Spectroscopic Studies and Electronic Structure Description of the High Potential Type 1 Copper Site in Fungal Laccase: Insight into the Effect of the Axial Ligand

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.

Mycoremediation of lead and cadmium by lignocellulosic enzymes of Pleurotus eryngii

This study aimed to investigate the ability of Pleurotus eryngii fungus to absorb lead and cadmium from industrial wastewater. After culturing the fungus on a potato dextrose agar (PDA) medium containing 0 (control), 150 mg L-1, 250 mg L-1, and 350 mg L-1 concentrations of lead and cademium for 30 days, the mycelia were isolated from the culture medium and their extracts were used to measure protein content and the activity of antioxidant enzymes. Also, heavy metal contents were analyzed by atomic absorption spectrometry using flame photometry. Results showed that the growth of mycelia was significantly affected by different concentrations of the two heavy metals. High tolerance of heavy metal pollution in the culture media and the ability to accumulate lead and cademium confirmed that Pleurotus eryngii is a favorable option for mycoremediation. Also, molecular studies for fungal sequencing were investigated using the trench method, the sequence of the fungus was recorded in the gene bank, and finally the fungus was identified in the study.

Resonance Raman spectra of blue copper proteins: Variable temperature spectra of Thermus thermophilus HB27 laccase

The resonance Raman (rR) spectra of the oxidized type 1 copper active site (CuT1) in Thermus thermophilus HB27 laccase (Tth-lac) has been determined in the 20 to 80 °C temperature range using 633-nm excitation. The positions and relative intensities of rR peaks are virtually independent of temperature, indicating that CuT1 ligation is robust over the investigated range. The intensity-weighted average of Tth-lac Cu-SCys vibrations (<ν(Cu-SCys)>) = 423 cm-1) is higher than those of most cupredoxins but is comparable to those of other multicopper oxidases (MCOs). <ν(Cu-SCys)> values for Tth-lac and several CuT1 centers in cupredoxins and MCOs do not correlate well with Cu-SCys bond lengths but do exhibit systematic trends with redox thermodynamic properties. PROLOGUE: F. Ann Walker was a great scholar and dear friend. While at Columbia in the early 1960s, I (HBG) followed her graduate work at Brown on the effects of axial ligands on vanadyl ion EPR spectra. Dick Carlin, her thesis adviser, invited me to serve as external member of her thesis committee. I joined, made my way to Providence, met her just before the exam, and greatly admired (enjoyed!) her thoughtful responses to questions from physical chemists about metal-oxo electronic structures. Our friendship grew stronger over the years, enhanced by lively discussions of heme protein chemistry in San Francisco, Pasadena, Tucson, and at Gordon Research Conferences. Ann was a superstar in biological inorganic chemistry. She will be sorely missed but not forgotten.

Primary and Secondary Coordination Sphere Effects on the Structure and Function of S-Nitrosylating Azurin

Much progress has been made in understanding the roles of the secondary coordination sphere (SCS) in tuning redox potentials of metalloproteins. In contrast, the impact of SCS on reactivity is much less understood. A primary example is how copper proteins can promote S-nitrosylation (SNO), which is one of the most important dynamic post-translational modifications, and is crucial in regulating nitric oxide storage and transportation. Specifically, the factors that instill CuII with S-nitrosylating capabilities and modulate activity are not well understood. To address this issue, we investigated the influence of the primary and secondary coordination sphere on CuII-catalyzed S-nitrosylation by developing a series of azurin variants with varying catalytic capabilities. We have employed a multidimensional approach involving electronic absorption, S and Cu K-edge XAS, EPR, and resonance Raman spectroscopies together with QM/MM computational analysis to examine the relationships between structure and molecular mechanism in this reaction. Our findings have revealed that kinetic competency is correlated with three balancing factors, namely Cu-S bond strength, Cu spin localization, and relative S(ps) vs S(pp) contributions to the ground state. Together, these results support a reaction pathway that proceeds through the attack of the Cu-S bond rather than electrophilic addition to CuII or radical attack of SCys. The insights gained from this work provide not only a deeper understanding of SNO in biology but also a basis for designing artificial and tunable SNO enzymes to regulate NO and prevent diseases due to SNO dysregulation.

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.

