The Structure of Rigidoporus lignosus Laccase Containing a Full Complement of Copper Ions, Reveals an Asymmetrical Arrangement for the T3 Copper Pair

Bioremediation of organic pollutants by laccase-metal-organic framework composites: A review of current knowledge and future perspective

Immobilized laccases are widely used as green biocatalysts for bioremediation of phenolic pollutants and wastewater treatment. Metal-organic frameworks (MOFs) show potential application for immobilization of laccase. Their unique adsorption properties provide a synergic effect of adsorption and biodegradation. This review focuses on bioremediation of wastewater pollutants using laccase-MOF composites, and summarizes the current knowledge and future perspective of their biodegradation and the enhancement strategies of enzyme immobilization. Mechanistic strategies of preparation of laccase-MOF composites were mainly investigated via physical adsorption, chemical binding, and de novo/co-precipitation approaches. The influence of architecture of MOFs on the efficiency of immobilization and bioremediation were discussed. Moreover, as sustainable technology, the integration of laccases and MOFs into wastewater treatment processes represents a promising approach to address the challenges posed by industrial pollution. The MOF-laccase composites can be promising and reliable alternative to conventional techniques for the treatment of wastewaters containing pharmaceuticals, dyes, and phenolic compounds. The detailed exploration of various immobilization techniques and the influence of MOF architecture on performance provides valuable insights for optimizing these composites, paving the way for future advancements in environmental biotechnology. The findings of this research have the potential to influence industrial wastewater treatment and promoting cleaner treatment processes and contributing to sustainability efforts.

Multicopper oxidase-2 mediated cuticle formation: Its contribution to evolution and success of insects as terrestrial organisms

The insect cuticle is a non-cellular matrix composed of polysaccharide chitins and proteins. The cuticle covers most of the body surface, including the trachea, foregut, and hindgut, and it is the body structure that separates the intraluminal environment from the external environment. The cuticle is essential to sustain their lives, both as a physical barrier to maintain homeostasis and as an exoskeleton that mechanically supports body shape and movement. Previously, we proposed a theory about the possibility that the cuticle-forming system contributes to the "evolution and success of insects." The main points of our theory are that 1) insects evolved an insect-specific system of cuticle formation and 2) the presence of this system may have provided insects with a competitive advantage in the early land ecosystems. The key to this theory is that insects utilize molecular oxygen abundant in the atmosphere, which differs from closely related crustaceans that form their cuticles with calcium ions. With newly obtained knowledge, this review revisits the significance of the insect-specific system for insects to adapt to terrestrial environments and also discusses the long-standing question in entomology as to why, despite their great success in terrestrial environments, they poorly adapt to marine environments.

Fungal Laccases: Fundamentals, Engineering and Classification Update

Multicopper oxidases (MCOs) share a common catalytic mechanism of activation by oxygen and cupredoxin-like folding, along with some common structural determinants. Laccases constitute the largest group of MCOs, with fungal laccases having the greatest biotechnological applicability due to their superior ability to oxidize a wide range of aromatic compounds and lignin, which is enhanced in the presence of redox mediators. The adaptation of these versatile enzymes to specific application processes can be achieved through the directed evolution of the recombinant enzymes. On the other hand, their substrate versatility and the low sequence homology among laccases make their exact classification difficult. Many of the ever-increasing amounts of MCO entries from fungal genomes are automatically (and often wrongly) annotated as laccases. In a recent comparative genomic study of 52 basidiomycete fungi, MCO classification was revised based on their phylogeny. The enzymes clustered according to common structural motifs and theoretical activities, revealing three novel groups of laccase-like enzymes. This review provides an overview of the structure, catalytic activity, and oxidative mechanism of fungal laccases and how their biotechnological potential as biocatalysts in industry can be greatly enhanced by protein engineering. Finally, recent information on newly identified MCOs with laccase-like activity is included.

