The mechanism of transmembrane delta muH+ generation in mitochondria by cytochrome c oxidase.

In rat liver mitochondria treated with rotenone, N-ethylmaleimide or oligomycin the expected alkalinization caused by proton consumption for aerobic oxidation of ferrocyanide was delayed with respect to ferrocyanide oxidation, unless carbonyl cyanide p-trifluoromethoxyphenylhydrazone was present. 2. When valinomycin or valinomycin plus antimycin were also present, ferricyanide, produced by oxidation of ferrocyanide, was re-reduced by hydrogenated endogenous reductants. Under these circumstances the expected net proton consumption caused by ferrocyanide oxidation was preceded by transient acidification. It is shown that re-reduction of formed ferricyanide and proton release derive from rotenone- and antimycin-resistant oxidation of endogenous reductants through the proton-translocating segments of the respiratory chain on the substrate side of cytochrome c. The number of protons released per electron flowing to ferricyanide varied, depending on the experimental conditions, from 3.6 to 1.5. 3. The antimycin-insensitive re-reduction of ferricyanide and proton release from mitochondria were strongly depressed by 2-n-heptyl-4-hydroxyquinoline N-oxide. This shows that the ferricyanide formed accepts electrons passing through the protonmotive segments of the respiratory chain at the level of cytochrome c and/or redox components of the cytochrome b-c1 complex situated on the oxygen side of the antimycin-inhibition site. Dibromothymoquinone depressed and duroquinol enhanced, in the presence of antimycin, the proton-release process induced by ferrocyanide respiration. Both quinones enhanced the rate of scalar proton production associated with ferrocyanide respiration, but lowered the number of protons released per electron flowing to the ferricyanide formed. 4. Net proton consumption caused by aerobic oxidation of exogenous ferrocytochrome c by antimycin-supplemented bovine heart mitochondria was preceded by scalar proton release, which was included in the stoicheiometry of 1 proton consumed per mol of ferrocytochrome c oxidized. This scalar proton production was associated with transition of cytochrome c from the reduced to the oxidized form and not to electron flow along cytochrome c oxidase. 5. It is concluded that cytochrome c oxidase only mediates vectorial electron flow from cytochrome c at the outer side to protons that enter the oxidase from the matrix side of the membrane. In addition to this consumption of protons the oxidase does not mediate vectorial proton translocation.     

A stopped-flow study of the reaction of reduced cytochrome oxidase with oxygen

The reaction of a reduced cytochrome oxidase system consisting of beef heart cytochrome oxidase, cytochrome c, and ascorbate with molecular oxygen was kinetically and thermodynamically investigated using a stopped-flow, rapid wavelength-scanning technique. Processes for oxidation of ferrocytochrome a, bound ferrocytochrome c, and free ferrocytochrome c have been identified, and their rate constants have been determined. Values of the activation energy for these reactions indicate that the oxidation of bound ferrocytochrome c is a simple chemical electron-transfer process and that oxidations of ferrocytochrome a and free ferrocytochrome c are complex processes involving changes in protein conformation.     

Characteristics of the redox-linked proton ejection in beef-heart cytochrome c oxidase reconstituted in liposomes

In this paper a study is presented of the characteristics of redox-linked proton ejection exhibited by isolated beef-heart cytochrome c oxidase incorporated in asolectin vesicles. The enzyme was 90% oriented 'right-side out' as in the mitochondrial membrane. The effects on the H+/e- stoichiometry of the modalities of activation of electron flow, the pH of the medium and its ionic composition were investigated. The results obtained show that, whilst ferrocytochrome c pulses of the aerobic oxidase vesicles at neutral pH and in the presence of saturating concentrations of valinomycin and K+ to ensure charge compensation produced H+/e- ratios around 1 (as has been shown previously), oxygen pulses of reduced anaerobic vesicles supplemented with cytochrome c, gave H+/e- ratios around 0.3. The H+/e- ratios exhibited, with both reductant and oxidant pulses, a marked pH dependence. Maximum values were observed at pH 7.0-7.7, which decreased to negligible values at acidic pH with apparent pKa of 6.7-6.3. Mg2+ and Ca2+ caused a marked depression of the H+/e- ratio, which in the presence of these cations and after a few ferrocytochrome pulses, became negligible. Analysis of cytochrome c oxidation showed that the modalities of activation of electron flow and divalent cations exerted profound effects on the kinetics of cytochrome c oxidation by oxidase vesicles. The observations presented seem to provide interesting clues for the nature and mechanism of redox-linked proton ejection in reconstituted cytochrome c oxidase.     