Catalysis and Electron Transfer in De Novo Designed Metalloproteins

One of the hallmark advances in our understanding of metalloprotein function is showcased in our ability to design new, non-native, catalytically active protein scaffolds. This review highlights progress and milestone achievements in the field of de novo metalloprotein design focused on reports from the past decade with special emphasis on de novo designs couched within common subfields of bioinorganic study: heme binding proteins, monometal- and dimetal-containing catalytic sites, and metal-containing electron transfer sites. Within each subfield, we highlight several of what we have identified as significant and important contributions to either our understanding of that subfield or de novo metalloprotein design as a discipline. These reports are placed in context both historically and scientifically. General suggestions for future directions that we feel will be important to advance our understanding or accelerate discovery are discussed.

Toward Selective Oxidation of Contaminants in Aqueous Systems

The presence of diverse pollutants in water has been threating human health and aquatic ecosystems on a global scale. For more than a century, chemical oxidation using strongly oxidizing species was one of the most effective technologies to destruct pollutants and to ensure a safe and clean water supply. However, the removal of increasing amount of pollutants with higher structural complexity, especially the emerging micropollutants with trace concentrations in the complicated water matrix, requires excessive dosage of oxidant and/or energy input, resulting in a low cost-effectiveness and possible secondary pollution. Consequently, it is of practical significance but scientifically challenging to achieve selective oxidation of pollutants of interest for water decontamination. Currently, there are a variety of examples concerning selective oxidation of pollutants in aqueous systems. However, a systematic understanding of the relationship between the origin of selectivity and its applicable water treatment scenarios, as well as the rational design of catalyst for selective catalytic oxidation, is still lacking. In this critical review, we summarize the state-of-the-art selective oxidation strategies in water decontamination and probe the origins of selectivity, that is, the selectivity resulting from the reactivity of either oxidants or target pollutants, the selectivity arising from the accessibility of pollutants to oxidants via adsorption and size exclusion, as well as the selectivity due to the interfacial electron transfer process and enzymatic oxidation. Finally, the challenges and perspectives are briefly outlined to stimulate future discussion and interest on selective oxidation for water decontamination, particularly toward application in real scenarios.

Electron Transfer to the Trinuclear Copper Cluster in Electrocatalysis by the Multicopper Oxidases

High-potential multicopper oxidases (MCOs) are excellent catalysts able to perform the oxygen reduction reaction (ORR) at remarkably low overpotentials. Moreover, MCOs are able to interact directly with the electrode surfaces via direct electron transfer (DET), that makes them the most commonly used electrocatalysts for oxygen reduction in biofuel cells. The central question in MCO electrocatalysis is whether the type 1 (T1) Cu is the primary electron acceptor site from the electrode, or whether electrons can be transferred directly to the trinuclear copper cluster (TNC), bypassing the rate-limiting intramolecular electron transfer step from the T1 site. Here, using site-directed mutagenesis and electrochemical methods combined with data modeling of electrode kinetics, we have found that there is no preferential superexchange pathway for DET to the T1 site. However, due to the high reorganization energy of the fully oxidized TNC, electron transfer from the electrode to the TNC does occur primarily through the T1 site. We have further demonstrated that the lower reorganization energy of the TNC in its two-electron reduced, alternative resting, form enables DET to the TNC, but this only occurs in the first turnover. This study provides insight into the factors that control the kinetics of electrocatalysis by the MCOs and a guide for the design of more efficient biocathodes for the ORR.

A [3Cu:2S] cluster provides insight into the assembly and function of the CuZ site of nitrous oxide reductase

Nitrous oxide reductase (N₂OR) is the only known enzyme reducing environmentally critical nitrous oxide (N₂O) to dinitrogen (N₂) as the final step of bacterial denitrification. The assembly process of its unique catalytic [4Cu:2S] cluster Cu_Z remains scarcely understood. Here we report on a mutagenesis study of all seven histidine ligands coordinating this copper center, followed by spectroscopic and structural characterization and based on an established, functional expression system for Pseudomonas stutzeri N₂OR in Escherichia coli. While no copper ion was found in the Cu_Z binding site of variants H129A, H130A, H178A, H326A, H433A and H494A, the H382A variant carried a catalytically inactive [3Cu:2S] center, in which one sulfur ligand, S_Z2, had relocated to form a weak hydrogen bond to the sidechain of the nearby lysine residue K454. This link provides sufficient stability to avoid the loss of the sulfide anion. The UV-vis spectra of this cluster are strikingly similar to those of the active enzyme, implying that the flexibility of S_Z2 may have been observed before, but not recognized. The sulfide shift changes the metal coordination in Cu_Z and is thus of high mechanistic interest.

