Crystal structure of a laccase from Melanocarpus albomyces with an intact trinuclear copper site

We have crystallized the ascomycete laccase from Melanocarpus albomyces with all four coppers present and determined the crystal structure at 2.4 A resolution. The enzyme is heavily glycosylated and consists of three cupredoxin-like domains, similar to those found in the Cu-depleted basidiomycete laccase from Coprinus cinereus. However, there are significant differences in the loops forming the substrate-binding pocket. In addition, the crystal structure of the M. albomyces laccase revealed elongated electron density between all three coppers in the trinuclear copper site, suggesting that an oxygen molecule binds with a novel geometry. This oxygen, required in the reaction, may enter the trinuclear site through the tunnel, which is open in the structure of the C. cinereus laccase. In contrast, the C-terminus on the M. albomyces laccase forms a plug that blocks this access.     

Crystal structure of a four-copper laccase complexed with an arylamine: insights into substrate recognition and correlation with kinetics

Laccases are multicopper oxidases that catalyze the oxidation of a wide range of phenols or arylamines, and their use in industrial oxidative processes is increasing. We purified from the white rot fungus Trametes versicolor a laccase that exists as five different isozymes, depending on glycosylation. The 2.4 A resolution structure of the most abundant isozyme of the glycosylated enzyme was solved. The four copper atoms are present, and it is the first crystal structure of a laccase in its active form. The crystallized enzyme binds 2,5-xylidine, which was used as a laccase inducer in the fungus culture. This arylamine is a very weak reducing substrate of the enzyme. The cavity enclosing 2,5-xylidine is rather wide, allowing the accommodation of substrates of various sizes. Several amino acid residues make hydrophobic interactions with the aromatic ring of the ligand. In addition, two charged or polar residues interact with its amino group. The first one is an histidine that also coordinates the copper that functions as the primary electron acceptor. The second is an aspartate conserved among fungal laccases. The purified enzyme can oxidize various hydroxylated compounds of the phenylurea family of herbicides that we synthesized. These phenolic substrates have better affinities at pH 5 than at pH 3, which could be related to the 2,5-xylidine binding by the aspartate. This is the first high-resolution structure of a multicopper oxidase complexed to a reducing substrate. It provides a model for engineering laccases that are either more efficient or with a wider substrate specificity.     

The Structure of the Small Laccase from Streptomyces coelicolor Reveals a Link between Laccases and Nitrite Reductases

The X-ray structure of the two-domain laccase (small laccase) from Streptomyces coelicolor A3(2) was solved at 2.7-Å resolution. The enzyme differs significantly from all laccases studied structurally so far. It consists of two domains and forms trimers and hence resembles the quaternary structure of nitrite reductases or ceruloplasmins more than that of large laccases. There are three trinuclear copper clusters in the enzyme localized between domains 1 and 2 of each pair of neighbor chains. In this way, a similar geometry of the active site as seen in large laccases is ensured, albeit by different arrangements of domains and protein chains. Three copper ions of type 1 lie close to one another near the surface of the central part of the trimer, and, effectively, a trimeric substrate binding site is formed in their vicinity.     

Oxygen-reducing enzyme cathodes produced from SLAC, a small laccase from Streptomyces coelicolor

The bacterially-expressed laccase, small laccase (SLAC) of Streptomyces coelicolor, was incorporated into electrodes of both direct electron transfer (DET) and mediated electron transfer (MET) designs for application in biofuel cells. Using the DET design, enzyme redox kinetics were directly observable using cyclic voltammetry, and a redox potential of 0.43 V (SHE) was observed. When mediated by an osmium redox polymer, the oxygen-reducing cathode retained maximum activity at pH 7, producing 1.5 mA/cm2 in a planar configuration at 900 rpm and 40 degrees C, thus outperforming enzyme electrodes produced using laccase from fungal Trametes versicolor (0.2 mA/cm2) under similar conditions. This improvement is directly attributable to differences in the kinetics of SLAC and fungal laccases. Maximum stability of the mediated SLAC electrode was observed at pH above the enzyme's relatively high isoelectric point, where the anionic enzyme molecules could form an electrostatic adduct with the cationic mediator. Porous composite SLAC electrodes with increased surface area produced a current density of 6.25 mA/cm2 at 0.3 V (SHE) under the above conditions.     

