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

Mechanistic and phenomenological features of proton pumps in the respiratory chain of mitochondria

Various direct, indirect (kinetic and thermodynamic), and combined mechanisms have been proposed to explain the conversion of redox energy into a transmembrane protonmotive force (delta p) by enzymatic complexes of respiratory chains. The conceptual evolution of these models is examined. The characteristics of thermodynamic coupling between redox transitions of electron carriers and scalar proton transfer in cytochrome c oxidase and its possible involvement in proton pumping is discussed. Other aspects dealt with in this paper are: (i) variability of <--H+/e- stoichiometries, in cytochrome c oxidase and cytochrome c reductase and its mechanistic implications; (ii) possible models by which the reduction of dioxygen to water at the binuclear heme-copper center of protonmotive oxidases can be directly involved in proton pumping. Finally a unifying concept for proton pumping by the redox complexes of respiratory chain is presented.

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.

Association of ferri- and ferro-cytochrome c with lipid multilayers: a 31P solid-state NMR study

The 31P nuclear magnetic resonance anisotropies of dispersions of diacylphosphatidic acid and diacylphosphatidylserine were slightly increased in the presence of cytochrome c: no increase was observed with cardiolipin. However, the 31P spin-lattice relaxation times (T1) for all of these lipids were reduced markedly by the protein. As similar effects were observed with ferri-cytochrome c and with the reduced protein, which is diamagnetic, we suggest that the changes in T1 reflect a reduction in the spectral density of fast motions for the lipid headgroups attendant on binding of protein, rather than paramagnetic relaxation of the phosphorus nuclear spin.

Measurements of the proton motive force generated by cytochrome c oxidase from Bacillus subtilis in proteoliposomes and membrane vesicles

Cytochrome c oxidase from Bacillus subtilis was reconstituted in liposomes and its energy-transducing properties were studied. The reconstitution procedure used included Ca2+-induced fusion of pre-formed membranes. The orientation of the enzyme in liposomes is influenced by the phospholipid composition of the membrane. Negatively charged phospholipids are essential for high oxidase activity and respiratory control. Analyses of the proteoliposomes by gel filtration, density gradient centrifugation and electron microscopy indicated a heterogeneity of the proteoliposomes with respect to size and respiratory control. Cytochrome c oxidase activity in the proteoliposomes resulted in the generation of a proton motive force, internally negative and alkaline. In the presence of the electron donor, ascorbate/N,N,N',N'-tetramethyl-p-phenylenediamine/cytochrome c or ascorbate/phenazine methosulphate, the reconstituted enzyme generated an electrical potential of 84 mV which was increased by the addition of nigericin to 95 mV and a pH gradient of 32 mV which was increased by the addition of valinomycin to 39 mV. Similar results were obtained with beef-heart cytochrome c oxidase reconstituted in liposomes. The maximal proton motive force which could be generated, assuming no endogenous ion leakage, varied over 110-140 mV. From this the efficiency of energy transduction by cytochrome c oxidase was calculated to be 18-23%, indicating that the oxidase is an efficient proton-motive-force-generating system.

