Reactions of cytochrome c oxidase with sodium dithionite

Brain Metabolism Monitoring through CCO Measurements Using All-Fiber-Integrated Super-Continuum Source

For monitoring of concussion, brain function, organ condition and other medical applications, what is needed is a non-invasive method of monitoring tissue metabolism. MRI-based functional imaging technology detects changes in blood oxygenation, a correlate of neural activity, and thus may offer a prediction of prognosis in cases of concussion and other cerebral traumas. Yet, potential relationships between perturbations to cerebral metabolism and patient outcomes cannot be effectively exploited clinically because we lack a practical, low-cost, non-invasive means to monitor cerebral oxygenation and metabolism in the emergency department, operating room, or medical facilities. We have developed a device to optically assay the redox state of Cytochrome-C-Oxidase (CCO), the mitochondrial enzyme responsible for the last step of the electron transport chain. Changes in CCO redox reflect changes in respiratory flux, and thus changes in the rate of oxidative adenosine triphosphate (ATP) synthesis. In other words, changes in CCO reflect brain cell's metabolic activity more directly than the traditional blood oxygenation measurement methods. To non-invasively measure changes in CCO as well as blood oxygenation, we have developed a Super-Continuum Infrared Spectroscopy of Cytochrome-C-Oxidase (SCISCCO) system that uses an all-fiber integrated, super-continuum light source to simultaneously measure both of the new (CCO) and the traditional (blood oxygenation) markers of neural metabolism. The SCISCCO system is validated by confirming the near-infrared spectrum of CCO in vitro. To demonstrate in vivo feasibility, the measured responses of oxygenation and CCO responses to acute ischemia (e.g., blood pressure tests) in human participants are compared to data from the literature. Furthermore, we show that the new device's measurements of oxygenated (HbO) and deoxygenated (HbR) hemoglobin in response to breath hold challenges are principled and consistent with previously reported findings. The validated SCISCCO system is finally applied to measure cerebral oxygenation and the redox state of CCO in participants during an attention test protocol. Twenty-five healthy adults completed an attention task that included nine 60-second periods of attention task, interleaved with 60-s periods of resting baseline. It has been well established that the frontal lobe of the human brain is active during tasks of attention. We therefore predicted that attention task should elicit an increase in HbO concentration accompanied by a decrease in redox state of CCO (e.g., ratio of oxidized CCO to reduced CCO) in frontal lobe brain regions as measured with the SCISCCO system. Our findings are consistent with our predictions: HbO concentration increases while CCO concentration decreases during the attention blocks relative to the resting baseline, thereby indicating an increase in oxidative metabolism of the frontal lobe brain regions of interest. Our systematic, multi-method approach thus validates the new device as well as the validity of the metabolic biomarkers that it measures. The SCISCCO system could be a new tool for monitoring brain and organ metabolism, which could be invaluable for screening concussion patients or use in an operating or emergency room to gauge patient's organ response to treatments.

Calcium ions inhibit reduction of heme a in bovine cytochrome c oxidase

The effect of Ca(2+) on the rate of heme a reduction by dithionite and hexaammineruthenium (RuAm) was studied in the cyanide-complexed bovine cytochrome oxidase (CcO). The rate of heme a reduction is proportional to RuAm concentration below 300 μM with kv of 0.53×10(6) M(-1) s(-1). Ca(2+) inhibits the rate of heme a reduction by dithionite by ∼25%. As the reaction speeds up with increased concentrations of RuAm, the inhibition by Ca(2+) disappears. The inhibition of heme a reduction may contribute to recently described partial inhibition of CcO by Ca(2+) in the enzymatic assays. The inhibitory effect of Ca(2+) on heme a reduction indicates that ET through heme a may be coupled to proton movement in the exit part of the proton channel H.

