Proton translocation reactions in the respiratory chains

Allosteric Cooperativity in Proton Energy Conversion in A1-Type Cytochrome c Oxidase

Cytochrome c oxidase (CcO), the Cu_A, heme a, heme a₃, Cu_B enzyme of respiratory chain, converts the free energy released by aerobic cytochrome c oxidation into a membrane electrochemical proton gradient (ΔμH⁺). ΔμH⁺ derives from the membrane anisotropic arrangement of dioxygen reduction to two water molecules and transmembrane proton pumping from a negative (N) space to a positive (P) space separated by the membrane. Spectroscopic, potentiometric, and X-ray crystallographic analyses characterize allosteric cooperativity of dioxygen binding and reduction with protonmotive conformational states of CcO. These studies show that allosteric cooperativity stabilizes the favorable conformational state for conversion of redox energy into a transmembrane ΔμH⁺.

Structural Origin of the Large Redox-Linked Reorganization in the 2-Oxoglutarate Dependent Oxygenase, TauD

2-Oxoglutarate (2OG)-dependent oxygenases catalyze a wide range of chemical transformations via C-H bond activation. Prior studies raised the question of whether substrate hydroxylation by these enzymes occurs via a hydroxyl rebound or alkoxide mechanism and highlighted the need to understand the thermodynamic properties of transient intermediates. A recent spectroelectrochemical investigation of the 2OG-dependent oxygenase, taurine hydroxylase (TauD), revealed a strong link between the redox potential of the Fe(II)/Fe(III) couple and conformational changes of the enzyme. In this study, we show that the redox potential of wild-type TauD varies by 468 mV between the reduction of 2OG-Fe(III)-TauD (-272 mV) and oxidation of 2OG-Fe(II)-TauD (+196 mV). We use active site variants to investigate the structural origin of the redox-linked reorganization and the contributions of the metal-bound residues to the dynamic tuning of the redox potential of TauD. Time-dependent redox titrations show that reorganization occurs as a multistep process. Transient optical absorption and infrared spectroelectrochemistry show that substitution of any metal ligand alters the kinetics and thermodynamics of the reorganization. The H99A variant shows the largest net redox change relative to the wild-type protein, suggesting that redox-coupled protonation of H99 is required for high redox potentials of the metal. The D101Q and H255Q variants also suppress the conformational change, supporting their involvement in the structural rearrangement. Similar redox-linked conformational changes are observed in another 2OG dependent oxygenase, ethylene-forming enzyme, indicating that dynamic structural flexibility and the associated thermodynamic tuning may be a common phenomenon in this family of enzymes.

Strongly Coupled Redox-Linked Conformational Switching at the Active Site of the Non-Heme Iron-Dependent Dioxygenase, TauD

2-Oxoglutarate (2OG)-dependent dioxygenases catalyze C-H activation while performing a wide range of chemical transformations. In contrast to their heme analogues, non-heme iron centers afford greater structural flexibility with important implications for their diverse catalytic mechanisms. We characterize an in situ structural model of the putative transient ferric intermediate of 2OG:taurine dioxygenase (TauD) by using a combination of spectroelectrochemical and semiempirical computational methods, demonstrating that the Fe(III/II) transition involves a substantial, fully reversible, redox-linked conformational change at the active site. This rearrangement alters the apparent redox potential of the active site between -127 mV for reduction of the ferric state and +171 mV for oxidation of the ferrous state of the 2OG-Fe-TauD complex. Structural perturbations exhibit limited sensitivity to mediator concentrations and potential pulse duration. Similar changes were observed in the Fe-TauD and taurine-2OG-Fe-TauD complexes, thus attributing the reorganization to the protein moiety rather than the cosubstrates. Redox-difference infrared spectra indicate a reorganization of the protein backbone in addition to the involvement of carboxylate and histidine ligands. Quantitative modeling of the transient redox response using two alternative reaction schemes across a variety of experimental conditions strongly supports the proposal for intrinsic protein reorganization as the origin of the experimental observations.