Structure and function of cytochrome-c oxidase

Recent works on the structure and the function of cytochrome-c oxidase are reviewed. The subunit composition of the mitochondrial enzyme depends on the species and is comprised of between 5 and 13 subunits. It is reduced to 1 to 3 subunits in prokaryotes. The complete amino acid composition has been derived from protein sequencing. Gene sequences are partially known in several eukaryote species. Metal centers are only located in subunits I and II. The mitochondrial cytochrome-c oxidase is Y-shaped; the arms of the Y cross the inner membrane, the stalk protrudes into the intermembrane space. The bacterial enzyme has a simpler, elongated shape. A number of data have been accumulated on the subunit topology and on their location within the protein. All available spectrometric techniques have been used to investigate the environment of the metal centers as well as their interactions. From the literature, attention must be paid to what may be considered or not as an active form. The steady improvement of the instrumentation has yielded evidence for different kinds of heterogeneities which could reflect the in vivo situation. The 'pulsed' and 'resting' conformers have been well characterized. The 'oxygenated' form has been identified as a peroxide derivative of the fully oxidized cytochrome-c oxidase. The mammalian enzyme has been isolated in fully active monomeric form which does not preclude the initially suggested dimeric behavior in situ. The role of the lipids is still largely investigated, mainly through reconstitution experiments. Kinetic studies of electron transfer between cytochrome c and cytochrome-c oxidase lead to a single catalytic site model to account for the multiphasic kinetics. Results related to the low temperature investigation of the intermediate steps in the reaction between oxygen and cytochrome-c oxidase received a sound confirmation by the resolution of compound A at room temperature. It is also pointed out that the so-called mixed valence state might not be a transient state in the catalytic reduction of oxygen. The functioning of cytochrome-c oxidase as a proton pump has been supported by a number of experimental results. Subunit III would be involved in this process. The redox link to the proton pump has been suggested to be at the Fea-CuA site. The molecular mechanism responsible for the proton pumping is still unknown.

Integrated functioning of the chloroplast coupling factor

This review is focused on some functional characteristics of the chloroplast coupling factor. The structure of the enzyme and the putative role of its subunits are recalled. An attempt is made to discriminate the driving force and the activator effects of the electrochemical proton gradient. Respective roles of delta pH, delta phi, external and internal pH are discussed with regard to mechanistic implications. The hypothesis of a functional switch of the enzyme between two states with better efficiency either in ATP synthesis or in ATP hydrolysis is also examined. A brief survey is made on some problems complicating quantitative studies of energy coupling, such as localized chemiosmosis, delta pH and delta phi computations, and scalar ATPases. The main data on the enzyme activation and the energy-dependent release of tightly bound nucleotides are summarized. The arguments for and against the catalytic competence of theses nucleotides are reviewed. Lastly, some prevailing models of the catalytic mechanism are presented. The relevance of nucleotides binding change events in this process is discussed.

Membrane potentials in reconstituted cytochrome c oxidase proteoliposomes determined by butyltriphenyl phosphonium cation distribution

Equilibration of the butyltriphenyl phosphonium (BTPP+) cation into cytochrome c oxidase reconstituted proteoliposomes was measured potentiometrically. The maximum membrane potential (delta psi) generated by oxidase activity was estimated to lie between -65 and -90 mV, vesicle interior negative, when internal BTPP+ binding is taken into account. Formation of delta psi was completely prevented by valinomycin and carbonyl-cyanide-p-trifluoromethoxyphenylhydrazone but only 10% inhibited by levels of N',N'-dicyclohexylcarbodiimide that abolish proton pumping by the oxidase. delta psi is thus maintained by at least one charge transfer process that does not involve proton movement. A nonlinear relationship was obtained between oxidase activity and steady-state delta psi. The value of delta psi estimated by BTPP+ distribution was lower than that calculated using the optical probes safranine and a carbocyanine dye. Possible reasons for this discrepancy are discussed.

Kinetics of redox-linked proton pumping activity of native and subunit III-depleted cytochrome c oxidase: a stopped-flow investigation

The kinetics of oxidation of reduced cytochrome c by cytochrome c oxidase reconstituted into unilamellar vesicles (COV) has been followed by stopped-flow method in the time range 3 msec-1 sec. In the presence of valinomycin, the oxidation of cytochrome c is linked to proton ejection in the external medium, with an apparent stoichiometry (H+/e-) of 0.93 +/- 0.22, under conditions in which the enzyme is in the more active "pulsed" state (i.e., having undergone oxidation-reduction cycles). The time course of reaction indicates that the conformational change(s) involved in coupling the redox reaction to proton translocation is fast. Similar experiments carried out with cytochrome c oxidase depleted of subunit III show that proton-pumping is maintained, although with a lower efficiency (H+/e- = 0.5). The number of protons ejected per electron appears to be correlated to the value of the respiratory control ratio; although this result is partly due to an increase in the rate of diffusion back into the vesicles, a relationship between the respiratory control ratio and the efficiency of the proton pump may be inferred, suggesting a control of the H+/e-ratio.