Crystal structures of E. coli laccase CueO at different copper concentrations

CueO protein is a hypothetical bacterial laccase and a good laccase candidate for large scale industrial application. Four CueO crystal structures were determined at different copper concentrations. Low copper occupancy in apo-CueO and slow copper reconstitution process in CueO with exogenous copper were demonstrated. These observations well explain the copper dependence of CueO oxidase activity. Structural comparison between CueO and other three fungal laccase proteins indicates that Glu106 in CueO constitutes the primary counter-work for reconstitution of the trinuclear copper site. Mutation of Glu106 to a Phe enhanced CueO oxidation activity and supported this hypothesis. In addition, an extra alpha-helix from Leu351 to Gly378 covers substrate biding pocket of CueO and might compromises the electron transfer from substrate to type I copper.     

Biochemical studies of the multicopper oxidase (small laccase) from Streptomyces coelicolor using bioactive phytochemicals and site‐directed mutagenesis

Multicopper oxidases can act on a broad spectrum of phenolic and non‐phenolic compounds. These enzymes include laccases, which are widely distributed in plants and fungi, and were more recently identified in bacteria. Here, we present the results of biochemical and mutational studies of small laccase (SLAC), a multicopper oxidase from Streptomyces coelicolor (SCO6712). In addition to typical laccase substrates, SLAC was tested using phenolic compounds that exhibit antioxidant activity. SLAC showed oxidase activity against 12 of 23 substrates tested, including caffeic acid, ferulic acid, resveratrol, quercetin, morin, kaempferol and myricetin. The kinetic parameters of SLAC were determined for 2,2′‐azino‐bis(3‐ethylbenzthiazoline‐6‐sulphonic acid), 2,6‐dimethoxyphenol, quercetin, morin and myricetin, and maximum reaction rates were observed with myricetin, where k_cat and K_m values at 60°C were 8.1 (± 0.8) s^?1 and 0.9 (± 0.3) mM respectively. SLAC had a broad pH optimum for activity (between pH 4 and 8) and temperature optimum at 60–70°C. It demonstrated remarkable thermostability with a half‐life of over 10 h at 80°C and over 7 h at 90°C. Site‐directed mutagenesis revealed 17 amino acid residues important for SLAC activity including the 10 His residues involved in copper coordination. Most notably, the Y229A and Y230A mutant proteins showed over 10‐fold increase in activity compared with the wild‐type SLAC, which was correlated to higher copper incorporation, while kinetic analyses with S929A predicts localization of this residue near the meta‐position of aromatic substrates.     

The structure of Rigidoporus lignosus Laccase containing a full complement of copper ions, reveals an asymmetrical arrangement for the T3 copper pair

Laccase is a multicopper blue oxidase that couples the four-electron reduction of oxygen with the oxidation of a broad range of organic substrates, including phenols and arylamines. The enzyme is the object of intense biotechnological research, due to its employment in bioremediation of soils and water as well as in other biotechnological applications. We report here the cDNA and protein sequences, the post-translational modifications, the crystallization and X-ray structure determination of a laccase from the white-rot fungus Rigidoporus lignosus. The amino acid residues sequence deduced from cDNA clearly identified a pre-sequence of 21 residues representing the signal for extra-cellular localization. Mass spectrometry analysis performed on the salvage enzyme, confirmed the deduced sequence and precisely mapped two glycosylation sites at Asn337 and Asn435, determining the nature of the bound glycosidic moieties. The crystal structure was determined at 1.7A resolution from perfectly hemihedrally twinned crystals, by molecular replacement technique. While the overall structure closely resembled those reported for other fungal laccases, the analysis of the T2/T3 trinuclear cluster revealed an unprecedented coordination sphere for the T3 copper pair. No bridging oxygen ligand was present between the two T3 copper ions, which were no longer symmetrically coordinated. The observed structure could represent an intermediate along the process of four-electron reduction of oxygen to water taking place at the trinuclear copper cluster.     

Spectroscopic characterization and O2 reactivity of the trinuclear Cu cluster of mutants of the multicopper oxidase Fet3p

