Characterization of an alkali- and halide-resistant laccase expressed in E. coli: CotA from Bacillus clausii

Biodegradation of bisphenol A by a Pichia pastoris whole-cell biocatalyst with overexpression of laccase from Bacillus pumilus and investigation of its potential degradation pathways

Bisphenol A (BPA), an endocrine disrupter with estrogen activity, can infiltrate animal and human bodies through the food chain. Enzymatic degradation of BPA holds promise as an environmentally friendly approach while it is limited due to lower stability and recycling challenges. In this study, laccase from Bacillus pumilus TCCC 11568 was expressed in Pichia pastoris (fLAC). The optimal catalytic conditions for fLAC were at pH 6.0 and 80 °C, with a half-life T1/2 of 120 min at 70 °C. fLAC achieved a 46 % degradation rate of BPA, and possible degradation pathways were proposed based on identified products and reported intermediates of BPA degradation. To improve its stability and degradation capacity, a whole-cell biocatalyst (WCB) was developed by displaying LAC (dLAC) on the surface of P. pastoris GS115. The functionally displayed LAC demonstrated enhanced thermostability and pH stability along with an improved BPA degradation ability, achieving a 91 % degradation rate. Additionally, dLAC maintained a degradation rate of over 50 % after the fourth successive cycles. This work provides a powerful catalyst for degrading BPA, which might decontaminate endocrine disruptor-contaminated water through nine possible pathways.

Optimizing laccase selection for enhanced outcomes: a comprehensive review

Despite their widespread applications in sectors such as pulp and paper, textile, food and beverage, pharmaceuticals, and biofuel production, laccases encounter challenges related to their activity and stability under varying reaction conditions. This review accumulates data on the complex interplay between laccase characteristics and reaction conditions for maximizing their efficacy in diverse biotechnological processes. Benefits of organic media such as improved substrate selectivity and reaction control, and their risks such as enzyme denaturation and reduced activity are reported. Additionally, the effect of reaction conditions such as pH and temperature on laccase activity and stability are gathered and reported. Sources like Bacillus pumilus, Alcaligenes faecalis, Bacillus clausii, and Bacillus tequilensis SN4 are producing laccases that are both thermo-active and alkali-active. Additionally, changes induced by the presence of various substances within reaction media such as metals, inhibitors, and organic solvents are also reported. Bacillus pumilus and Bacillus licheniformis LS04 produce the most resistant laccases in this case. Finally, the remarkable laccases have been highlighted and the proper laccase source for each industrial application is suggested.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-04015-5.

Harnessing the power of bacterial laccases for xenobiotic degradation in water: A 10-year overview

Industrialization and population growth are leading to the production of significant amounts of sewage containing hazardous xenobiotic compounds. These compounds pose a threat to human and animal health, as well as the overall ecosystem. To combat this issue, chemical, physical, and biological techniques have been used to remove these contaminants from water bodies affected by human activity. Biotechnological methods have proven effective in utilizing microorganisms and enzymes, particularly laccases, to address this problem. Laccases possess versatile enzymatic characteristics and have shown promise in degrading different xenobiotic compounds found in municipal, industrial, and medical wastewater. Both free enzymes and crude enzyme extracts have demonstrated success in the biotransformation of these compounds. Despite these advancements, the widespread use of laccases for bioremediation and wastewater treatment faces challenges due to the complex composition, high salt concentration, and extreme pH often present in contaminated media. These factors negatively impact protein stability, recovery, and recycling processes, hindering their large-scale application. These issues can be addressed by focusing on large-scale production, resolving operation problems, and utilizing cutting-edge genetic and protein engineering techniques. Additionally, finding novel sources of laccases, understanding their biochemical properties, enhancing their catalytic activity and thermostability, and improving their production processes are crucial steps towards overcoming these limitations. By doing so, enzyme-based biological degradation processes can be improved, resulting in more efficient removal of xenobiotics from water systems. This review summarizes the latest research on bacterial laccases over the past decade. It covers the advancements in identifying their structures, characterizing their biochemical properties, exploring their modes of action, and discovering their potential applications in the biotransformation and bioremediation of xenobiotic pollutants commonly present in water sources.

Research of Multicopper Oxidase and Its Degradation of Histamine in Lactiplantibacillus plantarum LPZN19

In order to explore the structural changes and products of histamine degradation by multicopper oxidase (MCO) in Lactiplantibacillus plantarum LPZN19, a 1500 bp MCO gene in L. plantarum LPZN19 was cloned, and the recombinant MCO was expressed in E. coli BL21 (DE3). After purification by Ni2+-NTA affinity chromatography, the obtained MCO has a molecular weight of 58 kDa, and it also has the highest enzyme activity at 50 °C and pH 3.5, with a relative enzyme activity of 100%, and it maintains 57.71% of the relative enzyme activity at 5% salt concentration. The secondary structure of MCO was determined by circular dichroism, in which the proportions of the α-helix, β-sheet, β-turn and random coil were 2.9%, 39.7%, 21.2% and 36.1%, respectively. The 6xj0.1.A with a credibility of 68.21% was selected as the template to predict the tertiary structure of MCO in L. plantarum LPZN19, and the results indicated that the main components of the tertiary structure of MCO were formed by the further coiling and folding of a random coil and β-sheet. Histamine could change the spatial structure of MCO by increasing the content of the α-helix and β-sheet. Finally, the LC-MS/MS identification results suggest that the histamine was degraded into imidazole acetaldehyde, hydrogen peroxide and ammonia.