Proton translocation by a native and subunit III-depleted cytochrome c oxidase reconstituted into phospholipid vesicles. Use of fluorescein-phosphatidylethanolamine as an intravesicular pH indicator

The existence of a proton pump associated with bovine cytochrome c oxidase (EC 1.9.3.1) has over the last few years been a matter of considerable dispute. In an attempt to resolve some of the problems with the measuring system we have synthesized fluorescein-phosphatidylethanolamine which when reconstituted with cytochrome c oxidase into phospholipid vesicles provided a reliable indicator of the intravesicular pH. It was observed that cytochrome c oxidase catalyzed the abstraction of almost 2 protons from the intravesicular medium/molecule of ferrocytochrome c oxidized. In parallel experiments whereby the extravesicular pH was measured with an electrode it was found that the enzyme appeared to be responsible for the appearance of almost 1.0 proton/molecule of ferrocytochrome c oxidized. Taken together these data unequivocally demonstrate that cytochrome c oxidase behaves as a proton pump. Furthermore, the other proton which was abstracted is believed to be used for the process of the reduction of oxygen. Similar experiments were performed with a cytochrome c oxidase preparation which was devoid of subunit III. Under these circumstances the enzyme appeared to be unable to translocate protons across the vesicular membrane but was competent to abstract protons from the intravesicular medium for the reduction of oxygen.     

Stopped-flow studies of cytochrome oxidase reconstituted into liposomes: proton pumping and control of activity

The transient kinetics of proton pumping and the electron transfer properties of cytochrome oxidase inserted into small unilamellar vesicles have been investigated by stopped-flow spectrophotometry. In the presence of valinomycin, proton pumping and cytochrome c oxidation by cytochrome oxidase are synchronous up to rate constants of approximately 9 sec-1. Moreover, the enzyme depleted of subunit III ("three-less oxidase") was also shown to pump protons, although with a significantly smaller stoichiometry. Thus, subunit III is not the only (or even the main) proton channel, although it may be involved in the regulation of activity. The kinetics of cytochrome c oxidation by COV in the absence and in the presence of ionophores have been investigated. Analysis of the time course of the process in the transient and steady state phases indicates that the onset of control by the electrochemical gradient follows the transfer of four electrons, i.e., one complete turnover of the oxidase. Two possible alternative interpretations for the control of the turnover phase are presented and discussed.     

Selective removal of subunit VIb increases the activity of cytochrome c oxidase.

Bovine heart cytochrome c oxidase was gel-filtered on Sephacryl S-300 in 0.05% dodecyl maltoside and in the presence or absence of 1 M KCl. The presence of KCl selectively removed subunit VIb from the enzyme complex, resulting in about doubling of enzymatic activity and an increase of the Km for ferrocytochrome c. In contrast, the proton pumping activity of the enzyme was unchanged. The increase of activity is due to removal of subunit VIb and not of lipids, because titration with asolectin or dodecyl maltoside could not abolish the difference in activity between the 12- and 13-subunit enzyme. Attempts to reconstitute cytochrome c oxidase from its separated components were unsuccessful. It is concluded that subunit VIb suppresses the activity of the mammalian enzyme complex by interaction with the active center.     

