Biochemical and biophysical studies on cytochrome c oxidase. XX. Reaction with sulphide.

1.Upon addition of sulphide to oxidized cytochrome c oxidase, a low-spin heme sulphide compound is formed with an EPR signal at gx = 2.54, gy = 2.23 and gz = 1.87. Concomitantly with the formation of this signal the EPR-detectable low-spin heme signal at g = 3 and the copper signal near g = 2 decrease in intensity, pointing to a partial reduction of the enzyme by sulphide. 2. The addition of sulphide to cytochrome c oxidase, previously reduced in the presence of azide or cyanide, brings about a disappearance of the azido-cytochrome c oxidase signal at gx = 2.9, gy = 2.2, and gz = 1.67 and a decrease of the signal at g = 3.6 of cyano-cytochrome c oxidase. Concomitantly the sulphide-induced EPR signal is formed. 3. These observations demonstrate that azide, cyanide and sulphide are competitive for an oxidized binding site on cytochrome c oxidase. Moreover, it is shown that the affinity of cyanide and sulphide for this site is greater than that of azide.     

Biochemical and biophysical studies on cytochrome c oxidase. XVIII. Potentiometric titrations of cytochrome c oxidase followed by circular dichroism

1. Potentiometric circular dichroism titrations of cytochrome c oxidase, carried out in the absence of cytochrome c, confirm the potentiometric equivalence of the two heme a groups of cytochrome c oxidase. In the presence of cytochrome c, two different midpoint potentials are found for the two heme a groups of cytochrome c oxidase.2. Circular dichroism difference spectra (reduced minus oxidized) of the two heme a components of cytochrome c oxidase have been obtained by means of this potentiometric titration. On reduction of the first heme a group a circular dichroism difference spectrum is obtained with peaks at 425, 442 and 602.5 nm; the second heme a group shows difference peaks at 434, 447 and 608 nm. Whereas both heme a groups contribute about equally to the absorbance difference spectrum, the second heme a group reduced contributes about twice as much to the circular dichroism difference spectrum as does the first heme a group.3. From these spectral and circular dichroism differences it is concluded that, on reduction of or ligand binding to cytochrome c oxidase, conformational changes occur which affect the symmetry of the environments of the heme a groups.     

Biochemical and biophysical studies on cytochrome c oxidase XVI. Circular dichroic study of cytochrome c oxidase and its ligand complexes

1. CD spectra of cytochrome c oxidase have been determined both in the absence and presence of the extrinsic ligands CO, NO, cyanide and azide.2. CO and NO affect the CD spectrum of cytochrome c oxidase in a similar way.3. Cyanide and azide also affect the CD spectrum of cytochrome c oxidase in a similar way, but distinctly different from CO and NO.4. From the CD spectra of the oxidized and reduced enzyme, in the presence and absence of extrinsic ligands, CD difference spectra (reduced minus oxidized) are calculated for the so-called cytochrome a and cytochrome a₃ moieties of the enzyme.5. These spectra are largely dependent on the extrinsic ligand used. It is therefore concluded that these spectra do not represent independent cytochrome a and cytochrome a₃ difference spectra, but that heme-heme interactions occur within the cytochrome c oxidase molecule, in such a way that binding of a ligand to one of the heme a groups of cytochrome c oxidase affects the spectral properties of the other heme a group.6. As a consequence, ligand-binding studies cannot give information as to the pre-existence of separate cytochrome a and cytochrome a₃ moieties in the absence of extrinsic ligands.     

Biochemical and biophysical studies on cytochrome c oxidase XV. Reaction with fluoride

1. Fluoride is a mixed-type inhibitor of the cytochrome c oxidase activity with a K_i for the free enzyme of 10 mM and a K_i for the cytochrome c-complexed enzyme of 35 mM.

2. Fluoride shifts the γ-band of the enzyme from 423 to 421 nm and the -band from 597 to 598 nm. The difference spectrum (oxidized enzyme in the presence of fluoride minus oxidized enzyme) has peaks at 400, 453, 482, 605 and 638 nm and troughs at 430, 520, 552 and 674 nm. The changes in absorbance are small (about 3% at absorbance maxima) with respect to those of other hemoproteins.

3. On addition of fluoride to isolated cytochrome c oxidase 3 reactions can be distinguished: (I) a bimolecular binding reaction (K_on = 4 M⁻¹ · s⁻¹ and k_off = 2.9 · 10⁻²s⁻¹ at 25 °C, pH 7.4) contributing at 638 nm and 430 nm; (II) a first-order reaction (k = 2.4 · 10⁻²) s⁻¹ at 22 °C, pH 7.2) visible mainly at 430 nm and (III) a very slow reaction with a half-time in the order of 10 min.

4. The spectroscopic dissociation constants for the fluoride binding, determined from Hill plots using the absorbance changes at 638 and 430 nm, are similar (7 and 10 mM, respectively, at 22 °C, pH 7.2).

5. A mechanism for the reaction is discussed in which the bimolecular binding reaction is followed by a conformational change of the enzyme-fluoride complex.     