Heterologous expression, purification, and characterization of thermo- and alkali-tolerant laccase-like multicopper oxidase from Bacillus mojavensis TH309 and determination of its antibiotic removal potential

Laccases or laccase-like multicopper oxidases have great potential in bioremediation to oxidase phenolic or non-phenolic substrates. However, their inability to maintain stability in harsh environmental conditions and against non-substrate compounds is one of the main reasons for their limited use. The gene (mco) encoding multicopper oxidase from Bacillus mojavensis TH309 were cloned into pET14b( +), expressed in Escherichia coli, and purified as histidine tagged enzyme (BmLMCO). The molecular weight of the enzyme was about 60 kDa. The enzyme exhibited laccase-like activity toward 2,6-dimethoxyphenol (2,6-DMP), syringaldazine (SGZ), and 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS). The highest enzyme activity was recorded at 80 °C and pH 8. BmLMCO showed a half-life of ~ 305, 99, 50, 46, 36, and 20 min at 40, 50, 60, 70, 80, and 90 °C, respectively. It retained more than 60% of its activity after pre-incubation in the range of pH 5-12 for 60 min. The enzyme activity significantly increased in the presence of 1 mM of Cu2+. Moreover, BmLMCO tolerated various chemicals and showed excellent compatibility with organic solvents. The Michaelis constant (Km) and the maximum velocity (Vmax) values of BmLMCO were 0.98 mM and 93.45 µmol/min, respectively, with 2,6-DMP as the substrate. BmLMCO reduced the antibacterial activity of cefprozil, gentamycin, and erythromycin by 72.3 ± 1.5%, 79.6 ± 6.4%, and 19.7 ± 4.1%, respectively. This is the first revealing shows the recombinant production of laccase-like multicopper oxidase from any B. mojavensis strains, its biochemical properties, and potential for use in bioremediation.

Isolation and Characterization of a Novel Laccase-Producing Bacteria Bhargavaea beijingensis from Paper and Pulp Effluent-Treated Soil Using In Silico Approaches

Laccases (EC 1.10.3.2) are considered one of the most prominent multicopper enzymes that exhibit the inherent properties of oxidizing a range of phenolic substrates. Mostly, reported laccases have been isolated from the plants and fungi species, whereas bacterial laccases are yet to be explored. Bacterial laccases have numerous distinctive properties over fungal laccases, including stability at high temperatures and high pH. This study includes the isolation of bacteria through the soil sample collected from the paper and pulp industry; the highest laccase-producing bacteria was identified as Bhargavaea bejingensis, using 16S rRNA gene sequencing. The extracellular and intracellular activities after 24 h incubation were 1.41 U/mL and 4.95 U/mL, respectively. The laccase-encoding gene of the bacteria was sequenced; moreover, the in vitro translated protein was bioinformatically characterized and asserted that the laccase produced by the bacteria Bhargavaea bejingensis was structurally and sequentially homologous to the CotA protein of Bacillus subtilis. The enzyme laccase produced from B. bejingensis was classified as three-domain laccase with several copper-binding residues, where a few crucial copper-binding residues of the laccase enzyme were also predicted.

A fast, sensitive and fluorescent LPMO activity assay

Lytic polysaccharide monooxygenases (LPMOs) are industrially relevant enzymes that utilize a copper co-factor and an oxygen species to break down recalcitrant polysaccharides. These enzymes are secreted by microorganisms and are used in lignocellulosic refineries. As such, they are interesting from both the ecological/biological and industrial perspectives. Here we describe the development of a new fluorescence-based kinetic LPMO activity assay. The assay is based on the enzymatic production of fluorescein from its reduced counterpart. The assay can detect as little as 1 nM LPMO with optimized assay conditions. Furthermore, the reduced fluorescein substrate can also be used to identify peroxidase activity as seen by the formation of fluorescein by horseradish peroxidase. The assay was shown to work well at relatively low H2O2 and dehydroascorbate concentrations. The applicability of the assay was demonstrated.

Improvement of thermoalkaliphilic laccase (CtLac) by a directed evolution and application to lignin degradation

Abstract

Thermoalkaliphilic laccase (CtLac) from the Caldalkalibacillus thermarum strain TA2.A1 has advantageous properties with potential industrial applications, such as high enzyme activity and stability at 70 °C and pH 8.0. In the present study, a directed evolution approach using a combination of random and site-directed mutagenesis was adopted to enhance the laccase activity of CtLac. Spectrophotometric assay and real-time oxygen measurement techniques were employed to compare and evaluate the enzyme activity among mutants. V243 was targeted for site-directed mutagenesis based on library screening. V243D showed a 25–35% higher laccase activity than wild-type CtLac in the spectrophotometric assay and oxygen consumption measurement results. V243D also showed higher catalytic efficiency than wild-type CtLac with decreased Km and increased kcat values. In addition, V243D enhanced oxidative degradation of the lignin model compound, guaiacylglycerol-β-guaiacyl ether (GGGE), by 10% and produced a 5–30% increase in high-value aldehydes than wild-type CtLac under optimal enzymatic conditions (i.e., 70 °C and pH 8.0). Considering the lack of protein structural information, we used the directed evolution approach to predict Val at the 243 position of CtLac as one of the critical amino acids contributing to the catalytic efficiency of the enzyme. Moreover, it found that the real-time oxygen measurement technique could overcome the limitations of the spectrophotometric assay, and apply to evaluate oxidase activity in mutagenesis research.

Key points

• CtLac was engineered for enhanced laccase activity through directed evolution approach

• V243D showed higher catalytic efficiency (kcat/Km) than wild-type CtLac

• V243D produced higher amounts of high-value aldehydes from rice straw than wild-type CtLac

Isolation of Bacillus sp. with high laccase activity for green biodecolorization of synthetic textile dyes

New Bacillus sp. strains with spore-laccase activity were isolated from rotten wood and soil samples and were identified as Bacillus sp. FM-78 and Bacillus paramycoides FM-86 by 16S rDNA gene sequence analysis. Both laccases were stable at broad pH range and high temperature. The laccase of strain FM-78 showed preferable activity and stability, with no loss of activity after 7 days incubation at pH 9.0, and 20.36% of its initial activity obtained after 10 h at 80 °C. 1 mmol/L EDTA, NaN₃ and SDS resulted in about 46-59% inactivation and strongly inhibition (87.88%) was caused by 1 mmol/L L-cysteine. However, the spore laccase could tolerate towards 0.5 mol/L NaCl as well as 10% of organic solvents. Reactive black 5, reactive blue 19 and crystal violet were decolorized by the spore laccase in the absence of mediator. The decolorization process was efficiently promoted with the presence of acetosyringone, and the color removal ratio was more than 80% in 1 h with the pH values of 6.6 or 9.0. Finally, the above unusual properties of Bacillus sp. spore laccase indicated it as a potential candidate in the dye decolorization in an ecofriendly and cost-effective way.