The oxidative phosphorylation system in mammalian mitochondria

The chapter provides a review of the state of art of the oxidative phosphorylation system in mammalian mitochondria. The sections of the paper deal with: (i) the respiratory chain as a whole: redox centers of the chain and protonic coupling in oxidative phosphorylation (ii) atomic structure and functional mechanism of protonmotive complexes I, III, IV and V of the oxidative phosphorylation system (iii) biogenesis of oxidative phosphorylation complexes: mitochondrial import of nuclear encoded subunits, assembly of oxidative phosphorylation complexes, transcriptional factors controlling biogenesis of the complexes. This advanced knowledge of the structure, functional mechanism and biogenesis of the oxidative phosphorylation system provides a background to understand the pathological impact of genetic and acquired dysfunctions of mitochondrial oxidative phosphorylation.

Redox Bohr effects and the role of heme a in the proton pump of bovine heart cytochrome c oxidase

Structural and functional observations are reviewed which provide evidence for a central role of redox Bohr effect linked to the low-spin heme a in the proton pump of bovine heart cytochrome c oxidase. Data on the membrane sidedness of Bohr protons linked to anaerobic oxido-reduction of the individual metal centers in the liposome reconstituted oxidase are analysed. Redox Bohr protons coupled to anaerobic oxido-reduction of heme a (and Cu(A)) and Cu(B) exhibit membrane vectoriality, i.e. protons are taken up from the inner space upon reduction of these centers and released in the outer space upon their oxidation. Redox Bohr protons coupled to anaerobic oxido-reduction of heme a(3) do not, on the contrary, exhibit vectorial nature: protons are exchanged only with the outer space. A model of the proton pump of the oxidase, in which redox Bohr protons linked to the low-spin heme a play a central role, is described. This article is part of a Special Issue entitled: Allosteric cooperativity in respiratory proteins.

Cytochrome b-559 and proton conductance in oxygenic photosynthesis

Although cytochrome b-559 has long been known as a membrane-bound redox component closely linked to the reaction center of the oxygen-generating photosystem (PSII), its role in photosynthesis has remained obscure. This paper reports evidence and outlines a hypothesis in support of a "b-559 cycle"-i.e., a light-induced, cytochrome b-559-dependent, cyclic electron transport pathway around PSII that promotes translocation of protons from plastoquinol into the aqueous domain (lumen) of photosynthetic membranes (thylakoids). Light-induced proton transport coupled to light-induced electron transport is an essential aspect of energy transduction in photosynthesis because it generates an electrochemical proton gradient that drives ATP synthesis by the process of photosynthetic phosphorylation. The principal carrier of electrons and protons in thylakoids is the plastoquinone/plastoquinol couple. We propose that the b-559 cycle functions as a redox-linked proton pump that may operate jointly with the Rieske iron-sulfur pathway in oxidizing plastoquinol. The overall effect of such concerted oxidation of plastoquinol would be the translocation into the thylakoid lumen of two protons for each electron transferred from water to plastocyanin via plastoquinone.

The inhibitory binding site(s) of Zn2+in cytochromecoxidase

EXAFS analysis of Zn binding site(s) in bovine-heart cytochrome c oxidase and characterization of the inhibitory effect of internal zinc on respiratory activity and proton pumping of the liposome reconstituted oxidase are presented. EXAFS identifies tetrahedral coordination site(s) for Zn(2+) with two N-histidine imidazoles, one N-histidine imidazol or N-lysine and one O-COOH (glutamate or aspartate), possibly located at the entry site of the proton conducting D pathway in the oxidase and involved in inhibition of the oxygen reduction catalysis and proton pumping by internally trapped zinc.

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.

