Improving the functional expression of a Bacillus licheniformis laccase by random and site-directed mutagenesis

Combined Strategies for Improving Aflatoxin B1 Degradation Ability and Yield of a Bacillus licheniformis CotA-Laccase

Aflatoxin B1 (AFB1) contamination is a serious threat to nutritional safety and public health. The CotA-laccase from Bacillus licheniformis ANSB821 previously reported by our laboratory showed great potential to degrade AFB1 without redox mediators. However, the use of this CotA-laccase to remove AFB1 in animal feed is limited because of its low catalytic efficiency and low expression level. In order to make better use of this excellent enzyme to effectively degrade AFB1, twelve mutants of CotA-laccase were constructed by site-directed mutagenesis. Among these mutants, E186A and E186R showed the best degradation ability of AFB1, with degradation ratios of 82.2% and 91.8% within 12 h, which were 1.6- and 1.8-times higher than those of the wild-type CotA-laccase, respectively. The catalytic efficiencies (kcat/Km) of E186A and E186R were found to be 1.8- and 3.2-times higher, respectively, than those of the wild-type CotA-laccase. Then the expression vectors pPICZαA-N-E186A and pPICZαA-N-E186R with an optimized signal peptide were constructed and transformed into Pichia pastoris GS115. The optimized signal peptide improved the secretory expressions of E186A and E186R in P. pastoris GS115. Collectively, the current study provided ideal candidate CotA-laccase mutants for AFB1 detoxification in food and animal feed and a feasible protocol, which was desperately needed for the industrial production of CotA-laccases.

Improving Degradation of Polycyclic Aromatic Hydrocarbons by Bacillus atrophaeus Laccase Fused with Vitreoscilla Hemoglobin and a Novel Strong Promoter Replacement

Laccases catalyze a variety of electron-rich substrates by reducing O2 to H2O, with O2 playing a vital role as the final electron acceptor in the reaction process. In the present study, a laccase gene, lach5, was identified from Bacillus atrophaeus through sequence-based screening. LacH5 was engineered for modification by fusion expression and promoter replacement. Results showed that the purified enzyme LacH5 exhibited strong oxidative activity towards 2,2'-azinobis(3-ehtylbenzothiazolin-6-sulfnic acid) ammonium salt (ABTS) under optimum pH and temperature conditions (pH 5.0, 60 °C) and displayed remarkable thermostability. The activity of the two fusion enzymes was enhanced significantly from 14.2 U/mg (LacH5) to 22.5 U/mg (LacH5-vgb) and 18.6 U/mg (Vgb-lacH5) toward ABTS after LacH5 fusing with Vitreoscilla hemoglobin (VHb). Three of six tested polycyclic aromatic hydrocarbons (PAHs) were significantly oxidized by two fusion laccases as compared with LacH5. More importantly, the expression level of LacH5 and fusion protein LacH5-vgb was augmented by 3.7-fold and 7.0-fold, respectively, by using a novel strong promoter replacement. The results from the current investigation provide new insights and strategies for improving the activity and expression level of bacterial laccases, and these strategies can be extended to other laccases and multicopper oxidases.

Decolorization of Textile Dye by Spore Surface Displayed Small Laccase for the Enhanced Thermal Stability and Robust Repeated Reaction

In this study, we tried to decolorize synthetic dyes using small laccase (SLAC) from Streptomyces coelicolor, which is resistant to pH, temperature change, and traditional inhibitors for the actual industrial applications using spore surface display system. We inserted SLAC-His6 tag at the C-terminal of CotE anchoring motif. The proper surface expression of CotE-SLAC fusion protein on the surface of Bacillus subtilis spore was verified with flow cytometry using FITC labeled anti-His6 tag antibody. After 6 h of reaction, more than 90% of Indigo carmine was decomposed using recombinant SLAC displaying Bacillus spore, whereas less than 10% of Indigo carmine was decomposed with wild type spore. Over 70% of laccase activity was retained with recombinant SLAC displaying spore, which was heat-treated for 3 h at 90°C. For eight rounds of repeated decomposition of Indigo carmine, no significant decrease of enzymatic activity was observed. This showed the robust characteristics of spore display format for repeated and harsh condition reactions.

