Enhanced expression of an industry applicable CotA laccase from Bacillus subtilis in Pichia pastoris by non-repressing carbon sources together with pH adjustment: Recombinant enzyme characterization and dye decolorization

Production and characterization of novel thermostable CotA-laccase from Bacillus altitudinis SL7 and its application for lignin degradation

Laccases are multi-copper oxidases and found in ligninolytic bacteria catalyzing the oxidation of both phenolic and non-phenolic compounds, however its application in lignin degradation suffers due to low oxidation rate, which have intensified the search for new laccases. In the present study, spore coat A protein (CotA) encoding gene having laccase like activity from Bacillus altitudinis SL7 (CotA-SL7) was cloned and expressed in Escherichia coli. The purified CotA-SL7 was active at wide range of temperature and pH with optimum activity at 55 °C and pH 5.0. The kinetic parameters of CotA-SL7 was determined with Km, Vmax, and kcat values 0.4 mM, 2777 μmol/min/mg, and 5194 s-1, respectively. Molecular docking revealed the presence of Pro, Phe, Asp, Asn, His, and Ile residues at the active site taking part in the oxidation of ABTS. The purified CotA-SL7 reduced lignin contents by 31 % and changes in lignin structure were analyzed through fourier transformed infrared spectroscopy (FTIR), scanning electron microsscopy (SEM) and gas chromatography mass-spectrometry (GC-MS). The appearance of low molecular size compounds clearly indicates the cleavage of lignin polymer and opening of the benzene ring by purified CotA-SL7. Thus, high catalytic efficiency of CotA-SL7 makes it a suitable bio-catalyst for remediation of lignin contaminated wastewater from pulp and paper industries with clear insights into lignin degradation at molecular level.

A comprehensive review on the potential of microbial enzymes in multipollutant bioremediation: Mechanisms, challenges, and future prospects

Industrialization and other human activity represent significant environmental hazards. Toxic contaminants can harm a comprehensive platform of living organisms in their particular environments. Bioremediation is an effective remediation process in which harmful pollutants are eliminated from the environment using microorganisms or their enzymes. Microorganisms in the environment often create a variety of enzymes that can eliminate hazardous contaminants by using them as a substrate for development and growth. Through their catalytic reaction mechanism, microbial enzymes may degrade and eliminate harmful environmental pollutants and transform them into non-toxic forms. The principal types of microbial enzymes which can degrade most hazardous environmental contaminants include hydrolases, lipases, oxidoreductases, oxygenases, and laccases. Several immobilizations, genetic engineering strategies, and nanotechnology applications have been developed to improve enzyme performance and reduce pollution removal process costs. Until now, the practically applicable microbial enzymes from various microbial sources and their ability to degrade multipollutant effectively or transformation potential and mechanisms are unknown. Hence, more research and further studies are required. Additionally, there is a gap in the suitable approaches considering toxic multipollutants bioremediation using enzymatic applications. This review focused on the enzymatic elimination of harmful contaminants in the environment, such as dyes, polyaromatic hydrocarbons, plastics, heavy metals, and pesticides. Recent trends and future growth for effectively removing harmful contaminants by enzymatic degradation are also thoroughly discussed.

A review on catalytic-enzyme degradation of toxic environmental pollutants: Microbial enzymes

Industrialization and other human anthropogenic activities cause serious threats to the environment. The toxic pollutants can cause detrimental diseases on diverse living beings in their respective ecosystems. Bioremediation is one of the efficient remediation methods in which the toxic pollutants are removed from the environment by the application of microorganisms or their biologically active products (enzymes). Typically, the microorganisms in the environment produce various enzymes to immobilize and degrade the toxic environmental pollutants by utilizing them as a substrate for their growth and development. Both the bacterial and fungal enzymes can degrade the toxic pollutants present in the environment and convert them into non-toxic forms through their catalytic reaction mechanism. Hydrolases, oxidoreductases, dehalogenases, oxygenases and transferases are the major classes of microbial enzymes responsible for the degradation of most of the toxic pollutants in the environment. Recently, there are different immobilizations and genetic engineering techniques have been developed to enhance enzyme efficiency and diminish the process cost for pollutant removal. This review focused on enzymatic removal of toxic pollutants such as heavy metals, dyes, plastics and pesticides in the environment. Current trends and further expansion for efficient removal of toxic pollutants through enzymatic degradation are also reviewed in detail.

