The invisible copper of cytochrome c oxidase. pH and ATP dependence of its midpoint potential and its role in the oxygen reaction

Cytochrome bd and Gaseous Ligands in Bacterial Physiology

Cytochrome bd is a unique prokaryotic respiratory terminal oxidase that does not belong to the extensively investigated family of haem-copper oxidases (HCOs). The enzyme catalyses the four-electron reduction of O₂ to 2H₂O, using quinols as physiological reducing substrates. The reaction is electrogenic and cytochrome bd therefore sustains bacterial energy metabolism by contributing to maintain the transmembrane proton motive force required for ATP synthesis. As compared to HCOs, cytochrome bd displays several distinctive features in terms of (i) metal composition (it lacks Cu and harbours a d-type haem in addition to two haems b), (ii) overall three-dimensional structure, that only recently has been solved, and arrangement of the redox cofactors, (iii) lesser energetic efficiency (it is not a proton pump), (iv) higher O₂ affinity, (v) higher resistance to inhibitors such as cyanide, nitric oxide (NO) and hydrogen sulphide (H₂S) and (vi) ability to efficiently metabolize potentially toxic reactive oxygen and nitrogen species like hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO^-). Compelling evidence suggests that, beyond its bioenergetic role, cytochrome bd plays multiple functions in bacterial physiology and affords protection against oxidative and nitrosative stress. Relevant to human pathophysiology, thanks to its peculiar properties, the enzyme has been shown to promote virulence in several bacterial pathogens, being currently recognized as a target for the development of new antibiotics. This review aims to give an update on our current understanding of bd-type oxidases with a focus on their reactivity with gaseous ligands and its potential impact on bacterial physiology and human pathophysiology.

Modeling the detailed kinetics of mitochondrial cytochrome c oxidase: Catalytic mechanism and nitric oxide inhibition

Cytochrome c oxidase (CcO) catalyzes the exothermic reduction of O₂ to H₂O by using electrons from cytochrome c, and hence plays a crucial role in ATP production. Although details on the enzyme structure and redox centers involved in O₂ reduction have been known, there still remains a considerable ambiguity on its mechanism of action, e.g., the number of sequential electrons donated to O₂ in each catalytic step, the sites of protonation and proton pumping, and nitric oxide (NO) inhibition mechanism. In this work, we developed a thermodynamically constrained mechanistic mathematical model for the catalytic action of CcO based on available kinetic data. The model considers a minimal number of redox centers on CcO and couples electron transfer and proton pumping driven by proton motive force (PMF), and accounts for the inhibitory effects of NO on the reaction kinetics. The model is able to fit well all the available kinetic data under diverse experimental conditions with a physiologically realistic unique parameter set. The model predictions show that: 1) the apparent K_m of O₂ varies considerably and increases from fully reduced to fully oxidized cytochrome c depending on pH and the energy state of mitochondria, and 2) the intermediate enzyme states depend on pH and cytochrome c redox fraction and play a central role in coupling mitochondrial respiration to PMF. The developed CcO model can easily be integrated into existing mitochondrial bioenergetics models to understand the role of the enzyme in controlling oxidative phosphorylation in normal and disease conditions.

Low concentrations of metal mixture exposures have adverse effects on selected biomarkers of Xenopus laevis tadpoles

Polluted ecosystems may contain mixtures of metals, such that the combinations of metals, even in low concentrations, may cause adverse effects. In the present study, we focused on toxic effects of mixtures of selected metals, the LC50 values, and also their safety limit in aquatic systems imposed by the European legislation using a model organism. Xenopus laevis tadpoles were used as test organisms. They were exposed to metals or their combinations due to 96-h LC50 values. Glutathione S-transferase (GST), glutathione reductase (GR), acetylcholinesterase (AChE), carboxylesterase (CaE), glutathione peroxidase (GPx), and catalase (CAT) levels were evaluated. Metallothionein concentrations were also determined. The LC50s for Cd, Pb, and Cu were calculated as 5.81mg AI/L, 123.05mg AI/L, and 0.85mg AI/L, respectively. Low lethality ratios were observed with unary exposure of each metal in lower concentrations. Double or triple combinations of LC50 and LC50/2 concentrations caused 100% lethality with Cd+Cu and Pb+Cd+Cu mixtures, while the Pb+Cu mixture also caused high lethal ratios. The selected enzyme activities were significantly affected by metals or mixtures, and dose-related effects were determined. The metallothionein levels generally increased as related to concentration in unary metals and mixtures. Acceptable limit values of unary metals and mixtures did not significantly change metallothionein levels. The results suggest that oxidative stress-related mechanisms are involved in the toxicity induced by selected metals with combinations of very low concentrations.

