Properties of bovine heart mitochondrial cytochrome b560

Computational Modeling Analysis of Kinetics of Fumarate Reductase Activity and ROS Production during Reverse Electron Transfer in Mitochondrial Respiratory Complex II

Reverse electron transfer in mitochondrial complex II (CII) plays an important role in hypoxia/anoxia, in particular, in ischemia, when the blood supply to an organ is disrupted and oxygen is not available. A computational model of CII was developed in this work to facilitate the quantitative analysis of the kinetics of quinol-fumarate reduction as well as ROS production during reverse electron transfer in CII. The model consists of 20 ordinary differential equations and 7 moiety conservation equations. The parameter values were determined at which the kinetics of electron transfer in CII in both forward and reverse directions would be explained simultaneously. The possibility of the existence of the "tunnel diode" behavior in the reverse electron transfer in CII, where the driving force is QH₂, was tested. It was found that any high concentrations of QH₂ and fumarate are insufficient for the appearance of a tunnel effect. The results of computer modeling show that the maximum rate of succinate production cannot provide a high concentration of succinate in ischemia. Furthermore, computational modeling results predict a very low rate of ROS production, about 50 pmol/min/mg mitochondrial protein, which is considerably less than 1000 pmol/min/mg protein observed in CII in forward direction.

Individual heme a and heme a3 contributions to the Soret absorption spectrum of the reduced bovine cytochrome c oxidase

Bovine cytochrome c oxidase (CcO) contains two hemes, a and a₃, chemically identical but differing in coordination and spin state. The Soret absorption band of reduced aa₃-type cytochrome c oxidase consists of overlapping bands of the hemes a²⁺ and a₃²⁺. It shows a peak at ∼444 nm and a distinct shoulder at ∼425 nm. However, attribution of individual spectral lineshapes to hemes a²⁺ and a₃²⁺ in the Soret is controversial. In the present work, we characterized spectral contributions of hemes a²⁺ and a₃²⁺ using two approaches. First, we reconstructed bovine CcO heme a²⁺ spectrum using a selective Ca²⁺-induced spectral shift of the heme a²⁺. Second, we investigated photobleaching of the reduced Thermus thermophilus ba₃- and bovine aa₃-oxidases in the Soret induced by femtosecond laser pulses in the Q-band. The resolved spectra show splitting of the electronic B_0x-, B_0y-transitions of both reduced hemes. The heme a²⁺ spectrum is shifted to the red relative to heme a₃²⁺ spectrum. The ∼425 nm shoulder is mostly attributed to heme a₃²⁺.

Modular structure of complex II: An evolutionary perspective

Succinate dehydrogenases (SDHs) and fumarate reductases (FRDs) catalyse the interconversion of succinate and fumarate, a reaction highly conserved in all domains of life. The current classification of SDH/FRDs is based on the structure of the membrane anchor subunits and their cofactors. It is, however, unknown whether this classification would hold in the context of evolution. In this work, a large-scale comparative genomic analysis of complex II addresses the questions of its taxonomic distribution and phylogeny. Our findings report that for types C, D, and F, structural classification and phylogeny go hand in hand, while for types A, B and E the situation is more complex, highlighting the possibility for their classification into subgroups. Based on these findings, we proposed a revised version of the evolutionary scenario for these enzymes in which a primordial soluble module, corresponding to the cytoplasmatic subunits, would give rise to the current diversity via several independent membrane anchor attachment events.

MITODYN: An Open Source Software for Quantitative Modeling of Mitochondrial and Cellular Energy Metabolic Flux Dynamics in Health and Disease

Mitochondrial respiratory chain (RC) transforms the reductive power of NADH or FADH2 oxidation into a proton gradient between the matrix and cytosolic sides of the inner mitochondrial membrane, that ATP synthase uses to generate ATP. This process constitutes a bridge between carbohydrates' central metabolism and ATP-consuming cellular functions. Moreover, the RC is responsible for a large part of reactive oxygen species (ROS) generation that play signaling and oxidizing roles in cells. Mathematical methods and computational analysis are required to understand and predict the possible behavior of this metabolic system. Here we propose a software tool that helps to analyze individual steps of respiratory electron transport in their dynamics, thus deepening understanding of the mechanism of energy transformation and ROS generation in the RC. This software's core is a kinetic model of the RC represented by a system of ordinary differential equations (ODEs). This model enables the analysis of complex dynamic behavior of the RC, including multistationarity and oscillations. The proposed RC modeling method can be applied to study respiration and ROS generation in various organisms and naturally extended to explore carbohydrates' metabolism and linked metabolic processes.

Crystallographic investigation of the ubiquinone binding site of respiratory Complex II and its inhibitors

The quinone binding site (Q-site) of Mitochondrial Complex II (succinate-ubiquinone oxidoreductase) is the target for a number of inhibitors useful for elucidating the mechanism of the enzyme. Some of these have been developed as fungicides or pesticides, and species-specific Q-site inhibitors may be useful against human pathogens. We report structures of chicken Complex II with six different Q-site inhibitors bound, at resolutions 2.0-2.4 Å. These structures show the common interactions between the inhibitors and their binding site. In every case a carbonyl or hydroxyl oxygen of the inhibitor is H-bonded to Tyr58 in subunit SdhD and Trp173 in subunit SdhB. Two of the inhibitors H-bond Ser39 in subunit SdhC directly, while two others do so via a water molecule. There is a distinct cavity that accepts the 2-substituent of the carboxylate ring in flutolanil and related inhibitors. A hydrophobic "tail pocket" opens to receive a side-chain of intermediate-length inhibitors. Shorter inhibitors fit entirely within the main binding cleft, while the long hydrophobic side chains of ferulenol and atpenin A5 protrude out of the cleft into the bulk lipid region, as presumably does that of ubiquinone. Comparison of mitochondrial and Escherichia coli Complex II shows a rotation of the membrane-anchor subunits by 7° relative to the iron‑sulfur protein. This rotation alters the geometry of the Q-site and the H-bonding pattern of SdhB:His216 and SdhD:Asp57. This conformational difference, rather than any active-site mutation, may be responsible for the different inhibitor sensitivity of the bacterial enzyme.

