Chemiosmotic coupling in cytochrome oxidase. Possible protonmotive O loop and O cycle mechanisms

Electron-transport-driven proton pumps display nonhyperbolic kinetics: Simulation of the steady-state kinetics of cytochrome c oxidase

A reaction cycle for electron-transportdriven proton pumps is proposed. It includes two distinct conformational states of the pump protein in which the primary electron acceptor has different reduction potentials. This has as an unavoidable consequence that the steady-state rate equation for the catalytic reaction driving the pump is nonhyperbolic. The model can be used to simulate experimental results for the kinetics of cytochrome oxidase (EC 1.9.3.1) in a wide range of experimental conditions (ionic strength, pH, temperature). It is thus not necessary to invoke more than one binding site for cytochrome c to account for the biphasic response of the oxidase activity to the concentration of this substrate.

Cytochrome c oxidase: chemistry of a molecular machine

The plethora of proposed chemical models attempting to explain the proton pumping reactions catalyzed by the CcO complex, especially the number of recent models, makes it clear that the problem is far from solved. Although we have not discussed all of the models proposed to date, we have described some of the more detailed models in order to illustrate the theoretical concepts introduced at the beginning of this section on proton pumping as well as to illustrate the rich possibilities available for effecting proton pumping. It is clear that proton pumping is effected by conformational changes induced by oxidation/reduction of the various redox centers in the CcO complex. It is for this reason that the CcO complex is called a redox-linked proton pump. The conformational changes of the proton pump cycle are usually envisioned to be some sort of ligand-exchange reaction arising from unstable geometries upon oxidation/reduction of the various redox centers. However, simple geometrical rearrangements, as in the Babcock and Mitchell models are also possible. In any model, however, hydrogen bonds must be broken and reformed due to conformational changes that result from oxidation/reduction of the linkage site during enzyme turnover. Perhaps the most important point emphasized in this discussion, however, is the fact that proton pumping is a directed process and it is electron and proton gating mechanisms that drive the proton pump cycle in the forward direction. Since many of the models discussed above lack effective electron and/or proton gating, it is clear that the major difficulty in developing a viable chemical model is not formulating a cyclic set of protein conformational changes effecting proton pumping (redox linkage) but rather constructing the model with a set of physical constraints so that the proposed cycle proceeds efficiently as postulated. In our discussion of these models, we have not been too concerned about which electron of the catalytic cycle was entering the site of linkage, but merely whether an ET to the binuclear center played a role. However, redox linkage only occurs if ET to the activated binuclear center is coupled to the proton pump. Since all of the models of proton pumping presented here, with the exception of the Rousseau expanded model and the Wikström model, have a maximum stoichiometry of 1 H+/e-, they inadequately explain the 2 H+/e- ratio for the third and fourth electrons of the dioxygen reduction cycle (see Section V.B). One way of interpreting this shortfall of protons is that the remaining protons are pumped by an as yet undefined indirectly coupled mechanism. In this scenario, the site of linkage could be coupled to the pumping of one proton in a direct fashion and one proton in an indirect fashion for a given electron. For a long time, it was assumed that at least some elements of such an indirect mechanism reside in subunit III. While recent evidence argues against the involvement of subunit III in the proton pump, subunit III may still participate in a regulatory and/or structural capacity (Section II.E). Attention has now focused on subunits I and II in the search for residues intimately involved in the proton pump mechanism and/or as part of a proton channel. In particular, the role of some of the highly conserved residues of helix VIII of subunit I are currently being studied by site directed mutagenesis. In our opinion, any model that invokes heme alpha 3 or CuB as the site of linkage must propose a very effective means by which the presumedly fast uncoupling ET to the dioxygen intermediates is prevented. It is difficult to imagine that ET over the short distance from heme alpha 3 or CuB to the dioxygen intermediate requires more than 1 ns. In addition, we expect the conformational changes of the proton pump to require much more than 1 ns (see Section V.B).

