Trinuclear Copper Cluster Research Articles

Role of External Loops of Human Ceruloplasmin in Copper Loading by ATP7B and Ccc2p

Ceruloplasmin is a multicopper oxidase required for correct iron homeostasis.Previously, we have identified a ceruloplasmin mutant associated with the iron overload disease aceruloplasminemia, which was unable to acquire copper from the mammalian pump ATP7B but could be produced in an enzymatically active form in yeast. Here, we report the expression of recombinant ceruloplasmin in the yeast Pichia pastoris and the study of the role of five surface-exposed loops in copper incorporation by comparing the efficiencies of mammalian ATP7B and yeast Ccc2p. The possibility to "mix and match" mammalian and yeast multicopper oxidases and copper ATPases can provide clues on the molecular features underlying the process of copper loading in multicopper oxidases.

Systematic Perturbation of the Trinuclear Copper Cluster in the Multicopper Oxidases: The Role of Active Site Asymmetry in Its Reduction of O2 to H2O

The multicopper oxidase Fet3p catalyzes the four-electron reduction of dioxygen to water, coupled to the one-electron oxidation of four equivalents of substrate. To carry out this process, the enzyme utilizes four Cu atoms: a type 1, a type 2, and a coupled binuclear, type 3 site. Substrates are oxidized at the T1 Cu, which rapidly transfers electrons, 13 A away, to a trinuclear copper cluster composed of the T2 and T3 sites, where dioxygen is reduced to water in two sequential 2e(-) steps. This study focuses on two variants of Fet3p, H126Q and H483Q, that perturb the two T3 Cu's, T3alpha and T3beta, respectively. The variants have been isolated in both holo and type 1 depleted (T1D) forms, T1DT3alphaQ and T1DT3betaQ, and their trinuclear copper clusters have been characterized in their oxidized and reduced states. While the variants are only mildly perturbed relative to T1D in the resting oxidized state, in contrast to T1D they are both found to have lost a ligand in their reduced states. Importantly, T1DT3alphaQ reacts with O(2), but T1DT3betaQ does not. Thus loss of a ligand at T3beta, but not at T3alpha, turns off O(2) reactivity, indicating that T3beta and T2 are required for the 2e(-) reduction of O(2) to form the peroxide intermediate (PI), whereas T3alpha remains reduced. This is supported by the spectroscopic features of PI in T1DT3alphaQ, which are identical to T1D PI. This selective redox activity of one edge of the trinuclear cluster demonstrates its asymmetry in O(2) reactivity. The structural origin of this asymmetry between the T3alpha and T3beta is discussed, as is its contribution to reactivity.

3D Coordination Framework with Uncommon Two‐Fold Interpenetrated {33⋅59⋅63}‐lcy Net and Coordinated Anion Exchange

Unusual {3(3)5(9)6(3)}-lcy topology has been found in an uncommon 3D six-connected, two-fold interpenetrated {3(3)5(9)6(3)}-lcy net (illustrated). The coordinated SO(4) (2-) anions in this framework can undergo a full exchange with Cl(-) anions, in the course of which the crystals change color as shown. The process has a solvent-mediated rather than a solid-state mechanism.Reaction of tetrazole-1-acetic acid with CuSO(4)5H(2)O produces two novel 3D coordination frameworks: ([Cu(3)(mu(3)-OH)(eta(1):eta(1):eta(1):mu(3)-SO(4))(tza)(3)]3 CH(3)OHH(2)O}(n) (1; Htza=tetrazole-1-acetic acid) and {[Cu(4)(mu(2)-OH)(eta(2):eta(2):mu(4)-SO(4))(tta)(5)]3 H(2)O}(n) (2; Htta=tetrazole). Framework 1 is constructed from a trinuclear copper cluster [Cu(3)(mu(3)-OH)(eta(1):eta(1):eta(1):mu(3)-SO(4))] and displays a six-connected framework with uncommon {3(3)5(9)6(3)}-lcy topology. Framework 2 presents a complicated five-nodal (3,4,6)-connected net with (4(2)6)(34(2)5(2)7)(4(3)6(2)8)(3(2)4(8)5(3)6(2))(4(6)8(6)10(3)) topology. The eta(2):eta(2):mu(4)-SO(4) bridging mode in 2 has not been found in the reported coordination polymers. The coordinated SO(4) (2-) anions in 1 can be replaced completely when the solid polymer is treated with an aqueous methanolic solution containing Cl(-) anions. Detailed atomic force microscopy studies indicate a solvent-mediated rather than a solid-state mechanism for the exchange process.