Solid Migration to Assemble a Flower-like Nanozyme with Highly Dense Single Copper Sites for Specific Phenol Oxidation

Nanozymes with high catalytic stability and sustainability have emerged as powerful competitors to natural enzymes for diverse biocatalytic applications. However, constructing a nanozyme with high specificity is one of their biggest challenges. Herein, we develop a facile solid migration strategy to access a flower-like single copper site nanozyme (Cu SSN) via direct transformation of copper foam activated by 2-methylimidazole. With highly clustered CuN₃ sites whose local structure is similar to that of natural polyphenol oxidase, the Cu SSN exhibits excellent activity and specificity to oxidize phenols without peroxidase-like activity. Furthermore, the Cu SSN shows high sensitivity in the colorimetric detection of epinephrine with a low detection limit of 0.10 μg mL^-1, exceeding that of most previously reported enzyme-mimicking catalysts. This work not only provides a simple method for the large-scale preparation of high-performance nanozymes but also offers an inspiration for the design of highly specific nanozymes by mimicking the synergy among sites in natural enzymes.

Involvement of Laccase2 in Cuticle Sclerotization of the Whitefly, Bemisia tabaci Middle East–Asia Minor 1

Cuticle sclerotization is critical for insect survival. Laccase2 (Lac2) is a phenol oxidase that plays a key role in cuticle formation and pigmentation in a variety of insects. However, the function of Lac2 in whitefly, Bemisia tabaci, remains unclear. In this study, we identified a BtLac2 gene in B. tabaci MEAM1 and found that BtLac2 was expressed in all stages. It was highly expressed in the egg stage, followed by nymph and adult. Moreover, the expression of BtLac2 was higher in the cuticle than in other tissues. Knockdown of BtLac2 in nymphs produced thinner and fragile cuticles, which significantly increased the mortality rate, extended the development duration of nymphs, and decreased the emergence rate of adults. This result demonstrates that BtLac2 plays an important role in the cuticle hardening of B. tabaci and suggests a potential management strategy using RNAi to knock down BtLac2 expression.

Analysis of laccase‐like enzymes secreted by fungi isolated from a cave in northern Spain

Laccases belong to a family of multicopper enzymes able to oxidize a broad spectrum of organic compounds. Despite the well‐known property of laccases to carry out bleaching and degradation of industrial dyes and polyphenolic compounds, their industrial use is often limited by the high cost, low efficiency, or instability of these enzymes. To look for new microorganisms which produce laccases that are potentially suitable for industrial applications, we have isolated several fungal strains from a cave in northern Spain. Their phenotypic analysis on agar plates supplemented with ABTS (2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid)) disclosed two laccase‐positive strains. Further genotyping revealed that they belonged to the Gliomastix murorum and Conidiobolus thromboides species. The secretion of G. murorum and C. thromboides laccase‐like enzymes was then confirmed by zymography. Further identification of these polypeptides by mass‐spectroscopy revealed the nature of the laccases and made it possible to predict their functional domains and other features. In addition, plate assays revealed that the laccases secreted by both G. murorum and C. thromboides were capable of degrading industrial dyes (Congo Red, Indigo, and Eriochrome Black T). Homology modeling and substrate docking predicted the putative structure of the currently uncrystallized G. murorum enzyme as well as its amino acid residues potentially involved in interactions with these dyes. In summary, new biochemical and structural insights into decolorization mediated by G. murorum laccase as well as identification of laccase‐like oxidase in C. thromboides point to a promising future for these enzymes in biotechnology.

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.

Supramolecule self-assembly synthesis of amyloid phenylalanine-Cu fibrils with laccase-like activity and their application for dopamine determination

Laccases are multicopper proteins for dioxygen-involved oxidation of a broad spectrum of organic compounds. I Novel amyloid-like phenylalanine-Cu (F-Cu(II)) fibrils were developed, which were obtained via supramolecular self-assembly of Cu²⁺ and phenylalanine (F) under basic condition. The obtained amyloid-like fibrils represented highly periodic structure, of which the lattice unit was constructed via alternating hydrophobic (aromatic environment) and hydrophilic (both hydrogen bonding and Cu(II) coordination) interactions. Relative to natural laccases, the amyloid-like F-Cu(II) architecture exhibited comparable substrate affinity (Michaelis constant, K_m = 0.75 mM) and higher catalytic efficiency (k_cat/K_m = 773.33 × 10^-3 g^-1 min^-1L). Moreover, it exhibited remarkable tolerances in pH (4 ~ 10), temperature (room temperature ~ 200 ℃), organic solvent, and long-term storage (> 15 days). These stabilities were superior among the reported nature and artificial laccases, presenting a more promising candidate in various chemo- or bio-applications. In addition, F-Cu(II) fibrils could catalyze the oxidation of dopamine (DA) to a brown product, in which a new absorption band at 470 nm was observed. Based on this, a simple colorimetric assay for the detection of DA could be performed. We reported a novel amyloid-like phenylalanine-Cu fibrils, in which F-Cu⁺ complex can mimick the T1 site of natural laccase to oxidize the substrates. Then electrons transferred to F-Cu²⁺ complex via N-H···O=C hydrogen binding pathway. Finally, the dioxygen was transformed to water though radical reaction.

