Reconstitution of Respiratory Chain Enzyme Systems

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.

An orphan cbb3-type cytochrome oxidase subunit supports Pseudomonas aeruginosa biofilm growth and virulence

Hypoxia is a common challenge faced by bacteria during associations with hosts due in part to the formation of densely packed communities (biofilms). cbb3-type cytochrome c oxidases, which catalyze the terminal step in respiration and have a high affinity for oxygen, have been linked to bacterial pathogenesis. The pseudomonads are unusual in that they often contain multiple full and partial (i.e. 'orphan') operons for cbb3-type oxidases and oxidase subunits. Here, we describe a unique role for the orphan catalytic subunit CcoN4 in colony biofilm development and respiration in the opportunistic pathogen Pseudomonas aeruginosa PA14. We also show that CcoN4 contributes to the reduction of phenazines, antibiotics that support redox balancing for cells in biofilms, and to virulence in a Caenorhabditis elegans model of infection. These results highlight the relevance of the colony biofilm model to pathogenicity and underscore the potential of cbb3-type oxidases as therapeutic targets.

The lysosomal H+ pump: 8-azido-ATP inhibition and the role of chloride in H+ transport

Lysosomes (tritosomes) were purified from the livers of rats injected with Triton WR 1339. The lysosomes developed an Mg2+-ATP-dependent pH gradient as measured by Acridine orange accumulation. H+ transport was supported by chloride, but not sulfate, and was independent of the cation used. H+ transport and Mg2+-stimulated ATPase was inhibited by diethylstilbesterol (K0.5 = 2 microM). N-Ethylmaleimide inhibited H+ transport (K0.5 = 30 microM). At low concentrations of N-ethylmaleimide, ATP partially protected H+ transport from inhibition with N-ethylmaleimide. Photolysis with 8-azido-ATP inhibited H+ transport and Mg2+-stimulated ATPase activity. Under these same conditions, 8-azido-[alpha-32P]ATP reacted with a number of polypeptides of the intact lysosome and lysosomal membranes. Pump-dependent potentials were measured using the fluorescent potential-sensitive dye, DiSC3(5) (3,3'-dipropylthiocarbocyanine) and ATP-dependent potential generation was inhibited by diethylstilbesterol. Chloride, but not sulfate reduced the magnitude of the ATP-dependent membrane potential, as measured using merocyanine 540. The chloride conductance, independent of ATP, was of sufficient magnitude to generate a H+ gradient driven by external chloride in the presence of tetrachlorosalicylanilide. In Cl- free media, ATP-dependent H+ transport was restored to control levels by outwardly directed K+ gradients in the presence of valinomycin. The role of cell Cl- is to provide the necessary conductance for supporting lysosomal acidification by the electrogenic proton pump.

Studies on the succinate dehydrogenating system. Interaction of the mitochondrial succinate-ubiquinone reductase with pyridoxal phosphate

The inhibitory effect of pyridoxal phosphate on the Triton X-100 solubilized purified bovine heart succinate-ubiquinone reductase (Choudhry, Z.M., Gavrikova, E.V., Kotlyar, A.B., Tushurashvilli, P.R. and Vinogradov, A.D. (1985) FEBS Lett. 182, 171-175) was studied. The kinetics of the enzyme inactivation by pyridoxal phosphate was found to be strongly dependent both qualitatively and quantitatively on the concentration of the protein-detergent complexes. In the diluted system the inactivation of the ubiquinone-depleted enzyme was completely prevented by the saturating concentrations of Q2, carboxin, thenoiltrifluoroacetone and pentachlorophenol, i.e., by the substrate and specific inhibitors of the enzyme. The protective effects of Q2 and the inhibitors was employed to quantitate the affinities of the ligands to their specific binding sites. Strong difference in the affinity of Q2 to the reduced and oxidized enzyme was found. When the soluble reconstitutively active succinate dehydrogenase was treated with pyridoxal phosphate, the reactivity of the enzyme towards low ferricyanide concentrations and its reconstitutive activity was significantly protected against aerobic inactivation.

