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

Modulation of the electron-proton coupling at cytochrome a by the ligation of the oxidized catalytic center in bovine cytochrome c oxidase

Cytochrome a was suggested as the key redox center in the proton pumping process of bovine cytochrome c oxidase (CcO). Recent studies showed that both the structure of heme a and its immediate vicinity are sensitive to the ligation and the redox state of the distant catalytic center composed of iron of cytochrome a₃ (Fe_a3) and copper (Cu_B). Here, the influence of the ligation at the oxidized Fe_a3³⁺-Cu_B²⁺ center on the electron-proton coupling at heme a was examined in the wide pH range (6.5-11). The strength of the coupling was evaluated by the determination of pH dependence of the midpoint potential of heme a (E_m(a)) for the cyanide (the low-spin Fe_a3³⁺) and the formate-ligated CcO (the high-spin Fe_a3³⁺). The measurements were performed under experimental conditions when other three redox centers of CcO are oxidized. Two slightly differing linear pH dependencies of E_m(a) were found for the CN- and the formate-ligated CcO with slopes of -13 mV/pH unit and -23 mV/pH unit, respectively. These linear dependencies indicate only a weak and unspecific electron-proton coupling at cytochrome a in both forms of CcO. The lack of the strong electron-proton coupling at the physiological pH values is also substantiated by the UV-Vis absorption and electron-paramagnetic resonance spectroscopy investigations of the cyanide-ligated oxidized CcO. It is shown that the ligand exchange at Fe_a³⁺ between His-Fe_a³⁺-His and His-Fe_a³⁺-OH^- occurs only at pH above 9.5 with the estimated pK >11.0.

Cytochrome c oxidase structures suggest a four-state stochastic pump mechanism

A simple stochastic model for a cytochrome c oxidase proton pump.

Oxygen Activation and Energy Conservation by Cytochrome c Oxidase

This review focuses on the type A cytochrome c oxidases (CcO), which are found in all mitochondria and also in several aerobic bacteria. CcO catalyzes the respiratory reduction of dioxygen (O₂) to water by an intriguing mechanism, the details of which are fairly well understood today as a result of research for over four decades. Perhaps even more intriguingly, the membrane-bound CcO couples the O₂ reduction chemistry to translocation of protons across the membrane, thus contributing to generation of the electrochemical proton gradient that is used to drive the synthesis of ATP as catalyzed by the rotary ATP synthase in the same membrane. After reviewing the structure of the core subunits of CcO, the active site, and the transfer paths of electrons, protons, oxygen, and water, we describe the states of the catalytic cycle and point out the few remaining uncertainties. Finally, we discuss the mechanism of proton translocation and the controversies in that area that still prevail.

Coupling between protonation and conformation in cytochrome c oxidase: Insights from constant-pH MD simulations

Cytochrome c oxidases (CcOs) are the terminal enzymes of the respiratory chain in mitochondria and most bacteria. These enzymes reduce dioxygen (O(2)) to water and, simultaneously, generate a transmembrane electrochemical proton gradient. Despite their importance in the aerobic metabolism and the large amount of structural and biochemical data available for the A1-type CcO family, there is still no consensually accepted description of the molecular mechanisms operating in this protein. A substantial number of questions about the CcO's working mechanism remain to be answered, including how the protonation behavior of some key residues is modulated during a reduction cycle and how is the conformation of the protein affected by protonation. The main objective of this work was to study the protonation-conformation coupling in CcOs and identify the molecular factors that control the protonation state of some key residues. In order to directly capture the interplay between protonation and conformational effects, we have performed constant-pH MD simulations of an A1-type CcO inserted into a lipid bilayer in two redox states (oxidized and reduced) at physiological pH. From the simulations, we were able to identify several groups with unusual titration behavior that are highly dependent on the protein redox state, including the A-propionate from heme a and the D-propionate from heme a3, two key groups possibly involved in proton pumping. The protonation state of these two groups is heavily influenced by subtle conformational changes in the protein (notably of R481(I) and R482(I)) and by small changes in the hydrogen bond network.

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.