Variants of all seven histidine ligands of the [4Cu:2S] active site of nitrous oxide reductase mostly result in loss of the metal site. However, a H382A variant retains a [3Cu:2S] cluster that hints towards a structural flexibility also present in the intact site.

Coordination geometry-induced optical imaging of l-cysteine in cancer cells using imidazopyridine-based copper(ii) complexes

Overexpression of cysteine cathepsins proteases has been documented in a wide variety of cancers, and enhances the l-cysteine concentration in tumor cells. We report the synthesis and characterization of copper(ii) complexes [Cu(L1)₂(H₂O)](SO₃CF₃)₂, 1, L1 = 3-phenyl-1-(pyridin-2-yl)imidazo[1,5-a]pyridine, [Cu(L2)₂(SO₃CF₃)]SO₃CF₃, 2, L2 = 3-(4-methoxyphenyl)-1-pyridin-2-yl-imidazo[1,5-a]pyridine, [Cu(L3)₂(H₂O)](SO₃CF₃)₂, 3, L3 = 3-(3,4-dimethoxy-phenyl)-1-pyridin-2-yl-imidazo[1,5-a]pyridine and [Cu(L4)₂(H₂O)](SO₃CF₃)₂, 4, L4 = dimethyl-[4-(1-pyridin-2-yl-imidazo[1,5-a]pyridin-3-yl)phenyl]amine as 'turn-on' optical imaging probes for l-cysteine in cancer cells. The molecular structure of complexes adopted distorted trigonal pyramidal geometry (τ, 0.68-0.87). Cu-N_py bonds (1.964-1.989 Å) were shorter than Cu-N_imi bonds (2.024-2.074 Å) for all complexes. Geometrical distortion was strongly revealed in EPR spectra, showing g_‖ (2.26-2.28) and A_‖ values (139-163 × 10^-4 cm^-1) at 70 K. The d-d transitions appeared around 680-741 and 882-932 nm in HEPES, which supported the existence of five-coordinate geometry in solution. The Cu(ii)/Cu(i) redox potential of 1 (0.221 V vs. NHE) was almost identical to that of 2 and 3 but lower than that of 4 (0.525 V vs. NHE) in HEPES buffer. The complexes were almost non-emissive in nature, but became emissive by the interaction of l-cysteine in 100% HEPES at pH 7.34 via reduction of Cu(ii) to Cu(i). Among the probes, probe 2 showed selective and efficient turn-on fluorescence behavior towards l-cysteine over natural amino acids with a limit of detection of 9.9 × 10^-8 M and binding constant of 2.3 × 10⁵ M^-1. The selectivity of 2 may have originated from a nearly perfect trigonal plane adopted around a copper(ii) center (∼120.70°), which required minimum structural change during the reduction of Cu(ii) to Cu(i) while imaging Cys. The other complexes, with their distorted trigonal planes, required more reorganizational energy, which resulted in poor selectivity. Probe 2 was employed for optical imaging of l-cysteine in HeLa cells and macrophages. It exhibited brighter fluorescent images by visualizing Cys at pH 7.34 and 37 °C. It showed relatively less toxicity for these cell lines as ascertained by the MTT assay.

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.

Two Decades of Laccases: Advancing Sustainability in the Chemical Industry

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

Molecular and biochemical characterization of a new thermostable bacterial laccase from Meiothermus ruber DSM 1279

A new bacterial laccase gene (mrlac) fromMeiothermus ruberDSM 1279 was successfully overexpressed to produce a laccase (Mrlac) in soluble form inEscherichia coliduring simultaneous overexpression of a chaperone protein (GroEL/ES).

Ultrafast excited-state charge-transfer dynamics in laccase type I copper site

Femtosecond pump-probe spectroscopy was used to investigate the excited state dynamics of the T1 copper site of laccase from Pleurotus ostreatus, by exciting its 600 nm charge transfer band with a 15-fs pulse and probing over a broad range in the visible region. The decay of the pump-induced ground-state bleaching occurs in a single step and is modulated by clearly visible oscillations. Global analysis of the two-dimensional differential transmission map shows that the excited state exponentially decays with a time constant of 375 fs, thus featuring a decay rate slower than those occurring in quite all the investigated T1 copper site proteins. The ultrashort pump pulse induces a vibrational coherence in the protein, which is mainly assigned to ground state activity, as expected in a system with fast excited state decay. Vibrational features are discussed also in comparison with the traditional resonance Raman spectrum of the enzyme. The results indicate that both excited state dynamics and vibrational modes associated with the T1 Cu laccase charge transfer have main characteristics similar to those of all the T1 copper site-containing proteins. On the other hand, the differences observed for laccase from P. ostreatus further confirm the peculiar hypothesized trigonal T1 Cu site geometry.

