The Thermodynamics, Kinetics, and Molecular Mechanism of Intramolecular Electron Transfer in Human Ceruloplasmin

Molecular response to the influences of Cu(II) and Fe(III) on forming biogenic manganese oxides by Pseudomonas putida MnB1

The biogeochemical cycle of biogenic manganese oxides (BioMnOx) is closely associated with the environmental behavior and fate of various pollutants. It is significantly interfered by many metals, such as Cu and Fe. However, the bacterial molecular responses are not clear. Here, the effects of Cu(II) and Fe(III) on oxidation of manganese by Pseudomonas putida MnB1 and the bacterial molecular response mechanisms have been studied. The bacterial oxidation of manganese were promoted by both Fe(III) and Cu(II) and the final manganese oxidation rate of the Cu(II) group exceeded 16 % that of the Fe(III) group. The results of transcriptome indicated that Cu(II) promoted manganese oxidation by up-regulating the expression levels of multicopper oxidase (MCO) and peroxidase(POD), and by stimulating electron transfer, while Fe(III) promoted this process by accelerating the electron transfer and nitrogen cycling, and activating POD. The protein-protein interaction (PPI) network indicated that the MCO genes (mnxG and mcoA) were directly linked to the copper homeostasis proteins (cusA, cusB, czcC and cusF). Cytochrome c was closely related to the genes related to nitrogen cycling (glnA, glnL, and putA) and electrons transfer (cycO, cycD, nuoA, nuoK, and nuoL), which also promoted manganese oxidation. This study provides a molecular level insight into the oxidation of Mn(II) by Pseudomonas putida MnB1 with Cu(II) and/or Fe(III) ions.

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.

A review on bacterial redox dependent iron transporters and their evolutionary relationship

Iron is an essential yet toxic micronutrient and its transport across biological membranes is tightly regulated in all living organisms. One such iron transporter, the Ftr-type permeases, is found in both eukaryotic and prokaryotic cells. These Ftr-type transporters are required for iron transport, predicted to form α-helical transmembrane structures, and conserve two ArgGluxxGlu (x = any amino acid) motifs. In the yeast Ftr transporter (Ftr1p), a ferroxidase (Fet3p) is required for iron transport in an oxidation coupled transport step. None of the bacterial Ftr-type transporters (EfeU and FetM from E. coli; cFtr from Campylobacter jejuni; FtrC from Brucella, Bordetella, and Burkholderia spp.) contain a ferroxidase protein. Bioinformatics report predicted periplasmic EfeO and FtrB (from the EfeUOB and FtrABCD systems) as novel cupredoxins. The Cu²⁺ binding and the ferrous oxidation properties of these proteins are uncharacterized and the other two bacterial Ftr-systems are expressed without any ferroxidase/cupredoxin, leading to controversy about the mode of function of these transporters. Here, we review published data on Ftr-type transporters to gain insight into their functional diversity. Based on original bioinformatics data presented here evolutionary relations between these systems are presented.

Ceruloplasmin in flatland: the relationship between enzyme catalytic activity and surface hydrophilicity

The effective immobilization of the enzyme on the substrate surface plays a key role especially in biocatalysis, medicine or industry. Herein, we showed the influence of substrate hydrophilicity on the activity of the physically immobilized ceruloplasmin. To control the hydrophilicity of the substrate, thiols with various terminal groups were used. We have found that the effectiveness of the catalytic process of multimeric protein is the highest in the situation of application of the highly hydrophilic substrate. In the case of physical adsorption, the orientation of the enzyme is random, however the application of the appropriate modifying layer enforces the desired enzyme orientation. The quartz crystal microbalance with dissipation (QCM-D) results showed that the crucial parameter for the highest and most durable catalytic activity of the enzyme is the orientation, not the amount of the physically adsorbed enzyme.

Surface hydrophilicity – the way to control the activity of the immobilized enzyme.

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.

Redox-Neutral S-nitrosation Mediated by a Dicopper Center

A redox-neutral S-nitrosation of thiol has been achieved at a dicopper(I,I) center. Treatment of dicopper (I,I) complex with excess NO^. and thiol generates a dicopper (I,I) di-S-nitrosothiol complex [Cu^I Cu^I (RSNO)₂ ]²⁺ or dicopper (I,I) mono-S-nitrosothiol complex [Cu^I Cu^I (RSNO)]²⁺ , which readily release RSNO in 88-94 % yield. The S-nitrosation proceeds by a mixed-valence [Cu^II Cu^III (μ-O)(μ-NO)]²⁺ species, which deprotonates RS-H at the basic μ-O site and nitrosates RS^- at the μ-NO site. The [Cu^II Cu^III (μ-O)(μ-NO)]²⁺ complex is also competent for O-nitrosation of MeOH. A rare [Cu^II Cu^II (μ-NO)(OMe)]²⁺ intermediate was isolated and fully characterized, suggesting the S-nitrosation may proceed through the intermediary of analogous [Cu^II Cu^II (μ-NO)(SR)]²⁺ species. This redox- and proton-neutral S-nitrosation process is the first functional model of ceruloplasmin in mediating S-nitrosation of external thiols, with implications for biological copper sites in the interconversion of NO^. /RSNO.

