Principles of coupling between electron transfer and proton translocation with special reference to proton-translocation mechanisms in cytochrome oxidase

Bacterial Oxidases of the Cytochrome bd Family: Redox Enzymes of Unique Structure, Function, and Utility As Drug Targets

Significance: Cytochrome bd is a ubiquinol:oxygen oxidoreductase of many prokaryotic respiratory chains with a unique structure and functional characteristics. Its primary role is to couple the reduction of molecular oxygen, even at submicromolar concentrations, to water with the generation of a proton motive force used for adenosine triphosphate production. Cytochrome bd is found in many bacterial pathogens and, surprisingly, in bacteria formally denoted as anaerobes. It endows bacteria with resistance to various stressors and is a potential drug target. Recent Advances: We summarize recent advances in the biochemistry, structure, and physiological functions of cytochrome bd in the light of exciting new three-dimensional structures of the oxidase. The newly discovered roles of cytochrome bd in contributing to bacterial protection against hydrogen peroxide, nitric oxide, peroxynitrite, and hydrogen sulfide are assessed. Critical Issues: Fundamental questions remain regarding the precise delineation of electron flow within this multihaem oxidase and how the extraordinarily high affinity for oxygen is accomplished, while endowing bacteria with resistance to other small ligands. Future Directions: It is clear that cytochrome bd is unique in its ability to confer resistance to toxic small molecules, a property that is significant for understanding the propensity of pathogens to possess this oxidase. Since cytochrome bd is a uniquely bacterial enzyme, future research should focus on harnessing fundamental knowledge of its structure and function to the development of novel and effective antibacterial agents.

Direct regulation of cytochrome c oxidase by calcium ions

Cytochrome c oxidase from bovine heart binds Ca²⁺ reversibly at a specific Cation Binding Site located near the outer face of the mitochondrial membrane. Ca²⁺ shifts the absorption spectrum of heme a, which allowed previously to determine the kinetics and equilibrium characteristics of the binding. However, no effect of Ca²⁺ on the functional characteristics of cytochrome oxidase was revealed earlier. Here we report that Ca²⁺ inhibits cytochrome oxidase activity of isolated bovine heart enzyme by 50–60% with K_i of ∼1 µM, close to K_d of calcium binding with the oxidase determined spectrophotometrically. The inhibition is observed only at low, but physiologically relevant, turnover rates of the enzyme (∼10 s⁻¹ or less). No inhibitory effect of Ca²⁺ is observed under conventional conditions of cytochrome c oxidase activity assays (turnover number >100 s⁻¹ at pH 8), which may explain why the effect was not noticed earlier. The inhibition is specific for Ca²⁺ and is reversed by EGTA. Na⁺ ions that compete with Ca²⁺ for binding with the Cation Binding Site, do not affect significantly activity of the enzyme but counteract the inhibitory effect of Ca²⁺. The Ca²⁺-induced inhibition of cytochrome c oxidase is observed also with the uncoupled mitochondria from several rat tissues. At the same time, calcium ions do not inhibit activity of the homologous bacterial cytochrome oxidases. Possible mechanisms of the inhibition are discussed as well as potential physiological role of Ca²⁺ binding with cytochrome oxidase. Ca²⁺- binding at the Cation Binding Site is proposed to inhibit proton-transfer through the exit part of the proton conducting pathway H in the mammalian oxidases.

Energy-converting respiratory Complex I: on the way to the molecular mechanism of the proton pump

In respiring organisms the major energy transduction flux employs the transmembrane electrochemical proton gradient as a physical link between exergonic redox reactions and endergonic ADP phosphorylation. Establishing the gradient involves electrogenic, transmembrane H(+) translocation by the membrane-embedded respiratory complexes. Among others, Complex I (NADH:ubiquinone oxidoreductase) is the most structurally complex and functionally enigmatic respiratory enzyme; its molecular mechanism is as yet unknown. Here we highlight recent progress and discuss the catalytic events during Complex I turnover in relation to their role in energy conversion and to the enzyme structure.