Enhancing the expression of recombinant small laccase in Pichia pastoris by a double promoter system and application in antibiotics degradation

Low-expression levels remain a challenge in the quest to use the small laccase (rSLAC) as a viable catalyst. In this study, a recombinant Pichia pastoris strain (rSLAC-GAP-AOX) producing rSLAC under both AOX and GAP promoters (located in two different plasmids) was generated and cultivated in the presence of methanol and mixed feed (methanol:glycerol). Induction with methanol resulted in a maximum laccase activity of 1200 U/L for rSLAC-GAP-AOX which was approximately 2.4-fold higher than rSLAC-AOX and 5.1-fold higher than rSLAC-GAP. The addition of methanol:glycerol in a stoichiometric ratio of 9:1 consistently improved biomass and led to a 1.5-fold increase in rSLAC production as compared to induction with methanol alone. The rSLAC removed 95% of 5 mg/L ciprofloxacin (CIP) and 99% of 100 mg/L tetracycline (TC) in the presence of a mediator. Removal of TC resulted in complete elimination of antibacterial activity while up to 48% reduction in antibacterial activity was observed when CIP was removed. Overall, the present study highlights the effectiveness of a double promoter system in enhancing SLAC production.

Enzymatic treatment of phenolic pollutants by a small laccase immobilized on APTES-functionalised magnetic nanoparticles

In this study, we have successfully synthesized magnetic nanoparticles (MNPs), functionalised them by silanization and used them for the covalent immobilization of a recombinant small laccase (rSLAC) from Streptomyces coelicolor. The immobilized recombinant laccase (MNP-rSLAC) was subsequently used for the treatment of phenol, 4-chlorophenol (4-CP) and 4-fluorophenol (4-FP). The enzyme completely degraded 80 µg/mL of the selected phenolic compounds within 2 h in the presence of a natural mediator, acetosyringone. The MNP-rSLAC retained > 73% of initial activity (2,6-dimethoxyphenol as substrate) after 10 catalytic cycles and could be easily recovered from the reaction mixture by the application of magnetic field. Furthermore, immobilised rSLAC exhibited better storage stability than its free counterpart. The Michaelis constant (K_m) value for the immobilised rSLAC was higher than free rSLAC, however the maximum velocity (V_max) of the immobilised SLAC was similar to that of the free rSLAC. Growth inhibition studies using Escherichia coli showed that rSLAC-mediated treatment of phenolic compounds reduced the toxicity of phenol, 4-CP and 4-FP by 90, 60 and 55%, respectively. Interestingly, the presence of selected metal ions (Co²⁺, Cu²⁺, Mn²⁺) greatly enhanced the catalytic activity of rSLAC and MNP-rSLAC. This study indicates that immobilized small laccase (MNP-rSLAC) has potential for treating wastewater contaminated with phenolic compounds.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02854-0.

Optimisation of the Production and Bleaching Process for a New Laccase from Madurella mycetomatis, Expressed in Pichia pastoris: from Secretion to Yielding Prominent

Laccases are polyphenol oxidoreductases used in a number of industrial applications. Due to the increasing demand for these "green catalysis" enzymes, the identification and biochemical characterisation of their novel properties is essential. In our study, cloned Madurella mycetomatis laccase (mmlac) genes were heterologously expressed in the methylotrophic yeast host Pichia pastoris. The high yield of the active recombinant protein in P. pastoris demonstrates the efficiency of a reliably constructed plasmid to express the laccase gene. The optimal biochemical conditions for the successfully expressed MmLac enzyme were identified. Detailed structural properties of the recombinant laccase were determined, and its utility in decolourisation and textile bleaching applications was examined. MmLac demonstrates good activity in an acidic pH range (4.0-6.0); is stable in the presence of cationic metals, organic solvents and under high temperatures (50-60 °C); and is stable for long-term storage at - 20 °C and - 80 °C for up to eight weeks. The structural analysis revealed that the catalytic residues are partially similar to other laccases. MmLac resulted in an increase in whiteness, whilst demonstrating high efficiency and stability and requiring the input of fewer chemicals. The performance of this enzyme makes it worthy of investigation for use in textile biotechnology applications, as well as within environmental and food technologies.