Autotolerant ceruloplasmin based biocathodes for implanted biological power sources

High-performance autotolerant bioelectrodes should be ideally suited to design implantable bioelectronic devices. Because of its high redox potential and ability to reduce oxygen directly to water, human ceruloplasmin, HCp, the only blue multicopper oxidase present in human plasma, appears to be the ultimate biocatalyst for oxygen biosensors and also biocathodes in biological power sources. In comparison to fungal and plant blue multicopper oxidases, e.g. Myrothecium verrucaria bilirubin oxidase and Rhus vernicifera laccase, respectively, the inflammatory response to HCp in human blood is significantly reduced. Partial purification of HCp allowed to preserve the native conformation of the enzyme and its biocatalytic activity. Therefore, electrochemical studies were carried out with the partially purified enzyme immobilised on nanostructured graphite electrodes at physiological pH and temperature. Amperometric investigations revealed low reductive current densities, i.e. about 1.65 µA cm^-2 in oxygenated electrolyte and in the absence of any mediator, demonstrating nevertheless direct electron transfer based O₂ bioelectroreduction by HCp for the first time. The reductive current density obtained in the mediated system was about 12 µA cm^-2. Even though the inflammatory response of HCp is diminished in human blood, inadequate bioelectrocatalytic performance hinders its use as a cathodic bioelement in a biofuel cell.

Role of a Tyrosine Radical in Human Ceruloplasmin Catalysis

Multicopper oxidases (MCOs) are a large family of diverse enzymes found in both eukaryotes and prokaryotes that couple one-electron oxidations of various substrates to the four-electron reduction of O₂ to H₂O, functioning through a set of metallocofactors consisting of one type 1 copper (T1 Cu) and one trinuclear copper cluster (TNC). Human serum ceruloplasmin (Cp) is a unique member of MCOs composed of six cupredoxin domains and harbors six Cu ions arranged as three T1 Cu and one TNC. The native substrate of Cp is Fe²⁺. It is an essential ferroxidase critical for iron homeostasis and is closely associated with metal-mediated diseases and metal neurotoxicity. In human serum, Cp operates under substrate-limiting low [Fe²⁺] but high [O₂] conditions, implying the possible involvement of partially reduced intermediates in Cp catalysis. In this work, we studied for the first time Cp reactivities at defined partially reduced states and discovered a tyrosine radical weakly magnetically coupled to the native intermediate (NI) of the TNC via a hydrogen bond. Our results lead to a new hypothesis that human iron transport is regulated as the paired transfer of iron from ferroportin to Cp to transferrin, and the tyrosine residue in Cp acts as a gate to avoid reactive oxygen species (ROS) formation when Fe²⁺ delivery is dysregulated.

Chloride Control of the Mechanism of Human Serum Ceruloplasmin (Cp) Catalysis

Unraveling the mechanism of ceruloplasmin (Cp) is fundamentally important toward understanding the pathogenesis of metal-mediated diseases and metal neurotoxicity. Here we report that Cl^-, the most abundant anion in blood plasma, is a key component of Cp catalysis. Based on detailed spectroscopic analyses, Cl^- preferentially interacts with the partially reduced trinuclear Cu cluster (TNC) in Cp under physiological conditions and shifts the electron equilibrium distribution among the two redox active type 1 (T1) Cu sites and the TNC. This shift in potential enables the intramolecular electron transfer (IET) from the T1 Cu to the native intermediate (NI) and accelerates the IET from the T1 Cu to the TNC, resulting in faster turnover in Cp catalysis.

Mn(III) species formed by the multi-copper oxidase MnxG investigated by electron paramagnetic resonance spectroscopy