Biochemical Characterization of a Novel Bacterial Laccase and Improvement of Its Efficiency by Directed Evolution on Dye Degradation

Laccase is a copper-containing polyphenol oxidase with a wide range of substrates, possessing a good application prospect in wastewater treatment and dye degradation. The purpose of this research is to study the degradation of various industrial dyes by recombinant laccase rlac1338 and the mutant enzyme lac2-9 with the highest enzyme activity after modification by error-prone PCR. Four enzyme activities improved mutant enzymes were obtained through preliminary screening and rescreening, of which lac2-9 has the highest enzyme activity. There are four mutation sites, including V281A, V281A, P309L, S318G, and D232V. The results showed that the expression of the optimized mutant enzyme also increased by 22 ± 2% compared to the unoptimized enzyme and the optimal reaction temperature of the mutant enzyme lac2-9 was 5°C higher than that of the rlac1338, and the optimal pH increased by 0.5 units. The thermal stability and pH stability of mutant enzyme lac2-9 were also improved. With ABTS as the substrate, the k_cat/K_m of rlac1338 and mutant strain lac2-9 are the largest than other substrates, 0.1638 and 0.618 s^–1M^–1, respectively, indicating that ABTS is the most suitable substrate for the recombinant enzyme and mutant enzyme. In addition, the K_m of the mutant strain lac2-9 (76 μM) was significantly lower, but the k_cat/K_m (0.618 s^–1M^–1) was significantly higher, and the specific enzyme activity (79.8 U/mg) increased by 3.5 times compared with the recombinant laccase (22.8 U/mg). The dye degradation results showed that the use of rlac1338 and lac2-9 alone had no degradation effect on the industrial dyes [indigo, amaranth, bromophenol blue, acid violet 7, Congo red, coomassie brilliant blue (G250)], however, adding small molecular mediators Ca²⁺ and ABTS at the same time can significantly improve the degradation ability. Compared to the rlac1338, the degradation rates with the simultaneous addition of Ca²⁺ and ABTS of mutant enzyme lac2-9 for acid violet 7, bromophenol blue and coomassie brilliant blue significantly improved by 8.3; 3.4 and 3.4 times. Therefore, the results indicated that the error-prone PCR was a feasible method to improve the degradation activity of laccase for environmental pollutants, which provided a basis for the application of laccase on dye degradation and other environmental pollutants.

Design of novel enzyme biocatalysts for industrial bioprocess: Harnessing the power of protein engineering, high throughput screening and synthetic biology

Biocatalysts have wider applications in various industries. Biocatalysts are generating bigger attention among researchers due to their unique catalytic properties like activity, specificity and stability. However the industrial use of many enzymes is hindered by low catalytic efficiency and stability during industrial processes. Properties of enzymes can be altered by protein engineering. Protein engineers are increasingly study the structure-function characteristics, engineering attributes, design of computational tools for enzyme engineering, and functional screening processes to improve the design and applications of enzymes. The potent and innovative techniques of enzyme engineering deliver outstanding opportunities for tailoring industrially important enzymes for the versatile production of biochemicals. An overview of the current trends in enzyme engineering is explored with important representative examples.

Improving decolorization of dyes by laccase from Bacillus licheniformis by random and site-directed mutagenesis

BACKGROUND

Dye wastewater increases cancer risk in humans. For the treatment of dyestuffs, biodegradation has the advantages of economy, high efficiency, and environmental protection compared with traditional physical and chemical methods. Laccase is the best candidate for dye degradation because of its multiple substrates and pollution-free products.

METHODS

Here, we modified the laccase gene of Bacillus licheniformis by error-prone PCR and site-directed mutagenesis and expressed in E. coli. The protein was purified by His-tagged protein purification kit. We tested the enzymatic properties of wild type and mutant laccase by single factor test, and further evaluated the decolorization ability of laccase to acid violet, alphazurine A, and methyl orange by spectrophotometry.

