Effect Of Glutathione On The Redox Transitions Of Naphthohydroquinone Derivatives Formed During Dt-Diaphorase Catalysis

The oxidation of GSH coupled to the redox transitions of 1, Cnaphthoquinone derivatives during DT-diaphorase catalysis was examined. The quinones studied included 1,4-naphthoquinone and its dimethoxy-and hydroxy derivatives and were selected according to their different ability to undergo nucleophilic addition with GSH and the dual effect of superoxide dismutase on hydroquinone autoxidation

GSH was oxidized to GSSG during the redox transitions of the above quinones, regardless of their substitution pattern. This effect was accompanied by an increase of total O₂ consumption, indicating the ability of GSH to support quinone redox cycling. The values for the relationship [O₂]_consumed[GSSG]_formde were, with every quinone examined, above unity. thus pointing to the occurrence of autoxidation reactions other than those involved during GSSG formation

These results are discussed in terms of the functional group chemistry of the quinones and the ther-modynamic properties of the reactions involved in the reduction of the semi- to the hydro-quinone by GSH     

Apparent inhibition of photoredox reactions of magnesium octaethylporphyrin at the lipid bilayer-water interface by neutral quinones.

Neutral quinones rapidly equilibrate across the lipid bilayer, hereby rendering the photoeffects seen in pigmented bilayers sensitive to the redox properties at both interfaces. The lack of photoeffect by quinones themselves and their apparent quenching reactions with aqueous acceptors is thus explained. An aqueous donor is needed on one side to break the symmetry and to allow vectorial electron transfer to be recorded. It is concluded that the neutral quinone accumulates on the polar side of the interface with respect to the hydrophobic pigment. The system may allow the study of kinetics of proton transfer accompanying the redox reactions of the quinones.     

Reactions of glutathione and glutathione radicals with benzoquinones.

The reactions of glutathione (GSH) and glutathione radicals with a series of methyl-substituted 1,4-benzoquinones and 1,4-benzoquinone have been studied. It was found that by mixing excess benzoquinone with glutathione at pH above 6.5, the products formed were complex and unstable. All of the other experiments were carried out at pH 6.0, where the main product was stable for several hours. Stopped-flow analysis allowed the measurement of the rates of the rapid reactions between GSH and the quinones, and the products were monitored by High Performance Liquid Chromatography (HPLC). The rates of the reactions vary by five orders of magnitude and must be influenced by steric factors as well as changes in the redox states. It was observed that simple hydroquinones were not formed when the different benzoquinones were mixed with excess GSH and suggests that the initial reaction is addition/reduction rather than electron transfer. In the presence of excess quinone, the hydroquinone of the glutathione conjugate is oxidized back to its quinone. The rates of the reaction were measured. By using the technique of pulse radiolysis, it was possible to measure the reduction of the quinones by GSSG.- and the oxidation of hydroquinones by GS(.). It is proposed that the appearance of GSSG in reactions of quinones with glutathione could be due to oxidation of the hydroquinone by oxygen and the subsequent superoxide or H2O2 promoting the oxidation of GSH to GSSG.     

Reduction of ferrylmyoglobin to metmyoglobin by quinonoid compounds

Several quinoid compounds mediated the reduction of ferrylmyoglobin (MbIV) to metmyoglobin (MbIII). The efficiency of the MbIV reduction to MbIII was accomplished by the quinones in the following order: p-benzoquinone greater than 1,4-naphthoquinone greater than 2-OH-1,4-naphthoquinone greater than 2,3-epoxy-1,4-naphthoquinone. The quinone-mediated reduction of MbIV to MbIII had the following characteristics: (a) it was stoichiometrically--rather than catalytically--related to the number of cycles of the MbIV----MbIII transition involving the reduction of H2O2. (b) It proceeded with similar efficiencies under aerobic and anaerobic conditions. (c) It did not require the free radical form of MbIV(.MbIV), thus excluding a two-electron oxidation of the quinone. (d) the nucleophilic addition of--NH2 groups of the apoprotein on the quinone seemed not to be involved through an alternative pathway in the reduction of MbIV, especially since 2-OH-1,4-naphthoquinone, a compound which cannot undergo nucleophilic addition, also facilitated the reduction of the ferryl compound. (e) No two-electron oxidation products of the unsubstituted quinones, such as quinone epoxides, were detected in the spent reaction mixture analyzed by HPLC with electrochemical detection. On the basis of these observations, it is suggested that the reduction of MbIV to MbIII by the above quinonoid compounds is a one-electron transfer process, with electron abstraction being probably accomplished at some site in the benzo ring of the quinone.     