Foundations of Vectorial Metabolism and Osmochemistry

Chemical transformations, like osmotic translocations, are transport processes when looked at in detail. In chemiosmotic systems, the pathways of specific ligand conduction are spatially orientated through osmoenzymes and porters in which the actions of chemical group, electron and solute transfer occur as vectorial (or higher tensorial order) diffusion processes down gradients of total potential energy that represent real spatially directed fields of force. Thus, it has been possible to describe classical bag-of-enzymes biochemistry as well as membrane biochemistry in terms of transport. But it would not have been possible to explain biological transport in terms of classical transformational biochemistry or chemistry. The recognition of this conceptual asymmetry in favour of transport has seemed to be upsetting to some biochemists and chemists; and they have resisted the shift towards thinking primarily in terms of the vectorial forces and co-linear displacements of ligands in place of their much less informative scalar products that correspond to the conventional scalar energies. Nevertheless, considerable progress has been made in establishing vectorial metabolism and osmochemistry as acceptable biochemical disciplines embracing transport and metabolism, and bioenergetics has been fundamentally transformed as a result.

Oxygen consumption and expression of the adenine nucleotide translocator in cells lacking mitochondrial DNA

It has been shown previously that human rho degrees cells, deprived of mitochondrial DNA and consequently of functional oxidative phosphorylation, maintain a mitochondrial membrane potential, which is necessary for their growth. The goal of our study was to determine the precise origin of this membrane potential in three rho degrees cell lines originating from the human HepG2, 143B, and HeLa S3 cell lines. Residual cyanide-sensitive oxygen consumption suggests the persistence of residual mitochondrial respiratory chain activity, about 8% of that of the corresponding parental cells. The fluorescence emitted by the three rho degrees cell lines in the presence of a mitochondrial specific fluorochrome was partially reduced by a protonophore, suggesting the existence of a proton gradient. The mitochondrial membrane potential is maintained both by a residual proton gradient (up to 45 to 50% of the potential) and by other ion movements such as the glycolytic ATP(4-) to mitochondrial ADP(3-) exchange. The ANT2 gene, encoding isoform 2 of the adenine nucleotide translocator, is overexpressed in rho degrees HepG2 and 143B cells strongly dependent on glycolytic ATP synthesis, as compared to the corresponding parental cells, which present a more oxidative metabolism. In rho degrees HeLa S3 cells, originating from the HeLa S3 cell line, which already displays a glycolytic energy status, ANT2 gene expression was not higher as in parental cells. Mitochondrial oxygen consumption and ANT2 gene overexpression vary in opposite ways and this suggests that these two parameters have complementary roles in the maintenance of the mitochondrial membrane potential in rho degrees cells.

Ferrocyanide-peroxidase activity of cytochrome c oxidase

Redox interaction of mitochondrial cytochrome c oxidase (COX) with ferrocyanide/ferricyanide couple is greatly accelerated by polycations, such as poly-l-lysine [Musatov et al. (1991) Biological Membranes 8, 229-234]. This has allowed us to study ferrocyanide oxidation by COX at very high redox potentials of the ferrocyanide/ferricyanide couple either following spectrophotometrically ferricyanide accumulation or measuring proton uptake associated with water formation in the reaction. At low [ferrocyanide]/[ferricyanide] ratios (Eh values around 500 mV) and ambient oxygen concentration, the ferrocyanide-oxidase activity of COX becomes negligibly small as compared to the reaction rate observed with pure ferrocyanide. Oxidation of ferrocyanide under these conditions, is greatly stimulated by H2O2 or ethylhydroperoxide indicating peroxidatic reaction involved. The ferrocyanide-peroxidase activity of COX is strictly polylysine-dependent and is inhibited by heme a3 ligands such as KCN and NaN3. Apparently the reaction involves normal electron pathway, i.e. electron donation through CuA and oxidation via heme a3. The peroxidase reaction shows a pH-dependence similar to that of the cytochrome c oxidase activity of COX. When COX is preequilibrated with excess H2O2, addition of ferrocyanide shifts the initial steady-state concentrations of the Ferryl-Oxo and Peroxy compounds towards approximately 2:1 ratio of the two intermediates. It is suggested that in the peroxidase cycleferrocyanide donates electrons to both P and F intermediates with a comparable efficiency. Isolation of a partial redox activity of COX opens a possibility to study separately proton translocation coupled to the peroxidase half-reaction of the COX reaction cycle. Copyright 1998

Peter Dennis Mitchell, 29 September 1920 - 10 April 1992

Peter Mitchell was a biochemist who developed a theory of the mechanism of energy transduction in biological systems that not only transformed the field of bioenergetics but that has had widespread ramifications in the whole field of cell biology.

