Overproduction of Thermus sp. Strain T2 beta-galactosidase in Escherichia coli and preparation by using tailor-made metal chelate supports

Dextran Aldehyde in Biocatalysis: More Than a Mere Immobilization System

Dextran aldehyde (dexOx), resulting from the periodate oxidative cleavage of 1,2-diol moiety inside dextran, is a polymer that is very useful in many areas, including as a macromolecular carrier for drug delivery and other biomedical applications. In particular, it has been widely used for chemical engineering of enzymes, with the aim of designing better biocatalysts that possess improved catalytic properties, making them more stable and/or active for different catalytic reactions. This polymer possesses a very flexible hydrophilic structure, which becomes inert after chemical reduction; therefore, dexOx comes to be highly versatile in a biocatalyst design. This paper presents an overview of the multiple applications of dexOx in applied biocatalysis, e.g., to modulate the adsorption of biomolecules on carrier surfaces in affinity chromatography and biosensors design, to serve as a spacer arm between a ligand and the support in biomacromolecule immobilization procedures or to generate artificial microenvironments around the enzyme molecules or to stabilize multimeric enzymes by intersubunit crosslinking, among many other applications.

Production Optimization of an Active β-Galactosidase of Bifidobacterium animalis in Heterologous Expression Systems

β-Galactosidase (E.C.3.2.1.23) catalyzes the hydrolysis of lactose into glucose and galactose and the synthesis of galacto-oligosaccharides as well. The β-galactosidases from bacteria, especially lactobacilli, and yeast have neutral pH and are much more likely to be developed as food additives. However, the challenges of cumbersome purification, product toxicity, and low yield in protein production have limited the commercialization of many excellent candidates. In this study, we identified a β-galactosidase gene (bg42-106) in Bifidobacterium animalis ACCC05790 and expressed the gene product in Escherichia coli BL21(DE3) and Pichia pastoris GS115, respectively. The recombinant bG42-106 purified from E. coli cells was found to be optimally active at pH 6.0 and 60°C and had excellent stability over a wide pH range (5.0–8.0) and at high temperature (60°C). The specific activity of bG42-106 reached up to 2351 U/mg under optimal conditions. The galacto-oligosaccharide yield was 24.45 g/L after incubation with bG42-106 at 60°C for 2 h. When recombinant bG42-106 was expressed in Pichia pastoris GS115, it was found in the culture medium but only at a concentration of 1.73 U/ml. To increase its production, three strategies were employed, including codon optimization, disulfide formation, and fusion with a Cherry tag, with Cherry-tag fusion being most effective. The culture medium of P. pastoris that expressed Cherry-tagged bG42-106 contained 24.4 U/mL of β-galactosidase activity, which is 14-fold greater than that produced by culture of P. pastoris harboring wild-type bG42-106.

One-Step Immobilization and Stabilization of a Recombinant Enterococcus faecium DBFIQ E36 L-Arabinose Isomerase for D-Tagatose Synthesis

A recombinant L-arabinose isomerase from Enterococcus faecium DBFIQ E36 was immobilized onto multifunctional epoxide supports by chemical adsorption and onto a chelate-activated support via polyhistidine-tag, located on the N-terminal (N-His-L-AI) or on the C-terminal (C-His-L-AI) sequence, followed by covalent bonding between the enzyme and the support. The results were compared to reversible L-AI immobilization by adsorption onto charged agarose supports with improved stability. All the derivatives presented immobilization yields of above 75%. The ionic interaction established between agarose gels containing monoaminoethyl-N-aminoethyl structures (MANAE) and the enzyme was the most suitable strategy for L-AI immobilization in comparison to the chelate-activated agarose. In addition, the immobilized biocatalysts by ionic interaction in MANAE showed to be the most stable, retaining up to 100% of enzyme activity for 60 min at 60 °C and with K_m values of 28 and 218 mM for MANAE-N-His-L-AI and MANAE-C-His-L-AI, respectively.

