Structural models of the redox centres in cytochrome oxidase.

Evolutionary conservation, predicted membrane topography of the subunits, and known chemical and physical properties of the catalytic metals in cytochrome oxidase provided the basis for plausible structural models of the enzyme's redox centres. Subunit II probably binds one of the copper ions (CuA) whilst subunit I is likely to bind the two haems (a and a3) and the other redox-active copper (CuB). Two cysteine and two histidine residues of subunit II are the likely ligands of CuA, forming a centre that may be structurally similar to that in azurin. The two haems may be sandwiched between two transmembranous segments of subunit I, one of which also provides a histidine ligand to CuB. A third segment may provide two more histidine ligands to the latter. The model was constructed with a 4 A Fe-Cu distance in the binuclear haem a3-CuB centre, and a 14 A distance between the haem irons. The subunit I model involves only three transmembranous helices which bind three catalytic metal groups. The fit of this model to several known physicochemical properties of the redox centres is analysed.     

Complete analysis of the cytochrome components of beef heart mitochondria in terms of spectra and redox properties. Cytochromes aa3.

Using newer techniques of data collection that accumulate entire spectra at a series of discrete voltages and newer techniques of analysis that utilize the additional data, we have re-examined the redox behavior and corresponding difference spectra of redox centers responsible for the alpha absorbance features of cytochromes aa3 in beef heart mitochondria. Our analysis reveals three Nernstian components with Em values of 200, 260, and 340 mV with n values of 2, 2, and 1, respectively. The maximum alpha absorbance in the difference spectra for each of these species is located at 602, 605, and 607 nm respectively. Titrations in the presence of carbon monoxide led to the identification of the lowest voltage species as cytochrome a3. The Em of the carbon monoxide-liganded species was not raised. This is contrary to the result expected when a ligand has a much stronger affinity for the reduced form of a redox couple than the oxidized form. It is, however, consistent with a proton-pumping model of cytochrome oxidase in which the binding of ligand results in the dissociation of protons.     

Spectral resolution of four cytochrome b components in mitochondria

The effects of the inhibitors antimycin, 2-heptyl-4-hydroxyquinoline N-oxide (NOQNO) and 2-nonyl 4-hydroxyquinoline N-oxide (NQNO) on the cytochrome b-c₁ region spectrum of submitochondrial particles are compared with those of a new inhibitor 2-ω-cyclohexyl pentyl 3-hydroxy 1,4-naphthoquinone. Dithiothreitol without electron mediators can cause selective reduction of part of the cytochromes b. Evidence for at least four cytochromes b is presented.     

Ubiquinone reduction pattern in pigeon heart mitochondria. Identification of three distinct ubiquinone pools.

Intact pigeon heart mitochondria showed 10-30% ubiquinone reduction in the absence of substrates. This reduction could not be ascribed to endogenous substrates, as judged by lack of effect of inhibitors and uncouplers and by the very low endogenous respiratory rate. Addition of NADH in the presence of antimycin caused further reduction of about 10% ubiquinone, apparently coupled to the rotenone- and antimycin-sensitive exo-NADH oxidase system [Rasmussen (1969) FEBS Lett. 2, 157-162]. Citric acid cycle substrates reduced most of the remaining ubiquinone in the presence of antimycin; 15-20% of the total ubiquinone content was still in the oxidized form under the most reducing conditions. Three pools of ubiquinone therefore appeared to be present in heart mitochondria: a metabolically inactive pool consisting of reduced as well as oxidized ubiquinone, a pool coupled to oxidation of added (cytoplasmic) NADH, and the well-known pool coupled to citric acid cycle oxidations. Ferricyanide selectively oxidized the ubiquinol reduced by added NADH, indicating that this pool is situated on the outer surface of the mitochondrial inner membrane. Ubiquinone reduction levels were determined with a new method, which is described in detail.     