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.

Mycoremediation of Benzo[a]pyrene by Pleurotus ostreatus in the presence of heavy metals and mediators

Benzo[a]pyrene is considered as a priority pollutant because of its carcinogenic, teratogenic and mutagenic effects. The highly recalcitrant nature of Benzo[a]pyrene poses a major problem for its degradation. White-rot fungi such as Pleurotus ostreatus can degrade Benzo[a]pyrene by enzymes like laccase and manganese peroxidase. The present investigation was carried out to determine the extent of Benzo[a]pyrene degradation by the PO-3, a native isolate of P. ostreatus, in the presence of heavy metals and ligninolytic enzyme mediators. Modified mineral salt medium was supplemented with 5 mM concentration of different heavy metal salts and ethylenediaminetetraacetic acid. Vanillin and 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (1 and 5 mM) were used to study the effect of mediators. Results indicated that P. ostreatus PO-3 degraded 71.2 % of Benzo[a]pyrene in the presence of copper ions. Moderate degradation was observed in the presence of zinc and manganese. Both biomass formation and degradation were severely affected in the presence of all other heavy metal salts used in the study. Copper at 15 mM concentration supported the best degradation (74.2 %), beyond which the degradation progressively reduced. Among the mediators, 1 mM 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) supported 78.7 % degradation and 83.6 % degradation was observed under the influence of 5 mM vanillin. Thus, metal ion like copper is essential for better biodegradation of Benzo[a]pyrene. Compared to synthetic laccase mediator like 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate), natural mediator such as vanillin may play a significant role in the degradation of aromatic compounds by white-rot fungi.

DFT studies of structural and some spectral parameters of copper(II) complexes with N,N,N',N″-tetrakis (2-hydroxyethyl/propyl) ethylenediamine and tris(2-hydroxyethyl)amine

The structures and some spectral parameters of three copper(II) complexes; [Cu(THEEN)(H2O)](PIC)2 (1), [Cu(THPEN)] (PIC)2 C3H8O (2) and [Cu(TEAH3)(PIC)] (PIC)⋅(H2O) (3), previously synthesized and characterized by X-ray diffraction, are here computationally studied by using density functional theory (DFT) in its hybrid form B3LYP. In these complexes, THEEN is N,N,N',N″-tetrakis(2-hydroxyethyl) ethylenediamine and THPEN is N,N,N',N″-tetrakis(2-hydroxypropyl) ethylenediamine, tetrapodal ligands and TEAH3 is tris(2-hydroxyethyl)amine, a tripodal ligand. The primary coordination sphere of copper(II) ion in complexes (1), (2) and (3) are optimized, structural parameters are calculated, vibrational bands are assigned and energy gaps of frontier orbital (HOMO-LUMO) have been calculated with B3LYP/6-31G/LANL2DZ level of theory using DMSO as solvent. The calculated geometric and spectral results reproduced the experimental data with well agreement. Theoretical calculated molecular orbitals (HOMO-LUMO) and their energies have been calculated that suggest charge transfer occurs within the complexes.

Axial interactions in the mixed-valent CuA active site and role of the axial methionine in electron transfer

Within Cu-containing electron transfer active sites, the role of the axial ligand in type 1 sites is well defined, yet its role in the binuclear mixed-valent CuA sites is less clear. Recently, the mutation of the axial Met to Leu in a CuA site engineered into azurin (CuA Az) was found to have a limited effect on E(0) relative to this mutation in blue copper (BC). Detailed low-temperature absorption and magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance studies on CuA Az (WT) and its M123X (X = Q, L, H) axial ligand variants indicated stronger axial ligation in M123L/H. Spectroscopically validated density functional theory calculations show that the smaller ΔE(0) is attributed to H2O coordination to the Cu center in the M123L mutant in CuA but not in the equivalent BC variant. The comparable stabilization energy of the oxidized over the reduced state in CuA and BC (CuA ∼ 180 mV; BC ∼ 250 mV) indicates that the S(Met) influences E(0) similarly in both. Electron delocalization over two Cu centers in CuA was found to minimize the Jahn-Teller distortion induced by the axial Met ligand and lower the inner-sphere reorganization energy. The Cu-S(Met) bond in oxidized CuA is weak (5.2 kcal/mol) but energetically similar to that of BC, which demonstrates that the protein matrix also serves an entatic role in keeping the Met bound to the active site to tune down E(0) while maintaining a low reorganization energy required for rapid electron transfer under physiological conditions.