Cooperativity and flexibility of the protonmotive activity of mitochondrial respiratory chain

Functional and structural data are reviewed which provide evidence that proton pumping in cytochrome c oxidase is associated with extended allosteric cooperativity involving the four redox centers in the enzyme . Data are also summarized showing that the H+/e- stoichiometry for proton pumping in the cytochrome span of the mitochondrial respiratory chain is flexible. The DeltapH component of the bulk-phase membrane electrochemical proton gradient exerts a decoupling effect on the proton pump of both the bc1 complex and cytochrome c oxidase. A slip in the pumping efficiency of the latter is also caused by high electron pressure. The mechanistic and physiological implications of proton-pump slips are examined. The easiness with which bulk phase DeltapH causes, at least above a threshold level, decoupling of proton pumping indicates that for active oxidative phosphorylation efficient protonic coupling between redox complexes and ATP synthase takes place at the membrane surface, likely in cristae, without significant formation of delocalized DeltamuH+. A role of slips in modulating oxygen free radical production by the respiratory chain and the mitochondrial pathway of apoptosis is discussed.

Role of cooperative H(+)/e(-) linkage (redox bohr effect) at heme a/Cu(A) and heme a(3)/Cu(B) in the proton pump of cytochrome c oxidase

It is a pleasure to contribute to the special issue published in honor of Vladimir Skulachev, a distinguished scientist who greatly contributes to maintain a high standard of biochemical research in Russia. A more particular reason can be found in his work, where observations anticipating some ideas presented in my article were reported. Cytochrome c oxidase exhibits protonmotive, redox linked allosteric cooperativity. Experimental observations on soluble bovine cytochrome c oxidase are presented showing that oxido-reduction of heme a/Cu(A) and heme a(3)/Cu(B) is linked to deprotonation/protonation of two clusters of protolytic groups, A(1) and A(2), respectively. This cooperative linkage (redox Bohr effect) results in the translocation of 1 H(+)/oxidase molecule upon oxido-reduction of heme a/Cu(A) and heme a(3)/Cu(B), respectively. Results on liposome-reconstituted oxidase show that upon oxidation of heme a/Cu(A) and heme a(3)/Cu(B) protons from A(1) and A(2) are released in the outer aqueous phase. A(1) but not A(2) appears to take up protons from the inner aqueous space upon reduction of the respective redox center. A cooperative model is presented in which the A(1) and A(2) clusters, operating in close sequence, constitute together the gate of the proton pump in cytochrome c oxidase.

Protonmotive cooperativity in cytochrome c oxidase

Cooperative linkage of solute binding at separate binding sites in allosteric proteins is an important functional attribute of soluble and membrane bound hemoproteins. Analysis of proton/electron coupling at the four redox centers, i.e. Cu(A), heme a, heme a(3) and Cu(B), in the purified bovine cytochrome c oxidase in the unliganded, CO-liganded and CN-liganded states is presented. These studies are based on direct measurement of scalar proton translocation associated with oxido-reduction of the metal centers and pH dependence of the midpoint potential of the redox centers. Heme a (and Cu(A)) exhibits a cooperative proton/electron linkage (Bohr effect). Bohr effect seems also to be associated with the oxygen-reduction chemistry at the heme a(3)-Cu(B) binuclear center. Data on electron transfer in cytochrome c oxidase are also presented, which, together with structural data, provide evidence showing the occurrence of direct electron transfer from Cu(A) to the binuclear center in addition to electron transfer via heme a. A survey of structural and functional data showing the essential role of cooperative proton/electron linkage at heme a in the proton pump of cytochrome c oxidase is presented. On the basis of this and related functional and structural information, variants for cooperative mechanisms in the proton pump of the oxidase are examined.

Thermodynamic and choreographic constraints for energy transduction by cytochrome c oxidase

Cooperative effects are fundamental for electroprotonic energy transduction processes, crucial to sustain much of life chemistry. However, the primary cooperative mechanism by which transmembrane proteins couple the downhill transfer of electrons to the uphill activation (acidification) of protic groups is still a matter of great controversy. To understand cooperative processes fully, it is necessary to obtain the microscopic thermodynamic parameters of the functional centres and relate them to the relevant structural features, a task difficult to achieve for large proteins. The approach discussed here explores how this may be done by extrapolation from mechanisms used by simpler proteins operative in similar processes. The detailed study of small, soluble cytochromes performing electroprotonic activation has shown how they use anti-electrostatic effects to control the synchronous movement of charges. These include negative e(-)/H(+) (redox-Bohr effect) cooperativities. This capacity is the basis to discuss an unorthodox mechanism consistent with the available experimental data on the process of electroprotonic energy transduction performed by cytochrome c oxidase (CcO).