Computational Modeling Analysis of Generation of Reactive Oxygen Species by Mitochondrial Assembled and Disintegrated Complex II

Reactive oxygen species (ROS) function as critical mediators in a broad range of cellular signaling processes. The mitochondrial electron transport chain is one of the major contributors to ROS formation in most cells. Increasing evidence indicates that the respiratory Complex II (CII) can be the predominant ROS generator under certain conditions. A computational, mechanistic model of electron transfer and ROS formation in CII was developed in the present study to facilitate quantitative analysis of mitochondrial ROS production. The model was calibrated by fitting the computer simulated results to experimental data obtained on submitochondrial particles (SMP) prepared from bovine and rat heart mitochondria upon inhibition of the ubiquinone (Q)-binding site by atpenin A5 (AA5) and Complex III by myxothiazol, respectively. The model predicts that only reduced flavin adenine dinucleotide (FADH₂) in the unoccupied dicarboxylate state and flavin semiquinone radical (FADH^•) feature the experimentally observed bell-shaped dependence of the rate of ROS production on the succinate concentration upon inhibition of respiratory Complex III (CIII) or Q-binding site of CII, i.e., suppression of succinate-Q reductase (SQR) activity. The other redox centers of CII such as Fe-S clusters and Q-binding site have a hyperbolic dependence of ROS formation on the succinate concentration with very small maximal rate under any condition and cannot be considered as substantial ROS generators in CII. Computer simulation results show that CII disintegration (which results in dissociation of the hydrophilic SDHA/SDHB subunits from the inner membrane to the mitochondrial matrix) causes crucial changes in the kinetics of ROS production by CII that are qualitatively and quantitatively close to changes in the kinetics of ROS production by assembled CII upon inhibition of CIII or Q-binding site of CII. Thus, the main conclusions from the present computational modeling study are the following: (i) the impairment of the SQR activity of CII resulting from inhibition of CIII or Q-binding site of CII and (ii) CII disintegration causes a transition in the succinate-dependence of ROS production from a small-amplitude sigmoid (hyperbolic) shape, determined by Q-binding site or [3Fe-4S] cluster to a high-amplitude bell-shaped kinetics with a shift to small subsaturated concentrations of succinate, determined by the flavin site.

The role of the myeloperoxidase-derived oxidant hypothiocyanous acid (HOSCN) in the induction of mitochondrial dysfunction in macrophages

A host of chronic inflammatory diseases are accelerated by the formation of the powerful oxidant hypochlorous acid (HOCl) by myeloperoxidase (MPO). In the presence of thiocyanate (SCN^-), the production of HOCl by MPO is decreased in favour of the formation of a milder oxidant, hypothiocyanous acid (HOSCN). The role of HOSCN in disease has not been fully elucidated, though there is increasing interest in using SCN^- therapeutically in different disease settings. Unlike HOCl, HOSCN can be detoxified by thioredoxin reductase, and reacts selectively with thiols to result in reversible modifications, which could potentially reduce the extent of MPO-induced damage during chronic inflammation. In this study, we show that exposure of macrophages, a key inflammatory cell type, to HOSCN results in the reversible modification of multiple mitochondrial proteins, leading to increased mitochondrial membrane permeability, decreased oxidative phosphorylation and reduced formation of ATP. The increased permeability and reduction in ATP could be reversed by pre-treatment of the macrophages with cyclosporine A, implicating a role for the mitochondrial permeability transition pore. HOSCN also drives cells to utilise fatty acids as an energetic substrate after the inhibition of oxidative phosphorylation. Raman imaging studies highlighted the ability of HOSCN to perturb the electron transport chain of mitochondria and redistribute these organelles within the cell. Taken together, these data provide new insight into the pathways by which HOSCN can induce cytotoxicity and cellular damage, which may have relevance for the development of inflammatory disease, and therapeutic strategies to reduce HOCl-induced damage by supplementation with SCN^-.

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HOSCN induces the oxidation of mitochondrial thiol proteins and cytochromes.

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HOSCN alters mitochondrial permeability and ATP production via MPTP formation.

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HOSCN increases the capacity of cells to use fatty acids as an energetic substrate.

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Raman imaging shows redistribution of mitochondria after cell exposure to HOSCN.

GATA-1 isoforms differently contribute to the production and compartmentation of reactive oxygen species in the myeloid leukemia cell line K562

Maintenance of a balanced expression of the two isoforms of the transcription factor GATA‐1, the full‐length protein (GATA‐1_FL) and a shorter isoform (GATA‐1 _S), contributes to control hematopoiesis, whereas their dysregulation can alter the differentiation/proliferation potential of hematopoietic precursors thereby eventually leading to a variety of hematopoietic disorders. Although it is well established that these isoforms play opposite roles in these remarkable processes, most of the molecular pathways involved remain unknown.

Here, we demonstrate that GATA‐1_FL and GATA‐1_S are able to differently influence intracellular redox states and reactive oxygen species (ROS) compartmentation in the erythroleukemic K562 cell line, thus shedding novel mechanistic insights into the processes of cell proliferation and apoptosis resistance in myeloid precursors. Furthermore, given the role played by ROS signaling as a strategy to escape apoptosis and evade cell‐mediated immunity in myeloid cells, this study highlights a mechanism through which aberrant expression of GATA‐1 isoforms could play a role in the leukemogenic process.

Electron-transfer pathways in the heme and quinone-binding domain of complex II (succinate dehydrogenase)

Single electron transfers have been examined in complex II (succinate:ubiquinone oxidoreductase) by the method of pulse radiolysis. Electrons are introduced into the enzyme initially at the [3Fe–4S] and ubiquinone sites followed by intramolecular equilibration with the b heme of the enzyme. To define thermodynamic and other controlling parameters for the pathways of electron transfer in complex II, site-directed variants were constructed and analyzed. Variants at SdhB-His207 and SdhB-Ile209 exhibit significantly perturbed electron transfer between the [3Fe–4S] cluster and ubiquinone. Analysis of the data using Marcus theory shows that the electronic coupling constants for wild-type and variant enzyme are all small, indicating that electron transfer occurs by diabatic tunneling. The presence of the ubiquinone is necessary for efficient electron transfer to the heme, which only slowly equilibrates with the [3Fe–4S] cluster in the absence of the quinone.

Acute mitochondrial inhibition by mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) 1/2 inhibitors regulates proliferation

The Ras-MEK1/2-ERK1/2 kinase signaling pathway regulates proliferation, survival, and differentiation and, because it is often aberrant in tumors, is a popular target for small molecule inhibition. A novel metabolic analysis that measures the real-time oxidation state of NAD(H) and the hemes of the electron transport chain and oxygen consumption within intact, living cells found that structurally distinct MEK1/2 inhibitors had an immediate, dose-dependent effect on mitochondrial metabolism. The inhibitors U0126, MIIC and PD98059 caused NAD(H) reduction, heme oxidation, and decreased oxygen consumption, characteristic of complex I inhibition. PD198306, an orally active MEK1/2 inhibitor, acted as an uncoupler. Each MEK1/2 inhibitor depleted phosphorylated ERK1/2 and inhibited proliferation, but the most robust antiproliferative effects always correlated with the metabolic failure which followed mitochondrial inhibition rather than inhibition of MEK1/2. This warrants rethinking the role of ERK1/2 in proliferation and emphasizes the importance of mitochondrial function in this process.