Solubilization and reconstitution of cholinephosphotransferase from sarcoplasmic reticulum: stabilization of solubilized enzyme by diacylglycerol and glycerol

Cholinephosphotransferase (CDPcholine: 1,2-diacylglycerol cholinephosphotransferase, EC 2.7.8.2), which catalyzes the terminal step in phosphatidylcholine synthesis via the CDPcholine pathway, is present in sarcoplasmic reticulum from rabbit skeletal muscle (Cornell, R. and MacLennan, D.H. (1985) Biochim. Biophys. Acta 835, 567-576). The conditions for solubilization and reconstitution of this enzyme were investigated as a preliminary step towards its eventual purification. The activity was not released by treatment of membranes with 1 M KCl, but was solubilized after dissolution of membranes with detergents. Cholinephosphotransferase was inactivated by cholate, deoxycholate, Triton X-100, octylglucoside, Tween-20 or SDS at concentrations which solubilize the membrane. However, the activity could be fully recovered after reconstituting the membrane by adding excess lipid (soybean) and removing detergent by gel filtration, dialysis or by absorption to Bio-Beads. When the membrane was solubilized with octylglucoside or cholate at weight ratios of detergent: membrane protein of at least 10, the activity was irreversibly lost unless stabilizers were added with detergent. The substrate diacylglycerol and glycerol were effective stabilizers.

Pyridoxal phosphate-induced dissociation of the succinate: ubiquinone reductase

Treatment of the soluble ubiquinone-deficient succinate: ubiquinone reductase with pyridoxal phosphate results in the inhibition of the carboxin-sensitive ubiquinone-reductase activity of the enzyme. The inactivation is prevented by the soluble homolog of ubiquinone (Q2) but is insensitive to the dicarboxylates interacting with the substrate binding site of succinate dehydrogenase. The reactivity of the pyridoxal phosphate-inhibited enzyme with different electron acceptors suggests that the observed inhibition is due to the dissociation of succinate dehydrogenase from the enzyme complex. The soluble succinate dehydrogenase was recovered in the supernatant after treatment of the insoluble succinate: ubiquinone reductase with pyridoxal phosphate. The data obtained strongly suggest the participation of amino groups in the interaction between succinate dehydrogenase and the ubiquinone reactivity conferring peptide within the complex.

Na+-Ca2+ exchange activity in rabbit lymphocyte plasma membranes

Plasma membranes of rabbit thymus lymphocytes accumulated Ca2+ when a Na+ gradient (intravesicular greater than extravesicular) was formed across the membranes. Dissipation of the Na+ gradient by the addition of Na+ to the external medium decreased Ca2+ uptake. Ca2+ preloaded into the lymphocytes was extruded when Na+ was added to the external medium. The Ca2+ uptake decreased at acidic pH but increased at alkaline pH (above 8) and the activity was saturable for Ca2+ (apparent Km for Ca2+ was 61 microM and apparent Vmax was 11.5 nmol/mg protein per min). Na+-dependent uptake of Ca2+ was inhibited by tetracaine and verapamil, and partially inhibited by La3+. The uptake was not influenced by orthovanadate.

Modification of the thermodynamic properties of the electron-transferring groups in mitochondrial succinate dehydrogenase upon binding of succinate

The redox properties of the covalently-bound flavin and of the tetrahedral iron-sulfur center S1 of succinate dehydrogenase were studied as a function of the binding of different ligands to the enzyme. The midpoint potential of both flavin and S1 increases by some 200 mV when protein binds succinate to a site having Kdsucc = 0.8-1.0 mM, thus different from the substrate binding site. Succinate binding increases the potential of the oxidized flavin/semiquinone half-cell more than that of the semiquinone/reduced flavin one: this results in higher semiquinone formation with increasing succinate. Malonate and fumarate appear to mimic, in this regard, the effect of succinate. The increase in midpoint potential of S1 upon binding of dicarboxylic acid is related to an increase in hydrophobicity of the cluster environment. The possible molecular basis for the modulation of the flavin potential is discussed together with the significance of this shift on the catalytic behaviour of the protein.

Studies on the succinate dehydrogenating system. II. Reconstitution of succinate-ubiquinone reductase from the soluble components

1. A protein fraction containing three polypeptides (the major one with Mr < 13 000) was isolated by means of Triton X-100 extraction of submitochondrial particles specifically treated to remove succinate dehydrogenase. 2. The mixing of the protein fraction with the soluble reconstitutively active succinate dehydrogenase results in formation of highly active succinate-DCIP reductase which is sensitive to thenoyltrifluoroacetone or carboxin. 3. The maximal turnover number of succinate dehydrogenase in the succinate-DCIP reductase reaction revealed in the presence of a saturating amount of the protein fraction is slightly higher than that measured with phenazine methosulfate as artificial electron acceptor. 4. The protein fraction greatly increases the stability of soluble succinate dehydrogenase under aerobic conditions. 5. The titration of soluble succinate dehydrogenase by the protein fraction shows that smaller amounts of the protein fraction are required to block the reduction of ferrycyanide by Hipip center than that required to reveal the maximal catalytic capacity of the enzyme. 6. The apparent Km of the reconstituted system for DCIP depends on the amount of protein fraction; the more protein fraction added to the enzyme, the lower the Km value obtained. 7. A comparison of different reconstituted succinate-ubiquinone reductases described in the literature is presented and the possible arrangement of the native and reconstituted succinate-ubiquinone region of the respiratory chain is discussed.