Degradation of Aflatoxin B1 and Zearalenone by Bacterial and Fungal Laccases in Presence of Structurally Defined Chemicals and Complex Natural Mediators

Aflatoxin B₁ (AFB₁) and zearalenone (ZEN) exert deleterious effects to human and animal health. In this study, the ability of a CotA laccase from Bacillus subtilis (BsCotA) to degrade these two mycotoxins was first investigated. Among the nine structurally defined chemical compounds, methyl syringate was the most efficient mediator assisting BsCotA to degrade AFB₁ (98.0%) and ZEN (100.0%). BsCotA could also use plant extracts, including the Epimedium brevicornu, Cucumis sativus L., Lavandula angustifolia, and Schizonepeta tenuifolia extracts to degrade AFB₁ and ZEN. Using hydra and BLYES as indicators, it was demonstrated that the degraded products of AFB₁ and ZEN using the laccase/mediator systems were detoxified. Finally, a laccase of fungal origin was also able to degrade AFB₁ and ZEN in the presence of the discovered mediators. The findings shed light on the possibility of using laccases and a mediator, particularly a natural plant-derived complex mediator, to simultaneously degrade AFB₁ and ZEN contaminants in food and feed.

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.

Textile Dye Decolorizing Synechococcus PCC7942 Engineered With CotA Laccase

Cyanobacteria are prokaryotic phototrophs capable of achieving high cellular densities with minimal inputs. These prokaryotic organisms can grow using sunlight as energy source and carbon dioxide as carbon source what makes them promising candidates as microbial cell factories for the production of numerous compounds such as chemicals, fuels, or biocatalysts. In this study, we have successfully designed and constructed using synthetic biology approach two recombinant strains of Synechococcus elongatus PCC7942 for heterologous expression of the industrially relevant Bacillus subtilis CotA laccase. One of the strains (PCC7942-NSI-CotA) was constructed through integration of the laccase gene into neutral site I of the cyanobacterial genome whilst the other (PCC7942-NSII-CotA) targeted neutral site II of the genome. Of the two strains the one with CotA laccase integrated in neutral site II (PCC7942-NSII-CotA) was superior in terms of growth rate and enzymatic activity toward typical laccase substrates: ABTS [2,2-azino-bis (3-ethylbenzothiazoline-6-sulfonate)] and syringaldazine. That may suggest that two of the traditionally used neutral sites of S. elongatus PCC7942 are not equally suitable for the expression of certain transgenes. The PCC7942-NSII-CotA produced protein was capable of decolourising three classes of dyes namely: anthraquinonic-, azo-, and indigoid-type over 7 days of incubation making the strain a potentially useful microbial cell factory for the production of broad-spectrum biodegradation agent. Interestingly, presence of additional synthetic redox mediator ABTS had no effect on the degradation of these dyes.

The development of CotA mediator cocktail system for dyes decolorization

AIMS

The increasing use of dyes leads to serious environmental concerns, it is significant to explore eco-friendly and economic approaches for dye decolorization. This study aimed to develop mediator cocktail (AS and ABTS) for enhancing the capability of laccase-mediator system in the removal of dyes.

METHODS AND RESULTS

By mediator screening, the mediators of ABTS and AS (ABTS, 2, 2'-azino-bis-(3-ethylbenzothiazo-thiazoline-6-sulphonic acid); AS, acetosyringone) were combined for dyes decolorization. The Box-Behnken Design and response surface analysis was performed to optimize experiment conditions. Comparing the CotA-ABTS-AS cocktail system with CotA-single mediator system showed that the coupling of ABTS and AS could increase the decolorization rate 15 times higher, save a third of the cost and shorten the reaction time by 50%. In addition, our studies revealed that sequential oxidation may occur in CotA-ABTS-AS system.

