Energy transduction function of the quinone reactions in cytochrome bc complexes.

Cytochrome bc1, a multi-subunit integral membrane protein complex found in mammalian mitochondria, plays a central role in the transfer of electrons and protons generated by the oxidation of ubiquinol. According to the classical chemiosmotic theory, quinones shuttle protons across the hydrophobic membrane bilayer with the net result of H+ transfer to the aqueous side and generation of an electrochemical proton gradient. Recently, high-resolution structures of the mitochondrial bc1 complex showed quinone binding sites at one of the transmembrane helices of cytochrome b, and two potentially protonatable histidine residues on the Rieske iron-sulfur protein. The modern biochemical refinements of the original chemiosmotic theory require electron and proton transfer from quinones to particular residues/redox centers of integral membrane proteins and subsequent transfer of H+ to the bulk aqueous phase outside the membrane.     

Redox and addition chemistry of quinoid compounds and its biological implications

The overall biological activity of quinones is a function of the physico-chemical properties of these compounds, which manifest themselves in a critical bimolecular reaction with bioconstituents. Attempts have been made to characterize this bimolecular reaction as a function of the redox properties of quinones in relation to hydrophobic or hydrophilic environments. The inborn physico-chemical properties of quinones are discussed on the basis of their reduction potential and dissociation constants, as well as the effect of environmental factors on these properties. Emphasis is given on the effect of methyl-, methoxy-, hydroxy-, and glutathionyl substituents on the reduction potential of quinones and the subsequent electron transfer processes. The redox chemistry of quinoid compounds is surveyed in terms of a) reactions involving only electron transfer, as those accomplished during the enzymic reduction of quinones and the non-enzymic interaction with redox couples generating semiquinones, and b) nucleophilic addition reactions. The addition of nucleophiles, entailing either oxidation or reduction of the quinone, are exemplified in reactions with oxygen- or sulfur nucleophiles, respectively. The former yields quinone epoxides, whereas the latter yields thioether-hydroquinone adducts as primary molecular products. The subsequent chemistry of these products is examined in terms of enzymic reduction, autoxidation, cross-oxidation, disproportionation, and free radical interactions. The detailed chemical mechanisms by which quinoid compounds exert cytotoxic, mutagenic and carcinogenic effects are considered individually in relation to redox cycling, alterations of thiol balance and Ca++ homeostasis, and covalent binding.     

The aprotic electrochemistry of quinones

Quinones play important roles in biological electron transfer reactions in almost all organisms, with specific roles in many physiological processes and chemotherapy. Quinones participate in two-electron, two-proton reactions in aqueous solution at equilibrium near neutral pH, but protons often lag behind the electron transfers. The relevant reactions in proteins are often sequential one electron redox processes without involving protons. Here we report the aprotic electrochemistry of the two half-couples, Q/Q^.- and Q^.?/Q⁼, of 11 parent quinones and 118 substituted 1,4-benzoquinones, 91 1,4-naphthoquinones, and 107 9,10-anthraquinones. The measured redox potentials are fit quite well with the Hammett para sigma (σ_para) parameter. Occasional exceptions can involve important groups, such as methoxy substituents in ubiquinone and hydroxy substituents in therapeutics. These can generally be explained by reasonable conjectures involving steric clashes and internal hydrogen bonds. We also provide data for 25 other quinones, 2 double quinones and 15 non-quinones, all measured under similar conditions.     

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.     

Structure and mechanism of tryptophylquinone enzymes

Tryptophylquinone cofactors are formed by posttranslational modifications that result in the incorporation of two oxygens into a tryptophan side chain, and the covalent cross-linking of that side chain to another amino acid residue. Tryptophylquinone enzymes catalyze the oxidative deamination of primary amines, and utilize other redox proteins as electron acceptors. Mechanistic and structural studies of these enzymes are providing insight into how these enzymes utilize these highly reactive protein-derived quinones in a controlled manner to facilitate biologically important catalytic and electron transfer reactions.     

