Reaction center and UQH2:cytc 2 oxidoreductase act as independent enzymes inRps. sphaeroides

Turnover of the ubiquinol oxidizing site of the UQH₂:cyt c₂ oxidoreductase (b/c ₁ complex) ofRps. sphaeroides can be assayed by measuring the rate of reduction of cytb ₅₆₁ in the presence of antimycin (AA). Oxidation of ubiquinol is a second-order process, with a value ofk ₂ of about 3 × 10⁵ M^–1. The reaction shows saturation at high quinol concentrations, with an apparentK _m of about 6–8 mM (with respect to the concentration of quinol in the membrane). When the quinone pool is oxidized before illumination, reduction of the complex shows a substantial lag (about 1 ms) after a flash, indicating that the quinol produced as a result of the photochemical reactions is not immediately available to the complex. We have suggested that the lag may be due to several factors, including the leaving time of the quinol from the reaction center, the diffusion time to the complex, and the time for the head group to cross the membrane. We have suggested aminimal value for the diffusion coefficient of ubiquinone in the membrane (assuming that the lag is due entirely to diffusion) of about 10^–9 cm^–2 sec^–1. The lag is reduced to about 100 µsec when the pool is significantly reduced, showing that quinol from the pool is more rapidly available to the complex than that from the reaction center. With the pool oxidized, similar kinetics are seen when the reduction of cytb ₅₆₁ occurs through the AA-sensitive site (with reactions at the quinol oxidizing site blocked by myxothiazol). These results show that there is no preferential reaction pathway for transfer of reducing equivalents from reaction center tob/c ₁ complex. Oxidation of cytb ₅₆₁ through the AA-sensitive site can be assayed from the slow phase of the carotenoid electrochromic change, and by comparison with the kinetics of cytb ₅₆₁. As long as the quinone pool is significantly oxidized, the reaction is not rate-determining for the electrogenic process. On reduction of the pool below 1 quinone per complex, a slowing of the electrogenic process occurs, which could reflect a dependence on the concentration of quinone. If the process is second-order, the rate constant must be about 2–5 times greater than that for quinol oxidation, since the effect on rate is relatively small compared with the effect seen at the quinol oxidizing site when the quinol concentration is changed over theE _h range where the first few quinols are produced on reductive titration. When the quinone pool is extracted (experiments in collaboration with G. Venturoli and B. A. Melandri), the slowing of the electrochromic change on reduction of the pool is not enhanced; we assume that this is due to the fact that a minimum of one quinone per active complex is produced by turnover of the quinol oxidizing site. Two lines of research lead us to revise our previous estimate for the minimal value of the quinone diffusion coefficient. These relate to the relation between the diffusion coefficient and the rate constants for processes involving the quinones: (a) The estimated rate constant for reaction of quinone at the AA-site approaches the calculated diffusion limited rate constant, implying an improbably efficient reaction. (b) From a preliminary set of experiments, the activation energy determined by measuring the variation of the rate constant for quinol oxidation with temperature, is about 8 kcal mol^–1. Although we do not know the contribution of entropic terms to the pre-exponential factor, the result is consistent with a considerably larger value for the diffusion coefficient than that previously suggested.     

Electron transfer from cytochromeb 5 to cytochromec

The reaction of cytochromeb ₅ with cytochromec has become a very prominent system for investigating fundamental questions regarding interprotein electron transfer. One of the first computer modeling studies of electron transfer and protein/protein interaction was reported using this system. Subsequently, numerous studies focused on the experimental determination of the features which control protein/protein interactions. Kinetic measurements of the intracomplex electron transfer reaction have only appeared in the last 10 years. The current review will provide a summary of the kinetic measurements and a critical assessment of the interpretation of these experiments.     

Modes of coenzyme Q function in electron transfer

Summary In the mitochondrial respiratory chain, coenzyme Q acts in different ways. A diffusable coenzyme Q pool as a common substrate-like intermediate links the low-potential complexes with complex III. Its diffusion in the lipids is not rate-limiting for electron transfer, but its content is not saturating for maximal rate of NADH oxidation. Protein-bound coenzyme Q is involved in energy conservation, and may be part of enzyme supercomplexes, as in succinate cytochromec reductase. The reason for lack of kinetic saturation of the respiratory chain by quinone concentration is in the low extent of solubility of monomeric coenzyme Q in the membrane lipids. Assays of respiratory enzymes are performed using water soluble coenzyme Q homologs and analogs; several problems exist in using oxidized quinones as acceptors of coenzyme Q reductases. In particular, for complex I no acceptor appears to favorably substitute the endogenous quinone. In addition, quinone reduction sites in complex III compete with the sites in the dehydrogenases, particularly when using duroquinone. The different extent by which these sites operate when different donor substrates (NADH, succinate, glycerol-3-phosphate) are used is best explained by different exposure of the quinone acceptor sites in the dehydrogenases.     

