Electron and proton transfers through quinones and cytochrome bc complexes

Role of protonatable groups of bovine heart bc(1) complex in ubiquinol binding and oxidation

The pH dependence of the initial reaction rate catalyzed by the isolated bovine heart ubiquinol-cytochrome c reductase (bc1 complex) varying decylbenzoquinol (DBH) and decylbenzoquinone (DB) concentrations was determined. The affinity for DBH was increased threefold by the protonation of a group with pKa = 5.7 +/- 0.2, while the inhibition constant (Ki) for DB decreased 22 and 2.8 times when groups with pKa = 5.2 +/- 0.6 and 7.7 +/- 0.2, respectively, were protonated. This suggests stabilization of the protonated form of the acidic group by DBH binding. Initial rates were best fitted to a kinetic model involving three protonatable groups. The protonation of the pKa approximately 5.7 group blocked catalysis, indicating its role in proton transfer. The kinetic model assumed that the deprotonation of two groups (pKa values of 7.5 +/- 0.03 and approximately 9.2) decreases the catalytic rate by diminishing the redox potential of the iron-sulfur (Fe-S) cluster. The protonation of the pKa approximately 7.5 group also decreased the reaction rate by 80-86%, suggesting its role as acceptor of a proton from ubiquinol. The lack of effect on the Km for DBH when the pKa 7.5-7.7 group is deprotonated suggests that hydrogen bonding to this residue is not the main factor that determines substrate binding to the Qo site. The possible relationship of the pKa 5.2-5.7 and pKa 7.5-7.7 groups with Glu272 of cytochrome b and His161 of the Fe-S protein is discussed.

Structures and proton-pumping strategies of mitochondrial respiratory enzymes

Enzymes of the mitochondrial respiratory chain serve as proton pumps, using the energy made available from electron transfer reactions to transport protons across the inner mitochondrial membrane and create an electrochemical gradient used for the production of ATP. The ATP synthase enzyme is reversible and can also serve as a proton pump by coupling ATP hydrolysis to proton translocation. Each of the respiratory enzymes uses a different strategy for performing proton pumping. In this work, the strategies are described and the structural bases for the action of these proteins are discussed in light of recent crystal structures of several respiratory enzymes. The mechanisms and efficiency of proton translocation are also analyzed in terms of the thermodynamics of the substrate transformations catalyzed by these enzymes.

Competitive decay pathways of the radical ions formed by photoinduced electron transfer between quinones and 4,4'-dimethoxydiphenylmethane in acetonitrile

The reactivity of the cation radical of (4-MeOC6H4)2CH2 photosensitized by 1,4-benzoquinone (BQ), 2,5-dichloro-1,4-benzoquinone (Cl2BQ), and tetrachloro-1,4-benzoquinone (chloranil, CA) was investigated in acetonitrile. The main photoreaction products obtained by steady-state irradiation were identified to be: (4-MeOC6H4)2-CHOC6H4OH, sensitized by BQ; (4-MeOC6H4)2CHCl, sensitized by Cl2BQ; (4-MeOC6H4)2CHOH, sensitized by CA. The mechanism of their formation was investigated by nanosecond laser flash photolysis that allowed transient species (radical ions, neutral radicals, and ions) to be detected and characterized in terms of absorption spectra, formation quantum yields, and decay rate constants. For all systems, the interaction between the triplet quinone (Q) and (4-MeOC6H4)2CH2 produced the corresponding radical ions (quantum yield phi > or = 0.72) which mainly decay by back electron transfer processes. Less efficient reaction routes for the radical ions Q*- and (4-MeOC6H4)2CH2*+ were also: i) the proton-transfer process with the formation of the radical (4-MeOC6H4)2CH* by use of Cl2BQ; ii) the hydrogen-transfer process with the formation of the cation (4-MeOC6H4)2CH+ in the case of CA. Instead. BQ sensitized a much higher yield of BOH* and (4-MeOC6H4)2CH*, mainly by the direct interaction of triplet BQ with (4-MeOC6H4)2CH2. It was also shown that the presence of salts decreases significantly the rate of the back electron transfer process and enhances the quantum yields of formation of the neutral radicals and ions when Cl2BQ and CA are used, respectively. The behavior of BQ*-, Cl2BQ*-, and CA*- appears to be mainly determined by the Mulliken charges on the oxygen atom obtained from quantum mechanical calculations with the model B3LYP/6-311G(d,p). Spin densities seem to be much less important.

The Na+-translocating NADH:quinone oxidoreductase (NDH I) from Klebsiella pneumoniae and Escherichia coli: implications for the mechanism of redox-driven cation translocation by complex I

Eukaryotic complex I integrated into the respiratory chain transports at least 4 H+ per NADH oxidized. Recent results indicate that the cation selectivity is altered to Na+ in complex I (NDH I) isolated from the enterobacteria Escherichia coli and Klebsiella pneumoniae. A sequence analysis illustrates the characteristic differences of the enterobacterial, Na+-translocating NDH I compared to the H+-translocating complex I from mitochondria. Special attention is given to the membranous NuoL (ND5, Nqo12) subunits that possess striking sequence similarities to secondary Na+/H+ antiporters and are proposed to participate in Na+ transport. A model of redox-linked Na+ (or H+) transport by complex I is discussed based on the ion-pair formation of a negatively charged ubisemiquinone anion with a positively charged Na+ (or H+).

Na(+) translocation by bacterial NADH:quinone oxidoreductases: an extension to the complex-I family of primary redox pumps

The current knowledge on the Na(+)-translocating NADH:ubiquinone oxidoreductase of the Na(+)-NQR type from Vibrio alginolyticus, and on Na(+) transport by the electrogenic NADH:Q oxidoreductases from Escherichia coli and Klebsiella pneumoniae (complex I, or NDH-I) is summarized. A general mode of redox-linked Na(+) transport by NADH:Q oxidoreductases is proposed that is based on the electrostatic attraction of a positively charged Na(+) towards a negatively charged, enzyme-bound ubisemiquinone anion in a medium of low dielectricity. A structural model of the [2Fe-2S]- and FAD-carrying NqrF subunit of the Na(+)-NQR from V. alginolyticus based on ferredoxin and ferredoxin:NADP(+) oxidoreductase suggests that a direct participation of the Fe/S center in Na(+) transport is rather unlikely. A ubisemiquinone-dependent mechanism of Na(+) translocation is proposed that results in the transport of two Na(+) ions per two electrons transferred. Whereas this stoichiometry of the pump is in accordance with in vivo determinations of Na(+) transport by the respiratory chain of V. alginolyticus, higher (Na(+) or H(+)) transport stoichiometries are expected for complex I, suggesting the presence of a second coupling site.