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.

The quinol:fumarate oxidoreductase from the sulphate reducing bacterium Desulfovibrio gigas: spectroscopic and redox studies

The membrane bound fumarate reductase (FRD) from the sulphate-reducer Desulfovibrio gigas was purified from cells grown on a fumarate/sulphate medium and extensively characterized. The FRD is isolated with three subunits of apparent molecular masses of 71, 31, and 22 kDa. The enzyme is capable of both fumarate reduction and succinate oxidation, exhibiting a higher specificity toward fumarate (Km for fumarate is 0.42 and for succinate 2 mM) and a reduction rate 30 times faster than that for oxidation. Studies by Visible and EPR spectroscopies allowed the identification of two B-type haems and the three iron-sulplur clusters usually found in FRDs and succinate dehydrogenases: [2Fe-2S]2+/1+ (S1), [4Fe-4S]2+/1+ (S2), and [3Fe-4S]1+/0 (S3). The apparent macroscopic reduction potentials for the metal centers, at pH 7.6, were determined by redox titrations: -45 and -175 mV for the two haems, and +20 and -140 mV for the S3 and SI clusters, respectively. The reduction potentials of the haem groups are pH dependent, supporting the proposal that fumarate reduction is associated with formation of the membrane proton gradient. Furthermore, co-reconstitution in liposomes of D. gigas FRD, duroquinone, and D. gigas cytochrome bd shows that this system is capable of coupling succinate oxidation with oxygen reduction to water.

Conformational component in the coupled transfer of multiple electrons and protons in a monomeric tetraheme cytochrome

Cell metabolism relies on energy transduction usually performed by complex membrane-spanning proteins that couple different chemical processes, e.g. electron and proton transfer in proton-pumps. There is great interest in determining at the molecular level the structural details that control these energy transduction events, particularly those involving multiple electrons and protons, because tight control is required to avoid the production of dangerous reactive intermediates. Tetraheme cytochrome c(3) is a small soluble and monomeric protein that performs a central step in the bioenergetic metabolism of sulfate reducing bacteria, termed "proton-thrusting," linking the oxidation of molecular hydrogen with the reduction of sulfate. The mechano-chemical coupling involved in the transfer of multiple electrons and protons in cytochrome c(3) from Desulfovibrio desulfuricans ATCC 27774 is described using results derived from the microscopic thermodynamic characterization of the redox and acid-base centers involved, crystallographic studies in the oxidized and reduced states of the cytochrome, and theoretical studies of the redox and acid-base transitions. This proton-assisted two-electron step involves very small, localized structural changes that are sufficient to generate the complex network of functional cooperativities leading to energy transduction, while using molecular mechanisms distinct from those established for other Desulfovibrio sp. cytochromes from the same structural family.

A cooperative model for protonmotive heme-copper oxidases. The role of heme a in the proton pump of cytochrome c oxidase

Oxido-reductions of metal centers in cytochrome c oxidase are linked to pK shifts of acidic groups in the enzyme (redox Bohr effects). The linkage at heme a results in proton uptake from the inner space upon reduction and proton release in the external space upon oxidation of the metal. The relationship of this process to the features of the proton pump in cytochrome c oxidase and its atomic structure revealed by X-ray crystallography to 2.8-2.3 A resolution is examined. A mechanism for the proton pump of cytochrome c oxidase, based on cooperative coupling at heme a, is proposed.