Comparative analysis of the performance of commonly available density functionals in the determination of geometrical parameters for copper complexes

In this study, a set of 50 transition-metal complexes of Cu(I) and Cu(II), were used in the evaluation of 18 density functionals in geometry determination. In addition, 14 different basis sets were considered, including four commonly used Pople's all-electron basis sets; four basis sets including popular types of effective-core potentials: Los Alamos, Steven-Basch-Krauss, and Stuttgart-Dresden; and six triple-ζ basis sets. The results illustrate the performance of different methodological alternatives for the treatment of geometrical properties in relevant copper complexes, pointing out Double-Hybrid (DH) and Long-range Correction (LC) Generalized Gradient Approximation (GGA) methods as better descriptors of the geometry of the evaluated systems. These however, are associated with a computational cost several times higher than some of the other methods employed, such as the M06 functional, which has also demonstrated a comparable performance. Regarding the basis sets, 6-31+G(d) and 6-31+G(d,p) were the best performing approaches. In addition, the results show that the use of effective-core potentials has a limited impact, in terms of the accuracy in the determination of metal-ligand bond-lengths and angles in our dataset of copper complexes. Hence, these could become a good alternative for the geometrical description of these systems, particularly CEP-121G and SDD basis sets, if one is considering larger copper complexes where the computational cost could be an issue.

Chemical modifications of laccase from white-rot basidiomycete Cerrena unicolor

Laccases belong to the group of phenol oxidizes and constitute one of the most promising classes of enzymes for future use in various fields. For industrial and biotechnological purposes, laccases were among the first enzymes providing larger-scale applications such as removal of polyphenols or conversion of toxic compounds. The wood-degrading basidiomycete Cerrena unicolor C-139, reported in this study, is one of the high-laccase producers. In order to facilitate novel and more efficient biocatalytic process applications, there is a need for laccases with improved biochemical properties, such as thermostability or stability in broad ranges of pH. In this work, modifications of laccase isoforms by hydrophobization, hydrophilization, and polymerization were performed. The hydrophobized and hydrophilized enzyme showed enhanced surface activity and higher ranges of pH and temperatures in comparison to its native form. However, performed modifications did not appear to noticeably alter enzyme’s native structure possibly due to the formation of coating by particles of saccharides around the molecule. Additionally, surface charge of modified laccase shifted towards the negative charge for the hydrophobized laccase forms. In all tested modifications, the size exclusion method led to average 80 % inhibition removal for hydrophilized samples after an hour of incubation with fluoride ions. Samples that were hydrophilized with lactose and cellobiose showed an additional 90 % reversibility of inhibition by fluoride ions after an hour of concluding the reaction and 40 % after 24 h. The hydrophobized laccase showed higher level of the reversibility after 1 h (above 80 %) and 24 h (above 70 %) incubation with fluoride ions. The addition of ascorbate to laccase solution before a fluoride spike resulted in more efficient reversibility of fluoride inhibitory effect in comparison to the treatments with reagents used in the reversed sequence.

Lignin-modifying enzymes in filamentous basidiomycetes--ecological, functional and phylogenetic review

Filamentous fungi owe powerful abilities for decomposition of the extensive plant material, lignocellulose, and thereby are indispensable for the Earth's carbon cycle, generation of soil humic matter and formation of soil fine structure. The filamentous wood-decaying fungi belong to the phyla Basidiomycota and Ascomycota, and are unique organisms specified to degradation of the xylem cell wall components (cellulose, hemicelluloses, lignins and extractives). The basidiomycetous wood-decaying fungi form brackets, caps or resupinaceous (corticioid) fruiting bodies when growing on wood for dissemination of their sexual basidiospores. In particular, the ability to decompose the aromatic lignin polymers in wood is mostly restricted to the white rot basidiomycetes. The white-rot decay of wood is possible due to secretion of organic acids, secondary metabolites, and oxidoreductive metalloenzymes, heme peroxidases and laccases, encoded by divergent gene families in these fungi. The brown rot basidiomycetes obviously depend more on a non-enzymatic strategy for decomposition of wood cellulose and modification of lignin. This review gives a current ecological, genomic, and protein functional and phylogenetic perspective of the wood and lignocellulose-decaying basidiomycetous fungi.

