Purification and properties of succinate-ubiquinone oxidoreductase complex from Paracoccus denitrificans

The Biological Role of the ζ Subunit as Unidirectional Inhibitor of the F1FO-ATPase of Paracoccus denitrificans

The biological roles of the three natural F₁F_O-ATPase inhibitors, ε, ζ, and IF₁, on cell physiology remain controversial. The ζ subunit is a useful model for deletion studies since it mimics mitochondrial IF₁, but in the F₁F_O-ATPase of Paracoccus denitrificans (PdF₁F_O), it is a monogenic and supernumerary subunit. Here, we constructed a P. denitrificans 1222 derivative (PdΔζ) with a deleted ζ gene to determine its role in cell growth and bioenergetics. The results show that the lack of ζ in vivo strongly restricts respiratory P. denitrificans growth, and this is restored by complementation in trans with an exogenous ζ gene. Removal of ζ increased the coupled PdF₁F_O-ATPase activity without affecting the PdF₁F_O-ATP synthase turnover, and the latter was not affected at all by ζ reconstitution in vitro. Therefore, ζ works as a unidirectional pawl-ratchet inhibitor of the PdF₁F_O-ATPase nanomotor favoring the ATP synthase turnover to improve respiratory cell growth and bioenergetics.

Isolation and Purification of Complex II from Proteus Mirabilis Strain ATCC 29245

A respiratory complex was isolated from plasma membrane of pathogenic Proteus mirabilis strain ATCC 29245. It was identified as complex II consisting of succinate:quinone oxidoreductase (EC 1.3.5.1) containing single heme b. The complex II was purified by ion-exchange chromatography and gel filtration. The molecular weight of purified complex was 116.5 kDa and it was composed of three subunits with molecular weights of 19 kDa, 29 kDa and 68.5 kDa. The complex II contained 9.5 nmoles of cytochrome b per mg protein. Heme staining indicated that the 19 kDa subunit was cytochrome b. Its reduced form showed absorptions peaks at 557.0, 524.8 and 424.4 nm. The α-band was shifted from 557.0 nm to 556.8 nm in pyridine ferrohemochrome spectrum. The succinate: quinone oxidoreductase activity was found to be high in this microorganism.

Electron transfer in Paracoccus denitrificans with the modified fbc operon

Membrane fragments of two mutant strains of Paracoccus denitrificans genetically modified in the bc(1) complex have been studied for comparison of enzymic activities of succinate-cytochrome-c reductase and its components, viz. succinate dehydrogenase (Complex II) and ubiquinol-cytochrome-c reductase (Complex III) and their response to changes in concentration of succinate, cytochrome c, ionic strength, pH, temperature and sensitivity to antimycin A. The mutants synthesized and assembled the b and c hemes in the ratio characteristic for the wild type strain. The mutant strain M 71 expressing the truncated copy of cytochrome c(1) (devoid of a stretch of 150 mainly acidic amino acids) was less sensitive to increasing concentration of cytochrome c and changes in ionic strength of the medium, but maintained the original affinity to succinate and sensitivity to antimycin A. The mutant strain M 36 with an overexpressed bc(1) content showed the highest response to changes in ionic strength and physical parameters, exhibited the lowest turnover number values with succinate-cytochrome-c reductase, but positively affected the succinate dehydrogenase. In view of the interaction of the redox components in native membranes the functional analyses of separated Complexes II and III should be regarded with caution.

A novel 11-kDa inhibitory subunit in the F1FO ATP synthase of Paracoccus denitrificans and related alpha-proteobacteria

