Structural and functional characterization of two-domain laccase from Streptomyces viridochromogenes

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.

A Novel Two-Domain Laccase with Middle Redox Potential: Physicochemical and Structural Properties

The gene for a previously unexplored two-domain laccase was identified in the genome of actinobacterium Streptomyces carpinensis VKM Ac-1300. The two-domain laccase, named ScaSL, was produced in a heterologous expression system (Escherichia coli strain M15 [pREP4]). The enzyme was purified to homogeneity using affinity chromatography. ScaSL laccase, like most two-domain laccases, exhibited activity in the homotrimer form. However, unlike the most two-domain laccases, it was also active in multimeric forms. The enzyme exhibited maximum activity at 80°C and was thermally stable. Half-inactivation time of ScaSL at 80°C was 40 min. The laccase was able to oxidize a non-phenolic organic compound ABTS at a maximum rate at pH 4.7, and to oxidized a phenolic compound 2,6-dimethoxyphenol at a maximum rate at pH 7.5. The laccase stability was observed in the pH range 9-11. At pH 7.5, laccase was slightly inhibited by sodium azide, sodium fluoride, and sodium chloride; at pH 4.5, the laccase was completely inhibited by 100 mM sodium azide. The determined Km and kcat of the enzyme for ABTS were 0.1 mM and 20 s-1, respectively. The Km and kcat for 2,6-dimethoxyphenol were 0.84 mM and 0.36 s-1, respectively. ScaSL catalyzed polymerization of humic acids and lignin. Redox potential of the laccase was 0.472 ± 0.007 V. Thus, the ScaSL laccase is the first characterized two-domain laccase with a middle redox potential. Crystal structure of ScaSL was determined with 2.35 Å resolution. Comparative analysis of the structures of ScaSL and other two-domain laccases suggested that the middle potential of ScaSL may be associated with conformational differences in the position of the side groups of amino acids at position 230 (in ScaSL numbering), which belong to the second coordination sphere of the copper atom of the T1 center.

Structural Insight into the Amino Acid Environment of the Two-Domain Laccase's Trinuclear Copper Cluster

Laccases are industrially relevant enzymes. However, their range of applications is limited by their functioning and stability. Most of the currently known laccases function in acidic conditions at temperatures below 60 °C, but two-domain laccases (2D) oxidize some substrates in alkaline conditions and above 70 °C. In this study, we aim to establish the structural factors affecting the alkaline activity of the 2D laccase from Streptomyces griseoflavus (SgfSL). The range of methods used allowed us to show that the alkaline activity of SgfSL is influenced by the polar residues located close to the trinuclear center (TNC). Structural and functional studies of the SgfSL mutants Met199Ala/Asp268Asn and Met199Gly/Asp268Asn revealed that the substitution Asp268Asn (11 Å from the TNC) affects the orientation of the Asn261 (the second coordination sphere of the TNC), resulting in hydrogen-bond-network reorganization, which leads to a change in the SgfSL-activity pH profile. The combination of the Met199Gly/Arg240His and Asp268Asn substitutions increased the efficiency (kcat/KM) of the 2,6-DMP oxidation by 34-fold compared with the SgfSL. Our results extend the knowledge about the structure and functioning of 2D laccases' TNC active sites and open up new possibilities for the directed engineering of laccases.

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.

Recombinant expression and characterization of a new laccase, bioinformatically identified, from the Antarctic thermophilic bacterium Geobacillus sp. ID17

Geobacillus sp. ID17 is a gram-positive thermophilic bacterium isolated from Deception Island, Antarctica, which has shown to exhibit remarkable laccase activity in crude extract at high temperatures. A bioinformatic search using local databases led to the identification of three putative multicopper oxidase sequences in the genome of this microorganism. Sequence analysis revealed that one of those sequences contains the four-essential copper-binding sites present in other well characterized laccases. The gene encoding this sequence was cloned and overexpressed in Escherichia coli, partially purified and preliminary biochemically characterized. The resulting recombinant enzyme was recovered in active and soluble form, exhibiting optimum copper-dependent laccase activity at 55 °C, pH 6.5 with syringaldazine substrate, retaining over 60% of its activity after 1 h at 55 and 60 °C. In addition, this thermophilic enzyme is not affected by common inhibitors SDS, NaCl and L-cysteine. Furthermore, biodecolorization assays revealed that this laccase is capable of degrading 60% of malachite green, 54% of Congo red, and 52% of Remazol Brilliant Blue R, after 6 h at 55 °C with aid of ABTS as redox mediator. The observed properties of this enzyme and the relatively straightforward overexpression and partial purification of it could be of great interest for future biotechnology applications.