Factors affecting the H+/e- stoichiometry in mitochondrial cytochrome c oxidase: influence of the rate of electron flow and transmembrane delta pH

A study is presented of the factors affecting the H+/e- stoichiometry of the proton pump of mitochondrial cytochrome c oxidase, isolated and reconstituted in phospholipid vesicles (COV). Under level flow conditions, i.e., in the absence of a transmembrane delta muH+, the H+/e- ratio, obtained from spectrophotometric measurements of the initial rates of electron flow and H+ release specifically elicited by cytochrome c, varied from around 0 to 1, depending on the actual rate of electron flow through the oxidase. At steady state the H+/e- ratio for the oxidase was specifically depressed by the transmembrane delta pH. The study of the H+/e- ratio of the pump was complemented by an analysis of the redox pattern of cytochrome c, CuA, and heme a. From both sets of results and recent structural data from other groups, it is concluded that the dependence of the H+/e- ratio on the rate of electron flow through the oxidase and transmembrane delta pH is associated with the possible occurrence of two electron transfer pathways in cytochrome c oxidase, a coupled one (cyt c-->CuA-->heme a-->heme a3-CuB) and a decoupled one (cyt c-->CuA-->heme a3-CuB). The contributions of the two pathways, differently affected by kinetics and thermodynamic factors, will determine the actual H+/e- ratio of the pump. A possible role of heme a in the proton pump and the physiological implication of the variable H+/e- ratio in the oxidase are discussed.

The histidine cycle: a new model for proton translocation in the respiratory heme-copper oxidases

A model of redox-linked proton translocation is presented for the terminal heme-copper oxidases. The new model, which is distinct both in principle and in detail from previously suggested mechanisms, is introduced in a historical perspective and outlined first as a set of general principles, and then as a more detailed chemical mechanism, adapted to what is known about the chemistry of dioxygen reduction in this family of enzymes. The model postulates a direct mechanistic role in proton-pumping of the oxygenous ligand on the iron in the binuclear heme-copper site through an electrostatic nonbonding interaction between this ligand and the doubly protonated imidazolium group of a conserved histidine residue nearby. In the model this histidine residue cycles between imidazolium and imidazolate states translocating two protons per event, the imidazolate state stabilized by bonding to the copper in the site. The model also suggests a key role in proton translocation for those protons that are taken up in reduction of O2 to water, in that their uptake to the oxygenous ligand unlatches the electrostatically stabilized imidazolium residue and promotes proton release.

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.

Fluorescence quenching of reconstituted NCD-4-labeled cytochrome c oxidase complex by DOXYL-stearic acids

It has been known for some time that dicyclohexylcarbodiimide (DCCD) inhibits the proton translocation function of the cytochrome c oxidase complex (CcO) and that there is one major site in subunit III which is modified upon reaction with DCCD (Glu-90 for the bovine enzyme). We have examined the reaction of bovine CcO with N-cyclohexyl-N'-(4-dimethylamino-alpha-napthyl)carbodiimide (NCD-4), a fluorescent analog of DCCD. NCD-4 labeling of CcO is strongly inhibited by DCCD implicating Glu-90 of subunit III as the site of chemical modification by NCD-4. The fluorescence of reconstituted NCD-4-labeled bovine CcO is strongly quenched by hydrophobic nitroxides, whereas hydrophilic nitroxides and iodide ions have a reduced quenching ability. It is concluded that the Glu-90 of subunit III resides near the protein-lipid interface of the membrane spanning region of the enzyme. Different quenching abilities of 5-, 7-, 10-, 12-, and 16-4,4-dimethyl-3-oxazolinyloxy-stearic acids suggest that the NCD-4 label is located in the membrane bilayer in the region near the middle of the hydrocarbon tail of stearic acid. In light of these results, it is unlikely that Glu-90 is part of a proton channel that is associated with the proton pumping machinery of the enzyme but the outcome of this study does not eliminate an allosteric regulatory role for this residue.