RESULTS

Mutant laccase Lacep69and D500G were superior to wild type laccase in enzyme activity, stability, and decolorization ability. Moreover, the laccase D500G obtained by site-directed mutagenesis had higher enzyme activity in both, and the specific activity of the purified enzyme was as high as 426.13 U/mg. Also, D500G has a higher optimum temperature of 70 °C and temperature stability, while it has a more neutral pH 4.5 and pH stability. D500G had the maximum enzyme activity at a copper ion concentration of 12 mM. The results of decolorization experiments showed that D500G had a strong overall decolorization ability, with a lower decolorization rate of 18% for methyl orange and a higher decolorization rate of 78% for acid violet.

CONCLUSION

Compared with the wild type laccase, the enzyme activity of D500G was significantly increased. At the same time, it has obvious advantages in the decolorization effect of different dyes. Also, the advantages of temperature and pH stability increase its tolerance to the environment of dye wastewater.

Enhancing the decolorization activity of Bacillus pumilus W3 CotA-laccase to Reactive Black 5 by site-saturation mutagenesis

Reactive Black 5 (RB5) is a typical refractory azo dye. Widespread utilization of RB5 has caused a variety of environmental and health problems. The enzymatic degradation of RB5 can be a promising solution due to its superiority as an eco-friendly and cost-competitive process. Bacterial CotA-laccase shows great application prospect to eliminate hazardous dyes from wastewater. However, efficient decolorization of RB5 CotA-laccase generally requires the participation of costly, toxic mediators. In the present study, we modified the amino acids Thr415 and Thr418 near the type 1 copper site and the amino acid Gln442 at the entrance of the substrate-binding pocket of Bacillus pumilus W3 CotA-laccase to boost its RB5 decolorization activity based on molecular docking analysis and site-saturation mutagenesis. Through the strategies, two double site mutants T415D/Q442A and T418K/Q442A obtained demonstrated 43.94 and 52.64% RB5 decolorization rates in the absence of a mediator at pH 10.0, respectively, which were about 3.70- and 4.43-fold higher compared with the wild-type CotA-laccase. Unexpectedly, the catalytic efficiency of the T418K/Q442A to ABTS was enhanced by 5.33-fold compared with the wild-type CotA-laccase. The mechanisms of conferring enhanced activity to the mutants were proposed by structural analysis. In summary, the mutants T415D/Q442A and T418K/Q442A have good application potentials for the biodegradation of RB5. KEY POINTS: • Three amino acids of CotA-laccase were manipulated by site-saturation mutagenesis. • Decolorization rate of two mutants to RB5 was enhanced 3.70- and 4.43-fold, respectively. • The mechanisms of awarding enhanced activity to the mutants were supposed.

Extracellular expression of mutant CotA-laccase SF in Escherichia coli and its degradation of malachite green

In this study, mutant CotA-laccase SF was successfully expressed in Escherichia coli by co-expression with phospholipase C. The optimized extracellular expression of CotA-laccase SF was 1257.22 U/L. Extracellularly expressed CotA-laccase SF exhibits enzymatic properties similar to intracellular CotA-laccase SF. CotA-laccase SF could decolorize malachite green (MG) under neutral and alkaline conditions. The K_m and k_cat values of CotA-laccase SF to MG were 39.6 mM and 18.36 s^-1. LC-MS analysis of degradation products showed that MG was finally transformed into 4-aminobenzophenone and 4-aminophenol by CotA-laccase. The toxicity experiment of garlic root tip cell showed that the toxicity of MG metabolites decreased. In summary, CotA-laccase SF had a good application prospect for degrading malachite green.

Enhanced catalytic efficiency of CotA-laccase by DNA shuffling

Bacterial CotA-laccases exhibit higher activity in alkaline pH and salt concentration conditions compared to laccases from white-rot fungi. They are considered as green catalysts in decolorizing of industrial dyes. However, CotA-laccases are limited due to the low yield and catalytic efficiency as the spore-bound nature of CotA. A DNA shuffling strategy was applied to generate a random mutation library. To improve laccase activities, a mutant (T232P/Q367R 5E29) with two amino acid substitutions was identified. The catalytic efficiency of mutant 5E29 was 1.21 fold higher compared with that of the wild-type. The Km and k_cat values of 5E29 for SGZ were of 20.3 ± 1.3 µM and 7.6 ± 2.7 s^-1. The thermal stability was a slight enhancement. Indigo Carmine and Congo red were efficiently decolorized by using this mutant at pH 9.0. These results provide that 5E29 CotA-laccase is a good candidate for biotechnology applications under alkaline condition, with an effective decolorization capability.