Dicyclohexylcarbodiimide binds to cytochrome b and subunit VIII in soluble complex III from beef heart mitochondria

Incubation of soluble complex III isolated from either yeast or beef heart mitochondria with 25-100 nmol of [14C]dicyclohexylcarbodiimide (DCCD)/nmol of cytochrome b followed by centrifugation through 10% sucrose or precipitation with trichloroacetic acid did not result in any changes in the appearance of the subunits of either complex. The [14C]DCCD was bound to cytochrome b and phospholipids in the yeast complex and with similar kinetics to both cytochrome b and subunit VIII (Mr = 4000-8000) plus phospholipids of the beef complex. Subunit VIII of the beef complex was partially extracted with chloroform:methanol; however, no subunit of this mobility was present in the yeast complex. Incubation of the beef complex in phosphate buffer for short times resulted in a doubling of the [14C]DCCD bound to cytochrome b relative to that to subunit VIII. Preincubation of both complexes with venturicidin prior to treatment with DCCD resulted in a 50% decrease in the binding of [14C]DCCD to cytochrome b. Reisolation of the beef complex III by precipitation with (NH4)2SO4 after incubation with [14C]DCCD resulted in the formation of a new band with an apparent molecular weight of 39,000 even in the zero time control. The [14C]DCCD was bound to subunit VIII and the core proteins but not to cytochrome b at all times, suggesting that precipitation with (NH)2SO4 in the presence of DCCD causes cross-linking of the subunits of complex III.     

Factors determining the special redox properties of photosynthetic cytochrome b559.

Factors controlling the redox properties of the two conventional forms of cytochrome b559, i.e. the unstable high-potential form and the stable low-potential form, have been further investigated using PSII-enriched membranes from pea and spinach chloroplasts. The redox potential of the stable form of cytochrome b559 is pH independent both above pH 7.5 (E'm approximately +110 mV) and below pH 6.0 (E'm approximately +203 mV), but it changes with a slope of 58 mV per pH unit between these two pH values. Thus, cytochrome b559 seems to have a single ionizing group influencing its redox potential, with a higher affinity for protons in the reduced form (pK(red) = 7.5) and a lower affinity in the oxidized form (pK(ox) = 6.0); consequently, one unprotonated low-potential form (LP) and one protonated intermediate-potential form (IP). The redox potential of the high-potential form (HP) is pH-independent between pH 5.0 and 8.0, but its relative content (compared to the total amount of protein) decreases progressively above pH 7.0. This conversion to the stable LP form is interpreted as corresponding to the loss of a proton by one ionizing group, the protonation of which is essential for maintaining the unstable HP state. According to chemical modification experiments with diethylpyrocarbonate, one of the two histidine ligands of the heme seems to be the ionizing group responsible for the existence of both the protonated IP and HP forms. It is proposed that the difference between the IP and HP forms is due to the formation of an additional hydrogen bond between the protonated histidine and the protein in the HP state that stabilizes a special hydrophobic heme environment responsible for its high redox potential.     

Complete analysis of the cytochrome components of beef heart mitochondria in terms of spectra and redox properties. The c1-cytochromes.

Using newer techniques for conducting and analyzing potentiometric titrations, we have studied the thermodynamic and spectral properties of cytochrome c1 in beef heart mitochondria. We find two species of cytochrome c1, both with n = 2 values for the number of electrons involved in their oxidation or reduction. One has an Em approximately 210 mV and a spectral peak near 555 nm and the other has an Em approximately 255 mV and a spectral peak nearer 553 nm. These Em values are pH-independent in the range of pH 6 to 8. The Em and n values of these two components are indistinguishable from those of two species of cytochrome aa3 (i.e. spectral feature of 605 nm).     

Steady-state redox behavior of cytochrome c, cytochrome a, and CuA of cytochrome c oxidase in intact rat liver mitochondria.

