Oxidative turnover increases the rate constant and extent of intramolecular electron transfer in the multicopper enzymes, ascorbate oxidase and laccase

Tracking light-induced electron transfer toward O2 in a hybrid photoredox-laccase system

Photobiocatalysis uses light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activities. Laccase, a robust multicopper oxidase, can be envisioned to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst. Here, we provide a detailed study of the coupling of a [Ru(bpy)₃]²⁺ photosensitizer to laccase. We demonstrate that efficient laccase reduction requires an electron relay like methyl viologen. In the presence of dioxygen, electrons transiently stored in superoxide ions are scavenged by laccase to form water instead of H₂O₂. The net result is the photo accumulation of highly oxidizing [Ru(bpy)₃]³⁺. This study provides ground for the use of laccase in tandem with a light-driven oxidative process and O₂ as one-electron transfer relay and as four-electron substrate to be a sustainable final electron acceptor in a photocatalytic process.

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An electron relay boosts photoreduction of laccase

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Superoxide is efficiently captured by laccase preventing formation of H₂O₂

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Light activation reveals information on elementary steps inside the enzyme

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Laccase enables O₂ as terminal electron acceptor for oxidative photocatalysis

The nature of the rate-limiting step of blue multicopper oxidases: Homogeneous studies versus heterogeneous

Multicopper oxidases (MCOs) catalyzed two half reactions (linked by an intramolecular electron transfer) through a Ping-Pong mechanism: the substrate oxidation followed by the O₂ reduction. MCOs have been characterized in details in solution or immobilized on electrode surfaces. The nature of the rate-limiting steps, which is controversial in the literature, is discussed in this mini review for both cases. Deciphering such rate-limiting steps is of particular importance to efficiently use MCOs in any applications requiring the reduction of O₂ to water.

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Limiting rate constant of multicopper oxidase in solution is discussed.

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Limiting rate constant of immobilized multicopper oxidase is discussed.

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Homogeneous and heterogeneous enzyme kinetics are compared.

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

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

Tracking electrons in biological macromolecules: from ensemble to single molecule

Nature utilizes oxido-reductases to cater to the energy demands of most biochemical processes in respiratory species. Oxido-reductases are capable of meeting this challenge by utilizing redox active sites, often containing transition metal ions, which facilitate movement and relocation of electrons/protons to create a potential gradient that is used to energize redox reactions. There has been a consistent struggle by researchers to estimate the electron transfer rate constants in physiologically relevant processes. This review provides a brief background on the measurements of electron transfer rates in biological molecules, in particular Cu-containing enzymes, and highlights the recent advances in monitoring these electron transfer events at the single molecule level or better to say, at the individual event level.

Intramolecular electron transfer in laccases

Rate constants and activation parameters have been determined for the internal electron transfer from type 1 (T1) to type 3 (T3) copper ions in laccase from both the fungus Trametes hirsuta and the lacquer tree Rhus vernicifera, using the pulse radiolysis method. The rate constant at 298 K and the enthalpy and entropy of activation were 25 ± 1 s(-1), 39.7 ± 5.0 kJ·mol(-1) and -87 ± 9 J·mol(-1) ·K(-1) for the fungal enzyme and 1.1 ± 0.1 s(-1), 9.8 ± 0.2 kJ·mol(-1) and -211 ± 3 J·mol(-1) ·K(-1) for the tree enzyme. The initial reduction of the T1 site by pulse radiolytically produced radicals was direct in the case of T. hirsuta laccase, but occured indirectly via a disulfide radical in R. vernicifera. The equilibrium constant that characterizes the electron transfer from T1 to T3 copper ions was 0.4 for T. hirsuta laccase and 1.5 for R. vernicifera laccase, leading to full reduction of the T1 site occurring at 2.9 ± 0.2 electron equivalents for T. hirsuta and 4 electron equivalents for R. vernicifera laccase. These results were compared with each other and with those for the same process in other multicopper oxidases, ascorbate oxidase and Streptomyces coelicolor laccase, using available structural information and electron transfer theory.