Cooperative coupling and role of heme a in the proton pump of heme-copper oxidases

In the last few years, evidence has accumulated supporting the applicability of the cooperative model of proton pumps in cytochrome systems, vectorial Bohr mechanisms, to heme-copper oxidases. The vectorial Bohr mechanism is based on short- and long-range protonmotive cooperative effects linked to redox transitions of the metal centers. The crystal structure of oxidized and reduced bovine-heart cytochrome c oxidase reveals, upon reduction, the occurrence of long-range conformational changes in subunit I of the oxidase. Analysis of the crystal structure of cytochrome c oxidase shows the existence of hydrogen-bonded networks of amino acid residues which could undergo redox-linked pK shifts resulting in transmembrane proton translocation. Our group has identified four proteolytic groups undergoing reversible redox-linked pK shifts. Two groups result in being linked to redox transitions of heme a3. One group is apparently linked to CuB. The fourth group is linked to oxido-reduction of heme a. We have shown that the proton transfer resulting from the redox Bohr effects linked to heme a and CuB in the bovine oxidase displays membrane vectorial asymmetry, i.e., protons are taken up from the inner aqueous space (N), upon reduction, and released in the external space (P), upon oxidation of the metals. This direction of proton uptake and release is just what is expected from the vectorial Bohr mechanism. The group linked to heme a, which can transfer up to 0.9 H+/e- at pHs around neutrality, can provide the major contribution to the proton pump. It is proposed that translocation of pumped protons, linked to electron flow through heme a, utilizes a channel (channel D) which extends from a conserved aspartate at the N entrance to a conserved glutamate located between heme a and the binuclear center. The carboxylic group of this glutamic acid, after having delivered, upon electron flow through heme a, pumped protons towards the P phase, once reprotonated from the N phase, moves to deliver, subsequently, to the binuclear center chemical protons consumed in the conversion of the peroxy to ferryl and of the latter to the oxy intermediate in the redox cycle. Site-directed mutagenesis of protolytic residues in subunit I of the aa3-600 quinol oxidase of Bacillus subtilis to non-polar residues revealed that the conserved Lys 304 is critical for the proton pumping activity of the oxidase. Crystal structures of cytochrome c oxidase show that this lysine is at the N entrance of a channel which translocates the protons consumed for the production of the peroxy intermediate. Inhibition of this pathway, by replacement of the lysine, short-circuits protons from channel D to the binuclear center, where they are utilized in the chemistry of oxygen reduction.

Vectorial nature of redox Bohr effects in bovine heart cytochrome c oxidase

The vectorial nature of redox Bohr effects (redox-linked pK shifts) in cytochrome c oxidase from bovine heart incorporated in liposomes has been analyzed. The Bohr effects linked to oxido-reduction of heme a and CuB display membrane vectorial asymmetry. This provides evidence for involvement of redox Bohr effects in the proton pump of the oxidase.

Redox-linked protolytic reactions in soluble cytochrome-c oxidase from beef-heart mitochondria: redox Bohr effects

A study is presented of co-operative redox-linked protolytic reactions (redox Bohr effects) in soluble cytochrome-c oxidase purified from bovine-heart mitochondria. Bohr effects were analyzed by direct measurement, with accurate spectrophotometric and potentiometric methods, of H+ uptake and release by the oxidase associated with reduction and oxidation of hemes a and a3. CuA and CuB in the unliganded and in the CN- or CO-liganded enzyme. The results show that there are in the bovine oxidase four protolytic groups undergoing reversible pK shifts upon oxido-reduction of the electron transfer metals. Two groups with pKox and pKred values around 7 and > 12 respectively appear to be linked to redox transitions of heme a3. One group with pKox and pKred around 6 and 7 is apparently linked to CuB, a fourth one with pKox and pKred of 6 and 9 appears to be linked to heme a. The possible nature of the amino acids involved in the redox Bohr effects and their role in H+ translocation is 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.