Slow ('resting') forms of mitochondrial cytochrome c oxidase consist of two kinetically distinct conformations of the binuclear CuB/a3 centre--relevance to the mechanism of proton translocation

We have purified slow ('resting') cytochrome oxidase from bovine heart, free of contamination with fast ('pulsed') enzyme. This form of the enzyme shows two kinetic phases of reduction of haem a3 by dithionite (k = 0.020 +/- 0.005 s-1 and k = 0.005 +/- 0.002 s-1). The presence of ligands that bind to the oxidized or reduced binuclear centre (formate or carbon monoxide respectively) has no effect on these rates. Varying the dithionite concentration also has no effect on either phase, although at low dithionite concentrations a lag phase is observed as the rate of haem a reduction is slower. The results are consistent with a model for reduction of the slow enzyme where the rate of electron transfer to the binuclear centre is the limiting step, rather than an equilibrium model where the haem a3 redox potential is low. Increasing the pH decreases the rate of the slower phase of dithionite reduction, but has no effect on the faster phase. EPR studies show that the slow phase (only) correlates with the disappearance of the g' = 12/g' = 2.95 signals, with the same pH dependence; again the presence of formate has no effect on these results. Deconvolution of the oxidized optical spectra shows that the enzyme reduced in the slow phase has a blue-shifted Soret band, relative to that reduced in the faster phase. Incubation of the oxidized enzyme at high pH causes a line-broadening of both the g' = 12 and g' = 2.95 EPR signals with no obvious effect on the amount of signal. The results are interpreted in a model where the presence of a carboxylate bridge between haem a3 and CuB defines the slow enzyme. It is suggested that the two rates of dithionite reduction are the result of different ligation to CuB--where water is the ligand the binuclear centre is FeIV/CuI (EPR-silent) and where hydroxide is the ligand the binuclear centre is FeIII/CuII (g' = 12/g' = 2.95 EPR signals).

pH changes associated with cytochrome c oxidase reaction with H2O2. Protonation state of the peroxy and oxoferryl intermediates

pH changes associated with the mitochondrial cytochrome oxidase reaction with H2O2 have been studied. In the presence of ferricyanide or Tris-phenanthroline complex of CoIII as electron acceptors, reaction of H2O2 with the oxidized cytochrome oxidase is accompanied by a steady proton release with a rate constant of ca. 3 M-1.s-1 at pH 6.8. The acidification is completely inhibited by superoxide dismutase and its pre-steady-state kinetics correlates with that of the oxoferryl compound (F) accumulation. Apparently, the proton release is linked to superoxide generation by cytochrome oxidase under these conditions. In the presence of superoxide dismutase and without the electron acceptors, the H2O2-induced transitions of cytochrome oxidase from the oxidized to the peroxy (P) and from the peroxy to the oxoferryl state are not associated with any significant proton release or uptake. The results point to the following mechanism of O2- generation and protonation states of the cytochrome oxidase compounds P and F: [formula: see text]

Cytochrome oxidase-catalyzed superoxide generation from hydrogen peroxide

Superoxide dismutase is shown to affect spectral changes observed upon cytochrome c oxidase reaction with H2O2, which indicates a possibility of O2- radicals being formed in the reaction. Using DMPO as a spin trap, generation of superoxide radicals from H2O2 in the presence of cytochrome oxidase is directly demonstrated. The process is inhibited by cyanide and is not observed with a heat-denatured enzyme pointing to a specific reaction in the oxygen-reducing centre of cytochrome c oxidase. The data support a hypothesis on a catalase cycle catalyzed by cytochrome c oxidase in the presence of excess H2O2 (Vygodina and Konstantinov (1988) Ann. NY Acad. Sci., 550, 124-138): (formula: see text)

Foundations of vectorial metabolism and osmochemistry

Chemical transformations, like osmotic translocations, are transport processes when looked at in detail. In chemiosmotic systems, the pathways of specific ligand conduction are spatially orientated through osmoenzymes and porters in which the actions of chemical group, electron and solute transfer occur as vectorial (or higher tensorial order) diffusion processes down gradients of total potential energy that represent real spatially-directed fields of force. Thus, it has been possible to describe classical bag-of-enzymes biochemistry as well as membrane biochemistry in terms of transport. But it would not have been possible to explain biological transport in terms of classical transformational biochemistry or chemistry. The recognition of this conceptual asymmetry in favour of transport has seemed to be upsetting to some biochemists and chemists; and they have resisted the shift towards thinking primarily in terms of the vectorial forces and co-linear displacements of ligands in place of their much less informative scalar products that correspond to the conventional scalar energies. Nevertheless, considerable progress has been made in establishing vectorial metabolism and osmochemistry as acceptable biochemical disciplines embracing transport and metabolism, and bioenergetics has been fundamentally transformed as a result.