Mitochondrial cytochrome c oxidase: mechanism of action and role in regulating oxidative phosphorylation

Mitochondrial oxidative phosphorylation has a central role in eukaryotic metabolism, providing the energy (ATP) required for survival. Regulation of this important pathway is, however, still not understood, largely due to limitations in the ability to measure the essential metabolites, including oxygen (pO2, oxygen pressure), ADP, and AMP. In addition, neither the mechanism of oxygen reduction by mitochondrial cytochrome c oxidase nor how its rate is controlled is understood, although this enzyme determines the rate of oxygen consumption and thereby the rate of ATP synthesis. Cytochrome c oxidase is responsible for reduction of molecular oxygen to water using reducing equivalents donated by cytochrome c and for site 3 energy coupling in oxidative phosphorylation. A mechanism-based model of the cytochrome c oxidase reaction is presented in which transfer of reducing equivalents from the lower- to the higher-potential region of the coupling site occurs against an opposing energy barrier, Q. The steady-state rate equation is fitted to data for the dependence of mitochondrial respiratory rate on cytochrome c reduction, oxygen pressure (pO2), and [ATP]/[ADP][Pi] at pH 6.5 to 8.35 (where Pi is inorganic phosphate). The fit of the rate expression to the experimental data is very good for all experimental conditions. Levels of the intermediates in oxygen reduction in the oxidase reaction site have been calculated. An intermediate in the reaction, tentatively identified as peroxide, bridged between the iron and copper atoms of the reaction site has a central role in coupling mitochondrial respiration to the [ATP]/[ADP][Pi].

Cytochrome bd oxidase and bacterial tolerance to oxidative and nitrosative stress

Cytochrome bd is a prokaryotic respiratory quinol:O2 oxidoreductase, phylogenetically unrelated to the extensively studied heme-copper oxidases (HCOs). The enzyme contributes to energy conservation by generating a proton motive force, though working with a lower energetic efficiency as compared to HCOs. Relevant to patho-physiology, members of the bd-family were shown to promote virulence in some pathogenic bacteria, which makes these enzymes of interest also as potential drug targets. Beyond its role in cell bioenergetics, cytochrome bd accomplishes several additional physiological functions, being apparently implicated in the response of the bacterial cell to a number of stress conditions. Compelling experimental evidence suggests that the enzyme enhances bacterial tolerance to oxidative and nitrosative stress conditions, owing to its unusually high nitric oxide (NO) dissociation rate and a notable catalase activity; the latter has been recently documented in one of the two bd-type oxidases of Escherichia coli. Current knowledge on cytochrome bd and its reactivity with O2, NO and H2O2 is summarized in this review in the light of the hypothesis that the preferential (over HCOs) expression of cytochrome bd in pathogenic bacteria may represent a strategy to evade the host immune attack based on production of NO and reactive oxygen species (ROS). This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

Cytochrome c oxidase and nitric oxide in action: molecular mechanisms and pathophysiological implications

BACKGROUND

The reactions between Complex IV (cytochrome c oxidase, CcOX) and nitric oxide (NO) were described in the early 60's. The perception, however, that NO could be responsible for physiological or pathological effects, including those on mitochondria, lags behind the 80's, when the identity of the endothelial derived relaxing factor (EDRF) and NO synthesis by the NO synthases were discovered. NO controls mitochondrial respiration, and cytotoxic as well as cytoprotective effects have been described. The depression of OXPHOS ATP synthesis has been observed, attributed to the inhibition of mitochondrial Complex I and IV particularly, found responsible of major effects.

SCOPE OF REVIEW

The review is focused on CcOX and NO with some hints about pathophysiological implications. The reactions of interest are reviewed, with special attention to the molecular mechanisms underlying the effects of NO observed on cytochrome c oxidase, particularly during turnover with oxygen and reductants.

MAJOR CONCLUSIONS AND GENERAL SIGNIFICANCE

The NO inhibition of CcOX is rapid and reversible and may occur in competition with oxygen. Inhibition takes place following two pathways leading to formation of either a relatively stable nitrosyl-derivative (CcOX-NO) of the enzyme reduced, or a more labile nitrite-derivative (CcOX-NO(2)(-)) of the enzyme oxidized, and during turnover. The pathway that prevails depends on the turnover conditions and concentration of NO and physiological substrates, cytochrome c and O(2). All evidence suggests that these parameters are crucial in determining the CcOX vs NO reaction pathway prevailing in vivo, with interesting physiological and pathological consequences for cells.

Nitric oxide and the respiratory enzyme

Available information on the molecular mechanisms by which nitric oxide (NO) controls the activity of the respiratory enzyme (cytochrome-c-oxidase) is reviewed. We report that, depending on absolute electron flux, NO at physiological concentrations reversibly inhibits cytochrome-c-oxidase by two alternative reaction pathways, yielding either a nitrosyl- or a nitrite-heme a3 derivative. We address a number of hypotheses, envisaging physiological and/or pathological effects of the reactions between NO and cytochrome-c-oxidase.