Peroxide-induced spectral perturbations of the 280-nm absorption band of cytochrome c oxidase

It is now widely believed that the first two electrons transferred to the dioxygen reduction site in cytochrome c oxidase (CcO) are not coupled to proton translocation. The activation of the pump cycle correlates with the binding of dioxygen to the binuclear center. In order to investigate conformational changes in CcO associated with the formation of dioxygen intermediates during the catalytic cycle of CcO, the effects of hydrogen peroxide binding to CcO have been examined using UV optical absorption and second derivative techniques. Our data indicates that in the presence low concentrations of H2O2 (2:1 molar ratio) an initial CcO-peroxide species is formed in which the 280-nm absorption band is red shifted. This red shift occurs prior to spectral changes associated with H2O2 binding to cytochrome a3. Upon addition of higher concentrations of H2O2 (> 10 equivalents of H2O2 per equivalent of CcO) oxidized CcO is converted to F-state enzyme with no corresponding shift at 280 nm. It is suggested that H2O2 initially binds to CuB2+ resulting in a conformational change in the enzyme giving rise to a red-shifted 280 nm band. The absence of any conformational changes in F-state enzyme is consistent with the lack of bridging interactions with CuB2+ in this intermediate.

The cytochrome chain of mitochondria exhibits variable H+/e- stoichiometry

A study is presented of the ----H+/e- stoichiometry for H+ pumping by the cytochrome chain in isolated rat liver mitochondria under level-flow and steady-state conditions. It is shown that the ----H+/e- stoichiometry for the cytochrome chain varies under the influence of the flow rate and transmembrane delta microH+. The rate-dependence is shown to be associated with cytochrome c oxidase, whose ----H+/e- ratio varies from 0 to 1, whilst the ----H+/e- ratio for the span covered by cytochrome c reductase is invariably 2.

H+/e- stoichiometry of mitochondrial cytochrome complexes reconstituted in liposomes. Rate-dependent changes of the stoichiometry in the cytochrome c oxidase vesicles

The H+/e- stoichiometry of protonmotive cytochrome c oxidase, isolated from bovine heart mitochondria and reconstituted in liposomes, has been determined by making use of direct spectrophotometric measurements of the initial rates of e- flow and H+ translocation. It is shown that the ----H+/e- ratio for redox-linked proton ejection by the oxidase varies from around 0 to a maximum of 1 as a function of the rate of overall electron flow in the complex.

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.

Role of supernumerary subunits in mitochondrial cytochrome c oxidase

Role of supernumerary subunits of bovine heart cytochrome c oxidase has been investigated by examining the influence on the enzymatic activity of their removal by chromatographic procedures or controlled digestion by trypsin. Is has been shown that partial proteolytic cleavage of subunit IV results in depression of respiratory activity and of redox-linked proton translocation. Selective removal by gel-filtration of subunit Vlb has no significant influence on the redox and protonmotive activity of the oxidase.

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.

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.

Redox-linked proton translocation in cytochrome oxidase: the importance of gating electron flow. The effects of slip in a model transducer

In at least one component of the mitochondrial respiratory chain, cytochrome c oxidase, exothermic electron transfer reactions are used to drive vectorial proton transport against an electrochemical hydrogen ion gradient across the mitochondrial inner membrane. The role of the gating of electrons (the regulation of the rates of electron transfer into and out of the proton transport site) in this coupling between electron transfer and proton pumping has been explored. The approach involves the solution of the steady-state rate equations pertinent to proton pump models which include, to various degrees, the uncoupled (i.e., not linked to proton pumping) electron transfer processes which are likely to occur in any real electron transfer-driven proton pump. This analysis furnishes a quantitative framework for examining the effects of variations in proton binding site pKas and metal center reduction potentials, the relationship between energy conservation efficiency and turnover rate, the conditions for maximum power output or minimum heat production, and required efficiency of the gating of electrons. Some novel conclusions emerge from the analysis, including: An efficient electron transfer-driven proton pump need not exhibit a pH-dependent reduction potential; Very efficient gating of electrons is required for efficient electron transfer driven proton pumping, especially when a reasonable correlation of electron transfer rate and electron transfer exoergonicity is assumed; and A consideration of the importance and possible mechanisms of the gating of electrons suggests that efficient proton pumping by CuA in cytochrome oxidase could, in principle, take place with structural changes confined to the immediate vicinity of the copper ion, while proton pumping by Fea would probably require conformational coupling between the iron and more remote structures in the enzyme. The conclusions are discussed with reference to proton pumping by cytochrome c oxidase, and some possible implications for oxidative phosphorylation are noted.