Enhancement in catalytic activity of CotA-laccase from Bacillus pumilus W3 via site-directed mutagenesis

CotA-laccases are potential enzymes that are widely used in decolorization of dyes and degradation of toxic substances. In this study, a novel CotA-laccase gene from Bacillus pumilus W3 was applied for rational design. After a series of site-directed genetic mutations, the mutant S208G/F227A showed a 5.1-fold higher catalytic efficiency (k_cat/K_m) than the wild-type CotA-laccase did. The optimal pH of S208G/F227A was 3.5 with ABTS as substrate. The residual activity of mutant S208G/F227A was more than 80% after incubated for 10 h at pH 7-11. Mutant S208G/F227A showed optimal temperature at 80°C with ABTS as substrate. The thermal stability of mutant laccase S208G/F227A was lower than that of wild-type CotA-laccase. This study showed that Gly208 and Ala227 play key roles in catalytic efficiency and it is possible to improve catalytic efficiency of CotA-laccase through site-directed mutagenesis.

Cohnella sp. A01 laccase: thermostable, detergent resistant, anti-environmental and industrial pollutants enzyme

Laccase (EC 1.10.3.2; benzenediol; oxygen oxidoreductases) is a multi-copper oxidase that catalyzes the oxidation of phenols, polyphenols, aromatic amines, and different non-phenolic substrates with concomitant reduction of O₂ to H₂O. Enzymatic oxidation techniques have the potential of implementation in different areas of industrial fields. In this study, the Cohnella sp. A01 laccase gene was cloned into pET-26 (b+) vector and was transformed to E. coli BL21. Then it was purified using His tag affinity (Ni sepharose resin) chromatography. The estimated molecular weight was approximately 60 kDa using SDS-PAGE. The highest enzyme activity and best pH for 2,6-dimethoxyphenol (DMP) oxidation were recorded as 8 at 90 °C respectively. The calculated half-life and kinetic values including K_m, V_max, turn over number (k_cat), and catalytic efficiency (k_cat/K_m) of the enzyme were 106 min at 90 °C and 686 μM, 10.69 U/ml, 20.3 S⁻, and 0.029 s⁻¹ μM⁻¹, respectively. The DMP was available as the substrate in all the calculations. Enzyme activity enhanced in the presence of Cu²⁺, NaCl, SDS, n-hexane, Triton X-100, tween 20, and tween 80, significantly. The binding residues were predicted and mapped upon the modeled tertiary structure of identified laccase. The remaining activity and structural properties of Cohnella sp. A01 laccase in extreme conditions such as high temperatures and presence of metals, detergents, and organic solvents suggest the potential of this enzyme in biotechnological and industrial applications. This process has been patented in Iranian Intellectual Property Centre under License No: 91325.

Bacterial laccases: promising biological green tools for industrial applications

Multicopper oxidases (MCOs) are a pervasive family of enzymes that oxidize a wide range of phenolic and nonphenolic aromatic substrates, concomitantly with the reduction of dioxygen to water. MCOs are usually divided into two functional classes: metalloxidases and laccases. Given their broad substrate specificity and eco-friendliness (molecular oxygen from air as is used as the final electron acceptor and they only release water as byproduct), laccases are regarded as promising biological green tools for an array of applications. Among these laccases, those of bacterial origin have attracted research attention because of their notable advantages, including broad substrate spectrum, wide pH range, high thermostability, and tolerance to alkaline environments. This review aims to summarize the significant research efforts on the properties, mechanisms and structures, laccase-mediator systems, genetic engineering, immobilization, and biotechnological applications of the bacteria-source laccases and laccase-like enzymes, which principally include Bacillus laccases, actinomycetic laccases and some other species of bacterial laccases. In addition, these enzymes may offer tremendous potential for environmental and industrial applications.