Reactions of mercaptans with cytochrome c oxidase and cytochrome c

1. The steady-state oxidation of ferrocytochrome c by dioxygen catalyzed by cytochrome c oxidase, is inhibited non-competitively towards cytochrome c by methanethiol, ethanethiol, 1-propanethiol and 1-butanethiol with K_i values of 4.5, 91, 200 and 330 μM, respectively.2. The inhibition constant K_i of ethanethiol is found to be constant between pH 5 and 8, which suggests that only the neutral form of the thiol inhibits the enzyme.3. The absorption spectrum of oxidized cytochrome c oxidase in the Soret region shows rapid absorbance changes upon addition of ethanethiol to the enzyme. This process is followed by a very slow reduction of the enzyme. The fast reaction, which represents a binding reaction of ethanethiol to cytochrome c oxidase, has a k₁ of 33 M^?1 · s^?1 and dissociation constant K_d of 3.9 mM.4. Ethanethiol induces fast spectral changes in the absorption spectrum of cytochrome c, which are followed by a very slow reduction of the heme. The rate constant for the fast ethanethiol reaction representing a bimolecular binding step is 50 M^?1 · s^?1 and the dissociation constant is about 2 mM. Addition of up to 25 mM ethanethiol to ferrocytochrome c does not cause spectral changes.5. EPR (electron paramagnetic resonance) spectra of cytochrome c oxidase, incubated with methanethiol or ethanethiol in the presence of cytochrome c and ascorbate, show the formation of low-spin cytochrome a₃-mercaptide compounds with g values of 2.39, 2.23, 1.93 and of 2.43, 2.24, 1.91, respectively.     

Reduction of oxygen-pulsed cytochrome c oxidase by cytochrome c and other electron donors

1. Stopped-flow experiments were performed in which solutions containing dithionite were mixed with air-saturated buffer. Cytochrome c oxidase present in the dithionite-containing syringe is fully oxidized within the mixing time and the oxygen-pulsed form of the oxidase is produced.2. The reduction of this form by dithionite, by dithionite plus cytochrome c and by dithionite plus methyl viologen or benzyl viologen was followed and compared with the corresponding reduction reactions of the ‘resting’ oxidized enzyme. Reduction by dithionite is relatively slow, but the rate of reduction is greatly increased by addition of cytochrome c or the viologens, which are even more effective than cytochrome c on a molar basis.3. Profound differences between the transient kinetics of the reduction of the two oxidized oxidase derivatives were observed. The results are consistent with a direct reduction of cytochrome a followed by an intramolecular electron transfer to cytochrome a₃ (k^obs₁ = 7.5 s^?1 for the oxygen-pulsed oxidase).4. The spectrum of the oxygen-pulsed oxidase formed within 5 ms of the mixing closely resembles that of the ‘oxygenated’ compound, but there were small differences between the two spectra.     

Anion and ionic strength effects upon the oxidation of cytochrome c by cytochrome c oxidase

Citrate and other polyanion binding to ferricytochrome c partially blocks reduction by ascorbate, but at constant ionic strength the citrate-cytochrome c complex remains reducible; reduction by TMPD is unaffected. At a constant high ionic strength citrate inhibits the cytochrome c oxidase reaction competitively with respect to cytochrome c, indicating that ferrocytochrome c also binds citrate, and that the citrateferrocytochrome c complex is rejected by the binding site at high ionic strength. At lower ionic strengths, citrate and other polyanions change the kinetic pattern of ferrocytochrome c oxidation from first-order towards zero-order, indicating preferential binding of the ferric species, followed by its exclusion from the binding site. The turnover at low cytochrome c concentrations is diminished by citrate but not the K_m (apparent non-competitive inhibition) or the rate of cytochrome a reduction by bound cytochrome c. Small effects of anions are seen in direct measurements of binding to the primary site on the enzyme, and larger effects upon secondary site binding. It is concluded that anion-cytochrome c complexes may be catalytically competent but that the redox potentials and/or intramolecular behaviour of such complexes may be affected when enzyme-bound. Increasing ionic strength diminishes cytochrome c binding not only by decreasing the ‘association’ rate but also by increasing the ‘dissociation’ rate for bound cytochrome c converting the ‘primary’ (T) site at high salt concentrations into a site similar kinetically to the ‘secondary’ (L) site at low ionic strength. A finite K_m of 170 μM at very high ionic strength indicates a ratio of $\text{K}{}^{\mathrm{\infty }}{}_{\mathrm{<mtext>M</mtext>}}\text{K}{}^0{}_{\mathrm{<mtext>M</mtext>}}$ of about 5000. It is proposed that anions either modify the E¹₀ of cytochrome c bound at the primary (T) site or that they perturb an equilibrium between two forms of bound c in favour of a less active form.     

Cytochrome c reactivity in its complexes with mammalian cytochrome c oxidase and yeast peroxidase

1.

1. The ascorbate reducibility of cytochrome c (beef or horse heart) in its complexes with cytochrome c oxidase (beef heart) and cytochrome c peroxidase (yeast) has been studied.     