The efficiencies of the component steps of oxidative phosphorylation. I. A simple steady state theory

Most earlier theoretical work on oxidative phosphorylation has emphasized the application of the formalism of nonequilibrium thermodynamics to the overall process. The resultant mathematical development and interpretation of some experimental data is complicated somewhat by the necessity of treating a system which is incompletely coupled (degree of coupling, q less than 1). Here a simple alternative approach is proposed which can be applied to many studies in the field. In this approach the overall process is broken up into sequential steps so that the product of the efficiencies of the steps is equal to the efficiency of the overall process. Steps of interest for which the degree of coupling may be quite close to unity can be "isolated" by this procedure. This approach results in a simple mathematical formalism emphasizing the power use (or energy use) at each step of the energy transduction process. The efficiencies of the steps of the process can be experimentally evaluated as is shown in the accompanying paper (B.D. Jensen, K. K. Gunter, and T. E. Gunter, 1986, Arch. Biochem. Biophys. 248, 305-323) where measurements are performed as dictated by the assumptions of the current theory. This alternative approach simplifies the analysis of changes induced in the process of oxidative phosphorylation as a result of agents added to the system or of changes in conditions. The locus (or loci) of such changes becomes rapidly apparent if the data is treated as suggested. Furthermore, the mathematical formalism lends itself both to the development of expressions and new experimental approaches which minimize the effects of a decrease in a value of q below unity and also to optimal statistical treatment of the data. As a concrete example of the use of this approach we reinvestigate the question of the equivalence of use of energy from the pH gradient and of the membrane potential in phosphorylation.

Organization and function of cytochrome b and ubiquinone in the cristae membrane of beef heart mitochondria

The arrangement and function of the redox centers of the mammalian bc1 complex is described on the basis of structural data derived from amino acid sequence studies and secondary structure predictions and on the basis of functional studies (i.e., EPR data, inhibitor studies, and kinetic experiments). Two ubiquinone reaction centers do exist--a QH2 oxidation center situated at the outer, cytosolic surface of the cristae membrane (Q0 center), and a Q reduction center (Qi center) situated more to the inner surface of the cristae membrane. The Q0 center is formed by the b-566 domain of cytochrome b, the FeS protein, and maybe an additional small subunit, whereas the Qi center is formed by the b-562 domain of cytochrome b and presumably the 13.4 kDa protein ("QP-C"). The "Q binding proteins" are proposed to be protein subunits of the Q reaction centers of various multiprotein complexes. The path of electron flow branches at the Q0 center, half of the electrons flowing via the high-potential cytochrome chain to oxygen and half of the electrons cycling back into the Q pool via the cytochrome b path connecting the two Q reaction centers. During oxidation of QH2, 2H+ are released to the cytosolic space and during reduction of Q, 2H+ are taken up from the matrix side, resulting in a net transport across the membrane of 2H+ per e- flown from QH2 to cytochrome c, the H+ being transported across the membrane as H (H+ + e-) by the mobile carrier Q. The authors correct their earlier view of cytochrome b functioning as a H+ pump, proposing that the redox-linked pK changes of the acidic groups of cytochrome b are involved in the protonation/deprotonation processes taking place during the reduction and oxidation of Q. The reviewers stress that cytochrome b is in equilibrium with the Q pool via the Qi center, but not via the Q0 center. Their view of the mechanisms taking place at the reductase is a Q cycle linked to a Q-pool where cytochrome b is acting as an electron pump.

Is there sufficient experimental evidence to consider the mitochondrial cytochrome bc1 complex a proton pump? Probably no

The electron flow through the cytochrome bc1 complex of the mitochondrial respiratory chain is accompanied by vectorial proton translocation, though the mechanism of the latter phenomenon has not yet been clarified. Several proposed hypotheses are briefly presented and discussed here. Recently, a number of papers have appeared claiming the existence of a proton pump in the enzyme mainly on the basis of the interaction of the complex with N,N'-dicyclohexylcarbodiimide. These data are reviewed here with the aim of showing their ability to fit multiple interpretations. This together with some other arguments leads to the conclusion that a proton pump in the mitochondrial bc1 complex has not yet been demonstrated.