Site-directed mutation of β-galactosidase from Aspergillus candidus to reduce galactose inhibition in lactose hydrolysis

β-Galactosidase is widely used for hydrolysis of whey lactose. However, galactose inhibition has acted as a major constraint on the catalytic process. Thus, it is sensible to improve upon this defect in β-galactosidase through protein modification. To reduce the galactose inhibition of Aspergillus candidus β-galactosidase (LACB), four amino acid positions were selected for mutation based on their molecular bindings with galactose. Four mutant libraries (Tyr96, Asn140, Glu142, and Tyr364) of the LACB were constructed using site-directed mutagenesis. Among all of the mutants, Y364F was superior to the wild-type enzyme. The Y364F mutant has a galactose inhibition constant (K_i) of 282 mM, 15.7-fold greater than that of the wild-type enzyme (K_i = 18 mM). When 18 mg/ml galactose was added, the activity of the wild-type enzyme fell to 57% of its initial activity, whereas Y364F activity was maintained at over 90% of its initial activity. The wild-type enzyme hydrolyzed 78% of the initial lactose (240 mg/ml) after 48 h, while the Y364F mutant had a hydrolysis rate greater than 90%. The β-galactosidase Y364F mutant with reduced galactose inhibition may have greater potential applications in whey treatment compared to wild-type LACB.

Engineering the l-Arabinose Isomerase from Enterococcus Faecium for d-Tagatose Synthesis

l-Arabinose isomerase (EC 5.3.1.4) (l-AI) from Enterococcus faecium DBFIQ E36 was overproduced in Escherichia coli by designing a codon-optimized synthetic araA gene. Using this optimized gene, two N- and C-terminal His-tagged-l-AI proteins were produced. The cloning of the two chimeric genes into regulated expression vectors resulted in the production of high amounts of recombinant N-His-l-AI and C-His-l-AI in soluble and active forms. Both His-tagged enzymes were purified in a single step through metal-affinity chromatography and showed different kinetic and structural characteristics. Analytical ultracentrifugation revealed that C-His-l-AI was preferentially hexameric in solution, whereas N-His-l-AI was mainly monomeric. The specific activity of the N-His-l-AI at acidic pH was higher than that of C-His-l-AI and showed a maximum bioconversion yield of 26% at 50 °C for d-tagatose biosynthesis, with Km and Vmax parameters of 252 mM and 0.092 U mg^-1, respectively. However, C-His-l-AI was more active and stable at alkaline pH than N-His-l-AI. N-His-l-AI follows a Michaelis-Menten kinetic, whereas C-His-l-AI fitted to a sigmoidal saturation curve.

New Heterofunctional Supports Based on Glutaraldehyde-Activation: A Tool for Enzyme Immobilization at Neutral pH

Immobilization is an exciting alternative to improve the stability of enzymatic processes. However, part of the applied covalent strategies for immobilization uses specific conditions, generally alkaline pH, where some enzymes are not stable. Here, a new generation of heterofunctional supports with application at neutral pH conditions was proposed. New supports were developed with different bifunctional groups (i.e., hydrophobic or carboxylic/metal) capable of adsorbing biocatalysts at different regions (hydrophobic or histidine richest place), together with a glutaraldehyde group that promotes an irreversible immobilization at neutral conditions. To verify these supports, a multi-protein model system (E. coli extract) and four enzymes (Candidarugosa lipase, metagenomic lipase, β-galactosidase and β-glucosidase) were used. The immobilization mechanism was tested and indicated that moderate ionic strength should be applied to avoid possible unspecific adsorption. The use of different supports allowed the immobilization of most of the proteins contained in a crude protein extract. In addition, different supports yielded catalysts of the tested enzymes with different catalytic properties. At neutral pH, the new supports were able to adsorb and covalently immobilize the four enzymes tested with different recovered activity values. Notably, the use of these supports proved to be an efficient alternative tool for enzyme immobilization at neutral pH.