Functional characterization of the lesion in the ubiquinol: Cytochromec oxidoreductase complex isolated from the nonphotosynthetic strain R126 ofRhodobacter capsulatus

The cytochromebc ₁ complexes from the nonphotosynthetic strain R126 ofRhodobacter capsulatus and from its revertant MR126 were purified. Between both preparations, no difference could be observed in the stoichiometries of the cytochromes, in their spectral properties, and in their midpoint redox potentials. Both also showed identical polypeptide patterns after electrophoresis on polyacrylamide gels in the presence of sodium dodecylsulfate. The ubiquinol: cytochromec oxidoreductase activity was strongly inhibited in the complex from the mutant compared to the one from the revertant. So was the oxidant-induced extra reduction of cytochromeb. Both preparations, however, showed an antimycin-induced red shift of cytochromeb, as well as antimycin-sensitive reduction of cytochromeb by ubiquinol. In accordance with a preceding study of chromatophores (Robertsonet al. (1986).J. Biol. Chem. 261, 584–591), it is concluded that the mutation affects specifically the ubiquinol oxidizing site, leaving the ubiquinol reducing site unchanged.     

The cytochromec reductase/oxidase respiratory pathway ofParacoccus denitrificans: Genetic and functional studies

Data are presented on three components of the quinol oxidation branch of theParacoccus respiratory chain: cytochromec reductase, cytochromec ₅₅₂, and thea-type terminal oxidase. Deletion mutants in thebc ₁ and theaa ₃ complex give insight into electron pathways, assembly processes, and stability of both redox complexes, and, moreover, are an important prerequisite for future site-directed mutagenesis experiments. In addition, evidence for a role of cytochromec ₅₅₂ in electron transport between complex III and IV is presented.     

The reactions of the oxidase and reductases ofParacoccus denitrificans with cytochromesc

Electron transport in theParacoccus denitrificans respiratory chain system is considerably more rapid when it includes the membrane-bound cytochromec ₅₅₂ than with either solubleParacoccus c ₅₅₀ or bovine cytochromec; a pool function for cytochromec is not necessary. Low concentrations ofParacoccus or bovine cytochromec stimulate the oxidase activity. This observation could explain the multiphasic Scatchard plots which are obtained. A negatively charged area on the back side ofParacoccus c which is not present in mitochondrialc could be a control mechanism forParacoccus reactions.Paracoccus oxidase and reductase reactions with bovinec show the same properties as mammalian systems; and this is true ofParacoccus oxidase reactions with its own soluble cytochromec if added polycation masks the negatively charged area. Evidence for different oxidase and reductase reaction sites on cytochromec include: (1) stimulation of the oxidase but not reductase by a polycation; (2) differences in the inhibition of the oxidase and reductases by monoclonal antibodies toParacoccus cytochromec; and (3) reaction of another bacterial cytochromec withParacoccus reductases but not oxidase. Rapid electron transport occurs in cytochromec-less mutants ofParacoccus, suggesting that the reactions result from collision of diffusing complexes.     

Cytochromec oxidase fromParacoccus denitrificans in Triton X-100: Aggregation state and kinetics

Cytochromec oxidase fromParacoccus denitrificans was homogenously dispersed in Triton X-100. Using gel exclusion chromatography and sucrose gradient centrifugation analysis a molecular weight of the detergent-protein complex of 155,000 was determined. After subtraction of the bound detergent (111 mol/mol hemeaa ₃) a molecular weight of 85,000 resulted, which agreed well with the model of a monomer containing two subunits. This monomer showed high cytochromec oxidase activity when measured spectrophotometrically in the presence of Triton X-100 (V _max=85 s^–1). The molecular activity, plotted according to Eadie-Hofstee, was monophasic as a function of the cytochromec concentration. AK _m of 3.6×10^–6 M was evaluated, similar to theK _m observed in the presence of dodecyl maltoside [Naeczet al. (1985).Biochim. Biophys. Acta 808, 259–272].     