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.

Iodide uptake by forest soils is principally related to the activity of extracellular oxidases

¹²⁹I is a nuclear fission decay product of concern because of its long half-life (16 Ma) and propensity to bioaccumulate. Microorganisms impact iodine mobility in soil systems by promoting iodination (covalent binding) of soil organic matter through processes that are not fully understood. Here, we examined iodide uptake by soils collected at two depths (0-10 and 10-20 cm) from 5 deciduous and coniferous forests in Japan and the United States. Autoclaved soils, and soils amended with an enzyme inhibitor (sodium azide) or an antibacterial agent (bronopol), bound significantly less ¹²⁵I tracer (93%, 81%, 61% decrease, respectively) than the untreated control soils, confirming a microbial role in soil iodide uptake. Correlation analyses identified the strongest significant correlation between ¹²⁵I uptake and three explanatory variables, actinobacteria soil biomass (p = 6.04E-04, 1.35E-02 for Kendall-Tau and regression analysis, respectively), soil nitrogen content (p = 4.86E-04, 4.24E-03), and soil oxidase enzyme activity at pH 7.0 using the substrate L-DOPA (p = 2.83E-03, 4.33E-04) and at pH 5.5 using the ABTS (p = 5.09E-03, 3.14E-03). Together, the results suggest that extracellular oxidases, primarily of bacterial origin, are the primary catalyst for soil iodination in aerobic, surface soils of deciduous and coniferous forests, and that soil N content may be indicative of the availability of binding sites for reactive iodine species.

Purification and biochemical characterization of a new thermostable laccase from Enterococcus faecium A2 by a three-phase partitioning method and investigation of its decolorization potential

Three-phase partitioning (TPP) is a simple, fast, cost-effective, and highly efficient process that can be used in the purification of laccases. In this study, microorganisms with laccase activity were isolated from water samples collected from the Agri-Diyadin hot spring. The isolate with the highest laccase activity was found to be the A2 strain. As a result of molecular (16S rRNA sequence) and conventional (morphological, biochemical, and physiological) analyses, it was determined that the A2 isolate was 99% similar to Enterococcus faecium (Genbank number: MH424896). The laccase was purified to 4.9-fold with 110% recovery using the TPP. The molecular mass of the enzyme was found by SDS-PAGE to be 50.11 kDa. Optimum pH 6.0 and optimum temperature for laccase were determined as 80 °C. The laccase exhibited pH stability over a wide range (pH 3.0-9.0) and a high thermostability, retaining over 90% of its activity after 1 h of incubation at 20-90 °C. The laccase exhibited high thermostability, with a heat inactivation half-life of approximately 24 h at 80 °C. The enzyme remained highly stable in the presence of surfactants and increased its activity in the presence of organic solvents, Cr²⁺, Cu²⁺, and Ag⁺ metal ions. The K_m, V_max, k_cat, and k_cat/K_m values of laccase for 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) substrate were 0.68 mM, 5.29 μmol mL^-1 min^-1, 110.2 s^-1, and 162.1 s^-1 mM^-1, respectively.

Engineering the Catalytic Properties of Two-Domain Laccase from Streptomyces griseoflavus Ac-993