Molecular mechanism of proton translocation by cytochrome c oxidase

Cytochrome c oxidase (CcO) is a terminal protein of the respiratory chain in eukaryotes and some bacteria. It catalyzes most of the biologic oxygen consumption on earth done by aerobic organisms. During the catalytic reaction, CcO reduces dioxygen to water and uses the energy released in this process to maintain the electrochemical proton gradient by functioning as a redox-linked proton pump. Even though the structures of several terminal oxidases are known, they are not sufficient in themselves to explain the molecular mechanism of proton pumping. Thus, additional extensive studies of CcO by varieties of biophysical and biochemical approaches are involved to shed light on the mechanism of proton translocation. In this review, we summarize the current level of knowledge about CcO, including the latest model developed to explain the CcO proton-pumping mechanism.

Redox properties of Thermus thermophilus ba3: different electron-proton coupling in oxygen reductases?

A comprehensive study of the thermodynamic redox behavior of the hemes of the ba3 enzyme from Thermus thermophilus, a B-type heme-copper oxygen reductase, is presented. This enzyme, in contrast to those having a single type of heme, allows the B- and A-type hemes to be monitored separately by visible spectroscopy and the reduction potential of each heme to be determined unequivocally. The relative order of the midpoint reduction potentials of each center changed in the pH range from 6 to 8.4, and both hemes present a significant redox-Bohr effect. For instance, at pH 7, the midpoint reduction potentials of the hemes B and A3 are 213 mV and 285 mV, respectively, whereas at pH 8.4, the order is reversed: 246 mV for heme B and 199 mV for heme A3. The existence of redox anticooperativity was established by introducing a redox interaction parameter in a model of pairwise interacting redox centers.

Energy diagrams and mechanism for proton pumping in cytochrome c oxidase

The powerful technique of energy diagrams has been used to analyze the mechanism for proton pumping in cytochrome c oxidase. Energy levels and barriers are derived starting out from recent kinetic experiments for the O to E transition, and are then refined using general criteria and a few additional experimental facts. Both allowed and non-allowed pathways are obtained in this way. A useful requirement is that the forward and backward rate should approach each other for the full membrane gradient. A key finding is that an electron on heme a (or the binuclear center) must have a significant lowering effect on the barrier for proton uptake, in order to prevent backflow from the pump-site to the N-side. While there is no structural gating in the present mechanism, there is thus an electronic gating provided by the electron on heme a. A quantitative analysis of the energy levels in the diagrams, leads to Prop-A of heme a(3) as the most likely position for the pump-site, and the Glu278 region as the place for the transition state for proton uptake. Variations of key redox potentials and pK(a) values during the pumping process are derived for comparison to experiments.

Control by cytochrome c oxidase of the cellular oxidative phosphorylation system depends on the mitochondrial energy state

Recent measurements of the flux control exerted by cytochrome c oxidase on the respiratory activity in intact cells have led to a re-appraisal of its regulatory function. We have further extended this in vivo study in the framework of the Metabolic Control Analysis and evaluated the impact of the mitochondrial transmembrane electrochemical potential (Deltamu(H+)) on the control strength of the oxidase. The results indicate that, under conditions mimicking the mitochondrial State 4 of respiration, both the flux control coefficient and the threshold value of cytochrome oxidase are modified with respect to the uncoupled condition. The results obtained are consistent with a model based on changes in the assembly state of the oxidative phosphorylation enzyme complexes and possible implications in the understanding of exercise-intolerance of human neuromuscular degenerative diseases are discussed.

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).

Structural elements involved in electron-coupled proton transfer in cytochrome c oxidase

Haem-copper oxidases are the last components of the respiratory chains in aerobic organisms. These membrane-bound enzymes energetically couple the electron transfer (eT) reactions associated with reduction of dioxygen to water, to proton pumping across the membrane. Even though the mechanism of proton pumping at the molecular level still remains to be uncovered, recent progress has presented us with the structural features of the pumping machinery and detailed information about the eT and proton-transfer reactions associated with the pumping process.