Energy transducing redox steps of the Na+-pumping NADH:quinone oxidoreductase from Vibrio cholerae

Na(+)-NQR is a unique respiratory enzyme that couples the free energy of electron transfer reactions to electrogenic pumping of sodium across the cell membrane. This enzyme is found in many marine and pathogenic bacteria where it plays an analogous role to the H(+)-pumping complex I. It has generally been assumed that the sodium pump of Na(+)-NQR operates on the basis of thermodynamic coupling between reduction of a single redox cofactor and the binding of sodium at a nearby site. In this study, we have defined the coupling to sodium translocation of individual steps in the redox reaction of Na(+)-NQR. Sodium uptake takes place in the reaction step in which an electron moves from the 2Fe-2S center to FMN(C), while the translocation of sodium across the membrane dielectric (and probably its release into the external medium) occurs when an electron moves from FMN(B) to riboflavin. This argues against a single-site coupling model because the redox steps that drive these two parts of the sodium pumping process do not have any redox cofactor in common. The significance of these results for the mechanism of coupling is discussed, and we proposed that Na(+)-NQR operates through a novel mechanism based on kinetic coupling, mediated by conformational changes.

Near-infrared light protect the photoreceptor from light-induced damage in rats

BACKGROUND

A project originally developed for NASA plant growth experiments in space demonstrating the Light-Emitting Diode (LED) could promote the wound healing. Further study showed that the LED's could protect cells by stimulating the basic energy processes in the mitochondria of each cell.

OBJECTIVE

The purpose of this study was to assess the effects of 670 nm LED to protect the photoreceptor from the light-induced damage in a rodent model.

METHODS

SD rats were randomly assigned to one of eight groups: untreated control group, the LED-treated control group, three light-induced damage groups, and three LED-protected groups. The rats were exposed to constant light for 3 h of different illuminations of 900, 1,800 and 2,700 lux, respectively. The LED treatment (50 mW) were done for 30 min, 3 h before the light damage and 0, 24 and 48 h after the light damage. Using the electroretinogram as a sensitive indicator of retinal function, and the histopathologic change was showed as a proof of the protective effect of LED treatment.

RESULTS

The 900 lux illumination for 3 h did not cause damage to the retina of rats, however, the 1,800 lux illumination for 3 h caused significant damage to ONL of an approximate half retina, which caused the swing of ERG b wave to be 431 muV. With the LED protection: the damage of ONL was near 1/6 of retina, which was significantly reduced than the ones without LED protection (P < 0.01); and the swing of ERG b wave was recorded to be 1,011 muV, which was increased significantly than the ones without LED protection (P < 0.01). The illumination of 2,700 lux for 3 h caused severe damage to the rats' retinas and the LED could not protect them significantly in both of morphology and function (P > 0.05, P > 0.05).

CONCLUSIONS

670 nm LED treatment has an evident protective effect on retinal cells against light-induced damage, which may be an innovative and non-invasive therapeutic approach to prevent or to delay age-related macular degeneration.

Functional biomimetic models for the active site in the respiratory enzyme cytochrome c oxidase

A functional analog of the active site in the respiratory enzyme, cytochrome c oxidase (CcO) reproduces every feature in CcO's active site: a myoglobin-like heme (heme a3), a distal tridentate imidazole copper complex (Cu(B)), a phenol (Tyr244), and a proximal imidazole. When covalently attached to a liquid-crystalline SAM film on an Au electrode, this functional model continuously catalyzes the selective four-electron reduction of dioxygen at physiological potential and pH, under rate-limiting electron flux (as occurs in CcO).

Foundations of Vectorial Metabolism and Osmochemistry

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

Thermodynamic and choreographic constraints for energy transduction by cytochrome c oxidase

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

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

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

The generation of proton electrochemical potential gradient by cytochrome c oxidase

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

Identification of essential amino acids within the proposed CuA binding site in subunit II of Cytochrome c oxidase

To explore the nature of proposed ligands to the CuA center in cytochrome c oxidase, site-directed mutagenesis has been initiated in subunit II of the enzyme. Mutations were introduced into the mitochondrial gene from the yeast Saccharomyces cerevisiae by high velocity microprojectile bombardment. A variety of single amino acid substitutions at each of the proposed cysteine and histidine ligands (His-161, Cys-196, Cys-200, and His-204 in the bovine numbering scheme), as well as at the conserved Met-207, all result in yeast which fails to grow on ethanol/glycerol medium. Similarly, all possible paired exchange Cys,His and Cys,Met mutants show the same phenotype. Furthermore, protein stability is severely reduced as evidenced by both the absence of an absorbance maximum at 600 nm in the spectra of mutant cells and the underaccumulation of subunit II, as observed by immunolabeling of mitochondrial extracts. In the same area of the protein, a variety of amino acid substitutions at one of the carboxylates previously implicated in binding cytochrome c, Glu-198, allow (reduced) growth on ethanol/glycerol medium, with normal intracellular levels of protein. These results suggest that a precise folding environment of the CuA site within subunit II is essential for assembly or stable accumulation of cytochrome c oxidase in yeast.