Multicopper oxidases: a workshop on copper coordination chemistry, electron transfer, and metallophysiology

Multicopper oxidases (MCOs) are unique among copper proteins in that they contain at least one each of the three types of biologic copper sites, type 1, type 2, and the binuclear type 3. MCOs are descended from the family of small blue copper proteins (cupredoxins) that likely arose as a complement to the heme-iron-based cytochromes involved in electron transport; this event corresponded to the aerobiosis of the biosphere that resulted in the conversion of Fe(II) to Fe(III) as the predominant redox state of this essential metal and the solubilization of copper from Cu(2)S to Cu(H(2)O)( n ) (2+). MCOs are encoded in genomes in all three kingdoms and play essential roles in the physiology of essentially all aerobes. With four redox-active copper centers, MCOs share with terminal copper-heme oxidases the ability to catalyze the four-electron reduction of O(2) to two molecules of water. The electron transfers associated with this reaction are both outer and inner sphere in nature and their mechanisms have been fairly well established. A subset of MCO proteins exhibit specificity for Fe(2+), Cu(+), and/or Mn(2+) as reducing substrates and have been designated as metallooxidases. These enzymes, in particular the ferroxidases found in all fungi and metazoans, play critical roles in the metal metabolism of the expressing organism.

Proximal mutations at the type 1 copper site of CotA laccase: spectroscopic, redox, kinetic and structural characterization of I494A and L386A mutants

In the present study the CotA laccase from Bacillus subtilis has been mutated at two hydrophobic residues in the vicinity of the type 1 copper site. The mutation of Leu(386) to an alanine residue appears to cause only very subtle alterations in the properties of the enzyme indicating minimal changes in the structure of the copper centres. However, the replacement of Ile(494) by an alanine residue leads to significant changes in the enzyme. Thus the major visible absorption band is upshifted by 16 nm to 625 nm and exhibits an increased intensity, whereas the intensity of the shoulder at approx. 330 nm is decreased by a factor of two. Simulation of the EPR spectrum of the I494A mutant reveals differences in the type 1 as well as in the type 2 copper centre reflecting modifications of the geometry of these centres. The intensity weighted frequencies <nu(Cu-S)>, calculated from resonance Raman spectra are 410 cm(-1) for the wild-type enzyme and 396 cm(-1) for the I494A mutant, indicating an increase of the Cu-S bond length in the type 1 copper site of the mutant. Overall the data clearly indicate that the Ile(494) mutation causes a major alteration of the structure near the type 1 copper site and this has been confirmed by X-ray crystallography. The crystal structure shows the presence of a fifth ligand, a solvent molecule, at the type 1 copper site leading to an approximate trigonal bipyramidal geometry. The redox potentials of the L386A and I494A mutants are shifted downwards by approx. 60 and 100 mV respectively. These changes correlate well with decreased catalytic efficiency of both mutants compared with the wild-type.

Structure and action mechanism of ligninolytic enzymes

Lignin is the most abundant renewable source of aromatic polymer in nature, and its decomposition is indispensable for carbon recycling. It is chemically recalcitrant to breakdown by most organisms because of the complex, heterogeneous structure. The white-rot fungi produce an array of extracellular oxidative enzymes that synergistically and efficiently degrade lignin. The major groups of ligninolytic enzymes include lignin peroxidases, manganese peroxidases, versatile peroxidases, and laccases. The peroxidases are heme-containing enzymes with catalytic cycles that involve the activation by H2O2 and substrate reduction of compound I and compound II intermediates. Lignin peroxidases have the unique ability to catalyze oxidative cleavage of C-C bonds and ether (C-O-C) bonds in non-phenolic aromatic substrates of high redox potential. Manganese peroxidases oxidize Mn(II) to Mn(III), which facilitates the degradation of phenolic compounds or, in turn, oxidizes a second mediator for the breakdown of non-phenolic compounds. Versatile peroxidases are hybrids of lignin peroxidase and manganese peroxidase with a bifunctional characteristic. Laccases are multi-copper-containing proteins that catalyze the oxidation of phenolic substrates with concomitant reduction of molecular oxygen to water. This review covers the chemical nature of lignin substrates and focuses on the biochemical properties, molecular structures, reaction mechanisms, and related structures/functions of these enzymes.