Effects of detergents and cytochrome c binding on scalar and vectorial proton ejection by proteoliposomes containing cytochrome oxidase

The detergent lauryl maltoside abolishes respiratory control and proton ejection by cytochrome c oxidase-containing proteoliposomes over a narrow concentration range. Expression of cryptic activity (inward-facing oxidase) is released over the same concentration range. Catalytic functions (Vmax. and Km) of the enzyme are not changed by the detergent. Lipid micelles containing detergent bind approximately the same amount of cytochrome c as do vesicles containing an equivalent amount of lipid. Uncoupler-insensitive proton release is seen when proteoliposomes are pulsed with ferrocytochrome c at low ionic strength. Such uncoupler-insensitive acidification is not seen at higher ionic strength, nor with oxygen pulses of anaerobic solutions previously incubated with cytochrome c. Vesicles at low ionic strength catalyse cytochrome c autoxidation; this process can mimic proton re-equilibration in systems that have pumped protons from inside to the bulk phase. Proton re-equilibration following a pulse of cytochrome c or oxygen is multiphasic. The slowest phases are attributed to vesicle heterogeneity, some internal alkali being retained within vesicles of low intrinsic proton permeability. This can be overcome by the addition of either very low levels of carbonyl cyanide p-trifluoromethoxyphenyl-hydrazone or high levels of valinomycin.

New insights on the cytochrome c oxidase proton pump

Cytochrome c oxidase vesicles were used to show that, under appropriate experimental conditions: (1) no net deprotonation of the vesicular membrane or of the incorporated enzyme occurs during the oxidation of ferrocytochrome c; (2) the pH equilibration kinetics of a respiration-induced pH gradient across the bilayer are a simple function of the ohmic proton-conductance properties of the membrane; (3) a fairly constant stoichiometry (0.8-0.7) of the numbers of protons pumped per molecule of ferrocytochrome c oxidized, i.e. the H+/e- ratio, over a wide range of dioxygen molecules reduced (1-12) is observed.

Characteristics and nature of redox-linked proton transfer reactions in cytochrome c oxidase of mitochondria

A discussion is presented of the characteristics of proton transfer reactions associated to redox catalysis in mitochondrial cytochrome c oxidase. These properties are examined in the light of the mechanisms proposed for the conversion of redox energy into a transmembrane proton gradient. It is concluded that this energy transfer process is first of all due to the anisotropic arrangement of the reduction of oxygen to H2O in the oxidase. The experimental observations available seem, on the other hand, to raise doubts on the capacity of cytochrome oxidase to function under steady-state conditions, in the native membrane, as a proton pump.

Effect of membrane potential and pH gradient on electron transfer in cytochrome oxidase

Steady-state spectra of cytochrome oxidase in phospholipid vesicles were obtained by using hexaammineruthenium(II) and ascorbate as reductants. Cytochrome a was up to 80% reduced in the steady state in coupled vesicles. Upon addition of nigericin or acetate, which decrease delta pH, resulting in an increase in delta psi, cytochrome a became more oxidized in the steady state with no change in the rate of respiration. On the other hand, uncouplers or valinomycin plus nigericin, which lower both delta psi and delta pH, stimulated respiration 2-8-fold and also lowered the steady-state level of reduction of cytochrome a. These experiments indicate that electron transfer between cytochromes a and a 3 is sensitive primarily to the pH gradient. Studies with the reconstituted and the soluble enzyme at various pH values indicated that the pH on the matrix side of the membrane, rather than delta pH, controlled the steady-state level of reduced cytochrome a. Hexaammineruthenium(II) substituted for cytochrome c in measurements of proton pumping by cytochrome oxidase. Dicyclohexylcarbodiimide, which eliminated proton pumping by cytochrome oxidase, decreased the effect of ionophores on the steady-state level of reduced cytochrome a.