Redox-driven proton pumping by heme-copper oxidases

One of the key problems of molecular bioenergetics is the understanding of the function of redox-driven proton pumps on a molecular level. One such class of proton pumps are the heme-copper oxidases. These enzymes are integral membrane proteins in which proton translocation across the membrane is driven by electron transfer from a low-potential donor, such as, e.g. cytochrome c, to a high-potential acceptor, O(2). Proton pumping is associated with distinct exergonic reaction steps that involve gradual reduction of oxygen to water. During the process of O(2) reduction, unprotonated high pK(a) proton acceptors are created at the catalytic site. Initially, these proton acceptors become protonated as a result of intramolecular proton transfer from a residue(s) located in the membrane-spanning part of the enzyme, but removed from the catalytic site. This residue is then reprotonated from the bulk solution. In cytochrome c oxidase from Rhodobacter sphaeroides, the proton is initially transferred from a glutamate, E(I-286), which has an apparent pK(a) of 9.4. According to a recently published structure of the enzyme, the deprotonation of E(I-286) is likely to result in minor structural changes that propagate to protonatable groups on the proton output (positive) side of the protein. We propose that in this way, the free energy available from the O(2) reduction is conserved during the proton transfer. On the basis of the observation of these structural changes, a possible proton-pumping model is presented in this paper. Initially, the structural changes associated with deprotonation of E(I-286) result in the transfer of a proton to an acceptor for pumped protons from the input (negative) side of the membrane. After reprotonation of E(I-286) this acceptor releases a proton to the output side of the membrane.

Proton transfer reactions associated with the reaction of the fully reduced, purified cytochrome C oxidase with molecular oxygen and ferricyanide

A study is presented on proton transfer associated with the reaction of the fully reduced, purified bovine heart cytochrome c oxidase with molecular oxygen or ferricyanide. The proton consumption associated with aerobic oxidation of the four metal centers changed significantly with pH going from approximately 3.0 H(+)/COX at pH 6.2-6.3 to approximately 1.2 H(+)/COX at pH 8.0-8.5. Rereduction of the metal centers was associated with further proton uptake which increased with pH from approximately 1.0 H(+)/COX at pH 6.2-6.3 to approximately 2.8 H(+)/COX at pH 8.0-8.5. Anaerobic oxidation of the four metal centers by ferricyanide resulted in the net release of 1.3-1.6 H(+)/COX in the pH range 6.2-8.2, which were taken up by the enzyme on rereduction of the metal centers. The proton transfer elicited by ferricyanide represents the net result of deprotonation/protonation reactions linked to anaerobic oxidoreduction of the metal centers. Correction for the ferricyanide-induced pH changes of the proton uptake observed in the oxidation and rereduction phase of the reaction of the reduced oxidase with oxygen gave a measure of the proton consumption in the reduction of O(2) to 2H(2)O. The results show that the expected stoichiometric proton consumption of 4H(+) in the reduction of O(2) to 2H(2)O is differently associated, depending on the actual pH, with the oxidation and reduction phase of COX. Two H(+)/COX are initially taken up in the reduction of O(2) to two OH(-) groups bound to the binuclear Fe a(3)-Cu(B) center. At acidic pHs the third and fourth protons are also taken up in the oxidative phase with formation of 2H(2)O. At alkaline pHs the third and fourth protons are taken up with formation of 2H(2)O only upon rereduction of COX.

A mechano-chemical model for energy transduction in cytochrome c oxidase: the work of a Maxwell's god

Cytochrome c3 has a central role in the energetics of Desulfovibrio sp., where it performs an electroprotonic energy transduction step. This process uses a network of cooperativities, largely based on anti-Coulomb components, resulting from a mechano-chemical energy coupling mechanism. This mechanism provides a model coherent with the data available for the redox chemistry of haem a of cytochrome c oxidase and its link to the activation of protons. A crucial feature of the model is an anti-Coulomb effect that sets the stage for a molecular ratchet, ensuring vectoriality for the redox-driven localised movement of protons across the membrane, against an electrochemical gradient.

