Targeted mutations in a Trametes villosa laccase. Axial perturbations of the T1 copper

Enzymes in "Green" Synthetic Chemistry: Laccase and Lipase

Enzymes play an important role in numerous natural processes and are increasingly being utilized as environmentally friendly substitutes and alternatives to many common catalysts. Their essential advantages are high catalytic efficiency, substrate specificity, minimal formation of byproducts, and low energy demand. All of these benefits make enzymes highly desirable targets of academic research and industrial development. This review has the modest aim of briefly overviewing the classification, mechanism of action, basic kinetics and reaction condition effects that are common across all six enzyme classes. Special attention is devoted to immobilization strategies as the main tools to improve the resistance to environmental stress factors (temperature, pH and solvents) and prolong the catalytic lifecycle of these biocatalysts. The advantages and drawbacks of methods such as macromolecular crosslinking, solid scaffold carriers, entrapment, and surface modification (covalent and physical) are discussed and illustrated using numerous examples. Among the hundreds and possibly thousands of known and recently discovered enzymes, hydrolases and oxidoreductases are distinguished by their relative availability, stability, and wide use in synthetic applications, which include pharmaceutics, food and beverage treatments, environmental clean-up, and polymerizations. Two representatives of those groups-laccase (an oxidoreductase) and lipase (a hydrolase)-are discussed at length, including their structure, catalytic mechanism, and diverse usage. Objective representation of the current status and emerging trends are provided in the main conclusions.

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.

Tuning the Type 1 Reduction Potential of Multicopper Oxidases: Uncoupling the Effects of Electrostatics and H-Bonding to Histidine Ligands

In multicopper oxidases (MCOs), the type 1 (T1) Cu accepts electrons from the substrate and transfers these to the trinuclear Cu cluster (TNC) where O₂ is reduced to H₂O. The T1 potential in MCOs varies from 340 to 780 mV, a range not explained by the existing literature. This study focused on the ∼350 mV difference in potential of the T1 center in Fet3p and Trametes versicolor laccase (TvL) that have the same 2His1Cys ligand set. A range of spectroscopies performed on the oxidized and reduced T1 sites in these MCOs shows that they have equivalent geometric and electronic structures. However, the two His ligands of the T1 Cu in Fet3p are H-bonded to carboxylate residues, while in TvL they are H-bonded to noncharged groups. Electron spin echo envelope modulation spectroscopy shows that there are significant differences in the second-sphere H-bonding interactions in the two T1 centers. Redox titrations on type 2-depleted derivatives of Fet3p and its D409A and E185A variants reveal that the two carboxylates (D409 and E185) lower the T1 potential by 110 and 255-285 mV, respectively. Density functional theory calculations uncouple the effects of the charge of the carboxylates and their difference in H-bonding interactions with the His ligands on the T1 potential, indicating 90-150 mV for anionic charge and ∼100 mV for a strong H-bond. Finally, this study provides an explanation for the generally low potentials of metallooxidases relative to the wide range of potentials of the organic oxidases in terms of different oxidized states of their TNCs involved in catalytic turnover.

Multicopper oxidases with laccase-ferroxidase activity: Classification and study of ferroxidase activity determinants in a member from Heterobasidion annosum s. l

Multi-copper oxidases (MCO) share a common molecular architecture and the use of copper ions as cofactors to reduce O₂ to H₂O, but show high sequence heterogeneity and functional diversity. Many new emerging MCO genes are wrongly annotated as laccases, the largest group of MCOs, with the widest range of biotechnological applications (particularly those from basidiomycete fungi) due to their ability to oxidise aromatic compounds and lignin. Thus, comprehensive studies for a better classification and structure-function characterisation of MCO families are required. Laccase-ferroxidases (LAC-FOXs) constitute a separate and unexplored group of MCOs with proposed dual features between laccases and ferroxidases. We aim to better define this cluster and the structural determinants underlying putative hybrid activity. We performed a phylogenetic analysis of the LAC-FOXs from basidiomycete fungi, that resulted in two subgroups. This division seemed to correlate with the presence or absence of some of the three acidic residues responsible for ferroxidase activity in Fet3p from Saccharomyces cerevisiae. One of these LAC-FOXs (with only one of these residues) from the fungus Heterobasidion annosum s. l. (HaLF) was synthesised, heterologously expressed and characterised to evaluate its catalytic activity. HaLF oxidised typical laccase substrates (phenols, aryl amines and N-heterocycles), but no Fe (II). The enzyme was subjected to site-directed mutagenesis to determine the key residues that confer ferroxidase activity. The mutated HaLF variant with full restoration of the three acidic residues exhibited efficient ferroxidase activity, while it partially retained the wide-range oxidative activity of the native enzyme associated to laccases sensu stricto.

