Mn(II) oxidation is the principal function of the extracellular Mn-peroxidase from Phanerochaete chrysosporium

Functional metalloenzymes based on myoglobin and neuroglobin that exploit covalent interactions

Globins, such as myoglobin (Mb) and neuroglobin (Ngb), are ideal protein scaffolds for the design of functional metalloenzymes. To date, numerous approaches have been developed for enzyme design. This review presents a summary of the progress made in the design of functional metalloenzymes based on Mb and Ngb, with a focus on the exploitation of covalent interactions, including coordination bonds and covalent modifications. These include the construction of a metal-binding site, the incorporation of a non-native metal cofactor, the formation of Cys/Tyr-heme covalent links, and the design of disulfide bonds, as well as other Cys-covalent modifications. As exemplified by recent studies from our group and others, the designed metalloenzymes have potential applications in biocatalysis and bioconversions. Furthermore, we discuss the current trends in the design of functional metalloenzymes and highlight the importance of covalent interactions in the design of functional metalloenzymes.

DypB peroxidase for aflatoxin removal: New insights into the toxin degradation process

Aflatoxin B1 (AFB1) is one of the most potent carcinogens and a widespread food and feed contaminant. As for other toxins, many efforts are devoted to find efficient and environmentally-friendly methods to degrade AFB1, such as enzymatic treatments, thus improving the safety of food and feed products. In this regard, the dye decolorizing peroxidase of type B (DypB) can efficiently degrade AFB1. The molecular mechanism, which is required to drive protein optimization in view of the usage of DypB as a mycotoxin reduction agent in large scale application, is unknown. Here, we focused on the role of four DypB residues in the degradation of AFB1 by alanine-scanning (residues 156, 215, 239 and 246), which were identified from biochemical assays to be kinetically relevant for the degradation. As a result of DypB degradation, AFB1 is converted into four products. Interestingly, the relative abundancy of these products depends on the replaced residues. Molecular dynamics simulations were used to investigate the role of these residues in the binding step between protein and manganese, a metal ion which is expected to be involved in the degradation process. We found that the size of the haem pocket as well as conformational changes in the protein structure could play a role in determining the kinetics of AFB1 removal and, consequently, guide the process towards specific degradation products.

Sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms in the aquatic environment

Microorganisms can oxidize Mn(II) to biogenic Mn oxides (BioMnOx), through enzyme-mediated processes and non-enzyme-mediated processes, which are generally considered as the source and sink of heavy metals due to highly reactive to sequestrate and oxidize heavy metals. Hence, the summary of interactions between Mn(II) oxidizing microorganisms (MnOM) and heavy metals is benefit for further work on microbial-mediated self-purification of water bodies. This review comprehensively summarizes the interactions between MnOM and heavy metals. The processes of BioMnOx production by MnOM has been firstly discussed. Moreover, the interactions between BioMnOx and various heavy metals are critically discussed. On the one hand, modes for heavy metals adsorbed on BioMnOx are summarized, such as electrostatic attraction, oxidative precipitation, ion exchange, surface complexation, and autocatalytic oxidation. On the other hand, adsorption and oxidation of representative heavy metals based on BioMnOx/Mn(II) are also discussed. Thirdly, the interactions between MnOM and heavy metals are also focused on. Finally, several perspectives which will contribute to future research are proposed. This review provides insight into the sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms. It might be helpful to understand the geochemical fate of heavy metals in the aquatic environment and the process of microbial-mediated water self-purification.

Involvement of Bacterial and Fungal Extracellular Products in Transformation of Manganese-Bearing Minerals and Its Environmental Impact

Manganese oxides are considered an essential component of natural geochemical barriers due to their redox and sorptive reactivity towards essential and potentially toxic trace elements. Despite the perception that they are in a relatively stable phase, microorganisms can actively alter the prevailing conditions in their microenvironment and initiate the dissolution of minerals, a process that is governed by various direct (enzymatic) or indirect mechanisms. Microorganisms are also capable of precipitating the bioavailable manganese ions via redox transformations into biogenic minerals, including manganese oxides (e.g., low-crystalline birnessite) or oxalates. Microbially mediated transformation influences the (bio)geochemistry of manganese and also the environmental chemistry of elements intimately associated with its oxides. Therefore, the biodeterioration of manganese-bearing phases and the subsequent biologically induced precipitation of new biogenic minerals may inevitably and severely impact the environment. This review highlights and discusses the role of microbially induced or catalyzed processes that affect the transformation of manganese oxides in the environment as relevant to the function of geochemical barriers.

Direct observation of Mn distribution/speciation within and surrounding a basidiomycete fungus in the production of Mn-oxides important in toxic element containment