Degradation of zearalenone and aflatoxin B1 by Lac2 from Pleurotus pulmonarius in the presence of mediators

The contamination of foods and feeds with mycotoxins has been an issue of global significance. For mycotoxin detoxification, enzymatic biodegradation using laccase has received much attention. In this study, a laccase gene lac2 from the fungus Pleurotus pulmonarius was expressed in the Pichia pastoris X33 yeast strain to produce recombinant proteins. Enzymatic properties of recombinant Lac2 and its ability to degrade zearalenone (ZEN) and Aflatoxin B1 (AFB1) in the presence of four mediators (ABTS, TEMPO, AS and SA) were investigated. Result showed that the optimum pH and temperature of recombinant Lac2 were 3.5 and 55 °C, respectively. Lac2 was not sensitive to heat and stable under both acidic and alkaline conditions. Lac2-ABTS and Lac2-AS were efficient systems for ZEN degradation over a wide range of pH (4-8) and temperature (40-60 °C). Lac2-AS was the most efficient system for AFB1 degradation, reaching 99.82% of degradation at pH 7 and 37 °C after 1 h of incubation. Finally, the Lac2-mediator oxidation products were structurally characterized. This study lays a solid foundation for the application of Lac2 laccase combined with AS for degrading mycotoxin in food and feed.

Review of advances in the development of laccases for the valorization of lignin to enable the production of lignocellulosic biofuels and bioproducts

Development and deployment of commercial biorefineries based on conversion of lignocellulosic biomass into biofuels and bioproducts faces many challenges that must be addressed before they are commercially viable. One of the biggest challenges faced is the efficient and scalable valorization of lignin, one of the three major components of the plant cell wall. Lignin is the most abundant aromatic biopolymer on earth, and its presence hinders the extraction of cellulose and hemicellulose that is essential to biochemical conversion of lignocellulose to fuels and chemicals. There has been a significant amount of work over the past 20 years that has sought to develop innovative processes designed to extract and recycle lignin into valuable compounds and help reduce the overall costs of the biorefinery process. Due to the complex matrix of lignin, which is essential for plant survival, the development of a reliable and efficient lignin conversion technology has been difficult to achieve. One approach that has received significant interest relies on the use of enzymes, notably laccases, a class of multi‑copper green oxidative enzymes that catalyze bond breaking in lignin to produce smaller oligomers. In this review, we first assess the different innovations of lignin valorization using laccases within the context of a biorefinery process, and then assess the latest economical advances that these innovations offered. Finally, we review laccase characterization and optimization, as well as the prospects and bottlenecks of this class of enzymes within the industrial and biorefining sectors.

Heterologous Expression, Engineering and Characterization of a Novel Laccase of Agrocybe pediades with Promising Properties as Biocatalyst

Agaricomycetes fungi responsible for decay of wood and other lignocellulosic substrates constitute a valuable source of lignin-degrading enzymes. Among these enzymes, laccases (multi-copper oxidases) present remarkable biotechnological potential as environmentally friendly biocatalysts able to oxidize a wide range of aromatic compounds using oxygen as the only requirement. Laccases from saprotrophic Agaricales species have been much less studied than laccases from Polyporales, despite the fact that the former fungi are excellent sources of laccases. Here, the gene of a novel laccase of Agrocybe pediades, that is secreted by the fungus during lignocellulose degradation, was synthesised de novo and expressed in Saccharomyces cerevisiae using an improved signal peptide previously obtained and enzyme directed evolution. The characterization of the new laccase variants provided new insights on the contribution of different amino acid residues to modulate laccase production, catalytic activity or optimal pH. The selected double-mutated variant also showed interesting properties as a biocatalyst, such as the ability to oxidise a wide range of substrates, including high-redox potential mediators and recalcitrant organic dyes, improved activity at neutral pH and high tolerance to inhibitors. Finally, we demonstrate the existence of three N-glycosylation sites in the laccase and their distinct effect on the secretion or catalytic activity of the enzyme.