Current Progress and Future Perspectives on the Use of Bacillus clausii

Bacillus clausii is a probiotic that benefits human health. Its key characteristics include the ability to form spores; the resulting tolerance to heat, acid, and salt ensures safe passage through the human gastrointestinal tract with no loss of cells. Although B. clausii has been widely used for many decades, the beneficial properties of other probiotics, such as Lactobacillus spp. and Bifidobacterium spp., are better disseminated in the literature. In this review, we summarize the physiological, antimicrobial, and immunomodulatory properties of probiotic B. clausii strains. We also describe findings from studies that have investigated B. clausii probiotics from the perspective of quality and safety. We highlight innovative properties based on biochemical investigations of non-probiotic strains of B. clausii, revealing that B. clausii may have further health benefits in other therapeutic areas.

Biochemical and Structural Properties of a High-Temperature-Active Laccase from Bacillus pumilus and Its Application in the Decolorization of Food Dyes

A novel laccase gene isolated from Bacillus pumilus TCCC 11568 was expressed, and the recombinant laccase (rLAC) displayed maximal activity at 80 °C and at pH 6.0 against ABTS. rLAC maintained its structural integrity at a high temperature (355 K) compared to its tertiary structure at a low temperature (325 K), except for some minor adjustments of certain loops. However, those adjustments were presumed to be responsible for the formation of a more open access aisle that facilitated the binding of ABTS in the active site, resulting in a shorter distance between the catalytic residue and the elevated binding energy. Additionally, rLAC showed good thermostability (≤70 °C) and pH stability over a wide range (3.0–10.0), and displayed high efficiency in decolorizing azo dyes that are applicable to the food industry. This work will improve our knowledge on the relationship of structure–function for thermophilic laccase, and provide a candidate for dye effluent treatment in the food industry.

Broad thermal spectrum metagenomic laccase with action for dye decolorization and fentin hydroxide treatment

Laccases are multicopper oxidases that act on various phenolic and non-phenolic compounds, enabling numerous applications including xenobiotic bioremediation, biofuel production, drug development, and cosmetic production, and they can be used as additives in the textile and food industries. This wide range of uses makes these enzymes extremely attractive for novel biotechnology applications. Here, we undertook the kinetic characterization of LacMeta, a predicted as homotrimeric (~ 107,93 kDa) small laccase, and demonstrated that this enzyme performs best at an acidic pH (pH 3–5) towards ABTS as substrate and has a broad thermal spectrum (10–60 °C), which can promote high plastic action potential through dynamic environmental temperature fluctuations. This enzyme showed following kinetic parameters: k_cat = 6.377 s⁻¹ ± 0.303, K_m = 4.219 mM, and V_max = 24.43 µM/min (against ABTS as substrate). LacMeta almost completely degraded malachite green (50 mg/mL) in only 2 h. Moreover, the enzyme was able to degrade seven dyes from four distinct classes and it respectively achieved 85% and 83% decolorization of methylene blue and trypan blue with ABTS as the mediator. In addition, LacMeta showed potential for the degradation of two thirds of an agricultural fungicide: fentin hydroxide, thus demonstrating its biotechnological aptitude for bioremediation. The results of this study suggest that LacMeta has potential in textile wastewater treatment and that it could help in the bioremediation of other human/environmental toxins such as pesticides and antibiotic compounds belonging to the same chemical classes as the degraded dyes.

LacMeta is a new two-domain laccase with activity over a wide temperature range

LacMeta maintained 50% activity after 5 months of storage at 4 °C.

Laccase was able to degrade in 2 h the Malachite Green dye, and had the potential to degrade fentin hydroxide

Cloning, expression and characterization of laccase from Bacillus licheniformis NS2324 in E. coli application in dye decolorization

Laccase gene from Bacillus licheniformis NS2324 was cloned and expressed in E. coli by using pUC 18 as cloning vector and pet 15b as expression vector. The purified recombinant laccase (rLacNS2324) showed a molecular mass of 66 KDa. The optimum pH and temperature for rLacNS2324 was found to be pH 8 and 40 °C respectively. The half life of rLacNS2324 at pH 7, 8 and 9 is 24 h. The half life of laccase at 45 °C is 8 h. Laccase activity was increased in the presence of Cu²⁺ (135.3%), Mn²⁺ (283.76%), and Co²⁺ (199.96%) at 5 mM of concentration, but inhibited to 17.01% in the presence of 5 mM Zn²⁺ ions. rLacNS2324 was found tolerant to NaCl and NaI. Among the inhibitors, it was found to be tolerant to EDTA, however, its activity was inhibited in the presence of sodium azide, dithiothreitol and β-mercapethanol. rLacNS2324 was able to decolorize a bromophenol blue by 85% and phenol red by 75% in 1 h without any mediator. Methylene blue was almost completely degraded (99.28% decolorization) by 10 IUml^-1 of laccase at 40 °C, pH 8.0 and in time 4 h. Overall rLacNS2324 showed ability to be used industrially to decolorize dyes in an eco-friendly and cost effective way.

Molecular Cloning, Functional and Biophysical Characterization of an Antimicrobial Peptide from Rhizosphere Soil

AIM

This study was designed to screen and identify an antimicrobial peptide from rhizosphere soil. The study was further focused towards overexpression, purification and characterization of this antimicrobial peptide, and to functionally validate its efficiency and efficacy as an antimicrobial agent. Yet the study was further aimed at corroborating structural and functional studies using biophysical tools.

BACKGROUND

Antimicrobial resistance is emerging as one of the top 10 global health crisis, it is multifaceted and is the second largest cause of mortality. According to the World Health Organization (WHO), around the world, an estimated 700,000 people die each year from infection caused by antibiotic-resistant microbes. Antimicrobial peptides offers best alternative to combat and overcome this crisis. In this manuscript, we report cloning, expression, purification and characterization of an antimicrobial peptide discovered from rhizosphere soil.

OBJECTIVE

Objectives of this study includes construction, screening and identification of antimicrobial peptide from metagenome followed by its expression, purification and functional and biophysical investigation. Yet another objective of the study was to determine antimicrobial efficacy and efficiency as an antimicrobial peptide towards MRSA strains.