A novel zebrafish mutant with wavy-notochord: an effective biological index for monitoring the copper pollution of water from natural resources

We identified a novel zebrafish mutant that has wavy-notochord phenotypes, such as severely twisted notochord and posterior malformations, but has normal melanocytes. Histological evidences showed that proliferating vacuolar cells extended their growth to the muscle region, and consequently caused the wavy-notochord phenotypes. Interestingly, those malformations can be greatly reversed by exposure with copper, suggesting that copper plays an important role on wavy-notochord phenotypes. In addition, after long-term copper exposure, the surviving larvae derived from wavy-notochord mutants displayed bone malformations, such as twisted axial skeleton and osteophyte. These phenotypic changes and molecular evidences of wavy-notochord mutants are highly similar to those embryos whose lysyl oxidases activities have been inactivated. Taken together, we propose that (i) the putative mutated genes of this wavy-notochord mutant might be highly associated with the lysyl oxidase genes in zebrafish; and (ii) this fish model is an effective tool for monitoring copper pollution of water from natural resources.

Kinetic studies on the reaction between Trametes villosa laccase and dioxygen

A laccase from the fungus Trametes villosa (TviL) was investigated in order to elucidate the reaction mechanism of the reduction of dioxygen to water performed by this blue multi-copper oxidase. The ability of TviL to activate dioxygen was studied by stopped-flow experiments and under steady-state conditions. In the stopped-flow experiments TviL was reduced with a small excess of 4-hydroxyphenylacetic acid and afterwards the re-oxidation process was monitored by stopped-flow techniques by mixing with excess dioxygen. The reaction between reduced TviL and dioxygen was studied in the temperature range 10-35 degrees Celsius and with the concentration of dioxygen between 30 and 240microM. The observed rate constant k(obs) is found to be linear dependent on the dioxygen concentration and the observed second-order rate constant for the re-oxidation of reduced TviL is, at 25 degrees Celsius, determined to be 1.14x10(6)M(-1)s(-1). The activation energy, E(a), is from the same data determined to be 22kJmol(-1). Oxidation of different phenols (4-hydroxyphenylacetic acid, 4-hydroxybenzoic acid, guaiacolsulfonic acid and hydroquinone) under steady state conditions was investigated at concentrations of dioxygen ranging from 60 to 250microM. This line of experiments showed that TviL follows a ping-pong mechanism, and an observed second-order rate constant for the reaction with dioxygen of 7.1x10(5)M(-1)s(-1) at 25 degrees Celsius was found with 4-hydroxyphenylacetic acid as reducing substrate. The two kinetic methods resulted in observed rate constants of equal magnitudes for the reaction with dioxygen, which suggests that the rate limiting step(s) is (are) included in both the reactions studied by the two different techniques.

Inhibition of intramolecular electron transfer in ascorbate oxidase by Ag+: redox state dependent binding

Intramolecular electron transfer within zucchini squash ascorbate oxidase is inhibited in a novel manner in the presence of an equimolar concentration of Ag(+). At pH 5.5 in acetate buffer reduction of the enzyme by laser flash photolytically generated 5-deazariboflavin semiquinone occurs at the Type I Cu with a rate constant of 5 x 10(8) M(-1)s(-1). Subsequent to this initial reduction step, equilibration of the reducing equivalent between the Type I Cu and the trinuclear Type II, III copper cluster (TNC) occurs with rate constant of 430 s(-1). The 41% of the reduced Type I Cu is oxidized by this intramolecular electron transfer reaction. When these reactions are performed in the presence of Ag(+) equimolar to dimeric AO, the bimolecular reduction of the enzyme by the 5-deazariboflavin semiquinone is not affected. As in the case of the native enzyme, intramolecular electron transfer between the Type I Cu and the TNC occurs, which continues until 25% of the reducing equivalent has been transferred. At that point, the reducing equivalent is observed to more slowly return to the Type I Cu, resulting a second reduction phase whose rate constant (100 s(-1)) is protein and Ag(+) concentration independent. The data suggest that partial reduction of the TNC results in Ag(+) binding to the enzyme which causes the apparent midpoint potential of the TNC as a whole to decrease thereby reversing the direction of electron flow. These results are consistent with the inhibitory effect of Ag(+) on the steady-state activity of ascorbate oxidase [S. Maritano, E. Malusa, A. Marchesini, presented at The Meeting on Metalloproteins, SERC Daresbury Laboratory, Warrington, England, 1992; A. Marchesini, XIX Convegno Nazionale SICA, Italian Society of Agricultural Chemistry, Reggio Calabria, Italy, September 2001.].