Mitochondrial complex II can generate reactive oxygen species at high rates in both the forward and reverse reactions

Respiratory complex II oxidizes succinate to fumarate as part of the Krebs cycle and reduces ubiquinone in the electron transport chain. Previous experimental evidence suggested that complex II is not a significant contributor to the production of reactive oxygen species (ROS) in isolated mitochondria or intact cells unless mutated. However, we find that when complex I and complex III are inhibited and succinate concentration is low, complex II in rat skeletal muscle mitochondria can generate superoxide or H(2)O(2) at high rates. These rates approach or exceed the maximum rates achieved by complex I or complex III. Complex II generates these ROS in both the forward reaction, with electrons supplied by succinate, and the reverse reaction, with electrons supplied from the reduced ubiquinone pool. ROS production in the reverse reaction is prevented by inhibition of complex II at either the ubiquinone-binding site (by atpenin A5) or the flavin (by malonate), whereas ROS production in the forward reaction is prevented by malonate but not by atpenin A5, showing that the ROS from complex II arises only from the flavin site (site II(F)). We propose a mechanism for ROS production by complex II that relies upon the occupancy of the substrate oxidation site and the reduction state of the enzyme. We suggest that complex II may be an important contributor to physiological and pathological ROS production.

Measurement of the mitochondrial membrane potential and pH gradient from the redox poise of the hemes of the bc1 complex

The redox potentials of the hemes of the mitochondrial bc(1) complex are dependent on the proton-motive force due to the energy transduction. This allows the membrane potential and pH gradient components to be calculated from the oxidation state of the hemes measured with multi-wavelength cell spectroscopy. Oxidation states were measured in living RAW 264.7 cells under varying electron flux and membrane potential obtained by a combination of oligomycin and titration with a proton ionophore. A stochastic model of bc(1) turnover was used to confirm that the membrane potential and redox potential of the ubiquinone pool could be measured from the redox poise of the b-hemes under physiological conditions assuming the redox couples are in equilibrium. The pH gradient was then calculated from the difference in redox potentials of cytochrome c and ubiquinone pool using the stochastic model to evaluate the ΔG of the bc(1) complex. The technique allows absolute quantification of the membrane potential, pH gradient, and proton-motive force without the need for genetic manipulation or exogenous compounds.

Out of plane distortions of the heme b of Escherichia coli succinate dehydrogenase

The role of the heme b in Escherichia coli succinate dehydrogenase is highly ambiguous and its role in catalysis is questionable. To examine whether heme reduction is an essential step of the catalytic mechanism, we generated a series of site-directed mutations around the heme binding pocket, creating a library of variants with a stepwise decrease in the midpoint potential of the heme from the wild-type value of +20 mV down to −80 mV. This difference in midpoint potential is enough to alter the reactivity of the heme towards succinate and thus its redox state under turnover conditions. Our results show both the steady state succinate oxidase and fumarate reductase catalytic activity of the enzyme are not a function of the redox potential of the heme. As well, lower heme potential did not cause an increase in the rate of superoxide production both in vitro and in vivo. The electron paramagnetic resonance (EPR) spectrum of the heme in the wild-type enzyme is a combination of two distinct signals. We link EPR spectra to structure, showing that one of the signals likely arises from an out-of-plane distortion of the heme, a saddled conformation, while the second signal originates from a more planar orientation of the porphyrin ring.

Review: studies of ferric heme proteins with highly anisotropic/highly axial low spin (S = 1/2) electron paramagnetic resonance signals with bis-histidine and histidine-methionine axial iron coordination

Six-coordinated heme groups are involved in a large variety of electron transfer reactions because of their ability to exist in both the ferrous (Fe(2+)) and ferric (Fe(3+)) state without any large differences in structure. Our studies on hemes coordinated by two histidines (bis-His) and hemes coordinated by histidine and methionine (His-Met) will be reviewed. In both of these coordination environments, the heme core can exhibit ferric low spin (electron paramagnetic resonance EPR) signals with large g(max) values (also called Type I, highly anisotropic low spin, or highly axial low spin, HALS species) as well as rhombic EPR (Type II) signals. In bis-His coordinated hemes rhombic and HALS envelopes are related to the orientation of the His groups with respect to each other such that (i) parallel His planes results in a rhombic signal and (ii) perpendicular His planes results in a HALS signal. Correlation between the structure of the heme and its ligands for heme with His-Met axial ligation and ligand-field parameters, as derived from a large series of cytochrome c variants, show, however, that for such a combination of axial ligands there is no clear-cut difference between the large g(max) and the "small g-anisotropy" cases as a result of the relative Met-His arrangements. Nonetheless, a new linear correlation links the average shift delta of the heme methyl groups with the g(max) values.

Fumarate in the preservation solution aggravates chronic allograft nephropathy

Data on a protective role of fumarate in acute ischemia of the rat heart led to the obvious hypothesis that addition of fumarate to the preservation solution for kidney transplantation may have beneficial value. This study was designed to test this hypothesis. Kidneys of Lewis or Fischer 344 rats were flushed with University of Wisconsin (UW) solution or UW solution containing 5 mM fumarate. Grafts were immediately transplanted to Lewis recipients or stored at 4 °C for 5 h before transplantation. Renal function was assessed on d 10 and monthly for 6 mo. One group of isografts was removed on d 10 post-transplantation, the other groups of isografts and allografts after 6 mo. We detected a modest protective effect regarding proteinuria 10 d after isogeneic transplantation, and exclude the possibility that fumarate exerts acute nephrotoxicity. Surprisingly, fumarate strongly promoted intimal hyperplasia of allograft arteries, thickening of the arterial media of isografts and allografts, tubulo-interstitial allograft damage, and allograft infiltration by macrophages on the long run. To date, we do not know the mechanism resulting in fumarate-induced chronic graft damage. We suggest, however, that addition of fumarate to the conservation fluid does not improve graft outcome.

Engineering heme binding sites in monomeric rop

Heme ligands were introduced in the hydrophobic core of an engineered monomeric ColE1 repressor of primer (rop-S55) in two different layers of the heptad repeat. Mutants rop-L63M/F121H (layer 1) and rop-L56H/L113H (layer 3) were found to bind heme with a K (D) of 1.1 +/- 0.2 and 0.47 +/- 0.07 microM, respectively. The unfolding of heme-bound and heme-free mutants, in the presence of guanidinium hydrochloride, was monitored by both circular dichroism and fluorescence spectroscopy. For the heme-bound rop mutants, the total free energy change was 0.5 kcal/mol higher in the layer 3 mutant compared with that in the layer1 mutant. Heme binding also stabilized these mutants by increasing the [DGobsH2O] by 1.4 and 1.8 kcal/mol in rop-L63M/F121H and rop-L56H/L113H, respectively. The reduction potentials measured by spectroelectrochemical titrations were calculated to be -154 +/- 2 mV for rop-56H/113H and -87.5 +/- 1.2 mV for rop-L63M/F121H. The mutant designed to bind heme in a more buried environment (layer 3) showed tighter heme binding, a higher stability, and a different reduction potential compared with the mutant designed to bind heme in layer 1.