Characterization of a thermostable glycoside hydrolase (CMbg0408) from the hyperthermophilic archaeon Caldivirga maquilingensis IC-167

BACKGROUND

Hyperthermophilic archaea capable of functioning optimally at very high temperatures are a good source of unique and industrially important thermostable enzymes.

RESULTS

A glycoside hydrolase family 1 β-galactosidase gene (BglB) from a hyperthermophilic archaeon Caldivirga maquilingensis IC-167 was cloned and expressed in Escherichia coli. The recombinant enzyme (CMbg0408) displayed optimum activity at 110 °C and pH 5.0. It also retained 92% and 70% of its maximal activity at 115 and 120 °C, respectively. The enzyme was completely thermostable and active after 120 min of incubation at 80 and 90 °C. It also showed broad substrate specificity with activities of 8876 ± 185 U mg^-1 for p-nitrophenyl-β-d-galactopyranoside, 4464 ± 172 U mg^-1 for p-nitrophenyl-β-d-glucopyranoside, 1486 ± 68 U mg^-1 for o-nitrophenyl-β-d-galactopyranoside, 2250 ± 86 U mg^-1 for o-nitrophenyl-β-d-xylopyranoside and 175 ± 4 U mg^-1 for lactose. A catalytic efficiency (k_cat /K_m ) of 3059 ± 122 mmol L^-1  s^-1 and K_m value of 8.1 ± 0.08 mmol L^-1 were displayed towards p-nitrophenyl-β-d-galactopyranoside.

CONCLUSION

As a result of its remarkable thermostability and high activity at high temperatures, this novel β-galactosidase may be useful for food and pharmaceutical applications. © 2016 Society of Chemical Industry.

Metagenomic approach for the isolation of a thermostable β-galactosidase with high tolerance of galactose and glucose from soil samples of Turpan Basin

Background

β-Galactosidases can be used to produce low-lactose milk and dairy products for lactose intolerant people. Although commercial β-galactosidases have outstanding lactose hydrolysis ability, their thermostability is low, and reaction products have strong inhibition to these enzymes. In addition, the β-galactosidases possessing simultaneously high thermostability and tolerance of galactose and glucose are still seldom reported until now. Therefore, identification of novel β-galactosidases with high thermostability and tolerance to reaction products from unculturable microorganisms accounting for over 99% of microorganisms in the environment via metagenomic strategy is still urgently in demand.

Results

In the present study, a novel β-galactosidase (Gal308) consisting of 658 amino acids was identified from a metagenomic library from soil samples of Turpan Basin in China by functional screening. After being overexpressed in Escherichia coli and purified to homogeneity, the enzymatic properties of Gal308 with N-terminal fusion tag were investigated. The recombinant enzyme displayed a pH optimum of 6.8 and a temperature optimum of 78°C, and was considerably stable in the temperature range of 40°C - 70°C with almost unchangeable activity after incubation for 60 min. Furthermore, Gal308 displayed a very high tolerance of galactose and glucose, with the highest inhibition constant K_i,gal (238 mM) and K_i,glu (1725 mM) among β-galactosidases. In addition, Gal308 also exhibited high enzymatic activity for its synthetic substrate o-nitrophenyl-β-D-galactopyranoside (ONPG, 185 U/mg) and natural substrate lactose (47.6 U/mg).

Conclusion

This study will enrich the source of β-galactosidases, and attract some attentions to β-galactosidases from extreme habitats and metagenomic library. Furthermore, the recombinant Gal308 fused with 156 amino acids exhibits many novel properties including high activity and thermostability at high temperatures, the pH optimum of 6.8, high enzyme activity for lactose, as well as high tolerance of galactose and glucose. These properties make it a good candidate in the production of low-lactose milk and dairy products after further study.