Glynn and the conceptual development of the chemiosmotic theory: a retrospective and prospective view

The origin and evolution of the chemiosmotic theory is described particularly in relation to Peter Mitchell's application of it to model oxidative phosphorylation. Much of the deployment, development and evaluation of the theory occurred at the independent laboratory of the Glynn Research Foundation; the value and future of such an institution is discussed. The role of models mediating between theories and phenomena is analyzed with regard to the growth of knowledge of chemiosmotic systems.

The osmochemistry of electron-transfer complexes

Detailed molecular mechanisms of electron transfer-driven translocation of ions and of the generation of electric fields across biological membranes are beginning to emerge. The ideas inherent in the early formulations of the chemiosmotic hypothesis have provided the framework for this understanding and have also been seminal in promoting many of the experimental approaches which have been successfully used. This article is an attempt to review present understanding of the structures and mechanisms of several osmoenzymes of central importance and to identify and define the underlying features which might be of general relevance to the study of chemiosmotic devices.

Cytochrome oxidase as a proton pump

The general structure of cytochrome oxidase is reviewed and evidence that the enzyme acts as a redox-linked proton pump outlined. The overall H+/e- stoichiometry of the pump is discussed and results [Wikström (1989), Nature 338, 293] which suggest that only the final two electrons which reduce the peroxide adduct to water are coupled to protein translocated are considered in terms of the restrictions they place on pump mechanisms. "Direct" and "indirect" mechanisms for proton translocation are discussed in the context of evidence for redox-linked conformational changes in the enzyme, the role of subunit III, and the nature of the CuA site.

Redox-linked proton translocation by direct-coupled ligand conduction

The term "direct-coupled" is considered in the context of redox-linked proton translocation mechanisms, and the origins of this concept, its philosophical implications, applications, and contributions to the development of bioenergetics, are discussed.

Effect of chemical modification of lysine amino groups on redox and protonmotive activity of bovine heart cytochrome c oxidase reconstituted in phospholipid membranes

A study is presented of the effect of chemical modification of lysine amino groups on the redox and protonmotive activity of bovine heart cytochrome c oxidase. Treatment of soluble oxidase with succinic acid anhydride resulted in succinylation of lysines in all the subunits of the enzyme. The consequent change of surface charges from positive to negative resulted in inversion of the orientation of the reconstituted enzyme from right-side-out to inside-out. Reconstitution of the oxidase in phospholipid vesicles prevented succinylation of subunits III and Vb and depressed that of other subunits with the exception of subunits II and IV which were predominantly labeled in a concentration-dependent manner by succinic acid anhydride. This modification of lysines produced a decoupling effect on redox-linked proton ejection, which was associated with a decrease of the respiratory control exerted by the delta pH component of PMF. The decoupling effect was directly shown to be exerted at the level of the pH-dependent rate-limiting step in intramolecular electron flow located on the oxygen side of heme a.

Protonmotive activity of cytochrome c oxidase: control of oxidoreduction of the heme centers by the protonmotive force in the reconstituted beef heart enzyme

This paper contributes to the characterization of partial steps of electron and proton transfer in mitochondrial cytochrome c oxidase with respect to their membrane arrangement and involvement in energy-linked protonmotive activity. It is shown that delta psi controls electron flow from cytochrome c to heme a is consistent with the view that the latter center is buried in the membrane in a central position. The pressure exerted by delta psi on oxidation of heme alpha 3 by O2 indicates also that this center is buried in the membrane at some distance from the inner side and is consistent with observations showing that protons consumed in the reduction of O2 to H2O derive from the inner space. Electron flow from heme alpha to heme alpha 3 is shown to be specifically controlled by delta pH and in particular by the pH of the inner phase. Analysis of the effect of DCCD treatment of oxidase vesicles reveals that concentrations of this reagent which result in selective modification of subunit III (Prochaska et al., 1981) produce inhibition of redox-linked proton release. Higher concentrations of DCCD which result also in modification of subunits II and IV (Prochaska et al., 1981) cause inhibition of the pH-dependent electron-transfer step from heme alpha to heme alpha 3.