Quinones and their interactions with enzyme complexes of energy-transducing biomembranes

The functionally essential properties of biomembrane quinones and the mechanism of their interaction with protein components are discussed. The hypotheses on the mobile quinone pool or the ability of protein-bound quinones to transfer redox equivalents in biomembranes are discussed. The idea of quinone domains is invoked, and evidence is provided for the presence of such domains in operative biomembranes.     

Bacterial tyrosinases and their applications

Tyrosinases with different physico-chemical properties have been identified from various bacterial phyla such as Actinobacteria and Proteobacteria and their production is often inducible by environmental stresses. Tyrosinases are enzymes catalysing the oxidation of mono- and di-phenolic compounds to corresponding quinones with the concomitant reduction of molecular oxygen to water. Since the quinone produced can further react non-enzymatically with other nucleophiles, e.g. amino groups, many tyrosinases have a recorded cross-linking activity on proteins. Various bacterial tyrosinases oxidise tyrosine, catechol, l/d-DOPA, caffeic acid and polyphenolic substrates such as catechins. This substrate specificity has been exploited to engineer biosensors able to detect even minimal amounts of different phenolic compounds. The physiological role of tyrosinases in the biosynthesis of melanins has been used for the production of coloured and dyeing agents. Moreover, the cross-linking activity of tyrosinases has found application in food processing and in the functionalisation of materials. Numerous tyrosinases with varying substrate specificities and stability features have been isolated from bacteria and they can constitute valuable alternatives to the well-studied tyrosinase from common mushroom.     

Protonmotive pathways and mechanisms in the cytochrome bc1 complex

The cytochrome bc(1) complex catalyzes electron transfer from ubiquinol to cytochrome c by a protonmotive Q cycle mechanism in which electron transfer is linked to proton translocation across the inner mitochondrial membrane. In the Q cycle mechanism proton translocation is the net result of topographically segregated reduction of quinone and reoxidation of quinol on opposite sides of the membrane, with protons being carried across the membrane as hydrogens on the quinol. The linkage of proton chemistry to electron transfer during quinol oxidation and quinone reduction requires pathways for moving protons to and from the aqueous phase and the hydrophobic environment in which the quinol and quinone redox reactions occur. Crystal structures of the mitochondrial cytochrome bc(1) complexes in various conformations allow insight into possible proton conduction pathways. In this review we discuss pathways for proton conduction linked to ubiquinone redox reactions with particular reference to recently determined structures of the yeast bc(1) complex.     

Two-electron reduction of quinones by Enterobacter cloacae NAD(P)H:nitroreductase: quantitative structure-activity relationships

Enterobacter cloacae NAD(P)H:nitroreductase (NR; EC 1.6.99.7) catalyzes two-electron reduction of a series of quinoidal compounds according to a "ping-pong" scheme, with marked substrate inhibition by quinones. The steady-state catalytic constants (k(cat)) range from 0.1 to 1600s(-1), and bimolecular rate constants (k(cat)/K(m)) range from 10(3) to 10(8)M(-1)s(-1). Quinones, nitroaromatic compounds and competitive to NADH inhibitor dicumarol, quench the flavin mononucleotide (FMN) fluorescence of nitroreductase. The reactivity of NR with single-electron acceptors is consistent with an "outer-sphere" electron transfer model, taking into account high potential of FMN semiquinone/FMNH(-) couple and good solvent accessibility of FMN. However, the single-electron acceptor 1,1(')-dibenzyl-4,4(')-bipyridinium was far less reactive than quinones possessing similar single-electron reduction potentials (E(1)(7)). For all quinoidal compounds except 2-hydroxy-1,4-naphthoquinones, there existed parabolic correlations between the log of rate constants of quinone reduction and their E(1)(7) or hydride-transfer potential (E(7)(Q/QH(-))). Based on pH dependence of rate constants, a single-step hydride transfer seems to be a more feasible quinone reduction mechanism. The reactivities of 2-hydroxy-1,4-naphthoquinones were much higher than expected from their reduction potential. Most probably, their enhanced reactivity was determined by their binding at or close to the binding site of NADH and dicumarol, whereas other quinones used the alternative, currently unidentified binding site.     