The F(1)F(O) and F(1)-ATPase complexes of Paracoccus denitrificans were isolated for the first time by ion exchange, gel filtration, and density gradient centrifugation into functional native preparations. The liposome-reconstituted holoenzyme preserves its tight coupling between F(1) and F(O) sectors, as evidenced by its high sensitivity to the F(O) inhibitors venturicidin and diciclohexylcarbodiimide. Comparison and N-terminal sequencing of the band profile in SDS-PAGE of the F(1) and F(1)F(O) preparations showed a novel 11-kDa protein in addition to the 5 canonical alpha, beta, gamma, delta, and epsilon subunits present in all known F(1)-ATPase complexes. BN-PAGE followed by 2D-SDS-PAGE confirmed the presence of this 11-kDa protein bound to the native F(1)F(O)-ATP synthase of P. denitrificans, as it was observed after being isolated. The recombinant 11 kDa and epsilon subunits of P. denitrificans were cloned, overexpressed, isolated, and reconstituted in particulate F(1)F(O) and soluble F(1)-ATPase complexes. The 11-kDa protein, but not the epsilon subunit, inhibited the F(1)F(O) and F(1)-ATPase activities of P. denitrificans. The 11-kDa protein was also found in Rhodobacter sphaeroides associated to its native F(1)F(O)-ATPase. Taken together, the data unveil a novel inhibitory mechanism exerted by this 11-kDa protein on the F(1)F(O)-ATPase nanomotor of P. denitrificans and closely related alpha-proteobacteria.

The succinate:menaquinone reductase of Bacillus cereus: characterization of the membrane-bound and purified enzyme

Utilization of external succinate by Bacillus cereus and the properties of the purified succinate:menaquinone-7 reductase (SQR) were studied. Bacillus cereus cells showed a poor ability for the uptake of and respiratory utilization of exogenous succinate, thus suggesting that B. cereus lacks a specific succinate uptake system. Indeed, the genes coding for a succinate-fumarate transport system were missing from the genome database of B. cereus. Kinetic studies of membranes indicated that the reduction of menaquinone-7 is the rate-limiting step in succinate respiration. In accordance with its molecular characteristics, the purified SQR of B. cereus belongs to the type-B group of SQR enzymes, consisting of a 65-kDa flavoprotein (SdhA), a 29-kDa iron-sulphur protein (SdhB), and a 19-kDa subunit containing 2 b-type cytochromes (SdhC). In agreement with this, we could identify the 4 conserved histidines in the SdhC subunit predicted by the B. cereus genome database. Succinate reduced half of the cytochrome b content. Redox titrations of SQR-cytochrome b-557 detected 2 components with apparent midpoint potential values at pH 7.6 of 79 and -68 mV, respectively; the components were not spectrally distinguishable by their maximal absorption bands as those of Bacillus subtilis. The physiological properties and genome database analyses of B. cereus are consistent with the cereus group ancestor being an opportunistic pathogen.

Assembly of respiratory complexes I, III, and IV into NADH oxidase supercomplex stabilizes complex I in Paracoccus denitrificans

Stable supercomplexes of bacterial respiratory chain complexes III (ubiquinol:cytochrome c oxidoreductase) and IV (cytochrome c oxidase) have been isolated as early as 1985 (Berry, E. A., and Trumpower, B. L. (1985) J. Biol. Chem. 260, 2458-2467). However, these assemblies did not comprise complex I (NADH:ubiquinone oxidoreductase). Using the mild detergent digitonin for solubilization of Paracoccus denitrificans membranes we could isolate NADH oxidase, assembled from complexes I, III, and IV in a 1:4:4 stoichiometry. This is the first chromatographic isolation of a complete "respirasome." Inactivation of the gene for tightly bound cytochrome c552 did not prevent formation of this supercomplex, indicating that this electron carrier protein is not essential for structurally linking complexes III and IV. Complex I activity was also found in the membranes of mutant strains lacking complexes III or IV. However, no assembled complex I but only dissociated subunits were observed following the same protocols used for electrophoretic separation or chromatographic isolation of the supercomplex from the wild-type strain. This indicates that the P. denitrificans complex I is stabilized by assembly into the NADH oxidase supercomplex. In addition to substrate channeling, structural stabilization of a membrane protein complex thus appears as one of the major functions of respiratory chain supercomplexes.