The oxidation of exogenous cytochrome c by mitochondria. Resolution of a long-standing controversy

Several reports in the past have dealt with the oxidation of cytochrome c added to suspensions of rat liver mitochondria. Yet, it is generally believed that the cytochrome cannot penetrate the outer membrane. Probably it has been assumed that the permeability of the outer membrane to cytochrome c is very low but finite, and that fast oxidation may be observed if time is allowed for sufficient penetration before initiation of electron flow. Here we show that this view is false. The main fraction of rat liver mitochondria, as isolated by conventional procedures, does not catalyse any significant oxidation of added cytochrome c, even after prolonged incubation. The observed appreciable oxidation of added cytochrome c is catalysed by a very small fraction (5-12%) of the mitochondria that apparently has a damaged outer membrane. Consequently, the turnover of cytochrome oxidase is very high in this fraction during oxidation of added cytochrome c. This finding readily explains why Moyle and Mitchell (e.g., FEBS Lett. 88 (1978) 268-272; 90 (1978) 361-365) have failed to observe proton translocation by cytochrome oxidase during oxidation of ferrocytochrome c added to rat liver mitochondria, which has been their main reason for rejecting the proton-pumping function of cytochrome oxidase.     

Concerted involvement of cooperative proton-electron linkage and water production in the proton pump of cytochrome c oxidase

In cytochrome c oxidase, oxido-reductions of heme a/Cu(A) and heme a3/Cu(B) are cooperatively linked to proton transfer at acid/base groups in the enzyme. H+/e- cooperative linkage at Fe(a3)/Cu(B) is envisaged to be involved in proton pump mechanisms confined to the binuclear center. Models have also been proposed which involve a role in proton pumping of cooperative H+/e- linkage at heme a (and Cu(A)). Observations will be presented on: (i) proton consumption in the reduction of molecular oxygen to H2O in soluble bovine heart cytochrome c oxidase; (ii) proton release/uptake associated with anaerobic oxidation/reduction of heme a/Cu(A) and heme a3/Cu(B) in the soluble oxidase; (iii) H+ release in the external phase (i.e. H+ pumping) associated with the oxidative (R-->O transition), reductive (O-->R transition) and a full catalytic cycle (R-->O-->R transition) of membrane-reconstituted cytochrome c oxidase. A model is presented in which cooperative H+/e- linkage at heme a/Cu(A) and heme a3/Cu(B) with acid/base clusters, C1 and C2 respectively, and protonmotive steps of the reduction of O2 to water are involved in proton pumping.     

Calmodulin stimulation of adenylate cyclase of intestinal epithelium

The effect of dicyclohexylcarbodiimide (DCCD) on the proton pumping two-subunit cytochrome c oxidase from Paracoccus denitrificans was investigated. Purified Paracoccus oxidase was reconstituted into phospholipid vesicles by cholate dialysis. Following incubation with increasing amounts of DCCD, proton ejection was recorded in response to reductant pulses with reduced cytochrome c. Concentrations of DCCD which greatly reduced proton pumping by bovine cytochrome c oxidase used as a control were found to exert only a minor effect on proton translocation by Paracoccus oxidase. Similarly, incubation of the bacterial enzyme with [14C]DCCD failed to reveal the specific covalent interaction previously demonstrated to occur with bovine cytochrome c oxidase, and here also shown for the oxidase of yeast. Thus, Paracoccus oxidase differs in its interaction with DCCD from the functionally analogous eukaryotic enzymes.     

Alternative hypotheses of proton ejection in cytochrome oxidase vesicles. Transmembrane proton pumping or redox-linked deprotonation of phospholipid-cytochrome c complex(es)

A review of published experimental and interpretative knowledge concerning proton ejection associated with cytochrome c oxidation by artificial phospholipid vesicles inlaid with cytochrome c oxidase indicates that the detailed characteristics of the redox-linked proton ejection cannot be simply explained by proton pumping. We propose an alternative hypothesis according to which proton ejection is due to the redox-linked deprotonation of a complex involving phospholipid and cytochrome c at the surface of the vesicles. The postulates upon which this hypothesis depends are explicitly outlined, and some methods of testing the hypothesis are suggested.     

