Characterization of a succinate dehydrogenase complex solubilized from the cytoplasmic membrane of Bacillus subtilis with the nonionic detergent Triton X-100

Dye-linked D-amino acid dehydrogenases: biochemical characteristics and applications in biotechnology

Dye-linked D-amino acid dehydrogenases (Dye-DADHs) catalyze the dehydrogenation of free D-amino acids in the presence of an artificial electron acceptor. Although Dye-DADHs functioning in catabolism of L-alanine and as primary enzymes in electron transport chains are widely distributed in mesophilic Gram-negative bacteria, biochemical and biotechnological information on these enzymes remains scanty. This is in large part due to their instability after isolation. On the other hand, in the last decade, several novel types of Dye-DADH have been found in thermophilic bacteria and hyperthermophilic archaea, where they contribute not only to L-alanine catabolism but also to the catabolism of other amino acids, including D-arginine and L-hydroxyproline. In this minireview, we summarize recent developments in our understanding of the biochemical characteristics of Dye-DADHs and their specific application to electrochemical biosensors.

Crystal structure of novel dye-linked L-proline dehydrogenase from hyperthermophilic archaeon Aeropyrum pernix

Two types of dye-linked L-proline dehydrogenase (PDH1, α4β4-type hetero-octamer, and PDH2, αβγδ-type heterotetramer) have been identified so far in hyperthermophilic archaea. Here, we report the crystal structure of a third type of L-proline dehydrogenase, found in the aerobic hyperthermophilic archaeon Aeropyrum pernix, whose structure (homodimer) is much simpler than those of previously studied L-proline dehydrogenases. The structure was determined at a resolution of 1.92 Å. The asymmetric unit contained one subunit, and a crystallographic 2-fold axis generated the functional dimer. The overall fold of the subunit showed similarity to that of the PDH1 β-subunit, which is responsible for catalyzing L-proline dehydrogenation. However, the situation at the subunit-subunit interface of the A. pernix enzyme was totally different from that in PDH1. The presence of additional surface elements in the A. pernix enzyme contributes to a unique dimer association. Moreover, the C-terminal Leu(428), which is provided by a tail extending from the FAD-binding domain, shielded the active site, and an L-proline molecule was entrapped within the active site cavity. The K(m) value of a Leu(428) deletion mutant for L-proline was about 800 times larger than the K(m) value of the wild-type enzyme, although the k(cat) values did not differ much between the two enzymes. This suggests the C-terminal Leu(428) is not directly involved in catalysis, but it is essential for maintaining a high affinity for the substrate. This is the first description of an LPDH structure with L-proline bound, and it provides new insight into the substrate binding of LPDH.

Physiological Characterization of Dicarboxylate-Induced Pleomorphic Forms of Bradyrhizobium japonicum

When Bradyrhizobium japonicum I-110 was transferred into medium containing 40 mM succinate or 40 mM fumarate, over 90% of the bacteria acquired a swollen, pleomorphic form similar to that of bacteroids. The induction of pleomorphism was dependent on the carbon substrate and concentration but was independent of the hydrogen ion and sodium ion concentration. Cell extracts of rod-shaped and pleomorphic cells contained enzymes required for sugar catabolism and gluconeogenesis. Variations in these enzyme profiles were correlated with the carbon source used and not with the conversion to the bacteroid-like morphology. Rod-shaped cells cultured on glucose or 10 mM succinate transported glucose and succinate; however, the pleomorphic cells behaved similarly to symbiotic bacteroids in that they lacked the ability to transport glucose and transported succinate at lower rates than did rod-shaped cells.

Possible relationship of succinate dehydrogenase and fatty acid synthetase activities to Aspergillus parasiticus (NRRL 5139) growth and aflatoxin production

Fatty acid synthetase (FAS) activity measured over time corresponded to aflatoxin B1 biosynthesis by Aspergillus parasiticus grown in minimal salts sucrose medium. Succinate dehydrogenase (SDH) activity, our primary metabolism indicator, decreased as FAS activity increased demonstrating that as primary metabolism slows, secondary metabolism and subsequently aflatoxin production begins. Fungal biomass, as measured by chitin, increased up to day 13 then stabilized. Calcium, potassium, magnesium, manganese, zinc, and a combination of these minerals were tested to determine their effect in culture on FAS and SDH activities. Cultures grown in broth supplemented with zinc had greater FAS activity and produced more aflatoxin B1 when compared to the unsupplemented control. To determine if enzyme activity in a complex substrate is altered due to mineral composition, peanuts were cultivated with gypsum (calcium sulfate) supplementation. The peanuts grown had higher calcium content but less zinc. All peanuts grown in gypsum treated fields had less aflatoxin produced on them when compared to unsupplemented peanuts. Also, FAS activity was lower and chitin content was less when compared to the unsupplemented control peanuts. The FAS activity observed in these experiments indirectly suggests that the FAS complex may be responsible for producing the precursor for aflatoxin synthesis. However, additional information is needed to validate this hypothesis.