Biogenic manganese (Mn) oxides occur ubiquitously in the environment including the uranium (U) mill tailings at the Ningyo-toge U mine in Okayama, Japan, being important in the sequestration of radioactive radium. To understand the nanoscale processes in Mn oxides formation at the U mill tailings site, Mn²⁺ absorption by a basidiomycete fungus, Coprinopsis urticicola, isolated from Ningyo-toge mine water samples, was investigated in the laboratory under controlled conditions utilizing electron microscopy, synchrotron-based X-ray analysis, and fluorescence microscopy with a molecular pH probe. The fungus' growth was first investigated in an agar-solidified medium supplemented with 1.0 mmol/L Mn²⁺, and Cu²⁺ (0-200 μM), Zn²⁺ (0-200 μM), or diphenyleneiodonium (DPI) chloride (0-100 μM) at 25 °C. The results revealed that Zn²⁺ has no significant effects on Mn oxide formation, whereas Cu²⁺ and DPI significantly inhibit both fungal growth and Mn oxidation, indicating superoxide-mediated Mn oxidation. Indeed, nitroblue tetrazolium and diaminobenzidine assays on the growing fungus revealed the production of superoxide and peroxide. During the interaction of Mn²⁺ with the fungus in solution medium at the initial pH of 5.67, a small fraction of Mn²⁺ infiltrated the fungal hyphae within 8 h, forming a few tens of nm-sized concentrates of soluble Mn²⁺ in the intracellular pH of ∼6.5. After 1 day of incubation, Mn oxides began to precipitate on the hyphae, which were characterized as fibrous nanocrystals with a hexagonal birnessite-structure, these forming spherical aggregates with a diameter of ∼1.5 μm. These nanoscale processes associated with the fungal species derived from the Ningyo-toge mine area provide additional insights into the existing mechanisms of Mn oxidation by filamentous fungi at other U mill tailings sites under circumneutral pH conditions. Such processes add to the class of reactions important to the sequestration of toxic elements.

Study on the effects of environmental factors on enzyme activities during growth of Hypsizygus marmoreus

The sensitivity of Hypsizygus marmoreus to environmental factors such as temperature, humidity, illumination and CO₂ concentration varies greatly in different growth stages. In this paper, the effects of various environmental factors on the growth and development of H. marmoreus were investigated by measuring the enzyme activities of H. marmoreus at different growth stages under different microenvironment conditions in the mushroom room, so as to confirm the influence mechanism of environmental factors on the growth of H. marmoreus. The results showed that at budding stage xylanase and laccase were found significantly positively correlated with CO₂ concentration and light intensity, and dramatically negatively correlated with humidity while carboxymethyl cellulose and manganese peroxidase were markedly positively correlated with humidity, and significantly negatively correlated with CO₂ concentration and light intensity. On the other hand, in mature fruit bodies xylanase activity was found significantly positively correlated with CO₂ concentration and light intensity, and dramatically negatively correlated with humidity while manganese peroxidase activities were found significantly positively correlated with humidity, and dramatically negatively correlated with light intensity. The activity of β-glucosidase in budding and mature fruiting bodies was markedly negatively correlated with CO₂ concentration and significantly positively correlated with humidity.

Contemporary proteomic research on lignocellulosic enzymes and enzymolysis: A review

This review overviewed the current researches on the isolation of novel strains, the development of novel identification protocols, the key enzymes and their synergistic interactions with other functional enzyme systems, and the strategies for enhancing enzymolysis efficiencies. The main obstacle for realizing biorefinery of lignocellulosic biomass to biofuels or biochemicals is the high cost of enzymolysis stage. Therefore, research prospects to reduce the costs for lignocellulose hydrolysis were outlined.

Effect of manganese peroxidase on the decomposition of cellulosic components: Direct cellulolytic activity and synergistic effect with cellulase

Herein, it was unearthed that manganese peroxidase (MnP) from Phanerochaete chrysosporium, a lignin-degrading enzyme, is capable of not only directly decomposing cellulosic components but also boosting cellulase activity. MnP decomposes various cellulosic substrates (carboxymethyl cellulose, cellobiose [CMC], and Avicel®) and produces reducing sugars rather than oxidized sugars such as lactone and ketoaldolase. MnP with Mn^II in acetate buffer evolves the Mn^III-acetate complex functioning as a strong oxidant, and the non-specificity of Mn^III-acetate enables cellulose-decomposition. The catalytic mechanism was proposed by analyzing catalytic products derived from MnP-treated cellopentaose. Notably, MnP also boosts cellulase activity on CMC and Avicel®, even considering the cellulolytic activity of MnP itself. To the best of the authors' knowledge, this is the first report demonstrating a previously unknown fungal MnP activity in cellulose-decomposition in addition to a known delignification activity. Consequently, the results provide a promising insight for further investigation of the versatility of lignin-degrading biocatalysts.

Bioprospection of ligninolytic enzymes from marine origin filamentous fungi

Fungi are excellent producers of extracellular enzymes. Therefore, the present study aimed to investigate the screening of marine fungi, which are laccase and manganese peroxidase potential producers, in solid fermentation for future applications in bioremediation processes of contaminated sites. For this purpose, two-level factorial planning was adopted, using time (6 and 15 days) and the absence or presence of oil (0 and 1%) as factors. The semi-quantitative evaluation was carried out by calculating radial growth, enzyme activity and enzyme index by measuring phenol red or syringaldazine oxidation halo. The results showed that all the studied strains showed a positive result for manganese peroxidase production, with an enzymatic activity in solid medium less than 0.61, indicating a strongly positive activity. Through the enzyme index, the study also showed prominence for Penicillium sp. strains, with values > 2. The enzyme index increase in oil presence and the inexpressive use of the genera studied for ligninolytic enzymes production from crude oil demonstrated these data importance for fermentative processes optimization. Considering the ability of these strains to develop into recalcitrant compounds and the potential for manganese peroxidase production, they are indicated for exploitation in various bioremediation technologies, as well as other biotechnological applications.