Bioelectronome. Integrated Approach to Receptor Chemistry,Radicals, Electrochemistry,Cell Signaling,and Physiological Effects Based on Electron Transfer

Bioelectronome refers to the host of electron transfer (ET) reactions that occur in living systems. This review presents an integrated approach to receptor chemistry based on electron transfer, radicals, electrochemistry, cell signaling, and end result. First, receptor activity is addressed from the unifying standpoint of redox transformations in which various receptors are discussed. After a listing of receptor-binding modes, receptor chemistry is treated with focus on generation of reactive oxygen species (ROS), activation by ROS, and subsequent cell signaling involving ROS. A general electrostatic mechanism is proposed for receptor-ligand action with supporting evidence. Cell-signaling processes appear to entail electron transfer, ROS, redox chains, and relays. The widespread involvement of phosphate from phosphorylation may be rationalized electrostatically by analogy with DNA phosphate. Extensive evidence supports important participation of ET functionalities in the mechanism of drugs and toxins. The integrated approach is applied to the main ET classes, namely, quinones, metal complexes, iminium species, and aromatic nitro compounds.     

Bioelectronome. Integrated approach to receptor chemistry, radicals, electrochemistry, cell signaling, and physiological effects based on electron transfer

Bioelectronome refers to the host of electron transfer (ET) reactions that occur in living systems. This review presents an integrated approach to receptor chemistry based on electron transfer, radicals, electrochemistry, cell signaling, and end result. First, receptor activity is addressed from the unifying standpoint of redox transformations in which various receptors are discussed. After a listing of receptor-binding modes, receptor chemistry is treated with focus on generation of reactive oxygen species (ROS), activation by ROS, and subsequent cell signaling involving ROS. A general electrostatic mechanism is proposed for receptor-ligand action with supporting evidence. Cell-signaling processes appear to entail electron transfer, ROS, redox chains, and relays. The widespread involvement of phosphate from phosphorylation may be rationalized electrostatically by analogy with DNA phosphate. Extensive evidence supports important participation of ET functionalities in the mechanism of drugs and toxins. The integrated approach is applied to the main ET classes, namely, quinones, metal complexes, iminium species, and aromatic nitro compounds.     

Enhancement of quinone redox cycling by ascorbate induces a caspase-3 independent cell death in human leukaemia cells. An in vitro comparative study

Since the higher redox potential of quinone molecules has been correlated with enhanced cellular deleterious effects, we studied the ability of the association of ascorbate with several quinones derivatives (having different redox potentials) to cause cell death in K562 human leukaemia cell line. The rationale is that the reduction of quinone by ascorbate should be dependent of the quinone half-redox potential thus determining if reactive oxygen species (ROS) are formed or not, leading ultimately to cell death or cell survival. Among different ROS that may be formed during redox cycling between ascorbate and the quinone, the use of different antioxidant compounds (mannitol, desferal, N-acetylcysteine, catalase and superoxide dismutase) led to support H2O2 as the main oxidizing agent. We observed that standard redox potentials, oxygen uptake, free ascorbyl radical formation and cell survival were linked. The oxidative stress induced by the mixture of ascorbate and the different quinones decreases cellular contents of ATP and GSH while caspase-3-like activity remains unchanged. Again, we observed that quinones having higher values of half-redox potential provoke a severe depletion of ATP and GSH when they were associated with ascorbate. Such a drop in ATP content may explain the lack of activation of caspase-3. In conclusion, our results indicate that the cytotoxicity of the association quinone/ascorbate on K562 cancer cells may be predicted on the basis of half-redox potentials of quinones.     