High vectorial proton stoichiometry by cytochrome c oxidase from the thermophilic bacterium PS3 reconstituted in liposomes

The stoichiometry of vectorial H+ ejection, coupled to ferrocytochrome c oxidation by a three-subunit bacterial cytochrome c oxidase (EC 1.9.3.1) from the thermophilic bacterium PS3, was measured. Three methods of measuring the H+/e- ratio were applied to proteoliposomes containing a relatively small amount of PS3 cytochrome oxidase, which showed a relatively low oxidation rate and a very low H+ leakage, as follows: (a) simultaneous measurements of H+ ejection and cytochrome c oxidation upon addition of a yeast ferrocytochrome c pulse, which enable us to calculate the H+/e- ratio as H+ ejected per cytochrome c oxidized; (b) computer simulations to find out the fit for the pH meter trace by changing the H+/e- ratio and the velocity constant of leakage; and (c) two successive measurements of initial rates of H+ movement in the absence and presence of carbonyl cyanide p-trifluoromethoxyphenylhydrazone. The H+/e- ratios obtained were 1.39, the 10-s value after ferrocytochrome c addition in (a), 1.35 in (b), and 1.33 in (c). This high H+/e- stoichiometry observed, exceeding 1 and as high as 1.4, is discussed with respect to the controversy of the H+/e- ratio at the cytochrome oxidase site.     

Contribution of peroxidized cardiolipin to inactivation of bovine heart cytochrome c oxidase

The lipid-soluble peroxides, tert-butyl hydroperoxide and peroxidized cardiolipin, each react with bovine cytochrome c oxidase and cause a loss of electron-transport activity. Coinciding with loss of activity is oxidation of Trp19 and Trp48 within subunits VIIc and IV, and partial dissociation of subunits VIa and VIIa. tert-Butyl hydroperoxide initiates these structural and functional changes of cytochrome c oxidase by three mechanisms: (1) radical generation at the binuclear center; (2) direct oxidation of Trp19 and Trp48; and (3) peroxidation of bound cardiolipin. All three mechanisms contribute to inactivation since blocking a single mechanism only partially prevents oxidative damage. The first mechanism is similar to that described for hydrogen peroxide [Biochemistry43:1003-1009; 2004], while the second and third mechanism are unique to organic hydroperoxides. Peroxidized cardiolipin inactivates cytochrome c oxidase in the absence of tert-butyl hydroperoxide and oxidizes the same tryptophans within the nuclear-encoded subunits. Peroxidized cardiolipin also inactivates cardiolipin-free cytochrome c oxidase rather than restoring full activity. Cardiolipin-free cytochrome c oxidase, although it does not contain cardiolipin, is still inactivated by tert-butyl hydroperoxide, indicating that the other oxidation products contribute to the inactivation of cytochrome c oxidase. We conclude that both peroxidized cardiolipin and tert-butyl hydroperoxide react with and triggers a cascade of structural alterations within cytochrome c oxidase. The summation of these events leads to cytochrome c oxidase inactivation.     

The mechanism of energy conservation and transduction by mitochondrial cytochrome c oxidase.

Oxidation of ferrocytochrome c by molecular oxygen catalysed by cytochrome c oxidase (cytochrome aa3) is coupled to translocation of H+ ions across the mitochondrial membrane. The proton pump is an intrinsic property of the cytochrome c oxidase complex as revealed by studies with phospholipid vesicles inlayed with the purified enzyme. As the conformation of cytochrome aa3 is specifically sensitive to the electrochemical proton gradient across the mitochondrial membrane, it is likely that redox energy is primarily conserved as a conformational "strain" in the cytochrome aa3 complex, followed by relaxation linked to proton translocation. Similar principles of energy conservation and transduction may apply on other respiratory chain complexes and on mitochondrial ATP synthase.     