Nature of the FeO2 bonding in myoglobin and hemoglobin: A new molecular paradigm

The iron(II)-dioxygen bond in myoglobin and hemoglobin is a subject of wide interest. Studies range from examinations of physical-chemical properties dependent on its electronic structure, to investigations of the stability as a function of oxygen supply. Among these, stability properties are of particular importance in vivo. Like all known dioxygen carriers synthesized so far with transition metals, the oxygenated forms of myoglobin and hemoglobin are known to be oxidized easily to their ferric met-forms, which cannot bind molecular oxygen and are therefore physiologically inactive. The mechanistic details of this autoxidation reaction, which are of clinical, as well as of physical-chemical, interest, have long been investigated by a number of authors, but a full understanding of the heme oxidation has not been reached so far. Recent kinetic and thermodynamic studies of the stability of oxymyoglobin (MbO2) and oxyhemoglobin (HbO2) have revealed new features in the FeO2 bonding. In vivo, the iron center is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the heme pocket from the surrounding solvent and thereby irreversibly displace the bound dioxygen from MbO2 or HbO2 in the form of O2- so that the iron is converted to the ferric met-form. Since the autoxidation reaction of MbO2 or HbO2 proceeds through a nucleophilic displacement following one-electron transfer from iron(II) to the bound O2, this reaction may be viewed as a meeting point of the stabilization and the activation of molecular oxygen performed by hemoproteins. Along with these lines of evidence, we finally discuss the stability property of human HbO2 and provide with the most recent state of hemoglobin research. The HbA molecule contains two types of alphabeta contacts and seems to differentiate them quite properly for its functional properties. The alpha1beta2 or alpha2beta1 contact is associated with the cooperative oxygen binding, whereas the alpha1beta1 or alpha2beta2 contact is used for controlling the stability of the bound O2. We can thus form a unified picture for hemoglobin function by closely integrating the cooperative and the stable binding of molecular oxygen with iron(II) in aqueous solvent. These new views on the nature of FeO2 bonding and the possible role of globin moiety in stabilizing MbO2 and HbO2 are of primary importance, not only for a full understanding of various hemoprotein reactions with O2, but also for planning new molecular designs for synthetic oxygen carriers which may be able to function in aqueous solvent and at physiological temperature.

Structure and function of a molecular machine: cytochrome c oxidase

Cytochrome c is responsible for over 90% of the dioxygen consumption in the living cell and contributes to the build-up of a proton electrochemical gradient derived by the vectorial transfer of electrons between cytochrome c and molecular oxygen. The metal ions found in cytochrome oxidases play a crucial role in these processes and have been extensively studied. In this review we present and discuss some of the relevant spectroscopic and kinetic properties of the prosthetic groups of cytochrome c oxidase.

Proton uptake by cytochrome c oxidase on reduction and on ligand binding

On reduction, cytochrome oxidase was found to take up 2.4 +/- 0.1 protons in the pH range 7.2-8.5, of which 2 are associated with the binuclear centre, and the remaining fractional proton with haem a/CuA. Ligation to oxidised cytochrome oxidase of the azide, formate, fluoride or cyanide anions is accompanied by uptake of one proton. In the case of the reduced enzyme, no protonation changes are observed on binding O2 (Hallén S. and Nilsson T. (1992) Biochemistry 31, 11853-11859) or CO. Cyanide binding to reduced oxidase is, in contrast, still accompanied by uptake of a proton. These findings are discussed in terms of our previously-published proposal for the ligand chemistry of the binuclear site. The results overall suggest a principle of electroneutrality of redox and ligand state changes of the binuclear centre, with charge compensations provided only by protonation reactions.

Flash photolysis of the carbon monoxide compounds of wild-type and mutant variants of cytochrome bo from Escherichia coli

The carbon monoxide compounds of the fully reduced and mixed valence forms of cytochrome bo from Escherichia coli were laser photolysed under anaerobic conditions at room temperature. The carbon monoxide recombined with characteristic rate constants of 50 s-1 or 35 s-1 in the fully reduced and mixed valence forms, respectively. Rates of CO recombination with the fully reduced enzyme were examined in a variety of mutant forms of cytochrome bo, produced by site-directed mutagenesis. A method was developed to deconvolute cytochromes bo and bd, leading to some reassessment of histidine ligands to the metals. Significant changes in the rate constant of recombination of carbon monoxide occurred in many of these mutants and these results could be rationalised generally in terms of our current working model of the folding structure of subunit I. In the mixed valence form of the enzyme the transient photolysis spectra in the visible region are consistent with a rapid electron redistribution from the binuclear centre to the low-spin haem. This electron transfer is biphasic, with rate constants of around 10(5) and 8000 s-1. The process was also examined in the His-333-Leu mutant, in which a putative histidine ligand to CuB is replaced by leucine, and which results in the loss of the CuB. It appeared that rapid haem-haem electron transfer could still occur. The observation that CuB is apparently not required for rapid haem-haem electron transfer is consistent with the recently proposed model in which the two haems are positioned on opposite sides of transmembrane helix X in subunit I of the oxidase.