Structure and function of cytochrome bc complexes

The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc1 complexes and the chloroplast and cyanobacterial b6f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b6f complex.

Kinetic modeling of the photosynthetic electron transport chain

A simulation model of the photosynthetic electron transport chain operating under steady state conditions is presented. The model enables the calculation of (1) the rates of electron transport and transmembrane proton translocation, (2) the proton/electron stoichiometry, (3) the number of electrons stored in the different redox centers and (4) the stationary transmembrane pH difference. Light intensity and proton permeability of the thylakoid membrane are varied in order to compare the predictions of the model with experimental data. The routes of electron transport and proton translocation are simulated by two coupled arithmetic loops. The first one represents the sequence of reaction steps making up the linear electron transport chain and the Q-cycle. This loop yields the electron flow rate and the proton/electron ratio. The second loop balances the H+ fluxes and yields the internal H+ concentration. The bifurcation of the electron transport pathways at the stage of plastoquinol oxidation is obligatory. The first electron enters always the linear branch and is transferred to photosystem I. The electron of the remaining semiquinone can enter the Q-cycle or, alternatively, the semiquinone can be lost from the cytochrome b6f complex. The competition between these two reactions explains the experimentally observed variability of the proton/electron ratio. We also investigated additional model variants, where the variation of the proton/electron stoichiometry is attributed to other loss reactions within the cytochrome b6f complex. However, the semiquinone detachment seems to be the best candidate for a satisfactory description of the experimental data. Additional calculations were done in order to assess the effects of the movement of the Rieske protein on linear electron transport; it was found that this conformational change does not limit the electron transport rate, if it occurs with a time constant of at least 1000 s(-1).

The cytochrome bc1 and cytochrome c oxidase complexes associate to form a single supracomplex in yeast mitochondria

The mitochondrial electron transport chain complexes are large multisubunit complexes embedded in the inner membrane. We report here that in the yeast Saccharomyces cerevisiae, the cytochrome bc(1) and cytochrome c oxidase complexes co-exist as a larger complex of approximately 1000 kDa in the mitochondrial membrane. Following solubilization with a mild detergent, the cytochrome bc(1)-cytochrome c oxidase complex remains stable. It was analyzed using the techniques of gel filtration and blue native-polyacrylamide gel electrophoresis. Direct physical association of subunits of the cytochrome bc(1) complex with those of the cytochrome c oxidase complex was verified by co-immunoprecipitation analysis. Our data indicate that the cytochrome bc(1) complex is exclusively in association with the cytochrome c oxidase complex in yeast mitochondria. We term this complex the cytochrome bc(1)-cytochrome c oxidase supracomplex.

Evidence for a concerted mechanism of ubiquinol oxidation by the cytochrome bc1 complex

To better understand the mechanism of divergent electron transfer from ubiquinol to the iron-sulfur protein and cytochrome b(L) within the cytochrome bc(1) complex, we have examined the effects of antimycin on the presteady state reduction kinetics of the bc(1) complex in the presence or absence of endogenous ubiquinone. When ubiquinone is present, antimycin slows the rate of cytochrome c(1) reduction by approximately 10-fold but had no effect upon the rate of cytochrome c(1) reduction in bc(1) complex lacking endogenous ubiquinone. In the absence of endogenous ubiquinone cytochrome c(1), reduction was slower than when ubiquinone was present and was similar to that in the presence of ubiquinone plus antimycin. These results indicate that the low potential redox components, cytochrome b(H) and b(L), exert negative control on the rate of reduction of cytochrome c(1) and the Rieske iron-sulfur protein at center P. If electrons cannot equilibrate from cytochrome b(H) and b(L) to ubiquinone, partial reduction of the low potential components slows reduction of the high potential components. We also examined the effects of decreasing the midpoint potential of the iron-sulfur protein on the rates of cytochrome b reduction. As the midpoint potential decreased, there was a parallel decrease in the rate of b reduction, demonstrating that the rate of b reduction is dependent upon the rate of ubiquinol oxidation by the iron-sulfur protein. Together these results indicate that ubiquinol oxidation is a concerted reaction in which both the low potential and high potential redox components control ubiquinol oxidation at center P, consistent with the protonmotive Q cycle mechanism.

Supercomplexes in the respiratory chains of yeast and mammalian mitochondria

Around 30-40 years after the first isolation of the five complexes of oxidative phosphorylation from mammalian mitochondria, we present data that fundamentally change the paradigm of how the yeast and mammalian system of oxidative phosphorylation is organized. The complexes are not randomly distributed within the inner mitochondrial membrane, but assemble into supramolecular structures. We show that all cytochrome c oxidase (complex IV) of Saccharomyces cerevisiae is bound to cytochrome c reductase (complex III), which exists in three forms: the free dimer, and two supercomplexes comprising an additional one or two complex IV monomers. The distribution between these forms varies with growth conditions. In mammalian mitochondria, almost all complex I is assembled into supercomplexes comprising complexes I and III and up to four copies of complex IV, which guided us to present a model for a network of respiratory chain complexes: a 'respirasome'. A fraction of total bovine ATP synthase (complex V) was isolated in dimeric form, suggesting that a dimeric state is not limited to S.cerevisiae, but also exists in mammalian mitochondria.