The mechanism of coupling between oxido‐reduction and proton translocation in respiratory chain enzymes

The respiratory chain of mitochondria and bacteria is made up of a set of membrane‐associated enzyme complexes which catalyse sequential, stepwise transfer of reducing equivalents from substrates to oxygen and convert redox energy into a transmembrane protonmotive force (PMF) by proton translocation from a negative (N) to a positive (P) aqueous phase separated by the coupling membrane. There are three basic mechanisms by which a membrane‐associated redox enzyme can generate a PMF. These are membrane anisotropic arrangement of the primary redox catalysis with: (i) vectorial electron transfer by redox metal centres from the P to the N side of the membrane; (ii) hydrogen transfer by movement of quinones across the membrane, from a reduction site at the N side to an oxidation site at the P side; (iii) a different type of mechanism based on co‐operative allosteric linkage between electron transfer at the metal redox centres and transmembrane electrogenic proton translocation by apoproteins. The results of advanced experimental and theoretical analyses and in particular X‐ray crystallography show that these three mechanisms contribute differently to the protonmotive activity of cytochrome c oxidase, ubiquinone‐cytochrome c oxidoreductase and NADH‐ubiquinone oxidoreductase of the respiratory chain. This review considers the main features, recent experimental advances and still unresolved problems in the molecular/atomic mechanism of coupling between the transfer of reducing equivalents and proton translocation in these three protonmotive redox complexes.     

Bioreductive Activation of Quinones: Redox Properties and Thiol Reactivity

Redox properties and thiol reactivity are central to the therapeutic and toxicological properties of qui-nones. The use of other physicochemical parameters to establish predictive relationships for redox properties of quinones is discussed. and attention drawn to situations where such relationships may be unreliable. The rates of reaction of semiquinone radicals with oxygen, including those of chemotherapeutic agents such as mitomycin and the anthracyclines. can be predicted with reasonable confidence from the redox properties. The reactions of quinones with thiols such as glutathione produces reduced quinones and radicals. but the reactions are complex and all the features are not well understood     

Redox compounds influence on the NAD(P)H:FMN-oxidoreductase-luciferase bioluminescent system.

A review of the mechanisms of the exogenous redox compounds influence on the bacterial coupled enzyme system: NAD(P)H:FMN-oxidoreductase-luciferase has been done. A series of quinones has been used as model organic oxidants. The three mechanisms of the quinones' effects on bioluminescence were suggested: (1) inhibition of the NADH-dependent redox reactions; (2) interactions between the compounds and the enzymes of the coupled enzyme system; and (3) intermolecular energy migration. The correlation between the kinetic parameters of bioluminescence and the standard redox potential of the quinones proved that the inhibition of redox reactions was the key mechanism by which the quinones decrease the light emission intensity. The changes in the fluorescence anisotropy decay of the endogenous flavin of the enzyme preparations showed the direct interaction between quinones and enzymes. It has been demonstrated that the intermolecular energy migration mechanism played a minor role in the effect of quinones on the bioluminescence. A comparative analysis of the effect of quinones, phenols and inorganic redox compounds on bioluminescent coupled enzyme systems has been carried out.     

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.     

Bioreductive Activation of Quinones: Redox Properties and Thiol Reactivity

Redox properties and thiol reactivity are central to the therapeutic and toxicological properties of qui-nones. The use of other physicochemical parameters to establish predictive relationships for redox properties of quinones is discussed. and attention drawn to situations where such relationships may be unreliable. The rates of reaction of semiquinone radicals with oxygen, including those of chemotherapeutic agents such as mitomycin and the anthracyclines. can be predicted with reasonable confidence from the redox properties. The reactions of quinones with thiols such as glutathione produces reduced quinones and radicals. but the reactions are complex and all the features are not well understood     

Polycyclic aromatic hydrocarbon quinone-mediated oxidation reduction cycling catalyzed by a human placental NADPH-linked carbonyl reductase.

