Electron transfer after flash photolysis of mixed-valence carboxycytochrome c oxidase

The catalytic reaction of cytochrome c oxidase probed by in situ gas titrations and FTIR difference spectroscopy

Cytochrome c oxidase (CcO) is a transmembrane heme‑copper metalloenzyme that catalyzes the reduction of O2 to H2O at the reducing end of the respiratory electron transport chain. To understand this reaction, we followed the conversion of CcO from Rhodobacter sphaeroides between several active-ready and carbon monoxide-inhibited states via attenuated total reflection Fourier-transform infrared (ATR FTIR) difference spectroscopy. Utilizing a novel gas titration setup, we prepared the mixed-valence, CO-inhibited R2CO state as well as the fully-reduced R4 and R4CO states and induced the "active ready" oxidized state OH. These experiments are performed in the dark yielding FTIR difference spectra exclusively triggered by exposure to O2, the natural substrate of CcO. Our data demonstrate that the presence of CO at heme a3 does not impair the catalytic oxidation of CcO when the cycle starts from the fully-reduced states. Interestingly, when starting from the R2CO state, the release of the CO ligand upon purging with inert gas yield a product that is indistinguishable from photolysis-induced states. The observed changes at heme a3 in the catalytic binuclear center (BNC) result from the loss of CO and are unrelated to electronic excitation upon illumination. Based on our experiments, we re-evaluate the assignment of marker bands that appear in time-resolved photolysis and perfusion-induced experiments on CcO.

Ligand Dynamics in an Electron Transfer Protein

Substitution of the heme coordination residue Met-80 of the electron transport protein yeast iso-1-cytochrome c allows external ligands like CO to bind and thus increase the effective redox potential. This mutation, in principle, turns the protein into a quasi-native photoactivable electron donor. We have studied the kinetic and spectral characteristics of geminate recombination of heme and CO in a series of single M80X (X = Ala, Ser, Asp, Arg) mutants, using femtosecond transient absorption spectroscopy. In these proteins, all geminate recombination occurs on the picosecond and early nanosecond time scale, in a multiphasic manner, in which heme relaxation takes place on the same time scale. The extent of geminate recombination varies from >99% (Ala, Ser) to approximately 70% (Arg), the latter value being in principle low enough for electron injection studies. The rates and extent of the CO geminate recombination phases are much higher than in functional ligand-binding proteins like myoglobin, presumably reflecting the rigid and hydrophobic properties of the heme environment, which are optimized for electron transfer. Thus, the dynamics of CO recombination in cytochrome c are a tool for studying the heme pocket, in a similar way as NO in myoglobin. We discuss the differences in the CO kinetics between the mutants in terms of the properties of the heme environment and strategies to enhance the CO escape yield. Experiments on double mutants in which Phe-82 is replaced by Asp or Gly as well as the M80D substitution indicate that such steric changes substantially increase the motional freedom-dissociated CO.

Cytochrome c oxidase: chemistry of a molecular machine

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

Electron transfer between hemes in mammalian cytochrome c oxidase

Fast intraprotein electron transfer reactions associated with enzymatic catalysis are often difficult to synchronize and therefore to monitor directly in non-light-driven systems. However, in the mitochondrial respiratory enzyme cytochrome oxidase aa(3), the kinetics of the final electron transfer step into the active site can be determined: reverse electron flow between the close-lying and chemically identical hemes a(3) and a can be initiated by flash photolysis of CO from reduced heme a(3) under conditions where heme a is initially oxidized. To follow this reaction, we used transient absorption spectroscopy, with femtosecond time resolution and a time window extending to 4 ns. Comparison of the picosecond heme a(3)-CO photodissociation spectra under different redox states of heme a shows significant spectral interaction between both hemes, a phenomenon complicating the interpretation of spectral studies with low time resolution. Most importantly, we show that the intrinsic electron equilibration, corresponding to a DeltaG(0) of 45-55 meV, occurs in 1.2 +/- 0.1 ns. This is 3 orders of magnitude faster than the previously established equilibration phase of approximately 3 mus, which we suggest to reflect a change in redox equilibrium closely following CO migration out of the active site. Our results allow testing a number of conflicting predictions regarding this reaction between both experimental and theoretical studies. We discuss the potential physiological relevance of fast equilibration associated with this low-driving-force redox reaction.

Time-resolved optical absorption studies of cytochrome oxidase dynamics

Time-resolved spectroscopic studies in our laboratory of bovine heart cytochrome c oxidase dynamics are summarized. Intramolecular electron transfer was investigated upon photolysis of CO from the mixed-valence enzyme, by pulse radiolysis, and upon light-induced electron injection into the cytochrome c/cytochrome oxidase complex from a novel photoactivatable dye. The reduction of dioxygen to water was monitored by a gated multichannel analyzer using the CO flow-flash method or a synthetic caged dioxygen carrier. The pH dependence of the intermediate spectra suggests a mechanism of dioxygen reduction more complex than the conventional unidirectional sequential scheme. A branched model is proposed, in which one branch produces the P form and the other branch the F form. The rate of exchange between the two branches is pH-dependent. A cross-linked histidine-phenol was synthesized and characterized to explore the role of the cross-linked His-Tyr cofactor in the function of the enzyme. Time-resolved optical absorption spectra, EPR and FTIR spectra of the compound generated after UV photolysis indicated the presence of a radical residing primarily on the phenoxyl ring. The relevance of these results to cytochrome oxidase function is discussed.