Na+ translocation by complex I (NADH:quinone oxidoreductase) of Escherichia coli

Following on from our previous discovery of Na+ pumping by the NADH:ubiquinone oxidoreductase (complex I) of Klebsiella pneumoniae, we show here that complex I from Escherichia coli is a Na+ pump as well. Our study object was the Escherichia coli mutant EP432, which lacks the Na+/H+ antiporter genes nhaA and nhaB and is therefore unable to grow on LB medium at elevated Na+ concentrations. During growth on mineral medium, the Na+ tolerance of E. coli EP432 was influenced by the organic substrate. NaCl up to 450 mM did not affect growth on glycerol and fumarate, but growth on glucose was inhibited. Correlated to the Na+ tolerance was an increased synthesis of complex I in the glycerol/fumarate medium. Inverted membrane vesicles catalysed respiratory Na+ uptake with NADH as electron donor. The sodium ion transport activity of vesicles from glycerol/fumarate-grown cells was 40 nmol mg-1 min-1 and was resistant to the uncoupler carbonyl-cyanide m-chlorophenylhydrazone (CCCP), but was inhibited by the complex I-specific inhibitor rotenone. With an E. coli mutant deficient in complex I, the Na+ transport activity was low (1-3 nmol mg-1 min-1), and rotenone was without effect.

Ubiquinone at center N is responsible for triphasic reduction of cytochrome b in the cytochrome bc(1) complex

We have examined the pre-steady state reduction kinetics of the Saccharomyces cerevisiae cytochrome bc(1) complex by menaquinol in the presence and absence of endogenous ubiquinone to elucidate the mechanism of triphasic cytochrome b reduction. With cytochrome bc(1) complex from wild type yeast, cytochrome b reduction was triphasic, consisting of a rapid partial reduction phase, an apparent partial reoxidation phase, and a slow rereduction phase. Absorbance spectra taken by rapid scanning spectroscopy at 1-ms intervals before, during, and after the apparent reoxidation phase showed that this was caused by a bona fide reoxidation of cytochrome b and not by any negative spectral contribution from cytochrome c(1). With cytochrome bc(1) complex from a yeast mutant that cannot synthesize ubiquinone, cytochrome b reduction by either menaquinol or ubiquinol was rapid and monophasic. Addition of ubiquinone restored triphasic cytochrome b reduction, and the duration of the reoxidation phase increased as the ubiquinone concentration increased. When reduction of the cytochrome bc(1) complex through center P was blocked, cytochrome b reduction through center N was biphasic and was slowed by the addition of exogenous ubiquinone. These results show that ubiquinone residing at center N in the oxidized cytochrome bc(1) complex is responsible for the triphasic reduction of cytochrome b.

Energy transduction function of the quinone reactions in cytochrome bc complexes

Cytochrome bc1, a multi-subunit integral membrane protein complex found in mammalian mitochondria, plays a central role in the transfer of electrons and protons generated by the oxidation of ubiquinol. According to the classical chemiosmotic theory, quinones shuttle protons across the hydrophobic membrane bilayer with the net result of H+ transfer to the aqueous side and generation of an electrochemical proton gradient. Recently, high-resolution structures of the mitochondrial bc1 complex showed quinone binding sites at one of the transmembrane helices of cytochrome b, and two potentially protonatable histidine residues on the Rieske iron-sulfur protein. The modern biochemical refinements of the original chemiosmotic theory require electron and proton transfer from quinones to particular residues/redox centers of integral membrane proteins and subsequent transfer of H+ to the bulk aqueous phase outside the membrane.

Proton translocation in the respiratory chain involving ubiquinone--a hypothetical semiquinone switch mechanism for complex I

The protonmotive Q-cycle is the generally accepted reaction scheme of the cytochrome bc1 complex of the respiratory chain. It employs the redox-dependent protonation and deprotonation of ubiquinone (coenzyme Q10) to translocate protons across the inner mitochondrial or bacterial plasma membrane. The requirements for the operation of a similar mechanism in proton translocating NADH:ubiquinone oxidoreductase (complex I) and the specific roles of the 'Rieske' iron-sulfur center in the cytochrome bc1 complex and iron-sulfur center N-2 in complex I are discussed. Recent results suggest that there is only one ubiquinone-reactive site in complex I which seems to exclude a classical Q-cycle type mechanism. Therefore, a "semiquinone switch" mechanism is proposed involving one tightly bound and one substrate quinone. It is based on the same principles as a Q-cycle, but is a localized rather than a ligand conduction type mechanism.

CD-monitored redox titration of the Rieske Fe-S protein of Rhodobacter sphaeroides: pH dependence of the midpoint potential in isolated bc1 complex and in membranes

The redox potential of the Rieske Fe-S protein has been investigated using circular dichroism (CD)-spectroscopy. The CD features characteristic of the purified bc1 complex and membranes of Rhodobacter sphaeroides were found in the region between 450 and 550 nm. The difference between reduced and oxidized CD-spectra shows a negative band at about 500 nm with a half of width 30 nm that corresponds to the specific dichroic absorption of the reduced Rieske protein (Fee, J.A. et al. (1984) J. Biol. Chem. 259, 124-133; Degli Esposti, M. et al. (1987) Biochem. J. 241, 285-290; Rich, P.R. and Wiggins, T.E. (1992) Biochem. Soc. Trans. 20, 241S). It was found that the redox potential at pH 7.0 for the Rieske center in the isolated bc1 complex and in chromatophore membranes from the R-26 strain of Rh. sphaeroides is 300 +/- 5 mV. In chromatophores from the BC17C strain of Rh. sphaeroides, the Em value measured for the Rieske iron-sulfur protein (ISP) was higher (315 +/- 5 mV), but the presence of carotenoids made measurement less accurate. The Em varied with pH in the range above pH 7, and the pH dependence was well fit either by one pK at approximately 7.5 in the range of titration, or by two pK values, pK1 = 7.6 and pK2 = 9.8. Similar titrations and pK values were found for the Rieske Fe-S protein in the isolated bc1 complex and membranes from the R-26 strain of Rb. sphaeroides. The results are discussed in the context of the mechanism of quinol oxidation by the bc1 complex, and the role of the iron sulfur protein in formation of a reaction complex at the Qo-site.

Thermodynamics of electron transfer in Escherichia coli cytochrome bo3

The proton translocation mechanism of the Escherichia coli cytochrome bo3 complex is intimately tied to the electron transfers within the enzyme. Herein we evaluate two models of proton translocation in this enzyme, a cytochrome c oxidase-type ion-pump and a Q-cycle mechanism, on the basis of the thermodynamics of electron transfer. We conclude that from a thermodynamic standpoint, a Q-cycle is the more favorable mechanism for proton translocation and is likely occurring in the enzyme.