Free radical induced activity of an anthracycline analogue and its MnII complex on biological targets through in situ electrochemical generation of semiquinone

Cytotoxicity by anthracycline antibiotics is attributed to several pathways. Important among them are formation of free-radical intermediates. However, their generation makes anthracyclines cardiotoxic which is a concern on their use as anticancer agents. Hence, any change in redox behavior that address cardiotoxicity is welcome. Modulation of redox behavior raises the fear that cytotoxicity could be compromised. Regarding the generation of free radical intermediates on anthracyclines, a lot depends on the surrounding environment (oxic or anoxic), polarity and pH of the medium. In case of anthracyclines, one-electron reduction to semiquinone or two-electron reduction to quinone-dianion are crucial both for cytotoxicity and for cardiotoxic side effects. The disproportion-comproportionation equilibria at play between quinone-dianion, free quinone and semiquinone control biological activity. Whatever is the form of reduction, semiquinones are generated as a consequence of the presence of anthracyclines and these interact with a biological target. Alizarin, a simpler anthracycline analogue and its Mn^II complex were subjected to electrochemical reduction to realize what happens when anthracyclines are reduced by compounds present in cells as members of the electron transport chain. Glassy carbon electrode maintained at the pre-determined reduction potential of a compound was used for reduction of the compounds. Nucleobases and calf thymus DNA that were maintained in immediate vicinity of such radical generation were used as biological targets. Changes due to the generated species under aerated/de-aerated conditions on nucleobases and on DNA helps one to realize the process by which alizarin and its Mn^II complex might affect DNA. The study reveals alizarin was more effective on nucleobases than the complex in the free radical pathway. Difference in damage caused by alizarin and the Mn^II complex on DNA is comparatively less than that observed on nucleobases; the complex makes up for any inefficacy in the free radical pathway by its other attributes.

Manganese peroxidase mediated oxidation of sulfamethoxazole: Integrating the computational analysis to reveal the reaction kinetics, mechanistic insights, and oxidation pathway

In this study, manganese peroxidase (MnP) was applied to induce the in vitro oxidation of sulfamethoxazole (SMX). The results indicated that 87.04% of the SMX was transformed and followed first-order kinetics (k_obs=0.438 h^-1) within 6 h when 40 U L^-1 of MnP was added. The reaction kinetics were investigated under different conditions, including pH, MnP activity, and H₂O₂ concentration. The active species Mn³⁺ was responsible for the oxidation of SMX, and the Mn³⁺ production rate was monitored to reveal the interaction among MnP, Mn³⁺, and SMX. By integrating the characterizations analysis of the MnP/H₂O₂ system with the density functional theory (DFT) calculations, the proton-coupled electron transfer (PCET) process dominated the catalytic circle of MnP and the transformation of Mn³⁺. Additionally, possible oxidation pathways of SMX were proposed based on single-electron transfer mechanism, which primarily included the S-N bond cleavage, the C-S bond cleavage, and one electron loss without bond breakage. It was then transformed to hydrolysis, N-H oxidation, self-coupling, and carboxylic acid coupling products. This study provides insights into the atomic-level mechanism of MnP and the transformation pathways of sulfamethoxazole, which lays a significant foundation for the potential of MnP in wastewater treatment applications.

Mechanisms of Manganese(II) Oxidation by Filamentous Ascomycete Fungi Vary With Species and Time as a Function of Secretome Composition

Manganese (Mn) oxides are among the strongest oxidants and sorbents in the environment, and Mn(II) oxidation to Mn(III/IV) (hydr)oxides includes both abiotic and microbially-mediated processes. While white-rot Basidiomycete fungi oxidize Mn(II) using laccases and manganese peroxidases in association with lignocellulose degradation, the mechanisms by which filamentous Ascomycete fungi oxidize Mn(II) and a physiological role for Mn(II) oxidation in these organisms remain poorly understood. Here we use a combination of chemical and in-gel assays and bulk mass spectrometry to demonstrate secretome-based Mn(II) oxidation in three phylogenetically diverse Ascomycetes that is mechanistically distinct from hyphal-associated Mn(II) oxidation on solid substrates. We show that Mn(II) oxidative capacity of these fungi is dictated by species-specific secreted enzymes and varies with secretome age, and we reveal the presence of both Cu-based and FAD-based Mn(II) oxidation mechanisms in all 3 species, demonstrating mechanistic redundancy. Specifically, we identify candidate Mn(II)-oxidizing enzymes as tyrosinase and glyoxal oxidase in Stagonospora sp. SRC1lsM3a, bilirubin oxidase in Stagonospora sp. and Paraconiothyrium sporulosum AP3s5-JAC2a, and GMC oxidoreductase in all 3 species, including Pyrenochaeta sp. DS3sAY3a. The diversity of the candidate Mn(II)-oxidizing enzymes identified in this study suggests that the ability of fungal secretomes to oxidize Mn(II) may be more widespread than previously thought.

Degradation of the Organochlorinated Herbicide Diuron by Rainforest Basidiomycetes

The main organochlorinated compounds used on agricultural crops are often recalcitrant, affecting nontarget organisms and contaminating rivers or groundwater. Diuron (N-(3,4-dichlorophenyl)-N′,N′-dimethylurea) is a chlorinated herbicide widely used in sugarcane plantations. Here, we evaluated the ability of 13 basidiomycete strains of growing in a contaminated culture medium and degrading the xenobiotic. Dissipation rates in culture medium with initial 25 mg/L of diuron ranged from 7.3 to 96.8%, being Pluteus cubensis SXS 320 the most efficient strain, leaving no detectable residues after diuron metabolism. Pycnoporus sanguineus MCA 16 removed 56% of diuron after 40 days of cultivation, producing three metabolites more polar than parental herbicide, two of them identified as being DCPU and DCPMU. Despite of the strong inductive effect of diuron upon laccase synthesis and secretion, the application of crude enzymatic extracts of P. sanguineus did not catalyzed the breakdown of the herbicide in vitro, indicating that diuron biodegradation was not related to this oxidative enzyme.