Crystal structure and electron transfer kinetics of CueO, a multicopper oxidase required for copper homeostasis in Escherichia coli

CueO (YacK), a multicopper oxidase, is part of the copper-regulatory cue operon in Escherichia coli. The crystal structure of CueO has been determined to 1.4-A resolution by using multiple anomalous dispersion phasing and an automated building procedure that yielded a nearly complete model without manual intervention. This is the highest resolution multicopper oxidase structure yet determined and provides a particularly clear view of the four coppers at the catalytic center. The overall structure is similar to those of laccase and ascorbate oxidase, but contains an extra 42-residue insert in domain 3 that includes 14 methionines, nine of which lie in a helix that covers the entrance to the type I (T1, blue) copper site. The trinuclear copper cluster has a conformation not previously seen: the Cu-O-Cu binuclear species is nearly linear (Cu-O-Cu bond angle = 170 degrees) and the third (type II) copper lies only 3.1 A from the bridging oxygen. CueO activity was maximal at pH 6.5 and in the presence of >100 microM Cu(II). Measurements of intermolecular and intramolecular electron transfer with laser flash photolysis in the absence of Cu(II) show that, in addition to the normal reduction of the T1 copper, which occurs with a slow rate (k = 4 x 10(7) M(-1)x (-1)), a second electron transfer process occurs to an unknown site, possibly the trinuclear cluster, with k = 9 x 10(7) M(-1) x (-1), followed by a slow intramolecular electron transfer to T1 copper (k approximately 10 s(-1)). These results suggest the methionine-rich helix blocks access to the T1 site in the absence of excess copper.

Human ceruloplasmin. Intramolecular electron transfer kinetics and equilibration

Pulse radiolytic reduction of disulfide bridges in ceruloplasmin yielding RSSR(-) radicals induces a cascade of intramolecular electron transfer (ET) processes. Based on the three-dimensional structure of ceruloplasmin identification of individual kinetically active disulfide groups and type 1 (T1) copper centers, the following is proposed. The first T1 copper(II) ion to be reduced in ceruloplasmin is the blue copper center of domain 6 (T1A) by ET from RSSR(-) of domain 5. The rate constant is 28 +/- 2 s(-1) at 279 K and pH 7.0. T1A is in close covalent contact with the type 3 copper pair and indeed electron equilibration between T1A and the trinuclear copper center in the domain 1-6 interface takes place with a rate constant of 2.9 +/- 0.6 s(-1). The equilibrium constant is 0.17. Following reduction of T1A Cu(II), another ET process takes place between RSSR(-) and T1B copper(II) of domain 4 with a rate constant of 3.9 +/- 0.8. No reoxidation of T1B Cu(I) could be resolved. It appears that the third T1 center (T1C of domain 2) is not participating in intramolecular ET, as it seems to be in a reduced state in the resting enzyme.

Novel enzymatic oxidation of Mn2+ to Mn3+ catalyzed by a fungal laccase

Fungal laccases are extracellular multinuclear copper-containing oxidases that have been proposed to be involved in ligninolysis and degradation of xenobiotics. Here, we show that an electrophoretically homogenous laccase preparation from the white rot fungus Trametes versicolor oxidized Mn2+ to Mn3+ in the presence of Na-pyrophosphate, with a Km value of 186 microM and a Vmax value of 0.11 micromol/min/mg protein at the optimal pH (5.0) and a Na-pyrophosphate concentration of 100 mM. The oxidation of Mn2+ involved concomitant reduction of the laccase type 1 copper site as usual for laccase reactions, thus providing the first evidence that laccase may directly utilize Mn2+ as a substrate.

Laser flash photolysis experiments on the effects of freezing and salt addition on intramolecular electron transfer within one-electron reduced ascorbate oxidase

