Laccases with Variable Properties from Different Strains of Steccherinum ochraceum: Does Glycosylation Matter?

Increasing Reduction Potentials of Type 1 Copper Center and Catalytic Efficiency of Small Laccase from Streptomyces coelicolor through Secondary Coordination Sphere Mutations

The key to type 1 copper (T1Cu) function lies in the fine tuning of the CuII/I reduction potential (E°'T1Cu ) to match those of its redox partners, enabling efficient electron transfer in a wide range of biological systems. While the secondary coordination sphere (SCS) effects have been used to tune E°'T1Cu in azurin over a wide range, these principles are yet to be generalized to other T1Cu-containing proteins to tune catalytic properties. To this end, we have examined the effects of Y229F, V290N and S292F mutations around the T1Cu of small laccase (SLAC) from Streptomyces coelicolor to match the high E°'T1Cu of fungal laccases. Using ultraviolet-visible absorption and electron paramagnetic resonance spectroscopies, together with X-ray crystallography and redox titrations, we have probed the influence of SCS mutations on the T1Cu and corresponding E°'T1Cu . While minimal and small E°'T1Cu increases are observed in Y229F- and S292F-SLAC, the V290N mutant exhibits a major E°'T1Cu increase. Moreover, the influence of these mutations on E°'T1Cu is additive, culminating in a triple mutant Y229F/V290N/S292F-SLAC with the highest E°'T1Cu of 556 mV vs. SHE reported to date. Further activity assays indicate that all mutants retain oxygen reduction reaction activity, and display improved catalytic efficiencies (kcat /KM ) relative to WT-SLAC.

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.

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.

Elucidating fungal Rigidoporus species FMD21 lignin-modifying enzyme genes and 2,3,7,8-tetrachlorodibenzo-p-dioxin degradation by laccase isozymes

White-rot fungus Rigidoporus sp. FMD21 is a lignin-modifying enzyme producing fungus that can degrade dioxin. Extracellular enzymes from FMD21 include laccase and manganese peroxidase which are promising enzymes for myco-remediation because of their wide substrate specificity and mild catalysis conditions. The FMD21 genome was sequenced using Ion Torrent technology and consists of 38.98 Mbps with a GC content of 47.4 %. Gene prediction using Augustus with Basidiomycota reference setting resulted in 8245 genes. Functional gene annotations were carried out by using several programs and databases. We focused on laccase and ligninolytic peroxidase genes, which are most likely involved in the degradation of aromatic pollutants. The genome of FMD21 contains 12 predicted laccase genes (10 out of 12 predicted as full length) and 13 putative ligninolytic peroxidases which were annotated as MnP or versatile peroxidases. Four predicted laccases showed a higher than 65 % binding chance to 2,3,7,8-TCDD with the highest at 72 % in in silico docking analysis. Heterologous expressed laccases showed activity towards three tested substrates included ABTS, guaiacol and 2,6-DMP. ABTS displayed two-stage oxidation which differed from natural FMD21 laccases. 2,3,7,8-TCDD was degraded by 50 % after two weeks of enzymatic treatment by three out of five laccase isozymes which were natural laccases secreted by FMD21. In this study, we provide direct evidence for the 2,3,7,8-TCDD biodegradation capability of fungal laccases.

Discovery of two novel laccase-like multicopper oxidases from Pleurotus citrinopileatus and their application in phenolic oligomer synthesis

BACKGROUND

Laccases and laccase-like multicopper oxidases (LMCOs) oxidize a vast array of phenolic compounds and amines, releasing water as a byproduct. Their low substrate specificity is responsible for their tremendous biotechnological interest, since they have been used for numerous applications. However, the laccases characterized so far correspond to only a small fraction of the laccase genes identified in fungal genomes. Therefore, the knowledge regarding the biochemistry and physiological role of minor laccase-like isoforms is still limited.

RESULTS

In the present work, we describe the isolation, purification and characterization of two novel LMCOs, PcLac1 and PcLac2, from Pleurotus citrinopileatus. Both LMCOs were purified with ion-exchange chromatographic methods. PcLac2 was found to oxidize a broader substrate range than PcLac1, but both LMCOs showed similar formal potentials, lower than those reported previously for laccases from white-rot fungi. Proteomic analysis of both proteins revealed their similarity with other well-characterized laccases from Pleurotus strains. Both LMCOs were applied to the oxidation of ferulic and sinapic acid, yielding oligomers with possible antioxidant activity.

CONCLUSIONS

Overall, the findings of the present work can offer new insights regarding the biochemistry and variability of low-redox potential laccases of fungal origin. Low-redox potential biocatalysts could offer higher substrate selectivity than their high-redox counterparts, and thus, they could be of applied value in the field of biocatalysis.