Tailor-made alkaliphilic and thermostable fungal laccases for industrial wood processing

BACKGROUND

During the kraft process to obtain cellulosic pulp from wood, most of the lignin is removed by high-temperature alkaline cooking, released in the black liquors and usually incinerated for energy. However, kraft lignins are a valuable source of phenolic compounds that can be valorized in new bio-based products. The aim of this work is to develop laccases capable of working under the extreme conditions of high temperature and pH, typical of the industrial conversion of wood into kraft pulp and fibreboard, in order to provide extremophilic biocatalysts for depolymerising kraft lignin, and enzyme-assisted technologies for kraft pulp and fibreboard production.

RESULTS

Through systematic enzyme engineering, combining enzyme-directed evolution and rational design, we changed the optimal pH of the laccase for oxidation of lignin phenols from acidic to basic, enhanced the catalytic activity at alkaline pH and increased the thermal tolerance of the enzyme by accumulating up to eight mutations in the protein sequence. The extremophilic laccase variants show maximum activity at 70 °C and oxidize kraft lignin at pH 10. Their integration into industrial-type processes saves energy and chemicals. As a pre-bleaching stage, the enzymes promote kraft pulp bleachability and significantly reduce the need for chlorine dioxide compared to the industrial sequence. Their application in wood chips during fibreboard production, facilitates the defibering stage, with less energy required.

CONCLUSIONS

A set of new alkaliphilic and thermophilic fungal laccases has been developed to operate under the extreme conditions of high temperature and pH typical of industrial wood conversion processes. For the first time basidiomycete laccases of high-redox potential show activity on lignin-derived phenols and polymeric lignin at pH 10. Considering the extreme conditions of current industrial processes for kraft pulp and fibreboard production, the new tailor-made laccases constitute a step forward towards turning kraft pulp mills into biorefineries. Their use as biocatalysts in the wood conversion sector is expected to support the development of more environmentally sound and efficient processes, and more sustainable products.

Designing Artificial Metalloenzymes by Tuning of the Environment beyond the Primary Coordination Sphere

Metalloenzymes catalyze a variety of reactions using a limited number of natural amino acids and metallocofactors. Therefore, the environment beyond the primary coordination sphere must play an important role in both conferring and tuning their phenomenal catalytic properties, enabling active sites with otherwise similar primary coordination environments to perform a diverse array of biological functions. However, since the interactions beyond the primary coordination sphere are numerous and weak, it has been difficult to pinpoint structural features responsible for the tuning of activities of native enzymes. Designing artificial metalloenzymes (ArMs) offers an excellent basis to elucidate the roles of these interactions and to further develop practical biological catalysts. In this review, we highlight how the secondary coordination spheres of ArMs influence metal binding and catalysis, with particular focus on the use of native protein scaffolds as templates for the design of ArMs by either rational design aided by computational modeling, directed evolution, or a combination of both approaches. In describing successes in designing heme, nonheme Fe, and Cu metalloenzymes, heteronuclear metalloenzymes containing heme, and those ArMs containing other metal centers (including those with non-native metal ions and metallocofactors), we have summarized insights gained on how careful controls of the interactions in the secondary coordination sphere, including hydrophobic and hydrogen bonding interactions, allow the generation and tuning of these respective systems to approach, rival, and, in a few cases, exceed those of native enzymes. We have also provided an outlook on the remaining challenges in the field and future directions that will allow for a deeper understanding of the secondary coordination sphere a deeper understanding of the secondary coordintion sphere to be gained, and in turn to guide the design of a broader and more efficient variety of ArMs.