The multi-copper oxidase (MCO) MnxG from marine Bacillus bacteria plays an essential role in geochemical cycling of manganese by oxidizing Mn²⁺(aq) to form manganese oxide minerals at rates that are three to five orders of magnitude faster than abiotic rates. The MCO MnxG protein is isolated as part of a multi-protein complex, denoted as Mnx, which includes one MnxG unit and a hexamer of MnxE₃F₃ subunit. During the oxidation of Mn²⁺(aq) catalyzed by the Mnx protein complex, an enzyme-bound Mn(III) species was trapped recently in the presence of pyrophosphate (PP) and analyzed using parallel-mode electron paramagnetic resonance (EPR) spectroscopy. Herein, we provide a full analysis of this enzyme-bound Mn(III) intermediate via temperature dependence studies and spectral simulations. This Mnx-bound Mn(III) species is characterized by a hyperfine-coupling value of A(⁵⁵Mn) = 4.2 mT (corresponding to 120 MHz) and a negative zero-field splitting (ZFS) value of D = - 2.0 cm^-1. These magnetic properties suggest that the Mnx-bound Mn(III) species could be either six-coordinate with a ⁵B_1g ground state or square-pyramidal five-coordinate with a ⁵B₁ ground state. In addition, as a control, Mn(III)PP is also analyzed by parallel-mode EPR spectroscopy. It exhibits distinctly different magnetic properties with a hyperfine-coupling value of A(⁵⁵Mn) = 4.8 mT (corresponding to 140 MHz) and a negative ZFS value of D = - 2.5 cm^-1. The different ZFS values suggest differences in ligand environment of Mnx-bound Mn(III) and aqueous Mn(III)PP species. These studies provide further insights into the mechanism of biological Mn²⁺(aq) oxidation.

Molecular Aspects of a Robust Assay for Ferroxidase Function of Ceruloplasmin

Ceruloplasmin (Cp) is one of the most complex multicopper oxidase enzymes and plays an essential role in the metabolism of iron in mammals. Ferrous ion supplied by the ferroportin exporter is converted by Cp to ferric ion that is accepted by plasma metallo-chaperone transferrin. Study of the enzyme at the atomic and molecular level has been hampered by the lack of a suitable ferrous substrate. We have developed the classic chromophoric complex Fe^IIH_x(Tar)₂ (H₂Tar, 4-(2-thiazolylazo)resorcinol; x = 0-2; overall charge omitted) as a robust substrate for evaluation of the ferroxidase function of Cp and related enzymes. The catalysis can be followed conveniently in real time by monitoring the solution absorbance at 720 nm, a fingerprint of Fe^IIH_x(Tar)₂. The complex is oxidized to its ferric form Fe^IIIH_x(Tar)₂ via the overall reaction sequence Fe^IIH_x(Tar)₂ → Fe^II-Cp → Fe^III-Cp → Fe^IIIH_x(Tar)₂: i.e., Fe(II) is transferred formally from Fe^IIH_x(Tar)₂ to the substrate docking/oxidation (SDO) site(s) in Cp, followed by oxidation to product Fe(III) that is trapped again by the ligand. Each Tar ligand in the above bis-complex coordinates the metal center in a meridional tridentate mode involving a pH-sensitive -OH group (pK_a > 12), and this imposes rapid Fe(II) and Fe(III) transfer kinetics to facilitate the catalytic process. The formation constants of both the ferrous and ferric complexes at pH 7.0 were determined (log β₂' = 13.6 and 21.6, respectively), as well as an average dissociation constant of the SDO site(s) in Cp (log K_D' = -7.2).

Biogenic Manganese-Oxide Mineralization is Enhanced by an Oxidative Priming Mechanism for the Multi-Copper Oxidase, MnxEFG

In a natural geochemical cycle, manganese-oxide minerals (MnO_x ) are principally formed through a microbial process, where a putative multicopper oxidase MnxG plays an essential role. Recent success in isolating the approximately 230 kDa, enzymatically active MnxEFG protein complex, has advanced our understanding of biogenic MnO_x mineralization. Here, the kinetics of MnO_x formation catalyzed by MnxEFG are examined using a quartz crystal microbalance (QCM), and the first electrochemical characterization of the MnxEFG complex is reported using Fourier transformed alternating current voltammetry. The voltammetric studies undertaken using near-neutral solutions (pH 7.8) establish the apparent reversible potentials for the Type 2 Cu sites in MnxEFG immobilized on a carboxy-terminated monolayer to be in the range 0.36-0.40 V versus a normal hydrogen electrode. Oxidative priming of the MnxEFG protein complex substantially enhances the enzymatic activity, as found by in situ electrochemical QCM analysis. The biogeochemical significance of this enzyme is clear, although the role of an oxidative priming of catalytic activity might be either an evolutionary advantage or an ancient relic of primordial existence.

Assembling Paramagnetic Ceruloplasmin at Electrode Surfaces Covered with Ferromagnetic Nanoparticles. Scanning Electrochemical Microscopy in the Presence of a Magnetic Field

Adsorption of ceruloplasmin (Cp) at a gold electrode modified with ferromagnetic iron nanoparticles encapsulated in carbon (Fe@C Nps) leads to a successful immobilization of the enzyme in its electroactive form. The proper placement of Cp at the electrode surface on top of the nanocapsules containing an iron core allowed a preorientation of the enzyme, hence allowing direct electron transfer between the electrode and the enzyme. Laser ablation coupled with inductively coupled plasma mass spectrometry indicated that Cp was predominantly located at the paramagnetic nanoparticles. Scanning electrochemical microscopy measurements in the sample-generation/tip-collection mode proved that Cp was ferrooxidative inactive if it was immobilized on the bare gold surface and reached the highest activity if it was adsorbed on Fe@C Nps in the presence of a magnetic field.