Anoxia induces matrix shrinkage accompanied by an increase in light scattering in isolated brain mitochondria

It is important to monitor mitochondrial conditions, and light scattering (LS) measurements have been applied to the detection of morphological changes in mitochondria in vivo. Little is known about the morphological and LS responses of brain mitochondria to oxygen withdrawal, a critical factor in cell death. We have therefore investigated the morphological and LS responses of isolated brain mitochondria to anoxia. Anoxia induced an increase in LS, reflecting mitochondrial matrix shrinkage. This response was reversible, but was reduced by adding digitonin, which disrupted the outer membrane selectively. This suggested that integrity of the outer membrane was necessary for the matrix response. We further examined the effects of Mg2+ and ATP on the responses because both exist in cells and modulate the changes in matrix volume. Although Mg2+ and ATP reduced the rates of increase and decrease in LS, respectively, the magnitudes of the increases in LS caused by anoxia stayed at over 80% of the control level (no Mg2+) in the presence of Mg2+ and ATP. This suggested that the increase in LS occurred in cells containing Mg2+ and ATP during anoxia. In contrast, that caused by inhibitors of the electron transport chain was reduced to below 30% of the control level in the presence of Mg2+. The present in vitro study provides a basis for interpretation of LS signals from mitochondria in brain research during oxygen withdrawal.

Modulation of the Electron Redistribution in Mixed Valence Cytochrome c Oxidase by Protein Conformational Changes

The redistribution of two electrons in the four redox centers of cytochrome c oxidase following photodissociation of CO from the CO-bound mixed valence species has been examined by resonance Raman spectroscopy. To account for both the kinetic data, obtained from 5 micros to 2 ms, and the equilibrium results, a model is proposed in which the electron redistribution is modulated by a protein conformation transition from a nascent P(1) state to a relaxed P(2) state in a time window longer than 2 ms. In this model, all six possible two-electron reduced species are considered. The high population of species with a one-electron reduced binuclear center, in which the spectrum of heme a(3) is perturbed by the redox state of Cu(B), accounts for the significant residuals in the fitting of the kinetic data with four standard spectra derived from redox species with either zero or two electrons in the binuclear center. Under equilibrium conditions, the conformational change to the P(2) state destabilizes the redox states with only one electron in the binuclear center with respect to those with either zero or two electrons. As a result, the redox equilibrium is perturbed, and the electrons are redistributed. A simulation based on the new kinetics scheme, in which the electron redistribution is modulated by the protein conformation, gives reasonable agreement with both the equilibrium and the kinetic data, demonstrating the validity of this model.

P(M) and P(R) forms of cytochrome c oxidase have different spectral properties

The reaction between bovine heart cytochrome c oxidase and dioxygen was monitored at room temperature in the visible and Soret regions following photolysis of the mixed-valence CO-bound enzyme. Time-resolved optical absorption difference spectra were collected between 50 ns and 1.7 ms by a gated multichannel analyzer. Singular value decomposition and global exponential fitting resolved three processes with apparent lifetimes of 2.2+/-0.5, 17+/-4 and 160+/-30 micros. The spectra of the intermediates were extracted based on a sequential kinetic mechanism and compared to the corresponding intermediate spectra observed during the reaction of the fully reduced enzyme with dioxygen. The first process is associated with a conformational change at heme a(3) upon dissociation of CO from Cu(B)(+) and concomitant back-electron transfer from heme a(3) to heme a. This is followed by O(2) binding to heme a(3) forming compound A (A(M)), with a spectrum identical to that observed upon O(2) binding to heme a(3) in the fully reduced enzyme (A(R)). Intermediate A(M) decays into P(M), the spectrum of which is equivalent to that of the 607 nm form, generated upon addition of H(2)O(2) to the oxidized enzyme at alkaline pH values (P(H)). However, the spectrum of P(M) is significantly different from the corresponding intermediate observed upon the reaction of dioxygen with the fully reduced enzyme (P(R)). The spectral differences between P(M) and P(R) may arise from the different number of redox equivalents at the binuclear site, with a tyrosine radical in the P(M) state, and tyrosine or tyrosinate in P(R), or may be the consequence of a more complex reaction mechanism in the case of the fully reduced enzyme.

Photochemically induced electron transfer

Biochemical reactions involving electron transfer between substrates or enzyme cofactors are both common and physiologically important; they have been studied by means of a variety of techniques. In this paper we review the application of photochemical methods to the study of intramolecular electron transfer in hemoproteins, thus selecting a small, well-defined sector of this otherwise enormous field. Photoexcitation of the heme populates short-lived excited states which decay by thermal conversion and do not usually transfer electrons, even when a suitable electron acceptor is readily available, e.g., in the form of a second oxidized heme group in the same protein; because of this, the experimental setup demands some manipulation of the hemoprotein. In this paper we review three approaches that have been studied in detail: (i) the covalent conjugation to the protein moiety of an organic ruthenium complex, which serves as the photoexcitable electron donor (in this case the heme acts as the electron acceptor); (ii) the replacement of the heme group with a phosphorescent metal-substituted porphyrin, which on photoexcitation populates long-lived excited states, capable of acting as electron donors (clearly the protein must contain some other cofactor acting as the electron acceptor, most often a second heme group in the oxidized state); (iii) the combination of the reduced heme with CO (the photochemical breakdown of the iron-CO bond yields transiently the ground-state reduced heme which is able to transfer one electron (or a fraction of it) to an oxidized electron acceptor in the protein; this method uses a "mixed-valence hybrid" state of the redox active hemoprotein and has the great advantage of populating on photoexcitation an electron donor at physiological redox potential).