Bis-histidine-coordinated hemes in four-helix bundles: how the geometry of the bundle controls the axial imidazole plane orientations in transmembrane cytochromes of mitochondrial complexes II and III and related proteins

Early investigation of the electron paramagnetic resonance spectra of bis-histidine-coordinated membrane-bound ferriheme proteins led to the description of a spectral signal that had only one resolved feature. These became known as "highly anisotropic low-spin" or "large g(max)" ferriheme centers. Extensive work with small-molecule model heme complexes showed that this spectroscopic signature occurs in bis-imidazole ferrihemes in which the planes of the imidazole ligands are nearly perpendicular, deltaphi = 57-90 degrees. In the last decade protein crystallographic studies have revealed the atomic structures of a number of examples of bis-histidine heme proteins. A frequent characteristic of these large g(max) ferrihemes in membrane-bound proteins is the occurrence of the heme within a four-helix bundle with a left-handed twist. The histidine ligands occur at the same level on two diametrically opposed helices of the bundle. These ligands have the same side-chain conformation and ligate heme iron on the bundle axis, resulting in a quasi-twofold symmetric structure. The two non-ligand-bearing helices also obey this symmetry, and have a conserved small residue, usually glycine, where the edge of the heme ring makes contact with the helix backbones. In many cases this small residue is preceded by a threonine or serine residue whose side-chain hydroxyl oxygen acts as a hydrogen-bond acceptor from the N(delta1) atom of the heme-ligating histidine. The deltaphi angle is thus determined by the common histidine side-chain conformation and the crossing angle of the ligand-bearing helices, in some cases constrained by hydrogen bonds to the serine/threonine residues on the non-ligand-bearing helices.

Escherichia coli succinate dehydrogenase variant lacking the heme b

The Escherichia coli enzyme succinate:ubiquinone oxidoreductase [(succinate dehydrogenase (SdhCDAB)] couples succinate oxidation to ubiquinone reduction and is structurally and functionally equivalent to mitochondrial complex II, an essential component of the aerobic respiratory chain and tricarboxylic acid cycle. All such enzymes contain a heme within their membrane anchor domain with a highly contentious, but as-yet-undetermined, function. Here, we report the generation of a complex II that lacks heme, which is confirmed by both optical and EPR spectroscopy. Despite the absence of heme, this mutant still assembles properly and retains physiological activity. However, the mutants lacking heme are highly sensitive to the presence of detergent. In addition, the heme does not appear to be involved in reactive oxygen species suppression. Our results indicate that redox cycling of the heme in complex II is not essential for the enzyme's ubiquinol reductase activity.

The Saccharomyces cerevisiae succinate dehydrogenase does not require heme for ubiquinone reduction

The coupling of succinate oxidation to the reduction of ubiquinone by succinate dehydrogenase (SDH) constitutes a pivotal reaction in the aerobic generation of energy. In Saccharomyces cerevisiae, SDH is a tetramer composed of a catalytic dimer comprising a flavoprotein subunit, Sdh1p and an iron-sulfur protein, Sdh2p and a heme b-containing membrane-anchoring dimer comprising the Sdh3p and Sdh4p subunits. In order to investigate the role of heme in SDH catalysis, we constructed an S. cerevisiae strain expressing a mutant enzyme lacking the two heme axial ligands, Sdh3p His-106 and Sdh4p Cys-78. The mutant enzyme was characterized for growth on a non-fermentable carbon source, for enzyme assembly, for succinate-dependent quinone reduction and for its heme b content. Replacement of both Sdh3p His-106 and Sdh4p Cys-78 with alanine residues leads to an undetectable level of cytochrome b(562). Although enzyme assembly is slightly impaired, the apocytochrome SDH retains a significant ability to reduce quinone. The enzyme has a reduced affinity for quinone and its catalytic efficiency is reduced by an order of magnitude. To better understand the effects of the mutations, we employed atomistic molecular dynamic simulations to investigate the enzyme's structure and stability in the absence of heme. Our results strongly suggest that heme is not required for electron transport from succinate to quinone nor is it necessary for assembly of the S. cerevisiae SDH.

Dicoumarol impairs mitochondrial electron transport and pyrimidine biosynthesis in human myeloid leukemia HL-60 cells

Dicoumarol, a competitive inhibitor of NAD(P)H:quinone oxidoreductase 1 (NQO1), increases intracellular superoxide and affects cell growth of tumor cells. This work was set to establish a mechanistic link between dicoumarol, superoxide and cell cycle alterations in HL-60 cells. Using ES936, a mechanism-based irreversible inhibitor of NQO1, we demonstrate that NQO1 inhibition is not a major factor involved in superoxide boost. Mitochondrial Complexes II, III and IV were directly inhibited by dicoumarol. Succinate, which inhibits superoxide generation by reversed electron flow in Complex II, significantly decreased superoxide boost in dicoumarol-treated cells and in isolated mitochondria incubated with dicoumarol and decylubiquinol. Superoxide generation in cells was strongly potentiated by blocking the quinone site of Complex II with thenoyltrifluoroacetone, supporting the involvement of cytochrome b560 to drive electrons for increasing superoxide. Simultaneous inhibition of the mitochondrial chain upstream ubiquinone and displacement of succinate from the Complex II active site is proposed as a major mechanism to explain how dicoumarol increases superoxide in HL-60 cells. Dicoumarol-treated cells accumulated in S phase due to the impairment of pyrimidine biosynthesis at dihydroorotate dehydrogenase step because blockade was overcome by addition of exogenous uridine or orotate, but not by dihydroorotate. We demonstrate for the first time that dicoumarol inhibits mitochondrial electron transport, induces superoxide release by reversed electron flow in Complex II, and inhibits pyrimidines biosynthesis. These actions must be taken into account when considering dicoumarol effects on cells.

Localization of superoxide anion production to mitochondrial electron transport chain in 3-NPA-treated cells

3-Nitropropionic acid (3-NPA), an inhibitor of succinate dehydrogenase (SDH) at complex II of the mitochondrial electron transport chain induces cellular energy deficit and oxidative stress-related neurotoxicity. In the present study, we identified the site of reactive oxygen species production in mitochondria. 3-NPA increased O2- generation in mitochondria respiring on the complex I substrates pyruvate+malate, an effect fully inhibited by rotenone. Antimycin A increased O2- production in the presence of complex I and/or II substrates. Addition of 3-NPA markedly increased antimycin A-induced O2- production by mitochondria incubated with complex I substrates, but 3-NPA inhibited O2- formation driven with the complex II substrate succinate. At 0.6 microM, myxothiazol inhibits complex III, but only partially decreases complex I activity, and allowed 3-NPA-induced O2- formation; however, at 40 microM myxothiazol (which completely inhibits both complexes I and III) eliminated O2- production from mitochondria respiring via complex I substrates. These results indicate that in the presence of 3-NPA, mitochondria generate O2- from a site between the ubiquinol pool and the 3-NPA block in the respiratory complex II.