Radical formation in cytochrome c oxidase

The formation of radicals in bovine cytochrome c oxidase (bCcO), during the O(2) redox chemistry and proton translocation, is an unresolved controversial issue. To determine if radicals are formed in the catalytic reaction of bCcO under single turnover conditions, the reaction of O(2) with the enzyme, reduced by either ascorbate or dithionite, was initiated in a custom-built rapid freeze quenching (RFQ) device and the products were trapped at 77K at reaction times ranging from 50μs to 6ms. Additional samples were hand mixed to attain multiple turnover conditions and quenched with a reaction time of minutes. X-band (9GHz) continuous wave electron paramagnetic resonance (CW-EPR) spectra of the reaction products revealed the formation of a narrow radical with both reductants. D-band (130GHz) pulsed EPR spectra allowed for the determination of the g-tensor principal values and revealed that when ascorbate was used as the reductant the dominant radical species was localized on the ascorbyl moiety, and when dithionite was used as the reductant the radical was the SO(2)(-) ion. When the contributions from the reductants are subtracted from the spectra, no evidence for a protein-based radical could be found in the reaction of O(2) with reduced bCcO. As a surrogate for radicals formed on reaction intermediates, the reaction of hydrogen peroxide (H(2)O(2)) with oxidized bCcO was studied at pH 6 and pH 8 by trapping the products at 50μs with the RFQ device to determine the initial reaction events. For comparison, radicals formed after several minutes of incubation were also examined, and X-band and D-band analysis led to the identification of radicals on Tyr-244 and Tyr-129. In the RFQ measurements, a peroxyl (ROO) species was formed, presumably by the reaction between O(2) and an amino acid-based radical. It is postulated that Tyr-129 may play a central role as a proton loading site during proton translocation by ejecting a proton upon formation of the radical species and then becoming reprotonated during its reduction via a chain of three water molecules originating from the region of the propionate groups of heme a(3). This article is part of a Special Issue entitled: "Allosteric cooperativity in respiratory proteins".

Filling the catalytic site of cytochrome c oxidase with electrons. Reduced CuB facilitates internal electron transfer to heme a3

In the reductive phase of its catalytic cycle, cytochrome c oxidase receives electrons from external electron donors. Two electrons have to be transferred into the catalytic center, composed of heme a(3) and Cu(B), before reaction with oxygen takes place. In addition, this phase of catalysis appears to be involved in proton translocation. Here, we report for the first time the kinetics of electron transfer to both heme a(3) and Cu(B) during the transition from the oxidized to the fully reduced state. The state of reduction of both heme a(3) and Cu(B) was monitored by a combination of EPR spectroscopy, the rapid freeze procedure, and the stopped-flow method. The kinetics of cytochrome c oxidase reduction by hexaamineruthenium under anaerobic conditions revealed that the rate-limiting step is the initial electron transfer to the catalytic site that proceeds with apparently identical rates to both heme a(3) and Cu(B). After Cu(B) is reduced, electron transfer to oxidized heme a(3) is enhanced relative to the rate of entry of the first electron.

Determination and novel features of the absolute absorption spectra of the heme a moieties in cytochrome c oxidase

The absolute absorption spectra of the two heme a moieties in cytochrome c oxidase were determined in the Soret region where spectral contributions from copper ions are negligible. This determination employs a set of absorption spectra of the enzyme recorded during anaerobic reduction with sodium dithionite, and does not require any other spectral data. The unique feature of the component spectra revealed in the present study suggests the existence of a specific interaction of heme a with the immediate environment as its origin. The usefulness of the absolute spectra in quantitative analysis of the spectral data is presented.

Functional properties of the quinol oxidase from Acidianus ambivalens and the possible catalytic role of its electron donor--studies on the membrane-integrated and purified enzyme

The aa3 quinol oxidase has been purified from the thermoacidophilic archaea Acidianus ambivalens as a three-redox-centers enzyme. The functional properties of this oxidase both as purified and in its most integral form (i.e. in native membranes and in intact cells) were investigated by stopped-flow spectrophotometry. The results suggest that the enzyme interacts in vivo with a redox-active molecule, which favours the electron entry via heme a and provides the fourth electron demanded for catalysis. We observe that the purified enzyme has two hemes with apparent redox potentials 215 +/- 20 mV and 415 +/- 20 mV at pH 5.4, showing redox-Bohr effect, and a heme a3-CuB center with an affinity for carbon monoxide (Ka = 5.7 x 10(4) M(-1) at 35 degrees C) much lower than that reported for the mammalian enzyme (Ka = 4 x 10(6) M(-1) at 20 degrees C). The reduction by dithionite is fast and monophasic when the quinol oxidase is in the native membranes, whereas it is slow and biphasic in the purified enzyme (with heme a3 being reduced faster than heme a). The oxygen reaction of the reduced purified enzyme is fast (few milliseconds), but yields an intermediate (likely ferryl) clearly different from the fully oxidized enzyme. In contrast, the same reaction performed in intact cells leads to the fully oxidized enzyme. We postulate that caldariella quinol, the physiological electron donor, is in vivo tightly bound to the enzyme, providing the fourth redox active center lacking in the purified enzyme.