Redox dependent conformational changes in the mixed valence form of the cytochrome c oxidase from p. The reorganization of glutamic acid 278 is coupled to the electron transfer from/to heme a and the binuclear center. denitrificans

In this work we present the separation of FTIR difference signals induced by electron transfer to/from the redox centers of the cytochrome c oxidase from P. denitrificans and compare electrochemically induced FTIR difference spectra with those induced by CO photolysis. FTIR difference spectra of rebinding of CO to the half reduced (mixed valence) form of the cytochrome c oxidase after photolysis reflect the conformational changes induced by the rebinding of CO and by electron transfer reactions from heme a3 to heme a and further on to CUA. During this process, heme a3 (and CUB) are oxidized, whereas heme a and CuA are reduced. By subtracting these difference spectra from an electrochemically induced FTIR difference spectrum, where all four cofactors are reduced, the contributions for heme a3 (and CuB) could be separated. Correspondingly, the spectral contributions of heme a and CuA have been separated. The comparison of these spectra with the spectra calculated for the hemes on the basis of their redox dependent changes previously published in Hellwig et al., (Biochemistry 38, (1999) 1685-1694) show a high degree of similarity, except for additional signals coupled to the reorganization of the binuclear center upon CO rebinding. The separated spectra clearly show that the signals attributed to Glu278, an amino acid discussed to be crucial for proton pumping, is coupled to electron transfer to/from heme a and the binuclear heme a3-CuB center.

Noninvasive assessment of changes in cytochrome-c oxidase oxidation in human subjects during visual stimulation

In this study the authors used a whole-spectrum near-infrared spectroscopy approach to noninvasively assess changes in hemoglobin oxygenation and cytochrome-c oxidase redox state (Cyt-Ox) in the occipital cortex during visual stimulation. The system uses a white light source (halogen lamp). The light reflected from the subject's head is spectrally resolved by a spectrograph and dispersed on a cooled charge-coupled device camera. The authors showed the following using this approach: (1) Changes in cerebral hemoglobin oxygenation (increase in concentration of oxygenated hemoglobin, decrease in concentration of deoxygenated hemoglobin) in the human occipital cortex during visual stimulation can be assessed quantitatively. (2) The spectral changes during functional activation cannot be completely explained by changes in hemoglobin oxygenation solely; Cyt-Ox has to be included in the analysis. Only if Cyt-Ox is considered can the spectral changes in response to increased brain activity be explained. (3) Cytochrome-c oxidase in the occipital cortex of human subjects is transiently oxidized during visual stimulation. This allows us to measure vascular and intracellular energy status simultaneously.

Use of mitochondrial inhibitors to demonstrate that cytochrome oxidase near-infrared spectroscopy can measure mitochondrial dysfunction noninvasively in the brain

The use of near-infrared spectroscopy to measure noninvasively changes in the redox state of cerebral cytochrome oxidase in vivo is controversial. We therefore tested these measurements using a multiwavelength detector in the neonatal pig brain. Exchange transfusion with perfluorocarbons revealed that the spectrum of cytochrome oxidase in the near-infrared was identical in the neonatal pig, the adult rat, and in the purified enzyme. Under normoxic conditions, the neonatal pig brain contained 15 micromol/L deoxyhemoglobin, 29 micromol/L oxyhemoglobin, and 1.2 micromol/L oxidized cytochrome oxidase. The mitochondrial inhibitor cyanide was used to determine whether redox changes in cytochrome oxidase could be detected in the presence of the larger cerebral hemoglobin concentration. Addition of cyanide induced full reduction of cytochrome oxidase in both blooded and bloodless animals. In the blooded animals, subsequent anoxia caused large changes in hemoglobin oxygenation and concentration but did not affect the cytochrome oxidase near-infrared signal. Simultaneous blood oxygenation level-dependent magnetic resonance imaging measurements showed a good correlation with near-infrared measurements of deoxyhemoglobin concentration. Possible interference in the near-infrared measurements from light scattering changes was discounted by simultaneous measurements of the optical pathlength using the cerebral water absorbance as a standard chromophore. We conclude that, under these conditions, near-infrared spectroscopy can accurately measure changes in the cerebral cytochrome oxidase redox state.

Pathways for electron tunneling in cytochrome c oxidase

Warburg showed in 1929 that the photochemical action spectrum for CO dissociation from cytochrome c oxidase is that of a heme protein. Keilin had shown that cytochrome a does not react with oxygen, so he did not accept Warburg's view until 1939, when he discovered cytochrome a3. The dinuclear cytochrome a3-CuB unit was found by EPR in 1967, whereas the dinuclear nature of the CuA site was not universally accepted until oxidase crystal structures were published in 1995. There are negative redox interactions between cytochrome a and the other redox sites in the oxidase, so that the reduction potential of a particular site depends on the redox states of the other sites. Calculated electron-tunneling pathways for internal electron transfer in the oxidase indicate that the coupling-limited rates are 9 x 10(5) (CuA-->a) and 7 x 10(6) s(-1) (a-->a3); these calculations are in reasonable agreement with experimental rates, after corrections are made for driving force and reorganization energy. The best CuA-a pathway starts from the ligand His204 and not from the bridging sulfur of Cys196, and an efficient a-a3 path involves the heme ligands His378 and His376 as well as the intervening Phe377 residue. All direct paths from CuA to a3 are poor, indicating that direct CuA-->a3 electron transfer is much slower than the CuA-->a reaction. The pathways model suggests a means for gating the electron flow in redox-linked proton pumps.

