Single-electron reduction of the oxidized state is coupled to proton uptake via the K pathway in Paracoccus denitrificans cytochrome c oxidase

The timing of proton migration in membrane-reconstituted cytochrome c oxidase

In mitochondria and aerobic bacteria energy conservation involves electron transfer through a number of membrane-bound protein complexes to O2. The reduction of O2, accompanied by the uptake of substrate protons to form H2O, is catalyzed by cytochrome c oxidase (CcO). This reaction is coupled to proton translocation (pumping) across the membrane such that each electron transfer to the catalytic site is linked to the uptake of two protons from one side and the release of one proton to the other side of the membrane. To address the mechanism of vectorial proton translocation, in this study we have investigated the solvent deuterium isotope effect of proton-transfer rates in CcO oriented in small unilamellar vesicles. Although in H2O the uptake and release reactions occur with the same rates, in D2O the substrate and pumped protons are taken up first (tau(D) congruent with 200 micros, "peroxy" to "ferryl" transition) followed by a significantly slower proton release to the other side of the membrane (tau(D) congruent with 1 ms). Thus, the results define the order and timing of the proton transfers during a pumping cycle. Furthermore, the results indicate that during CcO turnover internal electron transfer to the catalytic site is controlled by the release of the pumped proton, which suggests a mechanism by which CcO orchestrates a tight coupling between electron transfer and proton translocation.

Analysis of the kinetics of the membrane potential generated by cytochrome c oxidase upon single electron injection

In a recent work from this group (Popovic, D. M.; Stuchebrukhov A. A. FEBS Lett. 2004, 566, 126), a model of proton pumping by cytochrome c oxidase (CcO) was proposed. The key element of the model is His291 (bovine notation), a histidine ligand to enzyme's CuB redox center, which plays the role of the pump element. The model assumes that upon electron transfer between heme a and the binuclear catalytic center of the enzyme, two sequential proton transfers occur: First, a proton from Glu242 is transferred to an unprotonated His291, then a second proton, after reprotonation of Glu242 from the negative side of the membrane, is transferred to a hydroxyl group in the binuclear center, a water molecule is formed, and the first proton, due to proton-proton repulsion, is expelled from His291 to the positive side of the membrane, resulting in a pumping event. In the process the free energy of water formation (i.e., reduction of oxygen) is transformed into a proton gradient across the membrane. The model possesses specific kinetic features. It assumes, for example, that upon electron transfer the first proton is transferred to the proton-loading site of the pump, His291, and not to the catalytic center of the enzyme. Here, we analyze the kinetic properties of the proposed model, and calculate the time dependence of the membrane potential generated by CcO upon a single electron injection into the enzyme. These data are directly compared with recent experimental measurements of the membrane potential generated by CcO. Specifically, F to O, and O to E transitions will be discussed. Several enzymes from different organisms (bovine, two bacterial enzymes, and several mutants) are compared and discussed in detail. The kinetic description, however, is phenomenological, and does not include explicitly the nature of the groups involved in proton translocation, except in terms of their position depth within the membrane; thus, the kinetic equations developed here are in fact describe a generic model, similar, e.g., to that proposed earlier by Peter Rich (P.R. Rich, Towards an understanding of the chemistry of oxygen reduction and proton translocation in the iron-copper respiratory oxidases. Aust. J. Plant Physiol. 22 (1995) 479-486), and which is based on the idea of displacement of the pumped protons by the chemical ones.

Redox-induced Protein Structural Changes in Cytochrome bo Revealed by Fourier Transform Infrared Spectroscopy and [13C]Tyr Labeling