Radicals associated with the catalytic intermediates of bovine cytochrome c oxidase

Two radicals have been detected previously by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies in bovine cytochrome oxidase after reaction with hydrogen peroxide, but no correlation could be made with predicted levels of optically detectable intermediates (P(M), F and F(z.rad;)) that are formed. This work has been extended by optical quantitation of intermediates in the EPR/ENDOR sample tubes, and by comparison with an analysis of intermediates formed by reaction with carbon monoxide in the presence of oxygen. The narrow radical, attributed previously to a porphyrin cation, is detectable at low levels even in untreated oxidase and increases with hydrogen peroxide treatments generally. It is presumed to arise from a side-reaction unrelated to the catalytic intermediates. The broad radical, attributed previously to a tryptophan radical, is observed only in samples with a significant level of F(z.rad;) but when F(z.rad;) is generated with hydrogen peroxide, is always accompanied by the narrow radical. When P(M) is produced at high pH with CO/O(2), no EPR-detectable radicals are formed. Conversion of the CO/O(2)-generated P(M) into F(z.rad;) when pH is lowered is accompanied by the appearance of a broad radical whose ENDOR spectrum corresponds to a tryptophan cation. Quantitation of its EPR intensity indicates that it is around 3% of the level of F(z.rad;) determined optically. It is concluded that low pH causes a change of protonation pattern in P(M) which induces partial electron redistribution and tryptophan cation radical formation in F(z.rad;). These protonation changes may mimic a key step of the proton translocation process.

Structures and proton-pumping strategies of mitochondrial respiratory enzymes

Enzymes of the mitochondrial respiratory chain serve as proton pumps, using the energy made available from electron transfer reactions to transport protons across the inner mitochondrial membrane and create an electrochemical gradient used for the production of ATP. The ATP synthase enzyme is reversible and can also serve as a proton pump by coupling ATP hydrolysis to proton translocation. Each of the respiratory enzymes uses a different strategy for performing proton pumping. In this work, the strategies are described and the structural bases for the action of these proteins are discussed in light of recent crystal structures of several respiratory enzymes. The mechanisms and efficiency of proton translocation are also analyzed in terms of the thermodynamics of the substrate transformations catalyzed by these enzymes.

Proton linkage of cytochrome a oxidoreduction in carbon monoxide-treated cytochrome c oxidase

Oxidoreduction of the low spin haem a of cytochrome c oxidase was recently reported to be coupled to release/uptake of nearly one proton from/to the enzyme at pH 7.5 in the presence of CO to block oxidoreduction of the binuclear haem a3/CuB centre (N. Capitanio et al., Biochim. Biophys. Acta, 1318 (1997) 255-265). This is difficult to reconcile with earlier findings from several laboratories that the pH-dependence of the Em of haem a is ca. 10 mV/pH unit over a wide pH range in such conditions, which implies redox coupling of only ca. 0.17 H+/e-. In order to resolve this discrepancy, we have performed careful measurements of proton release coupled to oxidation of haem a and CuA in CO-inhibited cytochrome aa3 from bovine heart mitochondria. We find that oxidation of these centres by ferricyanide leads to release of a total of 0.20 protons per enzyme molecule at pH 7.7, increasing to 0.43 protons at pH 6.6, far short of a full 1 H+/e-. Using vesicles reconstituted with cytochrome c oxidase, we also found that all this proton release occurs towards the outside of the vesicles. The observed dependence can be explained by a model in which oxidoreduction of haem a is coupled to uptake and release of ca. 0.17 H+/e-, while oxidoreduction of CuA is linked to a protonatable group which has a pKa of 6.2 when CuA is in the reduced state. In agreement with existing data, this model predicts that the Em of CuA will only be slightly pH dependent in the pH range of these measurements.

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.