Conformational change due to reduction of cytochrome-c oxidase in lauryl maltoside: picosecond time-resolved tryptophan fluorescence studies on the native and heat modified enzyme

Detailed fluorescence studies on bovine heart cytochrome-c oxidase (CcO) has been carried out in lauryl maltoside solution. Steady-state fluorescence of the tryptophan residues of the enzyme showed that the fluorophores are embedded deep inside the hydrophobic protein cavity. Time resolved studies of tryptophan fluorescence of native and heat treated CcO have been carried out in both reduced and oxidised forms using synchronously pumped pulsed picosecond dye laser and single photon counting technique. Decay of the tryptophan fluorescence have been fitted using discrete four exponential model. Amplitude distribution of lifetimes also showed four distinct regions in the analysis of the decay profiles by maximum entropy method (MEM). The results indicate that controlled heat treatment of CcO affects the conformation of the enzyme near the active centers which makes it incapable of active proton pumping while the electron transfer property is still conserved. Reduction of the native CcO is associated with a large conformation change in lauryl maltoside near the active centers which is not observed in case of CcO encapsulated in vesicles. Reduction of the heat treated enzyme was found to have a conformation different from the reduced native CcO.

Selective interaction of valinomycin/K+ with the cytochrome bf complex of chloroplasts. Synergistic effect with MOA stilbene on extent of cytochrome b563 reduction in continuous light

Valinomycin/K+ is shown to selectively interact at sub-micromolar concentrations with the cytochrome bf complex in thylakoid membranes, inducing a red shift of the ferrohaem b absorbance alpha-band, a slow down of post-illumination b-reoxidation and a corresponding increase of b-reduction level in continuous light. These effects of valinomycin/K+ are not related to its field dissipating action, as they are not affected by nonactin. Presence of K+ is required. Phenomenologically the valinomycin/K+ effects are similar to those caused by 10 times higher MOA stilbene nonactin. Presence of K+ is required. Phenomenologically the valinomycin/K+ effects are similar to those caused by 10 times higher MOA stilbene concentrations. However, synergism is observed between the two inhibitors, suggesting different modes of action. When both inhibitors are combined more than one haem b can be reduced by illumination.

Time-resolved study of tryptophan fluorescence in vesicle reconstituted cytochrome oxidase. Effect of redox transition

Time-resolved study of fluorescence decay of the tryptophan residue in bovine cytochrome c oxidase in phospholipid vesicles is reported for the first time. The effect of the redox state of the protein on its conformation has been investigated using time-resolved decay of tryptophan fluorescence in the oxidised and reduced protein. The fluorescence decay was best fitted using a discrete three exponential model. Amplitude distribution of lifetimes also showed three distinct regions in the analysis of decay profiles by the maximum entropy method (MEM). Results of the time resolved studies showed that the amplitudes as well as the lifetimes of the tryptophan fluorescence remain the same for the oxidised and the reduced states of cytochrome c oxidase, indicating that the environment around tryptophan residues remains more or less unaltered on reduction of the protein. The results suggest that there is no global conformational change in the protein on electron transfer and support the possibility of the existence of local fluctuations in the protein during the redox cycle.

The history of proton-driven ATP formation

This article sets down the beginnings of some thoughts in bio-energetics. It illustrates how difficult it is in science as elsewhere to know how a new idea is generated. The literature needs very careful examination and separation from personalities.

Oxygen activation and the conservation of energy in cell respiration

Many of the membrane-associated oxidases that catalyse respiratory reduction of O2 to water simultaneously couple this exergonic reaction to the translocation of protons across the inner mitochondrial membrane, or the cell membrane in prokaryotes, a process by which metabolic energy is conserved for subsequent synthesis of ATP. The molecular mechanism of O2 reduction and its linkage to H+ translocation are now emerging. The bimetallic haem iron-copper reaction centre in this family of enzymes is the critical structure for catalysis of both these processes.