Multitargeting Antibacterial Activity of a Synthesized Mn2+ Complex of Curcumin on Gram-Positive and Gram-Negative Bacterial Strains

Curcumin is an important molecule with a plethora of pharmacological activities and therapeutic potentials. Despite its efficacy, it remained a potential drug candidate owing to hydrolytic instability and poor aqueous solubility. To overcome the limitations related to low solubility, low bioavailability, and the fact that curcumin is never present in solution as a "single unit", its complex was prepared with MnII with the idea that binding to a metal ion might help to resolve these issues. The complex was characterized by elemental and spectral analysis. The structure of the complex was determined by density functional theory calculations. The complex was stable at physiological buffer conditions, unlike curcumin. It did not have any detrimental effect on mammalian cells. There was a significant enhancement in the antibacterial activity of the complex compared to curcumin against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. It showed a strong affinity for deoxyribonucleic acid (DNA) evident from a high binding constant value with calf thymus DNA and also from the retarded electrophoretic mobility of bacterial plasmid DNA. The complex showed "superoxide dismutase-like" activity leading to the generation of reactive oxygen species (ROS). The complex caused bacterial membrane perturbation evident from calcein leakage assay, which was further corroborated by scanning and transmission electron microscopic experiments. Overall, the present study shows improved stability and antibacterial potency of a nontoxic complex over curcumin. Its multitargeting mode of action such as ROS-production, effective binding with DNA, and permeabilization of bacterial membrane together allows it to be an effective antibacterial agent that could be taken further for therapeutic use against bacterial infections.

Molecular Cloning and Heterologous Expression of Manganese(II)-Oxidizing Enzyme from Acremonium strictum Strain KR21-2

Diverse ascomycete fungi oxidize manganese(II) [Mn(II)] and produce Mn(III, IV) oxides in terrestrial and freshwater environments. Although multicopper oxidase (MCO) is considered to be a key catalyst in mediating Mn(II) oxidation in ascomycetes, the responsible gene and its product have not been identified. In this study, a gene, named mco1, encoding Mn(II)-oxidizing MCO from Acremonium strictum strain KR21-2 was cloned and heterologously expressed in the methylotrophic yeast Pichia pastoris. Based on the phylogenetic relationship, similarity of putative copper-binding motifs, and homology modeling, the gene product Mco1 was assigned to a bilirubin oxidase. Mature Mco1 was predicted to be composed of 565 amino acids with a molecular mass of 64.0 kDa. The recombinant enzyme oxidized Mn(II) to yield spherical Mn oxides, several micrometers in diameter. Zinc(II) ions added to the reaction mixture were incorporated by the Mn oxides at a Zn/Mn molar ratio of 0.36. The results suggested that Mco1 facilitates the growth of the micrometer-sized Mn oxides and affects metal sequestration through Mn(II) oxidation. This is the first report on heterologous expression and identification of the Mn(II) oxidase enzyme in Mn(II)-oxidizing ascomycetes. The cell-free, homogenous catalytic system with recombinant Mco1 could be useful for understanding Mn biomineralization by ascomycetes and the sequestration of metal ions in the environment

Functional Expression and One-Step Protein Purification of Manganese Peroxidase 1 (rMnP1) from Phanerochaete chrysosporium Using the E. coli-Expression System

Manganese peroxidases (MnP) from the white-rot fungi Phanerochaete chrysosporium catalyse the oxidation of Mn²⁺ to Mn³⁺, a strong oxidizer able to oxidize a wide variety of organic compounds. Different approaches have been used to unravel the enzymatic properties and potential applications of MnP. However, these efforts have been hampered by the limited production of native MnP by fungi. Heterologous expression of MnP has been achieved in both eukaryotic and prokaryotic expression systems, although with limited production and many disadvantages in the process. Here we described a novel molecular approach for the expression and purification of manganese peroxidase isoform 1 (MnP1) from P. chrysosporium using an E. coli-expression system. The proposed strategy involved the codon optimization and chemical synthesis of the MnP1 gene for optimised expression in the E. coli T7 shuffle host. Recombinant MnP1 (rMnP1) was expressed as a fusion protein, which was recovered from solubilised inclusion bodies. rMnP1 was purified from the fusion protein using intein-based protein purification techniques and a one-step affinity chromatography. The designated strategy allowed production of an active enzyme able to oxidize guaiacol or Mn²⁺.

Mn oxide formation by phototrophs: Spatial and temporal patterns, with evidence of an enzymatic superoxide-mediated pathway

Manganese (Mn) oxide minerals influence the availability of organic carbon, nutrients and metals in the environment. Oxidation of Mn(II) to Mn(III/IV) oxides is largely promoted by the direct and indirect activity of microorganisms. Studies of biogenic Mn(II) oxidation have focused on bacteria and fungi, with phototrophic organisms (phototrophs) being generally overlooked. Here, we isolated phototrophs from Mn removal beds in Pennsylvania, USA, including fourteen Chlorophyta (green algae), three Bacillariophyta (diatoms) and one cyanobacterium, all of which consistently formed Mn(III/IV) oxides. Isolates produced cell-specific oxides (coating some cells but not others), diffuse biofilm oxides, and internal diatom-specific Mn-rich nodules. Phototrophic Mn(II) oxidation had been previously attributed to abiotic oxidation mediated by photosynthesis-driven pH increases, but we found a decoupling of Mn oxide formation and pH alteration in several cases. Furthermore, cell-free filtrates of some isolates produced Mn oxides at specific time points, but this activity was not induced by Mn(II). Manganese oxide formation in cell-free filtrates occurred via reaction with the oxygen radical superoxide produced by soluble extracellular proteins. Given the known widespread ability of phototrophs to produce superoxide, the contribution of phototrophs to Mn(II) oxidation in the environment may be greater and more nuanced than previously thought.