Factors determining electron-transfer rates in cytochrome c oxidase: investigation of the oxygen reaction in the R. sphaeroides enzyme

We have investigated the kinetics of the single-turnover reaction of fully reduced solubilised cytochrome c oxidase (cytochrome aa3) from Rhodobacter sphaeroides with dioxygen using the flow-flash methodology and compared the results to those obtained with the well-characterised bovine mitochondrial enzyme. The overall reaction sequence was the same in the two enzymes, but the extents and rates of the electron-transfer reactions differed, implying differences in redox potentials, and/or interaction energies between electrons and protons during oxygen reduction. As with the bovine enzyme, the R. sphaeroides enzyme displayed two major kinetic phases of proton uptake with rate constants of approximately 5000 s-1 and approximately 500 s-1 at pH 7.9, concomitant with the peroxy to oxoferryl and oxoferryl to oxidised states. The net number of protons taken up in the R. sphaeroides enzyme was about approximately 1.9, which implies that upon reduction, the enzyme has to pick up approximately 2.1 H+ from the medium. On the basis of the comparison of electron-transfer reactions in the two enzymes, we conclude that the transfer rate of the fourth electron to the binuclear centre is not only determined by the electron-transfer rate from haem a to the binuclear centre, but also by the electron equilibrium between CuA and haem a. In addition, in contrast to the bovine enzyme, where the electron- and proton-transfer rates during oxidation of the fully reduced enzyme by O2 are all faster than the overall turnover rate, in the R. sphaeroides enzyme, the slowest kinetic phase was rate limiting for the overall turnover. Moreover, the comparison of the reactions in the two systems shows that in the R. sphaeroides enzyme, the electrons are more evenly distributed among the redox centres during oxygen reduction. This enables investigations of effects also of minor perturbations on, e.g., the electron-transfer characteristics in mutant enzymes, for which this study forms the basis.

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.

Cytochrome c oxidase: structure and spectroscopy

Cytochrome c oxidase, the terminal enzyme of the respiratory chains of mitochondria and aerobic bacteria, catalyzes electron transfer from cytochrome c to molecular oxygen, reducing the latter to water. Electron transfer is coupled to proton translocation across the membrane, resulting in a proton and charge gradient that is then employed by the F0F1-ATPase to synthesize ATP. Over the last years, substantial progress has been made in our understanding of the structure and function of this enzyme. Spectroscopic techniques such as EPR, absorbance and resonance Raman spectroscopy, in combination with site-directed mutagenesis work, have been successfully applied to elucidate the nature of the cofactors and their ligands, to identify key residues involved in proton transfer, and to gain insight into the catalytic cycle and the structures of its intermediates. Recently, the crystal structures of a bacterial and a mitochondrial cytochrome c oxidase have been determined. In this review, we provide an overview of the crystal structures, summarize recent spectroscopic work, and combine structural and spectroscopic data in discussing mechanistic aspects of the enzyme. For the latter, we focus on the structure of the oxygen intermediates, proton-transfer pathways, and the much-debated issue of how electron transfer in the enzyme might be coupled to proton translocation.

Redox-coupled crystal structural changes in bovine heart cytochrome c oxidase

Crystal structures of bovine heart cytochrome c oxidase in the fully oxidized, fully reduced, azide-bound, and carbon monoxide-bound states were determined at 2.30, 2.35, 2.9, and 2.8 angstrom resolution, respectively. An aspartate residue apart from the O2 reduction site exchanges its effective accessibility to the matrix aqueous phase for one to the cytosolic phase concomitantly with a significant decrease in the pK of its carboxyl group, on reduction of the metal sites. The movement indicates the aspartate as the proton pumping site. A tyrosine acidified by a covalently linked imidazole nitrogen is a possible proton donor for the O2 reduction by the enzyme.

The generation of proton electrochemical potential gradient by cytochrome c oxidase

Cytochrome c oxidase, the terminal oxidase of mitochondria and some bacteria, catalyzes the four electron reduction of oxygen, and generates a proton electrochemical potential gradient (Delta microH). The recently determined structures of the bacterial and the bovine enzymes, together with studies of site directed mutants of a bacterial cytochrome c oxidase and a closely related ubiquinol oxidase, have greatly advanced our understanding of the mechanism by which oxygen reduction is coupled to the generation of Delta microH. Two different mechanisms contribute to the generation of Delta microH: protons that are consumed by the reduction of oxygen, are taken exclusively from the mitochondrial matrix ('consumed' protons), while other protons are translocated by the enzyme across the membrane ('pumped' protons). It is suggested that both proton consumption and proton pumping are driven by the electrostatic charging of the enzyme reaction center by the reducing electrons. Proton consumption is suggested to result from the electrostatically driven ejection of hydroxyls into the matrix that is catalyzed by a tyrosine residue in the reaction center. Proton pumping is suggested to result from the electrostatically driven translocation of a glutamate residue near the reaction center, and is assisted by secondary acceptors that release the translocated protons.