METHODS

In this study, we used array of molecular biology tools that include genetic engineering, PCR amplification, construction of an expression construct and NI-NTA based purification of the recombinant peptide. We have also carried out antimicrobial activity assay to determine MIC and IC50 values of antimicrobial peptide. To establish structural and functional relationship, circular dichroism, and both extrinsic and intrinsic fluorescence spectroscopy studies were carried out.

RESULTS

Screening of metagenomic library resulted in identification of gene (~500bp) harbouring an open reading frame (ORF) consisting of 282 bp. Open reading frame identified in gene encodes an antimicrobial peptide which had shared ~95% sequence similarity with the antimicrobial peptide of Bacillus origin. Purification of recombinant protein using Ni-NTA column chromatography demonstrated a purified protein band of ~11 kDa on 14% SDS-PAGE which is well corroborated to theoretical deduced molecular weight of peptide from its amino acids sequence. Interestingly, the peptide exhibited antimicrobial activity in broad range of pH and temperature. MIC (minimum inhibitory concentration) determined against gram positive Bacillus sp. was found to be 0.015mg/ml, whereas in case of gram negative E. coli, it was calculated to be 0.062mg/ml. The peptide exhibited IC50 values corresponding to ~0.25mg/ml against Bacillus and ~0.5 mg/ml against E. coli. Antimicrobial susceptibility assay performed against methicillin resistant Staphylococcus aureus strain ATCC 3412 and standard strain of Staphylococcus aureus ATCC 9144 revealed its strong inhibitory activity against MRSA, whereby we observed a ~16mm clearance zone at higher peptide concentrations ~2mg/ml (~181.8µM). Biophysical investigation carried out using Trp fluorescence, ANS fluorescence and circular dichroism spectroscopy further revealed conformational stability in its secondary and tertiary structure at wide range of temperature and pH.

CONCLUSION

Altogether, the peptide discovered from rhizosphere metagenome hold potential in inhibiting the growth of both the gram positive and gram negative bacteria, and was equally effective in inhibiting the multidrug resistant pathogenic strains (MRSA).

Biochemical properties of CumA multicopper oxidase from plant pathogen, Pseudomonas syringae

Multicopper oxidases have a wide range of substrate specificity to be involved in various physiological reactions. Pseudomonas syringae, a plant pathogenic bacterium, has a multicopper oxidase, CumA. Multicopper oxidases have ability to degrade plant cell wall component, lignin. Once P. syringae enter apoplast and colonize, they start to disrupt plant immunity. Therefore, deeper understanding of multicopper oxidases from plant pathogens, help to invent measures to prevent invasion into plant cell, which bring agricultural benefits. Several biochemical studies have reported lower activity of CumA compared with other multicopper oxidase called CotA. However, the mechanisms underlying the difference in activity have not yet been revealed. In order to acquire insight into them, we conducted a biophysical characterization of PsCumA. Our results show that PsCumA has weak type I copper EPR signal, which is essential for oxidation activity. We propose that difference in the coordination of copper ions may decrease reaction frequency.

Functional expression and characterization of two laccases from the brown rot Fomitopsis pinicola

Laccase is predominantly found in lignin degrading filamentous white rot fungi, where it is involved in the oxidative degradation of this recalcitrant heteropolymer. In brown rot fungi it is much less prevalent: laccases from only a few brown rots have been detected and only two have been characterized. This study tries to understand the role of this ligninolytic enzyme in brown rots by investigating the catalytic properties of laccases secreted by Fomitopsis pinicola FP58527 SS1. When grown on either poplar or spruce wood blocks, several laccases were detected in the secretome. Two of them (FpLcc1 and FpLcc2) were heterologously produced using Trichoderma reesei QM9414 Δxyr1 as expression host and purified to homogeneity by consecutive steps of hydrophobic interaction, anion exchange and size exclusion chromatography. With the substrates 2,2-azino-bis(3-ethylthiazoline-6-sulfonate) (ABTS), 2,6-dimethoxyphenol (2,6-DMP) and guaiacol both laccases showed similar, low pH-optima below 3 for ABTS and 2,6-DMP and at pH 3.5 for guaiacol which is at the acidic end of laccases isolated from white rot fungi. The determined K_M values were low while k_cat values measured at acidic conditions were comparable to those reported for other laccases from white rot fungi. While both enzymes showed a moderate decrease in activity in the presence of oxalic and citric acid FpLcc2 was activated by acetic acid up to 3.7 times. This activation effect is much more pronounced at pH 5.0 compared to pH 3.0 and could already be observed at a concentration of 1 mM acetic acid.

Use of Copper as a Trigger for the in Vivo Activity of E. coli Laccase CueO: A Simple Tool for Biosynthetic Purposes

Laccases are multi-copper oxidases that catalyze the oxidation of various electron-rich substrates with concomitant reduction of molecular oxygen to water. The multi-copper oxidase/laccase CueO of Escherichia coli is responsible for the oxidation of Cu+ to the less harmful Cu2+ in the periplasm. CueO has a relatively broad substrate spectrum as laccase, and its activity is enhanced by copper excess. The aim of this study was to trigger CueO activity in vivo for the use in biocatalysis. The addition of 5 mM CuSO4 was proven effective in triggering CueO activity at need with minor toxic effects on E. coli cells. Cu-treated E. coli cells were able to convert several phenolic compounds to the corresponding dimers. Finally, the endogenous CueO activity was applied to a four-step cascade, in which coniferyl alcohol was converted to the valuable plant lignan (-)-matairesinol.