Laser flash photolysis has been used to investigate the effects of freezing protein solutions and of adding various salts on the kinetics of one-electron photoreduction by 5-deazariboflavin semiquinone (5-DRFH.) of oxidized ascorbate oxidase (AO) from zucchini in 100 mM phosphate buffer (pH 7.0). The initial reaction between oxidized AO and 5-DRFH. is quite rapid (k approximately 10(8) M-1 s-1) and occurs at the blue Type I Cu center. Subsequent to this, a slower, protein concentration-independent intramolecular reoxidation of the Type I Cu is observed, with kET approximately 150 s-1, resulting in 40-50% reoxidation of the blue Cu center and the establishment of an electron transfer (ET) equilibrium between the various Cu centers in AO. When such a sample of AO was frozen overnight at -30 degrees C, flash photolysis of the thawed sample showed no effect on the kinetics of reduction of the Type I Cu by 5-DRFH. However, the rate constant for intramolecular ET decreased to a value of 2.7 s-1, with only 20% reoxidation of the Type I center. Reduction of the enzyme with ascorbic acid, followed by O2 oxidation, resulted in restoration of rapid intramolecular reoxidation (kET = 130 s-1), with 33% of the Type I Cu reduced by 5-DRFH. being reoxidized. These results are consistent with previous work which showed that samples of AO with initially low activity can be reactivated by ascorbic acid turnover in the presence of O2. When AO was frozen in the presence of ascorbic acid, similar inhibition of intramolecular ET was obtained, whereas upon turnover of this sample by further addition of ascorbic acid and exposure to O2, activity was not restored. The effects of addition of (NH4)2SO4, Na2SO4, NH4Cl, NaCl, KCl, and KF on the kinetics of Type I Cu reduction by 5-deazariboflavin semiquinone and on the subsequent intramolecular ET were also examined. A twofold increase in the bimolecular rate constant for reduction of the Type I Cu was observed for the two sodium salts at high concentrations (500 mM). Intramolecular ET was also significantly affected upon addition of all three chloride salts. Although the intramolecular ET rate constant was not altered, the fraction of reduced Type I Cu reoxidized by the trinuclear cluster decreased with increasing Cl- concentration, regardless of the cation. Total inhibition of intramolecular ET was observed at a significantly lower concentration of KF than observed with the Cl- salts. Sulfate ion had no effect on either parameter. These changes are thus ion specific, suggesting that they are related to ion binding by the protein, possibly at one of the coppers of the trinuclear cluster.

The type 2 copper of ascorbate oxidase

Ascorbate oxidase, dissolved in Hepes or sodium phosphate buffers, was analyzed by EPR and activity measurements before and after storage at -30 degrees C and 77 K. The specific activity was somewhat higher in the phosphate buffer, about 3500-3700 Dawson units compared to about 3100 units of the enzyme dissolved in Hepes buffer. After storage at -30 degrees C the activity fell to 1400-2000 units in the phosphate buffer but only to 2600-2800 units in the Hepes buffer. Large changes occurred in the EPR spectrum of enzyme dissolved in the phosphate buffer after storing at -30 degrees C suggesting an alteration of the type 2 copper site. These changes were, however, reverted when the samples were thawed and rapidly frozen at 77 K. Copper analysis showed that about 50% of the total copper was EPR detected. The type 2 Cu2+ EPR intensity was in most samples close to 25% of the total EPR intensity. This low contribution of type 2 Cu2+ could not be changed if the enzyme was completely reduced and reoxidized, treated with Fe(CN)6(3), large excess of NaF, addition of 50% (v/v) ethylene glycol or dialyzed against 0.1 M Mes buffer (pH 5.5). Since the crystal structure shows that there are one each of types 1 and 2 copper in the monomers there must be another species with an EPR signal rather different from these two copper species. This signal is proposed to originate from some trinuclear centers. The EPR simulations show that it is possible to house a broad unresolved signal under the resolved type 1 and 2 signals so that the total integral becomes 50% of the total copper in the molecule.

Effects of redox potential and hydroxide inhibition on the pH activity profile of fungal laccases

The electronic absorption spectrum, susceptibility to fluoride inhibition, redox potential, and substrate turnover of several fungal laccases have been explored as a function of pH. The laccases showed a single spectrally detectable acid-base transition at pH 6-9 and a fluoride inhibition that diminished by increased pH (indicating a competition with hydroxide inhibition). Relatively small changes in the redox potentials (< or = 0.1 V) of laccase were observed over the pH 2.7-11. Under the catalysis of laccase, the apparent oxidation rates (kcat and kcat/Km) of two nonphenolic substrates, potassium ferrocyanide and 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid), decreased monotonically as the pH increased. In contrast, the apparent oxidation rates (kcat and kcat/Km) of three 2,6-dimethoxyphenols (whose pKa values range from 7.0 to 8.7) exhibited bell-shaped pH profiles whose maxima were distinct for each laccase but independent of the substrate. By correlating these pH dependences, it is proposed that the balance of two opposing effects, one generated by the redox potential difference between a reducing substrate and the type 1 copper of laccase (which correlates to the electron transfer rate and is favored for a phenolic substrate by higher pH) and another generated by the binding of a hydroxide anion to the type 2/type 3 coppers of laccase (which inhibits the activity at higher pH), contributes to the pH activity profile of the fungal laccases.