FAD semiquinone stability regulates single- and two-electron reduction of quinones by Anabaena PCC7119 ferredoxin:NADP+ reductase and its Glu301Ala mutant

Flavoenzymes may reduce quinones in a single-electron, mixed single- and two-electron, and two-electron way. The mechanisms of two-electron reduction of quinones are insufficiently understood. To get an insight into the role of flavin semiquinone stability in the regulation of single- vs. two-electron reduction of quinones, we studied the reactions of wild type Anabaena ferredoxin:NADP(+)reductase (FNR) with 48% FAD semiquinone (FADH*) stabilized at the equilibrium (pH 7.0), and its Glu301Ala mutant (8% FADH* at the equilibrium). We found that Glu301Ala substitution does not change the quinone substrate specificity of FNR. However, it confers the mixed single- and two-electron mechanism of quinone reduction (50% single-electron flux), whereas the wild type FNR reduces quinones in a single-electron way. During the oxidation of fully reduced wild type FNR by tetramethyl-1,4-benzoquinone, the first electron transfer (formation of FADH*) is about 40 times faster than the second one (oxidation of FADH*). In contrast, the first and second electron transfer proceeded at similar rates in Glu301Ala FNR. Thus, the change in the quinone reduction mechanism may be explained by the relative increase in the rate of second electron transfer. This enabled us to propose the unified scheme of single-, two- and mixed single- and two-electron reduction of quinones by flavoenzymes with the central role of the stability of flavin/quinone ion-radical pair.     

Plasticity of the Quinone-binding Site of the Complex II Homolog Quinol:Fumarate Reductase

Respiratory processes often use quinone oxidoreduction to generate a transmembrane proton gradient, making the 2H⁺/2e⁻ quinone chemistry important for ATP synthesis. There are a variety of quinones used as electron carriers between bioenergetic proteins, and some respiratory proteins can functionally interact with more than one quinone type. In the case of complex II homologs, which couple quinone chemistry to the interconversion of succinate and fumarate, the redox potentials of the biologically available ubiquinone and menaquinone aid in driving the chemical reaction in one direction. In the complex II homolog quinol:fumarate reductase, it has been demonstrated that menaquinol oxidation requires at least one proton shuttle, but many of the remaining mechanistic details of menaquinol oxidation are not fully understood, and little is known about ubiquinone reduction. In the current study, structural and computational studies suggest that the sequential removal of the two menaquinol protons may be accompanied by a rotation of the naphthoquinone ring to optimize the interaction with a second proton shuttling pathway. However, kinetic measurements of site-specific mutations of quinol:fumarate reductase variants show that ubiquinone reduction does not use the same pathway. Computational docking of ubiquinone followed by mutagenesis instead suggested redundant proton shuttles lining the ubiquinone-binding site or from direct transfer from solvent. These data show that the quinone-binding site provides an environment that allows multiple amino acid residues to participate in quinone oxidoreduction. This suggests that the quinone-binding site in complex II is inherently plastic and can robustly interact with different types of quinones.     

Electron transfer reactions between quinols and quinones in aqueous and aprotic media

The pathways of redox equilibration of quinols and quinones have been investigated. The rate-limiting reaction involves the couple QH^?/QH^· of the reducing quinol and the couple Q^?/Q of the oxidising quinone. Three general mechanistic points may be surmised: (i) protonation/deprotonation reactions are not rate-limiting; (ii) all transfers occur in one-equivalent steps; (iii) electron transfers, but not hydrogen atom transfers, are the dominant features. In aprotic media, no rapid route of equilibration is available since the ionic species which are necessary for thermodynamically feasible routes of electron transfer cannot exist to any great extent. The relation of these results to models of biological quinone systems is discussed.     