Stabilization-destabilization and redox properties of laccases from medicinal mushroom Ganoderma lucidum and human pathogen Yersinia enterocolitica

Laccases or benzenediol oxygen oxidoreductases (EC 1.10.3.2) are polyphenol multicopper oxidases that are known for their structural and functional diversity in various life forms. In the present study, the molecular and physico-chemical properties (redox-potential and secondary structures) of fungal laccase isozymes (FLIs) isolated from a medicinal mushroom Ganoderma lucidum were analyzed and compared with those of the recombinant bacterial laccases (rLac) obtained from different Yersinia enterocolitica strains. It was revealed that the FLIs contained His-Cys-His as the most conserved residue in its domain I Cu site, while the fourth and fifth residues were variable (Ile, Leu, or Phe). Evidently, the cyclic voltammetric measurements of Glac L2 at Type 1 Cu site revealed greater E° for ABTS/ABTS⁺ (0.312 V) and ABTS⁺/ABTS²⁺ (0.773 V) compared to the E° of rLac. Furthermore, circular dichroism-based conformational analysis revealed structural stability of the FLIs at acidic pH (3.0) and low temperature (<30 °C), while the isozymes were destabilized at neutral pH (7.0) and high-temperature conditions (>70 °C). The zymographic studies further confirmed the functional inactivation of FLIs at high temperatures (≥70 °C), predominantly due to domain unfolding. These findings provide novel insight into the evolution of the catalytic efficiency and redox properties of the FLIs, contributing to the existing knowledge regarding stress responses, metabolite production, and the biotechnological utilization of metabolites.

Multicopper oxidase (MCO) laccase from Stropharia sp. ITCC-8422: an apparent authentication using integrated experimental and in silico analysis

In the present study, specificity of laccase from Stropharia sp. ITCC-8422 against various substrates, i.e. 2,2-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), guaiacol (GCL) and syringaldazine (SYZ) was determined. It exhibited maximum affinity against ABTS, followed by DMP and negligible activity for GCL and SYZ. As the concentration of substrate increased from 0.5 to 1.5 mM (ABTS) and 1 to 5 mM (DMP), the activity increased from 301.1 to 567.8 U/L and 254.4 to 436.2 U/L. Further, quadrupole time-of-flight liquid chromatography mass spectrometry (QTOF-LCMS) analysis of the extracellular proteome of Stropharia sp. ITCC-8422 identified eighty-four (84) extracellular proteins. The peptide sequence for the enzyme of interest exhibited sequence similarity with laccase-5 of Trametes pubescens. Using high molecular mass sequence of laccase-5, the protein structure of laccase was modelled and binding energy of laccase with four substrates, i.e. ABTS (- 5.65), DMP (- 4.65), GCL (- 4.66) and SYZ (- 5.5) was determined using autodock tool. The experimental and in silico analyses revealed maximum activity of laccase and lowest binding energy with ABTS. Besides, laccase was purified and it exhibited 2.1-fold purification with purification yield of 20.4% and had stability of 70% at pH 5-9 and 30-40 ℃. In addition, the bioremediation potential of laccase was explored by in silico analysis, where the binding energy of laccase with alizarin cyanine green was - 6.37 and both in silico work and experimental work were in agreement.

Metal and metal-oxide nanozymes: bioenzymatic characteristics, catalytic mechanism, and eco-environmental applications

Phenolic contaminants (R-OH) are a category of highly toxic organic compounds that are widespread in aquatic ecosystems and can induce carcinogenic risk to wildlife and humans; natural enzymes as green catalysts are capable of step-polymerizing these compounds to produce diverse macromolecular self-coupling products via radical-mediated C-C and C-O-C bonding at either the ortho- or para-carbon position, thereby evading the bioavailability and ecotoxicity of these compounds. Intriguingly, certain artificial metal and metal-oxide nanomaterials are known as nanozymes. They not only possess the unique properties of nanomaterials but also display intrinsic enzyme-mimicking activities. These artificial nanozymes are expected to surmount the shortcomings, such as low stability, easy inactivation, difficult recycling, and high cost, of natural enzymes, thus contributing to eco-environmental restoration. This review highlights the available studies on the enzymatic characteristics and catalytic mechanisms of natural enzymes and artificial metal and metal-oxide nanozymes in the removal and transformation of R-OH. These advances will provide key research directions beneficial to the multifunctional applications of artificial nanozymes in aquatic ecosystems.