Protein Engineering Approaches to Enhance Fungal Laccase Production in S. cerevisiae

Laccases secreted by saprotrophic basidiomycete fungi are versatile biocatalysts able to oxidize a wide range of aromatic compounds using oxygen as the sole requirement. Saccharomyces cerevisiae is a preferred host for engineering fungal laccases. To assist the difficult secretion of active enzymes by yeast, the native signal peptide is usually replaced by the preproleader of S. cerevisiae alfa mating factor (MFα1). However, in most cases, only basal enzyme levels are obtained. During directed evolution in S. cerevisiae of laccases fused to the α-factor preproleader, we demonstrated that mutations accumulated in the signal peptide notably raised enzyme secretion. Here we describe different protein engineering approaches carried out to enhance the laccase activity detected in the liquid extracts of S. cerevisiae cultures. We demonstrate the improved secretion of native and engineered laccases by using the fittest mutated α-factor preproleader obtained through successive laccase evolution campaigns in our lab. Special attention is also paid to the role of protein N-glycosylation in laccase production and properties, and to the introduction of conserved amino acids through consensus design enabling the expression of certain laccases otherwise not produced by the yeast. Finally, we revise the contribution of mutations accumulated in laccase coding sequence (CDS) during previous directed evolution campaigns that facilitate enzyme production.

Purification and Characterization of Two Novel Laccases from Peniophora lycii

Although, currently, more than 100 laccases have been purified from basidiomycete fungi, the majority of these laccases were obtained from fungi of the Polyporales order, and only scarce data are available about the laccases from other fungi. In this article, laccase production by the white-rot basidiomycete fungus Peniophora lycii, belonging to the Russulales order, was investigated. It was shown that, under copper induction, this fungus secreted three different laccase isozymes. Two laccase isozymes—Lac5 and LacA—were purified and their corresponding nucleotide sequences were determined. Both purified laccases were relatively thermostable with periods of half-life at 70 °C of 10 and 8 min for Lac5 and LacA, respectively. The laccases demonstrated the highest activity toward ABTS (97 U·mg⁻¹ for Lac5 and 121 U·mg⁻¹ for LacA at pH 4.5); Lac5 demonstrated the lowest activity toward 2,6-DMP (2.5 U·mg⁻¹ at pH 4.5), while LacA demonstrated this towards gallic acid (1.4 U·mg⁻¹ at pH 4.5). Both Lac5 and LacA were able to efficiently decolorize such dyes as RBBR and Bromcresol Green. Additionally, phylogenetic relationships among laccases of Peniophora spp. were reconstructed, and groups of orthologous genes were determined. Based on these groups, all currently available data about laccases of Peniophora spp. were systematized.

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

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

Extracellular laccase from Monilinia fructicola: isolation, primary characterization and application

Laccases are enzymes belonging to the family of blue copper oxidases. Due to their broad substrate specificity, they are widely used in many industrial processes and environmental bioremediations for removal of a large number of pollutants. During last decades, laccases attracted scientific interest also as highly promising enzymes to be used in bioanalytics. The aim of this study is to obtain a highly purified laccase from an efficient fungal producer and to demonstrate the applicability of this enzyme for analytics and bioremediation. To select the best microbial source of laccase, a screening of fungal strains was carried out and the fungus Monilinia fructicola was chosen as a producer of an extracellular enzyme. Optimal cultivation conditions for the highest yield of laccase were established; the enzyme was purified by a column chromatography and partially characterized. Molecular mass of the laccase subunit was determined to be near 35 kDa; the optimal pH ranges for the highest activity and stability are 4.5-5.0 and 3.0-5.0, respectively; the optimal temperature for laccase activity is 30°C. Laccase preparation was successfully used as a biocatalyst in the amperometric biosensor for bisphenol A assay and in the bioreactor for bioremediation of some xenobiotics.

Substrate-Related Factors Affecting Cellulosome-Induced Hydrolysis for Lignocellulose Valorization

Cellulosomes are an extracellular supramolecular multienzyme complex that can efficiently degrade cellulose and hemicelluloses in plant cell walls. The structural and unique subunit arrangement of cellulosomes can promote its adhesion to the insoluble substrates, thus providing individual microbial cells with a direct competence in the utilization of cellulosic biomass. Significant progress has been achieved in revealing the structures and functions of cellulosomes, but a knowledge gap still exists in understanding the interaction between cellulosome and lignocellulosic substrate for those derived from biorefinery pretreatment of agricultural crops. The cellulosomic saccharification of lignocellulose is affected by various substrate-related physical and chemical factors, including native (untreated) wood lignin content, the extent of lignin and xylan removal by pretreatment, lignin structure, substrate size, and of course substrate pore surface area or substrate accessibility to cellulose. Herein, we summarize the cellulosome structure, substrate-related factors, and regulatory mechanisms in the host cells. We discuss the latest advances in specific strategies of cellulosome-induced hydrolysis, which can function in the reaction kinetics and the overall progress of biorefineries based on lignocellulosic feedstocks.