Discrete steps in dioxygen activation--the cytochrome oxidase/O2 reaction

The kinetic constraints that are imposed on cytochrome oxidase in its dual function as the terminal oxidant in the respiratory process and as a redox-linked proton pump provide a unique opportunity to investigate the molecular details of biological O2 activation. By using flow/flash techniques, it is possible to visualize individual steps in the O2-binding and reduction process, and results from a number of spectroscopic investigations on the oxidation of reduced cytochrome oxidase by O2 are now available. In this article, we use these results to synthesize a reaction mechanism for O2 activation in the enzyme and to simulate time-concentration profiles for a number of intermediates that have been observed experimentally. Kinetic manifestation of the consequences of coupling exergonic electron transfer to endergonic proton translocation emerge from this analysis. Energetic efficiency in this process apparently requires that potentially toxic intermediate oxidation states of dioxygen accumulate to substantial concentration during the reduction reaction.

Respiratory control in cytochrome oxidase vesicles is correlated with the rate of internal electron transfer

Cytochrome c oxidase, after reconstitution into phospholipid vesicles, displays respiratory control. This appears as an inhibition of substrate oxidation (cytochrome c) or reduction (O2) rates which, in the first few turnovers, can be largely removed upon addition of valinomycin, a specific K+ carrier. We report experiments designed to measure directly the internal electron transfer leading to the reduction of cytochrome a3/CuB, in the presence and the absence of a membrane potential. The results suggest that, after the complete oxidation and partial re-reduction of the protein, electron transfer to the binuclear site is valinomycin-sensitive, i.e. is inhibited by the membrane potential. The first-order rate constants calculated in the absence and presence of valinomycin were 0.5-0.6 and 5-6 s-1 respectively. Kinetic analysis of the reduction process is consistent with the conclusion that the membrane potential is below the critical threshold until the first electron is transferred to the cytochrome a3/CuB site. Furthermore, the respiratory control ratio obtained from the dependence of the internal electron transfer rate constant on valinomycin is always higher (by factor of 2) than that measured under turnover conditions either polarographically or spectrophotometrically. Two possible interpretations of this discrepancy are discussed.

Oxygen activation and the conservation of energy in cell respiration

Many of the membrane-associated oxidases that catalyse respiratory reduction of O2 to water simultaneously couple this exergonic reaction to the translocation of protons across the inner mitochondrial membrane, or the cell membrane in prokaryotes, a process by which metabolic energy is conserved for subsequent synthesis of ATP. The molecular mechanism of O2 reduction and its linkage to H+ translocation are now emerging. The bimetallic haem iron-copper reaction centre in this family of enzymes is the critical structure for catalysis of both these processes.

Can ferricyanide oxidize carbon monoxide-liganded cytochrome a3?

Evidence for the oxidation of CO-liganded cytochrome a3 by ferricyanide has been published recently. These observations conflict with the long-held belief that ferricyanide is thermodynamically incapable of oxidizing the CO complex. The present paper examines the facts on which the earlier idea was based. It is concluded that the earlier evidence did not establish that ferricyanide was incompetent as an oxidant for CO-liganded cytochrome a3.

The assignment of the 655 nm spectral band of cytochrome oxidase

The spectral characteristics of the '655 nm' band of cytochrome oxidase were found to be affected by ligands of the binuclear centre, including formate and chloride, and by the resting/pulsed transition. The band titrated with near n = 1 characteristics at a midpoint of about 400 mV, in contrast to haem a3, which exhibits strong redox interaction and a titration range at significantly lower potential. Thus, although the total reduced-oxidised difference spectrum of haem a3 shows a trough at about 655 nm, this characteristic is absent in the low potential region. The 655 nm feature may arise from a charge transfer band of ferric high-spin haem a3, which is modulated by the redox state of CuB, as suggested by Beinert et al. [(1976) Biochim. Biophys. Acta 423, 339-355].