A clarification of the effects of DCCD on the electron transfer and antimycin binding of the mitochondrialbc 1 complex

We have studied in detail the effects of dicyclohexylcarbodiimide (DCCD) on the redox activity of the mitochondrialbc ₁ complex, and on the binding of its most specific inhibitor antimycin. An inhibitory action of the reagent has been found only at high concentration of the diimide and/or at prolonged times of incubation. Under these conditions, DCCD also displaced antimycin from its specific binding site in thebc ₁ complex, but did not apparently change the antimycin sensitivity of the ubiquinol-cytochromec reductase activity. On the other hand, using lower DCCD concentrations and/or short times of incubation, i.e., conditions which usually lead to the specific inhibition of the proton-translocating activity of thebc ₁ complex, no inhibitory effect of DCCD could be detected in the ubiquinol-cytochromec reductase activity. However, a clear stimulation of the rate of cytochromeb reduction in parallel to an inhibition of cytochromeb oxidation has been found under these conditions. On the basis of the present work and of previous reports in the literature about the effects of DCCD on thebc ₁ complex, we propose a clarification of the various effects of the reagent depending on the experimental conditions employed.     

基于直接接触的微生物胞外电子传递

微生物电子传递在微生物的代谢繁殖和物质的生物地球化学循环中发挥着关键作用。其中基于直接接触的微生物胞外电子传递(Direct extracellular electron transfer,DEET)已成为微生物学、地球化学和生物物理学等学科共同关注的焦点,并在近几年取得了一系列重要发现和理论突破,包括微生物纳米导线、电缆细菌、微生物种间DEET等。伴随着这些新进展,更多的问题也需要研究者们在进一步的研究中解决,包括DEET的分子机制及其相关功能微生物种群等。不同学科理论和技术的交叉是进一步揭示DEET过程的关键。     

The type I/type II cytochrome c3 complex: an electron transfer link in the hydrogen-sulfate reduction pathway

In Desulfovibrio metabolism, periplasmic hydrogen oxidation is coupled to cytoplasmic sulfate reduction via transmembrane electron transfer complexes. Type II tetraheme cytochrome c3 (TpII-c3), nine-heme cytochrome c (9HcA) and 16-heme cytochrome c (HmcA) are periplasmic proteins associated to these membrane-bound redox complexes and exhibit analogous physiological function. Type I tetraheme cytochrome c3 (TpI-c3) is thought to act as a mediator for electron transfer from hydrogenase to these multihemic cytochromes. In the present work we have investigated Desulfovibrio africanus (Da) and Desulfovibrio vulgaris Hildenborough (DvH) TpI-c3/TpII-c3 complexes. Comparative kinetic experiments of Da TpI-c3 and TpII-c3 using electrochemistry confirm that TpI-c3 is much more efficient than TpII-c3 as an electron acceptor from hydrogenase (second order rate constant k = 9 x 10(8) M(-1) s(-1), K(m) = 0.5 microM as compared to k = 1.7 x 10(7) M(-1) s(-1), K(m) = 40 microM, for TpI-c3 and TpII-c3, respectively). The Da TpI-c3/TpII-c3 complex was characterized at low ionic strength by gel filtration, analytical ultracentrifugation and cross-linking experiments. The thermodynamic parameters were determined by isothermal calorimetry titrations. The formation of the complex is mainly driven by a positive entropy change (deltaS = 137(+/-7) J mol(-1) K(-1) and deltaH = 5.1(+/-1.3) kJ mol(-1)) and the value for the association constant is found to be (2.2(+/-0.5)) x 10(6) M(-1) at pH 5.5. Our thermodynamic results reveal that the net increase in enthalpy and entropy is dominantly produced by proton release in combination with water molecule exclusion. Electrostatic forces play an important role in stabilizing the complex between the two proteins, since no complex formation is detected at high ionic strength. The crystal structure of Da TpI-c3 has been solved at 1.5 angstroms resolution and structural models of the complex have been obtained by NMR and docking experiments. Similar experiments have been carried out on the DvH TpI-c3/TpII-c3 complex. In both complexes, heme IV of TpI-c3 faces heme I of TpII-c3 involving basic residues of TpI-c3 and acidic residues of TpII-c3. A secondary interacting site has been observed in the two complexes, involving heme II of Da TpII-c3 and heme III of DvH TpI-c3 giving rise to a TpI-c3/TpII-c3 molar ratio of 2:1 and 1:2 for Da and DvH complexes, respectively. The physiological significance of these alternative sites in multiheme cytochromes c is discussed.     