A flash-photolysis study of the reactions of a caa3-type cytochrome oxidase with dioxygen and carbon monoxide

The time course of absorbance changes following flash photolysis of the fully-reduced carboxycytochrome oxidase from Bacillus PS3 in the presence of O2 has been followed at 445, 550, 605, and 830 nm, and the results have been compared with the corresponding changes in bovine cytochrome oxidase. The PS3 enzyme has a covalently bound cytochrome c subunit and the fully-reduced species therefore accommodates five electrons instead of four as in the bovine enzyme. In the bovine enzyme, following CO dissociation, four phases were observed with time constants of about 10 microseconds, 30 microseconds, 100 microseconds, and I ms at 445 nm. The initial, 10-microsecond absorbance change at 445 nm is similar in the two enzymes. The subsequent phases involving heme a and CuA are not seen in the PS3 enzyme at 445 nm, because these redox centers are re-reduced by the covalently bound cytochrome c, as indicated by absorbance changes at 550 nm. A reaction scheme consistent with the experimental observations is presented. In addition, internal electron-transfer reactions in the absence of O2 were studied following flash-induced CO dissociation from the mixed-valence enzyme. Comparisons of the CO recombination rates in the mixed-valence and fully-reduced oxidases indicate that more electrons were transferred from heme a3 to a in PS3 oxidase compared to the bovine enzyme.

Kinetic coupling between electron and proton transfer in cytochrome c oxidase: simultaneous measurements of conductance and absorbance changes

Bovine heart cytochrome c oxidase is an electron-current driven proton pump. To investigate the mechanism by which this pump operates it is important to study individual electron- and proton-transfer reactions in the enzyme, and key reactions in which they are kinetically and thermodynamically coupled. In this work, we have simultaneously measured absorbance changes associated with electron-transfer reactions and conductance changes associated with protonation reactions following pulsed illumination of the photolabile complex of partly reduced bovine cytochrome c oxidase and carbon monoxide. Following CO dissociation, several kinetic phases in the absorbance changes were observed with time constants ranging from approximately 3 microseconds to several milliseconds, reflecting internal electron-transfer reactions within the enzyme. The data show that the rate of one of these electron-transfer reactions, from cytochrome a3 to a on a millisecond time scale, is controlled by a proton-transfer reaction. These results are discussed in terms of a model in which cytochrome a3 interacts electrostatically with a protonatable group, L, in the vicinity of the binuclear center, in equilibrium with the bulk through a proton-conducting pathway, which determines the rate of proton transfer (and indirectly also of electron transfer). The interaction energy of cytochrome a3 with L was determined independently from the pH dependence of the extent of the millisecond-electron transfer and the number of protons released, as determined from the conductance measurements. The magnitude of the interaction energy, 70 meV (1 eV = 1.602 x 10(-19) J), is consistent with a distance of 5-10 A between cytochrome a3 and L. Based on the recently determined high-resolution x-ray structures of bovine and a bacterial cytochrome c oxidase, possible candidates for L and a physiological role for L are discussed.

Structure and function of a molecular machine: cytochrome c oxidase

Cytochrome c is responsible for over 90% of the dioxygen consumption in the living cell and contributes to the build-up of a proton electrochemical gradient derived by the vectorial transfer of electrons between cytochrome c and molecular oxygen. The metal ions found in cytochrome oxidases play a crucial role in these processes and have been extensively studied. In this review we present and discuss some of the relevant spectroscopic and kinetic properties of the prosthetic groups of cytochrome c oxidase.

Electron transfer and proton pumping in cytochrome oxidase

This article presents an outlook on the structure and function of terminal oxidases, the respiratory enzymes which catalyze the reduction of dioxygen to water in aerobic organisms. The structure of the redox active metals, their interactions with the protein matrix, and their role in electron transfer ligand binding and proton pumping are briefly reviewed.

Electron transfer and ligand binding in terminal oxidases. Impact of recent structural information

A consensus structure for the active site of terminal oxidases has been recently proposed by Hosler et al. [(1993) J. Bioenerg. Biomem. 25, 121-135]. We exploit the novel structural information to propose a hypothesis for the large difference in the rate of internal electron transfer found when experiments are started either with the reduced or with the oxidized enzyme. This rationale also allows us to discuss the oxidation state of the prevailing oxygen reacting species with reference to the concentration of the two substrates (oxygen and cytochrome c) and to the structural state of the oxidase.