An alkaline thermostable laccase from termite gut associated strain of Bacillus stratosphericus

Laccase, an important oxidoreductase, is widely distributed in various organisms. Termites are known to decompose lignocellulose efficiently with the aid of gut microorganisms. However, few laccases have been characterized from termite or its gut microbes. We aimed to screen the strain capable of degrading lignocellulose from fungus-growing termites. In this study, Bacillus stratosphericus BCMC2 with lignocellulolytic activity was firstly isolated from the hindgut of fungus-growing termite Macrotermes barneyi. The laccase gene (BaCotA) was cloned both from the BCMC2 strain and termite intestinal metagenomic DNA. BaCotA was overexpressed in E. coli, and the recombinant BaCotA showed high specific activity (554.1 U/mg). BaCotA was thermostable with an optimum temperature of 70 °C, pH 5.0. Furthermore, BaCotA was resistant to alkali and organic solvents. The enzyme remained more than 70% residual activity at pH 8.0 for 120 min; and the organic solvents such as methanol, ethanol and acetone (10%) had no inhibitory effect on laccase activity. Additionally, BaCotA exhibited efficient decolorization ability towards indigo and crystal violet. The multiple enzymatic properties suggested the presented laccase as a potential candidate for industrial applications. Moreover, this study highlighted that termite intestine is a good resource for either new strains or enzymes.

Colorimetric LPMO assay with direct implication for cellulolytic activity

BACKGROUND

Lytic polysaccharide monooxygenases (LPMOs) are important industrial enzymes known for their catalytic degradation of recalcitrant polymers such as cellulose or chitin. Their activity can be measured by lengthy HPLC methods, while high-throughput methods are less specific. A fast and specific LPMO assay would simplify screening for new or engineered LPMOs and accelerate biochemical characterization.

RESULTS

A novel LPMO activity assay was developed based on the production of the dye phenolphthalein (PHP) from its reduced counterpart (rPHP). The colour response of rPHP oxidisation catalysed by the cellulose-specific LPMO from Thermoascus aurantiacus (TaAA9A), was found to increase tenfold by adding dehydroascorbate (DHA) as a co-substrate. The assay using a combination of rPHP and DHA was tested on 12 different metallo-enzymes, but only the LPMOs catalysed this reaction. The assay was optimized for characterization of TaAA9A and showed a sensitivity of 15 nM after 30 min incubation. It followed apparent Michaelis-Menten kinetics with kcat = 0.09 s-1 and KM = 244 µM, and the assay was used to confirm stoichiometric copper-enzyme binding and enzyme unfolding at a temperature of approximately 60 °C. DHA, glutathione and fructose were found to enhance LPMO oxidation of rPHP and in the optimized assay conditions these co-substrates also enabled cellulose degradation.

CONCLUSIONS

This novel and specific LPMO assay can be carried out in a convenient microtiter plate format ready for high-throughput screening and enzyme characterization. DHA was the best co-substrate tested for oxidation of rPHP and this preference appears to be LPMO-specific. The identified co-substrates DHA and fructose are not normally considered as LPMO co-substrates but here they are shown to facilitate both oxidation of rPHP and degradation of cellulose. This is a rare example of a finding from a high-throughput assay that directly translate into enzyme activity on an insoluble substrate. The rPHP-based assay thus expands our understanding of LPMO catalysed reactions and has the potential to characterize LPMO activity in industrial settings, where usual co-substrates such as ascorbate and oxygen are depleted.

A novel and highly active recombinant spore-coat bacterial laccase, able to rapidly biodecolorize azo, triarylmethane and anthraquinonic dyestuffs

Laccases are enzymes able to catalyze the oxidation of a wide array of phenolic and non-phenolic compounds using oxygen as co-substrate and releasing water as by-product. They are well known to have wide substrate specificity and in recent years, have gained great biotechnological importance. To date, most well studied laccases are from fungal and mesophilic origin, however, enzymes from extremophiles possess an even greater potential to withstand the extreme conditions present in many industrial processes. This research work presents the heterologous production and characterization of a novel laccase from a thermoalkaliphilic bacterium isolated from a hot spring in a geothermal site. This recombinant enzyme exhibits remarkably high specific activity (>450,000 U/mg) at 70 °C, pH 6.0, using syringaldazine substrate, it is active in a wide range of temperature (20-90 °C) and maintains over 60% of its activity after 2 h at 60 °C. Furthermore, this novel spore-coat laccase is able to biodecolorize eight structurally different recalcitrant synthetic dyes (Congo red, methyl orange, methyl red, Coomassie brilliant blue R250, bromophenol blue, malachite green, crystal violet and Remazol brilliant blue R), in just 30 min at 40 °C in the presence of the natural redox mediator acetosyringone.

Complete genome reveals genetic repertoire and potential metabolic strategies involved in lignin degradation by environmental ligninolytic Klebsiella variicola P1CD1

Lignin is a recalcitrant macromolecule formed by three alcohols (monolignols) predominantly connected by β-aryl ether linkages and is one of the most abundant organic macromolecules in the biosphere. However, the role played by environmental bacteria in lignin degradation is still not entirely understood. In this study, we identified an environmental Klebsiella strain isolated from sediment collected from an altitudinal region in a unique Brazilian biome called Caatinga. This organism can also grow in the presence of kraft lignin as a sole source of carbon and aromatic compounds. We performed whole-genome sequencing and conducted an extensive genome-based metabolic reconstruction to reveal the potential mechanisms used by the bacterium Klebsiella variicola P1CD1 for lignin utilization as a carbon source. We identified 262 genes associated with lignin-modifying enzymes (LMEs) and lignin-degrading auxiliary enzymes (LDAs) required for lignin and aromatic compound degradation. The presence of one DyP (Dye-decolorizing Peroxidase) gene suggests the ability of P1CD1 strain to access phenolic and nonphenolic structures of lignin molecules, resulting in the production of catechol and protocatechuate (via vanillin or syringate) along the peripheral pathways of lignin degradation. K. variicola P1CD1 uses aldehyde-alcohol dehydrogenase to perform direct conversion of vanillin to protocatechol. The upper funneling pathways are linked to the central pathways of the protocatechuate/catechol catabolic branches via β-ketoadipate pathways, connecting the more abundant catabolized aromatic compounds with essential cellular functions, such as energy cellular and biomass production (i.e., via acetyl-CoA formation). The combination of phenotypic and genomic approaches revealed the potential dissimilatory and assimilatory ability of K. variicola P1CD1 to perform base-catalyzed lignin degradation, acting on high- and low-molecular-weight lignin fragments. These findings will be relevant for developing metabolic models to predict the ligninolytic mechanism used by environmental bacteria and shedding light on the flux of carbon in the soil.