Cytochrome c oxidase as a proton-pumping peroxidase: reaction cycle and electrogenic mechanism

Cytochrome oxidase (COX) is considered to integrate in a single enzyme two consecutive mechanistically different redox activities--oxidase and peroxidase--that can be catalyzed elsewhere by separate hemoproteins. From the viewpoint of energy transduction, the enzyme is essentially a proton pumping peroxidase with a built-in auxiliary eu-oxidase module that activates oxygen and prepares in situ H2O2, a thermodynamically efficient but potentially hazardous electron acceptor for the proton pumping peroxidase. The eu-oxidase and peroxidase phases of the catalytic cycle may be performed by different structural states of COX. Resolution of the proton pumping peroxidase activity of COX and identification of individual charge translocation steps inherent in this reaction are discussed, as well as the specific role of the two input proton channels in proton translocation.

Protonmotive mechanism of heme-copper oxidases

The mechanism of coupling of proton and electron transfer in oxidases is reviewed and related to the structural information that is now available. A "glutamate trap" mechanism for proton/electron coupling is described.

The coupling of electron transfer and proton translocation: electrostatic calculations on Paracoccus denitrificans cytochrome c oxidase

We have calculated the electrostatic potential and interaction energies of ionizable groups and analyzed the response of the protein environment to redox changes in Paracoccus denitrificans cytochrome c oxidase by using a continuum dielectric model and finite difference technique. Subsequent Monte Carlo sampling of protonation states enabled us to calculate the titration curves of all protonatable groups in the enzyme complex. Inclusion of a model membrane allowed us to restrict the calculations to the functionally essential subunits I and II. Some residues were calculated to have complex titration curves, as a result of strong electrostatic coupling, desolvation, and dipolar interactions. Around the heme a3-CuB binuclear center, we have identified a cluster of 18 strongly interacting residues that account for most of the proton uptake linked to electron transfer. This was calculated to be between 0.7 and 1.1 H+ per electron, depending on the redox transition considered. A hydroxide ion bound to CuB was determined to become protonated to form water upon transfer of the first electron to the binuclear site. The bulk of the protonation changes linked to further reduction of the heme a3-CuB center was calculated to be due to proton uptake by the interacting cluster and Glu(II-78). Upon formation of the three-electron reduced state (P1), His325, modeled in an alternative orientation away from CuB, was determined to become protonated. The agreement of these results with experiment and their relevance in the light of possible mechanisms of redox-coupled proton transfer are discussed.

The role of electrostatic interactions for cytochrome c oxidase function

In recent years, the enormous increase in high-resolution three-dimensional structures of proteins together with the development of powerful theoretical techniques have provided the basis for a more detailed examination of the role of electrostatics in determining the midpoint potentials of redox-active metal centers and in influencing the protonation behavior of titratable groups in proteins. Based on the coordinates of the Paracoccus denitrificans cytochrome c oxidase, we have determined the electrostatic potential in and around the protein, calculated the titration curves for all ionizable residues in the protein, and analyzed the response of the protein environment to redox changes at the metal centers. The results of this study provide insight into how charged groups can be stabilized within a low-dielectric environment and how the range of their electrostatic effects can be modulated by the protein. A cluster of 18 titratable groups around the heme a3-CuB binuclear center, including a hydroxide ion bound to the copper, was identified that accounts for most of the proton uptake associated with redox changes at the binuclear site. Predicted changes in net protonation were in reasonable agreement with experimentally determined values. The relevance of these findings in the light of possible mechanisms of redox-coupled proton movement is discussed.

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.