Kinetic analysis and structural studies of a high-efficiency laccase from Cerrena sp. RSD1

A high‐efficiency laccase, DLac, was isolated from Cerrena sp. RSD1. The kinetic studies indicate that DLac is a diffusion‐limited enzyme. The crystal structure of DLac was determined to atomic resolution, and its overall structure shares high homology to monomeric laccases, but displays unique substrate‐binding loops from those in other laccases. The substrate‐binding residues with small side chain and the short substrate‐binding loop IV broaden the substrate‐binding cavity and may facilitate large substrate diffusion. Unlike highly glycosylated fungal laccases, the less‐glycosylated DLac contains one highly conserved glycosylation site at N432 and an unique glycosylation site at N468. The N‐glycans stabilize the substrate‐binding loops and the protein structure, and the first N‐acetylglucosamine is crucial for the catalytic efficiency. Additionally, a fivefold increase in protein yield is achieved via the submerged culture method for industrial applications.

Database

The atomic coordinates of the structure of DLac from Cerrena sp. RSD1 and structural factors have been deposited in the RCSB Protein Data Bank (PDB ID: 5Z1X).

Activity enhancement of CotA laccase by hydrophilic engineering, histidine tag optimization and static culture

CotA protein from Bacillus subtilis is of laccase activity. The solubility of recombinant CotA is low, which hinders its application. In this study, histidine tag position optimization and hydrophilic engineering were applied to increase the yield and activity of CotA protein. The results showed that the protein yield of CotA with his tag at C-terminal (CH6-CotA) was four times of that of NH6-CotA (His tag at N-terminal). Then, 23 single mutants were constructed by substitutions of hydrophobic residues with hydrophilic amino acids. Among them, the protein yield of the mutant F207Y was increased by 30%; the catalytic activity (kcat/Km) of V403T and P455S was two and three times higher than that of CH6-CotA, respectively. Finally, triple mutant F2071Y/V403T/P455S with C-terminal his-tag (CH6-TSY) was constructed. When the proteins were expressed in microanaerobic condition, the activities of mutants CH6-P455S and CH6-TSY were enhanced about 48- and 42-folds compared to that of NH6-CotA in non-static culture.

Bacterial laccase: recent update on production, properties and industrial applications

Laccases (benzenediol: oxygen oxidoreductase, EC 1.10.3.2) are multi-copper enzymes which catalyze the oxidation of a wide range of phenolic and non-phenolic aromatic compounds in the presence or absence of a mediator. Till date, laccases have mostly been isolated from fungi and plants, whereas laccase from bacteria has not been well studied. Bacterial laccases have several unique properties that are not characteristics of fungal laccases such as stability at high temperature and high pH. Bacteria produce these enzymes either extracellularly or intracellularly and their activity is in a wide range of temperature and pH. It has application in pulp biobleaching, bioremediation, textile dye decolorization, pollutant degradation, biosensors, etc. Hence, comprehensive information including sources, production conditions, characterization, cloning and biotechnological applications is needed for the effective understanding and application of these enzymes at the industrial level. The present review provides exhaustive information of bacterial laccases reported till date.

Enhancing the thermostability of α-L-rhamnosidase from Aspergillus terreus and the enzymatic conversion of rutin to isoquercitrin by adding sorbitol

Background

Thermally stable α-L-rhamnosidase with cleaving terminal α-L-rhamnose activity has great potential in industrial application. Therefore, it is necessary to find a proper method to improve the thermal stability of α-L-rhamnosidase.

Results

In this study, addition of sorbitol has been found to increase the thermostability of α-L-rhamnosidase from Aspergillus terreus at temperatures ranging from 65 °C to 80 °C. Half-life and activation free energy with addition of 2.0 M sorbitol at 70 °C were increased by 17.2-fold, 8.2 kJ/mol, respectively. The analyses of the results of fluorescence spectroscopy and CD have indicated that sorbitol helped to protect the tertiary and secondary structure of α-L-rhamnosidase. Moreover, the isoquercitrin yield increased from 60.01 to 96.43% with the addition of 1.5 M of sorbitol at 70 °C.