Crystal Structure of a Two-domain Multicopper Oxidase

The two-domain multicopper oxidases are proposed to be key intermediates in the evolution of three-domain multicopper oxidases. A number of two-domain multicopper oxidases have been identified from genome sequences and are classified as type A, type B, or type C on the basis of the predicted location of the type 1 copper center. The crystal structure of blue copper oxidase, a type C two-domain multicopper oxidase from Nitrosomonas europaea, has been determined to 1.9 A resolution. Blue copper oxidase is a trimer, of which each subunit comprises two cupredoxin domains. Each subunit houses a type 1 copper site in domain 1 and a type 2/type 3 trinuclear copper cluster at the subunit-subunit interface. The coordination geometry at the trinuclear copper site is consistent with reduction of the copper ions. Although the overall architecture of blue copper oxidase is similar to nitrite reductases, detailed structural alignments show that the fold and domain orientation more closely resemble the three-domain multicopper oxidases. These observations have important implications for the evolution of nitrite reductases and multicopper oxidases.

X-ray structure of a two-domain type laccase: A missing link in the evolution of multi-copper proteins

A multi-copper protein with two cupredoxin-like domains was identified from our in-house metagenomic database. The recombinant protein, mgLAC, contained four copper ions/subunits, oxidized various phenolic and non-phenolic substrates, and had spectroscopic properties similar to common laccases. X-ray structure analysis revealed a homotrimeric architecture for this enzyme, which resembles nitrite reductase (NIR). However, a difference in copper coordination was found at the domain interface. mgLAC contains a T2/T3 tri-nuclear copper cluster at this site, whereas a mononuclear T2 copper occupies this position in NIR. The trimer is thus an essential part of the architecture of two-domain multi-copper proteins, and mgLAC may be an evolutionary precursor of NIR.

Trinuclear Triangular Copper(II) Clusters – Synthesis, Electrochemical Studies and Catalytic Peroxidative Oxidation of Cycloalkanes

AbstractThe reactions of CuII carboxylates (valerate, 2‐methylbutyrate, hexanoate, heptanoate) with pyrazole (Hpz) in EtOH or EtOH/water solutions easily afford the triangular trinuclear copper derivatives [Cu3(μ3‐OH)(μ‐pz)3(RCOO)2(L)x] [R = CH3(CH2)3, L = H2O, x = 1 for 5; R = CH3CH2CH(CH3), L = EtOH, x = 2 for 6; R = CH3(CH2)4, L = EtOH, x = 1 for 7; R = CH3(CH2)5, L = EtOH, x = 1 for 8] as it has been previously found for R = H, L = Hpz, x = 2, (1); R = CH3, L = Hpz, x = 1, (2); R = CH3CH2, L = EtOH, x = 1, (3) and [Cu3(μ3‐OH)(μ‐pz)3(CH3(CH2)2COO)2(MeOH)(H2O)], (4). The trinuclear structure common to 5–8 has been assigned on the basis of magnetic susceptibility studies, ESI MS, IR and UV/Vis spectroscopy as well as 1H NMR measurements. The room temp. magnetic susceptibilities of 5–8 almost correspond to the presence of a single unpaired electron for each trinuclear unit. The IR spectra exhibit signals due to the bridging μ3‐OH in accordance with what was observed in the spectra of 1–4. Solid‐state and MeOH solution UV/Vis spectra show the same features previously reported for 1–4 and 1H NMR spectra of 1–8 show almost identical low field signals that can be assigned to pz– hydrogens. A detailed investigation of the supramolecular structures of 1 and 4 and the single‐crystal X‐ray study of the polymeric paddlewheel Cu(2‐methylbutyrate)2, A, are also reported. Electrochemical experiments show that in 1–8 the CuII ions can be reduced, in distinct steps, to CuI and Cu0. All the complexes act as catalysts or catalyst precursors for the efficient peroxidative oxidation, by aqueous hydrogen peroxide in acetonitrile and at room temp., of cycloalkanes RH (cyclohexane and cyclopentane) to the corresponding cyclic alcohols and ketones, with overall yields of up to 34 % and TONs up to 42. Radical pathways involving the formation of alkyl hydroperoxides (ROOH) are involved.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Biomimetic Modelling of Copper Enzymes: Synthesis, Characterization, EPR Analysis and Enantioselective Catalytic Oxidations by a New Chiral Trinuclear Copper(II) Complex