Laccases catalyze the oxidation of substrates with the concomitant reduction of oxygen to water. Recently, we found that polar residues located in tunnels leading to Cu2 and Cu3 ions control oxygen entrance (His 165) and proton transport (Arg 240) of two-domain laccase (2D) from Streptomyces griseoflavus (SgfSL). In this work, we have focused on optimizing the substrate-binding pocket (SBP) of SgfSL while simultaneously adjusting the oxygen reduction process. SgfSL variants with three single (Met199Ala, Met199Gly, and Tyr230Ala) and three double amino acid residues substitutions (Met199Gly/His165Ala, His165Ala/Arg240His, Met199Gly/Arg240His) were constructed, purified, and investigated. Combination of substitutions in the SBP and in the tunnel leading to Cu2 ion (Met199Gly/Arg240His) increased SgfSL catalytic activity towards ABTS by 5-fold, and towards 2.6-DMP by 16-fold. The high activity of the Met199Gly/Arg240His variant can be explained by the combined effect of the SBP geometry optimization (Met199Gly) and increased proton flux via the tunnel leading to Cu2 ion (Arg240His). Moreover, the variant with Met199Gly and His165Ala mutations did not significantly increase SgfSL's activity, but led to a drastic shift in the optimal pH of 2.6-DMP oxidation. These results indicate that His 165 not only regulates oxygen access, but it also participates in proton transport in 2D laccases.

Genomic Organization of Streptomyces flavotricini NGL1 and Streptomyces erythrochromogenes HMS4 Reveals Differential Plant Beneficial Attributes and Laccase Production Capabilities

The genus Streptomyces has been explored in industrial sectors due to its endurance to environmental stresses, the production of a plethora of biomolecules, the biological remediation of soils, and alleviating plant stresses. The whole genome of NGL1 and HMS4 was sequenced due to the specific laccase activity against 2,6-dimethoxyphenol (2,6-DMP) and differential plant beneficial attributes. The deduced genome of 8.85 Mbp and 7.73 Mbp in size with a G+C content of 72.03% and 72.3% was obtained for NGL1 and HMS4, respectively. A total of 8438 and 7322 protein coding genes, 155 (130 tRNA, 25 rRNA) and 145 tRNA (121 tRNA, 24 rRNA) coding genes were predicted in NGL1 and HMS4, respectively. The comparative genomics of NGL1 and HMS4 showed 185 and 162 genes encoding for carbohydrate-active enzymes, respectively. The genomic ability of these strains to encode carbohydrate-active enzymes, laccase, and diversity of BGCs, along with plant beneficial attributes to suppress the plant pathogens can be used for several industrial and agricultural applications.

A Novel Polyphenol Oxidoreductase OhLac from Ochrobactrum sp. J10 for Lignin Degradation

Identifying the enzymes involved in lignin degradation by bacteria is important in studying lignin valorization to produce renewable chemical products. In this paper, the catalytic oxidation of lignin by a novel multi-copper polyphenol oxidoreductase (OhLac) from the lignin degrader Ochrobactrum sp. J10 was explored. Following its expression, reconstitution, and purification, a recombinant enzyme OhLac was obtained. The OhLac enzyme was characterized kinetically against a range of substrates, including ABTS, guaiacol, and 2,6-DMP. Moreover, the effects of pH, temperature, and Cu²⁺ on OhLac activity and stability were determined. Gas chromatography-mass spectrometer (GC-MS) results indicated that the β-aryl ether lignin model compound guaiacylglycerol-β-guaiacyl ether (GGE) was oxidized by OhLac to generate guaiacol and vanillic acid. Molecular docking analysis of GGE and OhLac was then used to examine the significant amino residues and hydrogen bonding sites in the substrate–enzyme interaction. Altogether, we were able to investigate the mechanisms involved in lignin degradation. The breakdown of the lignocellulose materials wheat straw, corn stalk, and switchgrass by the recombinant OhLac was observed over 3 days, and the degradation results revealed that OhLac plays a key role in lignin degradation.

Expression of thermophilic two-domain laccase from Catenuloplanes japonicus in Escherichia coli and its activity against triarylmethane and azo dyes

BACKGROUND

Two-domain laccases are copper-containing oxidases found in bacteria in the beginning of 2000ths. Two-domain laccases are known for their thermal stability, wide substrate specificity and, the most important of all, their resistance to so-called «strong inhibitors» of classical fungal laccases (azides, fluorides). Low redox potential was found to be specific for all the two-domain laccases, due to which these enzymes lost the researchers' interest as potentially applicable for various biotechnological purposes, such as bioremediation. Searching, obtaining and studying the properties of novel two-domain laccases will help to obtain an enzyme with high redox-potential allowing its practical application.