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.

Effects of cytochrome c on the oxidation of reduced cytochrome c oxidase by hydrogen peroxide

The oxidation of the redox centres in reduced cytochrome c oxidase by hydrogen peroxide was studied by stopped-flow spectrophotometry in the absence and presence of reduced cytochrome c. The oxidation rate of cytochrome a decreased in the presence of cytochrome c. This effect was more pronounced at low than at high ionic strength. Cytochrome c did not influence the time-course of the oxidation of CuA or cytochrome a3. The oxidation of cytochrome c itself was faster at low ionic strength. The results suggest that the effect of cytochrome c is caused by re-reduction of cytochrome a by cytochrome c, the rate of which is dependent upon the ionic strength. We conclude that cytochrome a and cytochrome c are in equilibrium and that the equilibrium constant depends on the ionic strength. At low ionic strength, as a complex is formed between cytochrome c and cytochrome c oxidase, cytochrome a is more reduced than at high ionic strength conditions, when no such complex exists. Since CuA is oxidized at the same rate whether cytochrome c is present or not, we conclude that electron transfer from cytochrome a or cytochrome c to CuA is slower than electron transfer from CuA to cytochrome a or/and to the cytochrome a2-CuB couple.

Slow ('resting') forms of mitochondrial cytochrome c oxidase consist of two kinetically distinct conformations of the binuclear CuB/a3 centre--relevance to the mechanism of proton translocation

We have purified slow ('resting') cytochrome oxidase from bovine heart, free of contamination with fast ('pulsed') enzyme. This form of the enzyme shows two kinetic phases of reduction of haem a3 by dithionite (k = 0.020 +/- 0.005 s-1 and k = 0.005 +/- 0.002 s-1). The presence of ligands that bind to the oxidized or reduced binuclear centre (formate or carbon monoxide respectively) has no effect on these rates. Varying the dithionite concentration also has no effect on either phase, although at low dithionite concentrations a lag phase is observed as the rate of haem a reduction is slower. The results are consistent with a model for reduction of the slow enzyme where the rate of electron transfer to the binuclear centre is the limiting step, rather than an equilibrium model where the haem a3 redox potential is low. Increasing the pH decreases the rate of the slower phase of dithionite reduction, but has no effect on the faster phase. EPR studies show that the slow phase (only) correlates with the disappearance of the g' = 12/g' = 2.95 signals, with the same pH dependence; again the presence of formate has no effect on these results. Deconvolution of the oxidized optical spectra shows that the enzyme reduced in the slow phase has a blue-shifted Soret band, relative to that reduced in the faster phase. Incubation of the oxidized enzyme at high pH causes a line-broadening of both the g' = 12 and g' = 2.95 EPR signals with no obvious effect on the amount of signal. The results are interpreted in a model where the presence of a carboxylate bridge between haem a3 and CuB defines the slow enzyme. It is suggested that the two rates of dithionite reduction are the result of different ligation to CuB--where water is the ligand the binuclear centre is FeIV/CuI (EPR-silent) and where hydroxide is the ligand the binuclear centre is FeIII/CuII (g' = 12/g' = 2.95 EPR signals).

Kinetics of inhibition of purified and mitochondrial cytochrome c oxidase by psychosine (beta-galactosylsphingosine)

1. Psychosine (beta-galactosylsphingosine) is the toxic agent in Krabbe's disease (globoid cells leukodystrophy). It inhibits purified bovine heart mitochondrial cytochrome c oxidase; there is a rapid phase of inhibition (complete within 10-15 s) and a slower phase (complete within 10-15 min). Both phases are also seen in rat liver mitochondria. IC50 is about 200 microM psychosine in the purified enzyme and less than 20 microM in mitochondria. Psychosine inhibition is due to binding to cytochrome oxidase, not cytochrome c. 2. Bovine heart submitochondrial particles show inhibition similar to rat liver mitochondria. However, although proteoliposomes containing bovine heart cytochrome oxidase show an identical fast phase, they have no noticeable slow phase of inhibition. Addition of phospholipid liposomes to submitochondrial particles relieved the majority of psychosine inhibition, consistent with the removal of those molecules binding in the slow phase. Psychosine can inhibit cytochrome oxidase molecules facing in either direction in proteoliposomes and submitochondrial particles, suggesting that it can rapidly interact with both sides of a membrane when added externally. 3. At high ionic strength, the presence of psychosine decreases the Vmax. of cytochrome oxidase with little effect on the Km for cytochrome c. This non-competitive inhibition suggests that the psychosine-enzyme complex is kinetically inactive and not labile over the time course of the assay. Psychosine does not inhibit the reduction of haem a or haem a3 by artificial electron donors, but does inhibit the reduction of haem a by cytochrome c.