Fet3p is a multicopper oxidase that uses four copper ions (one type 1, one type 2, and one type 3 binuclear site) to couple substrate oxidation to the reduction of O(2) to H(2)O. The type 1 Cu site shuttles electrons between the substrate and the type 2/type 3 Cu sites which form a trinuclear Cu cluster that is the active site for O(2) reduction. This study extends the spectroscopic and reactivity studies that have been conducted with type 1-substituted Hg (T1Hg) laccase to Fet3p and a mutant of Fet3p in which the trinuclear Cu cluster is perturbed. To examine the reaction between the trinuclear Cu cluster and O(2), the type 1 Cu Cys(484) was mutated to Ser, resulting in a type 1-depleted (T1D) form of the enzyme. Additional His to Gln mutations were made at the trinuclear cluster to further probe specific contributions to reactivity. One of these mutants (His(126)Gln) produces the first stable but perturbed trinuclear Cu cluster (T1DT3' Fet3p). Spectroscopic characterization (absorption, circular dichroism, magnetic circular dichroism, and electron paramagnetic resonance) of the resting trinuclear sites in T1D and T1DT3' Fet3p reveal that the His(126)Gln mutation changes the electronic structure of both the type 3 and type 2 Cu sites. The trinuclear clusters in T1D and T1DT3' Fet3p react with O(2) to produce peroxide intermediates analogous to that observed in T1Hg laccase. Spectroscopic data on the peroxide intermediates in the three forms provide further insight into the structure of this intermediate. In T1D Fet3p, the decay of this peroxide intermediate is pH-dependent, and the rate of decay is 10-fold higher at low pH. In T1DT3' Fet3p, the decay of the peroxide intermediate is pH-independent and is slow at all pH's. This change in the pH dependence provides new insight into the mechanism of intermediate decay involving reductive cleavage of the O-O bond.     

Probing the dioxygen route in Melanocarpus albomyces laccase with pressurized xenon gas

Laccases catalyze the oxidation of phenolic substrates and the concominant reduction of dioxygen to water. We used xenon as an oxygen probe in search of routes for the entry of dioxygen into the catalytic center. Two xenon-pressurized crystal structures of recombinant Melanocarpus albomyces laccase were determined, showing three hydrophobic Xe-binding sites located in domain C. The analysis of hydrophobic cavities in other laccase structures further suggested the preference of domain C for binding of hydrophobic species such as dioxygen, thus suggesting that the hydrophobic core of domain C could function as a channel through which dioxygen can enter the trinuclear copper center.     

X-ray structural studies of the fungal laccase from Cerrena maxima

Laccases are members of the blue multi-copper oxidase family. These enzymes oxidize substrate molecules by accepting electrons at a mononuclear copper centre and transferring them to a trinuclear centre. Dioxygen binds to the trinuclear centre and following the transfer of four electrons is reduced to two molecules of water. The X-ray structure of a laccase from Cerrena maxima has been elucidated at 1.9 A resolution using synchrotron data and the molecular replacement technique. The final refinement coefficients are Rcryst = 16.8% and Rfree = 23.0%, with root mean square deviations on bond lengths and bond angles of 0.015 A and 1.51 degrees , respectively. The type 1 copper centre has an isoleucine residue at the axial position and the "resting" state of the trinuclear centre comprises a single oxygen (OH) moiety asymmetrically disposed between the two type 3 copper ions and a water molecule attached to the type 2 ion. Several carbohydrate binding sites have been identified and the glycan chains appear to promote the formation of well-ordered crystals. Two tyrosine residues near the protein surface have been found in a nitrated state.     

The blue oxidases, ascorbate oxidase, laccase and ceruloplasmin. Modelling and structural relationships

On the basis of the spatial structure of ascorbate oxidase [Messerschmidt, A., Rossi, A., Ladenstein, R., Huber, R., Bolognesi, M., Gatti, G., Marchesini, A., Petruzzelli, R. & Finazzi-Agro, A. (1989) J. Mol. Biol. 206, 513-529], an alignment of the amino acid sequence of the related blue oxidases, laccase and ceruloplasmin is proposed. This strongly suggests a three-domain structure for laccase closely related to ascorbate oxidase and a six-domain structure of ceruloplasmin. These domains demonstrate homology with the small blue copper proteins. The relationships suggest that laccase, like ascorbate oxidase, has a mononuclear blue copper in domain 3 and a trinuclear copper between domain 1 and 3 and ceruloplasmin has mononuclear copper ions in domains 2, 4 and 6 and a trinuclear copper between domains 1 and 6.     