A comparative structural analysis of the surface properties of asco-laccases

Laccases of different biological origins have been widely investigated and these studies have elucidated fundamentals of the generic catalytic mechanism. However, other features such as surface properties and residues located away from the catalytic centres may also have impact on enzyme function. Here we present the crystal structure of laccase from Myceliophthora thermophila (MtL) to a resolution of 1.62 Å together with a thorough structural comparison with other members of the CAZy family AA1_3 that comprises fungal laccases from ascomycetes. The recombinant protein produced in A. oryzae has a molecular mass of 75 kDa, a pI of 4.2 and carries 13.5 kDa N-linked glycans. In the crystal, MtL forms a dimer with the phenolic substrate binding pocket blocked, suggesting that the active form of the enzyme is monomeric. Overall, the MtL structure conforms with the canonical fold of fungal laccases as well as the features specific for the asco-laccases. However, the structural comparisons also reveal significant variations within this taxonomic subgroup. Notable differences in the T1-Cu active site topology and polar motifs imply molecular evolution to serve different functional roles. Very few surface residues are conserved and it is noticeable that they encompass residues that interact with the N-glycans and/or are located at domain interfaces. The N-glycosylation sites are surprisingly conserved among asco-laccases and in most cases the glycan displays extensive interactions with the protein. In particular, the glycans at Asn88 and Asn210 appear to have evolved as an integral part of the asco-laccase structure. An uneven distribution of the carbohydrates around the enzyme give unique properties to a distinct part of the surface of the asco-laccases which may have implication for laccase function–in particular towards large substrates.

Inactivation of Laccase by the Attack of As (III) Reaction in Water

Laccase is a multicopper oxidase containing four coppers as reaction sites, including one type 1, one type 2, and two type 3. We here provide the first experimental data showing that As (III) can be effectively removed from water and transformed to As (V) through reactions mediated by laccase with the presence of oxygen. To this end, the As (III) removal, As (V) yields, total protein, active laccase, and copper concentrations in the aqueous phase were determined, respectively. Additionally, electron paramagnetic resonance spectra and UV-vis spectra were applied to probe possible structural changes of the laccase during the reaction. The data offer the first evidence that laccase can be inactivated by As (III) attack thus leading to the release of type 2 copper. The released copper has no reactivity with the As (III). These findings provide new ideas into a significant pathway likely to master the environmental transformation of arsenite, and advance the understanding of laccase inactivation mechanisms, thus providing a foundation for optimization of enzyme-based processes and potential development for removal and remediation of arsenite contamination in the environment.

Nanostructured material-based biofuel cells: recent advances and future prospects

The review provides comprehensive discussions about electrode materials of BFCs and prospects of this technology for real-word applications.

Isolation, Purification and Characterization of Two Laccases from Carrot (Daucus carota L.) and Their Response to Abiotic and Metal Ions Stresses

Laccases, which belong to the blue copper oxidase enzyme family, oxidize many organic and inorganic compounds. The laccase-encoding genes DcLac1 and DcLac2 were isolated from the economically important tuberous root carrot, and their proteins were successfully expressed and purified using the Escherichia coli expression system BL21(DE3). DcLac1 and DcLac2 had molecular masses of approximately 64 and 61.9 kDa, respectively. With 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate acid) as the substrate, DcLac1 and DcLac2 had K m values of 3.9043 and 1.255 mM, respectively, and V max values of 54.0832 and 81.7996 μM mg(-1) min(-1), respectively. Moreover, DcLac1 and DcLac2 had optimal pH values of 2.8 and 2.6, respectively, and optimal temperatures of 45 and 40 °C, respectively. The activities of the two enzymes were promoted by Ca(2+), Mg(2+), Cu(2+), and Na(+) but inhibited by Fe(2+), Zn(2+), Mn(2+), K(+), SDS, and EDTA. Expression profiles showed that the two DcLac genes had almost identical responses to high and low temperature stresses but different responses to salt, drought, and metal stresses. This study provided insights into the characteristics and tolerance response mechanisms of laccase in carrot.