Hypoxic vasoconstriction of partial muscular intra-acinar pulmonary arteries in murine precision cut lung slices

Background

Acute alveolar hypoxia causes pulmonary vasoconstriction (HPV) which serves to match lung perfusion to ventilation. The underlying mechanisms are not fully resolved yet. The major vascular segment contributing to HPV, the intra-acinar artery, is mostly located in that part of the lung that cannot be selectively reached by the presently available techniques, e.g. hemodynamic studies of isolated perfused lungs, recordings from dissected proximal arterial segments or analysis of subpleural vessels. The aim of the present study was to establish a model which allows the investigation of HPV and its underlying mechanisms in small intra-acinar arteries.

Methods

Intra-acinar arteries of the mouse lung were studied in 200 μm thick precision-cut lung slices (PCLS). The organisation of the muscle coat of these vessels was characterized by α-smooth muscle actin immunohistochemistry. Basic features of intra-acinar HPV were characterized, and then the impact of reactive oxygen species (ROS) scavengers, inhibitors of the respiratory chain and Krebs cycle metabolites was analysed.

Results

Intra-acinar arteries are equipped with a discontinuous spiral of α-smooth muscle actin-immunoreactive cells. They exhibit a monophasic HPV (medium gassed with 1% O₂) that started to fade after 40 min and was lost after 80 min. This HPV, but not vasoconstriction induced by the thromboxane analogue U46619, was effectively blocked by nitro blue tetrazolium and diphenyleniodonium, indicating the involvement of ROS and flavoproteins. Inhibition of mitochondrial complexes II (3-nitropropionic acid, thenoyltrifluoroacetone) and III (antimycin A) specifically interfered with HPV, whereas blockade of complex IV (sodium azide) unspecifically inhibited both HPV and U46619-induced constriction. Succinate blocked HPV whereas fumarate had minor effects on vasoconstriction.

Conclusion

This study establishes the first model for investigation of basic characteristics of HPV directly in intra-acinar murine pulmonary vessels. The data are consistent with a critical involvement of ROS, flavoproteins, and of mitochondrial complexes II and III in intra-acinar HPV. In view of the lack of specificity of any of the classical inhibitors used in such types of experiments, validation awaits the use of appropriate knockout strains and siRNA interference, for which the present model represents a well-suited approach.

Electron tunneling chains of mitochondria

The single, simple concept that natural selection adjusts distances between redox cofactors goes a long way towards encompassing natural electron transfer protein design. Distances are short or long as required to direct or insulate promiscuously tunneling single electrons. Along a chain, distances are usually 14 A or less. Shorter distances are needed to allow climbing of added energetic barriers at paired-electron catalytic centers in which substrate and the required number of cofactors form a compact cluster. When there is a short-circuit danger, distances between shorting centers are relatively long. Distances much longer than 14 A will support only very slow electron tunneling, but could act as high impedance signals useful in regulation. Tunneling simulations of the respiratory complexes provide clear illustrations of this simple engineering.

Identification of a ubiquinone-binding site that affects autophosphorylation of the sensor kinase RegB

Rhodobacter capsulatus regulates many metabolic processes in response to the level of environmental oxygen and the energy state of the cell. One of the key global redox regulators of the cell's metabolic physiology is the sensor kinase RegB that controls the synthesis of numerous energy generation and utilization processes. In this study, we have succeeded in purifying full-length RegB containing six transmembrane-spanning elements. Exogenous addition of excess oxidized coenzyme Q1 is capable of inhibiting RegB autophosphorylation approximately 6-fold. However, the addition of reduced coenzyme Q1 exhibits no inhibitory effect on kinase activity. A ubiquinone-binding site, as defined by azidoquinone photo affinity cross-linking, was determined to lie within a periplasmic loop between transmembrane helices 3 and 4 that contains a fully conserved heptapeptide sequence of GGXXNPF. Mutation of the phenylalanine in this heptapeptide renders RegB constitutively active in vivo, indicating that this domain is responsible for sensing the redox state of the ubiquinone pool and subsequently controlling RegB autophosphorylation.

Electron Transfer within Complex II

Electron transfer within Escherichia coli succinate:ubiquinone oxidoreductase has been examined by the pulse radiolysis technique using spectrophotometric detection. Electrons have been introduced into the protein by the bimolecular reaction with quantified concentrations of the low potential N-methylnicotinamide radical at a rate constant of 7 x 10(8) M(-1) s(-1). Two redox-active centers in the protein are initially reduced, assigned as the high potential [3Fe-4S] center and the bound ubiquinone, followed by intramolecular equilibration with the b heme in both cases. Electron equilibration at 25 degrees C from the ubisemiquinone proceeds with an observed rate constant of 7,200 s(-1) and from the more distant [3Fe-4S] reduced center at a rate constant of 1,200 s(-1). Temperature dependence studies have revealed that both reactions have large free energies of activation, with deltaG(double dagger) values of +0.53 and +0.58 eV, respectively. Cumulative spectral changes, as well as accompanying decreases in the rates of intramolecular electron transfer, observed upon adding electrons to progressively reduced protein, indicate that 4 electrons must be introduced into the protein before the heme center is fully reduced. Overall, evidence is presented that the heme, far from being a bystander in the efficient transfer of reducing equivalents from succinate to the ubiquinone via the flavin-Fe/S centers, plays a pivotal role in providing a lower energy pathway for the transfer of an electron from the high potential [3Fe-4S] center to ubiquinone.

Crystal structure of mitochondrial respiratory membrane protein complex II

The mitochondrial respiratory Complex II or succinate:ubiquinone oxidoreductase (SQR) is an integral membrane protein complex in both the tricarboxylic acid cycle and aerobic respiration. Here we report the first crystal structure of Complex II from porcine heart at 2.4 A resolution and its complex structure with inhibitors 3-nitropropionate and 2-thenoyltrifluoroacetone (TTFA) at 3.5 A resolution. Complex II is comprised of two hydrophilic proteins, flavoprotein (Fp) and iron-sulfur protein (Ip), and two transmembrane proteins (CybL and CybS), as well as prosthetic groups required for electron transfer from succinate to ubiquinone. The structure correlates the protein environments around prosthetic groups with their unique midpoint redox potentials. Two ubiquinone binding sites are discussed and elucidated by TTFA binding. The Complex II structure provides a bona fide model for study of the mitochondrial respiratory system and human mitochondrial diseases related to mutations in this complex.