Preparation and synthetic dye decolorization ability of magnetic cross-linked enzyme aggregates of laccase from Bacillus amyloliquefaciens

Laccases are versatile oxidases that are capable of decolorizing various synthetic dyes. Recombinant Bacillus amyloliquefaciens laccase was immobilized as magnetic cross-linked enzyme aggregates (M-CLEAs) for application in dye decolorization. Several parameters influencing the activity recovery were evaluated during the synthesis of M-CLEAs. With ammonium sulfate as precipitant, maximum activity was recovered by cross-linking with 0.16% glutaraldehyde for 1 h. The prepared M-CLEAs exhibited improved activity under alkaline conditions. It remained 74% activity after incubation at 60 °C for 5 h. Enhanced tolerance towards NaCl was also observed for the M-CLEAs, with 68% activity remaining in the presence of 1 M NaCl. The immobilized laccase could rapidly decolorize more than 93% of reactive black 5 and indigo carmine in 1 h, while its catalytic efficiency towards reactive blue 19 was relatively low. After four cycles of consecutive reuse, the M-CLEAs could decolorize 92% of indigo carmine. The easy recovery and reusability of M-CLEAs facilitate the potential application of bacterial laccase in dye decolorization.

Enzymatic characterization, molecular dynamics simulation, and application of a novel Bacillus licheniformis laccase

A new laccase gene from newly isolated Bacillus licheniformis TCCC 111219 was actively expressed in Escherichia coli. This recombinant laccase (rLAC) exhibited a high stability towards a wide pH range and high temperatures. 170% of the initial activity was detected at pH 10.0 after 10-d incubation, and 60% of the initial activity was even kept after 2-h incubation at 70 °C. It indicated that only single type of extreme environment, such as strong alkaline environment (300 K, pH 12) or high temperature (370 K, pH 7), did not show obvious impact on the structural stability of rLAC during molecular dynamics simulation process. But the four loop regions of rLAC where the active site is situated were seriously destroyed when strong alkaline and high temperature environment existed simultaneously (370 K, pH 12) because of the damage of hydrogen bonds and salt bridges. Moreover, this thermo- and alkaline-stable enzyme could efficiently decolorize the structurally differing azo, triphenylmethane, and anthraquinone dyes with appropriate mediator at pH 3.0, 7.0, and 9.0 at 60 °C. These rare characteristics suggested its high potential in industrial applications to decolorize textile dyeing effluent.

Laccase isoform diversity on basal medium in Cyathus bulleri and role in decolorization/detoxification of textile dyes and effluent

Laccases (EC 1.10.3.2) are multi-copper oxidases that can degrade several xenobiotics, including textile dyes. Present study investigated the nature of laccase isoforms induced by 2,6-dimethylaniline in Cyathus bulleri cultivated on basal salt medium. Two isoforms, LacI and LacII were identified and purified by a combination of ultrafiltration and ion-exchange chromatography. The MS spectrum of the two proteins displayed a number of non-identical and identical molecular peaks (m/z), and, the latter were mapped to protein originating from the previously reported Laccase (Lcc) 1 gene. The LacI isoform exhibited higher catalytic efficiency (K_cat/K_m) towards 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid), 2,6-dimethoxyphenol, guaiacol and pyrogallol and was tolerant to high levels of chloride ions and resistant to EDTA. Higher decolorization of several dyes such as Direct Scarlet B (67%), Reactive Brilliant blue-R (96%), Direct Orange 34 (50%) and Reactive Red198 (95%) by the LacI isoform makes it a good candidate for degradation of synthetic dyes. The decolorization of Direct Orange 34 by laccases is being reported for the first time. Many of the properties exhibited by this isoform make it a good candidate for large scale production and applications for use in the dyeing industry.

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.

A non-blue laccase of Bacillus sp. GZB displays manganese-oxidase activity: A study of laccase characterization, Mn(II) oxidation and prediction of Mn(II) oxidation mechanism

Laccase, a unique class of multicopper oxidase, presents promising potential as a biocatalyst in many industrial and biotechnological applications. Recently, it has been significantly applied in many metal-polluted sites due to its Manganese (Mn)-oxidation ability. Here, we demonstrate the Mn(II)-oxidase activity of laccase obtained from Bacillus sp. GZB. The CotA gene of GZB was transformed in E. coli BL21 and overexpressed. The purified laccase (LACREC3-laccase) displayed the absence of a peak at 610 nm that is usually found in blue-laccase. Further, the LACREC3-laccase exhibited high activity and stability at different pH and temperatures with substrates 2, 2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonate) and syringaldazine, respectively. It also functioned in the presence of various metals and enzyme inhibitors. Most notably, LACREC3-laccase formed insoluble brown Mn(III)/Mn(IV)-oxide particles from Mn(II) mineral, exhibiting its Mn(II)-oxidase activity. In addition to native polyacrylamide gel electrophoresis and buffer test, we developed an 'agarose gel plate' assay to evaluate Mn(II) oxidation activity of laccase. Furthermore, using the leucoberbelin blue assay, a total of 44.45 ± 0.45% Mn(IV)-oxides were quantified, in which 5.87 ± 0.61% autoxidized after 24 h. The Mn(II) oxidation mechanisms were further predicted by trapping Mn(III) using pyrophosphate during Mn(II) to Mn(IV) conversion by LACREC3-laccase. Overall, the laccase of GZB has excellent activity and stability plus an ability to oxidize Mn(II). This study is the first report on a non-blue laccase, exhibiting Mn(II)-oxidase activity. Thus, it offers a novel finding of the Mn(II) oxidation processes that can be a valuable way of Mn(II)-mineralization in various metal-polluted environments.