METHODS

A gene encoding two-domain laccase was identified in Catenuloplanes japonicus genome, cloned and expressed in an Echerichia coli strain. The protein was purified to homogeneity by immobilized metal ion affinity chromatography. Its molecular properties were studied using electrophoresis in native and denaturing conditions. Physico-chemical properties, kinetic characteristics, substrate specificity and decolorization ability of laccase towards triphenylmethane dyes were measured spectrophotometrically.

RESULTS

A novel two-domain recombinant laccase CjSL appeared to be a multimer with a subunit molecular mass of 37 kDa. It oxidized a wide range of phenolic substrates (ferulic acid, caffeic acid, hydroquinone, catechol, etc.) at alkaline pH, while oxidizing of non phenolic substrates (K4[Fe(CN)6], ABTS) was optimal at acidic pH. The UV-visible absorption spectrum of the purified enzyme was specific for all two-domain laccases with peak of absorption at 600 nm and shoulder at 340 nm. The pH optima of CjSL for oxidation of ABTS and 2, 6-DMP substrates were 3.6 and 9.2 respectively. The temperature optimum was 70 °C. The enzyme was most stable in neutral-alkaline conditions. CjSL retained 53% activity after pre-incubation at 90 °C for 60 min. The enzyme retained 26% activity even after 60 min of boiling. The effects of NaF, NaN3, NaCl, EDTA and 1,10-phenanthroline on enzymatic activity were investigated. Only 1,10-phenanthroline reduced laccase activity under both acidic and alkaline conditions. Laccase was able to decolorize triphenylmethane dyes and azo-dyes. ABTS and syringaldehyde were effective mediators for decolorization. The efficacy of dye decolorization depended on pH of the reaction medium.

The role of positive charged residue in the proton-transfer mechanism of two-domain laccase from Streptomyces griseoflavus Ac-993

Multi-copper oxidases are capable of coupling the one-electron oxidation of four substrate equivalents to the four-electron reduction of dioxygen to two molecules of water. This process takes place at the trinuclear copper center of the enzymes. Previously, the main catalytic stages for three-domain (3D) laccases have been identified. However, for bacterial small two-domain (2D) laccases several questions remain to be answered. One of them is the nature of the protonation events upon the reductive cleavage of dioxygen to water. In 3D laccases, acidic residues play a key role in the protonation mechanisms. In this study, the role of the Arg240 residue, located within the T2 tunnel of 2D laccase from Streptomyces griseoflavus Ac-993, was investigated. X-ray structural analysis and kinetic characterization of two mutants, R240A and R240H, have provided support for a role of this residue in the protonation events. Communicated by Ramaswamy H. Sarma.

Characterization of a thermostable and solvent-tolerant laccase produced by Streptomyces sp. LAO

OBJECTIVES

Decaying wood samples were collected, and actinomycetes were isolated and screened for laccase production. The identity of the efficient laccase-producing isolate was confirmed by using a molecular approach. Fermentation conditions for laccase production were optimized, and laccase biochemical properties were studied.

RESULTS

Based on the 16S rRNA gene sequencing and phylogenetic analysis, the isolate coded as HWP₃ was identified as Streptomyces sp. LAO. The time-course study showed that the isolate optimally produced laccase at 84 h with 40.58 ± 2.35 U/mL activity. The optimized physicochemical conditions consisted of pH 5.0, ferulic acid (0.04%; v/v), pine back (0.2 g/L), urea (1.0 g/L), and lactose (1 g/L). Streptomyces sp. LAO laccase was optimally active at pH and temperature of 8.0 and 90 °C, respectively, with remarkable pH and thermal stability. Furthermore, the enzyme had a sufficient tolerance for organic solvents after 16 h of preincubation, with laccase activity > 70%. Additionally, the laccase maintained considerable residual activity after pretreatment with 100 mM of chemical agents, including sodium dodecyl sulphate (69.93 ± 0.89%), ethylenediaminetetraacetic acid (93.1 ± 7.85%), NaN₃ (96.28 ± 3.34%) and urea (106.03 ± 10.72%).