Complex II from phototrophic purple bacterium Rhodoferax fermentans displays rhodoquinol-fumarate reductase activity

It has long been accepted that bacterial quinol-fumarate reductase (QFR) generally uses a low-redox-potential naphthoquinone, menaquinone (MK), as the electron donor, whereas mitochondrial QFR from facultative and anaerobic eukaryotes uses a low-redox-potential benzoquinone, rhodoquinone (RQ), as the substrate. In the present study, we purified novel complex II from the RQ-containing phototrophic purple bacterium, Rhodoferax fermentans that exhibited high rhodoquinol-fumarate reductase activity in addition to succinate-ubiquinone reductase activity. SDS/PAGE indicated that the purified R. fermentans complex II comprises four subunits of 64.0, 28.6, 18.7 and 17.5 kDa and contains 1.3 nmol heme per mg protein. Phylogenetic analysis and comparison of the deduced amino acid sequences of R. fermentans complex II with pro/eukaryotic complex II indicate that the structure and the evolutional origins of R. fermentans complex II are closer to bacterial SQR than to mitochondrial rhodoquinol-fumarate reductase. The results strongly indicate that R. fermentans complex II and mitochondrial QFR might have evolved independently, although they both utilize RQ for fumarate reduction.

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.

The succinate dehydrogenase from the thermohalophilic bacterium Rhodothermus marinus: redox-Bohr effect on heme bL

The succinate dehydrogenase from the thermohalophilic bacterium Rhodothermus marinus is a member of the succinate:menaquinone oxidoreductases family. It is constituted by three subunits with apparent molecular masses of 70, 32, and 18 kDa. The optimum temperature for succinate dehydrogenase activity is 80 degrees C, higher than the optimum growth temperature of R. marinus, 65 degrees C. The enzyme shows a high affinity for both succinate (Km = 0.165 mM) and fumarate (Km = 0.10 mM). It contains the canonical iron-sulfur centers S1, S2, and S3, as well as two B-type hemes. In contrast to other succinate dehydrogenases, the S3 center has an unusually high reduction potential of +130 mV and is present in two different conformations, one of which presents an unusual EPR signal with g values at 2.035, 2.009, and 2.001. The apparent midpoint reduction potentials of the hemes, +75 and -65 mV at pH 7.5, are also higher than those reported for other enzymes. The heme with the lower potential (heme bL) presents a considerable dependence of the reduction potential with pH (redox-Bohr effect), having a pKa(OX) = 6.5 and a pKa(red) = 8.7. This behavior is consistent with the proposal that in these enzymes menaquinone reduction occurs close to heme bL, near to the periplasmic side of the membrane, and involving dissipation of the proton transmembrane gradient.

Molecular genetics of the genus Paracoccus: metabolically versatile bacteria with bioenergetic flexibility

Paracoccus denitrificans and its near relative Paracoccus versutus (formerly known as Thiobacilllus versutus) have been attracting increasing attention because the aerobic respiratory system of P. denitrificans has long been regarded as a model for that of the mitochondrion, with which there are many components (e.g., cytochrome aa3 oxidase) in common. Members of the genus exhibit a great range of metabolic flexibility, particularly with respect to processes involving respiration. Prominent examples of flexibility are the use in denitrification of nitrate, nitrite, nitrous oxide, and nitric oxide as alternative electron acceptors to oxygen and the ability to use C1 compounds (e.g., methanol and methylamine) as electron donors to the respiratory chains. The proteins required for these respiratory processes are not constitutive, and the underlying complex regulatory systems that regulate their expression are beginning to be unraveled. There has been uncertainty about whether transcription in a member of the alpha-3 Proteobacteria such as P. denitrificans involves a conventional sigma70-type RNA polymerase, especially since canonical -35 and -10 DNA binding sites have not been readily identified. In this review, we argue that many genes, in particular those encoding constitutive proteins, may be under the control of a sigma70 RNA polymerase very closely related to that of Rhodobacter capsulatus. While the main focus is on the structure and regulation of genes coding for products involved in respiratory processes in Paracoccus, the current state of knowledge of the components of such respiratory pathways, and their biogenesis, is also reviewed.