In situ magnetic separation of antibody fragments from Escherichia coli in complex media

Background

In situ magnetic separation (ISMS) has emerged as a powerful tool to overcome process constraints such as product degradation or inhibition of target production. In the present work, an integrated ISMS process was established for the production of his-tagged single chain fragment variable (scFv) D1.3 antibodies (“D1.3”) produced by E. coli in complex media. This study investigates the impact of ISMS on the overall product yield as well as its biocompatibility with the bioprocess when metal-chelate and triazine-functionalized magnetic beads were used.

Results

Both particle systems are well suited for separation of D1.3 during cultivation. While the triazine beads did not negatively impact the bioprocess, the application of metal-chelate particles caused leakage of divalent copper ions in the medium. After the ISMS step, elevated copper concentrations above 120 mg/L in the medium negatively influenced D1.3 production. Due to the stable nature of the model protein scFv D1.3 in the biosuspension, the application of ISMS could not increase the overall D1.3 yield as was shown by simulation and experiments.

Conclusions

We could demonstrate that triazine-functionalized beads are a suitable low-cost alternative to selectively adsorb D1.3 fragments, and measured maximum loads of 0.08 g D1.3 per g of beads. Although copper-loaded metal-chelate beads did adsorb his-tagged D1.3 well during cultivation, this particle system must be optimized by minimizing metal leakage from the beads in order to avoid negative inhibitory effects on growth of the microorganisms and target production. Hereby, other types of metal chelate complexes should be tested to demonstrate biocompatibility. Such optimized particle systems can be regarded as ISMS platform technology, especially for the production of antibodies and their fragments with low stability in the medium. The proposed model can be applied to design future ISMS experiments in order to maximize the overall product yield while the amount of particles being used is minimized as well as the number of required ISMS steps.

Immobilization of enzymes on monofunctional and heterofunctional epoxy-activated supports

The immobilization of proteins on epoxy activated supports is discussed in this chapter. Immobilization on epoxy supports is carried out as a two-step mechanism: in the first step the adsorption of the protein is promoted and in the second step the intramolecular covalent linkage among epoxy groups and nucleophiles of the protein is produced. Based on this mechanism of the need of a first adsorption of the protein on the support, different epoxy supports are described. The different supports are able to immobilize proteins through different orientations being obtained catalysts with different properties of activity, stability, and selectivity.

New biotechnological perspectives of a NADH oxidase variant from Thermus thermophilus HB27 as NAD+-recycling enzyme

Background

The number of biotransformations that use nicotinamide recycling systems is exponentially growing. For this reason one of the current challenges in biocatalysis is to develop and optimize more simple and efficient cofactor recycling systems. One promising approach to regenerate NAD⁺ pools is the use of NADH-oxidases that reduce oxygen to hydrogen peroxide while oxidizing NADH to NAD⁺. This class of enzymes may be applied to asymmetric reduction of prochiral substrates in order to obtain enantiopure compounds.

Results

The NADH-oxidase (NOX) presented here is a flavoenzyme which needs exogenous FAD or FMN to reach its maximum velocity. Interestingly, this enzyme is 6-fold hyperactivated by incubation at high temperatures (80°C) under limiting concentrations of flavin cofactor, a change that remains stable even at low temperatures (37°C). The hyperactivated form presented a high specific activity (37.5 U/mg) at low temperatures despite isolation from a thermophile source. Immobilization of NOX onto agarose activated with glyoxyl groups yielded the most stable enzyme preparation (6-fold more stable than the hyperactivated soluble enzyme). The immobilized derivative was able to be reactivated under physiological conditions after inactivation by high solvent concentrations. The inactivation/reactivation cycle could be repeated at least three times, recovering full NOX activity in all cases after the reactivation step. This immobilized catalyst is presented as a recycling partner for a thermophile alcohol dehydrogenase in order to perform the kinetic resolution secondary alcohols.

Conclusion

We have designed, developed and characterized a heterogeneous and robust biocatalyst which has been used as recycling partner in the kinetic resolution of rac-1-phenylethanol. The high stability along with its capability to be reactivated makes this biocatalyst highly re-useable for cofactor recycling in redox biotransformations.