On the mechanism of quinol oxidation in the bc1 complex

The question of whether significant levels of a semiquinone can be generated in the Qo site of the bc1 complex under conditions of oxidant-induced reduction is relevant to the mechanism of bifurcation of electron transfer in this site. It has already been reported that beef heart submitochondrial particles under such conditions exhibit an EPR-detectable semiquinone, which is distinct from Q-i and which was attributed to a semiquinone in the Qo site (de Vries, S., Albracht, S. P. J., Berden, J. A., and Slater, E. C. (1981) J. Biol. Chem. 256, 11996-11998). However, we show here that this signal, which can be generated to a level of around 0.1 per bc1 monomer, is insensitive to the Qo site inhibitors myxothiazol, E-beta-methoxyacrylate-stilbene, and stigmatellin, indicating that it does not arise from a Q-o species. Based on sensitivities to inhibitors of other Q sites, up to 60% of the signal may arise from semiquinones of complexes I and II. We further show that the iron-sulfur center remains EPR silent under oxidant-induced reduction conditions. Overall, the results indicate that, under conditions of oxidant-induced reduction, the Qo site is occupied primarily by quinol with the iron-sulfur center oxidized, or, possibly, by an antiferromagnetically coupled semiquinone/reduced iron-sulfur center pair, which are EPR silent. This is discussed in relation to proposed mechanisms of quinol oxidation in the Qo site, and we describe a minimal intermediate-controlled bifurcation model based on rate constants by which bifurcated electron transfer at the Qo site might occur.

Mechanism of ubiquinol oxidation by the cytochrome bc1 complex: pre-steady-state kinetics of cytochrome bc1 complexes containing site-directed mutants of the Rieske iron-sulfur protein

To facilitate characterization of mutated cytochrome bc1 complexes in S. cerevisiae we have developed a new approach using a rapid scanning monochromator to examine pre-steady-state reduction of the enzyme with menaquinol. The RSM records optical spectra of cytochromes b and c1 at 1-ms intervals after a dead time of 2 ms, and menaquinol fully reduces both cytochromes bH and c1 and a portion of cytochrome bL. The rapid-mixing, rapid-scanning monochromator methodology obviates limitations inherent in previous rapid kinetics methods and permits measurements of rates exceeding 200 s-1. To document the validity of this methodology we have examined the reduction kinetics of the cytochrome bc1 complexes from wild-type yeast and yeast that lack ubiquinone. The results establish that menaquinol reacts via the Q cycle pathway both in the presence and absence of ubiquinone. From analyzing bc1 complexes containing Rieske proteins in which the midpoint potential of the iron-sulfur cluster has been altered from +280 to +105 mV, we propose a mechanism in which the protonated quinol displaces a proton from the imidazole nitrogen of one of the histidines that is a ligand to the iron-sulfur cluster and forms a quinol-imidazolate complex that is the electron donor to the redox active iron.

The chemistry and mechanics of ubihydroquinone oxidation at center P (Qo) of the cytochrome bc1 complex

The emerging X-ray structures of the cytochrome bc1 complexes from bovine and chicken heart mitochondria support the protonmotive Q-cycle as the overall electron- and proton-pathway within the cytochrome bc1 complex. The energy conserving reaction within this reaction scheme is the unique bifurcation of electron flow into a high potential and a low potential pathway occurring at the ubihydroquinone-oxidation center (center P or Qo). This step is prerequisite for the 'recycling' of every second electron across the membrane onto the ubiquinone-reduction center, which results in vectorial proton translocation. It has been shown that during steady-state the step controlling this reaction is the first deprotonation of ubihydroquinone and not, as proposed earlier, the formation of a highly unstable semiquinone species. Ubiquinone has not yet been detected at the ubihydroquinone-oxidation center of the protein structures now available, but the pocket seems spacious enough to accommodate two ubiquinone molecules. This is in line with recent enzymological studies, which have shown that not only two ubiquinones, but also two inhibitor molecules can bind to center P. The most striking result from the structures is that the hydrophilic domain of the 'Rieske' protein can be found in two different positions which seem to allow electron transfer between the iron-sulfur cluster and either ubiquinone binding at center P or heme c1. This provides strong support for the 'catalytic switch' model proposed earlier based on detailed analysis of inhibitor binding to cytochrome bc1 complex in different redox states.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH2) as well as that of UQH2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH2.+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH. neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ.- resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ.- uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.

Inhibitor binding within the NarI subunit (cytochrome bnr) of Escherichia coli nitrate reductase A

We have used inhibitors and site-directed mutants to investigate quinol binding to the cytochrome bnr (NarI) of Escherichia coli nitrate reductase (NarGHI). Both stigmatellin and 2-n-heptyl-4-hydroxyquinoline-N-oxide (HOQNO) inhibit menadiol:nitrate oxidoreductase activity with I50 values of 0.25 and 6 microM, respectively, and prevent the generation of a NarGHI-dependent proton electrochemical potential across the cytoplasmic membrane. These inhibitors have little effect on the rate of reduction of the two hemes of NarI (bL and bH), but have an inhibitory effect on the extent of nitrate-dependent heme reoxidation. No quinol-dependent heme bH reduction is detected in a mutant lacking heme bL (NarI-H66Y), whereas a slow but complete heme bL reduction is detected in a mutant lacking heme bH (NarI-H56R). This is consistent with physiological quinol binding and oxidation occurring at a site (QP) associated with heme bL which is located toward the periplasmic side of NarI. Optical and EPR spectroscopies performed in the presence of stigmatellin or HOQNO provide further evidence that these inhibitors bind at a heme bL-associated QP site. These results suggest a model for electron transfer through NarGHI that involves quinol binding and oxidation in the vicinity of heme bL and electron transfer through heme bH to the cytoplasmically localized membrane-extrinsic catalytic NarGH dimer.