A quantitative characterisation of H+ translocation by cytochrome c oxidase vesicles

A quantitative analysis of H+ extrusion by reconstituted cytochrome c oxidase vesicles is presented with particular regard to the decay kinetics of the extruded proton pulse and to the structural heterogeneity of the vesicle preparation. The decay of the extruded H+ pulse under conditions typical of those used for its measurement is much slower than expected from the passive proton permeability of the vesicle membranes. It is shown that this apparent anomaly results from insufficient transmembrane charge equilibration via valinomycin and K+ during oxidase turnover. This situation can be remedied by increasing the valinomycin concentration or by replacing this counterion system with 1 mM tetraphenylphosphonium. Under these latter conditions, the decay kinetics can be described as the sum of two exponential terms. To facilitate interpretation of the proton pump decay kinetics, a structural analysis of the oxidase vesicle preparation is presented. The bulk of the reconstituted vesicles (i.e., those representing approx. 80% of the total oxidase and lipid) are 30-62 nm in diameter. At least 70% of the reconstituted oxidase molecules are contained individually in separate vesicles, indicating that the enzyme monomer is competent in H+ translocation.

Interactions in cytochrome oxidase: functions and structure

Mitochondrial cytochrome c oxidase is an exceedingly complex multistructural and multifunctional membranous enzyme. In this review, we will provide an overview of the many interactions of cytochrome oxidase, stressing developments not covered by the excellent monograph of Wikström, Krab, and Saraste (1981), and continuing into early 1983. First we describe its functions (both in the nominal sense, as a transporter of electrons between cytochrome c and oxygen, and in its role in energy transduction). Then we describe its structure, emphasizing the protein (its structure as a whole, the number and stoichiometry of its subunits, their biosynthetic origin, and their interactions with each other, with other components of the enzyme complex, and with the membrane as a whole). Finally, we present a model in which the protein conformation serves as the focus for the dynamic interaction of its two major functions.

Na+/H+ antiporters

Na+/H+ antiports or exchange reactions have been found widely, if not ubiquitously, in prokaryotic and eukaryotic membranes. In any given experimental system, the multiplicity of ion conductance pathways and the absence of specific inhibitors complicate efforts to establish that the antiport observed actually results from the activity of a specific secondary porter which catalyzes coupled exchanged of the two ions. Nevertheless, a large body of evidence suggests that at least some prokaryotes possess a delta psi-dependent, mutable Na+/H+ antiporter which catalyzes Na+ extrusion in exchange for H+; in other bacterial species, the antiporter my function electroneutrally, at least at some external pH values. The bacterial Na+/H+ antiporter constitutes a critical limb of Na+ circulation, functioning to maintain a delta mu Na+ for use by Na+-coupled bioenergetic processes. The prokaryotic antiporter is also involved in pH homeostasis in the alkaline pH range. Studies of mutant strains that are deficient in Na+/H+ antiporter activity also indicate the existence of a relationship, e.g., a common subunit or regulatory factor, between the Na+/H+ antiporter and Na+/solute symporters in several bacterial species. In eukaryotes, an electroneutral, amiloride-sensitive Na+/H+ antiport has been found in a wide variety of cell and tissue types. Generally, the normal direction of the antiport appears to be that of Na+ uptake and H+ extrusion. The activity is thus implicated as part of a complex system for Na+ circulation, e.g., in transepithelial transport, and might have some role in acidification in the renal proximal tubule. In many experimental systems, the Na+/H+ antiport appears to influence intracellular pH. In addition to a role in general pH homeostasis, such Na+-dependent changes in intracellular pH could be part of the early events in a variety of differentiating and proliferative systems. Reconstitution and structural studies, as well as detailed analysis of gene loci and products which affect the antiport activity, are in their very early stages. These studies will be important in further clarification of the precise structural nature and role(s) of the Na+/H+ antiporters. In neither prokaryotes nor eukaryotes systems is there yet incontrovertible evidence that a specific protein carrier, that catalyzes Na+/H+ antiport, is actually responsible for any of the multitude of effects attributed to such antiporters. The Na+-H+ exchange might turn out to be side reactions of other porters or the additive effects of several conductance pathways; or, as appears most likely in at least some bacteria and in renal tissue, the antiporter may be a discrete, complex carr