Electron transfer and reaction mechanism of laccases

Laccases are part of the family of multicopper oxidases (MCOs), which couple the oxidation of substrates to the four electron reduction of O2 to H2O. MCOs contain a minimum of four Cu's divided into Type 1 (T1), Type 2 (T2), and binuclear Type 3 (T3) Cu sites that are distinguished based on unique spectroscopic features. Substrate oxidation occurs near the T1, and electrons are transferred approximately 13 Å through the protein via the Cys-His pathway to the T2/T3 trinuclear copper cluster (TNC), where dioxygen reduction occurs. This review outlines the electron transfer (ET) process in laccases, and the mechanism of O2 reduction as elucidated through spectroscopic, kinetic, and computational data. Marcus theory is used to describe the relevant factors which impact ET rates including the driving force, reorganization energy, and electronic coupling matrix element. Then, the mechanism of O2 reaction is detailed with particular focus on the intermediates formed during the two 2e(-) reduction steps. The first 2e(-) step forms the peroxide intermediate, followed by the second 2e(-) step to form the native intermediate, which has been shown to be the catalytically relevant fully oxidized form of the enzyme.

Multicopper oxidase involvement in both Mn(II) and Mn(III) oxidation during bacterial formation of MnO(2)

Global cycling of environmental manganese requires catalysis by bacteria and fungi for MnO(2) formation, since abiotic Mn(II) oxidation is slow under ambient conditions. Genetic evidence from several bacteria indicates that multicopper oxidases (MCOs) are required for MnO(2) formation. However, MCOs catalyze one-electron oxidations, whereas the conversion of Mn(II) to MnO(2) is a two-electron process. Trapping experiments with pyrophosphate (PP), a Mn(III) chelator, have demonstrated that Mn(III) is an intermediate in Mn(II) oxidation when mediated by exosporium from the Mn-oxidizing bacterium Bacillus SG-1. The reaction of Mn(II) depends on O(2) and is inhibited by azide, consistent with MCO catalysis. We show that the subsequent conversion of Mn(III) to MnO(2) also depends on O(2) and is inhibited by azide. Thus, both oxidation steps appear to be MCO-mediated, likely by the same enzyme, which is indicated by genetic evidence to be the MnxG gene product. We propose a model of how the manganese oxidase active site may be organized to couple successive electron transfers to the formation of polynuclear Mn(IV) complexes as precursors to MnO(2) formation.

Channeling of electrons within SLAC, the small laccase from Streptomyces coelicolor

The reduction kinetics of the fluorescently labeled small laccase (SLAC) from Streptomyces coelicolor was studied by stopped flow kinetic measurements. The tryptophan fluorescence and the emission from a covalently attached label were used to selectively follow the progress of the reduction of the trinuclear copper center (TNC) and the type-1 (T1) Cu site in the enzyme as a function of time. A numerical analysis of the kinetic traces provided new insight into the midpoint potential difference between the T1 and the TNC site as the TNC becomes stepwise charged with electrons. The change in fluorescence of the TNC as the reduction of the TNC proceeds provided evidence that the type-3 dinuclear part of the TNC becomes charged prior to the reduction of the type-2 (T2) center of the TNC. The rate of reduction of the enzyme by dithionite (DT) appeared proportional to the square root of the DT concentration with a rate constant of k(red) = 0.28 +/- 0.02 microM(-1/2) s(-1).

Metallo-oxidase Enzymes: Design of their Active Sites

Multi-copper oxidases are a large family of enzymes prevalent in all three domains of life. They couple the one-electron oxidation of substrate to the four-electron reduction of dioxygen to water and feature at least four Cu atoms, traditionally divided into three sites: T1, T2, and (binuclear) T3. The T1 site catalyzes substrate oxidation while a trinuclear cluster (comprising combined T2 and T3 centres) catalyzes the reduction of dioxygen. Substrate oxidation at the T1 Cu site occurs via an outer-sphere mechanism and consequently substrate specificities are determined primarily by the nature of a substrate docking/oxidation (SDO) site associated with the T1 Cu centre. Many of these enzymes ‘moonlight’, i.e. display broad specificities towards many different substrates and may have multiple cellular functions. A sub-set are robust catalysts for the oxidation of low-valent transition metal ions such as FeII, CuI, and MnII and are termed ‘metallo-oxidases’. They play essential roles in nutrient metal uptake and homeostasis, with the ferroxidase ceruloplasmin being a prominent member. Their SDO sites are tailored to facilitate specific binding and facile oxidation of these low-valent metal ions and this is the focus of this review.

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.

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.