Time-resolved optical absorption studies of intramolecular electron transfer in cytochrome c oxidase

Intramolecular electron transfer and conformational changes in cytochrome c oxidase were studied at room temperature following the photodissociation of CO bound to mixed-valence enzyme (cytochrome a3(2+)-CO CuB+ cytochrome a3+ CuA2+) and fully reduced enzyme. Time-resolved optical absorption difference spectra were collected in the Soret region on time scales of nanoseconds to milliseconds using a gated optical spectrometric multichannel analyzer. A global exponential fitting procedure combined with a singular value decomposition method was used to analyze the transient difference spectra at various times following CO photolysis. The analysis shows that at least two processes, with apparent lifetimes of 1.4 microseconds and 11.1 ms, are present following the photodissociation of CO bound to the fully reduced enzyme. These are attributed to a conformational change and CO recombination at the cytochrome a3 site, respectively. Global analysis of the mixed-valence CO complex transient difference spectra showed the presence of five intermediates with apparent lifetimes of 1.0 microseconds, 5.2 microseconds, 83.7 microseconds, 10.5 ms, and 25.3 ms. The data on a microsecond time scale are consistent with a mechanism involving a conformational change at cytochrome a3, followed by electron transfer from cytochrome a3 to cytochrome a with subsequent electron transfer to CuA. One of the two processes on a millisecond time scale is attributed to CO recombination and the other to a structural rearrangement or heme-heme electron transfer. On the basis of this mechanism, the kinetics and the absorption spectra of the intermediates involved in the conformational and electron transfer dynamics of the mixed-valence enzyme were determined.

Light-induced structural changes in cytochrome c oxidase: implication for the mechanism of electron and proton gating

We have investigated electrogenic events and absorbance changes following pulsed illumination of partly reduced cytochrome c oxidase in the absence of dioxygen and carbon monoxide (Hallén et al. (1993) FEBS Lett. 318, 134-138). In both types of experiment similar kinetics were observed; a rapid (tau < 0.5 micros) change was followed by relaxations with time constants of approx. 7 micros and 80 micros. Both the time constant and the activation energy of the 80 micros component were, within the experimental error, the same as those of one of the steps in the reduction of dioxygen by reduced cytochrome c oxidase. The absorbance changes showed a rapid haem reduction, followed by reoxidation. They were affected by CN(-) and N(-)3, ligands which bind in the binuclear centre of cytochrome c oxidase; the absorbance changes were quenched by CN(-) and in the presence of N(-)3, the amplitude of the 7 micros component increased whereas that of the 80 micros decreased. Based on these findings, a model is proposed which involves electron transfer from Cu(+)B to Fe(3+)A3, as a response to structural changes upon pulsed illumination. The same structural changes are also suggested to take place in the oxygen reduction. These changes may play an important role in the gating of electrons as well as protons, an obligatory feature of a redox-linked proton pump.

Flash photolysis of the carbon monoxide compounds of wild-type and mutant variants of cytochrome bo from Escherichia coli

The carbon monoxide compounds of the fully reduced and mixed valence forms of cytochrome bo from Escherichia coli were laser photolysed under anaerobic conditions at room temperature. The carbon monoxide recombined with characteristic rate constants of 50 s-1 or 35 s-1 in the fully reduced and mixed valence forms, respectively. Rates of CO recombination with the fully reduced enzyme were examined in a variety of mutant forms of cytochrome bo, produced by site-directed mutagenesis. A method was developed to deconvolute cytochromes bo and bd, leading to some reassessment of histidine ligands to the metals. Significant changes in the rate constant of recombination of carbon monoxide occurred in many of these mutants and these results could be rationalised generally in terms of our current working model of the folding structure of subunit I. In the mixed valence form of the enzyme the transient photolysis spectra in the visible region are consistent with a rapid electron redistribution from the binuclear centre to the low-spin haem. This electron transfer is biphasic, with rate constants of around 10(5) and 8000 s-1. The process was also examined in the His-333-Leu mutant, in which a putative histidine ligand to CuB is replaced by leucine, and which results in the loss of the CuB. It appeared that rapid haem-haem electron transfer could still occur. The observation that CuB is apparently not required for rapid haem-haem electron transfer is consistent with the recently proposed model in which the two haems are positioned on opposite sides of transmembrane helix X in subunit I of the oxidase.

Light-induced structural changes in cytochrome c oxidase. Measurements of electrogenic events and absorbance changes

We have investigated flash-induced electrogenic events and absorbance changes in cytochrome c oxidase in the absence of dioxygen and carbon monoxide. Electrogenic events were studied using a Teflon-bound layer of cytochrome c oxidase oriented in a phospholipid monolayer. Absorbance changes were observed exclusively in partly reduced cytochrome c oxidase; the largest changes were found in the one-electron-reduced species. Electrogenic events were detected in all reduction states of the enzyme. Both types of experiments displayed a rapid (< 0.5 microseconds) event followed by a biphasic relaxation. The time constants of the relaxation were 6 +/- 2 microseconds and 70 +/- 10 microseconds in the electrogenicity, and 9 +/- 3 microseconds in the absorbance changes (at approximately 22 degrees C). The kinetic absorbance difference spectrum was consistent with that of reduced minus oxidized haem. The experimental results are discussed in terms of structural changes in the vicinity of cytochrome a3. These changes may play an important role in all studies that involve flash photolysis of cytochrome c oxidase-ligand complexes.