CONCLUSIONS

Compared with CotA laccase-single mediator system, the CotA-ABTS-AS cocktail system showed advantages including higher efficiency, lower cost and shorter reaction time.

SIGNIFICANCE AND IMPACT OF THE STUDY

This was the first report on the dyes decolorization by laccase mediator cocktail system. These results paved the curb for the application of laccase mediator system in various industrial processes.

Secretory expression of recombinant small laccase from Streptomyces coelicolor A3(2) in Pichia pastoris

This work reports for the first time the secretory expression of the small laccase (SLAC) from Streptomyces coelicolor A3(2) in Pichia pastoris. Using an AOX1 promoter and α factor as a secretion signal, the recombinant P. pastoris harbouring the laccase gene (rSLAC) produced high titres of extracellular laccase (500 ± 10 U/l), which were further increased seven fold by pre-incubation at 80 °C for 30 min. The enzyme (∼38 kDa) had an optimum activity at 80 °C, but optimum pH varied with substrate used. K_m values for ABTS, SGZ and 2,6-DMP were 142.85 μM, 10 μM and 54.55 μM and the corresponding k_cat values were 60.6 s^-1, 25.36 s^-1 and 27.84 s^-1, respectively. The t_1/2 values of the rSLAC at 60 °C, 70 °C, 80 °C were 60 h, 32 h and 10 h, respectively. The enzyme deactivation energy (E_d) was 117.275 kJ/mol while ΔG, ΔH and ΔS for thermal inactivation of the rSLAC were all positive. The rSLAC decolourised more than 90% of Brilliant Blue G and Trypan Blue dye in 6 h without the addition of a mediator. High titres of SLAC expressed in P. pastoris enhance its potential for various industrial applications.

Expression of a new laccase from Moniliophthora roreri at high levels in Pichia pastoris and its potential application in micropollutant degradation

Laccases have gained significant attention due to their emerging applications including bioremediation, biomass degradation and biofuel cells. One of the prerequisites for the industrial application of laccases is their sufficient availability. However, expression levels of recombinantly expressed laccases are often low. In this study Mrl2, a new laccase from the basidiomycete Moniliophthora roreri, was cloned in Pichia pastoris and produced in an optimized fed-batch process at an exceptionally high yield of 1.05 g l⁻¹. With a redox potential of 0.58 V, Mrl2 belongs to mid-redox potential laccases. However, Mrl2 demonstrated high k_cat values of 316, 20, 74, and 36 s⁻¹ towards 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), syringaldazine (SGZ), 2,6-dimethoxyphenol (2,6-DMP) and guaiacol, respectively. Mrl2 remained stable above pH 6 and in the presence of many metal ions, which is important for application in bioremediation. Mrl2 was investigated for the ability to degrade endocrine disrupting chemicals (EDCs) and non-steroidal anti-inflammatory drugs (NSDAIs) at neutral pH value. The enzyme accepted and converted estrone, 17β-estradiol, estriol, the synthetic contraceptive 17α-ethinyl estradiol and bisphenol A at pH 7 faster than high-potential laccases from Trametes versicolor. For example, within 30 min Mrl2 removed more than 90% bisphenol A, 17ß-estradiol, 17α-ethinyl estradiol and estriol, respectively. The concentration of the recalcitrant drug diclofenac dropped by 56% after 20 h incubation with Mrl2.

Characterization of a thermo-alkali-stable laccase from Bacillus subtilis cjp3 and its application in dyes decolorization

In this work, a novel bacterial strain exhibiting laccase activity was isolated from black liquor and identified as Bacillus subtilis cjp3. The CotA-laccase gene was cloned from strain cjp3 and expressed in Escherichia coli. The purified recombinant laccase has a maximum activity of 7320 U/L, maintaining high stabilities under a wide pH range and high temperature conditions. Nearly no loss of laccase activity was observed even at pH 9.0 after 10 h of incubation. Reactive blue 19, reactive black 5 and indigo carmine could be efficiently decolorized by the purified laccase in the presence of a mediator ABTS. More than 86% of tested dyes were removed in 4 h at pH = 9.0. The recombinant laccase can work well in a broad range of temperatures of 20-80°C(>80% relative activity). These special properties indicated the potential use of the CotA-laccase in treating wastewater containing synthetic dyes.