Conclusion

Our findings provide an effective approach for enhancing the thermostability of α-L-rhamnosidase from Aspergillus terreus, which makes it a good candidate for industrial processes of isoquercitrin preparation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0342-9) contains supplementary material, which is available to authorized users.

Yeast Hosts for the Production of Recombinant Laccases: A Review

Laccases are multi-copper oxidoreductases which catalyze the oxidation of a wide range of substrates during the simultaneous reduction of oxygen to water. These enzymes, originally found in fungi, plants, and other natural sources, have many industrial and biotechnological applications. They are used in the food, textile, pulp, and paper industries, as well as for bioremediation purposes. Although natural hosts can provide relatively high levels of active laccases after production optimization, heterologous expression can bring, moreover, engineered enzymes with desired properties, such as different substrate specificity or improved stability. Hence, diverse hosts suitable for laccase production are reviewed here, while the greatest emphasis is placed on yeasts which are commonly used for industrial production of various proteins. Different approaches to optimize the laccase expression and activity are also discussed in detail here.

Crystal structure of CotA laccase complexed with 2,2-azinobis-(3-ethylbenzothiazoline-6-sulfonate) at a novel binding site

The CotA laccase from Bacillus subtilis is an abundant component of the spore outer coat and has been characterized as a typical laccase. The crystal structure of CotA complexed with 2,2-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) in a hole motif has been solved. The novel binding site was about 26 Å away from the T1 binding pocket. Comparison with known structures of other laccases revealed that the hole is a specific feature of CotA. The key residues Arg476 and Ser360 were directly bound to ABTS. Site-directed mutagenesis studies revealed that the residues Arg146, Arg429 and Arg476, which are located at the bottom of the novel binding site, are essential for the oxidation of ABTS and syringaldazine. Specially, a Thr480Phe variant was identified to be almost 3.5 times more specific for ABTS than for syringaldazine compared with the wild type. These results suggest this novel binding site for ABTS could be a potential target for protein engineering of CotA laccases.

Comparison of different microbial laccases as tools for industrial uses

Laccases from different sources are employed in a number of biotechnological processes, each characterized by specific reaction constraints and thus requiring an enzyme with suitable properties. In order to avoid the bias generated by different assay methodologies, in this work we investigated the main properties of ten laccases from fungi and bacteria under identical conditions. As a general rule, the optimal activity was apparent at pH 3-4 and was lost at pH≥7.0 (all laccases were stable at pH≥7.0); enzymes active at neutral pH values were also identified. For all tested laccases, activity increased with temperature up to 80°C and stability was good at 25°C. Interestingly, laccases insensitive to high salt concentration were identified, this favoring their use in treating waste waters. Indeed, bacterial laccases retained a significant activity in the presence of DMSO (up to 40% final concentration) and of surfactants, suggesting that they can be applied in lignin degradation processes requiring solvents. The available laccases are versatile and satisfy requirements related to different processes. Notably, the recombinant laccase from Bacillus licheniformis favorably compares with the tested enzymes, indicating that it is well suited for different biotechnological applications.

Biochemical properties and yields of diverse bacterial laccase-like multicopper oxidases expressed in Escherichia coli

Laccases are multi-copper oxidases that oxidize a broad range of substrates at the expense of molecular oxygen, without any need for co-factor regeneration. These enzymes bear high potential for the sustainable synthesis of fine chemicals and the modification of (bio)polymers. Here we describe cloning and expression of five novel bacterial laccase-like multi copper oxidases (LMCOs) of diverse origin which were identified by homology searches in online databases. Activity yields under different expression conditions and temperature stabilities were compared to three previously described enzymes from Bacillus subtilis, Bacillus pumilus and Bacillus clausii. In almost all cases, a switch to oxygen-limited growth conditions after induction increased volumetric activity considerably. For proteins with predicted signal peptides for secretion, recombinant expression with and without signal sequence was investigated. Bacillus CotA-type LMCOs outperformed enzymes from Streptomyces and Gram-negative bacteria with respect to activity yields in Escherichia coli and application relevant biochemical properties. The novel Bacillus coagulans LMCO combined high activity yields in E. coli with unprecedented activity at strong alkaline pH and high storage stability, making it a promising candidate for further development.