Fungal Laccases: The Forefront of Enzymes for Sustainability

Enzymatic catalysis is one of the main pillars of sustainability for industrial production. Enzyme application allows minimization of the use of toxic solvents and to valorize the agro-industrial residues through reuse. In addition, they are safe and energy efficient. Nonetheless, their use in biotechnological processes is still hindered by the cost, stability, and low rate of recycling and reuse. Among the many industrial enzymes, fungal laccases (LCs) are perfect candidates to serve as a biotechnological tool as they are outstanding, versatile catalytic oxidants, only requiring molecular oxygen to function. LCs are able to degrade phenolic components of lignin, allowing them to efficiently reuse the lignocellulosic biomass for the production of enzymes, bioactive compounds, or clean energy, while minimizing the use of chemicals. Therefore, this review aims to give an overview of fungal LC, a promising green and sustainable enzyme, its mechanism of action, advantages, disadvantages, and solutions for its use as a tool to reduce the environmental and economic impact of industrial processes with a particular insight on the reuse of agro-wastes.

Recent advances in tuning redox properties of electron transfer centers in metalloenzymes catalyzing oxygen reduction reaction and H2 oxidation important for fuel cells design

Current fuel-cell catalysts for oxygen reduction reaction (ORR) and H₂ oxidation use precious metals and, for ORR, require high overpotentials. In contrast, metalloenzymes perform their respective reaction at low overpotentials using earth-abundant metals, making metalloenzymes ideal candidates for inspiring electrocatalytic design. Critical to the success of these enzymes are redox-active metal centers surrounding the enzyme active sites that ensure fast electron transfer (ET) to or away from the active site, by tuning the catalytic potential of the reaction as observed in multicopper oxidases but also in dictating the catalytic bias of the reaction as realized in hydrogenases. This review summarizes recent advances in studying these ET centers in multicopper oxidases and heme-copper oxidases that perform ORR and hydrogenases in carrying out H₂ oxidation. Insights gained from understanding how the reduction potential of the ET centers effects reactivity at the active site in both the enzymes and their models are provided.

Structural and functional study on cysteine 495, coordinating ligand to T1Cu site in multicopper oxidase CopA

Excessive intake of manganese seriously affects human health. Manganese oxidizing bacteria can efficiently remove manganese, among which manganese oxidase plays a decisive role. Multicopper oxidase, one of the manganese oxidases, has 4 copper binding sites, among them, T1Cu coordinates with two histidine, one cysteine and one axial residue, mainly transferring electrons from the substrate to T2Cu and T3Cu. Here, we conducted site-directed mutagenesis on T1Cu coordinating 495 amino acid site from cysteine to aspartic acid, histidine and methionine in multicopper oxidase CopA from Brevibacillus panacihumi MK-8, through the enzyme kinetics and structure models, finding that the enzyme catalytic efficiency (k_cat/K_m) of the mutated C495H with Mn²⁺ and ABTS reached 9.03 min^-1 mM^-1 and 8863 s^-1 mM^-1, 1.47 times and 1.67 times that of CopA. And it was found strain Rosetta-pET-copA^C495H could remove 91.67% manganese after 7-day culture, which was 11.65% higher than the original strain. To sum up, these results provide a vision for the future application of protein engineering in biological manganese removal.

Phenoloxidases in Plants-How Structural Diversity Enables Functional Specificity

The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism.

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.