Identification of the electron transfers in cytochrome oxidase that are coupled to proton-pumping

Mitochondrial cytochrome oxidase is a functionally complex, membrane-bound respiratory enzyme which catalyses both the reduction of O2 to water and proton-pumping. During respiration, an exogenous donor, cytochrome c, donates four electrons to O2 bound at the bimetallic haem alpha 3 Fe-Cu centre within the enzyme. These four electron transfers are mediated by the enzyme's haem alpha and CuA redox centres and result in the translocation of four protons across the inner mitochondrial membrane. The molecular mechanism of proton translocation has not yet been delineated, however, and in the absence of direct experimental evidence all four electron transfers have been assumed to couple equally to proton-pumping. Here, I report the effects of proton-motive force and membrane potential on two equilibria involving intermediates of the bimetallic centre at different levels of O2 reduction. The results show that only two of the electron transfers, to the 'peroxy' and 'oxyferryl' intermediates of the bimetallic centre, are linked to proton translocation, a finding which strongly constrains candidate mechanisms for proton-pumping.

Effect of pH on the spectrum of cytochrome c oxidase hydrogen peroxide complex

Hydrogen peroxide binding to ferric cytochrome c oxidase in proteoliposomes brings about a red-shift of the enzyme Soret band and increased absorption in the visible range with two prominent peaks at approx. 570 and 607 nm. The molar absorptivity of the H2O2-induced difference spectrum is virtually pH-independent in the Soret band and at 570 nm, whereas the peak at 607 nm increases approx. 3-fold upon alkalinization in a narrow pH range 6.0-7.2, the effect being reversible. The pH profile of this transition indicates ionization of two acid-base groups with close pK values of 6.7. The lineshape of the peroxide compound difference spectrum is found to respond to pH changes inside the proteoliposomes. It is suggested that peroxide-complexed enzyme can undergo a pH-dependent transition to a form with increased extinction at 605-607 nm, possibly corresponding to the 420 nm (or 'pulsed') conformer of the ferric cytochrome oxidase formed as an early product of the enzyme oxidation. Accordingly, relaxation of the '420 nm' form to the resting state would be linked to an uptake of two protons from the M-aqueous phase. This protolytic reaction might be a partial step of the cytochrome oxidase proton pumping mechanism or it could serve to regulate interconversion between the active 'pulsed' and less active 'resting' states of the enzyme in the membrane.

Activation by reduction of the resting form of cytochrome c oxidase: tests of different models and evidence for the involvement of CuB

(1) The reaction of the resting form of oxidised cytochrome c oxidase from ox heart with dithionite has been studied in the presence and absence of cyanide. In both cases, cytochrome a reduction in 0.1 M phosphate (pH 7) occurs at a rate of 8.2.10(4) M-1.s-1. In the absence of cyanide, ferrocytochrome a3 appears at a rate (kobs) of 0.016 s-1. Ferricytochrome a3 maintains its 418 nm Soret maximum until reduced. The rate of a3 reduction is independent of dithionite concentration over a range 0.9 mM-131 mM. In the presence or cyanide, visible and EPR spectral changes indicate the formation of a ferric a3/cyanide complex occurs at the same rate as a3 reduction in the absence of cyanide. A g = 3.6 signal appears at the same time as the decay of a g = 6 signal. No EPR signals which could be attributed to copper in any significant amounts could be detected after dithionite addition, either in the presence or absence of cyanide. (2) Addition of dithionite to cytochrome oxidase at various times following induction of turnover with ascorbate/TMPD, results in a biphasic reduction of cytochrome a3 with an increasing proportion of the fast phase of reduction occurring after longer turnover times. At the same time, the predominant steady state species of ferri-cytochrome a3 shifts from high to low spin and the steady-state level of reduction of cytochrome a drops indicating a shift in population of the enzyme molecules to a species with fast turnover. In the final activated form, oxygen is not required for fast internal electron transfer to cytochrome a3. In addition, oxygen does not induce further electron uptake in samples of resting cytochrome oxidase reduced under anaerobic conditions in the presence of cyanide. Both findings are contrary to predictions of certain O-loop types of mechanism for proton translocation. (3) A measurement of electron entry into the resting form of cytochrome oxidase in the presence of cyanide, using TMPD or cytochrome c under anaerobic conditions, shows that three electrons per oxidase enter below a redox potential of around +200 mV. An initial fast entry of two electrons is followed by a slow (kobs approximately 0.02 s) entry of a third electron.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of the protonmotive activity of mammalian cytochrome c oxidase and their modification by amino acid reagents