Production of active manganese peroxidase in Escherichia coli by co-expression of chaperones and in vitro maturation by ATP-dependent chaperone release

Manganese peroxidase (MnP) is a fungal heme-containing enzyme which oxidizes Mn²⁺ to Mn³⁺, a diffusible and strong non-specific oxidant capable of attacking bulky phenolic substrates. Therefore, MnP is indispensable in the polymer and paper industries. Previous attempts of MnP expression in Escherichia coli resulted in the formation of inclusion bodies which required in vitro refolding. Aiming to investigate the bacterial production of MnP, we have revealed an interesting mechanism underlying chaperone-assisted maturation of this enzyme to its active form. Since we previously found that in vitro expression of MnP in E. coli system depends on disulfide bond isomerase DsbC, we chose SHuffle T7 Express, an E. coli constitutively expressing DsbC, as the host for in vivo expression of MnP. Initially, only a low amount of the enzyme was present in the soluble fraction, with no detectable peroxidase activity. Co-expression of MnP with different chaperone revealed that DnaK, DnaJ, and GrpE contributed the most to the solubility improvement, however, remained in a complex with the MnP, preventing the enzyme to assume its active conformation. We resolved this by in vitro maturation, involving incubation of the MnP-chaperone complex with hemin, ATP, and ATP regeneration system. While ATP enables the chaperones to finish the refolding cycle and release the MnP in its correctly folded form, hemin supports the formation of the holo-enzyme with fully recovered peroxidase activity. We believe that the findings of this paper will serve as an important clue for establishing the bacterial production of fungal peroxidases in the future.

Emerging contaminants of high concern and their enzyme-assisted biodegradation - A review

The widespread occurrence and adverse environmental and health-related impacts of various types of emerging contaminants (ECs) have become an issue of high concern. With ever increasing scientific knowledge, socio-economic awareness, health-related problems and ecological apprehensions, people are more concerned about the widespread ECs, around the globe. Among ECs, biologically active compounds from pharmaceutical, cosmeceutical, biomedical, personal care products (PPCPs), endocrine-disrupting chemicals (EDCs), and flame-retardants are of paramount concern. The presence and persistence of ECs in water bodies are of continued and burning interest, worldwide. Various types of ECs are being discharged knowingly/unknowingly with/without partial treatments into the aquatic environments that pose serious health issues and affects the entire living ecosystem. So far, various approaches have been developed for ECs degradation and removal to diminish their adverse impact. Many previous and/or ongoing studies have focused on contaminants degradation and efficient removal via numerous treatment strategies, i.e. (1) physical, (2) chemical and (3) biological. However, the experimental evidence is lacking to enable specific predictions about ECs mechanistic degradation and removal fate across various in-practice systems. In this context, the deployment oxidoreductases such as peroxidases (lignin peroxidases, manganese-dependent peroxidases, and horseradish peroxidase), aromatic dioxygenases, various oxygenases, laccases, and tyrosinases have received considerable research attention. Immobilization is highlighted as a promising approach to improve enzyme catalytic performance and stabilization, as well as, to protect the three-dimensional structure of the enzyme against the undesirable consequences of harsh reaction environment. This work overviews the current and state-of-the-art critical aspect related to hazardous pollutants at large and ECs in particular by the immobilized oxidoreductase enzymes. The first part of the review focuses on the occurrence, physiochemical behavior, potent sources and significant routes of ECs. Following that, environmentally-related adverse impacts and health-related issues of ECs are discussed in the second part. In the third part, biodegradation and removal strategies with a comparative overview of several conventional vs. non-conventional methods are presented briefly. The fourth part majorly focuses on operational modes of different oxidoreductase enzyme-based biocatalytic processes for the biodegradation and biotransformation of a wide array of harmful environmental contaminants. Finally, the left behind research gaps, concluding remarks as well as future trends and recommendations in the use of carrier-immobilized oxidoreductases for environmental perspective are also discussed.

Linking Enzymatic Oxidative Degradation of Lignin to Organics Detoxification

The major enzymes involved in lignin degradation are laccase, class II peroxidases (lignin peroxidase, manganese peroxidase, and versatile peroxidase) and dye peroxidase, which use an oxidative or peroxidative mechanism to deconstruct the complex and recalcitrant lignin. Laccase and manganese peroxidase directly oxidize phenolic lignin components, while lignin peroxidase and versatile peroxidase can act on the more recalcitrant non-phenolic lignin compounds. Mediators or co-oxidants not only increase the catalytic ability of these enzymes, but also largely expand their substrate scope to those with higher redox potential or more complicated structures. Neither laccase nor the peroxidases are stringently selective of substrates. The promiscuous nature in substrate preference can be employed in detoxification of a range of organics.