CONCLUSION

The laccase's pH and thermal stability; and robust catalytic efficiency in the presence of organic solvents suggest its industrial and biotechnological application potentials for the sustainable development of green chemistry.

Transformation of low molecular compounds and soil humic acid by two domain laccase of Streptomyces puniceus in the presence of ferulic and caffeic acids

The two-domain bacterial laccases oxidize substrates at alkaline pH. The role of natural phenolic compounds in the oxidation of substrates by the enzyme is poorly understood. We have studied the role of ferulic and caffeic acids in the transformation of low molecular weight substrates and of soil humic acid (HA) by two-domain laccase of Streptomyces puniceus (SpSL, previously undescribed). A gene encoding a two-domain laccase was cloned from S. puniceus and over-expressed in Escherichia coli. The recombinant protein was purified by affinity chromatography to an electrophoretically homogeneous state. The enzyme showed high thermal stability, alkaline pH optimum for the oxidation of phenolic substrates and an acidic pH optimum for the oxidation of K4[Fe(CN)6] (potassium ferrocyanide) and ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt). Phenolic compounds were oxidized with lower efficiency than K4[Fe(CN)6] and ABTS. The SpSL did not oxidize 3.4-dimethoxybenzoic alcohol and p-hydroxybenzoic acid neither in the absence of phenolic acids nor in their presence. The enzyme polymerized HA-the amount of its high molecular weight fraction (>80 kDa) increased at the expense of low MW fraction (10 kDa). The addition of phenolic acids as potential mediators did not cause the destruction of HA by SpSL. In the absence of the HA, the enzyme polymerized caffeic and ferulic acids to macromolecular fractions (>80 kDa and 10-12 kDa). The interaction of SpSL with HA in the presence of phenolic acids caused an increase in the amount of HA high MW fraction and a two-fold increase in the molecular weight of its low MW fraction (from 10 to 20 kDa), suggesting a cross-coupling reaction. Infrared and solution-state 1H-NMR spectroscopy revealed an increase in the aromaticity of HA after its interaction with phenolic acids. The results of the study expand our knowledge on the transformation of natural substrates by two-domain bacterial laccases and indicate a potentially important role of the enzyme in the formation of soil organic matter (SOM) at alkaline pH values.

A Fungal Ascorbate Oxidase with Unexpected Laccase Activity

Ascorbate oxidases are an enzyme group that has not been explored to a large extent. So far, mainly ascorbate oxidases from plants and only a few from fungi have been described. Although ascorbate oxidases belong to the well-studied enzyme family of multi-copper oxidases, their function is still unclear. In this study, Af_AO1, an enzyme from the fungus Aspergillus flavus, was characterized. Sequence analyses and copper content determination demonstrated Af_AO1 to belong to the multi-copper oxidase family. Biochemical characterization and 3D-modeling revealed a similarity to ascorbate oxidases, but also to laccases. Af_AO1 had a 10-fold higher affinity to ascorbic acid (K_M = 0.16 ± 0.03 mM) than to ABTS (K_M = 1.89 ± 0.12 mM). Furthermore, the best fitting 3D-model was based on the ascorbate oxidase from Cucurbita pepo var. melopepo. The laccase-like activity of Af_AO1 on ABTS (V_max = 11.56 ± 0.15 µM/min/mg) was, however, not negligible. On the other hand, other typical laccase substrates, such as syringaldezine and guaiacol, were not oxidized by Af_AO1. According to the biochemical and structural characterization, Af_AO1 was classified as ascorbate oxidase with unusual, laccase-like activity.

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.

Investigations of Accessibility of T2/T3 Copper Center of Two-Domain Laccase from Streptomyces griseoflavus Ac-993

Laccases (EC 1.10.3.2) are multicopper oxidoreductases acting on diphenols and related substances. Laccases are highly important for biotechnology and environmental remediation. These enzymes contain mononuclear one T2 copper ion and two T3 copper ions (Cu3_α and Cu3_β), which form the so-called trinuclear center (TNC). Along with the typical three-domain laccases Bacteria produce two-domain (2D) enzymes, which are active at neutral and basic pH, thermostable, and resistant to inhibitors. In this work we present the comparative analysis of crystal structures and catalytic properties of recombinant 2D laccase from Streptomyces griseoflavus Ac-993 (SgfSL) and its four mutant forms with replacements of two amino acid residues, located at the narrowing of the presumable T3-solvent tunnels. We obtained inactive enzymes with substitutions of His165, with Phe, and Ile170 with Ala or Phe. His165Ala variant was more active than the wild type. We suggest that His165 is a “gateway” at the O₂-tunnel leading from solvent to the Cu3_β of the enzyme. The side chain of Ile170 could be indirectly involved in the coordination of copper ions at the T3 center by maintaining the position of the imidazole ring of His157 that belongs to the first coordination sphere of Cu3_α.