Direct electron transfer reactions between human ceruloplasmin and electrodes

In an effort to find conditions favouring bioelectrocatalytic reduction of oxygen by surface-immobilised human ceruloplasmin (Cp), direct electron transfer (DET) reactions between Cp and an extended range of surfaces were considered. Exploiting advances in surface nanotechnology, bare and carbon-nanotube-modified spectrographic graphite electrodes as well as bare, thiol- and gold-nanoparticle-modified gold electrodes were considered, and ellipsometry provided clues as to the amount and form of adsorbed Cp. DET was studied under different conditions by cyclic voltammetry and chronoamperometry. Two Faradaic processes with midpoint potentials of about 400 mV and 700 mV vs. NHE, corresponding to the redox transformation of copper sites of Cp, were clearly observed. In spite of the significant amount of Cp adsorbed on the electrode surfaces, as well as the quite fast DET reactions between the redox enzyme and electrodes, bioelectrocatalytic reduction of oxygen by immobilised Cp was never registered. The bioelectrocatalytic inertness of this complex multi-functional redox enzyme interacting with a variety of surfaces might be associated with a very complex mechanism of intramolecular electron transfer involving a kinetic trapping behaviour.

The Fox1 ferroxidase of Chlamydomonas reinhardtii: a new multicopper oxidase structural paradigm

Multicopper oxidases (MCO) contain at least four copper atoms arrayed in three distinct ligand fields supported by two canonical structural features: (1) multiples of the cupredoxin fold and (2) four unique sequence elements that include the ten histidine and one cysteine ligands to the four copper atoms. Ferroxidases are a subfamily of MCO proteins that contain residues supporting a specific reactivity towards ferrous iron; these MCOs play a vital role in iron metabolism in bacteria, algae, fungi, and mammals. In contrast to the fungal ferroxidases, e.g., Fet3p from Saccharomyces cerevisiae, the mammalian ceruloplasmin (Cp) is twice as large (six vs. three cupredoxin domains) and contains three type 1, or "blue," copper sites. Chlamydomonas reinhardtii expresses a putative ferroxidase, Fox1, which has sequence similarity to human Cp (hCp). Eschewing the standard sequence-based modeling paradigm, we have constructed a function-based model of the Fox1 protein which replicates hCp's six copper-site ligand arrays with an overall root mean square deviation of 1.4 A. Analysis of this model has led also to assignment of motifs in Fox1 that are unique to ferroxidases, the strongest evidence to date that the well-characterized fungal high-affinity iron uptake system is essential to iron homeostasis in green algae. The model of Fox1 also establishes a subfamily of MCO proteins with a noncanonical copper-ligand organization. These diverse structures suggest alternative mechanisms for intramolecular electron transfer and require a new trajectory for the evolution of the MCO superfamily.

Direct identification of a bacterial manganese(II) oxidase, the multicopper oxidase MnxG, from spores of several different marine Bacillus species

Microorganisms catalyze the formation of naturally occurring Mn oxides, but little is known about the biochemical mechanisms of this important biogeochemical process. We used tandem mass spectrometry to directly analyze the Mn(II)-oxidizing enzyme from marine Bacillus spores, identified as an Mn oxide band with an in-gel activity assay. Nine distinct peptides recovered from the Mn oxide band of two Bacillus species were unique to the multicopper oxidase MnxG, and one peptide was from the small hydrophobic protein MnxF. No other proteins were detected in the Mn oxide band, indicating that MnxG (or a MnxF/G complex) directly catalyzes biogenic Mn oxide formation. The Mn(II) oxidase was partially purified and found to be resistant to many proteases and active even at high concentrations of sodium dodecyl sulfate. Comparative analysis of the genes involved in Mn(II) oxidation from three diverse Bacillus species revealed a complement of conserved Cu-binding regions not present in well-characterized multicopper oxidases. Our results provide the first direct identification of a bacterial enzyme that catalyzes Mn(II) oxidation and suggest that MnxG catalyzes two sequential one-electron oxidations from Mn(II) to Mn(III) and from Mn(III) to Mn(IV), a novel type of reaction for a multicopper oxidase.

Electronic structure of the peroxy intermediate and its correlation to the native intermediate in the multicopper oxidases: insights into the reductive cleavage of the o-o bond