Cytochrome bo is a heme-copper terminal ubiquinol oxidase of Escherichia coli under highly aerated growth conditions. Tyr-288 present at the end of the K-channel forms a Cepsilon-Nepsilon covalent bond with one of the Cu(B) ligand histidines and has been proposed to be an acid-base catalyst essential for the O-O bond cleavage at the Oxy-to-P transition of the dioxygen reduction cycle (Uchida, T., Mogi, T., and Kitagawa, T. (2000) Biochemistry 39, 6669-6678). To probe structural changes at tyrosine residues, we examined redox difference Fourier transform infrared difference spectra of the wild-type enzyme in which either L-[1-13C]Tyr or L-[4-13C]Tyr has been biosynthetically incorporated in the tyrosine auxotroph. Spectral comparison between [1-13C]Tyr-labeled and unlabeled proteins indicated that substitution of the main chain carbonyl of a Tyr residue(s) significantly affected changes in the amide-I (approximately 1620-1680 cm(-1)) and -II ( approximately 1540-1560 cm(-1)) regions. In contrast, spectral comparison between [4-13C]Tyr-labeled and unlabeled proteins showed only negligible changes, which was the case for both the pulsed and the resting forms. Thus, protonation of an OH group of tyrosines including Tyr-288 in the vicinity of the heme o-Cu(B) binuclear center was not detected at pH 7.4 upon full reduction of cytochrome bo. Redox-induced main chain changes at a Tyr residue(s) are associated with structural changes at Glu-286 near the binuclear metal centers and may be related to switching of the K-channel operative at the reductive phase to D-channel at the oxidative phase of the dioxygen reduction cycle via conformational changes in the middle of helix VI.

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.

Insights into the Structure and Function of Redox-Active Tyrosines from Model Compounds

Redox-active tyrosine residues play important roles in long distance electron-transfer reactions in enzymes, including prostaglandin H synthase, ribonucleotide reductase, and photosystem II. In cytochrome c oxidase, a cross-linked tyrosine-histidine cofactor has been proposed to play a role in proton and electron transfer reactions. Studies of tyrosyl radicals in model compounds, generated by UV photolysis, have recently provided new information about the structure and function of these redox-active species. The results of these studies, which combine magnetic resonance and optical spectroscopies, are described in this review.

Probing the access of protons to the K pathway in the Paracoccus denitrificans cytochrome c oxidase

In recent studies on heme-copper oxidases a particular glutamate residue in subunit II has been suggested to constitute the entry point of the so-called K pathway. In contrast, mutations of this residue (E78(II)) in the Paracoccus denitrificans cytochrome c oxidase do not affect its catalytic activity at all (E78(II)Q) or reduce it to about 50% (E78(II)A); in the latter case, the mutation causes no drastic decrease in heme a(3) reduction kinetics under anaerobic conditions, when compared to typical K pathway mutants. Moreover, both mutant enzymes retain full proton-pumping competence. While oxidized-minus-reduced Fourier-transform infrared difference spectroscopy demonstrates that E78(II) is indeed addressed by the redox state of the enzyme, absence of variations in the spectral range characteristic for protonated aspartic and glutamic acids at approximately 1760 to 1710 cm(-1) excludes the protonation of E78(II) in the course of the redox reaction in the studied pH range, although shifts of vibrational modes at 1570 and 1400 cm(-1) reflect the reorganization of its deprotonated side chain at pH values greater than 4.8. We therefore conclude that protons do not enter the K channel via E78(II) in the Paracoccus enzyme.

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.

Transmembrane Charge Separation during the Ferryl-oxo → Oxidized Transition in a Nonpumping Mutant of Cytochrome c Oxidase