Limited reversibility of transmembrane proton transfer assisting transmembrane electron transfer in a dihaem-containing succinate:quinone oxidoreductase

Membrane protein complexes can support both the generation and utilisation of a transmembrane electrochemical proton potential (Δp), either by supporting transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane or by supporting transmembrane proton transfer. The first mechanism has been unequivocally demonstrated to be operational for Δp-dependent catalysis of succinate oxidation by quinone in the case of the dihaem-containing succinate:menaquinone reductase (SQR) from the Gram-positive bacterium Bacillus licheniformis. This is physiologically relevant in that it allows the transmembrane potential Δp to drive the endergonic oxidation of succinate by menaquinone by the dihaem-containing SQR of Gram-positive bacteria. In the case of a related but different respiratory membrane protein complex, the dihaem-containing quinol:fumarate reductase (QFR) of the ?-proteobacterium Wolinella succinogenes, evidence has been obtained that both mechanisms are combined, so as to facilitate transmembrane electron transfer by proton transfer via a both novel and essential compensatory transmembrane proton transfer pathway (“E-pathway”). Although the reduction of fumarate by menaquinol is exergonic, it is obviously not exergonic enough to support the generation of a Δp. This compensatory “E-pathway” appears to be required by all dihaem-containing QFR enzymes and results in the overall reaction being electroneutral. However, here we show that the reverse reaction, the oxidation of succinate by quinone, as catalysed by W. succinogenes QFR, is not electroneutral. The implications for transmembrane proton transfer via the E-pathway are discussed.     

Structural and functional studies on the tetraheme cytochrome subunit and its electron donor proteins: the possible docking mechanisms during the electron transfer reaction

The photosynthetic reaction centers (RCs) classified as the group II possess a peripheral cytochrome (Cyt) subunit, which serves as the electron mediator to the special-pair. In the cycle of the photosynthetic electron transfer reactions, the Cyt subunit accepts electrons from soluble electron carrier proteins, and re-reduces the photo-oxidized special-pair of the bacteriochlorophyll. Physiologically, high-potential cytochromes such as the cytochrome c₂ and the high-potential iron–sulfur protein (HiPIP) function as the electron donors to the Cyt subunit. Most of the Cyt subunits possess four heme c groups, and it was unclear which heme group first accepts the electron from the electron donor. The most distal heme to the special-pair, the heme-1, has a lower redox potential than the electron donors, which makes it difficult to understand the electron transfer mechanism mediated by the Cyt subunit. Extensive mutagenesis combined with kinetic studies has made a great contribution to our understanding of the molecular interaction mechanisms, and has demonstrated the importance of the region close to the heme-1 in the electron transfer. Moreover, crystallographic studies have elucidated two high-resolution three-dimensional structures for the RCs containing the Cyt subunit, the Blastochloris viridis and Thermochromatium tepidum RCs, as well as the structures of their electron donors. An examination of the structural data also suggested that the binding sites for both the cytochrome c₂ and the HiPIP are located adjacent to the solvent-accessible edge of the heme-1. In addition, it is also indicated by the structural and biochemical data that the cytochrome c₂ and the HiPIP dock with the Cyt subunit by different mechanisms although the two electron donors utilize the same region for the interactions; cytochrome c₂ is recognized through electrostatic interactions while hydrophobic interactions are important in the HiPIP docking.     

Cytochromec oxidase metal centers: Location and function

Cytochromec oxidase ofParacoccus denitrificans is spectroscopically and functionally very similar to the mammalian enzyme. However, it has a very much simpler quaternary structure, consisting of only three subunits instead of the 13 of the bovine enzyme. The known primary structure of theParacoccus denitrificans subunits, the knowledge of a large number of sequences from other species, and data on the controlled proteolytic digestion of the enzyme allow structural restrictions to be placed on the models describing the binding of the active metal centers to the polypeptide structure.     