Alteration of the midpoint potential and catalytic activity of the rieske iron-sulfur protein by changes of amino acids forming hydrogen bonds to the iron-sulfur cluster

The crystal structure of the bovine Rieske iron-sulfur protein indicates a sulfur atom (S-1) of the iron-sulfur cluster and the sulfur atom (Sgamma) of a cysteine residue that coordinates one of the iron atoms form hydrogen bonds with the hydroxyl groups of Ser-163 and Tyr-165, respectively. We have altered the equivalent Ser-183 and Tyr-185 in the Saccharomyces cerevisiae Rieske iron-sulfur protein by site-directed mutagenesis of the iron-sulfur protein gene to examine how these hydrogen bonds affect the midpoint potential of the iron-sulfur cluster and how changes in the midpoint potential affect the activity of the enzyme. Eliminating the hydrogen bond from the hydroxyl group of Ser-183 to S-1 of the cluster lowers the midpoint potential of the cluster by 130 mV, and eliminating the hydrogen bond from the hydroxyl group of Tyr-185 to Sgamma of Cys-159 lowers the midpoint potential by 65 mV. Eliminating both hydrogen bonds has an approximately additive effect, lowering the midpoint potential by 180 mV. Thus, these hydrogen bonds contribute significantly to the positive midpoint potential of the cluster but are not essential for its assembly. The activity of the bc1 complex decreases with the decrease in midpoint potential, confirming that oxidation of ubiquinol by the iron-sulfur protein is the rate-limiting partial reaction in the bc1 complex, and that the rate of this reaction is extensively influenced by the midpoint potential of the iron-sulfur cluster.

Electrochemical measurement of lateral diffusion coefficients of ubiquinones and plastoquinones of various isoprenoid chain lengths incorporated in model bilayers

The long-range diffusion coefficients of isoprenoid quinones in a model of lipid bilayer were determined by a method avoiding fluorescent probe labeling of the molecules. The quinone electron carriers were incorporated in supported dimyristoylphosphatidylcholine layers at physiological molar fractions (<3 mol%). The elaborate bilayer template contained a built-in gold electrode at which the redox molecules solubilized in the bilayer were reduced or oxidized. The lateral diffusion coefficient of a natural quinone like UQ10 or PQ9 was 2.0 +/- 0.4 x 10(-8) cm2 s(-1) at 30 degrees C, two to three times smaller than the diffusion coefficient of a lipid analog in the same artificial bilayer. The lateral mobilities of the oxidized or reduced forms could be determined separately and were found to be identical in the 4-13 pH range. For a series of isoprenoid quinones, UQ2 or PQ2 to UQ10, the diffusion coefficient exhibited a marked dependence on the length of the isoprenoid chain. The data fit very well the quantitative behavior predicted by a continuum fluid model in which the isoprenoid chains are taken as rigid particles moving in the less viscous part of the bilayer and rubbing against the more viscous layers of lipid heads. The present study supports the concept of a homogeneous pool of quinone located in the less viscous region of the bilayer.

The respiratory chain in yeast behaves as a single functional unit

Inhibitor titrations using antimycin have been used to study the pool behavior of ubiquinone and cytochrome c in the respiratory chain of the yeast Saccharomyces cerevisiae. If present in a homogeneous pool, these carriers should be able to diffuse freely through or along the membrane respectively and accept and subsequently donate electrons to an infinite number of the respective respiratory complex. However, we show that under physiological conditions neither ubiquinone nor cytochrome c exhibits pool behavior, implying that the respiratory chain in yeast is one functional unit. Pool behavior can be introduced for both small carriers by adding chaotropic agents to the reaction medium. We conclude that these agents disrupt the interaction between the respiratory complexes, thereby causing them to become randomly arranged in the membrane. In such a situation, ubiquinone and cytochrome c become mobile carriers, shuttling between the large respiratory complexes. Furthermore, we conclude from the respiratory activities found for different substrates that the respiratory units in yeast vary in composition with respect to the ubiquinone reducing enzyme. All units contain the cytochrome chain, supplemented with either succinate dehydrogenase or the internal or the external NADH dehydrogenase. This implies that when only one substrate is available, only a certain fraction of the cytochrome chain is used in respiration. The molecular organization of the respiratory chain in yeast is compared with that of higher eukaryotes and to the electron transfer systems of photosynthetic membranes. Differences between the organization of the respiratory chain of yeast and that of higher eukaryotes are discussed in terms of the ability of yeast to radically alter its metabolism in response to change of the available carbon source.

An electrochemical approach of the redox behavior of water insoluble ubiquinones or plastoquinones incorporated in supported phospholipid layers

Physiological mole fractions of long isoprenic chain ubiquinone (UQ[10]) and plastoquinone (PQ9) were incorporated inside a supported bilayer by vesicle fusion. The template of the bilayer was an especially designed microporous electrode that allows the direct electrochemistry of water insoluble molecules in a water environment. The artificial structure, made by self-assembly procedures, consisted of a bilayer laterally in contact with a built-in gold electrode at which direct electron transfers between the redox heads of the quinones molecules and the electrode can proceed. The mass balances of quinone and lipid in the structure were determined by radiolabeling and spectrophotometry. A dimyristoyl phosphatdylcholine stable surface concentration of 250 +/- 50 pmol x cm(-2), unaffected by the presence of the quinone, was measured in the fluid monolayer. The mole fraction of quinone was between 1 and 3 mol%, remaining unchanged when going from the vesicles to the supported layers. The lipid molecules and the quinone pool were both laterally mobile, and cyclic voltammetry was used to investigate the redox properties of UQ10 and PQ9 over a wide pH range. Below pH 12, the two electrons-two protons electrochemical process at the gold electrode appeared under kinetic control. Thus all thermodynamic deductions must be anchored in the observed reversibility of the quinone/hydroquinol anion transformation at pH > 13. Within the experimental uncertainty, the standard potentials and the pK(a)'s of the pertinent redox forms of UQ10 and PQ9 were found to be essentially identical. This differs slightly from the literature in which the constants were deduced from the studies of model quinones in mixed solvents or of isoprenic quinones without a lipidic environment.