The N139D mutant of cytochrome c oxidase from Rhodobacter sphaeroides retains full steady state oxidase activity but completely lacks proton translocation coupled to turnover in reconstituted liposomes (Pawate, A. S., Morgan, J., Namslauer, A., Mills, D., Brzezinski, P., Ferguson-Miller, S., and Gennis, R. B. (2002) Biochemistry 41, 13417-13423). Here, time-resolved electron transfer and vectorial charge translocation in the ferryl-oxo --> oxidized transition (transfer of the 4th electron in the catalytic cycle) have been studied with the N139D mutant using ruthenium(II)-tris-bipyridyl complex as a photoactive single-electron donor. With the wild type oxidase, the flash-induced generation of Deltaphi in the ferryl-oxo --> oxidized transition begins with rapid vectorial electron transfer from CuA to heme a (tau approximately 15 micros), followed by two protonic phases, referred to as the intermediate (0.4 ms) and slow electrogenic phases (1.5 ms). In the N139D mutant, only a single protonic phase (tau approximately 0.6 ms) is observed, which was associated with electron transfer from heme a to the heme a3/CuB site and decelerates approximately 4-fold in D2O. With the wild type oxidase, such a high H2O/D2O solvent isotope effect is characteristic of only the slow (1.5 ms) phase. Presumably, the 0.6-ms electrogenic phase in the N139D mutant reports proton transfer from the inner aqueous phase to Glu-286, replacing the "chemical" proton transferred from Glu-286 to the heme a3/CuB site. The transfer occurs through the D-channel, because it is observed also in the N139D/K362M double mutant in which the K-channel is blocked. It is concluded that the intermediate electrogenic phase observed in the wild type enzyme is missing in the N139D mutant and is because of translocation of the "pumped" proton from Glu-286 to the D-ring propionate of heme a3 or to release of this proton to the outer aqueous phase. Significantly, with the wild type oxidase, the protonic electrogenic phase associated with proton pumping (approximately 0.4 ms) precedes the electrogenic phase associated with the oxygen chemistry (approximately 1.5 ms).

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.

Cytochrome c oxidase: 25 years of the elusive proton pump

Since its discovery [Nature 266 (1977) 271], the function of cytochrome c oxidase (and other haem-copper oxidases) as a redox-driven proton pump has been subject of both intense research and controversy, and is one of the key unsolved issues of bioenergetics and of biochemistry more generally. Despite the fact that the mechanism of proton translocation is not yet fully understood on the molecular level, many important details and principles have been learned. In the hope of accelerating progress, some of these will be reviewed here, together with a brief presentation of a novel proton pump mechanism, and of the emergence of a molecular basis for control of its efficiency.

Surface-mediated proton-transfer reactions in membrane-bound proteins

As outlined by Peter Mitchell in the chemiosmotic theory, an intermediate in energy conversion in biological systems is a proton electrochemical potential difference ("proton gradient") across a membrane, generated by membrane-bound protein complexes. These protein complexes accommodate proton-transfer pathways through which protons are conducted. In this review, we focus specifically on the role of the protein-membrane surface and the surface-bulk water interface in the dynamics of proton delivery to these proton-transfer pathways. The general mechanisms are illustrated by experimental results from studies of bacterial photosynthetic reaction centres (RCs) and 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 catalytic cycle of cytochrome c oxidase is not the sum of its two halves

Membrane-bound cytochrome c oxidase catalyzes cell respiration in aerobic organisms and is a primary energy transducer in biology. The two halves of the catalytic cycle may be studied separately: in an oxidative phase, the enzyme is oxidized by O(2), and in a reductive phase, the oxidized enzyme is reduced before binding the next O(2) molecule. Here we show by time-resolved membrane potential and pH measurements with cytochrome oxidase liposomes that, with both phases in succession, two protons are translocated during each phase, one during each individual electron transfer step. However, when the reductive phase is not immediately preceded by oxidation, it follows a different reaction pathway no longer coupled to proton pumping. Metastable states with altered redox properties of the metal centers are accessed during turnover and relax when external electron donors are exhausted but recover after enzyme reduction and reoxidation by O(2). The efficiency of ATP synthesis might be regulated by switching between the two catalytic pathways.

Redox-coupled proton translocation in biological systems: proton shuttling in cytochrome c oxidase

In the respiratory chain free energy is conserved by linking the chemical reduction of dioxygen to the electrogenic translocation of protons across a membrane. Cytochrome c oxidase (CcO) is one of the sites where this linkage occurs. Although intensively studied, the molecular mechanism of proton pumping by this enzyme remains unknown. Here, we present data from an investigation of a mutant CcO from Rhodobacter sphaeroides [Asn-139 --> Asp, ND(I-139)] in which proton pumping is completely uncoupled from the catalytic turnover (i.e., reduction of O2). However, in this mutant CcO, the rate by which O2 is reduced to H2O is even slightly higher than that of the wild-type CcO. The data indicate that the disabling of the proton pump is a result of a perturbation of E(I-286), which is located 20 A from N(I-139) and is an internal proton donor to the catalytic site, located in the membrane-spanning part of CcO. The mutation results in raising the effective pKa of E(I-286) by 1.6 pH units. An explanation of how the mutation uncouples catalytic turnover from proton pumping is offered, which suggests a mechanism by which CcO pumps protons.