Two Decades of Laccases: Advancing Sustainability in the Chemical Industry

Given the current state of environmental affairs and that our future on this planet as we know it is in jeopardy, research and development into greener and more sustainable technologies within the chemical and forest products industries is at its peak. Given the global scale of these industries, the need for environmentally benign practices is propelling new green processes. These challenges are also impacting academic research and our reagents of interest are laccases. These enzymes are employed in a variety of biotechnological applications due to their native function as catalytic oxidants. They are about as green as it gets when it comes to chemical processes, requiring O₂ as their only co-substrate and producing H₂ O as the sole by-product. The following account will review our twenty year journey on the use of these enzymes within our research group, from their initial use in biobleaching of kraft pulps and for fiber modification within the pulp and paper industry, to their current application as green catalytic oxidants in the field of synthetic organic chemistry.

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

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

Diversity of laccase-coding genes in Fusarium oxysporum genomes

Multiple studies confirm laccase role in fungal pathogenicity and lignocellulose degradation. In spite of broad genomic research, laccases from plant wilt pathogen Fusarium oxysporum are still not characterized. The study aimed to identify F. oxysporum genes that may encode laccases sensu stricto and to characterize the proteins in silico in order to facilitate further research on their impact on the mentioned processes. Twelve sequenced F. oxysporum genomes available on Broad Institute of Harvard and MIT (2015) website were analyzed and three genes that may encode laccases sensu stricto were found. Their amino acid sequences possess all features essential for their catalytic activity, moreover, the homology models proved the characteristic 3D laccase structures. The study shades light on F. oxysporum as a new source of multicopper oxidases, enzymes with possible high redox potential and broad perspective in biotechnological applications.

Expression, refolding, and characterization of a small laccase from Thermus thermophilus HJ6

An open reading frame of the Thermus thermophilus HJ6 hypothetical laccase, which composed of 729 bases, was cloned and expressed as a fusion protein with six histidine residues in Escherichia coli SoluBL21™ cells. The resulting insoluble bodies were separated from cellular debris by centrifugation and solubilized with 6M guanidine HCl. The solubilized protein was refolded by a simple on-column refolding procedure using Ni-chelation affinity chromatography and then the refolded protein was purified by gel filtration chromatography. It showed a single band with a molecular mass of 27kDa in SDS-PAGE. The results from UV-visible absorption and electron paramagnetic resonance (EPR) analysis suggested that the enzyme had the typical copper sites, type-1, 2, and 3 Cu(II) of laccase. The purified enzyme exhibited the laccase activity with the optimal catalytic temperature at 75°C. The optimum pH for the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and syringaldazine was 4.5 and 6.0, respectively. The recombinant protein showed high thermostability, and the half-life of heat inactivation was about 50min at 85°C. The enzyme oxidized various known laccase substrates, its lowest Km value being for syringaldazine, highest kcat value for guaiacol, and highest kcat/Km for 2,6-dimethoxy-phenol. The enzyme reaction was strongly inhibited by the metal chelators and the thiol compounds.

Structure and molecular evolution of multicopper blue proteins

The multicopper blue protein family, which contains cupredoxin-like domains as a structural unit, is one of the most diverse groups of proteins. This protein family is divided into two functionally different types of enzymes: multicopper oxidase and nitrite reductase. Multicopper oxidase catalyzes the oxidation of the substrate and then reduces dioxygen. The structures of many multicopper oxidases are already known, and until recently they were classified into two main groups: the three- and six-domain types. Both function as monomers and have three spectroscopically different copper sites: Types I (blue), II, and III (tri-nuclear). Nitrite reductase is a closely related protein that contains Types I and II (mono-nuclear) coppers but reduces nitrite instead of dioxygen. Nitrite reductase, which consists of two domains, forms a homotrimer. Multicopper oxidase and nitrite reductase share similar structural architectures and also contain Type I copper. Therefore, it is proposed that they have a common ancestor protein. Recently, some two-domain type multicopper oxidases have been found and their crystal structures have been determined. They have a trimeric quaternary structure and contain an active site at the molecular interface such as nitrite reductase. These results support previous hypotheses and provide an insight into the molecular evolution of multicopper blue proteins.