Biogenesis of respiratory cytochromes in bacteria

Biogenesis of respiratory cytochromes is defined as consisting of the posttranslational processes that are necessary to assemble apoprotein, heme, and sometimes additional cofactors into mature enzyme complexes with electron transfer functions. Different biochemical reactions take place during maturation: (i) targeting of the apoprotein to or through the cytoplasmic membrane to its subcellular destination; (ii) proteolytic processing of precursor forms; (iii) assembly of subunits in the membrane and oligomerization; (iv) translocation and/or modification of heme and covalent or noncovalent binding to the protein moiety; (v) transport, processing, and incorporation of other cofactors; and (vi) folding and stabilization of the protein. These steps are discussed for the maturation of different oxidoreductase complexes, and they are arranged in a linear pathway to best account for experimental findings from studies concerning cytochrome biogenesis. The example of the best-studied case, i.e., maturation of cytochrome c, appears to consist of a pathway that requires at least nine specific genes and more general cellular functions such as protein secretion or the control of the redox state in the periplasm. Covalent attachment of heme appears to be enzyme catalyzed and takes place in the periplasm after translocation of the precursor through the membrane. The genetic characterization and the putative biochemical functions of cytochrome c-specific maturation proteins suggest that they may be organized in a membrane-bound maturase complex. Formation of the multisubunit cytochrome bc, complex and several terminal oxidases of the bo3, bd, aa3, and cbb3 types is discussed in detail, and models for linear maturation pathways are proposed wherever possible.

Electron paramagnetic resonance studies of succinate:ubiquinone oxidoreductase from Paracoccus denitrificans. Evidence for a magnetic interaction between the 3Fe-4S cluster and cytochrome b

Electron paramagnetic resonance (EPR) studies of succinate:ubiquinone oxidoreductase (SQR) from Paracoccus denitrificans have been undertaken in the purified and membrane-bound states. Spectroscopic "signatures" accounting for the three iron-sulfur clusters (2Fe-2S, 3Fe-4S, and 4Fe-4S), cytochrome b, flavin, and protein-bound ubisemiquinone radicals have been obtained in air-oxidized, succinate-reduced, and dithionite-reduced preparations at 4-10 K. Spectra obtained at 170 K in the presence of excess succinate showed a signal typical of that of a flavin radical, but superimposed with another signal. The superimposed signal originated from two bound ubisemiquinones, as shown by spectral simulations. Power saturation measurements performed on the air-oxidized enzyme provided evidence for a weak magnetic dipolar interaction operating between the oxidized 3Fe-4S cluster and the oxidized cytochrome b. Power saturation experiments performed on the succinate- and dithionite-reduced forms of the enzyme demonstrated that the 4Fe-4S cluster is coupled weakly to both the 2Fe-2S and the 3Fe-4S clusters. Quantitative interpretation of these power saturation experiments has been achieved through redox calculations. They revealed that a spin-spin interaction between the reduced 3Fe-4S cluster and the cytochrome b (oxidized) may also exist. These findings form the first direct EPR evidence for a close proximity (</=2 nm) of the high potential 3Fe-4S cluster, situated in the succinate dehydrogenase part of the enzyme, and the low potential, low spin b-heme in the membrane anchor of the enzyme.

Purification and characterization of the succinate dehydrogenase complex and CO-reactive b-type cytochromes from the facultative alkaliphile Bacillus firmus OF4

The presence of a cytochrome bo-type terminal oxidase in Bacillus firmus OF4 had been suggested from the effects of CO on the spectra of reduced membrane cytochromes (Hicks, D.B., Plass, R.J. and Quirk, P.G. (1991) J. Bacteriol. 173, 5010-5016). In that study the CO-binding b-type cytochrome was partially purified by anion exchange chromatography. No further purification was attempted but later HPLC analysis indicated the absence of significant heme O in the B. firmus OF4 membranes. The current work shows that the partially purified cytochrome b is actually composed of three different b-type cytochromes which can be separated and purified by a combination of ion-exchange, hydroxyapatite and gel filtration chromatographies. Two of the cytochromes were CO-reactive but lacked the characteristic multisubunit composition of known terminal oxidases. Neither purified cytochrome catalyzed quinol or ferrocytochrome c oxidation. The more abundant CO-reactive b-type cytochrome (cytochrome b560) had an apparent molecular mass of 10 kDa, whereas the other, more minor component (cytochrome b558), was partially purified and showed two bands of 23 and 17 kDa on SDS-PAGE. The functions of the cytochromes b560 and b558 remain unknown but together they account for the spectrum originally attributed to cytochrome bo. The third, non-CO reactive, cytochrome b was associated with substantial succinate dehydrogenase activity and was purified as a three subunit succinate dehydrogenase complex with high specific activity (17.7 mumol/min/mg). Limited N-terminal sequence of each subunit demonstrated marked similarity to the complex from Bacillus subtilis. The cytochrome b of the alkaliphile enzyme was reduced about 50% by succinate compared to the level of reduction achieved by dithionite. The enzyme reacted with both napthoquinones and benzoquinones. The results presented indicate that Bacillus firmus OF4 contains a succinate dehydrogenase complex with very similar properties to the enzyme from Bacillus subtilis, but does not contain a cytochrome o-type terminal oxidase under the growth conditions studied.