Hypoxic increase in nitric oxidegeneration of rat sensory neurons requires activation of mitochondrial complex II and voltage-gated calcium channels

Recently, we have demonstrated that sensory neurons of rat lumbar dorsal root ganglia (DRG) respond to hypoxia with an activation of endothelial nitric oxide (NO) synthase (eNOS) resulting in enhanced NO production associated with mitochondria which contributes to resistance against hypoxia. Extracellular calcium is essential to this effect. In the present study on rat DRG slices, we set out to determine what types of calcium channels operate under hypoxia, and which upstream events contribute to their activation, thereby focusing upon mitochondrial complex II. Both the metallic ions Cd2+ and Ni2+, known to inhibit voltage-gated calcium channels and T-type channels, respectively, and verapamil and nifedipine, typical blocker of L-type calcium channels completely prevented the hypoxic neuronal NO generation. Inhibition of complex II by thenoyltrifluoroacetone at the ubiquinon binding site or by 3-nitropropionic acid at the substrate binding site largely diminished hypoxic-induced NO production while having an opposite effect under normoxia. An additional blockade of voltage-gated calcium channels entirely abolished the hypoxic response. The complex II inhibitor malonate inhibited both normoxic and hypoxic NO generation. These data show that complex II activity is required for increased hypoxic NO production. Since succinate dehydrogenase activity of complex II decreased at hypoxia, as measured by histochemistry and densitometry, we propose a hypoxia-induced functional switch of complex II from succinate dehydrogenase to fumarate reductase, which subsequently leads to activation of voltage-gated calcium channels resulting in increased NO production by eNOS.

Identification of the heme axial ligands in the cytochrome b562 of the Saccharomyces cerevisiae succinate dehydrogenase

Succinate dehydrogenase (SDH) plays a key role in energy generation by coupling the oxidation of succinate to the reduction of ubiquinone in the mitochondrial electron transport chain. The Saccharomyces cerevisiae SDH is composed of a catalytic dimer of the Sdh1p and Sdh2p subunits containing flavin adenine dinucleotide (FAD) and iron-sulfur clusters and a heme b-containing membrane-anchoring domain comprised of the Sdh3p and Sdh4p subunits. We systematically mutated all the histidine and cysteine residues in Sdh3p and Sdh4p to identify the residues involved in axial heme ligation. The mutants were characterized for growth on a non-fermentable carbon source, for enzyme assembly, for succinate-dependent quinone reduction, for heme b content, and for heme spectral properties. Mutation of Sdh3p His-46 or His-113 leads to a marked reduction in the catalytic efficiency of the enzyme for quinone reduction, suggesting that these residues form part of a quinone-binding site. We identified Sdh3p His-106 and Sdh4p Cys-78 as the most probable axial ligands for cytochrome b(562). Replacement of His-106 or Cys-78 with an alanine residue leads to a marked reduction in cytochrome b(562) content and to altered heme spectral characteristics that are consistent with a direct perturbation of heme b environment. This is the first identification of a cysteine residue serving as an axial ligand for heme b in the SDH family of enzymes. Loss of cytochrome b(562) has no effect on enzyme assembly and quinone reduction; the role of the heme in enzyme structure and function is discussed.

Function and structure of complex II of the respiratory chain

Complex II is the only membrane-bound component of the Krebs cycle and in addition functions as a member of the electron transport chain in mitochondria and in many bacteria. A recent X-ray structural solution of members of the complex II family of proteins has provided important insights into their function. One feature of the complex II structures is a linear electron transport chain that extends from the flavin and iron-sulfur redox cofactors in the membrane extrinsic domain to the quinone and b heme cofactors in the membrane domain. Exciting recent developments in relation to disease in humans and the formation of reactive oxygen species by complex II point to its overall importance in cellular physiology.

Isolation and characterization of the stage-specific cytochrome b small subunit (CybS) of Ascaris suum complex II from the aerobic respiratory chain of larval mitochondria

We recently reported that Ascaris suum mitochondria express stage-specific isoforms of complex II: the flavoprotein subunit and the small subunit of cytochrome b (CybS) of the larval complex II differ from those of adult enzyme, while two complex IIs share a common iron-sulfur cluster subunit (Ip). In the present study, A. suum larval complex II was highly purified to characterize the larval cytochrome b subunits in more detail. Peptide mass fingerprinting and N-terminal amino acid sequencing showed that the larval and adult cytochrome b (CybL) proteins are identical. In contrast, cDNA sequences revealed that the small subunit of larval cytochrome b (CybS(L)) is distinct from the adult CybS (CybS(A)). Furthermore, Northern analysis and immunoblotting showed stage-specific expression of CybS(L) and CybS(A) in larval and adult mitochondria, respectively. Enzymatic assays revealed that the ratio of rhodoquinol-fumarate reductase (RQFR) to succinate-ubiquinone reductase (SQR) activities and the K(m) values for quinones are almost identical for the adult and larval complex IIs, but that the fumarate reductase (FRD) activity is higher for the adult form than for the larval form. These results indicate that the adult and larval A. suum complex IIs have different properties than the complex II of the mammalian host and that the larval complex II is able to function as a RQFR. Such RQFR activity of the larval complex II would be essential for rapid adaptation to the dramatic change of oxygen availability during infection of the host.

Essential role of complex II of the respiratory chain in hypoxia-induced ROS generation in the pulmonary vasculature

In the pulmonary vasculature, the mechanisms responsible for oxygen sensing and the initiation of hypoxia-induced vasoconstriction and vascular remodeling are still unclear. Nitric oxide (NO) and reactive oxygen species (ROS) are discussed as early mediators of the hypoxic response. Here, we describe a quantitative analysis of NO- and ROS-producing cells within the vascular walls of murine lung sections cultured at normoxia or hypoxia. Whereas the number of NO-producing cells was not changed by hypoxia, the number of ROS-generating cells was significantly increased. Addition of specific inhibitors revealed that mitochondria were the source of ROS. The participation of the individual mitochondrial complexes differed in normoxic and hypoxic ROS generation. Whereas normoxic ROS production required complexes I and III, hypoxic ROS generation additionally demanded complex II. Histochemically demonstrable succinate dehydrogenase activity of complex II in the arterial wall decreased during hypoxia. Inhibition of the reversed enzymatic reaction, i.e., fumarate reductase, by application of succinate, specifically abolished hypoxic, but not normoxic, ROS generation. Thus complex II plays an essential role in hypoxic ROS production. Presumably, its catalytic activity switches from succinate dehydrogenase to fumarate reductase at reduced oxygen tension, thereby modulating the directionality of the electron flow.

Architecture of succinate dehydrogenase and reactive oxygen species generation

The structure of Escherichia coli succinate dehydrogenase (SQR), analogous to the mitochondrial respiratory complex II, has been determined, revealing the electron transport pathway from the electron donor, succinate, to the terminal electron acceptor, ubiquinone. It was found that the SQR redox centers are arranged in a manner that aids the prevention of reactive oxygen species (ROS) formation at the flavin adenine dinucleotide. This is likely to be the main reason SQR is expressed during aerobic respiration rather than the related enzyme fumarate reductase, which produces high levels of ROS. Furthermore, symptoms of genetic disorders associated with mitochondrial SQR mutations may be a result of ROS formation resulting from impaired electron transport in the enzyme.