New transients in the electron-transfer dynamics of photolyzed mixed-valence CO-cytochrome c oxidase

Electron transfer following photolysis of CO from mixed-valence (cytochrome a3+ Cu2+A cytochrome a2+3-CO Cu+B) cytochrome oxidase (ferrocytochrome-c; oxygen oxidoreductase, EC 1.9.3.1) was studied on time scales of nanoseconds to milliseconds by multichannel time-resolved optical absorption spectroscopy. In this method, the optical absorption was measured at many wavelengths simultaneously by using an optical spectrometric multichannel analyzer system. The high-quality time-resolved difference spectra showed a large increase on a microsecond time scale in the visible region centered at approximately 520 nm and in the UV region centered at approximately 390 nm. These absorbance changes were not observed after photodissociation of CO from the fully reduced enzyme and therefore are attributed to intramolecular electron transfer. Simultaneously, there was a blue shift and a small increase in the alpha band, which is attributed to the reduction of cytochrome alpha. Approximately one-third of the absorbance change at 520 nm can be attributed to reduction of cytochrome a. The absorbance changes associated with the 520- and the 390-nm bands are on the same time scale (t1/2 approximately 2 microseconds) as the dissociation of CO from Cu+B reported previously by time-resolved infrared spectroscopy. The position and shape of these bands are reasonable for charge-transfer transitions involving copper(II). We suggest that the absorbance increase at 520 nm, which cannot be attributed to a reduction of cytochrome a, may represent a charge transfer involving Cu2+B accompanying the oxidation of Cu+B to Cu2+B. The absorbance increase at 390 nm is also partially attributed to this transition. These results suggest that Cu2+B may be observed spectrophotometrically in the electron-transfer dynamics of cytochrome oxidase.

Evidence for a band III analogue in the near-infrared absorption spectra of cytochrome c oxidase

Ground state near-infrared absorption spectra of fully reduced unliganded and fully reduced CO (a2+ CuA+ a3(2+)-CO CuB+) cytochrome c oxidase were investigated. Flash-photolysis time-resolved absorption difference spectra of the mixed-valence (a3+ CuA2+ a3(2+)-CO CuB+) and the fully reduced CO complexes were also studied. A band near 785 nm (epsilon approximately 50 M-1cm-1) was observed in the fully reduced unliganded enzyme and the CO photoproducts. The time-resolved 785 nm band disappeared on the same timescale (t1/2 approximately 7 ms) as CO recombined with cytochrome a3(2+). This band, which is attributed to the unliganded five coordinate ferrous cytochrome a3(2+), has some characteristics of band III in deoxy-hemoglobin and deoxy-myoglobin. A second band was observed at approximately 710 nm (epsilon approximately 80 M-1cm-1) in the fully reduced unliganded and the fully reduced CO complexes. This band, which we assign to the low spin ferrous cytochrome a, appears to be affected by the ligation state at the cytochrome a3(2+) site.

The effect of pH and temperature on the reaction of fully reduced and mixed-valence cytochrome c oxidase with dioxygen

The reaction of fully reduced and mixed-valence cytochrome oxidase with O2 has been followed in flow-flash experiments, starting from the CO complexes, at 428, 445, 605 and 830 nm between pH 5.8b and 9.0 in the temperature range of 2-40 degrees C. With the fully reduced enzyme, four kinetic phase with rate constants at pH 7.4 and 25 degrees C of 9 x 10(4), 2.5 x 10(4), 1.0 x 10(4) and 800 s(-1), respectively, are observed. The rates of the three last phases display a very small temperature dependence, corresponding to activation energies in the range 13-54 kJ x mol(-1). The rates of the third and fourth phases decrease at high pH due to the deprotonation of groups with pKa values of 8.3 and 8.8, respectively, but also the second phase appears to have a small pH dependence. In the reaction of the mixed-valence enzyme, three kinetic phases with rate constants at pH 7.4 and 25 degrees C of 9 x 10(4), 6000 and 150 s(-1), respectively, are observed. The third phase only has a small temperature dependence, corresponding to an activation energy of 20 kJ x mol(-1). No pH dependence could be detected for any phase. Reaction schemes consistent with the experimental observations are presented. The pH dependencies of the rates of the two final phase in the reaction of the fully reduced enzyme are proposed to be related to the involvement of protons in the reduction of a peroxide intermediate. The temperature dependence data suggest that the reorganization energies and driving forces are closely matched in all electron transfer steps with both enzyme forms. It is suggested that the slowest step in the reaction of the mixed-valence enzyme is a conformation change involved in the reaction cycle of cytochrome oxidase as a proton pump.

Electron transfer between cytochrome a and copper A in cytochrome c oxidase: a perturbed equilibrium study

Intramolecular electron transfer in partially reduced cytochrome c oxidase has been studied by the perturbed equilibrium method. We have prepared a three-electron-reduced, CO-inhibited form of the enzyme in which cytochrome a and copper A are partially reduced and in an intramolecular redox equilibrium. When these samples were irradiated with a nitrogen laser (0.6-ns, 1.0-mJ pulses) to photodissociate the bound CO, changes in absorbance at 598 and 830 nm were observed which were consistent with a fast electron transfer from cytochrome a to copper A. The absorbance changes at 598 nm gave an apparent rate of 17,000 +/- 2000 s-1 (1 sigma), at pH 7.0 and 25.5 degrees C. These changes were not observed in either the CO mixed-valence or the CO-inhibited fully reduced forms of the enzyme. The rate was fastest at about pH 8.0, falling off toward both lower and higher pHs. There was a small but clear temperature dependence. The process was also observed in the cytochrome c-cytochrome c oxidase high-affinity complex. The electron equilibration measured between cytochrome a and copper A is far faster than any rate measured or inferred previously for this process.