Lignocellulose degrading extremozymes produced by Pichia pastoris: current status and future prospects

In this review article, extremophilic lignocellulosic enzymes with special interest on xylanases, β-mannanases, laccases and finally cellulases, namely, endoglucanases, exoglucanases and β-glucosidases produced by Pichia pastoris are reviewed for the first time. Recombinant lignocellulosic extremozymes are discussed from the perspectives of their potential application areas; characteristics of recombinant and native enzymes; the effects of P. pastoris expression system on recombinant extremozymes; and their expression levels and applied strategies to increase the enzyme expression yield. Further, effects of enzyme domains on activity and stability, protein engineering via molecular dynamics simulation and computational prediction, and site-directed mutagenesis and amino acid modifications done are also focused. Superior enzyme characteristics and improved stability due to the proper post-translational modifications and better protein folding performed by P. pastoris make this host favourable for extremozyme production. Especially, glycosylation contributes to the structure, function and stability of enzymes, as generally glycosylated enzymes produced by P. pastoris exhibit better thermostability than non-glycosylated enzymes. However, there has been limited study on enzyme engineering to improve catalytic efficiency and stability of lignocellulosic enzymes. Thus, in the future, studies should focus on protein engineering to improve stability and catalytic efficiency via computational modelling, mutations, domain replacements and fusion enzyme technology. Also metagenomic data need to be used more extensively to produce novel enzymes with extreme characteristics and stability.

A strategy for soluble overexpression and biochemical characterization of halo-thermotolerant Bacillus laccase in modified E. coli

An efficient method was introduced for soluble expression of recombinant laccase (rpCotA(SL-1)) from a newly isolated halo-thermotolerant Bacillus sp. SL-1 in modified Escherichia coli, trxB2/gor2 mutant (Origami™ B (DE3)). The yield of purified soluble laccase in Origami strain under micro-aerobic condition was ∼20mg/L of bacterial culture, showing significant improvement over the laccase produced in E.coli BL21 strain under aerobic condition. The specific activity of 13U/mg for purified laccase produced in micro-aerobic condition was higher than that of 1.07U/mg observed for the purified enzyme obtained in aerobic condition in Origami. The kinetic Km and kcat parameters for laccase-induced oxidation reactions were 46μM and 23s(-1) for ABTS (2,2'-Azino-bis(3-ethylbenzthiazoline-6-sulphonic acid), and 19.6μM and 24s(-1) for SGZ (syringaldazine) substrates, respectively. The rpCotA(SL-1) displayed thermostability at 70°C and tolerance to specified concentrations of NaCl, NaN3, EDTA and SDS as inhibitors. The enzyme was relatively stable in the presence of different concentration of organic solvents, however the residual activity was adversely affected as the dipole moment of the solvents increase. Here we successfully report the production of soluble and functional laccase in Origami at the expression level suitable for industrial application.

Fungal laccases as tools for biodegradation of industrial dyes

Laccases are blue copper oxidases, found in some plants and secreted by a wide range of ligninolytic fungi. These enzymes are well known for their ability in oxidizing several organic compounds, mainly phenolics and aromatic amines, at the expenses of molecular oxygen. Therefore, they could find application in the field of enzymatic bioremediation of many industrial wastewaters, and in particular to bleach and/or detoxify dye-containing effluents. Not all industrial dyes behave as laccase substrates, but this limitation is often overcome by the judicious use of redox mediators. These could substantially widen the application range of laccases as bioremediation tools. The present study encompasses the main properties of the most used industrial dyes as related to their chemical classification, fungal laccases and their molecular and catalytic features, the use of redox mediators, limitations and perspectives of the use of fungal laccases for industrial dye bleaching.