A structural model for the membrane-integral domain of succinate: quinone oxidoreductases

Many succinate:quinone oxidoreductases in bacteria and mitochondria, i.e. succinate:quinone reductases and fumarate reductases, contain in the membrane anchor a cytochrome b whose structure and function is poorly understood. Based on biochemical data and polypeptide sequence information, we show that the anchors in different organisms are related despite an apparent diversity in polypeptide and heme composition. A general structural model for the membrane-integral domain of the anchors is proposed. It is an antiparallel four-helix bundle with a novel arrangement of hexa-coordinated protoheme IX. The structure can be applied to a larger group of membrane-integral cytochromes of b-type and has evolutionary and functional implications.

Succinate:quinone oxidoreductase (complex II) containing a single heme b in facultative alkaliphilic Bacillus sp. strain YN-2000

Succinate:quinone oxidoreductase (EC 1.3.5.1) was first purified from the facultative alkaliphilic Bacillus sp. strain YN-2000 in the presence of Triton X-100. The isolated enzyme showed high succinate-ubiquinone oxidoreductase activity at pH 8.5. The Km for ubiquinone 1 and the Vmax of the enzyme were determined to be about 5 microM and 48 micromol of ubiquinone 1 per min per mg, respectively. The catalytic activity of the enzyme was 50% inhibited by 9 microM 2-thenoyltrifluoroacetone or 0.8 microM 2-n-heptyl-4-hydroxyquinoline- N-oxide. The enzyme consisted of three kinds of subunits with molecular masses of 66, 26, and 15 kDa, respectively, and contained 1.28 mol of covalently bound flavin adenine dinucleotide, 0.9 mol of heme b, 1.35 mol of menaquinone, 8.3 mol of nonheme iron, and 7.5 mol of inorganic sulfide per mol of enzyme. The enzyme showed symmetrical alpha absorption peaks at 556.5 and 554 nm in the reduced state at room temperature and 77 K, respectively. The potentiometric analysis of the enzyme yielded an Em,7 of heme b of about -64 mV (n = 1). Furthermore, the content of the enzyme was increased up to fivefold when the bacterium was grown at pH 10 compared with pH 7. These results indicate that the succinate:quinone oxidoreductase with a single heme b is involved in the respiratory chain of the alkaliphile at a very alkaline pH.

Resolution of the aerobic respiratory system of the thermoacidophilic archaeon, Sulfolobus sp. strain 7. III. The archaeal novel respiratory complex II (succinate:caldariellaquinone oxidoreductase complex) inherently lacks heme group

An active respiratory complex II (succinate:quinone oxidoreductase) has been purified from tetraether lipid membranes of the thermoacidophilic archaeon, Sulfolobus sp. strain 7. It consists of four different subunits with apparent molecular masses of 66, 37, 33, and 12 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 66-kDa subunit contains a covalently bound flavin, the 37-kDa subunit is a possible iron-sulfur protein carrying three distinct types of EPR-visible FeS cluster, and the 33- and 12-kDa subunits are putative membrane-anchor subunits, respectively. While no heme group is detected in the purified complex II, it catalyzes succinate-dependent reduction of ubiquinone-1 and 2,6-dichlorophenolindophenol in the absence of phenazine methosulfate. The respiratory complex II of Sulfolobus sp. strain 7 appears to be novel in that it functions as a true succinate:caldariellaquinone oxidoreductase, although inherently lacking any heme group. This further indicates that the heme group of several respiratory complexes II may not be involved in the redox intermediates of the electron transfer from succinate to quinone.