Location of ubiquinone-10 (CoQ-10) in phospholipid vesicles

Egg yolk phosphatidylcholine (PC) liposomes were prepared by ultrasonic irradiation. At least 25 mol% of coenzyme Q-10 (CoQ-10) can be incorporated nonstoichiometrically into PC liposomes. Electron microscopy showed no visible influence of CoQ-10 on the membrane structure. Nuclear magnetic resonance spectra of sonicated PC liposomes containing CoQ-10 showed two peaks (3.82 and 3.98 ppm) due to CoQ-10 methoxyl protons and a 'high-field component' (1.52 and 1.58 ppm). The areas of these peaks were inversely related and influenced by the time of ultrasonic irradiation. After short sonication the low-field positions (3.98/1.58 ppm) are favoured, after long sonication the high-field positions (3.82/1.52 ppm). No gradual shift of the two peaks is observed. The 'critical' liposome diameter was found to be between 500 to 700 A. Lanthanide induced pseudocontact shift on CoQ-10 resonances ('high-field component' and methoxyl) does not lead to a split of the peaks and the difference between them remains constant. It is concluded that CoQ-10 is incorporated into the membrane core, beyond C-2 of the PC acyl chains, with two bilayer curvature-dependent resonance positions.

Purification of cytochrome b from complex III of beef heart mitochondria

Two cytochrome b preparations have been prepared from Complex III of beef heart mitochondria, by detergent-exchange chromatography on a butyl-Toyopearl column. One was eluted from the column with buffer containing Tween 20 after most of other subunits of Complex III were eluted with buffer containing guanidine-HCl, and the other was eluted from the column with buffer containing sodium dodecyl sulfate. The former is consisted of a single polypeptide (subunit III) and contained 37.5 nmol of heme b/mg of protein, and the latter consisted of subunits III and IX and contained 19.5 nmol of heme b/mg of protein. The former was labile when it was reduced by dithionite, whereas the latter was stable. Subunit IX in the latter is associated with cytochrome b even after gel filtration and density gradient centrifugation. These results suggest that subunit IX plays a role in stabilizing cytochrome b.

Effect of papain digestion on redox-linked proton translocation in b-c1 complex from beef heart reconstituted into liposomes

Papain treatment of the cytochrome b-c1 complex from beef heart results in partial proteolysis of core protein II, the iron-sulphur protein and the 15-kDa subunit. Under these conditions a significant inhibition of electron flow and complete suppression of proton translocation in the complex reconstituted into liposomes are observed. Kinetic experiments indicate a correlation between the digestion of core protein II and 15-kDa subunit and the suppression of proton translocation. The results suggest an active involvement of polypeptides of the complex in stabilizing the semiquinone species and/or providing pathways to exchange protons between bound quinone systems and aqueous phases.

Effects of Stevia rebaudiana natural products on rat liver mitochondria

The effects of several natural products extracted from the leaves of Stevia rebaudiana on rat liver mitochondria were investigated. The compounds used were stevioside (a non-caloric sweetener), steviolbioside, isosteviol and steviol. Total aqueous extracts of the leaves were also investigated. S. rebaudiana natural products inhibited oxidative phosphorylation, ATPase activity NADH-oxidase activity, succinate-oxidase activity, succinate dehydrogenase, and L-glutamate dehydrogenase. The ADP/O ratio was decreased. Substrate respiration (state II respiration) was increased at low concentrations (up to 0.5 mM) and inhibited at higher concentrations (1 mM or more). In uncoupled mitochondria, inhibition of substrate respiration was the only effect observed. Net proton ejection induced by succinate and swelling induced by several substrates were inhibited. Of the compounds investigated, the sweet principle stevioside was less active. It was concluded that, in addition to the inhibitory effects, S. rebaudiana natural products may also act as uncouplers of oxidative phosphorylation. The possible physiologic consequences of the ingestion of stevioside and S. rebaudiana aqueous extracts are discussed.