The multicopper oxidases (MCOs) utilize a blue type 1 (T1) copper site and a trinuclear Cu cluster composed of a type 2 (T2) and a binuclear type 3 (T3) site that together catalyze the four-electron reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 proceeds via two sequential two-electron steps generating the peroxy intermediate (PI) and the native intermediate (NI). While a detailed description of the geometric and electronic structure of NI has been developed, this has been more elusive for PI largely due to the diamagnetic nature of its ground state. Density functional theory (DFT) calculations have been used to correlate to spectroscopic data to generate a description of the geometric and electronic structure of PI. A highly conserved carboxylate residue near the T2 site is found to play a critical role in stabilizing the PI structure, which induces oxidation of the T2 and one T3 Cu center and strong superexchange stabilization via the peroxide bridge, allowing irreversible binding of O2 at the trinuclear Cu site. Correlation of PI to NI is achieved using a two-dimensional potential energy surface generated to describe the catalytic two-electron reduction of the peroxide O-O bond by the MCOs. It is found that the reaction is thermodynamically driven by the relative stability of NI and the involvement of the simultaneous two-electron-transfer process. A low activation barrier (calculated approximately 5-6 kcal/mol and experimental approximately 3-5 kcal/mol) is produced by the triangular topology of the trinuclear Cu cluster site, as this symmetry provides good donor-acceptor frontier molecular orbital (FMO) overlap. Finally, the O-O bond cleavage in the trinuclear Cu cluster can be achieved via either a proton-assisted or a proton-unassisted process, allowing the MCOs to function over a wide range of pH. It is found that while the proton helps to stabilize the acceptor O22- sigma* orbital in the proton-assisted process for better donor-acceptor FMO overlap, the third oxidized Cu center in the trinuclear site assumes the role as a Lewis acid in the proton-unassisted process for similarly efficient O-O bond cleavage.

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 degrees 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 degrees C). It emerges that native CP (i.e., with six coppers) is rather unstable and converts to I in <1 day at 37 degrees 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 degrees C.

The two oxidized forms of the trinuclear Cu cluster in the multicopper oxidases and mechanism for the decay of the native intermediate

Multicopper oxidases (MCOs) catalyze the 4e(-) reduction of O(2) to H(2)O. The reaction of the fully reduced enzyme with O(2) generates the native intermediate (NI), which undergoes a slow decay to the resting enzyme in the absence of substrate. NI is a fully oxidized form, but its spectral features are very different from those of the resting form (also fully oxidized), because the type 2 and the coupled-binuclear type 3 Cu centers in the O(2)-reducing trinuclear Cu cluster site are isolated in the resting enzyme, whereas these are all bridged by a micro(3)-oxo ligand in NI. Notably, the one azide-bound NI (NI(Az)) exhibits spectral features very similar to those of NI, in which the micro(3)-oxo ligand in NI has been replaced by a micro(3)-bridged azide. Comparison of the spectral features of NI and NI(Az), combined with density functional theory (DFT) calculations, allows refinement of the NI structure. The decay of NI to the resting enzyme proceeds via successive proton-assisted steps, whereas the rate-limiting step involves structural rearrangement of the micro(3)-oxo-bridge from inside to outside the cluster. This phenomenon is consistent with the slow rate of NI decay that uncouples the resting enzyme from the catalytic cycle, leaving NI as the catalytically relevant fully oxidized form of the MCO active site. The all-bridged structure of NI would facilitate electron transfer to all three Cu centers of the trinuclear cluster for rapid proton-coupled reduction of NI to the fully reduced form for catalytic turnover.

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.

Atx1-like chaperones and their cognate P-type ATPases: copper-binding and transfer

Copper is an essential yet toxic metal ion. To satisfy cellular requirements, while, at the same time, minimizing toxicity, complex systems of copper trafficking have evolved in all cell types. The best conserved and most widely distributed of these involve Atx1-like chaperones and P(1B)-type ATPase transporters. Here, we discuss current understanding of how these chaperones bind Cu(I) and transfer it to the Atx1-like N-terminal domains of their cognate transporter.

Structure-function analysis of the cuprous oxidase activity in Fet3p from Saccharomyces cerevisiae

The Fet3 protein from Saccharomyces cerevisiae is a multicopper oxidase with specificity toward Fe(II) and Cu(I). Fet3p turnover of Fe(II) supports high affinity iron uptake across the yeast plasma membrane, whereas its turnover of Cu(I) contributes to copper resistance in yeast. The structure of Fet3p has been used to identify possible amino acid residues responsible for this protein's reactivity with Cu(I), and structure-function analyses have confirmed this assignment. Fet3p Met(345) is required for the enzyme's reactivity toward Cu(I). Although the Fet3pM345A mutant exhibits wild type spectral and electrochemical behavior, the kinetic constants for Cu(I) turnover and for single-turnover electron transfer from Cu(I) to the enzyme are significantly reduced. The specificity constant with Cu(I) as substrate is reduced by one-fifth, whereas the electron transfer rate from Cu(I) is reduced 50-fold. This mutation has little effect on the reactivity toward Fe(II), indicating that Met(345) contributes specifically to Fet3p reactivity with the cuprous ion. These kinetic defects render the Fet3pM345A unable to support wild type cellular copper resistance, suggesting that there is a finely tuned copper redox balance at the yeast plasma membrane.