Stage-specific isoforms of complex II (succinate-ubiquinone oxidoreductase) in mitochondria from the parasitic nematode, Ascaris suum

Complex II from mitochondria of the adult parasitic nematode, Ascaris suum, exhibits high fumarate reductase activity and plays a key role in the anaerobic electron transport observed in these organelles. In contrast, mitochondria isolated from free living second stage larvae (L2) of A. suum show much lower fumarate reductase activity than those from adults, whereas succinate dehydrogenase activities of mitochondria in both stages are comparable. In the present study, biochemical and antigenic properties of the partially purified enzymes from both larval and adult mitochondria were compared. Larval complex II eluted from the DEAE-Cellulofine column chromatography at a lower salt concentration than adult enzyme, whereas the apparent molecular size of both enzyme complexes estimated by gel permeation column chromatography was the same. The fumarate reductase activity of larval complex II was less than 3% of that of adult enzyme, and the Km values for substrates were significantly different between the two complexes. The flavoprotein subunit of larval complex II could be distinguished from that of adult complex II by two-dimensional gel electrophoresis and peptide mapping. The antibody against the smallest subunit (small subunit of cytochrome b558) of the adult enzyme did not cross-react with that of the larval enzyme. These results suggest that larval complex II differs from adult enzyme and is more similar to aerobic mammalian enzymes with low fumarate reductase activity. This is the first direct indication of the two different stage-specific forms of mitochondrial complex II.

Identification of the axial heme ligands of cytochrome b556 in succinate: ubiquinone oxidoreductase from Escherichia coli

Electron paramagnetic resonance (EPR) and near-infrared magnetic circular dichroism (MCD) have been used to identify the ligands to the cytochrome b556 component of succinate: ubiquinone oxidoreductase (succinate dehydrogenase) from Escherichia coli. The 'highly axial low spin' (HALS) EPR spectrum suggests bis(histidine) ligation of the heme with the histidines in a staggered configuration. The near-infrared MCD spectrum exhibits a low energy maximum at 1600 nm which is also clearly indicative of bis(histidine) ligation of the heme iron. The data unambiguously demonstrate that the heme b556 is ligated to E. coli succinate dehydrogenase via two histidines.

Fumarate reductase activity of bovine heart succinate-ubiquinone reductase. New assay system and overall properties of the reaction

A simple system for aerobic assay of the quinol-fumarate reductase reaction catalyzed by purified soluble bovine heart succinate-ubiquinone reductase in the presence of NADH, NAD(P)H-quinone reductase (DT-diaphorase) and an appropriate quinone is described. The reaction is inhibited by carboxin, suggesting that the same quinone/quinol binding site is involved in electron transfer from succinate to ubiquinone and from ubiquinol to fumarate. The kinetic properties of the reaction in both directions and comparative affinities of the substrate binding sites of the enzyme to substrates (products) and competitive inhibitors are reported. Considerable difference in affinity of the substrates binding site to oxaloacetate was demonstrated when the enzyme was assayed in the direct and reverse directions. These results were taken to indicate that the oxidized dicarboxylate-free enzyme is an intermediate during the steady-state succinate-ubiquinone reductase reaction, whereas the reduced dicarboxylate-free enzyme is an intermediate of the steady-state ubiquinol-fumarate reductase reaction. No difference in the reactivity of the substrate-protected cysteine and arginine residues was found when the pseudo-first-order rate constants for N-ethylmaleimide and phenylglyoxal inhibition were determined for oxidized and quinol-reduced enzyme. Quinol-fumarate reductase activity was reconstituted from the soluble succinate dehydrogenase and low-molecular-mass ubiquinone reactivity conferring protein(s). No reduction of cytochrome b was observed in the presence of quinol generating system, whereas S-3 low temperature EPR-detectable iron-sulfur center was completely reduced by quinol under equilibrium (without fumarate) or steady-state (in the presence of fumarate). No significant reduction of ferredoxin type iron-sulfur centers was detected during the steady-state quinol-fumarate oxidoreductase reaction. The data obtained eliminate participation of cytochrome b in the quinol-fumarate reductase reaction and show that the rate limiting step of the overall reaction lies between iron-sulfur center S-3 and lower midpoint potential redox components of the enzyme.