Redox chains in chloroplast envelope membranes: spectroscopic evidence for the presence of electron carriers, including iron-sulfur centers

We have shown that envelope membranes from spinach chloroplasts contain (i) semiquinone and flavosemiquinone radicals, (ii) a series of iron-containing electron-transfer centers, and (iii) flavins (mostly FAD) loosely associated with proteins. In contrast, we were unable to detect any cytochrome in spinach chloroplast envelope membranes. In addition to a high spin [1Fe]3+ type protein associated with an EPR signal at g = 4.3, we observed two iron-sulfur centers, a [4Fe-4S]1+ and a [2Fe-2S]1+, associated with features, respectively, at g = 1.921 and g = 1.935, which were detected after reduction by NADPH and NADH, respectively. The [4Fe-4S] center, but not the [2Fe-2S] center, was also reduced by dithionite or 5-deazaflavin/oxalate. An unusual Fe-S center, named X, associated with a signal at g = 2.057, was also detected, which was reduced by dithionite but not by NADH or NADPH. Extremely fast spin-relaxation rates of flavin- and quinone-free radicals suggest their close proximity to the [4Fe-4S] cluster or the high-spin [1Fe]3+ center. Envelope membranes probably contain enzymatic activities involved in the formation and reduction of semiquinone radicals (quinol oxidase, NADPH-quinone, and NADPH-semiquinone reductases). The physiological significance of our results is discussed with respect to (i) the presence of desaturase activities in envelope membranes and (ii) the mechanisms involved in the export of protons to the cytosol, which partially regulate the stromal pH during photosynthesis. The characterization of such a wide variety of electron carriers in envelope membranes opens new fields of research on the functions of this membrane system within the plant cell.

Proton-translocation by membrane-bound NADH:ubiquinone-oxidoreductase (complex I) through redox-gated ligand conduction

For the catalytic mechanism of proton-translocating NADH-dehydrogenase (complex I, EC 1.6.99.3) a number of hypothetical models have been proposed over the last three decades. These models are discussed in the light of recent substantial progress on the structure and function of this very complicated multiprotein complex. Only the high-potential iron-sulfur center N-2 and ubiquinone seem to contribute to the proton-translocating machinery of complex I: Based on the pH dependent midpoint potential of iron-sulfur cluster N-2 and the physical properties of ubiquinone intermediates a novel mechanism is proposed. The model builds on a series of defined chemical reactions taking place at three different ubiquinone-binding sites. Therefore, some aspects of this redox-gated ligand conduction mechanism are reminiscent to the proton-motive Q-cycle. However, its central feature is the abstraction of a proton from ubihydroquinone by a redox-Bohr group associated with iron-sulfur cluster N-2. Thus, in the proposed mechanism proton translocation is driven by a direct linkage between redox dependent protonation of iron-sulfur cluster N-2 and the redox chemistry of ubiquinone.

Energy conservation by bifurcated electron-transfer in the cytochrome-bc1 complex

The overall electron- and proton-pathways within the cytochrome-bc1 complex are described by a widely accepted mechanism known as the protonmotive Q-cycle. Within this reaction scheme, the unique bifurcation of electron flow into a high potential and a low potential pathway occurring at the ubihydroquinone-oxidation center is the energy conserving reaction. It is this reaction, which results in vectorial proton translocation, as it allows the 'recycling' of every second electron across the membrane onto the ubiquinone-reduction center. However, the Q-cycle reaction scheme does not address the detailed chemistry of this central step. Based on a structural model of the ubihydroquinone-oxidation pocket and the assumption that the reaction involves two ubiquinone molecules in a stacked configuration, here I propose a detailed chemical model for the reactions occurring during steady-state catalysis. In this proton-gated charge-transfer mechanism the reaction is controlled by the deprotonation of the substrate ubihydroquinone and not, as proposed earlier, by the formation of a highly unstable semiquinone species.

Bifurcated ubihydroquinone oxidation in the cytochrome bc1 complex by proton-gated charge transfer

The unique bifurcation of electron flow at the ubihydroquinone-oxidation center of the cytochrome bc1 complex is the energy-conserving reaction of the protonmotive Q- cycle and is prerequisite to vectorial proton translocation. The widely accepted Q-cycle reaction scheme describes the overall electron and proton pathways, but does not address the detailed chemistry of this central step. Based on a model of the ubihydroquinone-oxidation pocket containing two ubiquinone molecules in a stacked configuration, a detailed model for the reactions during steady-state catalysis is proposed. In this proton-gated charge-transfer mechanism the reaction is controlled by the deprotonation of the substrate ubihydroquinone.

Reaction of the Escherichia coli quinol oxidase cytochrome bo3 with dioxygen: the role of a bound ubiquinone molecule

We have studied the kinetics of the oxygen reaction of the fully reduced quinol oxidase, cytochrome bo3, using flow-flash and stopped flow techniques. This enzyme belongs to the heme-copper oxidase family but lacks the CuA center of the cytochrome c oxidases. Depending on the isolation procedure, the kinetics are found to be either nearly monophasic and very different from those of cytochrome c oxidase or multiphasic and quite similar to cytochrome c oxidase. The multiphasic kinetics in cytochrome c oxidase can largely be attributed to the presence Of CuA as the donor of a fourth electron, which rereduces the originally oxidized low-spin heme and completes the reduction of O2 to water. Monophasic kinetics would thus be expected, a priori, for cytochrome bo3 since it lacks the CuA center, and in this case we show that the oxygen reaction is incomplete and ends with the ferryl intermediate. Multiphasic kinetics thus suggest the presence of an extra electron donor (analogous to CuA). We observe such kinetics exclusively with cytochrome bo3 that contains a single equivalent of bound ubiquinone-8, whereas we find no bound ubiquinone in an enzyme exhibiting monophasic kinetics. Reconstitution with ubiquinone-8 converts the reaction kinetics from monophasic to multiphasic. We conclude that a single bound ubiquinone molecule in cytochrome bo3 is capable of fast rereduction of heme b and that the reaction with O2 is quite similar in quinol and cytochrome c oxidases.