Laccases: Versatile Biocatalysts for the Synthesis of Heterocyclic Cores

Laccases are multicopper oxidases that have shown a great potential in various biotechnological and green chemistry processes mainly due to their high relative non-specific oxidation of phenols, arylamines and some inorganic metals, and their high redox potentials that can span from 500 to 800 mV vs. SHE. Other advantages of laccases include the use of readily available oxygen as a second substrate, the formation of water as a side-product and no requirement for cofactors. Importantly, addition of low-molecular-weight redox mediators that act as electron shuttles, promoting the oxidation of complex bulky substrates and/or of higher redox potential than the enzymes themselves, can further expand their substrate scope, in the so-called laccase-mediated systems (LMS). Laccase bioprocesses can be designed for efficiency at both acidic and basic conditions since it is known that fungal and bacterial laccases exhibit distinct optimal pH values for the similar phenolic and aromatic amines. This review covers studies on the synthesis of five- and six-membered ring heterocyclic cores, such as benzimidazoles, benzofurans, benzothiazoles, quinazoline and quinazolinone, phenazine, phenoxazine, phenoxazinone and phenothiazine derivatives. The enzymes used and the reaction protocols are briefly outlined, and the mechanistic pathways described.

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.

Site-Directed Mutagenesis of Multicopper Oxidase from Hyperthermophilic Archaea for High-Voltage Biofuel Cells

Enzymes from hyperthermophilic archaea are potential candidates for industrial use because of their superior pH, thermal, and long-term stability, and are expected to improve the long-term stability of biofuel cells (BFCs). However, the reported multicopper oxidase (MCO) from hyperthermophilic archaea has lower redox potential than MCOs from other organisms, which leads to a decrease in the cell voltage of BFCs. In this study, we attempted to positively shift the redox potential of the MCO from hyperthermophilic archaeon Pyrobaculum aerophilum (McoP). Mutations (M470L and M470F) were introduced into the axial ligand of the T1 copper atom of McoP, and the enzymatic chemistry and redox potentials were compared with that of the parent (M470). The redox potentials of M470L and M470F shifted positively by about 0.07 V compared with that of M470. In addition, the catalytic activity of the mutants towards 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) increased 1.2-1.3-fold. The thermal stability of the mutants and the electrocatalytic performance for O₂ reduction of M470F was slightly reduced compared with that of M470. This research provides useful enzymes for application as biocathode catalysts for high-voltage BFCs.

Stabilizing Enzymes within Polymersomes by Coencapsulation of Trehalose

Enzymes are essential biocatalysts and very attractive as therapeutics. However, their functionality is strictly related to their stability, which is significantly affected by the environmental changes occurring during their usage or long-term storage. Therefore, maintaining the activity of enzymes is essential when they are exposed to high temperature during usage or when they are stored for extended periods of time. Here, we stabilize and protect enzymes by coencapsulating them with trehalose into polymersomes. The anhydrobiotic disaccharide preserved up to about 81% of the enzyme's original activity when laccase/trehalose-loaded nanoreactors were kept desiccated for 2 months at room temperature and 75% of its activity when heated at 50 °C for 3 weeks. Moreover, the applicability of laccase/trehalose-loaded nanoreactors as catalysts for bleaching of the textile dyes orange G, toluidine blue O, and indigo was proven. Our results demonstrate the advantages of coencapsulating trehalose within polymersomes to stabilize enzymes in dehydrated state for extended periods of time, preserving their activity even when heated to elevated temperature.

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.

Correlation between the T1 copper reduction potential and catalytic activity of a small laccase

Laccases are multicopper enzymes that catalyze oxidation of electron-rich substrates coupled to reduction of molecular oxygen to water. Since the Type 1 copper (T1 Cu) is the site where electrons are withdrawn from the substrate, it is assumed that the reduction potential of this copper correlates with enzyme activity. Herein, we studied the correlation of the T1 Cu reduction potential and the enzymatic activity of the small two-domain laccase Ssl1 from Streptomyces sviceus. For a systematic approach, we aimed to minimize any effects other than the reduction potential difference. To this end, we constructed a series of Ssl1 mutants with reduction potentials varying from <290 to 560 mV. Along with the hydrophobicity of the axial ligand of the T1 Cu also structural changes in the substrate binding site and additional hydrogen bonding increased the reduction potential. Enzyme activity experiments demonstrated that the T1 Cu reduction potential has a different effect on oxidation of different substrates. Whereas there was no obvious correlation between the T1 Cu reduction potential and kinetic parameters for the oxidation of syringaldazine (with a reduction potential of 390 mV), a good correlation was observed between the T1 Cu reduction potential and the conversion of substituted phenols with reduction potentials between 660 and 820 mV. This correlation was pronounced for the Ssl1 variants with reduction potentials above 470 mV, which demonstrated increased activities also during the oxidation of two dyes, alizarin red S and indigo carmine.