Could the tyrosine-histidine ligand to CuB in cytochrome c oxidase be coordinatively labile? Implications from a quantum chemical model study of histidine substitutional lability and the effects of the covalent tyrosine-histidine cross-link

Density functional theory calculations have been used to evaluate the effects of inter-ring interactions within a covalently linked histidine-tyrosine cofactor such as that which is a ligand to the Cu(B) centre in cytochrome c oxidases and to investigate the energetics of histidine substitution at the Cu(B) centre. Small, but significant, perturbations of the redox potentials and/or p K(a) values of the histidine imidazole, the tyrosine phenol and the copper ion are found. The Cu(B)-N(cofactor) bond is estimated to be weaker than the Cu(B)-N(histidine coligand) bonds in the Cu(B)(I) state and in the Cu(B) (II) state when the cofactor is oxidized, by approximately 13 kJ/mol and approximately 23 kJ/mol, respectively. The calculations reveal that displacement of a histidine ligand from the Cu(B) centre, as is suggested in proposals of "histidine cycle" mechanisms for proton pumping in cytochrome c oxidases, is only energetically feasible if accompanied by protonation of the histidine imidazole and coupled to an endothermic process. It is proposed that the histidine-tyrosine cofactor ought to be considered as the substitutionally labile ligand to Cu(B) as the covalent crosslink would ensure displacement of the cofactor from Cu(B)-driven helix deformation. It is estimated that this process could store up to approximately 70 kJ/mol, which, based upon thermodynamic considerations, is sufficient for the pumping of two protons in the later steps (reductive phase) of the catalytic cycle. Ramifications of this proposition for the mechanism of proton pumping in cytochrome c oxidases are discussed.

Direct observation of protonation reactions during the catalytic cycle of cytochrome c oxidase

Cytochrome c oxidase, the terminal protein in the respiratory chain, converts oxygen into water and helps generate the electrochemical gradient used in the synthesis of ATP. The catalytic action of cytochrome c oxidase involves electron transfer, proton transfer, and O2 reduction. These events trigger specific molecular changes at the active site, which, in turn, influence changes throughout the protein, including alterations of amino acid side chain orientations, hydrogen bond patterns, and protonation states. We have used IR difference spectroscopy to investigate such modulations for the functional intermediate states E, R2,Pm, and F. These spectra reveal deprotonation of its key glutamic acid E286 in the E and in the Pm states. The consecutive deprotonation and reprotonation of E286 twice within one catalytic turnover illustrates the role of this residue as a proton shuttle. In addition, the spectra point toward deprotonation of a redox-active tyrosine, plausibly Y288, in the F intermediate. Structural insights into the molecular mechanism of catalysis based on the subtle molecular changes observed with IR difference spectroscopy are discussed.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

The influence of temperature and osmolyte on the catalytic cycle of cytochrome c oxidase