Purification and characterisation of an archaebacterial succinate dehydrogenase complex from the plasma membrane of the thermoacidophile Sulfolobus acidocaldarius

A succinate dehydrogenase complex was isolated in a three-step purification from plasma membranes of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius. It consists of four subunits: a, 66 kDa; b, 31 kDa; c, 28 kDa and d, 12.8 kDa. In the 141-kDa native protein, the four subunits are present in an equimolar stoichiometry. The complex contains acid-non-extractable flavin, iron and acid-labile sulphide. Maximal succinate dehydrogenase activities were recorded at pH 6.5, which coincides with the internal pH of Sulfolobus cells. The temperature optimum of 81 degrees C defines the Sulfolobus succinate dehydrogenase as a thermophilic enzyme complex. The Km for succinate was found to be 1.42 mM (55 degrees C). Similar to the mitochondrial soluble succinate dehydrogenase, this enzyme is capable of transferring electrons to artificial electron acceptors, for instance phenazine methosulfate, N,N,N',N'-tetramethyl-p-phenylenediamine and ferricyanide. In contrast to the mitochondrial succinate dehydrogenase, the archaebacterial enzyme reduces 1,4-dichloroindophenol also in the absence of phenazine methosulfate. Caldariella quinone, the physiological electron mediator in the Sulfolobus respiratory chain, was only slowly reduced under adjusted conditions. The succinate--phenazine methosulfate-(1,4-dichloroindophenol) oxidoreductase of the isolated complex was strongly inhibited by tetrachlorobenzoquinone. In plasma membranes the complex reduces molecular oxygen in a cyanide-sensitive reaction. Polyclonal Sulfolobus anti-a antibodies crossreacted with 66-67-kDa polypeptides from membranes of Thermoplasma acidophilium, Sulfolobus solfataricus and beef heart submitochondrial particles.

The F0 subunits of bovine mitochondrial ATP synthase complex: purification, antibody production, and interspecies cross-immunoreactivity

The known subunits of the membrane sector F0 of the bovine mitochondrial ATP synthase complex are subunits b, d, 6, F6, OSCP (oligomycin sensitivity-conferring protein), the DCCD (dicyclohexylcarbodiimide) binding proteolipid, and A6L. The first six subunits were purified from SMP or preparations of the ATP synthase complex, and monospecific antibodies were raised against each. The antisera were shown to be competent for immuno-blotting, and each antiserum recognized a single polypeptide of the expected Mr in preparations of the ATP synthase complex. Immunoblots utilizing antibodies to OSCP and subunits d and 6, which exhibit the same Mr on dodecyl sulfate-polyacrylamide gels, showed clearly that these polypeptides are immunologically distinct. Immunological cross-reactivity was demonstrated between bovine, human, rat, Saccharomyces cerevisiae, Paracoccus denitrificans, and Escherichia coli for subunit 6; between bovine, human, and rat for subunits b, d, OSCP, and F6; and between bovine and rat for the DCCD binding proteolipid. Anti-subunit 6 antiserum, before or after immunopurification against the ATP synthase complex, recognized a single polypeptide in the bovine ATP synthase complex and S. cerevisiae mitochondria, but two polypeptides of different Mr in bovine SMP, human, and rat mitochondria, and Paracoccus and E. coli membranes.