Cytopathies involving mitochondrial complex II

Complex II (succinate-ubiquinone oxidoreductase) is the smallest complex in the respiratory chain and contains four nuclear-encoded subunits SdhA, SdhB, SdhC, and SdhD. It functions both as a respiratory chain component and an essential enzyme of the TCA cycle. Electrons derived from succinate can thus be directly transferred to the ubiquinone pool. Major insights into the workings of complex II have been provided by crystal structures of closely related bacterial enzymes, which have also been genetically manipulated to answer questions of structure-function not approachable using the mammalian system. This information, together with that accrued over the years on bovine complex II and by recent advances in understanding in vivo synthesis of the non-heme iron co-factors of the enzyme, is allowing better recognition of improper functioning of human complex II in diseased states. The discussion in this review is thus limited to cytopathies arising because the enzyme itself is defective or depleted by lack of iron-sulfur clusters. There is a clear dichotomy of effects. Enzyme depletion and mutations in SDHA compromise TCA activity and energy production, whereas mutations in SDHB, SDHC, and SDHD induce paraganglioma. SDHC and SDHD are the first tumor suppressor genes of mitochondrial proteins.

Succinate dehydrogenase and fumarate reductase from Escherichia coli

Succinate-ubiquinone oxidoreductase (SQR) as part of the trichloroacetic acid cycle and menaquinol-fumarate oxidoreductase (QFR) used for anaerobic respiration by Escherichia coli are structurally and functionally related membrane-bound enzyme complexes. Each enzyme complex is composed of four distinct subunits. The recent solution of the X-ray structure of QFR has provided new insights into the function of these enzymes. Both enzyme complexes contain a catalytic domain composed of a subunit with a covalently bound flavin cofactor, the dicarboxylate binding site, and an iron-sulfur subunit which contains three distinct iron-sulfur clusters. The catalytic domain is bound to the cytoplasmic membrane by two hydrophobic membrane anchor subunits that also form the site(s) for interaction with quinones. The membrane domain of E. coli SQR is also the site where the heme b556 is located. The structure and function of SQR and QFR are briefly summarized in this communication and the similarities and differences in the membrane domain of the two enzymes are discussed.

Succinate:quinone oxidoreductase in the bacteria Paracoccus denitrificans and Bacillus subtilis

An overview of the present knowledge about succinate:quinone oxidoreductase in Paracoccus denitrificans and Bacillus subtilis is presented. P. denitrificans contains a monoheme succinate:ubiquinone oxidoreductase that is similar to that of mammalian mitochondria with respect to composition and sensitivity to carboxin. Results obtained with carboxin-resistant P. denitrificans mutants provide information about quinone-binding sites on the enzyme and the molecular basis for the resistance. B. subtilis contains a diheme succinate:menaquinone oxidoreductase whose activity is dependent on the electrochemical gradient across the cytoplasmic membrane. Data from studies of mutant variants of the B. subtilis enzyme combined with available crystal structures of a similar enzyme, Wolinella succinogenes fumarate reductase, substantiate a proposed explanation for the mechanism of coupling between quinone reductase activity and transmembrane potential.

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.

Retention of heme in axial ligand mutants of succinate-ubiquinone xxidoreductase (complex II) from Escherichia coli

Succinate-ubiquinone oxidoreductase (SdhCDAB, complex II) from Escherichia coli is a four-subunit membrane-bound respiratory complex that catalyzes ubiquinone reduction by succinate. In the E. coli enzyme, heme b(556) is ligated between SdhC His(84) and SdhD His(71). Contrary to a previous report (Vibat, C. R. T., Cecchini, G., Nakamura, K., Kita, K., and Gennis, R. B. (1998) Biochemistry 37, 4148-4159), we demonstrate the presence of heme in both SdhC H84L and SdhD H71Q mutants of SdhCDAB. EPR spectroscopy reveals the presence of low spin heme in the SdhC H84L (g(z) = 2.92) mutant and high spin heme in the SdhD H71Q mutant (g = 6.0). The presence of low spin heme in the SdhC H84L mutant suggests that the heme b(556) is able to pick up another ligand from the protein. CO binds to the reduced form of the mutants, indicating that it is able to displace one of the ligands to the low spin heme of the SdhC H84L mutant. The g = 2.92 signal of the SdhC H84L mutant titrates with a redox potential at pH 7.0 (E(m)(,7)) of approximately +15 mV, whereas the g = 6.0 signal of the SdhD H71Q mutant titrates with an E(m)(,7) of approximately -100 mV. The quinone site inhibitor pentachlorophenol perturbs the heme optical spectrum of the wild-type and SdhD H71Q mutant enzymes but not the SdhC H84L mutant. This finding suggests that the latter residue also plays an important role in defining the quinone binding site of the enzyme. The SdhC H84L mutation also results in a significant increase in the K(m) and a decrease in the k(cat) for ubiquinone-1, whereas the SdhD H71Q mutant has little effect on these parameters. Overall, these data indicate that SdhC His(84) has an important role in defining the interaction of SdhCDAB with both quinones and heme b(556).

Purification and characterization of Plasmodium falciparum succinate dehydrogenase

Succinate dehydrogenase (SDH), a Krebs cycle enzyme and complex II of the mitochondrial electron transport system was purified to near homogeneity from the human malarial parasite Plasmodium falciparum cultivated in vitro by FPLC on Mono Q, Mono S and Superose 6 gel filtration columns. The malarial SDH activity was found to be extremely labile. Based on Superose 6 FPLC, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and nondenaturing-PAGE analyses, it was demonstrated that the malarial enzyme had an apparent native molecular mass of 90 +/- 8 kDa and contained two major subunits with molecular masses of 55 +/- 6 and 35 +/- 4 kDa (n = 8). The enzymatic reaction required both succinate and coenzyme Q (CoQ) for its maximal catalysis with Km values of 3 and 0.2 microM, and k(cat) values of 0.11 and 0.06 min(-1), respectively. Catalytic efficiency of the malarial SDH for both substrates were found to be relatively low (approximately 600-5000 M(-1) s(-1)). Fumarate, malonate and oxaloacetate were found to inhibit the malarial enzyme with Ki values of 81, 13 and 12 microM, respectively. The malarial enzyme activity was also inhibited by substrate analog of CoQ, 5-hydroxy-2-methyl-1,4-naphthoquinone, with a 50% inhibitory concentration of 5 microM. The quinone had antimalarial activity against the in vitro growth of P. falciparum with a 50% inhibitory concentration of 0.27 microM and was found to completely inhibit oxygen uptake of the parasite at a concentration of 0.88 microM. A known inhibitor of mammalian mitochondrial SDH, 2-thenoyltrifluoroacetone. had no inhibitory effect on both the malarial SDH activity and the oxygen uptake of the parasite at a concentration of 50 microM. Many properties observed in the malarial SDH were found to be different from the host mammalian enzyme.