Transmembrane proton translocation by cytochrome c oxidase

Respiratory heme-copper oxidases are integral membrane proteins that catalyze the reduction of molecular oxygen to water using electrons donated by either quinol (quinol oxidases) or cytochrome c (cytochrome c oxidases, CcOs). Even though the X-ray crystal structures of several heme-copper oxidases and results from functional studies have provided significant insights into the mechanisms of O₂-reduction and, electron and proton transfer, the design of the proton-pumping machinery is not known. Here, we summarize the current knowledge on the identity of the structural elements involved in proton transfer in CcO. Furthermore, we discuss the order and timing of electron-transfer reactions in CcO during O₂ reduction and how these reactions might be energetically coupled to proton pumping across the membrane.     

Kinetics of cytochrome c oxidase from yeast. Membrane-facilitated electrostatic binding of cytochrome c showing a specific interaction with cytochrome c oxidase and inhibition by ATP (With an appendix on the occurrence of two-dimensional diffusion of cytochrome c on the membrane surface)

The steady-state kinetics of purified yeast cytochrome c oxidase were investigated at low ionic strength where the electrostatic interaction with cytochrome c is maximized. In 10 mM cacodylate/Tris (pH 6.5) the oxidation kinetics of yeast iso-1-cytochrome c were sigmoidal with a Hill coefficient of 2.35, suggesting cooperative binding. The half-saturation point was 1.14 μM. Horse cytochrome c exhibited Michaelis-Menten kinetics with a higher affinity (K_m = 0.35 μM) and a 100% higher maximal velocity.In 67 mM phosphate the Hill coefficient for yeast cytochrome c decreased to 1.42, and the species differences in Hill coefficients were lessened. Under the latter conditions, a yeast enzyme preparation partially depleted of phospholipids was activated on addition of diphosphatidylglycerol liposomes. When the enzyme was incorporated into sonicated yeast promitochondrial particles the apparent K_m for horse cytochrome c was considerably lower than the value for the isolated enzyme.ATP was found to inhibit both the isolated oxidase and the membrane-bound enzyme. With the isolated enzyme in 10 mM cacodylate/Tris, 3 mM ATP increased the half-saturation point with yeast cytochrome c 3-fold, without altering the maximal velocity or the Hill coefficient. 67 mM phosphate abolished the inhibition of the isolated oxidase by ATP.The increase in affinity for cytochrome c produced by binding the oxidase to the membrane was not observed in the presence of 3 mM ATP, with the result that the membrane-bound enzyme was more sensitive to inhibition by ATP. ADP was a less effective inhibitor than ATP, and did not prevent the inhibition by ATP.It is proposed that non-specific electrostatic binding of cytochrome c to phospholipid membranes, followed by rapid lateral diffusion, is responsible for the dependence of the affinity on the amount and nature of the phospholipids and on the ionic strength.ATP may interfere with the membrane-facilitated binding of cytochrome c by a specific electrostatic interaction with the membrane or by binding to cytochrome c.     

Spectroscopic and electrochemical studies of novel model compounds for cytochrome c oxidase

Different metalated porphyrin compounds were studied as model complexes for cytochrome c oxidase. All models contain a tyrosine molecule and a copper binding site. Two of the compounds are bearing an axial pyridine ligand that could possibly coordinate with Fe porphyrins. All complexes were studied using NMR and UV-Vis spectroscopies and it was found that the coordination of the axial ligand is possible only in one of the porphyrins. Moreover, the synthesized catalysts were studied as promising enzyme mimics using a rotating disc electrode in the presence of molecular oxygen.     

Reaction of CO with cytochrome c oxidase. Titration of the reaction site with chemical oxidant and reductant

The number of reducing equivalents required to form the reduced cytochrome a₃-CO compound has been determined for suspensions of submitochondrial particles and for isolated cytochrome c oxidase. Anaerobic preparations were titrated reductively with NADH and oxidatively with O₂ in the presence of high concentrations of CO (0.4 to 0.8 mM) while monitoring reduction of cytochrome a and the formation of the reduced cytochrome a₃-CO compound by their characteristic absorbance changes. Analysis of the titration data show that 2.0±0.3 and 2.1±0.2 reducing equivalents per mol of cytochrome oxidase (per cytochrome a) are required for formation of the reduced cytochrome a₃-CO compound in submitochondrial particles and isolated cytochrome c oxidase, respectively. In each case, the formation of the CO compound is proportional to the number of equivalents accepted by the preparation, indicating that the two equivalents are equal and the effective n value for the reaction is 2.0. Potentiometric titrations of cytochrome c oxidase using the cobalt orthophenanthrolene complex (E_{m, 7.0} = 0.37 V) as mediator give the same half-reduction potential values for cytochrome a and a₃ as those obtained using the ferro-ferricyanide couple. The formation of the reduced cytochrome a₃-CO compound at pH 7.0, in the presence of 0.6 mM CO and with CO-orthophenanthrolene as mediator occurs with a half-reduction potential of 0.45 V and requires two electrons. These data confirm and extend the observation of Lindsay et al. (Arch. Biochim. Biophys. (1975) 169, 492–505) that both the “invisible” copper and cytochrome a₃ must be reduced in order for CO to bind with high affinity.