Magnetic state of the alpha 3 center of cytochrome c oxidase and some of its derivatives

The temperature dependence of the magnetic susceptibility was used to investigate the nature of the coupling between cytochrome alpha 3 and CuB in resting and oxidized cyanide- and formate-bound cytochrome oxidase. Resting and formate-bound enzymes were found to have strong antiferromagnetic coupling with an S = 5/2 cytochrome alpha 3, results that were independent of the dispersing detergent and the enzyme isolation method. The cyanide-bound enzyme was heterogeneous, with a minor fraction showing intermediate strength antiferromagnetic coupling. The magnitude of this coupling was independent of the enzyme isolation method and depended moderately on the identity of the dispersing detergent. The major fraction of the cyanide-bound enzyme had a lowest energy state of Ms = 0. The coupling constant for this fraction did not depend on the isolation technique or on the identity of the dispersing detergent. The use of glucose-glucose oxidase to deoxygenate samples influenced the susceptibility behavior of some preparations of both the resting and formate-bound enzymes, with results indicating an S = 3/2 cytochrome alpha 3 in the resting enzyme samples. Retention of a 417-nm Soret band for formate-bound enzyme concomitant with peroxide-induced changes in susceptibility behavior indicates different sites of enzyme interactions for the formate ion and hydrogen peroxide.

Activation by reduction of the resting form of cytochrome c oxidase: tests of different models and evidence for the involvement of CuB

(1) The reaction of the resting form of oxidised cytochrome c oxidase from ox heart with dithionite has been studied in the presence and absence of cyanide. In both cases, cytochrome a reduction in 0.1 M phosphate (pH 7) occurs at a rate of 8.2.10(4) M-1.s-1. In the absence of cyanide, ferrocytochrome a3 appears at a rate (kobs) of 0.016 s-1. Ferricytochrome a3 maintains its 418 nm Soret maximum until reduced. The rate of a3 reduction is independent of dithionite concentration over a range 0.9 mM-131 mM. In the presence or cyanide, visible and EPR spectral changes indicate the formation of a ferric a3/cyanide complex occurs at the same rate as a3 reduction in the absence of cyanide. A g = 3.6 signal appears at the same time as the decay of a g = 6 signal. No EPR signals which could be attributed to copper in any significant amounts could be detected after dithionite addition, either in the presence or absence of cyanide. (2) Addition of dithionite to cytochrome oxidase at various times following induction of turnover with ascorbate/TMPD, results in a biphasic reduction of cytochrome a3 with an increasing proportion of the fast phase of reduction occurring after longer turnover times. At the same time, the predominant steady state species of ferri-cytochrome a3 shifts from high to low spin and the steady-state level of reduction of cytochrome a drops indicating a shift in population of the enzyme molecules to a species with fast turnover. In the final activated form, oxygen is not required for fast internal electron transfer to cytochrome a3. In addition, oxygen does not induce further electron uptake in samples of resting cytochrome oxidase reduced under anaerobic conditions in the presence of cyanide. Both findings are contrary to predictions of certain O-loop types of mechanism for proton translocation. (3) A measurement of electron entry into the resting form of cytochrome oxidase in the presence of cyanide, using TMPD or cytochrome c under anaerobic conditions, shows that three electrons per oxidase enter below a redox potential of around +200 mV. An initial fast entry of two electrons is followed by a slow (kobs approximately 0.02 s) entry of a third electron.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectrophotometric characterization of intermediate redox states of cytochrome oxidase

The spectrophotometric characteristics of hemes a and a3 in cytochrome oxidase have been examined over the range 380 nm to 900 nm. Difference spectra (relative to the oxidized form) are presented for ferrous, high-spin oxidized, low-spin oxidized, early "pulsed," late "pulsed," and two-peroxide-treated states of the enzyme. Comparisons indicate that the decay product of the initial peroxide complex of the enzyme is identical to a low-spin pulsed form of the enzyme. A high-spin pulsed form of the enzyme persists for several hours to days after preparation.