AbstractThe new octadentate ligand (R)‐(+)‐N,N′‐dimethyl‐N,N′‐bis{‐2‐[bis(1‐methyl‐2‐benzimidazolylmethyl)]‐2‐methylaminoethyl}‐1,1′‐binaphthyl‐2,2′diamine [(R)‐(+)‐DABN‐L‐Ala‐Bz4; L] was employed for the synthesis of dinuclear and trinuclear copper(II) complexes. The ligand design is based on the insertion of chiral residues derived from L‐alanine between the diaminobinaphthyl (R)‐(+)‐DABN spacer and the aminobis(benzimidazole) metal binding units. The chiroptical properties of the ligand and the complexes are described. EPR experiments were performed on [Cu2L]4+ and [Cu3L]6+ at low temperatures. In the case of [Cu2L]4+, a weak dipolar interaction between the two spin centres was found. A similar weak spin interaction occurred in the trinuclear copper cluster, which could be treated likewise as a weakly coupled three‐spin system. The analysis was substantiated by studying the complex EPR temperature behaviour, where population of the quartet state occurred only at high temperature (77 K), whereas at cryogenic temperatures (4 K) the system adopted a doublet state as a ground‐state spin configuration. Titration with sodium azide of [Cu2L]4+ was consistent with terminal binding of one N3– molecule to each copper ion. Furthermore, dipolar interactions between spin centres were strongly suppressed in this case. For [Cu3L]6+, the adduct formed by the interaction with two azido molecules induced formation of a μ‐azido bridges among the three Cu2+ ions and led to population of the quartet state even at cryogenic temperatures. This bridged configuration was, however, lost upon further addition of azido molecules. The copper(II) complexes were tested as catalysts in the oxidation of biogenic catechols and flavonoids by dioxygen to give the corresponding quinones, which were trapped as adducts with MBTH. The dinuclear complex [Cu2L]4+ displays poor substrate enantiodifferentiating ability, even though it exhibits catalytic activity comparable to that of [Cu3L]6+. The trinuclear complex [Cu3L]6+ exhibits significant enantioselectivity in the oxidations of the catecholamines L‐/D‐dopa methyl ester and L‐/D‐norepinephrine. The origin of this enantioselectivity must be associated with the mode of substrate binding, as it depends almost entirely on KM.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A two-dimensional copper polymer constructed from rod-shaped ferromagnetic secondary building units

On the basis of rod-shaped secondary building units (SBUs), a novel two-dimensional (2D) azide–copper polymer, [Cu3(N3)4(anol)2]∞ (1; anol=aminoethanol), was synthesized and structurally characterized. Complex 1 is a neutral 2D coordination network consisting of trinuclear copper(II) clusters [Cu3(N3)4(anol)2] connected with the bridge of end-end (EE) azide group. Magnetic data analysis shows that a metamagnetic behavior is observed for 1 which is associated with significant ferromagnetic coupling intracluster over 4K.

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.