The influence of temperature on cytochrome c oxidase (CCO) catalytic activity was studied in the temperature range 240-308 K. Temperatures below 273 K required the inclusion of the osmolyte ethylene glycol. For steady-state activity between 278 and 308 K the activation energy was 12 kcal x mol-1; the molecular activity or turnover number was 12 s-1 at 280 K in the absence of ethylene glycol. CCO activity was studied between 240 and 277 K in the presence of ethylene glycol. The activation energy was 30 kcal x mol-1; the molecular activity was 1 s-1 at 280 K. Ethylene glycol inhibits CCO by lowering the activity of water. The rate limitation in electron transfer (ET) was not associated with ET into the CCO as cytochrome a was predominantly reduced in the aerobic steady state. The activity of CCO in flash-induced oxidation experiments was studied in the low temperature range in the presence of ethylene glycol. Flash photolysis of the reduced CO complex in the presence of oxygen resulted in three discernable processes. At 273 K the rate constants were 1500 s-1, 150 s-1 and 30 s-1 and these dropped to 220 s-1, 27 s-1 and 3 s-1 at 240 K. The activation energies were 5 kcal.mol-1, 7 kcal.mol-1, and 8 kcal.mol-1, respectively. The fastest rate we ascribe to the oxidation of cytochrome a3, the intermediate rate to cytochrome a oxidation and the slowest rate to the re-reduction of cytochrome a followed by its oxidation. There are two comparisons that are important: (a). with vs. without ethylene glycol and (b). steady state vs. flash-induced oxidation. When one makes these two comparisons it is clear that the CCO only senses the presence of osmolyte during the reductive portion of the catalytic cycle. In the present work that would mean after a flash-induced oxidation and the start of the next reduction/oxidation cycle.

Influence of structure, pH and membrane potential on proton movement in cytochrome oxidase

Cytochrome c oxidase (CcO) reconstituted into phospholipid vesicles and subject to a membrane potential, exhibits different characteristics than the free enzyme, with respect to effects of mutations, pH, inhibitors, and native structural differences between CcO from different species. The results indicate that the membrane potential influences the conformation of CcO and the direction of proton movement in the exit path. The importance of the protein structure above the hemes in proton exit, back leak and respiratory control is discussed.

Proton translocation by cytochrome c oxidase in different phases of the catalytic cycle

Since mitochondrial cytochrome c oxidase was found to be a redox-linked proton pump, most enzymes of the haem-copper oxidase family have been shown to share this function. Here, the most recent knowledge of how the individual reactions of the enzyme's catalytic cycle are coupled to proton translocation is reviewed. Two protons each are pumped during the oxidative and reductive halves of the cycle, respectively. An apparent controversy that concerns proton translocation during the reductive half is resolved. If the oxidised enzyme is allowed to relax in the absence of reductant, the binuclear haem-copper centre attains a state that lies outside the main catalytic cycle. Reduction of this form of the enzyme is not linked to proton translocation, but is necessary for a return to the main cycle. This phenomenon might be related to the previously described "pulsed" vs. "resting" and "fast" vs."slow" forms of haem-copper oxidases.

Nitric oxide reacts with the single-electron reduced active site of cytochrome c oxidase

The reduction kinetics of the mutants K354M and D124N of the Paracoccus denitrificans cytochrome oxidase (heme aa(3)) by ruthenium hexamine was investigated by stopped-flow spectrophotometry in the absence/presence of NO. Quick heme a reduction precedes the biphasic heme a(3) reduction, which is extremely slow in the K354M mutant (k(1) = 0.09 +/- 0.01 s(-1); k(2) = 0.005 +/- 0.001 s(-1)) but much faster in the D124N aa(3) (k(1) = 21 +/- 6 s(-1); k(2) = 2.2 +/- 0.5 s(-1)). NO causes a very large increase (>100-fold) in the rate constant of heme a(3) reduction in the K354M mutant but only a approximately 5-fold increase in the D124N mutant. The K354M enzyme reacts rapidly with O(2) when fully reduced but is essentially inactive in turnover; thus, it was proposed that impaired reduction of the active site is the cause of activity loss. Since at saturating [NO], heme a(3) reduction is approximately 100-fold faster than the extremely low turnover rate, we conclude that, contrary to O(2), NO can react not only with the two-electron but also with the single-electron reduced active site. This mechanism would account for the efficient inhibition of cytochrome oxidase activity by NO in the wild-type enzyme, both from P. denitrificans and from beef heart. Results also suggest that the H(+)-conducting K pathway, but not the D pathway, controls the kinetics of the single-electron reduction of the active site.