A cooperative model for proton pumping in cytochrome c oxidase

In this paper, the mechanism of proton pumping in cytochrome c oxidase is examined. Data on cooperative linkage of vectorial proton translocation to oxido-reduction of Cu(A) and heme a in the CO-inhibited, liposome-reconstituted bovine cytochrome c oxidase are reviewed. Results on proton translocation associated to single-turnover oxido-reduction of the four metal centers in the unliganded, membrane-reconstituted oxidase are also presented. On the basis of these results, X-ray crystallographic structures and spectrometric data for a proton pumping model in cytochrome c oxidase is proposed. This model, which is specifically derived from data available for the bovine cytochrome c oxidase, is intended to illustrate the essential features of cooperative coupling of proton translocation at the low potential redox site. Variants will have to be introduced for those members of the heme copper oxidase family which differ in the redox components of the low potential site and in the amino acid network connected to this site. The model we present describes in detail steps of cooperative coupling of proton pumping at the low potential Cu(A)-heme a site in the bovine enzyme. It is then outlined how this cooperative proton transfer can be thermodynamically and kinetically coupled to the chemistry of oxygen reduction to water at the high potential Cu(B)-heme a(3) center, so as to result in proton pumping, in the turning-over enzyme, against a transmembrane electrochemical proton gradient of some 250 mV.     

Impaired proton pumping in cytochrome c oxidase upon structural alteration of the D pathway

Cytochrome c oxidase is a membrane-bound enzyme, which catalyses the one-electron oxidation of four molecules of cytochrome c and the four-electron reduction of O(2) to water. Electron transfer through the enzyme is coupled to proton pumping across the membrane. Protons that are pumped as well as those that are used for O(2) reduction are transferred though a specific intraprotein (D) pathway. Results from earlier studies have shown that replacement of residue Asn139 by an Asp, at the beginning of the D pathway, results in blocking proton pumping without slowing uptake of substrate protons used for O(2) reduction. Furthermore, introduction of the acidic residue results in an increase of the apparent pK(a) of E286, an internal proton donor to the catalytic site, from 9.4 to ~11. In this study we have investigated intramolecular electron and proton transfer in a mutant cytochrome c oxidase in which a neutral residue, Thr, was introduced at the 139 site. The mutation results in uncoupling of proton pumping from O(2) reduction, but a decrease in the apparent pK(a) of E286 from 9.4 to 7.6. The data provide insights into the mechanism by which cytochrome c oxidase pumps protons and the structural elements involved in this process.     

Proton-pumping mechanism of cytochrome c oxidase: a kinetic master-equation approach

Cytochrome c oxidase is an efficient energy transducer that reduces oxygen to water and converts the released chemical energy into an electrochemical membrane potential. As a true proton pump, cytochrome c oxidase translocates protons across the membrane against this potential. Based on a wealth of experiments and calculations, an increasingly detailed picture of the reaction intermediates in the redox cycle has emerged. However, the fundamental mechanism of proton pumping coupled to redox chemistry remains largely unresolved. Here we examine and extend a kinetic master-equation approach to gain insight into redox-coupled proton pumping in cytochrome c oxidase. Basic principles of the cytochrome c oxidase proton pump emerge from an analysis of the simplest kinetic models that retain essential elements of the experimentally determined structure, energetics, and kinetics, and that satisfy fundamental physical principles. The master-equation models allow us to address the question of how pumping can be achieved in a system in which all reaction steps are reversible. Whereas proton pumping does not require the direct modulation of microscopic reaction barriers, such kinetic gating greatly increases the pumping efficiency. Further efficiency gains can be achieved by partially decoupling the proton uptake pathway from the active-site region. Such a mechanism is consistent with the proposed Glu valve, in which the side chain of a key glutamic acid shuttles between the D channel and the active-site region. We also show that the models predict only small proton leaks even in the absence of turnover. The design principles identified here for cytochrome c oxidase provide a blueprint for novel biology-inspired fuel cells, and the master-equation formulation should prove useful also for other molecular machines. .     

Understanding the cytochrome c oxidase proton pump: thermodynamics of redox linkage.