Enhancing the functional properties of thermophilic enzymes by chemical modification and immobilization

The immobilization of proteins (mostly typically enzymes) onto solid supports is mature technology and has been used successfully to enhance biocatalytic processes in a wide range of industrial applications. However, continued developments in immobilization technology have led to more sophisticated and specialized applications of the process. A combination of targeted chemistries, for both the support and the protein, sometimes in combination with additional chemical and/or genetic engineering, has led to the development of methods for the modification of protein functional properties, for enhancing protein stability and for the recovery of specific proteins from complex mixtures. In particular, the development of effective methods for immobilizing large multi-subunit proteins with multiple covalent linkages (multi-point immobilization) has been effective in stabilizing proteins where subunit dissociation is the initial step in enzyme inactivation. In some instances, multiple benefits are achievable in a single process. Here we comprehensively review the literature pertaining to immobilization and chemical modification of different enzyme classes from thermophiles, with emphasis on the chemistries involved and their implications for modification of the enzyme functional properties. We also highlight the potential for synergies in the combined use of immobilization and other chemical modifications.

Potential Applications of Immobilized β-Galactosidase in Food Processing Industries

The enzyme β-galactosidase can be obtained from a wide variety of sources such as microorganisms, plants, and animals. The use of β-galactosidase for the hydrolysis of lactose in milk and whey is one of the promising enzymatic applications in food and dairy processing industries. The enzyme can be used in either soluble or immobilized forms but the soluble enzyme can be used only for batch processes and the immobilized form has the advantage of being used in batch wise as well as in continuous operation. Immobilization has been found to be convenient method to make enzyme thermostable and to prevent the loss of enzyme activity. This review has been focused on the different types of techniques used for the immobilization of β-galactosidase and its potential applications in food industry.

New Cationic Exchanger Support for Reversible Immobilization of Proteins

New tailor-made cationic exchange resins have been prepared by covalently binding aspartic-dextran polymers (e.g. MW 15 000-20 000) to porous supports (aminated agarose and Sepabeads). More than 80% of the proteins contained in crude extracts from Escherichia coli and Acetobacter turbidans have been strongly adsorbed on these porous materials at pH 5. This interaction was stronger than in conventional carboxymethyl cellulose (e.g., at pH 7 and 25 degrees C, all proteins previously adsorbed at pH 5 were released from carboxymethyl cellulose, whereas no protein was released from the new supports under similar conditions). Ionic exchange properties of such composites were strongly dependent on the size of the aspartic-dextran polymers as well as on the exact conditions of the covalent coating of the solids with the polymer (optimal conditions: 100 mg aspartic-dextran 20 000/(mL of support); room temperature). Finally, some industrially relevant enzymes (Kluyveromices lactis, Aspergillus oryzae, and Thermus sp. beta-galactosidases, Candida antarctica B lipase, and bovine pancreas trypsin and chymotrypsin) have been immobilized on these supports with very high activity recovery and immobilization rates. After enzyme inactivation, the enzyme can be fully desorbed from the support and the support could be reused for several cycles.

High-level overproduction of Thermus enzymes in Streptomyces lividans

Biotechnology needs to explore the capacity of different organisms to overproduce proteins of interest at low cost. In this paper, we show that Streptomyces lividans is a suitable host for the expression of Thermus thermophilus genes and report the overproduction of the corresponding proteins. This capacity was corroborated after cloning the genes corresponding to an alkaline phosphatase (a periplasmic enzyme in T. thermophilus) and that corresponding to a beta-glycosidase (an intracellular enzyme) in Escherichia coli and in S. lividans. Comparison of the production in both hosts revealed that the expression of active protein achieved in S. lividans was much higher than in E. coli, especially in the case of the periplasmic enzyme. In fact, the native signal peptide of the T. thermophilus phosphatase was functional in S. lividans, being processed at the same peptide bond in both organisms, allowing the overproduction and secretion of this protein to the S. lividans culture supernatant. As in E. coli, the thermostability of the expressed proteins allowed a huge purification factor upon thermal denaturation and precipitation of the host proteins. We conclude that S. lividans is a very efficient and industry-friendly host for the expression of thermophilic proteins from Thermus spp.