Laccases of prokaryotic origin: enzymes at the interface of protein science and protein technology

The ubiquitous members of the multicopper oxidase family of enzymes oxidize a range of aromatic substrates such as polyphenols, methoxy-substituted phenols, amines and inorganic compounds, concomitantly with the reduction of molecular dioxygen to water. This family of enzymes can be broadly divided into two functional classes: metalloxidases and laccases. Several prokaryotic metalloxidases have been described in the last decade showing a robust activity towards metals, such as Cu(I), Fe(II) or Mn(II) and have been implicated in the metal metabolism of the corresponding microorganisms. Many laccases, with a superior efficiency for oxidation of organic compounds when compared with metals, have also been identified and characterized from prokaryotes, playing roles that more closely conform to those of intermediary metabolism. This review aims to present an update of current knowledge on prokaryotic multicopper oxidases, with a special emphasis on laccases, anticipating their enormous potential for industrial and environmental applications.

Laccase engineering by rational and evolutionary design

Laccases are considered as green catalysts of great biotechnological potential. This has attracted a great interest in designing laccases a la carte with enhanced stabilities or activities tailored to specific conditions for different fields of application. Over 20 years, numerous efforts have been taken to engineer these multicopper oxidases and to understand their reaction mechanisms by site-directed mutagenesis, and more recently, using computational calculations and directed evolution tools. In this work, we review the most relevant contributions made in the field of laccase engineering, from the comprehensive study of their structure–function relationships to the tailoring of outstanding biocatalysts.

In vitro engineering of microbial enzymes with multifarious applications: prospects and perspectives

The discovery of a novel enzyme from a microbial source takes anywhere between months to years, and therefore, there has been an immense interest in modifying the existing microbial enzymes to suit the present day needs of the industry. The redesigning of industrially useful enzymes for improving their performance has become a challenge because bioinformatics databases have been revealing new facts on a day-to-day basis. Modification of the existing enzymes has become a trend for fine tuning of biocatalysts in the biotech industry. Hydrolases are employed in pharmaceutical, biofuel, detergent, food and feed industries that significantly contribute to the global annual revenue, and therefore, the emphasis has been on engineering them. Although a large data is accumulating on making alterations in microbial enzymes, there is a lack of definite information on redesigning industrial enzymes. This review focuses on the recent developments in improving the characteristics of various biotechnologically important enzymes.

Structural insights into 2,2'-azino-Bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)-mediated degradation of reactive blue 21 by engineered Cyathus bulleri Laccase and characterization of degradation products

Advanced oxidation processes are currently used for the treatment of different reactive dyes which involve use of toxic catalysts. Peroxidases are reported to be effective on such dyes and require hydrogen peroxide and/or metal ions. Cyathus bulleri laccase, expressed in Pichia pastoris, catalyzes efficient degradation (78 to 85%) of reactive azo dyes (reactive black 5, reactive orange 16, and reactive red 198) in the presence of synthetic mediator ABTS [2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)]. This laccase was engineered to degrade effectively reactive blue 21 (RB21), a phthalocyanine dye reported to be decolorized only by peroxidases. The 816-bp segment (toward the C terminus) of the lcc gene was subjected to random mutagenesis and enzyme variants (Lcc35, Lcc61, and Lcc62) were selected based on increased ABTS oxidizing ability. Around 78 to 95% decolorization of RB21 was observed with the ABTS-supplemented Lcc variants in 30 min. Analysis of the degradation products by mass spectrometry indicated the formation of several low-molecular-weight compounds. Mapping the mutations on the modeled structure implicated residues both near and far from the T1 Cu site that affected the catalytic efficiency of the mutant enzymes on ABTS and, in turn, the rate of oxidation of RB21. Several inactive clones were also mapped. The importance of geometry as well as electronic changes on the reactivity of laccases was indicated.