Enzymatic and spectroscopic studies on the activation or inhibition effects by substituted phenolic compounds in the oxidation of aryldiamines and catechols catalyzed by Rhus vernicifera laccase

The effect of various phenolic compounds on the activity of Rhus vernicifera laccase (Lc) has been evaluated using two different substrates, N,N-dimethyl-p-phenylenediamine and p-tert-butylcatechol. The observed effect strongly depends on the phenol employed and involves either a moderate activation, by halophenols, or inhibition, by acidic phenols. The collective data are consistent with an open active site in Lc, which is capable of accommodating more than one substrate or phenol molecule. According to NMR relaxation experiments, a phenol molecule binds at an average distance from type 1 Cu of about 6 Å, while evidence from electron paramagnetic resonance (EPR) experiments shows that binding of another phenol molecule induces a change, and probably occurs close to, the type 2/type 3 cluster. The effect of phenolic compounds on Lc reactivity is related to a modification of the substrate affinity for the enzyme. This affinity can either be increased, probably through π-stacking or other types of interactions, or decreased, due to competition for the same site. In addition, the alteration induced in the trinuclear copper cluster has a marked effect on the enzyme reactivity. The inhibition observed with acidic phenols is probably due to the protonation of an enzyme intermediate produced at the trinuclear site, e.g. the peroxy intermediate, that causes the release of hydrogen peroxide and prevents the reaction of this intermediate with the substrate.     

Discrete roles of copper ions in chemical unfolding of human ceruloplasmin

Human ceruloplasmin (CP) is a multicopper oxidase essential for normal iron homeostasis. The protein has six beta-barrel domains with one type 1 copper in each of domains 2, 4, and 6; the remaining copper ions form a catalytic trinuclear cluster, one type 2 and two type 3 coppers, at the interface between domains 1 and 6. We have characterized urea-induced unfolding of holo- and apo-forms of CP by far-UV circular dichroism, intrinsic fluorescence, 8-anilinonaphthalene-1-sulfonic acid binding, visible absorption, copper content, and oxidase activity probes (pH 7, 23 degrees C). We find that holo-CP unfolds in a complex reaction with at least one intermediate. The formation of the intermediate correlates with decreased secondary structure, exposure of aromatics, loss of two coppers, and reduced oxidase activity; this step is reversible, indicating that the trinuclear cluster remains intact. Further additions of urea trigger complete protein unfolding and loss of all coppers. Attempts to refold this species result in an inactive apoprotein with molten-globule characteristics. The apo-form of CP also unfolds in a multistep reaction, albeit the intermediate appears at a slightly lower urea concentration. Again, correct refolding is possible from the intermediate but not the unfolded state. Our study demonstrates that in vitro equilibrium unfolding of CP involves intermediates and that the copper ions are removed in stages. When the catalytic site is finally destroyed, refolding is not possible at neutral pH. This implies a mechanistic role for the trinuclear metal cluster as a nucleation point, aligning domains 1 and 6, during CP folding in vivo.     

Purification and characterization of a phenoloxidase (laccase) from the lignin-degrading basidiomycete PM1 (CECT 2971).

A new lignin-degrading basidiomycete, strain PM1 (= CECT 2971), was isolated from the wastewater of a paper factory. The major ligninolytic activity detected in the basidiomycete PM1 culture supernatant was a phenoloxidase (laccase). This activity was produced constitutively in defined or complex media and appeared as two protein bands in native gel electrophoresis preparations. No enzyme induction was found after treatment with certain potential laccase inducers. Laccase I was purified to homogeneity by gel filtration chromatography, anion-exchange chromatography, and hydrophobicity chromatography. The enzyme is a monomeric glycoprotein containing 6.5% carbohydrate and having a molecular weight of 64,000. It has an isoelectric point of 3.6, it is stable in a pH range from 3 to 9, and its optimum pH is 4.5. The laccase optimal reaction temperature is 80 degrees C, the laccase is stable for 1 h at 60 degrees C, and its activity increases with temperature. Spectroscopic analysis revealed that the enzyme has four bound copper atoms, a type I copper, a type II copper, and a type III binuclear copper. The amino-terminal sequence of the protein is very similar to the amino-terminal sequences of laccases from Coriolus hirsutus and Phlebia radiata.     