Photodissociation of cytochrome c oxidase-nitric oxide complexes

The dissociation of cytochrome c oxidase-nitric oxide complexes was studied by optical spectroscopy at cryogenic temperatures (15 degrees K). With the reduced cytochrome c oxidase-nitric oxide complex, the observations that were reported by Yoshida et al. were confirmed. Photodissociation of the oxidized cytochrome c oxidase-nitric oxide complex did not induce any significant absorbance changes between 350 and 875 nm. With the azide-nitrosyl-cytochrome c oxidase complex, the illumination caused the dissociation of the a2+(3).NO complex to the unligated state a2+(3). Increasing the temperature to 77 degrees K led to the formation of a new complex, probably a3+(3).N3-. The N3(-)-NO-cytochrome c oxidase complex was the only compound for which appreciable photodissociation was achieved by continuous illumination at room temperature (20 degrees C). The effect of illumination was biphasic. In the first phase the a2+(3).NO complex is dissociated and cytochrome a3 oxidized by an electron transfer to CuB. In the second phase nitric oxide, which is still bound to CuB after the first phase, is expelled from the complex by azide, with a concomitant electron transfer from CuB to cytochrome a.

Electron redistribution in mixed valence cytochrome oxidase following photolysis of carboxy-oxidase

Absorbance changes at 446 nm in purified cytochrome oxidase following flash photolysis of carboxy-oxidase poised in the mixed valance state at +220 mV show biphasic kinetics. One phase corresponds to CO recombination to ferrous cytochrome a3 with an energy of activation of 9 kcal/mol; the second phase is 3-5 times faster with an energy of activation of 9.15 kcal/mol. Following flash photolysis at approximately -60 degrees C, cytochromes a and c and the 840-nm CuA species are observed to undergo reduction as electrons from ferrous unliganded cytochrome a3 equilibrate with the equipotential redox centers of the oxidase; as CO recombines with ferrous cyochrome a3, these centers are oxidized and the mixed valence carboxy-oxidase is regenerated. Electron redistribution between centers of the oxidase in the forward and reverse directions occurs faster than does the binding of CO.

The possible role of the closed-open transition in proton pumping by cytochrome c oxidase: the pH dependence of cyanide inhibition

The rate of oxidation of reduced cytochrome c catalyzed by cytochrome oxidase in the presence and absence of cyanide has been measured spectrophotometrically at pH 5.5, 6.4, 7.4 and 8.3. At the cytochrome c concentration used (272 microM), the uninhibited rate is maximal at pH 6.4 and drops to a value about one sixth of this maximum at pH 8.3. In the presence of cyanide, the rate initially drops rapidly, but with the cyanide concentration used (5.5 microM) there is still a measurable rate of oxidation when maximal inhibition has been reached. This inhibited rate decreases as the pH increases, whereas the apparent rate constant for cyanide binding is almost independent of pH. The results have been analyzed on the basis of a model in which two-electron reduction of the oxidized enzyme triggers a transition from a closed to an open conformation. It is assumed that cyanide can only bind to the open conformation and, furthermore, that rapid internal electron transfer to the dioxygen-reducing site occurs in this state alone. The analysis shows that the true constant for cyanide binding decreases with decreasing pH to a constant value at low pH. It also indicates that the increase in the catalytic constant with decreasing pH is associated with an increase in the rate of the closed-open conformational transition on protonation of the enzyme, and it is proposed that this transition is operative in electron gating in the proton-pump function of the enzyme.

Rate enhancement of the internal electron transfer in cytochrome c oxidase by the formation of a peroxide complex; its implication on the reaction mechanism of cytochrome c oxidase

The oxidation of reduced cytochrome c oxidase by hydrogen peroxide was investigated with stopped-flow methods. It was reported by us previously (A.C.F. Gorren, H. Dekker and R. Wever (1986) Biochim. Biophys. Acta 852, 81-92) that at low H2O2 concentrations cytochrome a is oxidised simultaneously with cytochrome a3, but that at higher H2O2 concentrations the oxidation of cytochrome a is slower than that of cytochrome a3. We now report that for high peroxide concentrations (10-45 mM) the oxidation rate of cytochrome a increased linearly with the concentration of H2O2 (k = 700 M-1.S-1). Upon extrapolation to zero H2O2 concentration an intercept with a value of 16 s-1 (at 20 degrees C and pH 7.4) was found. A reaction sequence is described to explain these results; according to this model the rate constant (16 S-1) at zero H2O2 concentration represents the true value of the rate of electron transfer from cytochrome a to cytochrome a3 when the a3-CuB site is oxidised and unligated. However, when a complex of hydrogen peroxide with oxidised cytochrome a3 is formed, this rate is strongly enhanced. The slope (700 M-1.S-1) would then represent the rate of cytochrome a3(3+)-H2O2 complex formation. From experiments in which the pH was varied, we conclude that the reaction of H2O2 with cytochrome a3(2+) is independent of pH, whereas the electron-transfer rate from cytochrome a to cytochrome a3 gradually decreases with increasing pH. From the temperature dependence we could calculate values of 23 kJ.mol-1 and 45 kJ.mol-1 for the activation energies of the oxidations by H2O2 of cytochrome a3(2+) and cytochrome a2+, respectively. The similarity of the values that were obtained for cytochrome a oxidation both with H2O2 and with O2 as the electron acceptor suggests that the reactions share the same mechanism. In 2H2O the reactions studied decreased in rate. For the reaction of 2H2O2 with reduced cytochrome a3 in 2H2O, a small effect was found (15% decrease in rate constant). However, the internal electron-transfer rate from cytochrome a to cytochrome a3 decreased by 50%, Our results suggest that the internal electron transfer is associated with proton translocation.