Controlled Oxidation of Hydrocarbons by the Membrane-Bound Methane Monooxygenase: The Case for a Tricopper Cluster

[Figure: see text]. The growing need for inexpensive methods to convert methane to methanol has sparked considerable interest in methods that catalyze this process. The integral membrane protein particulate methane monooxygenase (pMMO) mediates the facile conversion of methane to methanol in methanotrophic bacteria. Most evidence indicates that pMMO is a multicopper enzyme, and these copper ions support redox, dioxygen, and oxo-transfer chemistry. However, the exact identity of the copper species that mediates the oxo-transfer chemistry remains an area of intense debate. This highly complex enzyme is notoriously difficult to purify because of its instability outside the lipid bilayer and tendency to lose its essential metal cofactors. For this reason, pMMO has resisted both initial identification and subsequent isolation and purification for biochemical and biophysical characterization. In this Account, we describe evidence that pMMO is a multicopper protein. Its unique trinuclear copper cluster mediates dioxygen chemistry and O-atom transfer during alkane hydroxylation. Although a recent crystal structure did not show this tricopper cluster, we provide compelling evidence for such a cluster through redox potentiometry and EPR experiments on the "holo" enzyme in pMMO-enriched membranes. We also identify a site in the structure of pMMO that could accommodate this cluster. A hydrophobic pocket capable of harboring pentane, the enzyme's largest known substrate, lies adjacent to this site. In addition, we have designed and synthesized model tricopper clusters to provide further chemical evidence that a tricopper cluster mediates the enzyme's oxo-transfer chemistry. These biomimetic models exhibit similar spectroscopic properties and chemical reactivity to the putative tricopper cluster in pMMO. Based on computational analysis using density functional theory (DFT), triangular tricopper clusters are capable of harnessing a "singlet oxene" upon activation by dioxygen. An oxygen atom is then inserted via a concerted process into the C-H bond of an alkane in the transition state during hydroxylation. The turnover frequency and kinetic isotope effect predicted by DFT show excellent agreement with experimental data.

Role of Copper in Thermal Stability of Human Ceruloplasmin

Human ceruloplasmin (CP) is a multicopper oxidase essential for normal iron homeostasis. The protein has six domains with one type-1 copper in each of domains 2, 4, and 6; the remaining coppers form a catalytic trinuclear cluster at the interface between domains 1 and 6. To assess the role of the coppers in CP thermal stability, we have probed the thermal unfolding process as a function of scan rate of holo- and apo-forms using several detection methods (circular dichroism, aromatic and 8-anilino-naphthalene-1-sulfonic acid fluorescence, visible absorption, activity, and differential scanning calorimetry). Both species of CP undergo irreversible thermal reactions to denatured states with significant residual structure. For identical scan rates, the thermal midpoint appears at temperatures 15–20° higher for the holo- as compared with the apo- form. The thermal data for both forms were fit by a mechanistic model involving two consecutive, irreversible steps ( N → I → D). The holo-intermediate, I, has lost one oxidized type-1 copper and secondary structure in at least one domain; however, the trinuclear copper cluster remains intact as it is functional in oxidase activity. The activation parameters obtained from the fits to the thermal transitions were used to assess the kinetic stability of apo- and holo-CP at physiological temperatures (i.e., at 37°C). It emerges that native CP (i.e., with six coppers) is rather unstable and converts to I in <1 day at 37°C. Nonetheless, this form remains intact for more than 2 weeks and may thus be a biologically relevant state of CP in vivo. In contrast, apo-CP unfolds rapidly: the denatured state is reached in <2 days at 37°C.

Altering the laccase functionality by in vivo assembly of mutant libraries with different mutational spectra

The generation of diversity for directed protein evolution experiments shows an important bottleneck in the in vitro random mutagenesis protocols. Most of them are biased towards specific changes that eventually confer a predicted and conservative mutational spectrum, limiting the exploration of the vast protein space. The current work describes a simple methodology to in vivo recombine mutant libraries with different nucleotide bias created by in vitro methods. This in vivo assembly was based on the accurate physiology of Saccharomyces cerevisiae, which as host, provided its high homologous recombination frequency to shuffle the libraries in a nonmutagenic way. The fungal thermophilic laccase from Myceliophthora thermophila expressed in S. cerevisiae was submitted to this protocol under the selective pressure of high concentrations of organic solvents. Mutant 2E9 with approximately 3-fold better kinetics than parent type showed two consecutive amino acid changes (G614D -GGC/GAC- and E615K -GAG/AAG-) because of the in vivo shuffling of the mutant libraries. Both mutations are located in the C-terminal tail that is specifically processed at the Golgi during the maturation of the protein by the Kex2 protease. Notoriously, the oxygen consumption at the T2/T3 trinuclear copper cluster was altered and the catalytic copper at the T1 site was perturbed showing differences in its redox potential and geometry. The change in the isoelectric point of C-terminal extension upon mutations seems to affect the folding of the protein at the posttranslational processing steps providing new insights in the significance of the C-terminal tail for the functionality of the ascomycete laccases.