Zinc ions inhibit the QP center of bovine heart mitochondrial bc1 complex by blocking a protonatable group

Bovine heart bc1 complex is reversibly inhibited by zinc ions with an inhibition constant KI of 10(-7) M at pH > or = 7.0. Binding of zinc is at least a factor of 10 tighter than binding of any other metal ion tested. Essentially complete inhibition of ubihydroquinone:cytochrome c oxidoreductase activity is observed at concentrations of [Zn2+] > 5 microM. Zinc does not affect the Km for the substrates, ubihydroquinone or cytochrome c, but zinc inhibits reduction of the cytochromes by ubihydroquinone through the QP center. A radioactive binding assay using 65Zn revealed one high affinity binding site per bc1 complex with KD < or = 10(-7) M at pH = 7.0 and 3-4 additional low affinity binding sites (KD > 2 x 10(-6) M). Zinc binding does not depend on the redox state of the high potential chain (iron-sulfur protein and cytochrome c1). Zinc binds 3 times tighter to Fe-S-depleted bc1 complex indicating that the zinc binding site is not on the "Rieske" iron-sulfur protein in contrast to a recent report by Lorusso et al. (Lorusso, M., Cocco, T., Sardanella, A.M., Minuto, M., Bonomi, F., and Papa, S. (1991) Eur. J. Biochem. 197, 555-561). Zinc binds to a site which has the same affinity for zinc as for protons. We conclude that the zinc binding site is close to a protonatable group of the bc1 complex with pKa = 7.2 which has not been identified previously. We propose that this group is part of the proton channel at the hydroquinone oxidation center of the bc1 complex.

Electrochemical and spectral analysis of the long-range interactions between the Qo and Qi sites and the heme prosthetic groups in ubiquinol-cytochrome c oxidoreductase

The results are presented of an electrochemical and high-resolution spectral analysis of the heme prosthetic groups in the bc1 complex from mouse cells. To study the long-range interactions between the Qo and Qi quinone redox sites and the b heme groups, we analyzed the effects on the proximal and distal b heme groups, and the c1 heme, of inhibitors that tightly and specifically bind to the Qi or Qo redox site. A number of results emerged from these studies. (1) There is inhomogeneous broadening of the b heme alpha band absorption spectra. Furthermore, contrary to the conclusion from low-resolution spectral analysis, the higher energy transition in the split-alpha band spectrum of the bL heme is more intense than the lower energy transition. (2) Inhibitors that bind at the Qi site have significant effects upon the electronic environment of the distal bL heme. Conversely, Qo site inhibitors induced changes in the electronic environment of the distal bH heme. (3) In contrast, inhibitor binding at either site has little effect upon the midpoint potential of the distal heme. (4) Experiments in which both a Qi and a Qo inhibitor are bound at the redox sites indicate that the long-range effects of one inhibitor are not blocked by the second inhibitor; enhanced effects are often observed. (5) In the double-inhibitor titrations involving the Qo inhibitor myxothiazol, there is evidence for two electrochemically and spectrally distinct species of the bL heme group, a phenomenon not observed previously. (6) The high-resolution deconvolutions of alpha band absorption spectra allow an interpretation of these inhibitor-induced changes in terms of homogeneous broadening, inhomogeneous broadening, and changes in x-y degeneracy. The general conclusion from these experiments is that when an inhibitor binds to a quinone redox site of the cytochrome b protein, it produces local conformational changes that, in turn, are transmitted to distal regions of the protein. The ligation of the bH and bL hemes between two parallel transmembrane helices provides a mechanism by which long-distance interactions can be propagated. The lack of long-range effects upon the midpoint potentials of the heme groups suggests, however, that protein conformational changes are unlikely to be a major control mechanism for the transmembrane electron- and proton-transfer steps of the Q cycle.

Structural characterization of isolated mitochondrial cytochrome c1

Resonance Raman spectroscopy (RRS) has been employed to characterize cytochromes c1 isolated from bc1 complexes of beef heart mitochondria and Rhodopseudomonas sphaeroides. The data obtained in this study extend the physical characterization of cytochromes c1 and focus on the effects of the local protein environment on the heme active site. While the general characteristics of the cytochromes c1 are similar to those of smaller soluble cytochromes c, the behavior of several core-size and ligation-sensitive heme modes reveal that significant systematic differences exist between those species. These, most likely, result from changes in the heme axial-ligand interactions.

A proposed pathway of proton translocation through the bc complexes of mitochondria and chloroplasts

The cytochrome bc complexes of the electron transport chain from a wide variety of organisms generate an electrochemical proton gradient which is used for the synthesis of ATP. Proton translocation studies with radiolabeled N,N'-dicyclohexylcarbodiimide (DCCD), the well-established carboxyl-modifying reagent, inhibited proton-translocation 50-70% with minimal effect on electron transfer in the cytochrome bc1 and cytochrome bf complexes reconstituted into liposomes. Subsequent binding studies with cytochrome bc1 and cytochrome bf complexes indicate that DCCD specifically binds to the subunit b and subunit b6, respectively, in a time and concentration dependent manner. Further analyses of the results with cyanogen bromide and protease digestion suggest that the probable site of DCCD binding is aspartate 160 of yeast cytochrome b and aspartate 155 or glutamate 166 of spinach cytochrome b6. Moreover, similar inhibition of proton translocating activity and binding to cytochrome b and cytochrome b6 were noticed with N-cyclo-N-(4-dimethylamino-napthyl)carbodiimide (NCD-4), a fluorescent analogue of DCCD. The spin-label quenching experiments provide further evidence that the binding site for NCD-4 on helix cd of both cytochrome b and cytochrome b6 is localized near the surface of the membrane but shielded from the external medium.

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O2 in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b 6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b 6-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and 'poises' the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO2 can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.

Random mutant generation and its utility in uncovering structural and functional features of cytochrome b in Saccharomyces cerevisiae

The generation of random mutations in the mitochondrial cytochrome b gene of Saccharomyces cerevisiae has been used as a most fruitful means of identifying subregions that play a key role in the bc1 complex mechanism, best explained by the protonmotive Q cycle originally proposed by Peter Mitchell. Selection for center i and center o inhibitor resistance mutants, in particular, has yielded much information. The combined approaches of genetics and structural predictions have led to a two-dimensional folding model for cytochrome b that is most compatible with current knowledge of the protonmotive Q cycle. A three-dimensional model is emerging from studies of distant reversions of deficient mutants. Finally, interactions between cytochrome b and the other subunits of the bc1 complex, such as the iron-sulfur protein, can be affected by a single amino acid change.