The mechanism of electron gating in proton pumping cytochrome c oxidase: the effect of pH and temperature on internal electron transfer

Electron-transfer reactions following flash photolysis of the mixed-valence cytochrome oxidase-CO complex have been measured at 445, 598 and 830 nm between pH 5.2 and 9.0 in the temperature range of 0-25 degrees C. There is a rapid electron transfer from the cytochrome a3-CuB pair to CuA (time constant: 14200 s-1), which is followed by a slower electron transfer to cytochrome a. Both the rate and the amplitude of the rapid phase are independent of pH, and the rate in the direction from CuA to cytochrome a3-CuB is practically independent of temperature. The second phase depends strongly on pH due to the titration of an acid-base group with pKa = 7.6. The equilibrium at pH 7.4 corresponds to reduction potentials of 225 and 345 mV for cytochrome a and a3, respectively, from which it is concluded that the enzyme is in a different conformation compared to the fully oxidized form. The results have been used to suggest a series of reaction steps in a cycle of the oxidase as a proton pump. Application of the electron-transfer theory to the temperature-dependence data suggests a mechanism for electron gating in the pump. Reduction of both cytochrome a and CuA leads to a conformational change, which changes the structure of cytochrome a3-CuB in such a way that the reorganizational barrier for electron transfer is removed and the driving force is increased.

Structure and function of cytochrome-c oxidase

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

The oxidation of cytochrome c oxidase by hydrogen peroxide

The reaction of H2O2 with mixed-valence and fully reduced cytochrome c oxidase was investigated by photolysis of fully reduced and mixed-valence carboxy-cytochrome c oxidase in the presence of H2O2 under anaerobic conditions. The results showed that H2O2 reacted rapidly (k = (2.5-3.1) X 10(4) M-1 X s-1) with both enzyme species. With the mixed-valence enzyme, the fully oxidised enzyme was reformed. On the time-scale of our experiments, no spectroscopically detectable intermediate was observed. This demonstrates that mixed-valence cytochrome c oxidase is able to use H2O2 as a two-electron acceptor, suggesting that cytochrome c oxidase may under suitable conditions act as a peroxidase. Upon reaction of H2O2 with the fully reduced enzyme, cytochrome a was oxidised before cytochrome a3. From this observation it was possible to estimate that the rate of electron transfer from cytochrome a to a3 is about 0.5-5 s-1.

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

Using the principle of specific vectorial ligand conduction, we outline directly coupled protonmotive O loop and O cycle mechanisms of cytochrome oxidase action that are analogous to protonmotive Q loop and Q cycle mechanisms of QH2 dehydrogenase action. We discuss these directly coupled mechanisms in the light of available experimental knowledge, and suggest that they may stimulate useful new research initiatives designed to elucidate the osmochemistry of protonmotive oxygen reduction in cytochrome oxidase.

Interactions in cytochrome oxidase: functions and structure

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

Spectroscopic evidence for the participation of compound A (Fea32+-O2) in the reaction of mixed-valence cytochrome c oxidase with oxygen at room temperature

1. The reaction of the partially reduced mixed-valence state of cytochrome c oxidase (a3+CuA2+a3(2+)COCuB+) with O2 was studied by the rapid-reaction technique of flow-flash spectrophotometry at room temperature. Biphasic absorption records are observed in the time range up to 2 ms in both the Soret and visible spectral regions. The fast-phase rate is O2-concentration-dependent and reaches a pseudo-first-order value of 4.5 X 10(4)s-1 at 680 microM-O2 at 20 degrees C. Under the same conditions the second-phase rate is limited at 6.0 X 10(3)s-1. Kinetic difference spectra of the two species in the Soret region are not markedly different in form, whereas in the visible region two spectroscopically different species are clearly distinguished. 4. The first intermediate has a peak at 595 nm and a trough at 605 nm. The form of this spectrum resembles that seen in low-temperature studies and assigned to an O2-bound form of ferrocytochrome a3. This evidence supports a structure for oxycytochrome c oxidase with O2 bound only to cytochrome a3 and not bridged between cytochrome a3 and CuB. The second intermediate has a difference spectrum with a trough at 592 nm and a peak at 610 nm. Again, the form of this spectrum is similar to that observed during the O2 reaction at low temperature and is though to be a result of electron transfer from the oxidase to bound O2. 5. The oxygen profile of the fast phase suggests that a spectroscopically silent species may precede the formation of compound A. These data represent the first spectroscopic distinction, in the physiological temperature range, between O2 binding and electron transfer during the O2 reaction of mammalian cytochrome c oxidase. 6. A mechanism is presented for the O2 reaction of the mixed-valence state of cytochrome c oxidase involving four intermediate species. Electron transfer during this reaction is slow, relative to that seen with the fully reduced enzyme, and probably accounts for the detectability of the oxyferro species under these conditions.