8 g/L and 87.46%, respectively. Further experiments showed that the resulting CLEAs also had much higher operational stability than the commercial Lipozyme TLIM. The present work has paved a new path for the large-scale bioproduction of S-6-a with immobilized lipase in the future.     

Improved biochemical characteristics of crosslinked β-glucosidase on nanoporous silica foams

Large mesoporous cellular foam (LMCF) materials were synthesized using the microemulsion templating route. For the enzyme stabilization, β-glucosidase was immobilized onto mesocellular silica foams (MCFs) in a simple and effective way, a process achieved using enzyme adsorption followed by glutaraldehyde (GA) crosslinking. This resulted in the formation of crosslinked enzyme aggregates (CLEAs) of nanometer scale. The structural and chemical properties of these prepared materials were characterized by TG, CPMAS NMR and nitrogen adsorption measurements. The crosslinked immobilizates retained activity over wider ranges of temperature and pH than those of the free enzyme. Kinetic parameter (K_m) of the immobilized β-glucosidase is lower than that of its free counterpart. The resulting CLEA was proved to be active and recyclable up to 10 cycles without much loss in activity. This demonstrates its prospects for commercial applications. The immobilizate exhibited enhanced storage stability characteristics than the native enzyme. In contrast to adsorbed GL and covalently bound glucosidase, the resulting crosslinked enzyme aggregates (CLEAs) showed an impressive stability with high enzyme loadings.     

Catalytic performance of cross-linked enzyme aggregates of Penicillium expansum lipase and their use as catalyst for biodiesel production

Cross-linked enzyme aggregates (CLEAs) of lipase from Penicillium expansum (PEL) were prepared directly from fermentation broth, a more practical and economically viable procedure than the generally used methods that require purified or partially purified enzymes for CLEA preparation. A systematic study of the activity and stability of PEL-CLEAs was undertaken in aqueous solution, organic solvents, and ionic liquids (ILs). Immobilization of the enzyme resulted in a significantly enhanced stability in aqueous solution with regard to pH and temperature. PEL-CLEAs showed an improved activity in the IL [BMIm][PF₆] relative to that observed in hexane, both keeping increased with temperature (up to 90 °C in the IL and 60 °C in hexane). The effect of water content and water activity in these two nonaqueous media showed similar patterns as for the uncrosslinked enzyme. The half life of the CLEAs was higher in hydrophobic organic solvents (hexane and chloroform) than in aqueous solution, and presented a sigmoid relationship with the log P of the organic solvent tested. PEL-CLEAs catalyzed biodiesel production from microalgal oil in the IL [BMIm][PF₆] with a conversion of 85.7%, demonstrating that they can be taken as a promising catalyst for this application.     

Preparation and characterization of combi-CLEAs catalyzing multiple non-cascade reactions

A novel cross-linked enzyme aggregates (CLEA) concept called combi-CLEA has been described. It is based upon the fact that CLEA can be made from heterogeneous populations of proteins/enzymes. Porcine pancreatic acetone powder crude extract was used for preparing CLEA in such a way that lipase, -amylase, phospholipase A₂ activities were retained upto 100%. The lipase present in the CLEA showed greater thermal stability at 50 °C as compared to free enzyme. For lipase and phospholipase A₂, V_max/K_m showed no significant change upon combi-CLEA formation but decreased significantly for -amylase activity from 190 to 114 min⁻¹. The lipase activity and -amylase activity in CLEA were completely retained upto three cycles of use. The scanning electron microscopic (SEM) studies showed that morphology of CLEA changed upon inactivation by reuses.     

1,3‐Regiospecific ethanolysis of soybean oil catalyzed by crosslinked porcine pancreas lipase aggregates

The preparation of crosslinked aggregates of pancreatic porcine lipase (PPL‐CLEA) was systematically studied, evaluating the influence of three precipitants and two crosslinking agents, as well as the use of soy protein as an alternative feeder protein on the catalytic properties and stability of the immobilized PPL. Standard CLEAs showed a global yield (CLEA’ observed activity/offered total activity) of less than 4%, whereas with the addition of soy protein (PPL:soy protein mass ratio of 1:3) the global yield was approximately fivefold higher. The CLEA of PPL prepared with soy protein as feeder (PPL:soy protein mass ratio of 1:3) and glutaraldehyde as crosslinking reagent (10 μmol of aldehyde groups/mg of total protein) was more active mainly because of the reduced enzyme leaching in the washing step. This CLEA, named PPL‐SOY‐CLEA, had an immobilization yield around 60% and an expressed activity around 40%. In the ethanolysis of soybean oil, the PPL‐SOY‐CLEA yielded maximum fatty acid ethyl ester (FAEE) concentration around 12‐fold higher than that achieved using soluble PPL (34 h reaction at 30°C, 300 rpm stirring, soybean oil/ethanol molar ratio of 1:5) with an enzyme load around 2‐fold lower (very likely due to free enzyme inactivation). The operational stability of the PPL‐SOY‐CLEA in the ethanolysis of soybean oil in a vortex flow type reactor showed that FAEE yield was higher than 50% during ten reaction cycles of 24 h. This reactor configuration may be an attractive alternative to the conventional stirred reactors for biotransformations in industrial plants using carrier‐free biocatalysts. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:910–920, 2018     

Quantitative characteristic of the catalytic properties and microstructure of cross-linked enzyme aggregates of penicillin acylase

The microstructure and the catalytic properties of cross-linked enzyme aggregates (CLEA) of penicillin acylase (PA) obtained under different conditions were investigated. The period of time left between the enzyme precipitation and the cross-linking step was found to influence the structural organization of the resulting enzyme preparation. Confocal fluorescent microscopy of the so-called “fresh” and “mature” CLEAs PA allowed to estimate the “characteristic” diameter of CLEA PA particles, which appeared to be about 1.6 μm, and revealed that the “mature” type was composed of relatively big particles as compared to the “fresh” type. Complementary kinetic studies showed that the “mature” CLEA PA were more effective in both hydrolytic and synthetic reactions. It was suggested that the aggregate size might regulate the extent of covalent modification of PA and thereby influence the catalytic properties of CLEA.