Autoxidation of oxymyoglobin: a meeting point of the stabilization and the activation of molecular oxygen

1. The primary events of haemoprotein reactions with molecular oxygen have been re-examined by placing special emphasis upon the reduction properties of dioxygen. 2. In the stepwise reduction of O2 to water via hydrogen peroxide, the addition of the first electron is an unfavourable, uphill process with the midpoint potential of -0.33 V, all the subsequent steps being downhill. This thermodynamic barrier to the first step is, therefore, a most crucial ridge located between the stabilization and the activation of dioxygen performed by haemoproteins. 3. If the proteins have a redox potential much higher than -0.33 V, molecular oxygen must bind to the proteins stably and reversibly. In Mb or Hb, however, the FeO2 centre is always subject to a nucleophilic attack of the water molecule or hydroxyl ion, which can enter the haem pocket from the surrounding solvent. These can cause irreversible oxidation of the FeO2 bonding to the ferric met-form with generation of the superoxide anion. 4. In cases of the oxygen activation, if haemoproteins have a redox potential lower than or close to -0.33 V, the first reduction of O2 to O2- would be a spontaneous process. Cytochrome P-450 provides such an example and can facilitate the subsequent addition of electrons that leads to the breaking of the O-O bond to yield the hydroxylating species. 5. As to the proteins whose redox potential is not facilitative and appreciably higher than -0.33 V, a bimetallic, concerted, two-equivalent reduction of the bound dioxygen to the peroxide level would be much more favoured without the intermediate formation of O2-. This is probably the case of cytochrome c oxidase for the reduction of O2 to water. 6. The redox potential diagrams thus visualize various aspects of the ways haemoproteins overcome their thermodynamic constraints and carry out their specific functions in the stabilization and the activation of molecular oxygen.

Electron transfer to the binuclear center in cytochrome oxidase: catalytic significance and evidence for an additional intermediate

We have followed, by transient kinetics, the reduction of cytochrome a3 in the presence of carbon monoxide under different experimental conditions. We have observed that the internal electron transfer rate accounts for the turnover number, and both display the same pH and temperature dependence [pKa = 7.4 and activation energy (Ea) = 14.7 +/- 0.1 kcal/mol]. Moreover, comparison of the time course of cytochrome c oxidation and cytochrome a3 reduction indicates that two electrons are transferred internally and with different rates to the oxygen-binding site. A kinetic model based on sequential internal electron transfer pathways, describing quantitatively the experimental data, is presented and discussed.

Characterization of two low Em forms of cytochrome a3 and their carbon monoxide complexes in mammalian cytochrome c oxidase

Evidence is presented for the existence of two forms of low-potential cytochrome a3. One appears to be similar to the low-spin form reported by Nicholls, P., and V. Hildebrandt (1978 Biochem. J. 173:65-72) and Wrigglesworth, J. M., J. Elsden, A. Chapman, N. Van der Water, and M. F. Grahn (1988. Biochim. Biophys. Acta. 936:452-464). It has a reduced Soret peak near 428 nm and a prominent alpha peak near 602 nm. This form is seen when the enzyme is either supplemented with lipoprotein or incorporated into a liposomal membrane, preexposed to a voltage greater than 400 mV for at least 30 min, and titrated in the presence of approximately 1 mM K3Fe(CN)6. The other form has a reduced Soret peak near 446 nm, and no prominent alpha peak. The 428-nm form has an Em near 175 mV and forms a CO complex with an Em near 225 mV. The 446-nm form has an Em near 200 mV and forms a CO complex with an Em near 335 mV.

The effect of pH on redox titrations of haem a in cyanide-liganded cytochrome-c oxidase: experimental and modelling studies

Isolated cytochrome-c oxidase ligated with cyanide was titrated by Flash-Induced chemical photoREduction (FIRE) (Moody, A.J. and Rich, P.R. (1988) EBEC Short Rep. 5, 69) using cytochrome c as a redox indicator. Haem a is found to titrate in a complex manner consistent with its interacting anticooperatively with at least two other components. We assign CuB as the major interactant at neutral pH, and CuA as the minor interactant. In the pH range 7.0-8.1 the strength of the interaction with CuB is found to decrease with increasing pH, while the interaction with CuA remains essentially constant. The decrease in the interaction with CuB appears to continue above pH 8.1 such that at pH 9.2 the titration curve for haem a is only slightly distorted from an 'n = 1' shape, although it is not possible from the titration data to assess the relative contributions of CuB and CuA to the total interaction observed at pH values greater than 8.1. Haem a and CuB show similar pH-dependence and, to account for this, we present a model in which the oxidoreductions of both haem a and CuB are linked to the (de)protonation of a common acid/base group. The model predicts a pH-dependent indirect cooperative interaction between haem a and CuB in addition to the direct anticooperative interaction, thereby explaining the observed pH-dependence of the redox interaction between haem a and CuB.