Interactions of quinones with thioredoxin reductase: a challenge to the antioxidant role of the mammalian selenoprotein

Mammalian thioredoxin reductases (TrxR) are important selenium-dependent antioxidant enzymes. Quinones, a wide group of natural substances, human drugs, and environmental pollutants may act either as TrxR substrates or inhibitors. Here we systematically analyzed the interactions of TrxR with different classes of quinone compounds. We found that TrxR catalyzed mixed single- and two-electron reduction of quinones, involving both the selenium-containing motif and a second redox center, presumably FAD. Compared with other related pyridine nucleotide-disulfide oxidoreductases such as glutathione reductase or trypanothione reductase, the k(ca)(t)/K(m) value for quinone reduction by TrxR was about 1 order of magnitude higher, and it was not directly related to the one-electron reduction potential of the quinones. A number of quinones were reduced about as efficiently as the natural substrate thioredoxin. We show that TrxR mainly cycles between the four-electron reduced (EH(4)) and two-electron reduced (EH(2)) states in quinone reduction. The redox potential of the EH(2)/EH(4) couple of TrxR calculated according to the Haldane relationship with NADPH/NADP(+) was -0.294 V at pH 7.0. Antitumor aziridinylbenzoquinones and daunorubicin were poor substrates and almost inactive as reversible TrxR inhibitors. However, phenanthrene quinone was a potent inhibitor (approximate K(i) = 6.3 +/- 1 microm). As with other flavoenzymes, quinones could confer superoxide-producing NADPH oxidase activity to mammalian TrxR. A unique feature of this enzyme was, however, the fact that upon selenocysteine-targeted covalent modification, which inactivates its normal activity, reduction of some quinones was not affected, whereas that of others was severely impaired. We conclude that interactions with TrxR may play a considerable role in the complex mechanisms underlying the diverse biological effects of quinones.     

The Proton Trap Technology—Toward High Potential Quinone‐Based Organic Energy Storage

An organic cathode material based on a copolymer of poly(3,4‐ethylenedioxythiophene) containing pyridine and hydroquinone functionalities is described as a proton trap technology. Utilizing the quinone to hydroquinone redox conversion, this technology leads to electrode materials compatible with lithium and sodium cycling chemistries. These materials have high inherent potentials that in combination with lithium give a reversible output voltage of above 3.5 V (vs Li^0/+) without relying on lithiation of the material, something that is not showed for quinones previously. Key to success stems from coupling an intrapolymeric proton transfer, realized by an incorporated pyridine proton donor/acceptor functionality, with the hydroquinone redox reactions. Trapping of protons in the cathode material effectively decouples the quinone redox chemistry from the cycling chemistry of the anode, which makes the material insensitive to the nature of the electrolyte cation and hence compatible with several anode materials. Furthermore, the conducting polymer backbone allows assembly without any additives for electronic conductivity. The concept is demonstrated by electrochemical characterization in several electrolytes and finally by employing the proton trap material as the cathode in lithium and sodium batteries. These findings represent a new concept for enabling high potential organic materials for the next generation of energy storage systems.     

Full replacement of the function of the secondary electron acceptor phylloquinone(= vitamin K1) by non-quinone carbonyl compounds in green plant photosystem I photosynthetic reaction centers

One-carbonyl quinonoid compounds, fluorenone (fluoren-9-one), anthrone, and their derivatives are introduced into spinach photosystem (PS) I reaction centers in place of the intrinsic secondary electron acceptor phylloquinone (= vitamin K1). Anthrone and 2-nitrofluorenone fully mediated the electron-transfer reaction between the reduced primary electron acceptor chlorophyll A0- and the tertiary electron acceptor iron-sulfur centers. It is concluded that the PS I phylloquinone-binding site has a structure that enables various compounds with different molecular structures to function as the secondary acceptor and that the reactions of incorporated compounds are mainly determined by their redox properties rather than by their molecular structure. Carbonyl groups increase the binding affinity of the quinone/quinonoid compounds but do not seem to be essential to their function. The quinonoid compounds as well as quinones incorporated into the PS I phylloquinone-binding sites are estimated to function at redox potentials more negative than in organic solvents.     