Microgel Bioreactors for Cancer Cell Targeting by pH-Dependent Generation of Radicals

The lack of specificity of traditional cytostatics and increasing resistance of cancer cells represent important challenges in cancer therapy. One of the characteristics of cancer cells is their intrinsic oxidative stress caused by higher metabolic activity, mitochondrial malfunction, and oncogene stimulation. This feature can be exploited in the pursuit of more selective cancer therapy, as there is increasing evidence that cancer cells are more sensitive to elevated concentrations of reactive oxygen species than normal cells. In this study, we demonstrate a new concept for cancer cell targeting by in situ production of radicals under physiological conditions. The biologically active radicals are produced in the milieu of cancer cells by enzymatic conversion from an inactive precursor, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)diammonium salt, by using miniature bioreactors represented by cell-sized microgels containing immobilized laccase. We utilize the pH-dependent activity of laccase to generate radicals only at a lower pH (5.7-6.1) that is characteristic of the tumor microenvironment. The composition of the microgels was optimized so as to allow sufficient substrate and radical diffusion, high enzyme activity, and stability under physiological conditions. The functionality of this system was evaluated on three cancer cell lines (HeLa, HT-29, and DLD1) and the cytotoxicity of in situ-produced radicals was successfully proven in all cases. These results demonstrate that cancer cell targeting by in situ-generated radicals using miniature enzymatic reactors may represent an alternative to traditional cytostatics. In particular, the pH-dependence of radical generation and their short-lived nature can ensure localized functionality in the tumor microenvironment and thereby reduce systemic side-effects.

Axial bonds at the T1 Cu site of Thermus thermophilus SG0.5JP17-16 laccase influence enzymatic properties

Laccase is a multi‐copper oxidase which oxidizes substrate at the type 1 copper site, simultaneously coupling the reduction of dioxygen to water at the trinuclear copper center. In this study, we used site‐directed mutagenesis to study the effect of axial bonds between the metal and amino acid residue side chains in lacTT. Our kinetic and spectral data showed that the replacement of the axial residue with non‐coordinating residues resulted in higher efficiency (k_cat/K_m) and a lower Cu²⁺ population at the type 1 copper site, while substitution with strongly coordinating residues resulted in lower efficiency and a higher Cu²⁺ population, as compared with the wild‐type. The redox potentials of mutants with hydrophobic axial residues (Ala and Phe) were higher than that of the wild‐type. In conclusion, these insights into the catalytic mechanism of laccase may be of use in protein engineering to fine‐tune its enzymatic properties for industrial application.

De novo transcriptome assembly: a new laccase multigene family from the marine-derived basidiomycete Peniophora sp. CBMAI 1063

Laccases are multicopper oxidases that are able to catalyze reactions involving a range of substrates, including phenols and amines, and this ability is related to the existence of different laccases. Basidiomycetes usually have more than one gene for laccase, but until now, this feature has not been demonstrated in a marine-derived fungus. Peniophora sp. CBMAI 1063 is a basidiomycete fungus isolated from a marine sponge that exhibits the ability to secrete significant amounts of laccase in saline conditions. In the present study, we identified laccase sequences from the transcriptome of Peniophora sp. CBMAI 1063 and used them to perform different molecular in silico analyses. The results revealed the presence of at least eight putative genes, which may encode ten different laccases with peptide lengths ranging from 482 to 588 aa and molecular weights ranging from 53.5 to 64.4 kDa. These laccases seem to perform extracellular activities, with the exception of one that may represent an intracellular laccase. The 10 predicted laccases expressed by Peniophora sp. CBMAI 1063 in laccase-induced media showed different patterns of N-glycosylation and isoelectric points and are divided into two classes based on the residue associated with the regulation of the redox potential of the enzyme. None of the predicted laccases showed more than 61% similarity to other fungal laccases. Based on the differences among the laccases expressed by Peniophora sp. CBMAI 1063, this marine-derived basidiomycete represents a valuable resource with strong potential for biotechnological exploitation.