Electron transfer between cytochrome a and copper A in cytochrome c oxidase: a perturbed equilibrium study

Intramolecular electron transfer in partially reduced cytochrome c oxidase has been studied by the perturbed equilibrium method. We have prepared a three-electron-reduced, CO-inhibited form of the enzyme in which cytochrome a and copper A are partially reduced and in an intramolecular redox equilibrium. When these samples were irradiated with a nitrogen laser (0.6-ns, 1.0-mJ pulses) to photodissociate the bound CO, changes in absorbance at 598 and 830 nm were observed which were consistent with a fast electron transfer from cytochrome a to copper A. The absorbance changes at 598 nm gave an apparent rate of 17,000 +/- 2000 s-1 (1 sigma), at pH 7.0 and 25.5 degrees C. These changes were not observed in either the CO mixed-valence or the CO-inhibited fully reduced forms of the enzyme. The rate was fastest at about pH 8.0, falling off toward both lower and higher pHs. There was a small but clear temperature dependence. The process was also observed in the cytochrome c-cytochrome c oxidase high-affinity complex. The electron equilibration measured between cytochrome a and copper A is far faster than any rate measured or inferred previously for this process.

Routes of cytochrome a3 reduction. The neoclassical model revisited

A model for cytochrome oxidase is presented in which cytochrome a, cytochrome a3, and CuB are mutally interacting centers. Cytochrome a3, at equilibrium, is always reduced after CuB. The redox potential of cytochrome a declines progressively as the a3 CuB center is reduced. Dithionite reduction involves up to five steps: (i) reduction of cytochrome a and CuA; (ii) reduction of CuB; (iii) dissociation of ligands (exogenous and endogenous) from cytochrome a3; (iv) spin state changes (high to low) in cyt. a3 and (v) reduction of cytochrome a3. Any of (ii), (iii), and (iv) may be implicated as part of the slow step in this process, which is seen in the resting enzyme.

The location of CuA in mammalian cytochrome c oxidase

Imposition of a protonmotive force across the inner membrane of coupled cyanide-inhibited, beef heart mitochondria by addition of ATP causes reduction of cytochrome c and CuA with concomitant oxidation of haem aA. The data are consistent with previous demonstrations of an intramembrane location of haem aA but further indicate that CuA is very close to the cytosolic surface of the membrane. The implications of this finding for electron transfer route and the site of the proton pumping chemistry are discussed.

Strategy of research on the chemiosmotic mechanism of cytochrome oxidase

In recent papers on protonmotive redox mechanisms in cytochrome oxidase in [(1987) FEBS Lett. 222, 235-245] and [Glynn Biological Research Reports (1987) 3, 1-7], I have suggested that a copper centre may enable the H2O/OH or H2O/O couple to act as the hydrogen-carrying arm of a redox loop by means of a (CuOH2)+/(CuOH)+ or (CuOH2)+/(CuO)+ system at the centre. I here explain that critical comments by Malmström [(1988) FEBS Lett. 231, 268-269] on the first of these papers, which might also be levelled at the second, depend on a misunderstanding. I also respond to Malmström's comment about testing conformationally coupled proton-pump mechanisms.

A new redox loop formality involving metal-catalysed hydroxide-ion translocation. A hypothetical Cu loop mechanism for cytochrome oxidase

A new hypothetical type of redox loop is described, which translocates hydroxide instead of protons. Conventional protonmotive redox loops use carriers of protons with electrons (e.g. QH2/Q systems) to couple electron transfer to the translocation of protons. The putative hydroxidemotive redox loop uses carriers of hydroxide ions against electrons (e.g. transition-metal centres) to couple electron transfer to the translocation of hydroxide ions. This simple idea leads to the proposal of a hydroxidemotive Cu loop mechanism that may possibly be applicable to the CuA or CuB centre of cytochrome oxidase, and might thus account for the coupling of electron transfer to net proton translocation in that osmoenzyme.

The distance between cytochromes a and a3 in the azide compound of bovine-heart cytochrome oxidase

The electron-spin relaxation rates of the two species of cytochrome a3(3+)-azide found in the azide compound of bovine-heart cytochrome oxidase were measured by progressive microwave saturation at T = 10 K. It has been shown previously that Cyt a3(3+)-azide gives rise to two distinct EPR resonances, depending upon the oxidation state of Cyt a. When Cyt a is ferrous, Cyt a3(3+)-azide has g = 2.88, 2.19 and 1.64; upon oxidation of Cyt a, the a3(3+)-azide g-values become g = 2.77, 2.18, and 1.74 (Goodman, G. (1984) J Biol. Chem. 259, 15094-15099). The relaxation effect of Cyt a on Cyt a3 could be measured as the difference in microwave field saturation parameter H1/2 between the g = 2.77 and g = 2.88 species. For each signal the spin-lattice relaxation time T1 was determined from H1/2 using the transverse relaxation time T2. The value of T2 at 10 K was extrapolated from a plot of line-width vs. temperature at higher temperature. The dipolar contribution to T1 was related to the Cyt a-Cyt a3 spin-spin distance utilizing available information on the relative orientation of Cyt a3-azide and Cyt a (Erecińska, M., Wilson, D.F. and Blasie, J.K. (1979) Biochim. Biophys. Acta 545, 352-364). By taking into account the relaxation parameters for both gx and gz components of the Cyt a3-azide g-tensor, the angle between the gz components of the Cyt a and Cyt a3 g-tensors was determined to be between 0 and 18 degrees, and the Cyt a-Cyt a3 spin-spin distance was found to be 19 +/- 8 A.