Fungi-Based Microbial Fuel Cells

Fungi are among the microorganisms able to generate electricity as a result of their metabolic processes. Throughout the last several years, a large number of papers on various microorganisms for current production in microbial fuel cells (MFCs) have been published; however, fungi still lack sufficient evaluation in this regard. In this review, we focus on fungi, paying special attention to their potential applicability to MFCs. Fungi used as anodic or cathodic catalysts, in different reactor configurations, with or without the addition of an exogenous mediator, are described. Contrary to bacteria, in which the mechanism of electron transfer is pretty well known, the mechanism of electron transfer in fungi-based MFCs has not been studied intensively. Thus, here we describe the main findings, which can be used as the starting point for future investigations. We show that fungi have the potential to act as electrogens or cathode catalysts, but MFCs based on bacteria–fungus interactions are especially interesting. The review presents the current state-of-the-art in the field of MFC systems exploiting fungi.

Characterization and use of a bacterial lignin peroxidase with an improved manganese-oxidative activity

Peroxidases are well-known biocatalysts produced by all organisms, especially microorganisms, and used in a number of biotechnological applications. The enzyme DypB from the lignin-degrading bacterium Rhodococcus jostii was recently shown to degrade solvent-obtained fractions of a Kraft lignin. In order to promote the practical use, the N246A variant of DypB, named Rh_DypB, was overexpressed in E. coli using a designed synthetic gene: by employing optimized conditions, the enzyme was fully produced as folded holoenzyme, thus avoiding the need for a further time-consuming and expensive reconstitution step. By a single chromatographic purification step, > 100 mg enzyme/L fermentation broth with a > 90% purity was produced. Rh_DypB shows a classical peroxidase activity which is significantly increased by adding Mn²⁺ ions: kinetic parameters for H₂O₂, Mn²⁺, ABTS, and 2,6-DMP were determined. The recombinant enzyme shows a good thermostability (melting temperature of 63-65 °C), is stable at pH 6-7, and maintains a large part of the starting activity following incubation for 24 h at 25-37 °C. Rh_DypB activity is not affected by 1 M NaCl, 10% DMSO, and 5% Tween-80, i.e., compounds used for dye decolorization or lignin-solubilization processes. The enzyme shows broad dye-decolorization activity, especially in the presence of Mn²⁺, oxidizes various aromatic monomers from lignin, and cleaves the guaiacylglycerol-β-guaiacyl ether (GGE), i.e., the Cα-Cβ bond of the dimeric lignin model molecule of β-O-4 linkages. Under optimized conditions, 2 mM GGE was fully cleaved by recombinant Rh_DypB, generating guaiacol in only 10 min, at a rate of 12.5 μmol/min mg enzyme.

Influence of light on lignin-degrading activities of fungal genus Polyporus s. str

Six strains belonging to five species of Polyporus (P. arcularius, P. arcularioides, P. tricholoma, P. cfr. tricholoma, and P. varius), collected from an Atlantic Forest area in Misiones (Argentina), where species usually grow exposed to high temperatures and humidity, were identified by morphological and molecular analyses. P. tricholoma (BAFC 4536) and P. arcularioides (BAFC 4534) were selected by their lignin-degrading enzyme production, their ability to produce primordial of basidiomes under submerged fermentation, and the decrease in lignin content caused in Poplar wood (up to 29% after 45 days). Among several variables evaluated with a Plackett-Burman design (glucose, copper, vanillic acid and manganese concentration, incubation period, and light incidence), the most important factor affecting laccase and Mn-peroxidase (MnP) production by both strains, was light incidence. Light induced fruit body development but diminished laccase and MnP production. Moreover, a modified isoenzymatic laccase pattern was observed, showing additional isoenzymes when fungi were cultivated under darkness and differences in optimal temperature. Although the studied strains did not produce high laccase and MnP titers (uppermost detected 4230 and 90 U L^-1 , respectively), their laccases showed thermal stability and optimal temperature above 70 °C, representing an interesting source in the search of thermo-tolerant enzymes for biotechnological applications.

Developments in support materials for immobilization of oxidoreductases: A comprehensive review

Bioremediation, a biologically mediated transformation or degradation of persistent chemicals into nonhazardous or less-hazardous substances, has been recognized as a key strategy to control levels of pollutants in water and soils. The use of enzymes, notably oxidoreductases such as laccases, tyrosinases, various oxygenases, aromatic dioxygenases, and different peroxidases (all of EC class 1) is receiving significant research attention in this regard. It should be stated that immobilization is emphasized as a powerful tool for enhancement of enzyme activity and stability as well as for protection of the enzyme proteins against negative effects of harsh reaction conditions. As proper selection of support materials for immobilization and their performance is overlooked when it comes to comparing performance of immobilized enzyme in academic studies, this review summarizes the current state of knowledge regarding the materials used for enzyme immobilization of these oxidoreductase enzymes for environmental applications. In the presented study, thorough physicochemical characteristics of the support materials was presented. Moreover, various types of reactions and notably operational modes of enzymatic processes for biodegradation of harmful pollutants are summarized, and future trends in use of immobilized oxidoreductases for environmental applications are discussed. Our goal is to provide an improved foundation on which new technological advancements can be made to achieve efficient enzyme-assisted bioremediation.