Reduction of cytochrome c oxidase by a second electron leads to proton translocation

Cytochrome c oxidase, the terminal enzyme of cellular respiration in mitochondria and many bacteria, reduces O(2) to water. This four-electron reduction process is coupled to translocation (pumping) of four protons across the mitochondrial or bacterial membrane; however, proton pumping is poorly understood. Proton pumping was thought to be linked exclusively to the oxidative phase, that is, to the transfer of the third and fourth electron. Upon re-evaluation of these data, however, this proposal has been questioned, and a transport mechanism including proton pumping in the reductive phase--that is, during the transfer of the first two electrons--was suggested. Subsequently, additional studies reported that proton pumping during the reductive phase can occur, but only when it is immediately preceded by an oxidative phase. To help clarify the issue we have measured the generation of the electric potential across the membrane, starting from a defined one-electron reduced state. Here we show that a second electron transfer into the enzyme leads to charge translocation corresponding to pumping of one proton without necessity for a preceding turnover.

Electron transfer at the low-spin heme b of cytochrome bo(3) induces an environmental change of the catalytic enhancer glutamic acid-286

Intramolecular proton transfer of heme-copper oxidases is performed via the K- and the transmembrane D-channels. A carboxyl group conserved in a subgroup of heme-copper oxidases, located within the D-channel close to the binuclear center (=glutamic acid-286 in cytochrome bo(3) from Escherichia coli) is essential for proton pumping. Upon electron transfer to the fully oxidized (FO) enzyme, this amino acid has been shown to undergo a cyanide-independent environmental change. The redox-induced environmental transition of glutamic acid-286 is preserved in the site-directed mutant Y288F, which has lost its Cu(B) binding capacity. Furthermore, the mixed-valence (MV) redox state of cytochrome bo(3) (in which Cu(B) and high-spin heme are reduced, whereas the low-spin heme stays oxidized) was prepared by anaerobic exposure of the protein to carbon monoxide. This complex was converted (i) to the FO state by reaction with the caged dioxygen donor mu-peroxo) (mu-hydroxo) bis [bis (bipyridyl) cobalt (III)] and (ii) to the fully reduced (FR) state via caged electron donors; the environmental change of glutamic acid-286 could be observed only upon reduction. Taken together, these results from two different lines of evidence clearly show that the redox transition of the low-spin heme b center alone triggers the change in the chemical environment of this acidic side chain. It is suggested that glutamic acid-286 is a kinetic enhancer of proton translocation, which is energetically favoured in mesophilic oxidases.

Zn(2+) binding to the cytoplasmic side of Paracoccus denitrificans cytochrome c oxidase selectively uncouples electron transfer and proton translocation

Using a combination of stopped-flow spectrophotometric proton pumping measurements and time-resolved potential measurements on black lipid membranes, we have investigated the effect of Zn(2+) ions on the proton transfer properties of Paracoccus denitrificans cytochrome c oxidase. When zinc was enclosed in the interior of cytochrome c oxidase containing liposomes, the H/e stoichiometry was found to gradually decrease with increasing Zn(2+) concentration. Half-inhibition of proton pumping was observed at [Zn(2+)](i)=75 microM corresponding to about 5-6 Zn(2+) ions per oxidase molecule. In addition, there was a significant increase in the respiratory control ratio of the proteoliposomes upon incorporation of Zn(2+). Time-resolved potential measurements on a black lipid membrane showed that the electrogenic phases slowed down in the presence of Zn(2+) correspond to phases that have been attributed to proton uptake from the cytoplasmic side and to proton pumping. We conclude that Zn(2+) ions bind close to or within the two proton transfer pathways of the bacterial cytochrome c oxidase.

Cytochrome C oxidase and the regulation of oxidative phosphorylation

Life of higher organisms is essentially dependent on the efficient synthesis of ATP by oxidative phosphorylation in mitochondria. An important and as yet unsolved question of energy metabolism is how are the variable rates of ATP synthesis at maximal work load during exercise or mental work and at rest or during sleep regulated. This article reviews our present knowledge on the structure of bacterial and eukaryotic cytochrome c oxidases and correlates it with recent results on the regulatory functions of nuclear-coded subunits of the eukaryotic enzyme, which are absent from the bacterial enzyme. A new molecular hypothesis on the physiological regulation of oxidative phosphorylation is proposed, assuming a hormonally controlled dynamic equilibrium in vivo between two states of energy metabolism, a relaxed state with low ROS (reactive oxygen species) formation, and an excited state with elevated formation of ROS, which are known to accelerate aging and to cause degenerative diseases and cancer. The hypothesis is based on the allosteric ATP inhibition of cytochrome c oxidase at high intramitochondrial ATP/ADP ratios ("second mechanism of respiratory control"), which is switched on by cAMP-dependent phosphorylation and switched off by calcium-induced dephosphorylation of the enzyme.