Evolving stability and pH-dependent activity of the high redox potential Botrytis aclada laccase for enzymatic fuel cells

Fungal high redox potential laccases are proposed as cathodic biocatalysts in implantable enzymatic fuel cells to generate high cell voltages. Their application is limited mainly through their acidic pH optimum and chloride inhibition. This work investigates evolutionary and engineering strategies to increase the pH optimum of a chloride-tolerant, high redox potential laccase from the ascomycete Botrytis aclada. The laccase was subjected to two rounds of directed evolution and the clones screened for increased stability and activity at pH 6.5. Beneficial mutation sites were investigated by semi-rational and combinatorial mutagenesis. Fourteen variants were characterised in detail to evaluate changes of the kinetic constants. Mutations increasing thermostability were distributed over the entire structure. Among them, T383I showed a 2.6-fold increased half-life by preventing the loss of the T2 copper through unfolding of a loop. Mutations affecting the pH-dependence cluster around the T1 copper and categorise in three types of altered pH profiles: pH-type I changes the monotonic decreasing pH profile into a bell-shaped profile, pH-type II describes increased specific activity below pH 6.5, and pH-type III increased specific activity above pH 6.5. Specific activities of the best variants were up to 5-fold higher (13 U mg⁻¹) than BaL WT at pH 7.5.

Simulating Substrate Recognition and Oxidation in Laccases: From Description to Design

To meet the very specific requirements demanded by industry, proteins must be appropriately tailored. Engineering laccases, to improve the oxidation of small molecules, with applications in multiple fields, is, however, a difficult task. Most efforts have concentrated on increasing the redox potential of the enzyme, but in recent work, we have pursued an alternate strategy to engineering these biocatalysts. In particular, we have found that redesigning substrate binding at the T1 pocket, guided by in silico methodologies, to be a more consistent option. In this work, we evaluate the robustness of our computational approach to estimate activity, emphasizing the importance of the binding event in laccase reactivity. Strengths and weaknesses of the protocol are discussed along with its potential for scoring large numbers of protein sequences and thus its significance in protein engineering.

Repurposing designed mutants: a valuable strategy for computer-aided laccase engineering – the case of POXA1b

The broad specificity of laccases, a direct consequence of their shallow binding site, makes this class of enzymes a suitable template to build specificity toward putative substrates.

Electrochemical Characterization of Graphene and MWCNT Screen-Printed Electrodes Modified with AuNPs for Laccase Biosensor Development

The aim of this work is to show how the integration of gold nanoparticles (AuNPs) into multi-wall-carbon-nanotubes (MWCNTs) based screen-printed electrodes and into graphene-based screen-printed electrodes (GPHs) could represent a potential way to further enhance the electrochemical properties of those electrodes based on nanoparticles. Laccase from Trametes versicolor (TvL) was immobilized over MWCNTs and GPH previously modified with AuNPs (of 5 and 10 nm). The characterization of the modified electrode surface has been carried out by cyclic voltammetry. The results showed that the use of AuNPs for modification of both graphene and MWCNTs screen-printed electrode surfaces would increase the electrochemical performances of the electrodes. MWCNTs showed better results than GPH in terms of higher electroactive area formation after modification with AuNPs. The two modified nanostructured electrodes were successively proven to efficiently immobilize the TvL; the electrochemical sensing properties of the GPH- and MWCNT-based AuNPs-TvL biosensors were investigated by choosing 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic-acid diammonium salt (ABTS), catechol and caffeic acid as laccase mediators; and the kinetic parameters of the laccase biosensor were carefully evaluated.