Three-dimensional structures of laccases

Laccases are phenol oxidases that belong to the family of multi-copper oxidases and the superfamily of cupredoxins. A number of potential industrial applications for laccases have led to intensive structure-function studies and an increased amount of crystal structures has been solved. The objective of this review is to summarize and analyze available crystal structures of laccases. The experimental crystallographic data are now easily available from the websites and electron density maps can be used for the interpretation of the structural models. The crystal structures can give valuable insights into the functional mechanisms and may serve as the basis for the development of laccases for industrial applications.

Laccase engineering by rational and evolutionary design

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

Decolorization of Alizarin Red and other synthetic dyes by a recombinant laccase from Pichia pastoris

A cDNA encoding for a laccase was isolated from the white-rot fungus Lenzites gibbosa by RT-PCR and expressed in the Pichia pastoris. The laccase native signal peptide efficiently directed the secretion of the recombinant laccase in an active form. Factors influencing laccase expression, such as pH, cultivation temperature, copper concentration and methanol concentration, were optimized. The recombinant enzyme was purified to electrophoretic homogeneity, and was estimated to have a MW of ~61.5 kDa. The purified enzyme behaved similarly to the native laccase produced by L. gibbosa and efficiently decolorized Alizarin Red, Neutral Red, Congo Red and Crystal Violet, without the addition of redox mediators. The decolorization capacity of this recombinant enzyme suggests that it could be a useful biocatalyst for the treatment of dye-containing effluents. This study is the first report on the synthetic dye decolorization by a recombinant L. gibbosa laccase.

Purification and characterization of a novel laccase from Fomitopsis pinicola mycelia

A novel laccase was isolated from the culture filtrate of the brown-rot fungus, Fomitopsis pinicola. Enzyme production reached its highest level after cultivation for 8 days at 25°C. The enzyme was purified by ultrafiltration, ion exchange chromatography, gelfiltration chromatography, and hydrophobic interaction chromatography. Zymography analysis of the purified enzyme showed a laccase band with a molecular mass of 92 kDa. The molecular weight of the enzyme was 92 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration chromatography. The enzyme also had an isoelectric point of 3.8. The optimum temperature and pH for enzyme activity were 80°C and 3.0, respectively. Enzyme activity was relatively stable in the pH range from 1.5 to 11.0 and at temperatures below 40°C. The N-terminal amino acid sequence of the enzyme was DTHKAEIACRFKDLG. Enzyme activity was potently inhibited by NaN3 and SDS. The enzyme showed the highest specific activity for 2,2-azino-bis(3-ethylthiazoline-6-sulfonate) (ABTS) as a substrate. The Km value of the enzyme for ABTS substrate was 0.28 mM with a Vmax value of 4.5 U/min. The enzyme degraded several recalcitrant dyes at different time intervals during decolorization. Therefore, the novel laccase from F. pinicola may be potentially useful in industry.

Effective mutations in a high redox potential laccase from Pleurotus ostreatus

Since the first report on a laccase, there has been a notable development in the interest towards this class of enzymes, highlighted from the number of scientific papers and patents about them. At the same time, interest in exploiting laccases-mainly high redox potential-for various functions has been growing exponentially over the last 10 years. Despite decades of work, the molecular determinants of the redox potential are far to be fully understood. For this reason, interest in tuning laccase redox potential to provide more efficient catalysts has been growing since the last years. The work herein described takes advantage of the filamentous fungus Aspergillus niger as host for the heterologous production of the high redox potential laccase POXA1b from Pleurotus ostreatus and of one of its in vitro selected variants (1H6C). The system herein developed allowed to obtain a production level of 35,000 U/L (583.3 μkat/L) for POXA1b and 60,000 U/L (1,000 μkat/L) for 1H6C, corresponding to 13 and 20 mg/L for POXA1b and 1H6C, respectively. The characterised proteins exhibit very similar characteristics, with some exceptions regarding catalytic behaviour, stability and spectro-electrochemical properties. Remarkably, the 1H6C variant shows a higher redox potential with respect to POXA1b. Furthermore, the spectro-electrochemical results obtained for 1H6C make it tempting to claim that we spectro-electrochemically determined the redox potential of the 1H6C T2 site, which has not been studied in any detail by spectro-electrochemistry yet.