Experimental analysis of the protonmotive activity of reconstituted cytochrome c oxidase from beef heart reveals the following features: (1) The observed H+:e- ratio for redox-linked proton ejection from oxidase vesicles is variable, being affected by various effectors that also influence the catalytic process. (2) Proton ejection appears to be associated with electron transfer from heme a (+CuA) to heme a3 (+CuB). (3) Chemical modification studies contribute to indentification of proton-conduction pathways in the protein and/or residues involved in the coupling process between redox and protonmotive activity. In intact rat liver mitochondria, under physiological conditions of dehydrogenase activity and delta microH+ generation by the respiratory chain cytochrome oxidase, does not appear to contribute significant H+ pumping. The relevance of what is observed is discussed in terms of possible mechanisms and physiological role.

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.

Redox loops and proton pumps

In a Review-Hypothesis, Mitchell [(1987) FEBS Lett. 222,235-245] has recently suggested possible molecular mechanisms for proton translocation by cytochrome oxidase. In describing these mechanisms, he extended his own concept of a redox loop in a manner expected to lead to confusion. He also stated that the term redox-linked proton pump implies an indirect coupling between electron transfer and proton translocation, and that this type of coupling is very difficult to test experimentally. Here it is argued that the original meaning of a redox loop should be maintained, and proper definitions of the terms redox-linked proton pump and direct or indirect coupling are formulated. In addition, it is reasoned that both types of coupling are amenable to experimental tests.

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.

NN'-dicyclohexylcarbodi-imide-sensitivity of bovine heart mitochondrial NADH: ubiquinone oxidoreductase. Inhibition of activity and binding to subunits

Dicyclohexylcarbodi-imide (DCCD) inhibition of NADH: ubiquinone oxidoreductase was studied in submitochondrial particles and in the isolated form, together with the binding of the reagent to the enzyme. DCCD inhibited the isolated enzyme in a time- and concentration-dependent manner. Over the concentration range studied, a maximum inhibition of 85% was attained within 60 min. The time course for the binding of DCCD to the enzyme was similar to that of activity inhibition. The NADH:ubiquinone oxidoreductase activity of the submitochondrial particles was also sensitive to DCCD, and the locus of binding of the inhibitor was studied by subsequent resolution of the enzyme into subunit polypeptides. Only two subunits (molecular masses 13.7 and 21.5 kDa) were labelled by [14C]DCCD, whereas, when the enzyme in its isolated form was treated with [14C]DCCD, six subunits (13.7, 16.1, 21.5, 39, 43 and 53 kDa) were labelled. Comparison with the subunit labelling of F1F0-ATPase and ubiquinol:cytochrome c oxidoreductase indicated that the labelling pattern of NADH:ubiquinone oxidoreductase, and enzyme complex with a multitude of subunits, is unique and not due to contamination by other inner-membrane proteins. The correlation between the electron- and proton-transport functions and the DCCD-binding components remains to be established.