Spectroscopic and Kinetic Studies of Perturbed Trinuclear Copper Clusters: The Role of Protons in Reductive Cleavage of the O−O Bond in the Multicopper Oxidase Fet3p

The multicopper oxidase Fet3p couples four 1e(-) oxidations of substrate to the 4e(-) reduction of O2 to H2O. Fet3p uses four Cu atoms to accomplish this reaction: the type 1, type 2, and coupled binuclear type 3 sites. The type 2 and type 3 sites together form a trinuclear Cu cluster (TNC) which is the site of O2 reduction. This study focuses on mutants of two residues, E487 and D94, which lie in the second coordination sphere of the TNC and defines the role that each plays in the structural integrity of the TNC, its reactivity with O2, and in the directional movement of protons during reductive cleavage of the O-O bond. The E487D, E487A, and D94E mutants have been studied in the holo and type 1 depleted (T1D) forms. Residue E487, located near the T3 center, is found to be responsible for donation of a proton during the reductive cleavage of the O-O bond in the peroxide intermediate and an inverse kinetic solvent isotope effect, which indicates that this proton is already transferred when the O-O bond is cleaved. Residue D94, near the T2 site, plays a key role in the reaction of the reduced TNC with O2 and drives electron transfer from the T2 Cu to cleave the O-O bond by deprotonating the T2 Cu water ligand. A mechanism is developed where these second sphere residues participate in the proton assisted reductive cleavage of the O-O bond at the TNC.

Self‐Assembly and Anion‐Exchange Properties of a Discrete Cage and 3D Coordination Networks Based on Cage Structures

By using tridentate ligand 4-(3-pyridinyl)-1,2,4-triazole (pytrz), cage-like complexes of {[Cu(mu2-pytrz)2](ClO4)(SO4)0.5C2H5OH.0.25 H2O}6 (1), {[Cu3(mu3-pytrz)4(mu2-Cl)2(H2O)2](ClO4)2Cl(2).2 H2O}n (2), and {[Cu3(mu3-pytrz)3(mu3-O)(H2O)3](ClO4)2.5(BF4)(1.5)5.25 H2O}n (3) have been synthesized with different copper(II) salts. Complex 1 represents the second example of a M6L12 metal-organic octahedron with an overall Th symmetry. Complex 2 is constructed from a 3(8) cage-building unit (CBU) and each CBU connects six neighboring cages to give the first 3D metal-organic framework (MOF) based on octahedral M6L12. Complex 3 is built from Cu24(pytrz)12 CBUs with the trinuclear copper clusters serving as second building units (SBUs) and decorating each corner of the M24L12 polyhedron. The Cu24(pytrz)12 building unit is linked by extra ligands to give an extended 3D framework that has the formula Cu24(pytrz)24 and possesses a CaB6 topology. The mixed anions ClO4- and BF4- in 3 are both included in the inner cavity of the cage and can be completely exchanged by ClO4- through the open windows of the cage, as evidenced by the crystal structure of the 3D MOF {[Cu3(mu3-pytrz)3(mu3-O)(H2O)3](ClO4)(4)4.5 H2O}n (4). Complex 4 can also be synthesized when employing 1 as a precursor in an extensive study of the anion-exchange reaction. This represents the first successful conversion of a discrete cage into a 3D coordination network based on a cage structure. Complex 2 remains invariable during anion-exchange reactions because uncoordinated Cl- ions are located in the comparatively small inner cavity.