We have examined the steady-state redox behavior of cytochrome c (Fec), Fea, and CuA of cytochrome c oxidase during steady-state turnover in intact rat liver mitochondria under coupled and uncoupled conditions. Ascorbate was used as the reductant and TMPD (N,N,N',N'-tetramethyl-1,4-phenylenediamine) as the redox mediator. After elimination of spectroscopic interference from the oxidized form of TMPD, we found that Fea remains significantly more oxidized than previously thought. During coupled turnover, CuA always appears to be close to redox equilibrium with Fec. By increasing the amount of TMPD, both centers can be driven to fairly high levels of reduction while Fea remains relatively oxidized. The reduction level at Fea is close to a linear function of the enzyme turnover rate, but the levels at Fec and CuA do not keep pace with enzyme turnover. This behavior can be explained in terms of a redox equilibrium among Fec, CuA, and Fea, where Fea is the electron donor to the oxygen reduction site, but only if Fea has an effective Em (redox midpoint potential) of 195 mV. This is too low to be accounted for on the basis of nonturnover measurements and the effects of the membrane potential. However, if there is no equilibrium, the internal CuA----Fea electron-transfer rate constant must be slow in the time average (about 200 s-1). Other factors which might contribute to such a low Em are discussed. In the presence of uncoupler, this situation changes dramatically. Both Fec and CuA are much less reduced; within the resolution of our measurements (about 10%), we were unable to measure any reduction of CuA. Fea and CuA remain too oxidized to be in redox equilibrium with Fec during steady-state turnover. Furthermore, our results indicate that, in the uncoupled system, the (time-averaged) internal electron-transfer rate constants in cytochrome oxidase must be of the order of 2500 s-1 or higher. When turnover is slowed by azide, the relative redox levels at Fea and Fec are much closer to those predicted from nonturnover measurements. In presence of uncouplers, Fea is always more reduced than Fec, but in the absence of uncouplers, the two centers track together. Unlike the uninhibited, coupled system, the redox behavior here is consistent with the known effect of the electrical membrane potential on electron distribution in the enzyme. Interestingly, in these circumstances (azide and uncoupler present), Fea behaves as if it were no longer the kinetically controlling electron donor to the bimetallic center.     

Ceramide induces cytochrome c release from isolated mitochondria. Importance of mitochondrial redox state

In the present study we show that N-acetylsphingosine (C2-ceramide), N-hexanoylsphingosine (C6-ceramide), and, to a much lesser extent, C2-dihydroceramide induce cytochrome c (cyto c) release from isolated rat liver mitochondria. Ceramide-induced cyto c release is prevented by preincubation of mitochondria with a low concentration (40 nM) of Bcl-2. The release takes place when cyto c is oxidized but not when it is reduced. Upon cyto c loss, mitochondrial oxygen consumption, mitochondrial transmembrane potential (Delta Psi), and Ca2+ retention are diminished. Incubation with Bcl-2 prevents, and addition of cyto c reverses the alteration of these mitochondrial functions. In ATP-energized mitochondria, ceramides do not alter Delta Psi, neither when cyto c is oxidized nor when it is reduced, ruling out a nonspecific disturbance by ceramides of mitochondrial membrane integrity. Furthermore, ceramides decrease the reducibility of cyto c. We conclude that the apoptogenic properties of ceramides are in part mediated via their interaction with mitochondrial cyto c followed by its release and that the redox state of cyto c influences its detachment by ceramide from the inner mitochondrial membrane.     

Isolation and partial charcterization of a cytochrome b complex and cytochrome oxidase from yeast mitochondria.

A cytochrome b complex and cytochrome oxidase have been purified 14- and 20-fold respectively from yeast submitochondrial particles by a simple procedure involving their spontaneous precipitation from a deoxycholate extract. The recovery of both proteins was almost quantitative. The specific heme contents were 11 and 8 nmoles/mg protein for the cytochrome b complex and cytochrome oxidase respectively and both were spectrally pure. Sodium dodecyl sulfate gel electrophoresis resolved the cytochrome b complex into seven distinct subunits with molecular weights 42,000, 33,000, 27,500, 23,000, 15,500, 13,000 and 10,500. Cytochrome oxidase contained five bands with molecular weights 42,000, 26,500, 21,000, 14,000 and 10,500.     