Purification, Biochemical Characterization, and Facile Immobilization of Laccase from Sphingobacterium ksn-11 and its Application in Transformation of Diclofenac

An extracellular laccase enzyme secreted from Sphingobacterium ksn-11 was purified to electrophoretic homogeneity, showing a molecular weight of 90 kDa. The purified enzyme was monomeric in nature confirmed by sodium dodecyl gel electrophoresis. The optimum temperature and pH were found to be 40 °C and 4.5 respectively. The enzyme showed highest substrate specificity for 2,2 azino-bis (ethylthiozoline-6-sulfonate) (ABTS), followed by syringaldazine. The K_m value for ABTS was 2.12 mM with a V_max value of 33.33 U/mg which was higher when compared with syringaldazine and guaiacol substrates. Sodium azide and EDTA inhibited the activity by 30%, whereas presence of Ca²⁺ and iron increased activity by 50%. The purified enzyme was immobilized in sodium alginate-silicon dioxide-polyvinyl alcohol beads and evaluated for diclofenac transformation studies. LC-MS analysis confirmed that immobilized laccase transformed diclofenac to 4-OH diclofenac after 4 h of incubation. 45 % of diclofenac was able to transform even at 3rd cycle of immobilized laccase use. Therefore, immobilized laccase can be used to transform or degrade several recalcitrant compounds from industrial effluents.

The heterologous expression, characterization, and application of a novel laccase from Bacillus velezensis

Laccases have a huge potential in numerous environmental and industrial applications due to the ability to oxidized a wide range of substrates. Here, a novel laccase gene from the identified Bacillus velezensis TCCC 111904 was heterologously expressed in Escherichia coli. The optimal temperature and pH for oxidation by recombinant laccase (rLac) were 80 °C and 5.5, respectively, in the case of the substrate 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and 80 °C and 7.0, respectively, in the case of 2,6-dimethoxyphenol (2,6-DMP). rLac exhibited high thermostability and pH stability over a wide range (pH 3.0, 7.0, and 9.0). Additionally, most of the metal ions did not inhibit the activity of rLac significantly. rLac showed great tolerance against high concentration of NaCl, and 50.8% of its initial activity remained in the reaction system containing 500 mM NaCl compared to the control. Moreover, rLac showed a high efficiency in decolorizing different types of dyes including azo, anthraquinonic, and triphenylmethane dyes at a high temperature (60 °C) and over an extensive pH range (pH 5.5, 7.0, and 9.0). These unique characteristics of rLac indicated that it could be a potential candidate for applications in treatment of dye effluents and other industrial processes.

Inhibition in multicopper oxidases: a critical review

This review critiques the literature on inhibition of O₂-reduction catalysis in multicopper oxidases like laccase and bilirubin oxidase and provide recommendations for best practice when carrying out experiments and interpreting published data.

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.

Laccases: structure, function, and potential application in water bioremediation

The global rise in urbanization and industrial activity has led to the production and incorporation of foreign contaminant molecules into ecosystems, distorting them and impacting human and animal health. Physical, chemical, and biological strategies have been adopted to eliminate these contaminants from water bodies under anthropogenic stress. Biotechnological processes involving microorganisms and enzymes have been used for this purpose; specifically, laccases, which are broad spectrum biocatalysts, have been used to degrade several compounds, such as those that can be found in the effluents from industries and hospitals. Laccases have shown high potential in the biotransformation of diverse pollutants using crude enzyme extracts or free enzymes. However, their application in bioremediation and water treatment at a large scale is limited by the complex composition and high salt concentration and pH values of contaminated media that affect protein stability, recovery and recycling. These issues are also associated with operational problems and the necessity of large-scale production of laccase. Hence, more knowledge on the molecular characteristics of water bodies is required to identify and develop new laccases that can be used under complex conditions and to develop novel strategies and processes to achieve their efficient application in treating contaminated water. Recently, stability, efficiency, separation and reuse issues have been overcome by the immobilization of enzymes and development of novel biocatalytic materials. This review provides recent information on laccases from different sources, their structures and biochemical properties, mechanisms of action, and application in the bioremediation and biotransformation of contaminant molecules in water. Moreover, we discuss a series of improvements that have been attempted for better organic solvent tolerance, thermo-tolerance, and operational stability of laccases, as per process requirements.

Characterization and Dye Decolorization Potential of Two Laccases from the Marine-Derived Fungus Pestalotiopsis sp

Two laccase-encoding genes from the marine-derived fungus Pestalotiopsis sp. have been cloned in Aspergillus niger for heterologous production, and the recombinant enzymes have been characterized to study their physicochemical properties, their ability to decolorize textile dyes for potential biotechnological applications, and their activity in the presence of sea salt. The optimal pH and temperature of PsLac1 and PsLac2 differed in relation to the substrates tested, and both enzymes were shown to be extremely stable at temperatures up to 50 °C, retaining 100% activity after 3 h at 50 °C. Both enzymes were stable between pH 4–6. Different substrate specificities were exhibited, and the lowest K_m and highest catalytic efficiency values were obtained against syringaldazine and 2,6-dimethoxyphenol (DMP) for PsLac1 and PsLac2, respectively. The industrially important dyes—Acid Yellow, Bromo Cresol Purple, Nitrosulfonazo III, and Reactive Black 5—were more efficiently decolorized by PsLac1 in the presence of the redox mediator 1-hydroxybenzotriazole (HBT). Activities were compared in saline conditions, and PsLac2 seemed more adapted to the presence of sea salt than PsLac1. The overall surface charges of the predicted PsLac three-dimensional models showed large negatively charged surfaces for PsLac2, as found in proteins for marine organisms, and more balanced solvent exposed charges for PsLac1, as seen in proteins from terrestrial organisms.

Production of polyextremotolerant laccase by Achromobacter xylosoxidans HWN16 and Citrobacter freundii LLJ16

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The biochemical properties of two proteobacteria laccases were assessed.

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Polyextremotolerant qualities of the laccases were identified.

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Multiple laccase-encoding genes were observed in laccase-producing strains.

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Their implication in biotechnological applications was deliberated.

Given the upwelling of a variety of potential applications laccases could participate in, it would be fitting to equally make available laccases that are well suited for the aforementioned. Therefore historian understanding of the catalytic and physicochemical properties is desirable. Owing to this, the biochemical properties of the crude laccases from Achromobacter xylosoxidans HWN16 (Hb9c) and Citrobacter freundii LLJ 16 (Ie1c) were assessed. Furthermore, a hint of the molecular basis for their production from respective organisms was presented. Results showed that both laccases were tolerant, and sometimes had their activities improved by the set of parameters tested. They were active at broad range of temperature (0–90 °C), pH (3–11), and were equally thermo- and pH-stable. Their activities were either improved, or left unabated by cations, detergents, and chloride (5–40%), however, the highlight of the study was their augmented activity, when they were incubated with certain concentrations of fluoride (2–20%), a potent inhibitor. They were depicted to have multiple homologous laccase encoding genes, on molecular evaluation, which may be responsible the conferral of these remarkable qualities they possess. Therefore, the laccases might be beneficial, if employed in formulations for a wide range of environmental and biotechnological applications. Moreover, the molecular machinery of their production be exploited for economical benefits in the immediate future.