Resolution and reconstitution of succinate-cytochrome c reductase: preparations and properties of high purity succinate dehydrogenase and ubiquinol-cytochrome c reductase

An improved method was developed to sequentially fractionate succinate-cytochrome c reductase into three reconstitutive active enzyme systems with good yield: pure succinate dehydrogenase, ubiquinone-binding protein fraction and a highly purified ubiquinol-cytochrome c reductase (cytochrome b-c1 III complex). An extensively dialyzed succinate-cytochrome c reductase was first separated into a succinae dehydrogenase fraction and the cytochrome b-c1 complex by alkali treatment. The resulting succinate dehydrogenase fraction was further purified to homogeneity by the treatment of butanol, calcium phosphate gel adsorption and ammonium sulfate fractionation under anaerobic condition in the presence of succinate and dithiothreitol. The cytochrome b-c1 complex was separated into chtochrome b-c1 III complex and ubiquinone-binding protein fractions by careful ammonium acetate fractionation in the presence of deoxycholate. The purified succinate dehydrogenase contained only two polypeptides with molecular weights of 70 000 anbd 27 000 as revealed by the sodium dodecyl sulfate polyacrylamide gel electrophoretic pattern. The enzyme has the reconstitutive activity and a low Km ferricyanide reductase activity of 85 mumol succinate oxidized per min per mg protein at 38 degrees C. Chemical composition analysis of cytochrome b-c1 III complex showed that the preparation was completely free of contamination of succinate dehydrogenase and ubiquinone-binding protein and was 30% more pure than the available preparation. When these three components were mixed in a proper ratio, a thenoyltrifluoroacetone- and antimycin A-sensitive succinate-cytochrome c reductase was reconstituted.

The prosthetic groups in succinate dehydrogenase. Number and stoichiometry

I. Succinate:Q oxidoreductase (EC 1.3.99.1) as present in beef-heart submitochondrial particles contains equal amounts of FAD, a [2Fe-2S] cluster and a [4Fe-4S] cluster. Both Fe-S clusters are reducible by succinate. 2. A second type of [2Fe-2S] cluster, called center S-2, that has been proposed to be present in purified preparations of succinate dehydrogenase and isolated Complex II (Ohnishi, T., Winter, D.B., Lim, J. and King, T.E. (1973) Biochem. Biophys. Res. Commun. 53, 231--237) is an artifact introduced by the purification procedure. 3. It is suggested that the 70 000 dalton subunit which is known to bind the flavin, accomodates also the [4Fe-4S] cluster whereas the 28 000 dalton subunit contains the [2Fe-2S] cluster.

The solubilisation of the membrane-bound D-alanyl-D-alanine carboxypeptidase of Bacillus coagulans NCIB 9365

Protoplast membranes and the particulate D,D-carboxypeptidase of Bacillus coagulans NCIB 9365 were extremely resistant to disruption by either detergents or urea. A combination of urea and the non-ionic detergent Genapol X-100 was required to achieve a significant solubilisation of membrane protein and D,D-carboxypeptidase in an active form; the pH optimum for this treatment was pH 7.5. Solubilisation of the enzyme was accompanied by a two-fold enhancement of activity. Kinetic results indicated that the enhancement may be due to an alteration in the conformation of the enzyme following disruption of membrane structure.

Studies on the succinate dehydrogenating system. I. Kinetics of the succinate dehydrogenase interaction with a semiquindiimine radical of N,N,N',N'-tetramethyl-p-phenylenediamine