Reduction of cytochrome b in mitochondria from yeast lacking coenzyme Q

Mitochondria isolated from coenzyme Q deficient yeast cells had no detectable NADH:cytochrome c reductase or succinate:cytochrome c reductase but had comparable amounts of cytochromes b and c1 as wild-type mitochondria. Addition of succinate to the mutant mitochondria resulted in a slight reduction of cytochrome b; however, the subsequent addition of antimycin resulted in a biphasic reduction of cytochrome b, leading to reduction of 68% of the total dithionite-reducible cytochrome b. No "red" shift in the absorption maximum was observed, and no cytochrome c1 was reduced. The addition of either myxothiazol or alkylhydroxynaphthoquinone blocked the reduction of cytochrome b observed with succinate and antimycin, suggesting that the reduction of cytochrome b-562 in the mitochondria lacking coenzyme Q may proceed by a pathway involving cytochrome b at center o where these inhibitors block. Cyanide did not prevent the reduction of cytochrome b by succinate and antimycin the the mutant mitochondria. These results suggest that the succinate dehydrogenase complex can transfer electrons directly to cytochrome b in the absence of coenzyme Q in a reaction that is enhanced by antimycin. Reduced dichlorophenolindophenol (DCIP) acted as an effective bypass of the antimycin block in complex III, resulting in oxygen uptake with succinate in antimycin-treated mitochondria. By contrast, reduced DCIP did not restore oxygen uptake in the mutant mitochondria, suggesting that coenzyme Q is necessary for the bypass. The addition of low concentrations of DCIP to both wild-type and mutant mitochondria reduced with succinate in the presence of antimycin resulted in a rapid oxidation of cytochrome b perhaps by the pathway involving center o, which does not require coenzyme Q.     

Effect of pH on the redox state of cytochrome a in anaerobic, ATP-treated intact mitochondria.

Addition of ATP to anaerobic, glutamate-reduced coupled mitochondria from rat liver or heart caused oxidation of cytochrome a having a peak at 608 nm. Subsequent increase in pH from 7.0 to 8.4 reversed the effect of ATP and subsequent decrease in pH from 8.4 to 7.0 induced reoxidation. This reversible effect of pH on the redox state of cytochrome a may reflect an electrochemical event in the energy-conserving mechanism of the terminal coupling site.     

Rapid redox equilibrium between the mitochondrial Q pool and cytochrome b during triphasic reduction of cytochrome b by succinate

The reliability of monitoring the redox reactions of cytochrome b using the different wavelengths employed by different authors has been reexamined. It was found that 562-575 nm is suitable in succinate: cytochrome c reductase but not in mitochondria, in which case 562-540 nm is a better pair. Direct optical measurements of the redox reaction kinetics of the mitochondrial Q pool using a commercial dual-wavelength spectrophotometer are possible when succinate is used as the electron donor. Using the correct wavelength pair, and with malonate to slow down the electron input, the reduction course of cytochrome b was still triphasic but a plateau or a turn replaced the oxidation phase previously reported by several authors. At the same time, the reduction course of the Q pool was also triphasic, and in perfect match with that of cytochrome b. Destruction of the Rieske iron-sulfur cluster by British anti-Lewisite (BAL) + O2 treatment or prereduction of the high-potential components made the reduction of both Q and b monophasic. The plot of log (Q/QH2) against log (b3+/b2+) gave a straight line with an n value of 1.7 for cytochrome b at pH 7.4. This n value rose to 2.0 at pH 6.5 and dropped to 1.4 at pH 8.5. On the other hand, the mid-point potential of cytochrome b relative to that of the Q pool remained essentially unchanged between pH 6.5 and 8.4. BAL treatment had a small effect on the midpoint potential of cytochrome b relative to that of the Q pool and had no effect on the n value. Addition of quinone homologues and analogues extended the plateau phase in the reduction of cytochrome b, but exogenous quinones did not equilibrate rapidly with cytochrome b. It was concluded that the appearance of the plateau between the two reduction phases of Q and b is caused by the rapid delivery of electrons to the high-potential components of the respiratory chain as envisaged in the Q cycle; the unexpected n value for cytochrome b suggests a concerted reduction by QH2 of two species of cytochromes b-562.