Principles of coupling between electron transfer and proton translocation with special reference to proton-translocation mechanisms in cytochrome oxidase

The recent general acceptance of the proton-pumping function of cytochrome oxidase has stimulated discussion and experiment on possible underlying molecular mechanisms. Adequate experimental design requires clear understanding of the theoretical principles governing such a linked function. The increasing structural knowledge of cytochrome oxidase also contributes to a present-day requirement of more precise chemical and physical description of redox-linked proton translocation, which is the fundamental process underlying conservation of energy from aerobic metabolism in all eukaryotes and many bacteria. This essay is based on our original theoretical treatment of this problem, which is expanded here to include discussion of more recent analyses by others, classification of different types of coupling principles, as well as some concrete proposed molecular mechanisms. The latter will be analysed qualitatively, and in some cases quantitatively where this is possible, using a common theoretical framework to help comparison between models. Experimental findings relevant to this problem will be critically reviewed, and some suggestions will be made to stimulate further experiments dedicated to clarify the problem.

Complete analysis of the cytochrome components of beef heart mitochondria in terms of spectra and redox properties. Cytochromes aa3

Using newer techniques of data collection that accumulate entire spectra at a series of discrete voltages and newer techniques of analysis that utilize the additional data, we have re-examined the redox behavior and corresponding difference spectra of redox centers responsible for the alpha absorbance features of cytochromes aa3 in beef heart mitochondria. Our analysis reveals three Nernstian components with Em values of 200, 260, and 340 mV with n values of 2, 2, and 1, respectively. The maximum alpha absorbance in the difference spectra for each of these species is located at 602, 605, and 607 nm respectively. Titrations in the presence of carbon monoxide led to the identification of the lowest voltage species as cytochrome a3. The Em of the carbon monoxide-liganded species was not raised. This is contrary to the result expected when a ligand has a much stronger affinity for the reduced form of a redox couple than the oxidized form. It is, however, consistent with a proton-pumping model of cytochrome oxidase in which the binding of ligand results in the dissociation of protons.

Coulometric and potentiometric evaluation of the redox components of cytochrome c oxidase in situ

A new coulometric-potentiometric titration cuvette is described which permits accurate measurements of oxidation-reduction components in membranous systems. This cuvette has been utilized to measure the properties of cytochrome c oxidase in intact membranes of pigeon breast muscle mitochondria. The reducing equivalents accepted and donated by the portion of the respiratory chain with half-reduction potentials greater than 200 mV are equal to those required for the known components (cytochrome a3 and the high-potential copper plus cytochrome a, 'visible copper', cytochrome c1, cytochrome c, and the Rieske iron-sulfur protein). Titrations in the presence of CO show that formation of the reduced cytochrome a3-CO complex requires two reducing equivalents per cytochrome a3 (coulometric titration). Potentiometric titrations indicate (Lindsay, J.G., Owen, C.S. and Wilson, D.F. (1975) Arch. Biochem. Biophys. 169, 492--505) that both cytochromes a3 and the high-potential copper must be reduced in order to form the CO complex (n = 2.0 with a CO concentration-dependent half-reduction potential, Em). By contrast, titrations in the presence of azide show that the Em value of the high-potential copper is unchanged by the presence of azide and thus azide binds with nearly equal affinity whether the copper is reduced or oxidized.

Cytochrome c oxidase binding of hydrogen peroxide

Oxidized cytochrome c oxidase can bind hydrogen peroxide, as evidenced by changes in its spectrum and its ability to use hydrogen peroxide as an electron acceptor in cytochrome c oxidation. The affinity of the oxidized enzyme for hydrogen peroxide is high, with a Kd of less than 10 microM, and the binding is inhibited by ligands of cytochrome a3. Oxidized cytochrome c oxidase, in submitochondrial particles or solubilized in several ionic and nonionic detergents, binds peroxide with comparable affinities. The size of the spectral shift observed upon peroxide binding depends on the pH of the solution and differs in extinction coefficient between preparations, but all preparations tested appeared to bind peroxide. The differences in the magnitude of the spectral shift upon peroxide binding to different preparations suggest that oxidized cytochrome c oxidase as prepared may be made up of more than one species and that the proportion of the species which binds peroxide varies with the preparation. These studies of the binding of peroxide clarify the mechanism by which cytochrome c oxidase catalyzes the reduction of oxygen to water without the formation of free-radical intermediates.