Identification of the NADH-binding subunit of NADH-ubiquinone oxidoreductase of Paracoccus denitrificans

The NADH dehydrogenase complex isolated from Paracoccus denitrificans is composed of approximately 10 unlike polypeptides and contains noncovalently bound FMN, non-heme iron, and acid-labile sulfide [Yagi, T. (1986) Arch. Biochem. Biophys. 250, 302-311]. When the Paracoccus NADH dehydrogenase complex was irradiated by UV light in the presence of [adenylate-32P]NAD, radioactivity was incorporated exclusively into one of three polypeptides of Mr approximately 50,000. Similar results were obtained when [adenylate-32P]NADH was used. The labeling of the Mr 50,000 polypeptide was diminished when UV irradiation of the enzyme with [adenylate-32P]NAD was performed in the presence of NADH, but not in the presence of NADP(H). The labeled polypeptide was isolated by preparative sodium dodecyl sulfate gel electrophoresis and was shown to cross-react with antiserum to the NADH-binding subunit (Mr = 51,000) of bovine NADH-ubiquinone oxidoreductase. Its amino acid composition was also very similar to that of the bovine NADH-binding subunit. These chemical and immunological results indicate that the Mr 50,000 polypeptide is an NADH-binding subunit of the Paracoccus NADH dehydrogenase complex.

Prediction and comparison of the haem-binding sites in membrane haemoproteins

This article contains a comparative review of the structural properties of membrane haemoproteins, with particular emphasis on the possible similarities of the haem-binding peptides. A procedure is suggested for identifying the peptides which may bind membrane-buried haems on the basis of the primary sequences of the proteins. The integration of this procedure with the information deduced by refined hydropathy analysis indicates that the basic structural model for the haemoproteins which interact with quinones may be a transmembrane helical bundle containing the haem(s) at its centre. Structural similarities exist in the sequence of hydrophobic segments that are predicted to bind the membrane-buried haems of b-cytochromes which interact with quinones. The predicted haem-binding sites show similarities also with the peptides that bind the non-haem iron in the bacterial reaction centres, and this may be correlated to the common function of interacting with quinones and their intermediates. The analysis of the amino-acid composition of the proposed ligand peptides in the membrane haemoproteins examined has provided a molecular rationale for explaining the highly anisotropic low-spin EPR signal which is characteristic of many membrane-bound b-cytochromes.

Nuclearly inherited diuron-resistant mutations conferring a deficiency in the NADH--or succinate--ubiquinone oxidoreductase activity in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, diuron, antimycin and myxothiazol block the respiratory pathway at the bc1 complex level. Nuclearly inherited mutations located at the DIU3 and DIU4 loci confer in vitro resistance to diuron and cross-resistance to antimycin and myxothiazol at the NADH oxidase level. The mutant strains do not exhibit diuron resistance at the quinol-cytochrome-c oxidoreductase level. Thus, the apparent resistance does not seem to be the result of a modification of the inhibitory sites. Instead, the quinone reduction rate was found to be altered in the mutant. The diu3 mutations lead to a deficiency of the NADH--ubiquinone oxidoreductase activity, and the diu4 mutations to a deficiency of the succinate--ubiquinone oxidoreductase activity. On the basis of the model of Kröger and Klingenberg, a decrease of quinone reduction could explain the resistance to the bc1 complex inhibitors. Thus, the apparent resistance to the bc1 complex inhibitors was found to be due to a modification of the electron transfer kinetics.

Electron transport reactions in a cytochrome c-deficient mutant of Paracoccus denitrificans

A mutant of Paracoccus denitrificans which is deficient in c-type cytochromes grows aerobically with generation times similar to those obtained with a wild-type strain. The aa3-type oxidase is functional in the mutant as judged by spectrophotometric assays of cytochrome c oxidation using the membrane particles and cytochrome aa3 reduction in whole cells. The cytochrome c oxidase (aa3-type) of the c-less mutant oxidizes soluble cytochrome c at rates equivalent to those obtained with the wild-type. NADH and succinate oxidase activities of the membrane preparations of the mutant and wild-type are also comparable in the absence of detergent treatment. Exogenous soluble cytochrome c can be both reduced by NADH- and succinate-linked systems and oxidized by cytochrome aa3 present in membranes of the mutant strain. Rapid overall electron transport can occur in the c-less mutant, suggesting that reactions result from collision of diffusing complexes.