Spore cells from BPA degrading bacteria Bacillus sp. GZB displaying high laccase activity and stability for BPA degradation

Laccase has been applied extensively as a biocatalyst to remove different organic pollutants. This study characterized a spore-laccase from the bisphenol A (BPA)-degrading strain Bacillus sp. GZB. The spore-laccase was encoded with 513 amino acids, containing spore coat protein A (CotA). It showed optimal activity at 70 °C and pH = 7.2 in presence of 2, 6-dimethoxyphenol. At 60 °C, optimal activity was also seen at pH = 3.0 and pH = 6.8 with 2, 2'-azino-bis (3-ethylbenzothiazoline-6-sulfonate) and syringaldazine, respectively. The spore-laccase was stable at high temperature, at acidic to alkaline pH values, and in the presence of different organic solvents. Spore-laccase activity was increased by introducing Cu²⁺, Mg²⁺, and Na⁺, but was strongly inhibited by Fe²⁺, Ag⁺, l-cysteine, dithiothreitol, and NaN₃. The cotA gene was cloned and expressed in E. coli BL21 (DE3); the purified protein was estimated as having a molecular weight of ~63 kDa. Different synthetic dyes and BPA were effectively decolorized or degraded both by the spore laccase and recombinant laccase. When BPA oxidation was catalyzed using laccase, there was an initial formation of phenoxy radicals and further oxidation or CC bond cleavage of the radicals produced different organic acids. Detailed reaction pathways were developed based on nine identified intermediates. The acute toxicity decreased gradually during BPA degradation by laccase. This study is the first report about a genus of Bacillus that can produce a highly active and stable laccase to degrade BPA.

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.

Structural and functional characterisation of multi-copper oxidase CueO from lignin-degrading bacterium Ochrobactrum sp. reveal its activity towards lignin model compounds and lignosulfonate

The identification of enzymes responsible for oxidation of lignin in lignin-degrading bacteria is of interest for biotechnological valorization of lignin to renewable chemical products. The genome sequences of two lignin-degrading bacteria, Ochrobactrum sp., and Paenibacillus sp., contain no B-type DyP peroxidases implicated in lignin degradation in other bacteria, but contain putative multicopper oxidase genes. Multi-copper oxidase CueO from Ochrobactrum sp. was expressed and reconstituted as a recombinant laccase-like enzyme, and kinetically characterized. Ochrobactrum CueO shows activity for oxidation of β-aryl ether and biphenyl lignin dimer model compounds, generating oxidized dimeric products, and shows activity for oxidation of Ca-lignosulfonate, generating vanillic acid as a low molecular weight product. The crystal structure of Ochrobactrum CueO (OcCueO) has been determined at 1.1 Å resolution (PDB: 6EVG), showing a four-coordinate mononuclear type I copper center with ligands His495, His434 and Cys490 with Met500 as an axial ligand, similar to that of Escherichia coli CueO and bacterial azurin proteins, whereas fungal laccase enzymes contain a three-coordinate type I copper metal center. A trinuclear type 2/3 copper cluster was modeled into the active site, showing similar structure to E. coli CueO and fungal laccases, and three solvent channels leading to the active site. Site-directed mutagenesis was carried out on amino acid residues found in the solvent channels, indicating the importance for residues Asp102, Gly103, Arg221, Arg223, and Asp462 for catalytic activity. The work identifies a new bacterial multicopper enzyme with activity for lignin oxidation, and implicates a role for bacterial laccase-like multicopper oxidases in some lignin-degrading bacteria.

DATABASE

Structural data are available in the PDB under the accession number 6EVG.

Heterologous expression of a Streptomyces cyaneus laccase for biomass modification applications

Laccases are used for the conversion of biomass into fermentable sugars but it is difficult to produce high yields of active laccases in heterologous expression systems. We overcame this challenge by expressing Streptomyces cyaneus CECT 3335 laccase in Escherichia coli (ScLac) and we achieved a yield of up to 104 mg L-1 following purification by one-step affinity chromatography. Stability and activity assays using simple lignin model substrates showed that the purified enzyme preparation was active over a broad pH range and at high temperatures, suggesting it would be suitable for biomass degradation. The reusability of ScLac was also demonstrated by immobilizing the enzyme on agarose beads with a binding yield of 33%, and by the synthesis of cross-linked enzyme aggregates with an initial activity recovery of 72%.

Polyphenol oxidase from Pectobacterium atrosepticum: identification and cloning of gene and characteristics of the enzyme

In the present study, we attempted to elucidate if the harmful phytopathogenic bacteria of Pectobacterium genus (P. atrosepticum) possess the enzymes for oxidation of phenolic compounds. Polyphenol oxidase (laccase) activity was revealed in P. atrosepticum cell lysates. Using bioinformatic analysis, an ORF encoding a putative copper-containing polyphenol oxidase of 241 amino acids with a predicted molecular mass of 25.9 kDa was found. This protein (named Pal1) shares significant level of identity with laccases of a new type described for several bacterial species. Cloning and expression of the pal1 gene and the analysis of corresponding recombinant protein confirmed that Pal1 possessed laccase activity. The recombinant Pal1 protein was characterized in terms of substrate specificity, kinetic parameters, pH and temperature optimum, sensitivity to inhibitors and metal content. Pal1 demonstrated alkali- and thermo-tolerance. The kinetic parameters K_m and kcat for 2,6-dimethoxyphenol were 0.353 ± 0.062 mM and 98.79 ± 4.9 s^-1 , respectively. The protein displayed high tolerance to sodium azide, sodium fluoride, NaCl, SDS and cinnamic acid. The transcript level of the pal1 gene in P. atrosepticum was shown to be induced by plant-derived phenolic compound (ferulic acid) and copper sulfate.