Oxidation-reduction potentials of respiratory chain components in Thiobacillus A2

(1) Cells of Thiobacillus A2 grown chemoautotrophically on thiosulfate or heterotrophically on succinate with oxygen contained b-, c-, o-, a- and a3-type cytochromes. The amount of cytochrome per mg of cell protein was much greater in thiosulfate-grown cells and differences in the relative concentrations of cytochromes were observed for the different growth conditions. (2) The half-reduction potentials at pH 7.0 (Em,7.0) and spectral maxima of c-, b-, a- and a3-type cytochromes were similar in cells grown aerobically with thiosulfate or with succinate as the growth substrate. (3) The half-reduction potential of the 'invisible', or high-potential copper, as determined from the potentiometric behavior of the carbon monoxide-reduced cytochrome a3 complex at pH 8.0, was 365 mV. (4) Reducing equivalents from thiosulfate appear to enter the respiratory chain at the cytochrome c level; however, studies in cell-free extracts were limited due to a loss in respiratory activity with thiosulfate as a substrate upon cell disruption.

EPR studies of the photodissociation reactions of cytochrome c oxidase-nitric oxide complexes

Three complexes of NO with cytochrome c oxidase are described which are all photodissociable at low temperatures as measured by EPR. The EPR parameters of the cytochrome a2+(3)-NO complex are the same both in the fully reduced enzyme and in the mixed-valence enzyme. The kinetics of photodissociation of cytochrome a2+(3)-NO and recombination of NO with cytochrome a2+(3) (in the 30-70 K region) revealed no differences in structure between cytochrome a2+(3) in the fully reduced and the mixed-valence states. The action spectrum of the photodissociation of cytochrome a2+(3)-NO as measured by EPR has maxima at 595, 560 and 430 nm, and corresponds to the absorbance spectrum of cytochrome a2+(3)-NO. Photodissociation of cytochrome a2+(3)-NO in the mixed-valence enzyme changes the EPR intensity at g 3.03, due to electron transfer from cytochrome a2+(3) to cytochrome a3+. The extent of electron transfer was found to be temperature dependent. This suggests that a conformational change is coupled to this electron transfer. The complex of NO with oxidized cytochrome c oxidase shows a photodissociation reaction and recombination of NO (in the 20-40 K region) which differ completely from those observed in cytochrome a2+(3)-NO. The observed recombination occurs at a temperature 15 K lower than that found for the cytochrome a2+(3)-NO complex. The action spectrum of the oxidized complex shows a novel spectrum with maxima at 640 and below 400 nm; it is assigned to a Cu2+B-NO compound. The triplet species with delta ms = 2 EPR signals at g 4 and delta ms = 1 signals at g 2.69 and 1.67, that is observed in partially reduced cytochrome c oxidase treated with azide and NO, can also be photodissociated.

Energy-dependent reversal of the cytochrome oxidase reaction

Energization of isolated rat liver mitochondria with ATP under conditions in which cytochrome c is poised in a highly oxidized state shifts the state of cytochrome oxidase (cytochrome c oxidase; ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) from fully oxidized to two new spectroscopically distinguishable states depending on the applied phosphorylation potential and redox potential at cytochrome c. Both new states are spectrally similar or identical to two previously described intermediates in the reaction between reduced enzyme and O2. The data suggest that the energy-dependent transitions are due to reversed electron transfer from water to ferricytochrome c linked to accumulation of intermediates of O2 reduction at the catalytic heme a3/copper center. Titrations with redox potential indicate that each transition is a one-electron step, a finding that would identify the second observed compound as enzyme-bound peroxide or its equivalent. This is consistent with this compound being spectrally identical to "Compound C," previously described as the reaction product between half-reduced oxidase (two electrons) and O2. On the basis of these data a catalytic scheme of O2 reduction is proposed for the heme a3/copper center of cytochrome oxidase.

Midpoint potentials of cytochromes in vesicles of anaerobically-grown Paracoccus denitrificans determined by the indirect coulometric titration method

1. Multiplicity of redox components with spectral properties similar to b-type cytochromes was established in vesicles derived fro anaerobically-grown Paracoccus denitrificans. 2. Multiplicity of c-type cytochromes was not apparent either from low temperature spectroscopy or potentiometric titrations. 3. Cytochromes a + a3 and a component, only observable at liquid nitrogen temperature, with a spectral maximum at 582.5 nm were detected. 4. Redox cycling of electron transport components using the indirect coulometric titration method was a convenient means of pairing redox potentials and was reproducible in total absorbance changes, midpoint potentials and spectral maxima.