Structural basis of the ferrous iron specificity of the yeast ferroxidase, Fet3p

Fet3p is a multicopper oxidase (MCO) that functions together with the iron permease, Ftr1p, to support high-affinity Fe uptake in yeast. Fet3p is a ferroxidase that, like ceruloplasmin and hephaestin, couples the oxidation of 4 equiv of Fe(II) to the reduction of O2 to 2 H2O. The ferrous iron specificity of this subclass of MCO proteins has not been delineated by rigorous structure-function analysis. Here the crystal structure of Fet3p has been used as a template to identify the amino acid residues that confer this substrate specificity and then to quantify the contributions they make to this specific reactivity by thermodynamic and kinetic analyses. In terms of the Marcus theory of outer-sphere electron transfer, we show here that D283, E185, and D409 in Fet3p provide a Fe(II) binding site that actually favors ferric iron; this site thus reduces the reduction potential of the bound Fe(II) in comparison to that of aqueous ferrous iron, providing a thermodynamically more robust driving force for electron transfer. In addition, E185 and D409 constitute parts of the electron-transfer pathway from the bound Fe(II) to the protein's type 1 Cu(II). This electronic matrix coupling relies on H-bonds from the carboxylate OD2 atom of each residue to the NE2 NH group of the two histidine ligands at the type 1 Cu site. These two acidic residues and this H-bond network appear to distinguish a fungal ferroxidase from a fungal laccase since the specificity that Fet3p has for Fe(II) is completely lost in a Fet3pE185A/D409A mutant. Indeed, this double mutant functions kinetically better as a laccase, albeit a relatively inefficient one.

Purification and characterization of hydrosoluble components from the sap of Chinese lacquer tree Rhus vernicifera

Continuous gradient elution chromatography (CGEC) was employed to purify and separate enzymes and polysaccharides from the sap of Rhus vernicifera Chinese lacquer tree. There are three different molecules with laccase enzyme activity. Two are enzymes of each other (L1, and L2), whereas the third (RL) is an entirely separate entity. Two polysaccharides (GP1 and GP2) were also found. The Rhus laccase (RL), and isoenzymes L1 and L2, have peak molecular masses of 109,100, 120,000, 103,000 respectively; each has four copper atoms per molecule, and the pI values were 8.2, 8.6, and 9.1, respectively. The structure of the laccases was studied by Fourier-transform infrared (FT-IR) and Matrix-assisted laser desorption/ionization time-of flight (MALDI-TOF) mass spectrometry. The typical amide I (1646cm(-1)) and amide II (1545cm(-1)) bands were observed. The results from MALDI-TOF were similar to those from CGEC, but the molecular mass from the MALDI-TOF was significantly different from that obtained from sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).

Variable-temperature, variable-field magnetic circular dichroism studies of tris-hydroxy- and mu3-oxo-bridged trinuclear Cu(II) complexes: evaluation of proposed structures of the native intermediate of the multicopper oxidases

Multicopper oxidases catalyze the 4e- reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 produces the native intermediate (NI) that consists of four oxidized Cu centers, three of which form a trinuclear cluster site, all bridged by the product of full O2 reduction. The most characteristic feature of NI is the intense magnetic circular dichroism pseudo-A feature (a pair of temperature-dependent C-terms with opposite signs) associated with O --> Cu(II) ligand-to-metal charge transfer (LMCT) that derives from the strong Cu-O bonds in the trinuclear site. In this study, the two most plausible Cu-O structures of the trinuclear site, the tris-mu2-hydroxy-bridged and the mu3-oxo-bridged structures, are evaluated through spectroscopic and electronic structure studies on relevant model complexes, TrisOH and mu3O. It is found that the two components of a pseudo-A-term for TrisOH are associated with LMCT to the same Cu that are coupled by a metal-centered excited-state spin-orbit coupling (SOC), whereas for mu3O they are associated with LMCT to different Cu centers that are coupled by oxo-centered excited state SOC. Based on this analysis of the two candidate models, only the mu3-oxo-bridged structure is consistent with the spectroscopic properties of NI. The Cu-O sigma-bonds in the mu3-oxo-bridged structure would provide the thermodynamic driving force for the 4e- reduction of O2 and would allow the facile electron transfer to all Cu centers in the trinuclear cluster that is consistent with its involvement in the catalytic cycle.