Crystal structure of a bacterial endospore coat component. A laccase with enhanced thermostability properties

Endospores produced by the Gram-positive soil bacterium Bacillus subtilis are shielded by a proteinaceous coat formed by over 30 structural components, which self-assemble into a lamellar inner coat and a thicker striated electrodense outer coat. The 65-kDa CotA protein is an abundant component of the outer coat layer. CotA is a highly thermostable laccase, assembly of which into the coat is required for spore resistance against hydrogen peroxide and UV light. Here, we report the structure of CotA at 1.7-A resolution, as determined by x-ray crystallography. This is the first structure of an endospore coat component, and also the first structure of a bacterial laccase. The overall fold of CotA comprises three cupredoxin-like domains and includes one mononuclear and one trinuclear copper center. This arrangement is similar to that of other multicopper oxidases and most similar to that of the copper tolerance protein CueO of Escherichia coli. However, the three cupredoxin domains in CotA are further linked by external interdomain loops, which increase the packing level of the structure. We propose that these interdomain loops contribute to the remarkable thermostability of the enzyme, but our results suggest that additional factors are likely to play a role. Comparisons with the structure of other monomeric multicopper oxidases containing four copper atoms suggest that CotA may accept the largest substrates of any known laccase. Moreover, and unlike other laccases, CotA appears to have a flexible lidlike region close to the substrate-binding site that may mediate substrate accessibility. The implications of these findings for the properties of CotA, its assembly and the properties of the bacterial spore coat structure are discussed.     

Characterisation of a novel white laccase from the deuteromycete fungus Myrothecium verrucaria NF-05 and its decolourisation of dyes

A novel 'white' laccase was purified from the deuteromycete fungus, Myrothecium verrucaria NF-05, which was a high laccase-producing strain (40.2 U·ml(-1) on the thirteenth day during fermentation). SDS-PAGE and native-PAGE revealed a single band with laccase activity corresponding to a molecular weight of approximately 66 kDa. The enzyme had three copper and one iron atoms per protein molecule determined by ICP-AES. Furthermore, both UV/visible and EPR spectroscopy remained silence, indicating the enzyme a novel laccase with new metal compositions of active centre and spectral properties. The N-terminal amino acid sequence of the purified protein was APQISPQYPM. Together with MALDI-TOF analysis, the protein revealed a high homology of the protein with that from reported M. verrucaria. The highest activity was detected at pH 4.0 and at 30°C. The enzyme activity was significantly enhanced by Na(+), Mn(2+), Cu(2+) and Zn(2+) while inhibited by DTT, NaN(3) and halogen anions. The kinetic constant (Km) showed the enzyme was more affinitive to ABTS than other tested aromatic substrates. Twelve structurally different dyes could be effectively decolourised by the laccase within 10 min. The high production of the strain and novel properties of the laccase suggested its potential for biotechnological applications.     

The reversible depletion and reconstitution of a copper ion in Coprinus cinereus laccase followed by spectroscopic techniques

The specific activities of crude and purified Coprinus cinereus laccase preparations could be enhanced by a factor of 10-12 by activation with copper ions. The copper to protein contents of purified non-activated laccase were 2.3 ± 0.1 compared to 3.3 ± 0.1 in purified activated laccase indicating that only a fraction of the laccase can be activated. Purified laccase not activated with copper ions shows in isoelectric focusing four bands in order of decreasing pI in a ratio 1/5/3/1 where only bands I and II had laccase activity. Purified activated laccase showed only three bands (I, II and III) in the ratio 5/4/1 all with some laccase activity. The pH profile of the activity for activated and non-activated laccase showed identical behavior indicating that the active forms were the same. The change in UV-Vis around 330 nm following the depletion and reconstitution of the enzyme combined with activity measurements supports the reversibility of the selective removal and insertion of copper ions at the type 2 site. The circular dichroism spectrum of activated purified laccase has characteristic changes around 350 nm relative to non-activated laccase indicative of changes at the type 2/type 3 sites. The difference between the electron paramagnetic resonance spectra of non-activated and activated C. cinereus laccase indicates that a fraction of the non-activated purified laccase contained a copper(II) signal with a coupling constant between a type 1 and a type 2 copper(II). This electron paramagnetic resonance signal could be explained by an induced asymmetry in the type 3 site due to a missing type 2 copper ion.     

Which Copper is Paramagnetic in the Type 2/Type 3 Cluster of Laccase?

 Understanding the structure and function of the three copper atoms in the dioxygen reduction site of the blue oxidases such as laccase has been a long standing challenge. In the case of a widely studied derivative, known as type 2-depleted laccase, the removal of one copper from the cluster abolishes the EPR signal of the so-called type 2 copper. However, the present studies of isotopically enriched protein from Polyporus versicolor show that the readily replaceable copper is not active in the low-temperature EPR spectrum of fungal laccase or its difluoride adduct. The same is true for the difluoride adduct of the tree enzyme. Thus, in type 2-depleted laccase the pattern of antiferromagnetic coupling is quite different from that of the native protein or the difluoride adduct. Received: 5 October 1998 / Accepted: 13 January 1999