Photo-oxidation of membrane-bound and soluble cytochromec in the green sulfur bacteriumChlorobium tepidum

We studied the photosynthetic electron transfer system of membrane-bound and soluble cytochromec inChlorobium tepidum, a thermophilic green sulfur bacterium, using whole cells and membrane preparations. Sulfide and thiosulfate, physiological electron donors, enhanced flash-induced photo-oxidation ofc-type cytochromes in whole cells. In membranes,c-553 cytochromes with two (or three) heme groups served as immediate electron donors for photo-oxidized bacteriochlorophyll (P840) in the reaction center, and appeared to be closely associated with the reaction center complex. The membrane-bound cytochromec-553 had anE _m-value of 180 mV. When isolated soluble cytochromec-553, which has an apparent molecular weight of 10 kDa and seems to correspond to the cytochromec-555 inChlorobium limicola andChlorobium vibrioforme, was added to a membrane suspension, rapid photo-oxidation of both soluble and membrane-bound cytochromesc-553 was observed. The oxidation of soluble cytochromec-553 was inhibited by high salt concentrations. In whole cells, photo-oxidation was observed in the absence of exogenous electron donors and re-reduction was inhibited by stigmatellin, an inhibitor of the cytochromebc complex. These results suggest that the role of membrane-bound and soluble cytochromec inC. tepidum is similar to the role of cytochromec in the photosynthetic electron transfer system of purple bacteria.     

Dark-interval relaxation kinetics (DIRK) of absorbance changes as a quantitative probe of steady-state electron transfer

We introduce a new, non-invasive technique to measure linear electron transfer in intact leaves under steady-state illumination. Dark-interval relaxation kinetic or ‘DIRK’ analysis is based on measurements of the initial rates of relaxation of steady-state absorbance signals upon a rapid light-dark transition. We show that estimates of electron flux by DIRK analysis of absorbance signals, reflecting redox changes in the photosynthetic electron transfer chain, can yield quantitative information about photosynthetic flux when the light-dependent partitioning of electrons among redox components of the electron transfer chain are considered. This concept is modeled in computer simulations and then demonstrated in vivo with tobacco plants under non-photorespiratory conditions resulting in linear relationships between DIRK analysis and gross carbon assimilation (A_G). Estimation based on DIRK analysis of the number of electrons transferred through the photosynthetic apparatus for each CO₂ fixed was within 20% of the theoretical value. Possible errors and future improvements are discussed. We conclude that the DIRK method represents a useful tool to address issues such as plant stress and photosynthetic regulation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evidence for two independent pathways of electron transfer in mitochondrial NADH:Q oxidoreductase. I. Pre-steady-state kinetics with NADPH

The reduction of NADH:Q oxidoreductase by NADPH occurring in submitochondrial particles has been studied with the freeze-quench technique. It was found that 50% of the Fe-S clusters 2, 3 and 4 could be reduced by NADPH within 30 ms at pH 6.5. The remainder of the clusters, including cluster 1, were reduced slowly and incompletely; it was concluded that these clusters play no role in the NADPH oxidase activity. Nearly the same results were obtained at pH 8 under anaerobic conditions, demonstrating that the rate of reaction of NADPH with the enzyme was essentially the same at both pH values. The rate and extent of reduction of half of the clusters 2 by NADPH at pH 8 were not affected by the presence of O₂ of rotenone. This implies a pH-dependent oxidation of the enzyme as the cause for the absence of the NADPH oxidase activity at this pH. A dimeric model of the enzyme is proposed in which one protomer, containing FMN and the Fe-S clusters 1–4 in stoichiometric amounts, is responsible for NADH oxidation at pH 8. This protomer cannot react with NADPH. The other protomer, containing only FMN and the clusters 2, 3 and 4, is supposed to catalyse the oxidation of NADPH. The oxidation of this protomer by ubiquinone is expected to be strongly dependent on pH. This protomer might also catalyse NADH oxidation at pH 6–6.5.     

On the role of cytochrome c8 in photosynthetic electron transfer of the purple non-sulfur bacterium Rhodoferax fermentans

We report on the isolation, purification and functional characterization of a soluble c-type cytochrome from light-grown cells of the purple phototroph Rhodoferax fermentans. This cytochrome is basic (pI = 8), has a molecular mass of 12 kDa, and is characterized by a midpoint reduction potential of +285 mV. Partial analysis of the N-terminus amino-acid sequence shows a high similarity with cytochromes of c₈ type (formerly called Pseudomonas cytochrome c-551 type). Time-resolved spectrophotometric studies show that this cytochrome c₈ reduces the tetraheme subunit of the photosynthetic reaction center, in a fast (sub-ms) and a slow (ms) phase. Competition experiments in the presence of both cytochrome c₈ and high potential iron-sulfur protein (HiPIP), isolated from the same microorganism, show that cytochrome c₈ oxidation is decreased upon addition of HiPIP. These observations suggest that cytochrome c₈ and HiPIP might play alternative roles in the photosynthetic electron flow of Rhodoferax fermentans.     