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) by fungal enzymes: A review

Polycyclic aromatic hydrocarbons (PAHs) are a large group of chemicals. They represent an important concern due to their widespread distribution in the environment, their resistance to biodegradation, their potential to bioaccumulate and their harmful effects. Several pilot treatments have been implemented to prevent economic consequences and deterioration of soil and water quality. As a promising option, fungal enzymes are regarded as a powerful choice for degradation of PAHs. Phanerochaete chrysosporium, Pleurotus ostreatus and Bjerkandera adusta are most commonly used for the degradation of such compounds due to their production of ligninolytic enzymes such as lignin peroxidase, manganese peroxidase and laccase. The rate of biodegradation depends on many culture conditions, such as temperature, oxygen, accessibility of nutrients and agitated or shallow culture. Moreover, the addition of biosurfactants can strongly modify the enzyme activity. The removal of PAHs is dependent on the ionization potential. The study of the kinetics is not completely comprehended, and it becomes more challenging when fungi are applied for bioremediation. Degradation studies in soil are much more complicated than liquid cultures because of the heterogeneity of soil, thus, many factors should be considered when studying soil bioremediation, such as desorption and bioavailability of PAHs. Different degradation pathways can be suggested. The peroxidases are heme-containing enzymes having common catalytic cycles. One molecule of hydrogen peroxide oxidizes the resting enzyme withdrawing two electrons. Subsequently, the peroxidase is reduced back in two steps of one electron oxidation. Laccases are copper-containing oxidases. They reduce molecular oxygen to water and oxidize phenolic compounds.

Comparative Analysis of Secretome Profiles of Manganese(II)-Oxidizing Ascomycete Fungi

Fungal secretomes contain a wide range of hydrolytic and oxidative enzymes, including cellulases, hemicellulases, pectinases, and lignin-degrading accessory enzymes, that synergistically drive litter decomposition in the environment. While secretome studies of model organisms such as Phanerochaete chrysosporium and Aspergillus species have greatly expanded our knowledge of these enzymes, few have extended secretome characterization to environmental isolates or conducted side-by-side comparisons of diverse species. Thus, the mechanisms of carbon degradation by many ubiquitous soil fungi remain poorly understood. Here we use a combination of LC-MS/MS, genomic, and bioinformatic analyses to characterize and compare the protein composition of the secretomes of four recently isolated, cosmopolitan, Mn(II)-oxidizing Ascomycetes (Alternaria alternata SRC1lrK2f, Stagonospora sp. SRC1lsM3a, Pyrenochaeta sp. DS3sAY3a, and Paraconiothyrium sporulosum AP3s5-JAC2a). We demonstrate that the organisms produce a rich yet functionally similar suite of extracellular enzymes, with species-specific differences in secretome composition arising from unique amino acid sequences rather than overall protein function. Furthermore, we identify not only a wide range of carbohydrate-active enzymes that can directly oxidize recalcitrant carbon, but also an impressive suite of redox-active accessory enzymes that suggests a role for Fenton-based hydroxyl radical formation in indirect, non-specific lignocellulose attack. Our findings highlight the diverse oxidative capacity of these environmental isolates and enhance our understanding of the role of filamentous Ascomycetes in carbon turnover in the environment.

A greener approach for resource recycling: Manganese bioleaching

In view of unremitting diminution of mineral resources, rising energy economics along with increasing global consumption of Manganese (Mn), development of environment friendly technologies for tapping alternate sources of Mn has gained importance lately. Mn recovery from mining residues using conventional approaches is extremely expensive due to high capital and energy costs involved. However lean grade ores present in millions of tons awaits the development of competent and cost effective extractive process. Mn recovery by biomining with diverse microbes is thereby recommended as a superior and green alternative to the current pyro metallurgical techniques. The synergistic effects of different factors are known to influence microbial leaching of mineral ores which includes microbiological, mineralogical, physicochemical and process parameters. Bacterial bioleaching is mostly due to enzymatic influence, however fungal bioleaching is non enzymatic. Genomic studies on microbial diversity and an insight of its metabolic pathways provides unique dimension to the mechanism of biomining microorganisms. The extraction of Mn has a massive future prospective and will play a remarkable role in altering the situation of ever-decreasing grades of ore. This review aims to encompass the different aspects of Mn bioleaching, the plethora of organisms involved, the mechanisms driving the process and the recent trends and future prospects of this green technology.

Biological Low-pH Mn(II) Oxidation in a Manganese Deposit Influenced by Metal-Rich Groundwater

The mechanisms, key organisms, and geochemical significance of biological low-pH Mn(II) oxidation are largely unexplored. Here, we investigated the structure of indigenous Mn(II)-oxidizing microbial communities in a secondary subsurface Mn oxide deposit influenced by acidic (pH 4.8) metal-rich groundwater in a former uranium mining area. Microbial diversity was highest in the Mn deposit compared to the adjacent soil layers and included the majority of known Mn(II)-oxidizing bacteria (MOB) and two genera of known Mn(II)-oxidizing fungi (MOF). Electron X-ray microanalysis showed that romanechite [(Ba,H2O)2(Mn(4+),Mn(3+))5O10] was conspicuously enriched in the deposit. Canonical correspondence analysis revealed that certain fungal, bacterial, and archaeal groups were firmly associated with the autochthonous Mn oxides. Eight MOB within the Proteobacteria, Actinobacteria, and Bacteroidetes and one MOF strain belonging to Ascomycota were isolated at pH 5.5 or 7.2 from the acidic Mn deposit. Soil-groundwater microcosms demonstrated 2.5-fold-faster Mn(II) depletion in the Mn deposit than adjacent soil layers. No depletion was observed in the abiotic controls, suggesting that biological contribution is the main driver for Mn(II) oxidation at low pH. The composition and species specificity of the native low-pH Mn(II) oxidizers were highly adapted to in situ conditions, and these organisms may play a central role in the fundamental biogeochemical processes (e.g., metal natural attenuation) occurring in the acidic, oligotrophic, and metalliferous subsoil ecosystems.