Facile O-atom insertion into CC and CH bonds by a trinuclear copper complex designed to harness a singlet oxene

Two trinuclear copper [Cu(I)Cu(I)Cu(I)(L)](1+) complexes have been prepared with the multidentate ligands (L) 3,3'-(1,4-diazepane-1,4-diyl)bis(1-((2-(dimethylamino)ethyl)(methyl)amino)propan-2-ol) (7-Me) and (3,3'-(1,4-diazepane-1,4-diyl)bis(1-((2-(diethylamino) ethyl)(ethyl) amino)propan-2-ol) (7-Et) as models for the active site of the particulate methane monooxygenase (pMMO). The ligands were designed to form the proper spatial and electronic geometry to harness a "singlet oxene," according to the mechanism previously suggested by our laboratory. Consistent with the design strategy, both [Cu(I)Cu(I)Cu(I)(L)](1+) reacted with dioxygen to form a putative bis(mu(3)-oxo)Cu(II)Cu(II)Cu(III) species, capable of facile O-atom insertion across the central C-C bond of benzil and 2,3-butanedione at ambient temperature and pressure. These complexes also catalyze facile O-atom transfer to the C-H bond of CH(3)CN to form glycolonitrile. These results, together with our recent biochemical studies on pMMO, provide support for our hypothesis that the hydroxylation site of pMMO contains a trinuclear copper cluster that mediates C-H bond activation by a singlet oxene mechanism.

A two-dimensional coordination polymer containing linear trinuclear copper(II) clusters

The title coordination polymer, poly[diaqua­(1,10-phen­an­throline)(μ3-pyrazole-3,5-dicarboxyl­ato)tricopper(II)], [Cu3(C5HN2O4)2(C12H8N2)2(H2O)2]n, was hydro­thermally synthesized and structurally characterized. It consists of linear trinuclear copper(II) clusters of [Cu3(pdc)2(phen)2(H2O)2] units (H3pdc = pyrazole-3,5-dicarboxylic acid and phen = 1,10-phenanthroline), in which two pdc3− ligands chelate three CuII ions with the central CuII ion on an inversion center. An infinite two-dimensional sheet structure is constructed by connecting adjacent linear trinuclear copper(II) units through coordination bonds between the central CuII atom and an O atom of the carboxyl­ate group coordinated to the terminal CuII atom from the pdc3− ligand.

Redox Potentiometry Studies of Particulate Methane Monooxygenase: Support for a Trinuclear Copper Cluster Active Site

Eine Tasche voller Kupfer: Redoxpotentiometrie- und EPR-Experimente belegen, dass das aktive Zentrum der partikulären Methan-Monooxygenase (pMMO), eines membrangebundenen Enzyms, das unter Umgebungsbedingungen Methan zu Methanol hydroxyliert, zusätzlich zu dem zweikernigen Kupfercluster, der früher röntgenographisch nachgewiesen wurde, aus einem dreikernigen Kupfercluster (siehe Bild) und einem Typ-2-Kupferzentrum besteht. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2007/z604647_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Redox Potentiometry Studies of Particulate Methane Monooxygenase: Support for a Trinuclear Copper Cluster Active Site

Redox Potentiometry Studies of Particulate Methane Monooxygenase: Support for a Trinuclear Copper Cluster Active Site

Combinatorial Saturation Mutagenesis by In Vivo Overlap Extension for the Engineering of Fungal Laccases

Combinatorial saturation mutagenesis -CSM- is a valuable tool for improving enzymatic properties from hot-spot residues discovered by directed enzyme evolution or performing semi-rational studies. CSM coupled to a reliable high-throughput screening assay -coefficient of variance below 10%- has been used to enhance turnover rates in the fungal laccase variant T2 from Myceliophthora thermophila. The influence of the highly conserved pentapeptide 509-513 on the redox potential of blue-copper containing enzymes is well described. We focused combinatorial saturation mutagenesis in residues Ser510 and Leu513. Libraries were constructed in Saccharomyces cerevisiae by in vivo overlap extension -IVOE- of the PCR products. This methodology provides a simple manner to build CSM libraries avoiding extra PCR reactions, by-products formation and in vitro ligation steps. After exploring more than 1,700 clones, mutant (7E1) with approximately 3-fold higher kinetics than parent type was found. 7E1 showed one synonymous mutation (L513L, CGT/TTG) and one beneficial mutation S510G (TCG/GGG) that can not be achieved by conventional error-prone PCR techniques. Mutation S510G seems to affect the C-terminal plug, which modulates the transit of water and oxygen to the trinuclear copper cluster.

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.