Aerobic and anaerobic functioning of superoxide-producing cytochrome b-559 reconstituted with phospholipids

Cytochrome b-559 reconstituted with phospholipids and FAD represents the simplest model of the respiratory burst NADPH oxidase and reproduces the main catalytic features of this system (Koshkin, V. and Pick, E. (1993) FEBS Lett. 327, 57-62; (1994) FEBS Lett. 338, 285-289). In the present report it is shown that activation by oxygen, characteristic of the NADPH oxidase complex, is an intrinsic property of flavocytochrome b-559, in principle independent of its complexation with the other components of NADPH oxidase. Facilitation of electron transfer from NADPH to FAD is found to be the reason for this phenomenon. Kinetic studies of anaerobic operation of flavocytochrome b-559 revealed the functional heterogeneity of two hemes, manifested as a dramatic difference in their reducibility under these conditions.

The reduction of membrane-bound dopamine beta-monooxygenase in resealed chromaffin granule ghosts. Is intragranular ascorbic acid a mediator for extragranular reducing equivalents?

The role of internal and external reductants in the dopamine beta-monooxygenase (D beta M)-catalyzed conversion of dopamine to norepinephrine has been investigated in resealed chromaffin granule ghosts. The rate of norepinephrine production was not affected by the exclusion of internal ascorbate. The omission of ascorbate from the external medium drastically reduced the norepinephrine production without affecting the net rate of dopamine uptake. In the presence of the external reductant, the internal ascorbate levels were constant throughout the incubation period. The rate of norepinephrine production was not affected when ghosts were resealed to contain the D beta M reduction site inhibitor, imino-D-glucoascorbate. Ghosts incubated with external imino-D-glucoascorbate reduced the norepinephrine production. The weak D beta M reductant, 6-amino-L-ascorbic acid, was found to be a good external reductant for granule ghosts. The outcome of the above experiments was not altered when dopamine was replaced with the reductively inactive D beta M substrate, tyramine. These results and the known topology of membrane-bound D beta M disfavor the direct reduction of the enzyme by the external reductant. Our observations are consistent with the hypothesis that external ascorbate is the sole source of reducing equivalents for D beta M monooxygenation and that internal soluble ascorbate (or dopamine) may not directly reduce or mediate the reduction of membrane-bound D beta M in resealed granule ghosts.

Intramolecular electron transfer in yeast flavocytochrome b2 upon one-electron photooxidation of the fully reduced enzyme: evidence for redox state control of heme-flavin communication

Flavocytochrome b2, which has been fully reduced using L-lactate, can be rapidly oxidized by 1 equiv using the laser-generated triplet state of 5-deazariboflavin. Parallel photoinduced oxidation occurs at the reduced heme and at the fully reduced FMN (FMNH2) prosthetic groups of different enzyme monomers, producing the anion semiquinone of FMN and a ferric heme. Following the initial oxidation reaction, rapid intramolecular reduction of the ferric heme occurs with concomitant oxidation of FMNH2, generating the neutral FMN semiquinone. The observed rate constant for this intramolecular electron transfer is 2200 s-1, which is 1 order of magnitude larger than the turnover number under these conditions. A slower reduction of the heme prosthetic group also occurs with an observed rate constant of approximately 10 s-1, perhaps due to intersubunit electron transfer from reduced FMN to heme. The rapid intramolecular electron transfer between the FMNH2 and ferric heme is eliminated upon addition of excess pyruvate (Ki = 3.8 mM). This latter result indicates that pyruvate inhibition of catalytic turnover apparently can occur at the FMNH2-->heme electron transfer step. These results markedly differ from those previously obtained (Walker, M. C., & Tollin, G. (1991) Biochemistry 30, 5546-5555) and confirmed here for electron transfer within the one-electron reduced enzyme and for the effect of pyruvate binding, suggesting that intramolecular communication between the heme and flavin prosthetic groups can be controlled by the redox state of the enzyme and by ligand binding to the active site.

Direct measurement by laser flash photolysis of intramolecular electron transfer in the three-electron reduced form of ascorbate oxidase from zucchini

Ascorbate oxidase, which has been fully reduced by its substrate, can rapidly transfer a single electron to the laser-generated triplet state of 5-deazariboflavin. Subsequent to this, intramolecular electron transfer occurs resulting in the oxidation of the blue type I copper center. This latter process proceeds via biphasic kinetics, with observed rate constants of 9500 s-1 and 1400 s-1, both of which are protein concentration independent. This indicates that the initial oxidation reaction involves the type II, III trinuclear center, probably occurring via parallel reactions of two of the three copper atoms. The rate constants for intramolecular electron transfer in the three-electron reduced enzyme are one to two orders of magnitude larger than previously observed for the one-electron reduced enzyme, indicating a dramatic effect of the redox state of the enzyme on the intramolecular communication between the copper centers.