IMPORTANCE

This study provides multiple lines of evidence to show that microbes are the main drivers of Mn(II) oxidation even at acidic pH, offering new insights into Mn biogeochemical cycling. A distinct, highly adapted microbial community inhabits acidic, oligotrophic Mn deposits and mediates biological Mn oxidation. These data highlight the importance of biological processes for Mn biogeochemical cycling and show the potential for new bioremediation strategies aimed at enhancing biological Mn oxidation in low-pH environments for contaminant mitigation.

The Role of Bacterial Spores in Metal Cycling and Their Potential Application in Metal Contaminant Bioremediation

Bacteria are one of the premier biological forces that, in combination with chemical and physical forces, drive metal availability in the environment. Bacterial spores, when found in the environment, are often considered to be dormant and metabolically inactive, in a resting state waiting for favorable conditions for them to germinate. However, this is a highly oversimplified view of spores in the environment. The surface of bacterial spores represents a potential site for chemical reactions to occur. Additionally, proteins in the outer layers (spore coats or exosporium) may also have more specific catalytic activity. As a consequence, bacterial spores can play a role in geochemical processes and may indeed find uses in various biotechnological applications. The aim of this review is to introduce the role of bacteria and bacterial spores in biogeochemical cycles and their potential use as toxic metal bioremediation agents.

A comparison of the sorption reactivity of bacteriogenic and mycogenic Mn oxide nanoparticles

Biogenic MnO2 minerals affect metal fate and transport in natural and engineered systems by strongly sorbing metals ions. The ability to produce MnO2 is widely dispersed in the microbial tree of life, leading to potential differences in the minerals produced by different organisms. In this study, we compare the structure and reactivity of biogenic Mn oxides produced by the biofilm-forming bacterium Pseudomonas putida GB-1 and the white-rot fungus Coprinellus sp. The rate of Mn(II) oxidation, and thus biomineral production, was 45 times lower for Coprinellus sp. (5.1 × 10(-2) mM d(-1)) than for P. putida (2.32 mM d(-1)). Both organisms produced predominantly Mn(IV) oxides with hexagonal-sheet symmetry, low sheet stacking, small particle size, and Mn(II/III) in the interlayer. However, we found that mycogenic MnO2 could support a significantly lower quantity of Ni sorbed via inner-sphere coordination at vacancy sites than the bacteriogenic MnO2: 0.09 versus 0.14 mol Ni mol(-1) Mn. In addition, 50-100% of the adsorbed Ni partitioned to the MnO2, which accounts for less than 20% of the sorbent on a mass basis. The vacancy content, which appears to increase with the kinetics of MnO2 precipitation, exerts significant control on biomineral reactivity.

Effect of Diffusion on Discoloration of Congo Red by Alginate Entrapped Turnip (Brassica rapa) Peroxidase

Enzymatic discoloration of the diazo dye, Congo red (CR), by immobilized plant peroxidase from turnip “Brassica rapa” is investigated. Partially purified turnip peroxidase (TP) was immobilized by entrapment in spherical particles of calcium alginate and was assayed for the discoloration of aqueous CR solution. Experimental data revealed that pH, reaction time, temperature, colorant, and H₂O₂ concentration play a significant role in dye degradation. Maximum CR removal was found at pH 2.0, constant temperature of 40°C in the presence of 10 mM H₂O₂, and 180 mg/L of CR. More than 94% of CR was removed by alginate immobilized TP after 1 h of incubation in a batch process under optimal conditions. About 74% removal efficiency was retained after four recycles. Diffusional limitations in alginate beads such as effectiveness factor η, Thiele modulus Φ, and effective diffusion coefficients (D_e) of Congo red were predicted assuming a first-order biodegradation kinetic. Results showed that intraparticle diffusion resistance has a significant effect on the CR biodegradation rate.

Profiling microbial communities in manganese remediation systems treating coal mine drainage

Water discharging from abandoned coal mines can contain extremely high manganese levels. Removing this metal is an ongoing challenge. Passive Mn(II) removal beds (MRBs) contain microorganisms that oxidize soluble Mn(II) to insoluble Mn(III/IV) minerals, but system performance is unpredictable. Using amplicon pyrosequencing, we profiled the bacterial, fungal, algal, and archaeal communities in four MRBs, performing at different levels, in Pennsylvania to determine whether they differed among MRBs and from surrounding soil and to establish the relative abundance of known Mn(II) oxidizers. Archaea were not detected; PCRs with archaeal primers returned only nontarget bacterial sequences. Fungal taxonomic profiles differed starkly between sites that remove the majority of influent Mn and those that do not, with the former being dominated by Ascomycota (mostly Dothideomycetes) and the latter by Basidiomycota (almost entirely Agaricomycetes). Taxonomic profiles for the other groups did not differ significantly between MRBs, but operational taxonomic unit-based analyses showed significant clustering by MRB with all three groups (P < 0.05). Soil samples clustered separately from MRBs in all groups except fungi, whose soil samples clustered loosely with their respective MRB. Known Mn(II) oxidizers accounted for a minor proportion of bacterial sequences (up to 0.20%) but a greater proportion of fungal sequences (up to 14.78%). MRB communities are more diverse than previously thought, and more organisms may be capable of Mn(II) oxidation than are currently known.