Energy coupling and Hill cycles in enzymatic processes

We review how Hill's work on enzyme catalysis has nurtured our understanding of the mechanism by which enzymes can couple downhill processes to uphill processes. More specifically, we discuss the following questions: (i) Does it make sense to distinguish the chemical potential of the bound ligand from that of the binding enzyme? (ii) To what extent can free-energy transduction be localized at some crucial step in the catalytic cycle? (iii) Need enzymes be optimized so as to even out the profile of basic free energy along the catalytic cycle? (iv) How do continuous models of conformational transitions relate to discrete state diagrams and their kinetic elaborations? We conclude that (1) only in very special cases is it useful to designate a portion of the free energy of the enzyme-ligand complex as the free energy of the bound ligand; (2) only for some mechanisms can free-energy transduction be localized within a part of the catalytic cycle; (3) only in special cases should one expect enzymes to be "optimized" so as to have smooth basic free-energy profiles; and (4) transition rate constants can often be related to conformational diffusion constants, although in certain situations the kinetic description of an enzyme as if jumping between discrete states is impracticable; a diffusion-type description may then be preferable.

Protonmotive stoichiometry of rat liver cytochrome c oxidase: determination by a new rate/pulse method

The stoichoimetry of vectorial H+ ejection coupled to electron flow through the cytochrome c oxidase (EC 1.9.3.1) of rat liver mitochondria was determined by a new rate/pulse method. This is a modification of the oxygen-pulse method. Electron flow through the oxidase is initiated by adding oxygen to suspensions of anaerobic mitochondria at a known and constant rate. Cytochrome c oxidase was examined directly or in combination with cytochrome c reductase (ubiquinol:ferricytochrome c oxidoreductase). In both cases the----H0+/2e- ratio was found to be constant during the time-course of oxygen reduction, and thus independent of delta pH. The stoichiometries observed were consistent with mechanistic stoichiometries of 2 and 6 for cytochrome c oxidase alone and cytochrome c oxidase together with cytochrome c reductase, respectively. The stoichiometry of cytochrome c reductase alone was also examined, by using ferricyanide in place of oxygen. The results obtained were consistent with the accepted mechanistic stoichiometry of 4 for this enzyme.

Cytochrome-c oxidase. Subunit structure and proton pumping

This article reviews the significance of the subunit structure of cytochrome-c oxidase in proton pumping and in particular summarizes available evidences for or against a role of subunit III in the control of this important function of the enzyme.

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.

Kinetic characterization of cytochrome c oxidase from Bacillus subtilis

Bacillus subtilis aa3-type cytochrome c oxidase is capable of oxidizing cytochrome c from different origins. The kinetic properties of the enzyme are influenced by ionic strength. The affinity for Saccharomyces cerevisiae cytochrome c declines with increasing ionic strength whereas the Vmax remains almost constant. An increase of Vmax is observed when the enzyme is incorporated in artificial membranes. Negatively charged phospholipids allow high turnover rates of the aa3-type oxidase. The effect of ionic strength on oxidation of horse heart cytochrome c results in significant changes of both Km and Vmax. These effects can be explained by disturbances of enzyme-substrate interactions and are not related to changes in the aggregation state of the enzyme. The respiration control index of the enzyme reconstituted in artificial membranes appeared to be dependent on phospholipid composition, protein/lipid ratios and also on the external pH. The action of the ionophores nigericin and valinomycin, at various pH values, on the enzyme activity and proton-permeability measurements of the membranes indicate that both components of the proton-motive force, the membrane potential and the pH gradient, can in principle regulate enzyme activity in the reconstituted state.

Evidence for two H2O2-binding sites in ferric cytochrome c oxidase. Indication to the O-cycle?

H2O2 addition to the oxidized cytochrome c oxidase reconstituted in liposomes brings about a red shift of the Soret band of the enzyme and an increased absorption in the visible region with two distinct peaks at approximately 570 and 605 nm. Throughout pH range 6-8.5, the spectral changes at 570 nm and in the Soret band titrate with very similar pH-independent Kd values of 2-3 microM. At the same time, Kd of the peroxide complex measured at 605 nm increases markedly with increased H+ activity reaching the value of 18 +/- 2 microM at pH 6.0. This finding may indicate the presence of two different H2O2-binding sites in the enzyme with different affinity for the ligand at acid pH. The Soret and 570 nm band effects are suggested to report H2O2 coordination to heme iron of alpha 3, whereas the maximum at 605 nm could arise from H2O2 binding to Cu alpha 3 followed by the enzyme transition into the 'pulsed' (or '420/605') conformation. Possible implication of the two H2O2-binding sites for the cytochrome oxidase redox and proton-pumping mechanisms are discussed.

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.