Multiconfigurational g tensor calculations as a probe for the covalency of the copper-ligand bonds in copper(II) complexes: [CuCl4]2-, [Cu(NH3)4]2+, and plastocyanin

Calculations of the g tensor of three copper(II) complexes [Cu(NH3)4]2+, [CuCl4]2-, and plastocyanin are presented. Two different sum-over-states-based approaches are considered, making use of the multistate CASPT2 method for excitation energies and PMCAS (perturbation modified CAS) wave functions for the computation of the angular momentum and spin-orbit coupling matrix elements. Test calculations on [Cu(NH3)4]2+ and [CuCl4]2- point to the need of including in the MS-CASPT2 treatment the specific charge-transfer state with an electron excited out of the bonding counterpart of the ground-state SOMO. The computed g shifts for these two molecules present a considerable improvement with respect to the results obtained from our previous g tensor calculations based instead on CASSCF/CASPT2. This is shown to be related to an improved description of the covalency of the Cu-L bonds. For the calculations on plastocyanin, different models are used, taken from a recent (QM/MM) DFT study by Sinnecker and Neese. The effect of the surrounding protein is taken into account by surrounding the central cluster either with a dielectric continuum (epsilon = 4) or with a set of point charges. The second approach is found to be indispensable for an accurate description of environmental effects. With this approach, the calculated g values compare to within 30 ppt with the experimental data of plastocyanin.

Copper incorporation into recombinant CotA laccase from Bacillus subtilis: characterization of fully copper loaded enzymes

The copper content of recombinant CotA laccase from Bacillus subtilis produced by Escherichia coli cells is shown to be strongly dependent on the presence of copper and oxygen in the culture media. In copper-supplemented media, a switch from aerobic to microaerobic conditions leads to the synthesis of a recombinant holoenzyme, while the maintenance of aerobic conditions results in the synthesis of a copper-depleted population of proteins. Strikingly, cells grown under microaerobic conditions accumulate up to 80-fold more copper than aerobically grown cells. In vitro copper incorporation into apoenzymes was monitored by optical and electron paramagnetic resonance (EPR) spectroscopy. This analysis reveals that copper incorporation into CotA laccase is a sequential process, with the type 1 copper center being the first to be reconstituted, followed by the type 2 and the type 3 copper centers. The copper reconstitution of holoCotA derivatives depleted in vitro with EDTA results in the complete recovery of the native conformation as monitored by spectroscopic, kinetic and thermal stability analysis. However, the reconstitution of copper to apo forms produced in cultures under aerobic and copper-deficient conditions resulted in incomplete recovery of biochemical properties of the holoenzyme. EPR and resonance Raman data indicate that, presumably, folding in the presence of copper is indispensable for the correct structure of the trinuclear copper-containing site.

Spectroscopic characterization of a high-potential lipo-cupredoxin found in Streptomyces coelicolor

For many streptomycetes, a distinct dependence on the "bioavailability" of copper ions for their morphological development has been reported. Analysis of the Streptomyces coelicolor genome reveals a number of gene products encoding for putative copper-binding proteins. One of these appears as an unusual copper-binding protein with a lipoprotein signal sequence and a cupredoxin-like domain harboring a putative Type-1 copper-binding motif. Cloning of this gene from S. coelicolor and subsequent heterologous expression in Escherichia coli has allowed for a thorough spectroscopic interrogation of this putative copper-binding protein. Optical and electron paramagnetic resonance spectroscopies have confirmed the presence of a "classic" Type-1 copper site with the axial ligand to the copper a methionine. Paramagnetic NMR spectroscopy on both the native Cu(II) form and Co(II)-substituted protein has yielded active-site structural information, which on comparison with that of other cupredoxin active sites reveals metal-ligand interactions most similar to the "classic" Type-1 copper site found in the amicyanin family of cupredoxins. Despite this high structural similarity, the Cu(II)/(I) midpoint potential of the S. coelicolor protein is an unprecedented +605 mV vs normal hydrogen electrode at neutral pH (amicyanin approximately +250 mV), with no active-site protonation of the N-terminal His ligand observed. Suggestions for the physiological role/function of this high-potential cupredoxin are discussed.