Stability Mechanisms of Laccase Isoforms using a Modified FoldX Protocol Applicable to Widely Different Proteins

A recent computational protocol that accurately predicts and rationalizes protein multisite mutant stabilities has been extended to handle widely different isoforms of laccases. We apply the protocol to four isoenzymes of Trametes versicolor laccase (TvL) with variable lengths (498-503 residues) and thermostability (Topt ∼ 45-80 °C) and with 67-77% sequence identity. The extended protocol uses (i) statistical averaging, (ii) a molecular-dynamics-validated "compromise" homology model to minimize bias that causes proteins close in sequence to a structural template to be too stable due to having the benefits of the better sampled template (typically from a crystal structure), (iii) correction for hysteresis that favors the input template to overdestabilize, and (iv) a preparative protocol to provide robust input sequences of equal length. The computed ΔΔG values are in good agreement with the major trends in experimental stabilities; that is, the approach may be applicable for fast estimates of the relative stabilities of proteins with as little as 70% identity, something that is currently extremely challenging. The computed stability changes associated with variations are Gaussian-distributed, in good agreement with experimental distributions of stability effects from mutation. The residues causing the differential stability of the four isoforms are consistent with a range of compiled laccase wild type data, suggesting that we may have identified general drivers of laccase stability. Several sites near Cu, notably 79, 241, and 245, or near substrate, mainly 265, are identified that contribute to stability-function trade-offs, of relevance to the search for new proficient and stable variants of these important industrial enzymes.

Structural and phylogenetic analysis of laccases from Trichoderma: a bioinformatic approach

The genus Trichoderma includes species of great biotechnological value, both for their mycoparasitic activities and for their ability to produce extracellular hydrolytic enzymes. Although activity of extracellular laccase has previously been reported in Trichoderma spp., the possible number of isoenzymes is still unknown, as are the structural and functional characteristics of both the genes and the putative proteins. In this study, the system of laccases sensu stricto in the Trichoderma species, the genomes of which are publicly available, were analyzed using bioinformatic tools. The intron/exon structure of the genes and the identification of specific motifs in the sequence of amino acids of the proteins generated in silico allow for clear differentiation between extracellular and intracellular enzymes. Phylogenetic analysis suggests that the common ancestor of the genus possessed a functional gene for each one of these enzymes, which is a characteristic preserved in T. atroviride and T. virens. This analysis also reveals that T. harzianum and T. reesei only retained the intracellular activity, whereas T. asperellum added an extracellular isoenzyme acquired through horizontal gene transfer during the mycoparasitic process. The evolutionary analysis shows that in general, extracellular laccases are subjected to purifying selection, and intracellular laccases show neutral evolution. The data provided by the present study will enable the generation of experimental approximations to better understand the physiological role of laccases in the genus Trichoderma and to increase their biotechnological potential.

Laccase immobilization and insolubilization: from fundamentals to applications for the elimination of emerging contaminants in wastewater treatment

Over the last few decades many attempts have been made to use biocatalysts for the biotransformation of emerging contaminants in environmental matrices. Laccase, a multicopper oxidoreductase enzyme, has shown great potential in oxidizing a large number of phenolic and non-phenolic emerging contaminants. However, laccases and more broadly enzymes in their free form are biocatalysts whose applications in solution have many drawbacks rendering them currently unsuitable for large scale use. To circumvent these limitations, the enzyme can be immobilized onto carriers or entrapped within capsules; these two immobilization techniques have the disadvantage of generating a large mass of non-catalytic product. Insolubilization of the free enzymes as cross-linked enzymes (CLEAs) is found to yield a greater volume ratio of biocatalyst while improving the characteristics of the biocatalyst. Ultimately, novel techniques of enzymes insolubilization and stabilization are feasible with the combination of cross-linked enzyme aggregates (combi-CLEAs) and enzyme polymer engineered structures (EPESs) for the elimination of emerging micropollutants in wastewater. In this review, fundamental features of laccases are provided in order to elucidate their catalytic mechanism, followed by different chemical aspects of the immobilization and insolubilization techniques applicable to laccases. Finally, kinetic and reactor design effects for enzymes in relation with the potential applications of laccases as combi-CLEAs and EPESs for the biotransformation of micropollutants in wastewater treatment are discussed.