The interaction between cytochrome c2 and the cytochrome bc1 complex in the photosynthetic purple bacteria Rhodobacter capsulatus and Rhodopseudomonas viridis

The rates of electron transfer from a ubiquinol analogue to cytochrome c2 catalyzed by the cytochrome bc1 complexes of Rhodobacter capsulatus and Rhodopseudomonas viridis were measured as a function of ionic strength. The effects of ionic strength on the kinetic parameters for the reactions are consistent with a role for electrostatic complex formation between cytochrome c2 and the cytochrome bc1 complex in the electron-transfer pathways in both photosynthetic purple non-sulfur bacteria. Additional support for a docking model in which positively charged lysines on cytochrome c2 interact with negatively charged groups on the Rb. capsulatus cytochrome bc1 complex was obtained from kinetic experiments using Rb. capsulatus cytochrome c2 and equine cytochrome c in which specific lysine residues were altered by site-directed mutagenesis and chemical modification, respectively. Equine cytochrome c, which is a poor electron donor to the reaction center of Rps. viridis, is an effective electron acceptor for the Rps. viridis cytochrome bc1 complex. Chemical modification of lysine residues on Rps. viridis cytochrome c2 has a substantially greater effect on the reduction of the Rps. viridis reaction center by ferrocytochrome c2 than on the oxidation of the Rps. viridis cytochrome bc1 complex by ferricytochrome c2. These data suggest that the docking site for Rps. viridis cytochrome c2 on the Rps. viridis reaction center tetraheme subunit differs in structure from the docking site for the cytochrome on the Rps. viridis cytochrome bc1 complex to a significant extent. In this respect, Rps. viridis differs from photosynthetic purple non-sulfur bacteria in which the reaction center does not contain a tetraheme subunit, where the binding sites for cytochrome c2 on the reaction center and the cytochrome bc1 complex appear to be quite similar.

Electrogenic steps during electron transfer via the cytochrome bc1 complex of Rhodobacter sphaeroides chromatophores

The results of the flash-induced electrometrical investigation on the functioning of the photosynthetic bacterial cytochrome bc1 complex are discussed. The data suggest possible arrangement of redox centers in the bc1 complex and propose that the total electrogenesis within the bc1 complex includes: (i) electron transfer between the low- and the high-potential cytochrome b hemes, (ii) proton binding by doubly reduced Q2- at ubiquinone reducing center 'C', and (iii) proton release on oxidation of QH2 at ubiquinol-oxidizing center 'Z'.

Hydroubiquinone-cytochrome c2 oxidoreductase from Rhodobacter capsulatus: definition of a minimal, functional isolated preparation

The hydroubiquinone-cytochrome c2 oxidoreductase (cyt bc1) from Rhodobacter capsulatus has been solubilized according to the dodecyl maltoside method and isolated, and its minimal functional composition has been characterized. We find the complex to be composed of three protein subunits corresponding to polypeptides of cyt b (44 kDa), cyt c1 (33 kDa), and 2Fe2S cluster (24 kDa). A fourth band sometimes discernable at 22 kDa appears to be an artifact of the polyacrylamide gel electrophoresis procedure. Its appearance is shown to be derived from the 2Fe2S cluster subunit by the similarity of the binding of subunit-specific monoclonal antibodies and the identical N-terminal sequence of the 24- and 22-kDa bands. The cofactors of cyt bc1, namely, cyt bH, cyt bL, cyt c1, and the 2Fe2S center, the Qos and Qow domains of the Qo site, and the Qi site appear intact as indicated by their optical and EPR spectral signatures, redox properties, and inhibitor binding. The electron paramagnetic resonance spectrum of the cyt bH heme is altered by antimycin, consistent with a change in the dihedral angle between the ligating histidine imidazoles, while the spectrum of the cyt bL heme is broadened by stigmatellin. The ubiquinone-10 content is variable, ranging from 0.8 to 3 molecules/cyt bc1. Activity studies define this three-subunit cyt bc1 complex as a minimal structure, equipped as the enzyme in the native state and capable of full catalytic activity.

The cytochrome bc 1 complexes of photosynthetic purple bacteria

Complete nucleotide sequences are now available for the pet (fbc) operons coding for the three electron carrying protein subunits of the cytochrome bc 1 complexes of four photosynthetic purple non-sulfur bacteria. It has been demonstrated that, although the complex from one of these bacteria may contain a fourth subunit, three subunit complexes appear to be fully functional. The ligands to the three hemes and the one [2Fe-2S] cluster in the complex have been identified and considerable progress has been made in mapping the two quinone-binding sites present in the complex, as well as the binding sites for quinone analog inhibitors. Hydropathy analyses and alkaline phosphatase fusion experiments have provided considerable insight into the likely folding pattern of the cytochrome b peptide of the complex and identification of the electrogenic steps associated with electron transport through the complex has allowed the orientation within the membrane of the electron-carrying groups of the complex to be modeled.

Photosystem I cyclic electron transport: Measurement of ferredoxin-plastoquinone reductase activity

Absorbance changes of ferredoxin measured at 463 nm in isolated thylakoids were shown to arise from the activity of the enzyme ferredoxin-plastoquinone reductase (FQR) in cyclic electron transport. Under anaerobic conditions in the presence of DCMU and an appropriate concentration of reduced ferredoxin, a light-induced absorbance decrease due to further reduction of Fd was assigned to the oxidation of the other components in the cyclic pathway, primarily plastoquinone. When the light was turned off, Fd was reoxidised and this gave a direct quantitative measurement of the rate of cyclic electron transport due to the activity of FQR. This activity was sensitive to the classical inhibitor of cyclic electron transport, antimycin, and also to J820 and DBMIB. Antimycin had no effect on Fd reduction although this was inhibited by stigmatellin. This provides further evidence that there is a quinone reduction site outside the cytochrome bf complex. The effect of inhibitors of ferredoxin-NADP(+) reductase and experiments involving the modification of ferredoxin suggest that there may be some role for the reductase as a component of FQR. Contrary to expectations, NADPH2 inhibited FQR activity; ATP and ADP had no effect.