The cytochrome c oxidase complex (CcO) catalyzes the four-electron reduction of dioxygen to water by using electrons from ferrocytochrome c. Redox free energy released in this highly exergonic process is utilized to drive the translocation of protons across a transmembrane electrochemical gradient. Although numerous chemical models of proton pumping have been developed, few attempts have been made to explain the stepwise transfer of energy in the context of proposed protein conformational changes. A model is described that seeks to clarify the thermodynamics of the proton pumping function of CcO and that illustrates the importance of electron and proton gating to prevent the occurrence of the more exergonic electron leak and proton slip reactions. The redox energy of the CcO-membrane system is formulated in terms of a multidimensional energy surface projected into two dimensions, a nuclear coordinate associated with electron transfer and a nuclear coordinate associated with elements of the proton pump. This model provides an understanding of how a transmembrane electrochemical gradient affects the efficiency of the proton pumping process. Specifically, electron leak and proton slip reactions become kinetically viable as a result of the greater energy barriers that develop for the desired reactions in the presence of a transmembrane potential.     

Electron transfer between soluble and immobilized mammalian cytochrome c. Equilibrium and kinetic studies on immobilized cytochrome c.

Horse heart cytochrome c was covalently bound to Sepharose 4B and its redox properties were measured under various experimental conditions. The equilibrium constant for the electron exchange between the oxidized and the reduced form of cytochrome c when one of the two forms was in the semi-solid state and the other one in solution was close to 1. Matrix-bound ferrocytochrome c is very stable to autoxidation and is not oxidized by O2 even in the presence of mammalian cytochrome oxidase. Oxidation occurs if catalytic amounts of soluble cytochrome c are added to the reaction mixture. The rate of oxidation of matrix-bound ferrocytochrome c in the presence of cytochrome oxidase and catalytic amounts of soluble cytochrome c may be correlated with the rate of electron transfer between soluble and matrix-bound cytochrome c. This rate is more than two orders of magnitude lower than that reported for the homonuclear (between identical species) electron transfer in solution.     

Bound cytochrome C as proton donor and acceptor during enzymic oxidoreduction

When ferrocytochrome $\text{c}$ reacts with delipidated cytochrome oxidase under conditions which prevent oxidation, one proton is taken up per molecule of ferrocytochrome $\text{c}$ bound to cytochrome oxidase. When ferricytochrome $\text{c}$ reacts with delipidated Complex III, one proton is released per molecule of ferricytochrome $\text{c}$ bound to Complex III. From these data it can be concluded that the oxidation of ferrocytochrome $\text{c}$ by cytochrome oxidase leads to the release of a proton and an electron, whereas the reduction of ferricytochrome $\text{c}$ by Complex III leads to the uptake of a proton and an electron. Thus ferrocytochrome $\text{c}$ like QH₂ and NADH is both an electron and proton donor, and ferricytochrome $\text{c}$ like Q and O₂ is both an electron and proton acceptor. The pattern for the three mitochondrial electron transfer sequences NADH → Q, QH₂ → ferricytochrome $\text{c}$ and ferrocytochrome $\text{c}$ → O₂ involves separation of an electron and proton on the side of the membrane where electron transfer is initiated and recombination of an electron and a proton in the terminal acceptor on the side of the membrane where electron transfer terminates.     

Elementary steps of proton translocation in the catalytic cycle of cytochrome oxidase

Proton translocation in the catalytic cycle of cytochrome c oxidase (CcO) proceeds sequentially in a four-stroke manner. Every electron donated by cytochrome c drives the enzyme from one of four relatively stable intermediates to another, and each of these transitions is coupled to proton translocation across the membrane, and to uptake of another proton for production of water in the catalytic site. Using cytochrome c oxidase from Paracoccus denitrificans we have studied the kinetics of electron transfer and electric potential generation during several such transitions, two of which are reported here. The extent of electric potential generation during initial electron equilibration between CuA and heme a confirms that this reaction is not kinetically linked to vectorial proton transfer, whereas oxidation of heme a is kinetically coupled to the main proton translocation events during functioning of the proton pump. We find that the rates and amplitudes in multiphase heme a oxidation are different in the OH-->EH and PM-->F steps of the catalytic cycle, and that this is reflected in the kinetics of electric potential generation. We discuss this difference in terms of different driving forces and relate our results, and data from the literature, to proposed mechanisms of proton pumping in cytochrome c oxidase.