Features of idebenone and related short-chain quinones that rescue ATP levels under conditions of impaired mitochondrial complex I

Short-chain quinones have been investigated as therapeutic molecules due to their ability to modulate cellular redox reactions, mitochondrial electron transfer and oxidative stress, which are pathologically altered in many mitochondrial and neuromuscular disorders. Recently, we and others described that certain short-chain quinones are able to bypass a deficiency in complex I by shuttling electrons directly from the cytoplasm to complex III of the mitochondrial respiratory chain to produce ATP. Although this energy rescue activity is highly interesting for the therapy of disorders associated with complex I dysfunction, no structure-activity-relationship has been reported for short-chain quinones so far. Using a panel of 70 quinones, we observed that the capacity for this cellular energy rescue as well as their effect on lipid peroxidation was influenced more by the physicochemical properties (in particular logD) of the whole molecule than the quinone moiety itself. Thus, the observed correlations allow us to explain the differential biological activities and therapeutic potential of short-chain quinones for the therapy of disorders associated with mitochondrial complex I dysfunction and/or oxidative stress.     

The effect of metal ions on the electrochemistry of the antitumor antibiotic streptonigrin

The effect of transition metal ions on the electrochemistry of 6-methoxy-5,8-quinolinedione (L1), 7-amino-6-methoxy-5,8-quinolinedione (L2) and the antitumor antibiotic streptonigrin (SN) was studied. In 10% methanol/water, the one-electron reduction of quinones L1 and L2 to the corresponding semiquinones is shifted to more positive potentials upon addition of one equivalent of Zn(II), Ni(II), Co(II) or Cd(II) and is consistent with formation of a 1:1 complex involving the quinone(N) and adjacent quinone(O). Similar results are observed for Cu(II) and Mn(II), but the redox chemistry is also complicated by metal-based redox chemistry. The addition of further equivalents of M(II) results in a number of different coordination and electrochemical processes including formation of 1:1 and 2:1 complexes of the quinone, semiquinone and dianion. Under similar conditions, the 1:1 SN 2,2'-bipyridyl metal complex undergoes a reversible one-electron reduction to the semiquinone. The redox potential of the quinone in SN was shifted positive in the presence of the metal ions, but both the magnitude of the shift, and the relative influence of the metals was different to ligands L1 and L2. The changes in redox chemistry of SN compared with L1 and L2 are consistent with the formation of the 2,2-bipyridyl complexes in which there is weaker coordination to the quinone(O) in ring A of SN. These results suggest that in vivo, metal ions such as Zn(II), Cu(II) and Mn(II) facilitate the initial reduction of streptonigrin to the semiquinone by capturing the semiquinone after SN is reduced by biological reductants.     

Redox potential of quinones in both electron transfer branches of photosystem I

The redox potentials of the two electron transfer (ET) active quinones in the central part of photosystem I (PSI) were determined by evaluating the electrostatic energies from the solution of the Poisson-Boltzmann equation based on the crystal structure. The calculated redox potentials are -531 mV for A1A and -686 mV for A1B. From these results we conclude the following. (i) Both branches are active with a much faster ET in the B-branch than in the A-branch. (ii) The measured lifetime of 200-290 ns of reduced quinones agrees with the estimate for the A-branch and corroborates with an uphill ET from this quinone to the iron-sulfur cluster as observed in recent kinetic measurements. (iii) The electron paramagnetic resonance spectroscopic data refer to the A-branch quinone where the corresponding ET is uphill in energy. The negative redox potential of A1 in PSI is primarily because of the influence from the negatively charged FX, in contrast to the positive shift on the quinone redox potential in bacterial reaction center and PSII that is attributed to the positively charged non-heme iron atom. The conserved residue Asp-B575 changes its protonation state after quinone reduction. The difference of 155 mV in the quinone redox potentials of the two branches were attributed to the conformation of the backbone with a large contribution from Ser-A692 and Ser-B672 and to the side chain of Asp-B575, whose protonation state couples differently with the formation of the quinone radicals.