Reduction of CuA induces a conformational change in cytochrome c oxidase from Paracoccus denitrificans

Cytochrome c oxidase (cytochrome aa3) from Paracoccus denitrificans contains a tightly bound manganese(II) ion, which responds to reduction of the enzyme by a change in its EPR signal (Seelig et al. (1981) Biochim. Biophys. Acta 636, 162-167). In this paper, the nature of this phenomenon is studied and the bound manganese is used as a reporter group to monitor a redox-linked conformational change in the protein. A reductive titration of the cyanide-inhibited enzyme shows that the change in the manganese EPR signal is associated with reduction of CuA. The change appears to reflect a rearrangement in the rhombic octahedral coordination environment of the central Mn2+ atom and is indicative of a redox-linked conformational transition in the enzyme. The manganese is likely to reside at the interface of subunits I and II, near the periplasmic side of the membrane. One of its ligands may be provided by the transmembrane segment X of subunit I, which has been suggested to contribute ligands to cytochrome a and CuB as well. Another manganese ligand is a water oxygen, as indicated by broadening of the manganese EPR signal in the presence of H2(17)O.

Foundations of vectorial metabolism and osmochemistry

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

The osmochemistry of electron-transfer complexes

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

Cytochrome oxidase as a proton pump

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

Redox-linked proton translocation by direct-coupled ligand conduction

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

Structural features of cytochrome oxidase

This article tries to be a compact summary of some recent research on cytochromecoxidase (EC 1.9.3.1), an important enzyme in membrane bioenergetics. Cytochrome oxidase is the terminal catalyst of the mitochondrial respiratory chain. It uses the electrons flowing through the chain to reduce oxygen molecules to water. Four electrons and four protons are consumed in the reduction of O2to two molecules of water (Fig. 1). Cytochrome oxidase contains four redoxactive metal centres. Two of these are copper atoms, two haem A groups. These four centres are employed in the dioxygen-binding site and in the electron-transferring pathways from cytochromec. The enzyme is also called cytochromeaa3, because the protein-bound haems are functionally and spectroscopically different.

The effect of pH on redox titrations of haem a in cyanide-liganded cytochrome-c oxidase: experimental and modelling studies

Isolated cytochrome-c oxidase ligated with cyanide was titrated by Flash-Induced chemical photoREduction (FIRE) (Moody, A.J. and Rich, P.R. (1988) EBEC Short Rep. 5, 69) using cytochrome c as a redox indicator. Haem a is found to titrate in a complex manner consistent with its interacting anticooperatively with at least two other components. We assign CuB as the major interactant at neutral pH, and CuA as the minor interactant. In the pH range 7.0-8.1 the strength of the interaction with CuB is found to decrease with increasing pH, while the interaction with CuA remains essentially constant. The decrease in the interaction with CuB appears to continue above pH 8.1 such that at pH 9.2 the titration curve for haem a is only slightly distorted from an 'n = 1' shape, although it is not possible from the titration data to assess the relative contributions of CuB and CuA to the total interaction observed at pH values greater than 8.1. Haem a and CuB show similar pH-dependence and, to account for this, we present a model in which the oxidoreductions of both haem a and CuB are linked to the (de)protonation of a common acid/base group. The model predicts a pH-dependent indirect cooperative interaction between haem a and CuB in addition to the direct anticooperative interaction, thereby explaining the observed pH-dependence of the redox interaction between haem a and CuB.

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.

Characterization of reductant-induced, tryptophan fluorescence changes in cytochrome oxidase

We have measured the steady-state tryptophan fluorescence spectrum of cytochrome oxidase in its oxidized and fully reduced states. Reduction of the oxidized enzyme by sodium dithionite causes an apparent shift in the fluorescence emission maximum from 328 nm, in the oxidized enzyme, to 348 nm, in the reduced enzyme. This spectroscopic change has been observed previously and assigned to a redox-linked, conformational change in cytochrome oxidase [Copeland, R. A., Smith, P. A., & Chan, S. I. (1987) Biochemistry 26, 7311-7316]. When dithionite-reduced enzyme sits in an open cuvette, the enzyme returns to the oxidized state, and the fluorescence maximum shifts back to 328 nm. However, the time course of the fluorescence change does not follow the redox state of the enzyme, monitored spectrophotometrically at 445,605, and 820 nm, but follows the disappearance of dithionite, which absorbs at 315 nm. Moreover, when the fluorescence emission spectrum of the dithionite-reduced enzyme is corrected for the absorbance due to dithionite, the fluorescence maximum is found 2 nm blue shifted, relative to that of the oxidized enzyme, at 326 nm. This dithionite-induced, red-shifted steady-state tryptophan fluorescence is also seen with the non-heme-containing enzyme carboxypeptidase A. The tryptophan emission spectrum of untreated carboxypeptidase A is at 332 nm, whereas in the presence of dithionite the emission spectrum of carboxypeptidase A is at 350 nm. When corrected for the absorbance of dithionite, the tryptophan emission maximum is at 332 nm. We have also used the photoreductant 3,10-dimethyl-5-deazaisoalloxazine (deazaflavin) to reduce cytochrome oxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of the phosphate-stimulated cytochrome c oxidase activity by thiophosphate