Polycyclic aromatic hydrocarbon quinones, hydroquinones, and glutathionyl adducts of quinones undergo oxidation-reduction (redox) cycling in the presence of NADPH and the NADPH-linked human placental carbonyl reductase. K-region and non-K-region o-quinones and their glutathione adducts are the best substrates of this enzyme; they are reduced to hydroquinones. Under aerobic conditions, the hydroquinones are autoxidized with the formation of potentially hazardous semiquinones and the superoxide anion. Because of these reactions it is unlikely that polycyclic aromatic hydrocarbon quinones or their glutathione adducts are inert products of detoxication in tissues that contain the carbonyl reductase or another enzyme with similar substrate specificity. If superoxide dismutase is added to reaction mixtures containing the carbonyl reductase and quinones, it inhibits redox cycling. Presumably this results from destruction of the superoxide anion which acts as a chain propagator in these reactions.     

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.     

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.     

Comparison of Quinone‐Based Catholytes for Aqueous Redox Flow Batteries and Demonstration of Long‐Term Stability with Tetrasubstituted Quinones

Quinones are appealing targets as organic charge carriers for aqueous redox flow batteries (RFBs), but their utility continues to be constrained by limited stability under operating conditions. The present study evaluates the stability of a series of water‐soluble quinones, with redox potentials ranging from 605–885 mV versus NHE, under acidic aqueous conditions (1 m H₂SO₄). Four of the quinones are examined as cathodic electrolytes in an aqueous RFB, paired with anthraquinone‐2,7‐disulfonate as the anodic electrolyte. The RFB data complement other solution stability tests and show that the most stable electrolyte is a tetrasubstituted quinone containing four sulfonated thioether substituents. The results highlight the importance of substituting all C–H positions of the quinone in order to maximize the quinone stability and set the stage for design of improved organic electrolytes for aqueous RFBs.     

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.     

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     

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.     

Effect of quinones on enzymatic bioluminescence of NADH-dependent systems

The effects of a number of quinones on the bioluminescence characteristics of a three-component enzymatic system containing alcohol dehydrogenase, bacterial luciferase, and NADH-FMN oxidoreductase were studied to find the most sensitive kinetic parameters of the system intended to be used in biological testing. Both direct and back reactions catalyzed by alcohol dehydrogenase were studied in the presence and in the absence of quinones. The kinetic parameters of the bioluminescent system were found to depend on the redox potentials and concentrations of quinones. The quinone-induced effects were shown to be associated with changes in the NAD+/NADH ratio in the chain of NADH-dependent enzymes. The three-enzyme system based on alcohol dehydrogenase is suggested as a bioluminescence test for ecological monitoring of waste water.     

Perturbation of the quinone-binding site of complex II alters the electronic properties of the proximal [3Fe-4S] iron-sulfur cluster

Succinate-ubiquinone oxidoreductase (SQR) and menaquinol-fumarate oxidoreductase (QFR) from Escherichia coli are members of the complex II family of enzymes. SQR and QFR catalyze similar reactions with quinones; however, SQR preferentially reacts with higher potential ubiquinones, and QFR preferentially reacts with lower potential naphthoquinones. Both enzymes have a single functional quinone-binding site proximal to a [3Fe-4S] iron-sulfur cluster. A difference between SQR and QFR is that the redox potential of the [3Fe-4S] cluster in SQR is 140 mV higher than that found in QFR. This may reflect the character of the different quinones with which the two enzymes preferentially react. To investigate how the environment around the [3Fe-4S] cluster affects its redox properties and catalysis with quinones, a conserved amino acid proximal to the cluster was mutated in both enzymes. It was found that substitution of SdhB His-207 by threonine (as found in QFR) resulted in a 70-mV lowering of the redox potential of the cluster as measured by EPR. The converse substitution in QFR raised the redox potential of the cluster. X-ray structural analysis suggests that placing a charged residue near the [3Fe-4S] cluster is a primary reason for the alteration in redox potential with the hydrogen bonding environment having a lesser effect. Steady state enzyme kinetic characterization of the mutant enzymes shows that the redox properties of the [3Fe-4S] cluster have only a minor effect on catalysis.