Recovery of laccase-producing gammaproteobacteria from wastewater

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Selective enrichment was used to isolate active biodegradative bacteria.

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The role of chemotaxis in xenobiotic metabolism was elucidated.

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Wastewater mesocosms were identified as a repository for biodegradative bacteria.

Wastewater environment is a rich source of microorganisms with the capability for the degradation of malicious aromatic pollutants. Although wastewater could be regarded as both a resource and a problem, we intended to elucidate its beneficial aspect in this study sourcing for laccase-producing proteobacteria. Different wastewater samples, from selected wastewater treatment plants (WWTPs), were selectively enriched with some model compounds (vanillin, lignin and potassium hydrogen phthalate) to screen out bacterial isolates that possess excellent degradation potentials. Thereafter, positive isolates were screened for the production of laccase and degradation on phenolic (guaiacol, α-naphthol and syringaldazine) and non-phenolic (ABTS; 2,2 azino-bis -(3-ethylbenzothiazoline 6 sulphonic acid) and PFC; potassium ferrocyanoferrate) substrates characteristic of laccase oxidation. Remarkable laccase producers were identified based on their 16 S rRNA sequences and their secreted enzymes were subjected to substrate specificity test, employing laccase substrates; ABTS, PFC, guaiacol, α-naphthol, 2,6-dimethoxyphenol and pyrogallol. Results showed that wastewater and selective enrichment, in tandem, produced the gammaproteobacteria Pseudomonas aeruginosa DEJ16, Pseudomonas mendocina AEN16 and Stenotrophomonas maltophila BIJ16, which preferably oxidized the non-phenolic substrates. Units of extracellular laccase activity ranging between cca. 490 and cca. 600 U/mL were recorded for ABTS whereas outputs recorded from PFC catalysis ranged from cca. 320 to cca. 430 U/mL. Stenotrophomonas maltophila BIJ16 presented an unparalleled high laccase activity and had a responsive substrate specificity to aromatic and inorganic substrates, thereby suggesting its employment for in situ biodegradation studies. In conclusion, wastewater serves as an ideal milieu for the isolation of laccase producing bacteria.

Streptomyces spp. in the biocatalysis toolbox

About 20,100 research publications dated 2000-2017 were recovered searching the PubMed and Web of Science databases for Streptomyces, which are the richest known source of bioactive molecules. However, these bacteria with versatile metabolism are powerful suppliers of biocatalytic tools (enzymes) for advanced biotechnological applications such as green chemical transformations and biopharmaceutical and biofuel production. The recent technological advances, especially in DNA sequencing coupled with computational tools for protein functional and structural prediction, and the improved access to microbial diversity enabled the easier access to enzymes and the ability to engineer them to suit a wider range of biotechnological processes. The major driver behind a dramatic increase in the utilization of biocatalysis is sustainable development and the shift toward bioeconomy that will, in accordance to the UN policy agenda "Bioeconomy to 2030," become a global effort in the near future. Streptomyces spp. already play a significant role among industrial microorganisms. The intention of this minireview is to highlight the presence of Streptomyces in the toolbox of biocatalysis and to give an overview of the most important advances in novel biocatalyst discovery and applications. Judging by the steady increase in a number of recent references (228 for the 2000-2017 period), it is clear that biocatalysts from Streptomyces spp. hold promises in terms of valuable properties and applicative industrial potential.

Laccase SilA from Streptomyces ipomoeae CECT 3341, a key enzyme for the degradation of lignin from agricultural residues?