Magnetostructural Studies on Ferromagnetically Coupled Copper(II) Cubanes of Schiff-Base Ligands

Three cubane copper(II) clusters, namely [Cu(4)(HL')4] (1), [Cu4L2(OH)2] (2), and [Cu4L2(OMe)2] (3), of two pentadentate Schiff-base ligands N,N'-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine) (H3L') and N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L), are prepared, structurally characterized by X-ray crystallography, and their variable-temperature magnetic properties studied. Complex 1 has a metal-to-ligand stoichiometry of 1:1 and it crystallizes in the cubic space group P43n with a structure that consists of a tetranuclear core with metal centers linked by a mu(3)-alkoxo oxygen atom to form a cubic arrangement of the metal and oxygen atoms. Each ligand displays a tridentate binding mode which means that a total of eight pendant binding sites remain per cubane molecule. Complexes [Cu4L2(OH)2] (2) and [Cu4L2(OMe)2] (3) crystallize in the orthorhombic space group Pccn and have a cubane structure that is formed by the self-assembly of two {Cu2L}+ units. The variable-temperature magnetic susceptibility data in the range 300-18 K show ferromagnetic exchange interactions in the complexes. Along with the ferromagnetic exchange pathway, there is also a weak antiferromagnetic exchange between the copper centers. The theoretical fitting of the magnetic data gives the following parameters: J1 = 38.5 and J2 = -18 cm(-1) for 1 with a triplet (S = 1) ground state and quintet (S = 2) lowest excited state; J1 = 14.7 and J2 = -18.4 cm(-1) for 2 with a triplet ground state and singlet (S = 0) lowest excited state; and J1 = 33.3 and J2 = -15.6 cm(-1) for 3 with a triplet ground state and quintet lowest excited state, where J1 and J2 are two different exchange pathways in the cubane {Cu4O4} core. The crystal structures of 2 * 6 H2O and 3 * 2 H2O * THF show the presence of channels containing the lattice solvent molecules.

Ferrous Binding to the Multicopper OxidasesSaccharomyces cerevisiaeFet3p and Human Ceruloplasmin: Contributions to Ferroxidase Activity

The multicopper oxidases are a family of enzymes that couple the reduction of O(2) to H(2)O with the oxidation of a range of substrates. Saccharomyces cerevisiae Fet3p and human ceruloplasmin (hCp) are members of this family that exhibit ferroxidase activity. Their high specificity for Fe(II) has been attributed to the existence of a binding site for iron. In this study, mutations at the E185 and Y354 residues, which are putative ligands for iron in Fet3p, have been generated and characterized. The effects of these mutations on the electronic structure of the T1 Cu site have been assessed, and the reactivities of this site toward 1,4-hydroquinone (a weak binding substrate) and Fe(II) have been evaluated and interpreted in terms of the semiclassical Marcus theory for electron transfer. The electronic and geometric structure of the Fe(II) substrate bound to Fet3p and hCp has been studied for the first time, using variable-temperature variable field magnetic circular dichroism (VTVH MCD) spectroscopy. The iron binding sites in Fet3p and hCp appear to be very similar in nature, and their contributions to the ferroxidase activity of these proteins have been analyzed. It is found that these iron binding sites play a major role in tuning the reduction potential of iron to provide a large driving force for the ferroxidase reaction, while still supporting the delivery of the Fe(III) product to the acceptor protein. Finally, the analysis of possible electron-transfer (ET) pathways from the protein-bound Fe(II) to the T1 Cu site indicates that the E185 residue not only plays a role in iron binding, but also provides the dominant ET pathway to the T1 Cu site.

Involvement of Glycosylphosphatidylinositol-linked Ceruloplasmin in the Copper/Zinc-Nitric Oxide-dependent Degradation of Glypican-1 Heparan Sulfate in Rat C6 Glioma Cells

The core protein of glypican-1, a glycosylphosphatidylinositol-linked heparan sulfate proteoglycan, can bind Cu(II) or Zn(II) ions and undergo S-nitrosylation in the presence of nitric oxide. Cu(II)-to-Cu(I)-reduction supports extensive and permanent nitrosothiol formation, whereas Zn(II) ions appear to support a more limited, possibly transient one. Ascorbate induces release of nitric oxide, which catalyzes deaminative degradation of the heparan sulfate chains on the same core protein. Although free Zn(II) ions support a more limited degradation, Cu(II) ions support a more extensive self-pruning process. Here, we have investigated processing of glypican-1 in rat C6 glioma cells and the possible participation of the copper-containing glycosylphosphatidylinositol-linked splice variant of ceruloplasmin in nitrosothiol formation. Confocal microscopy demonstrated colocalization of glypican-1 and ceruloplasmin in endosomal compartments. Ascorbate induced extensive, Zn(II)-supported heparan sulfate degradation, which could be demonstrated using a specific zinc probe. RNA interference silencing of ceruloplasmin expression reduced the extent of Zn(II)-supported degradation. In cell-free experiments, the presence of free Zn(II) ions prevented free Cu(II) ion from binding to glypican-1 and precluded extensive heparan sulfate autodegradation. However, in the presence of Cu(II)-loaded ceruloplasmin, heparan sulfate in Zn(II)-loaded glypican-1 underwent extensive, ascorbate-induced degradation. We propose that the Cu(II)-to-Cu(I)-reduction that is required for S-nitrosylation of glypican-1 can take place on ceruloplasmin and thereby ensure extensive glypican-1 processing in the presence of free Zn(II) ions.