Evidence for two independent pathways of electron transfer in mitochondrial NADH:Q oxidoreductase. II. Kinetics of reoxidation of the reduced enzyme

The pre-steady-state kinetics of reoxidation of NADH:Q oxidoreductase present in submitochondrial particles has been studied by the freeze-quench method. It was found that at pH 8 only 50% of the Fe-S clusters 2 and 4 and 75% of the clusters 3 were rapidly reoxidised after transient and complete reduction by a pulse of NADH in the presence of excess NADPH. Thus, NADPH keeps 50% of the clusters 2 and 4 and 25% of the clusters 3 permanently reduced at this pH. Since NADH oxidation is nearly optimal at this pH, whereas NADPH oxidation is virtually absent, it was concluded that these permanently reduced clusters were not involved in the NADH oxidation activity. Incomplete reoxidation of the clusters 2, 3 and 4 after a pulse of NADH was also found in the absence of NADPH, both at pH 6.5 and at pH 8. A pulse of NADPH given at pH 6.5, where NADPH oxidation by oxygen is nearly optimal, caused a slow reduction of 50% of clusters 2 and 4 and 30% of the clusters 3, which persisted for a period of at least 15 s. It was concluded that these clusters were not involved in the oxidation of NADPH by oxygen, as catalysed by the particles. As a working hypothesis a dimeric model for NAD(P)H:Q oxidoreductase is proposed, consisting of two different protomers. One of the protomers, containing FMN and the Fe-S clusters 1–4 in stoichiometric amounts, only reacts with NADH, and its oxidation by ubiquinone is rapid at pH but slow at pH 6.5. The other protomer, containing FMN and the clusters 2, 3 and 4, reacts with both NADH and NADPH and has a pH optimum at 6–6.5 for the reaction with ubiquinone.     

Plastocyanin: Structural and functional analysis

Plastocyanin is one of the best characterized of the photosynthetic electron transfer proteins. Since the determination of the structure of poplar plastocyanin in 1978, the structure of algal (Scenedesmus, Enteromorpha, Chlamydomonas) and plant (French bean) plastocyanins has been determined either by crystallographic or NMR methods, and the poplar structure has been refined to 1.33 Å resolution. Despite the sequence divergence among plastocyanins of algae and vascular plants (e.g., 62% sequence identity between theChlamydomonas and poplar proteins), the three-dimensional structures are remarkably conserved (e.g., 0.76 Å rms deviation in the C positions between theChlamydomonas and poplar proteins). Structural features include a distorted tetrahedral copper binding site at one end of an eight-stranded antiparallel -barrel, a pronounced negative patch, and a flat hydrophobic surface. The copper site is optimized for its electron transfer function, and the negative and hydrophobic patches are proposed to be involved in recognition of physiological reaction partners. Chemical modification, cross-linking, and site-directed mutagenesis experiments have confirmed the importance of the negative and hydrophobic patches in binding interactions with cytochromef and Photosystem I, and validated the model of two functionally significant electron transfer paths in plastocyanin. One putative electron transfer path is relatively short (4 Å) and involves the solvent-exposed copper ligand His-87 in the hydrophobic patch, while the other is more lengthy (12–15 Å) and involves the nearly conserved residue Tyr-83 in the negative patch.     

Structure of the Avian Mitochondrial Cytochrome bc 1 Complex

There are now four structures of vertebrate mitochondrial bc ₁ complexes available in theprotein databases and structures from yeast and bacterial sources are expected soon. Thisreview summarizes the new information with emphasis on the avian cytochrome bc ₁ complex(PDB entries 1BCC and 3BCC). The Rieske iron–sulfur protein is mobile and this has beenproposed to be important for catalysis. The binding sites for quinone have been located basedon structures containing inhibitors and, in the case of the quinone reduction site Qi, thequinone itself.