Bacillus amyloliquefaciens laccase--from soil bacteria to recombinant enzyme for wastewater decolorization

One hundred wild type strains of Bacillus sp. were isolated from industrial and agricultural soil across Serbia and screened for laccase activity. Three strains showed high laccase activity temperature optimum of 65 and 80 °C towards ABTS. A new laccase gene from the strain with highest temperature optimum, namely Bacillus amyloliquefaciens 12B was cloned and expressed in Escherichia coli. Recombinant laccase degraded dye Reactive blue 52 at pH 7.0 and pH 4.0 and at elevated temperature, while fungal laccases was unable to act on this substrate at pH higher than 4.0 and was quickly inactivated at temperatures higher than 45 °C. Degradation of dye was monitored by HPLC-DAD and resulting precipitate was analyzed by FTIR spectroscopy. Single product peak without chromophore was detected in solution, while water insoluble aggregate, presumably dye polymer is formed retaining blue color.

Enhancement of catalysis and functional expression of a bacterial laccase by single amino acid replacement

Structure-function relationships underlying laccases properties are very limited that makes these enzymes interesting for protein engineering approaches. Therefore in the current study, a thermostable laccase that was isolated from Bacillus sp. HR03 with the ability of bilirubin oxidation besides its laccase and tyrosinase activity is used. The extensive application of this enzyme is limited by its low expression level in Escherichia coli. Based on sequence alignments and structural studies, three single amino acid substitutions, D500G, D500E, D500S and a glycine insertion, are introduced using site-directed mutagenesis to evaluate the role of Asp(500) located in the C-terminal segment close to the T1 copper center. Substitution of aspartic acid with less sterically hindered, conserved residue such as glycine increase kcat (2.3 fold) and total activity (7.3 fold) which is accompanied by a significant increase in the expression level up to 3 fold. Biochemical characterization and structural studies using far-UV CD and fluorescence spectroscopy reveal the importance of C-terminal copper-binding loop in the laccase functional expression and catalytic efficiency. Kinetic characterization of the purified mutants toward 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), syringaldazine (SGZ) and bilirubin, shows that substrate specificity is left unchanged.

CotA, a multicopper oxidase from Bacillus pumilus WH4, exhibits manganese-oxidase activity

Multicopper oxidases (MCOs) are a family of enzymes that use copper ions as cofactors to oxidize various substrates. Previous research has demonstrated that several MCOs such as MnxG, MofA and MoxA can act as putative Mn(II) oxidases. Meanwhile, the endospore coat protein CotA from Bacillus species has been confirmed as a typical MCO. To study the relationship between CotA and the Mn(II) oxidation, the cotA gene from a highly active Mn(II)-oxidizing strain Bacillus pumilus WH4 was cloned and overexpressed in Escherichia coli strain M15. The purified CotA contained approximately four copper atoms per molecule and showed spectroscopic properties typical of blue copper oxidases. Importantly, apart from the laccase activities, the CotA also displayed substantial Mn(II)-oxidase activities both in liquid culture system and native polyacrylamide gel electrophoresis. The optimum Mn(II) oxidase activity was obtained at 53°C in HEPES buffer (pH 8.0) supplemented with 0.8 mM CuCl2. Besides, the addition of o-phenanthroline and EDTA both led to a complete suppression of Mn(II)-oxidizing activity. The specific activity of purified CotA towards Mn(II) was 0.27 U/mg. The Km, Vmax and kcat values towards Mn(II) were 14.85±1.17 mM, 3.01×10(-6)±0.21 M·min(-1) and 0.32±0.02 s(-1), respectively. Moreover, the Mn(II)-oxidizing activity of the recombinant E. coli strain M15-pQE-cotA was significantly increased when cultured both in Mn-containing K liquid medium and on agar plates. After 7-day liquid cultivation, M15-pQE-cotA resulted in 18.2% removal of Mn(II) from the medium. Furthermore, the biogenic Mn oxides were clearly observed on the cell surfaces of M15-pQE-cotA by scanning electron microscopy. To our knowledge, this is the first report that provides the direct observation of Mn(II) oxidation with the heterologously expressed protein CotA, Therefore, this novel finding not only establishes the foundation for in-depth study of Mn(II) oxidation mechanisms, but also offers a potential biocatalyst for Mn(II) removal.