Yeast and mammalian cytochrome c oxidase activity is inhibited by thiophosphate. This inhibition was observed when using either whole mitochondria or the isolated or reconstituted enzyme. The kinetics of the reduction reaction enabled us to demonstrate that thiophosphate acted on the electron transfer between hemes a and a3. With whole mitochondria, phosphate alone stimulated respiration. The inhibition induced by thiophosphate was suppressed by phosphate only in mitochondria, but not when the isolated enzyme was used. The possibility of a kinetic regulation is discussed.

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

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

Intrinsic uncoupling in proton-pumping cytochrome c oxidase: pH dependence of cytochrome c oxidation in coupled and uncoupled phospholipid vesicles

The pH dependence of the transient aerobic kinetics of cytochromes c and a has been investigated with cytochrome oxidase reconstituted in phospholipid vesicles in the absence and presence of an uncoupler and an ionophore. The cytochrome a reduction level immediately after the burst phase was 60-80% and was not significantly changed by the addition of uncoupler and/or ionophore. The coupled rate of ferro-cytochrome c oxidation increases linearly with decreasing pH in the range 8.4-5.4. The increase in rate on uncoupling becomes less with decreasing pH and low cytochrome c concentration, being almost zero at pH 5.4. The coupled rate is increased by a lowering of the outside pH when the inside pH is constant. Varying the inside pH with a constant outside pH of 7.4 has little effect on the rate. It is suggested that the electrochemical potential has two separate effects on the coupled rate: the pH gradient mainly slows down the intramolecular electron transfer, but the membrane potential also lowers the second-order rate constant for the reaction with cytochrome c. The results are interpreted in terms of a model in which protonation of an acid-base group with a pKa of 6.4 from the inside increases the catalytic constant. Protonation from the outside, on the other hand, leads to an intrinsic uncoupling, because the protonated enzyme in the output state can return to the input state. This has no adverse physiological effect, since it becomes significant only at pH values well below 7.

The effect of pH changes on the optical spectrum of oxidised cytochrome oxidase

The sensitivity to pH changes of oxidised cytochrome oxidase, oxidised oxidase-cyanide and oxidase-azide complexes was investigated by optical difference spectroscopy in the pH region 6.0-8.2. The responses of the optical spectrum of oxidised oxidase and the oxidase-cyanide complex are almost fully developed within some minutes after a pH change of 1-2 units. A blue shift of the spectrum of oxidised oxidase and the oxidase-cyanide complex is observed with decreasing pH, whereas the oxidase-azide complex is almost insensitive to the pH change. From the pH insensitivity of the oxidase-azide complex and the differences in the pH-induced spectral changes of the oxidase-cyanide complex relative to unliganded oxidase, it is concluded that the spectral pH sensitivity in the oxidised enzyme is probably associated with proton binding at or near cytochrome a3 only. Similar absorption changes are observed with the oxidised oxidase on increasing the pH and on the binding of azide to oxidised oxidase at constant pH. It is suggested that azide binding is probably associated with the deprotonation of some ionizable group(s) in the vicinity of the cytochrome a3-CuB site.

The location of CuA in mammalian cytochrome c oxidase

Imposition of a protonmotive force across the inner membrane of coupled cyanide-inhibited, beef heart mitochondria by addition of ATP causes reduction of cytochrome c and CuA with concomitant oxidation of haem aA. The data are consistent with previous demonstrations of an intramembrane location of haem aA but further indicate that CuA is very close to the cytosolic surface of the membrane. The implications of this finding for electron transfer route and the site of the proton pumping chemistry are discussed.