Comparisons of coenzyme Q bound to mitochondrial membrane proteins among different mammalian species

The objective of this study was to elucidate the mechanisms that govern the variations in the rates of mitochondrial superoxide anion radical (O2-*) generation in different species. The amounts of coenzyme Q (CoQ) associated with mitochondrial membrane proteins were compared in five different mammalian species, namely mouse, rat, rabbit, pig, and cow. Micelles of cardiac mitochondria were prepared using Triton X-100 or deoxycholate (DOC) as detergents, and the micelles containing mitochondrial proteins were sedimented by sucrose density ultracentrifugation. The amount of CoQ present in both types of micelles varied in different species, whereas alpha-tocopherol, another lipoidal molecule in mitochondrial membranes, could not be detected in the micelles of any of these species. The amounts of CoQ bound to mitochondrial proteins in DOC micelles were higher in those mammalian species where CoQ10 was the predominant CoQ homologue, and the amounts were found to be inversely correlated with the rate of mitochondrial 02-* generation among different species. Results also indicated that mitochondrial CoQ exists in at least two distinct pools, one of which is associated with the membrane proteins. The degree of association between CoQ and membrane proteins appears to be a factor determining the rate of mitochondrial O2-* generation.

Identification of quinone-binding and heme-ligating residues of the smallest membrane-anchoring subunit (QPs3) of bovine heart mitochondrial succinate:ubiquinone reductase

The smallest membrane-anchoring subunit (QPs3) of bovine heart succinate:ubiquinone reductase was overexpressed in Escherichia coli JM109 as a glutathione S-transferase fusion protein using the expression vector pGEX2T/QPs3. The yield of soluble active recombinant glutathione S-transferase-QPs3 fusion protein was isopropyl-1-thio-beta-D-galactopyranoside concentration-, induction growth time-, temperature-, and medium-dependent. Maximum yield of soluble recombinant fusion protein was obtained from cells harvested 3.5 h post-isopropyl-1-thio-beta-D-galactopyranoside (0.4 mM)-induction growth at 25 degrees C in 2.0% tryptone, 0.5% yeast extract, 10 mM NaCl, 2.5 mM KCl, 10 mM MgCl2, 20 mM glucose (SOC medium) containing 440 mM sorbitol and 2.5 mM betaine. QPs3 was released from the fusion protein by proteolytic cleavage with thrombin. Isolated recombinant QPs3 shows one protein band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis that corresponds to subunit V of mitochondrial succinate:ubiquinone reductase. Although purified recombinant QPs3 is dispersed in 0.01% dodecylmaltoside, it is in a highly aggregated form, with an apparent molecular mass of more than 1 million. The recombinant QPs3 binds ubiquinone, causing a spectral blue shift. Upon titration of the recombinant protein with ubiquinone, a saturation behavior is observed, suggesting that the binding is specific and that recombinant QPs3 may be in the functionally active state. Two amino acid residues, serine 33 and tyrosine 37, in the putative ubiquinone binding domain of QPs3 are involved in ubiquinone binding because the S33A- or Y37A-substituted recombinant QPs3s do not cause the spectral blue shift of ubiquinone. Although recombinant QPs3 contains little cytochrome b560 heme, the spectral characteristics of cytochrome b560 are reconstituted upon addition of hemin chloride. Reconstituted cytochrome b560 in recombinant QPs3 shows a EPR signal at g = 2.92. Histidine residues at positions 46 and 60 are responsible for heme ligation because the H46N- or H60N-substituted QPs3 fail to restore cytochrome b560 upon addition of hemin chloride.

The Saccharomyces cerevisiae succinate-ubiquinone reductase contains a stoichiometric amount of cytochrome b562

The Saccharomyces cerevisiae succinate-ubiquinone reductase or succinate dehydrogenase (SDH) is a tetramer of non-equivalent subunits encoded by the SDH1, SDH2, SDH3, and SDH4 genes. In most organisms, SDH contains one or two endogenous b-type hemes. However, it is widely believed that the yeast SDH does not contain heme. In this report, we demonstrate the presence of a stoichiometric amount of cytochrome b562 in the yeast SDH. The cytochrome is detected as a peak present in fumarate-oxidized, dithionite-reduced mitochondria. The peak is centered at 562 nm and is present at a heme:covalent FAD molar ratio of 0.92+/-0.11. The cytochrome is not detectable in mitochondria isolated from SDH3 and SDH4 deletion strains. These observations strongly support our conclusion that cytochrome b562 is a component of the yeast SDH.

Generation of superoxide anion by succinate-cytochrome c reductase from bovine heart mitochondria

Production of superoxide anion (O-2), measured as the chemiluminescence of the 2-methyl-6-(p-methoxyphenyl)-3, 7-dihydroimidazo[1,2-a]pyrazin-3-one hydrochloride (MCLA)-O-2 adduct, was observed during electron transfer from succinate to cytochrome c by reconstituted succinate-cytochrome c reductase-phospholipid vesicles replenished with succinate dehydrogenase. Addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone or detergent to the reconstituted reductase-phospholipid vesicles abolished O-2 production, suggesting that O-2 generation is caused by the membrane potential generated during electron transfer through the cytochrome bc1 complex. Production of O-2 was also observed during electron transfer from succinate to cytochrome c by antimycin-treated reductase, in which approximately 99.7% of the reductase activity was inhibited. The rate of O-2 production was closely related to the rate of antimycin-insensitive cytochrome c reduction. Factors affecting antimycin-insensitive reduction of cytochrome c also affected O-2 production and vice versa. When the oxygen concentration in the system was decreased, the rate of O-2 production and cytochrome c reduction by antimycin-treated reductase decreased. When the concentrations of MCLA and cytochrome c were increased, the rate of O-2 production and cytochrome c reduction by antimycin-treated reductase increased. The rate of antimycin-insensitive cytochrome c reduction was sensitive to Qo site inhibitors such as 5-undecyl-6-hydroxy-4,7-dioxobenzothiazole. These results indicate that generation of O-2 during the oxidation of ubiquinol by the cytochrome bc1 complex results from a leakage of the second electron of ubiquinol from its Q cycle electron transfer pathway to interact with oxygen. The electron-leaking site is located at the reduced cytochrome b566 or ubisemiquinone of the Qo site because addition of MCLA to antimycin-treated cytochrome bc1 complex, in the presence of catalytic amounts of succinate-cytochrome c reductase, delayed cytochrome b reduction by succinate. In the presence of oxidized cytochrome c, purified succinate dehydrogenase also catalyzed oxidation of succinate to generate O-2. When succinate dehydrogenase was reconstituted with the bc1 particles to form succinate-cytochrome c reductase, the production of O-2 diminished. These results suggest that reduced FAD of succinate dehydrogenase is the electron donor for oxygen to produce O-2 in the absence of their immediate electron acceptor and in the presence of cytochrome c.