The role of laccase SilA produced by Streptomyces ipomoeae CECT 3341 in lignocellulose degradation was investigated. A comparison of the properties and activities of a laccase-negative mutant strain (SilA-) with that of the wild-type was studied in terms of their ability to degrade lignin from grass lignocellulose. The yields of solubilized lignin (acid precipitable polymeric lignin, APPL) obtained from wheat straw by both strains in Solid State Fermentation (SSF) conditions demonstrated the importance of SilA laccase in lignin degradation with the wild-type showing 5-fold more APPL produced compared with the mutant strain (SilA-). Analytical pyrolysis and FT-IR (Fourier Transform Infrared Spectroscopy) confirmed that the APPL obtained from the substrate fermented by wild-type strain was dominated by lignin derived methoxyphenols whereas those from SilA- and control APPLs were composed mainly of polysaccharides. This is the first report highlighting the role of this laccase in lignin degradation.

Enhanced delignification of steam-pretreated poplar by a bacterial laccase

The recalcitrance of woody biomass, particularly its lignin component, hinders its sustainable transformation to fuels and biomaterials. Although the recent discovery of several bacterial ligninases promises the development of novel biocatalysts, these enzymes have largely been characterized using model substrates: direct evidence for their action on biomass is lacking. Herein, we report the delignification of woody biomass by a small laccase (sLac) from Amycolatopsis sp. 75iv3. Incubation of steam-pretreated poplar (SPP) with sLac enhanced the release of acid-precipitable polymeric lignin (APPL) by ~6-fold, and reduced the amount of acid-soluble lignin by ~15%. NMR spectrometry revealed that the APPL was significantly syringyl-enriched relative to the original material (~16:1 vs. ~3:1), and that sLac preferentially oxidized syringyl units and altered interunit linkage distributions. sLac's substrate preference among monoaryls was also consistent with this observation. In addition, sLac treatment reduced the molar mass of the APPL by over 50%, as determined by gel-permeation chromatography coupled with multi-angle light scattering. Finally, sLac acted synergistically with a commercial cellulase cocktail to increase glucose production from SPP ~8%. Overall, this study establishes the lignolytic activity of sLac on woody biomass and highlights the biocatalytic potential of bacterial enzymes.

The Rise of Radicals in Bioinorganic Chemistry

Prior to 1950, the consensus was that biological transformations occurred in two-electron steps, thereby avoiding the generation of free radicals. Dramatic advances in spectroscopy, biochemistry, and molecular biology have led to the realization that protein-based radicals participate in a vast array of vital biological mechanisms. Redox processes involving high-potential intermediates formed in reactions with O₂ are particularly susceptible to radical formation. Clusters of tyrosine (Tyr) and tryptophan (Trp) residues have been found in many O₂-reactive enzymes, raising the possibility that they play an antioxidant protective role. In blue copper proteins with plastocyanin-like domains, Tyr/Trp clusters are uncommon in the low-potential single-domain electron-transfer proteins and in the two-domain copper nitrite reductases. The two-domain muticopper oxidases, however, exhibit clusters of Tyr and Trp residues near the trinuclear copper active site where O₂ is reduced. These clusters may play a protective role to ensure that reactive oxygen species are not liberated during O₂ reduction.

Crystallization and X-ray diffraction studies of a two-domain laccase from Streptomyces griseoflavus

Laccase (EC 1.10.3.2) is one of the most common copper-containing oxidases; it is found in many organisms and catalyzes the oxidation of primarily phenolic compounds by oxygen. Two-domain laccases have unusual thermostability, resistance to inhibitors and an alkaline optimum of activity. The causes of these properties in two-domain laccases are poorly understood. A recombinant two-domain laccase (SgfSL) was cloned from the genome of Streptomyces griseoflavus Ac-993, expressed in Escherichia coli and purified to homogeneity. The crystals of SgfSL belonged to the monoclinic space group P21, with unit-cell parameters a = 74.64, b = 94.72, c = 117.40 Å, β = 90.672°, and diffraction data were collected to 2.0 Å resolution using a synchrotron-radiation source. Two functional trimers per asymmetric unit correspond to a Matthews coefficient of 1.99 Å(3) Da(-1) according to the monomer molecular weight of 35.6 kDa.