Redox-linked proton translocation in the b-c1 complex from beef-heart mitochondria reconstituted into phospholipid vesicles. General characteristics and control of electron flow by delta micro H+

A study is presented of the characteristics of redox-linked proton translocation in the b-c1 complex isolated from beef-heart mitochondria and reconstituted into phospholipid vesicles. Measurements of the H+/e- stoichiometry, with three different methods, show that four protons are released from the vesicles per 2e- flowing from quinols to cytochrome c, two of these protons formally deriving from scalar oxidation of quinols by cytochrome c. This H+/e- stoicheiometry is independent of the initial redox state of the b-c1 complex (fully reduced or oxidized) and the rate of electron flow through the complex. It does not change in the pH range 6.0 - 7.2, but declines to 1.5 going with pH from 7.2 - 8.3. This decrease is accompanied by enhancement of the rate of electron flow in the coupled state. Collapse of delta psi effected by valinomycin addition to turning-over b-c1 vesicles resulted in substantial oxidation of cytochrome b-566 and comparable reduction of cytochrome c1, with little oxidation of cytochrome b-562. Nigericin alone had no effect on the steady-state redox levels of b and c cytochromes. Its addition in the presence of valinomycin caused oxidation of b cytochromes but no change in the redox state of cytochrome c1. Valinomycin alone caused a marked enhancement of the rate of electron flow through the complex. Nigericin alone was ineffective, but caused further stimulation of electron flow when added in the presence of valinomycin. The data presented are discussed in terms of two mechanisms: the Q cycle and a model based on combination of protonmotive catalysis by special bound quinone and proton conduction along pathways in the apoproteins.

Light-activated proton-motive force generation in lipid vesicles containing cytochrome b-c1 complex and bacterial reaction centres

(1) Purified bovine heart mitochondrial cytochrome b-c1 complex (ubiquinone-cytochrome c oxidoreductase) and photosynthetic reaction centres isolated from Rhodopseudomonas sphaeroides strain R-26 have been incorporated into lipid vesicles. In the presence of cytochrome c and ubiquinone-2, light activation caused a cyclic electron transfer involving both components. (2) Since cytochrome c is added outside the vesicles, it is both reduced by the cytochrome b-c1 complex and oxidised by the reaction centre on the outside of the vesicles. Ubiquinone-2, however, is reduced by the reaction centres at a site in contact with the inside of the vesicles, but the reduced form, ubiquinol-2, is oxidised by the cytochrome b-c1 complex at a site in contact with the outer aqueous phase. (3) In the presence of valinomycin plus K+, initiation of cyclic electron flow causes protons to move from inside the vesicles to the outer medium and the H +/2e- ratio was calculated to be close to 4.

Modification of the catalytic function of the mitochondrial cytochrome b-c1 complex by dicyclohexylcarbodiimide