1. The activities of the soluble reconstitutively active succinate dehydrogenase (EC 1.3.99.1) measured with three artificial electron acceptors, e.g. ferricyanide, phenazine methosulfate and free radical of N,N,N',N'-tetramethyl-p-phenylenediamine (WB), have been compared. The values estimated by extrapolation to infinite acceptor concentration using double reciprocal plots 1/v versus 1/[acceptor] are nearly the same for ferricyanide and phenazine methosulfate and about twice as high for the WB. 2. The double reciprocal plots 1/v versus 1/[succinate] in the presence of malonate at various concentrations of WB give a series of straight lines intercepting in the third quadrant. The data support the mechanism of the overall reaction, in which the reduced enzyme is oxidized by WB before dissociation of the enzyme-product complex. 3. The dependence of the rate of the overall reaction on WB concentration shows that only one kinetically significant redox site of the soluble succinate dehydrogenase is involved in the reduction of WB. 4. Studies of the change of V and Km values during aerobic inactivation of the soluble enzyme suggest that only 'the low Km ferricyanide reactive site' (Vinogradov, A.D., Gavrikova, E.V. and Goloveshkina, V.G. (1975) Biochem. Biophys, Res. Commun. 65, 1264--1269) is involved in reoxidation of the reduced enzyme by WB. 5. The pH dependence of V for the succinate-WB reductase reaction shows that the group of the enzyme with the pKa value of 6.7 at 22 degrees C is responsible for the reduction of dehydrogenase in the enzyme-substrate complex. 6. When WB interacts with the succinate-ubiquinone region of the respiratory chain, the double reciprocal plot 1/v versus 1/[WB] gives a straight line. The thenoyltrifluoroacetone inhibition of succinate-ubiquinone reductase or extraction of ubiquinone alter the 1/v versus 1/[WB] plots for the curves with a positive initial slope intercepting the ordinate at the same V as in the native particles. The data support the mechanism of succinate-ubiquinone reduction, in which no positive modulation of succinate dehydrogenase by ubiquinone exist in the membrane.

Rhodanese-Mediated sulfur transfer to succinate dehydrogenase

The interaction of the sulfurtransferase rhodanese (EC 2.8.1.1) with succinate dehydrogenase (EC 1.3.99.1), yeast alcohol dehydrogenase (EC 1.1.1.1) and bovine serum albumin was studied. Succinate dehydrogenase incorporates the sulfane sulfur of [35S]rhodanese and, in the presence of unlabelled rhodanese, also incorporates that of [35S]thiosulfate. Rhodanese releases most of its transferable sulfur and is re-loaded in the presence of thiosulfate. Rhodanese undergoes similar modifications with yeast alcohol dehydrogenase but this latter does not bind 35S in amounts comparable to those incorporated in succinate dehydrogenase: nearly all the 35S released by [35S]rhodanese is with low-molecular-weight compounds. Bovine serum albumin also binds very little sulfur and [35S]rhodanese present in the reaction mixture does not discharge its radioactive sulfur nor does it take up sulfur from thiosulfate. Sulfur release from rhodanese appears to depend on the presence of - SH groups in the acceptor protein. Sulfur incorporated into succinate dehydrogenase was analytically determined as sulfide. A comparison of the optical spectra of succinate dehydrogenase preparations incubated with or without rhodanese indicates that there is an effect of the sulfurtransferase on the iron-sulfur absorption of the flavorprotein. The interaction of rhodanese with succinate dehydrogenase greatly decreases the catalytic activity of rhodanese with respect to thiocyanate formation. This is attributed to modifications in rhodanese associated with the reduction of sulfane sulfur to sulfide. Thiosulfate in part protects from this deactivation. The reconstitutive capacity of succinate dehydrogenase increased in parallel with sulfur incorporated in that enzyme following its interaction with rhodanese.

The role of protein and lipids in stabilizing the activity of bovine heart succinate dehydrogenase

When incubated in an air atmosphere, solubilized succinate dehydrogenase (succinate:(acceptor) oxidoreductase, EC 1.3.99.1) quickly loses the capability to recombine with membrane components to catalyze mitochondrial related electron transport activities. At 0 degrees the loss in reconstitution capability is a first-order process; the half-life of the enzyme is 1.6 hr at this temperature. The enzyme is stabilized by recombining it with submitochondrial particles or with a cytochrome b preparation-phospholipid mixture. The presence of the cytochrome b preparation in the succinate dehydrogenase-cytochrome b-phospholipid complex is obligatory, indicating that protein-protein interactions between succinate dehydrogenase and other membrane components are important in stabilizing the capability of the flavoprotein to transfer electrons to other respiratory components. Treatment of this complex with phospholipase C results in loss of most of the succinate-dichlorophenolindophenol reductase activity and almost complete hydrolysis of phospholipid. Succinate dehydrogenase maintains its capability to participate in mitochondrial electron transport for several hours if the phospholipase treated complex is reconstituted with lysolecithin at the time of assay. Phospholipids are therefore not required for the stabilization process, but rather for formation of an active reductase complex. A lipophilic environment, if required for stabilization, can be provided by diglycerides. Diglycerides also can provide an environment conducive to electron transfer from succinate to ubiquinone but do so less efficiently than intact phospholipids.