Influence of the protein oligomericity on final yield after affinity tag removal in purification of recombinant proteins

The new aspect concerning the applicability of histidine and other affinity tags for the purification of oligomeric proteins, with particular emphasis on cleavage efficiency and final yield, is presented in this study. The final yield depends on both the cleavage efficiency and the degree of oligomerization of the protein. Cleavage procedures that are good enough for monomeric proteins can be problematic for oligomeric proteins. Random distribution of uncleaved or partially cleaved affinity tags among oligomers is the main cause of reduced yields. A trimeric protein, tumour necrosis factor alpha (TNF-alpha), bearing different histidine tags, was used as a model protein to explore and confirm this theoretical concept. Analysis of mixed TNF trimers, prepared from tag-free TNF doped with various amounts of histidine-tagged TNF, revealed an increased retention of the trimeric protein on immobilized metal-ion affinity chromatography (IMAC) columns. When 20% of histidine-tagged TNF was added, more than 50% of the protein was retained on the IMAC column. Thus, the applicability of histidine- and other affinity tags for purifying oligomeric proteins is significantly prejudiced in the case of higher oligomers. Various histidine-tags were fused to the N-terminus of full-length TNF-alpha and to the truncated form (dN6) of TNF-alpha. Two-step IMAC separation was used for purification. In the first step, IMAC-1, over 95% purity of histidine-tagged protein was achieved in all cases. Endo- and exoproteolytic removal of histidine tags with enterokinase (EKmax) and aminodipeptidase (DAPase) was studied and the major parameters affecting cleavage efficiency, microheterogeneity and final yield are critically discussed. IMAC-2 was used as the second and final step for removing the cleavage enzyme, cleaved tags, unprocessed protein and some other impurities. Selection of the optimal cleavage enzyme depends on the amino acid composition of the N-terminus and the intended use of the purified protein. The main conclusion is that special caution should be taken when introducing affinity tags to oligomeric proteins, with the final goal to produce pure, tag-free protein with acceptable yields. Given the same enzyme cleavage efficiency one can expect progressively reduced final protein yields with increasing degree of oligomerization. This should be considered as a general rule.

Selective and mild adsorption of large proteins on lowly activated immobilized metal ion affinity chromatography matrices

A strategy to selectively adsorb large proteins on immobilized metal ion affinity chromatography supports is presented. It is based on the fact that large proteins have a large surface that permits the long distance interaction with groups placed quite far apart (very dispersed onto the support surface) in the support, therefore, even using lowly activated supports, these proteins may be able to yield multiple interactions with the support, which is not possible for smaller proteins. This has been shown using a crude extract from Escherichia coli, where only large proteins were adsorbed on supports having 0.25 micromol of metallic groups/g of support. Then, these lowly activated supports have been used for purifying multimeric enzymes from thermophilic organisms (alpha- and beta-galactosidases from Thermus sp. strain T2) cloned and over-expressed in mesophilic ones. A previous heating step of the crude extract destroyed the quaternary structure of all multimeric enzymes from the host (E. coli). Thus, the only large protein remaining in the supernatant of this heated extract are the cloned multimeric thermophilic enzymes, permitting their very simple purification by using only one chromatographic step.

Characterization of a thermostable recombinant beta-galactosidase from Thermotoga maritima

AIMS

Characterization of a thermostable recombinant beta-galactosidase from Thermotoga maritima for the hydrolysis of lactose and the production of galacto-oligosaccharides.