Routes of cytochrome a3 reduction. The neoclassical model revisited

A model for cytochrome oxidase is presented in which cytochrome a, cytochrome a3, and CuB are mutally interacting centers. Cytochrome a3, at equilibrium, is always reduced after CuB. The redox potential of cytochrome a declines progressively as the a3 CuB center is reduced. Dithionite reduction involves up to five steps: (i) reduction of cytochrome a and CuA; (ii) reduction of CuB; (iii) dissociation of ligands (exogenous and endogenous) from cytochrome a3; (iv) spin state changes (high to low) in cyt. a3 and (v) reduction of cytochrome a3. Any of (ii), (iii), and (iv) may be implicated as part of the slow step in this process, which is seen in the resting enzyme.

Reduction of lactoperoxidase by the dithionite anion monomer

The reduction of lactoperoxidase with sodium dithionite has been studied by means of stopped-flow spectrophotometry in an anaerobic system. Under pseudo-first-order conditions the rate constant was found to be linearly dependent on the square root of the dithionite concentration, which confirms the monomeric radical, SO2- as the reducing species. The second-order rate constant is moderately influenced by increased ionic strength but drastically increased at lower pH. The pH dependence supports the previously suggested existence of a carboxyl group, essential to the different enzymatic functions of lactoperoxidase. The second-order rate constant for the reduction of lactoperoxidase at pH 7.0 (kappa 1 = 1.3 X 10(5) M-1 s-1) was about three times higher than the rate constant for the reduction of cyanide-bound lactoperoxidase and two times the rate constant for the reduction of the fluoride-lactoperoxidase complex.

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.

Cyanide inhibition of cytochrome c oxidase. A rapid-freeze e.p.r. investigation

The inhibition of cytochrome c oxidase by cyanide, starting either with the resting or the pulsed enzyme, was studied by rapid-freeze quenching followed by quantitative e.p.r. It is found that a partial reduction of cytochrome oxidase by transfer of 2 electron equivalents from ferrocytochrome c to cytochrome a and CuA will induce a transition from a closed to an open enzyme conformation, rendering the cytochrome a3-CuB site accessible for cyanide binding, possibly as a bridging ligand. A heterogeneity in the enzyme is observed in that an e.p.r. signal from the cytochrome a3 3+-HCN complex is only found in 20% of the molecules, whereas the remaining cyanide-bound a3-CuB sites are e.p.r.-silent.

A comparison of the reactions of cytochrome c oxidase, cytochrome c and azurin with Cr2+ ions

The reduction of cytochrome c oxidase (EC 1.9.3.1) by Cr2+ ions has been studied by stopped-flow spectrophotometry and is compared with the Cr2+ reduction of cytochrome c and azurin. The effects of temperature, pH, and added anions (e.g., SCN-) have been investigated. The behavior of the electron acceptor site of cytochrome c oxidase stands in contrast to that of the other redox proteins with regard to the effects of added anions and we suggest that this reflects the more buried nature of this site in the complex enzyme.

A re-examination of the reactions of cyanide with cytochrome c oxidase

Experiments were performed to examine the cyanide-binding properties of resting and pulsed cytochrome c oxidase in both their stable and transient turnover states. Inhibition of the oxidation of ferrocytochrome c was monitored as a function of cyanide concentration. Cyanide binding to partially reduced forms produced by mixing cytochrome c oxidase with sodium dithionite was also examined. A model is presented that accounts fully for cyanide inhibition of the enzyme, the essential feature of which is the rapid, tight, binding of cyanide to transient, partially reduced, forms of the enzyme populated during turnover. Computer fitting of the experimentally obtained data to the kinetic predictions given by this model indicate that the cyanide-sensitive form of the enzyme binds the ligand with combination constants in excess of 10(6) M-1 X s-1 and with KD values of 50 nM or less. Kinetic difference spectra indicate that cyanide binds to oxidized cytochrome a33+ and that this occurs rapidly only when cytochrome a and CuA are reduced.