N,N'-Dicyclohexylcarbodiimide (DCCD) induces a complex set of effects on the succinate-cytochrome c span of the mitochondrial respiratory chain. At concentrations below 1000 mol per mol of cytochrome c1, DCCD is able to block the proton-translocating activity associated to succinate or ubiquinol oxidation without inhibiting the steady-state redox activity of the b-c1 complex either in intact mitochondrial particles or in the isolated ubiquinol-cytochrome c reductase reconstituted in phospholipid vesicles. In parallel to this, DCCD modifies the redox responses of the endogenous cytochrome b, which becomes more rapidly reduced by succinate, and more slowly oxidized when previously reduced by substrates. At similar concentrations the inhibitor apparently stimulates the redox activity of the succinate-ubiquinone reductase. Moreover, DCCD, at concentrations about one order of magnitude higher than those blocking proton translocation, produces inactivation of the redox function of the b-c1 complex. The binding of [14C]DCCD to the isolated b-c1 complex has shown that under conditions leading to the inhibition of the proton-translocating activity of the enzyme, a subunit of about 9500 Da, namely Band VIII, is the most heavily labelled polypeptide of the complex. The possible correlations between the various effects of DCCD and its modification of the b-c1 complex are discussed.

The effect of specific antibodies on oxygen uptake and H+ pumping by cytochrome c oxidase vesicles

Antibodies to solubilized cytochrome c oxidase and to subunit III were incubated with liposomal oxidase. In oxygen uptake experiments, the inhibiting effects on RCI of anti-oxidase (primarily anti- subunits II and IV) and anti-III were by different mechanisms: the former, by inhibiting the uncoupled rate; the later, by stimulating the coupled rate. In experiments with H+ translocation, anti-oxidase was without effect, while anti-III was a potent inhibitor of proton pumping. These results are conclusive evidence for redox-linked proton extrusion from the vesicles by the oxidase (and its subunit III).

Characteristics of redox-linked proton ejection in cytochrome c oxidase reconstituted in phospholipid vesicles. New observations support mechanisms different from proton pumping

Experimental observations reveal a number of characteristics of the redox-linked proton ejection from cytochrome c oxidase vesicles, which apparently cannot be explained by a proton pumping activity of the oxidase. These observations seem, on the other hand, to provide useful elements for alternative explanation(s) of the proton ejection. It is proposed here that the process is scalar and not vectorial and can derive from redox-linked rupture of protonated salt-bridges in the oxidase-lipid complex.

Redox-linked hydrogen bond strength changes in cytochrome a: implications for a cytochrome oxidase proton pump

The heme a formyl group of cytochrome a in cytochrome oxidase appears to be involved in a hydrogen-bond interaction with a proton donor associated with the polypeptide backbone [Callahan, P.M., & Babcock, G.T. (1983) Biochemistry 22, 452-461]. Resonance Raman and optical absorption spectroscopies have been applied to the beef heart and Thermus thermophilus proteins and to heme a and copper porphyrin a models in order to assess the spectroscopic manifestations and the energetics of the hydrogen-bond interaction. We find a linear relationship between optical absorption red shift and carbonyl vibrational frequency decrease for a series of hydrogen-bonded model complexes; the magnitude of both changes increases as the hydrogen-bond strength increases. Comparison of the model compound data with analogous data for the proteins indicates that the strength of the formyl hydrogen bond in situ increases by 2-2.5 kcal/mol upon reduction of ferric cytochrome a. The selective stabilization of reduced cytochrome a by the stronger hydrogen bond is expected to increase the redox potential of this center; the energy made available as the hydrogen bond strengthens during reduction may be used to drive redox-coupled events in the protein. Thus, the linkage between cytochrome a redox state and chromophore/protein interaction energy provides a mechanism by which electron-transfer events and protein structure are coupled. Two models, which incorporate this linkage into a redox-driven proton pump centered at cytochrome a in cytochrome oxidase, are presented.

An evaluation of the evidence for H+ pumping by reconstituted cytochrome c oxidase in the light of recent criticism

We review the evidence for H+ translocation by reconstituted cytochrome c oxidase; attention is paid to those aspects which we feel most open to criticism. Possible alternative hypotheses are assessed, with regard to experiments carried out to test them directly and with regard to published data. We conclude that, whilst certain aspects of this system are worthy of further clarification, the reported observations are all consistent with proton translocation by reconstituted cytochrome c oxidase; most of these observations provide a positive indication of such an activity.