Functional characterization of a yellow laccase from Leucoagaricus gongylophorus

In this work we have identified, using mass spectrometry, two laccases produced by Leucoagaricus gongylophorus. One of them, Lac1Lg, was isolated, purified and characterized. Lac1Lg, a monomeric enzyme, was studied using ABTS and syringaldazine substrates. Lac1Lg presented kcat/Km almost threefold higher for syringaldazine than for ABTS, showing a higher catalytic efficiency of Lac1Lg for syringaldazine. The interference of several metal ions and substances in the laccase activity were evaluated. Lac1Lg did not absorb at 600 nm, which is a characteristic of so-called yellow laccases. Lac1Lg also was able to oxidize non-phenolic substrate (anthracene) in the absence of an exogenous mediator, showing that the enzyme has potential to explore in biotechnological processes. Our Lac1Lg three-dimensional molecular model, constructed using homology modeling, showed that the Lac1Lg catalytic site is very closed to blue laccases.

Insights into Laccase Engineering from Molecular Simulations: Toward a Binding-Focused Strategy

Understanding the molecular determinants of enzyme performance is of primary importance for the rational design of ad hoc mutants. A novel approach, which combines efficient conformational sampling and quick reactivity scoring, is used here to shed light on how substrate oxidation was improved during the directed evolution experiment of a fungal laccase (from Pycnoporus cinnabarinus), an industrially relevant class of oxidoreductases. It is found that the enhanced activity of the evolved enzyme is mainly the result of substrate arrangement in the active site, with no important change in the redox potential of the T1 copper. Mutations at the active site shift the binding mode into a more buried substrate position and provide a more favorable electrostatic environment for substrate oxidation. As a consequence, engineering the binding event seems to be a viable way to in silico evolution of oxidoreductases.

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

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

Laccase engineering by rational and evolutionary design

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

Electron transfer and reaction mechanism of laccases

Laccases are part of the family of multicopper oxidases (MCOs), which couple the oxidation of substrates to the four electron reduction of O2 to H2O. MCOs contain a minimum of four Cu's divided into Type 1 (T1), Type 2 (T2), and binuclear Type 3 (T3) Cu sites that are distinguished based on unique spectroscopic features. Substrate oxidation occurs near the T1, and electrons are transferred approximately 13 Å through the protein via the Cys-His pathway to the T2/T3 trinuclear copper cluster (TNC), where dioxygen reduction occurs. This review outlines the electron transfer (ET) process in laccases, and the mechanism of O2 reduction as elucidated through spectroscopic, kinetic, and computational data. Marcus theory is used to describe the relevant factors which impact ET rates including the driving force, reorganization energy, and electronic coupling matrix element. Then, the mechanism of O2 reaction is detailed with particular focus on the intermediates formed during the two 2e(-) reduction steps. The first 2e(-) step forms the peroxide intermediate, followed by the second 2e(-) step to form the native intermediate, which has been shown to be the catalytically relevant fully oxidized form of the enzyme.

Effect of the L499M mutation of the ascomycetous Botrytis aclada laccase on redox potential and catalytic properties

Laccases are members of a large family of multicopper oxidases that catalyze the oxidation of a wide range of organic and inorganic substrates accompanied by the reduction of dioxygen to water. These enzymes contain four Cu atoms per molecule organized into three sites: T1, T2 and T3. In all laccases, the T1 copper ion is coordinated by two histidines and one cysteine in the equatorial plane and is covered by the side chains of hydrophobic residues in the axial positions. The redox potential of the T1 copper ion influences the enzymatic reaction and is determined by the nature of the axial ligands and the structure of the second coordination sphere. In this work, the laccase from the ascomycete Botrytis aclada was studied, which contains conserved Ile491 and nonconserved Leu499 residues in the axial positions. The three-dimensional structures of the wild-type enzyme and the L499M mutant were determined by X-ray crystallography at 1.7 Å resolution. Crystals suitable for X-ray analysis could only be grown after deglycosylation. Both structures did not contain the T2 copper ion. The catalytic properties of the enzyme were characterized and the redox potentials of both enzyme forms were determined: E0 = 720 and 580 mV for the wild-type enzyme and the mutant, respectively. Since the structures of the wild-type and mutant forms are very similar, the change in the redox potential can be related to the L499M mutation in the T1 site of the enzyme.

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

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