METHODS AND RESULTS

A putative beta-galactosidase gene of Thermotoga maritima was expressed in Escherichia coli as a carboxyl terminal His-tagged recombinant enzyme. The gene encoded a 1100-amino acid protein with a calculated molecular weight of 129,501. The expressed enzyme was purified by heat treatment, His-tag affinity chromatography, and gel filtration. The optimum temperatures for beta-galactosidase activity were 85 and 80 degrees C with oNPG and lactose, respectively. The optimum pH value was 6.5 for both oNPG and lactose. In thermostability experiments, the enzyme followed first-order kinetics of thermal inactivation and its half-life times at 80 and 90 degrees C were 16 h and 16 min, respectively. Mn2+ was the most effective divalent cation for beta-galactosidase activity on both oNPG and lactose. The Km and Vmax values of the thermostable enzyme for oNPG at 80 degrees C were 0.33 mm and 79.6 micromol oNP min(-1) mg(-1). For lactose, the Km and Vmax values were dependent on substrate concentrations; 1.6 and 63.3 at lower concentrations up to 10 mm of lactose and 27.8 mm and 139 micromol glucose min(-1) mg(-1) at higher concentrations, respectively. The enzyme displayed non-Michaelis-Menten reaction kinetics with substrate activation, which was explained by simultaneous reactions of hydrolysis and transgalactosylation.

CONCLUSIONS

The results suggest that the thermostable enzyme may be suitable for both the hydrolysis of lactose and the production of galacto-oligosaccharides.

SIGNIFICANCE AND IMPACT OF THE STUDY

The findings of this work contribute to the knowledge of hydrolysis and transgalactosylation performed by beta-galactosidase of hyperthermophilic bacteria.

Stabilization of a multimeric beta-galactosidase from Thermus sp. strain T2 by immobilization on novel heterofunctional epoxy supports plus aldehyde-dextran cross-linking

This work exemplifies the advantages of using a battery of new heterofunctional epoxy supports to immobilize enzymes. We have compared the performance of a standard Sepabeads-epoxy support with other Sepabeads-epoxy supports partially modified with boronate, iminodiacetic, metal chelates, and ethylenediamine in the immobilization of the thermostable beta-galactosidase from Thermus sp. strain T2 as a model system. Immobilization yields depended on the support, ranging from 95% using Sepabeads-epoxy-chelate, Sepabeads-epoxy-amino, or Sepabeads-epoxy-boronic to 5% using Sepabeads-epoxy-IDA. Moreover, immobilization rates were also very different when using different supports. Remarkably, the immobilized beta-galactosidase derivatives showed very improved but different stabilities after favoring multipoint covalent attachment by long-term alkaline incubation, the enzyme immobilized on Sepabeads-epoxy-boronic being the most stable. This derivative had some subunits of the enzyme not covalently attached to the support (detected by SDS-PAGE). This is a problem if the biocatalysts were to be used in food technology. The optimization of the cross-linking with aldehyde-dextran permitted the full stabilization of the quaternary structure of the enzyme. The optimal derivative was very active in lactose hydrolysis even at 70 degrees C (over 1000 IU/g), maintaining its activity after long incubation times under these conditions and with no risk of product contamination with enzyme subunits.

Ion exchange using poorly activated supports, an easy way for purification of large proteins

Ion-exchange chromatography using commercial ionic supports is a commonly used technique for protein purification. However, selective adsorption of a target protein from a given extract onto commercial ion exchangers seems to be quite complex since they are designed to adsorb the maximum percentage of proteins with the opposite charge. In this paper, ion-exchanger supports with different activation degrees (from 1 to 40 micromol of amino groups per g of agarose) have been prepared and used for the purification of large proteins. These kinds of proteins have large surfaces to interact by many points with the support. Therefore, it was possible to purify large proteins as beta-galactosidase from Thermus sp. strain T2 from a crude extract from Escherichia coli or bovine liver catalase from a commercial preparation, with tailor-made ion-exchanger supports. A simple step of adsorption/desorption on lowly activated supports rendered both enzymes rather pure as confirmed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Moreover, this strategy makes also easy the desorption step that requires rather low NaCl concentrations, which may become a serious problem for desorption of large proteins when using conventional supports, due to their ability of generating a very strong adsorption.