On the role of the K-proton transfer pathway in cytochrome c oxidase

Cytochrome c oxidase is a membrane-bound enzyme that catalyzes the four-electron reduction of oxygen to water. This highly exergonic reaction drives proton pumping across the membrane. One of the key questions associated with the function of cytochrome c oxidase is how the transfer of electrons and protons is coupled and how proton transfer is controlled by the enzyme. In this study we focus on the function of one of the proton transfer pathways of the R. sphaeroides enzyme, the so-called K-proton transfer pathway (containing a highly conserved Lys(I-362) residue), leading from the protein surface to the catalytic site. We have investigated the kinetics of the reaction of the reduced enzyme with oxygen in mutants of the enzyme in which a residue [Ser(I-299)] near the entry point of the pathway was modified with the use of site-directed mutagenesis. The results show that during the initial steps of oxygen reduction, electron transfer to the catalytic site (to form the "peroxy" state, P(r)) requires charge compensation through the proton pathway, but no proton uptake from the bulk solution. The charge compensation is proposed to involve a movement of the K(I-362) side chain toward the binuclear center. Thus, in contrast to what has been assumed previously, the results indicate that the K-pathway is used during oxygen reduction and that K(I-362) is charged at pH approximately 7.5. The movement of the Lys is proposed to regulate proton transfer by "shutting off" the protonic connectivity through the K-pathway after initiation of the O(2) reduction chemistry. This "shutoff" prevents a short-circuit of the proton-pumping machinery of the enzyme during the subsequent reaction steps.

On the mechanism of proton transport by the neuronal excitatory amino acid carrier 1

Uptake of glutamate from the synaptic cleft is mediated by high affinity transporters and is driven by Na(+), K(+), and H(+) concentration gradients across the membrane. Here, we characterize the molecular mechanism of the intracellular pH change associated with glutamate transport by combining current recordings from excitatory amino acid carrier 1 (EAAC1)-expressing HEK293 cells with a rapid kinetic technique with a 100-micros time resolution. Under conditions of steady state transport, the affinity of EAAC1 for glutamate in both the forward and reverse modes is strongly dependent on the pH on the cis-side of the membrane, whereas the currents at saturating glutamate concentrations are hardly affected by the pH. Consistent with this, the kinetics of the pre-steady state currents, measured after saturating glutamate concentration jumps, are not a function of the pH. In addition, we determined the deuterium isotope effect on EAAC1 kinetics, which is in agreement with proton cotransport but not OH(-) countertransport. The results can be quantitatively explained with an ordered binding model that includes a rapid proton binding step to the empty transporter followed by glutamate binding and translocation of the proton-glutamate-transporter complex. The apparent pK of the extracellular proton binding site is approximately 8. This value is shifted to approximately 6.5 when the substrate binding site is exposed to the cytoplasm.

The role of the D- and K-pathways of proton transfer in the function of the haem-copper oxidases

The X-ray structures of several haem-copper oxidases now at hand have given important constraints on how these enzymes function. Yet, dynamic data are required to elucidate the mechanisms of electron and proton transfer, the activation of O(2) and its reduction to water, as well as the still enigmatic mechanism by which these enzymes couple the redox reaction to proton translocation. Here, some recent observations will be briefly reviewed with special emphasis on the functioning of the so-called D- and K-pathways of proton transfer. It turns out that only one of the eight protons taken up by the enzyme during its catalytic cycle is transferred via the K-pathway. The D-pathway is probably responsible for the transfer of all other protons, including the four that are pumped across the membrane. The unique K-pathway proton may be specifically required to aid O-O bond scission by the haem-copper oxidases.