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 properties of succinate-ubiquinone oxidoreductase complex from Paracoccus denitrificans

Highly active succinate-ubiquinone reductase has been purified from cytoplasmic membranes of aerobically grown Paracoccus denitrificans. The purified enzyme has a specific activity of 100 units per mg protein, and a turnover number of 305 s-1. Succinate-ubiquinone reductase activity of the purified enzyme is inhibited by 3'-methylcarboxin and thenoyltrifluoroacetone. Four subunits, with apparent molecular masses of 64.9, 28.9, 13.4 and 12.5 kDa, were observed on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme contains 5.62 nmol covalently bound flavin and 3.79 nmol cytochrome b per mg protein. The 64.9 kDa subunit was shown to be a flavoprotein by its fluorescence. Polyclonal antibodies raised against this protein cross-reacted with the flavoprotein subunit of bovine heart mitochondrial succinate-ubiquinone reductase. The 28.9 kDa subunit is likely analogous to the bovine heart iron protein, and the cytochrome b heme is probably associated with one or both of the low-molecular-weight polypeptides. The cytochrome b is not reducible with succinate but is reoxidized with fumarate after prereduction with dithionite. Iron-sulfur clusters S-1 and S-3 of the Paracoccus oxidoreductase exhibit EPR spectra very similar to their mitochondrial counterparts. Paracoccus succinate-ubiquinone reductase complex is thus similar to the bovine heart mitochondrial enzyme with respect to prosthetic groups, enzymatic activity, inhibitor sensitivities, and polypeptide subunit composition.

Fumarate reductase of Escherichia coli: an investigation of function and assembly using in vivo complementation

Recombinant plasmids which carried portions of the Escherichia coli frd operon were constructed and their expression examined by in vivo complementation of E. coli MI1443. This strain lacked a chromosomal frd operon and was unable to grow anaerobically on glycerol and fumarate. Introduction of all four fumarate reductase subunits into E. coli MI1443 was essential for the restoration of growth. The FRD A, FRD B dimer (but neither subunit alone) was active in the benzyl viologen oxidase assay. Both FRD C and FRD D were required for membrane association of fumarate reductase and for the oxidation of reduced quinone analogues. Introduction into E. coli MI1443 of the frdABC and frdD genes on two separate plasmid vectors failed to restore anaerobic growth on glycerol and fumarate. Thus separation of the DNA coding for the FRD C and FRD D proteins affected the ability of fumarate reductase to assemble into a functional complex.

Genetic and biochemical characterization of Bacillus subtilis mutants defective in expression and function of cytochrome b-558

Bacillus subtilis succinate dehydrogenase is bound to the cytoplasmic membrane by cytochrome b-558, a 23-kDa transmembrane protein which also functions as electron acceptor to the dehydrogenase. The structural gene for the apocytochrome, sdhC, has previously been cloned and sequenced. In this work the structure and translation of cytochrome b-558 was studied in different sdhC mutants. Mutant cytochrome was analyzed both in B. subtilis and after amplification in Escherichia coli. It is concluded that amino acid substitutions in the C-terminal half of the cytochrome can prevent the binding of succinate dehydrogenase without affecting membrane binding of the cytochrome protein or heme ligation. Mutagenesis of His-113 excludes this residue as an axial heme ligand. A base-pair exchange of G to A in the ribosome-binding sequence of sdhC was found to reduce cytochrome b-558 translation about tenfold in B. subtilis, whereas the mutation had no effect on translation in E. coli. Translation of the two succinate dehydrogenase genes from the sdhCAB polycistronic transcript does not seem to be coupled to translation of sdhC. Less than 10% of the wild-type amount of membrane-bound succinate dehydrogenase in B. subtilis still allows growth on non-fermentable substrate, but makes the dehydrogenase a limiting enzyme in the tricarboxylic acid cycle and leads to succinate accumulation.

Spectral and potentiometric analysis of cytochromes from Bacillus subtilis

Bacillus subtilis cytoplasmic membranes contain several cytochromes which are linked to the respiratory chain. At least six different cytochromes have been separated and identified by ammonium sulphate fractionation and ion-exchange chromatography. They include two terminal oxidases with CO-binding properties and cyanide sensitivity. One of these is an aa3-type cytochrome c oxidase which has characteristic absorption maxima in the reduced-oxidized difference spectrum at 601 nm in the alpha-band and at 443 nm in the Soret band regions. In the alpha-band two separate electron transitions with Em = +205 mV and Em = +335 mV can be discriminated by redox potentiometric titration. The other CO-binding cytochrome c oxidase contains two cytochrome b components with alpha-band maxima at 556 nm and 559 nm. Cytochrome b556 can be reduced by ascorbate and has an Em + +215 mV, whereas cytochrome b559 has an Em = +140 mV. Furthermore a complex consisting of a cytochrome b564 (Em = +140 mV) associated with a cytochrome c554 (Em = +250 mV) was found. This cytochrome c554, which can be reduced by ascorbate, appears to have an asymmetrical alpha-peak and stains for heme-catalyzed peroxidase activity on SDS-containing polyacrylamide gels. A protein with a molecular mass of about 30 kDa is responsible for this activity. A cytochrome b559 (Em = +65 mV) appears to be an essential part of succinate dehydrogenase. Finally a cytochrome c550 component with an apparent mid-point potential of Em = +195 mV has been detected.

Immunochemical probing of the structure and cofactor of NADH dehydrogenase from Paracoccus denitrificans

Monospecific antibody to the respiratory NADH dehydrogenase from Paracoccus denitrificans was prepared by using as antigen specific immunoprecipitates containing NADH dehydrogenase which were excised from crossed-immunoelectrophoresis plates. The latter were run with selectively solubilized plasma membranes and antibodies against plasma membranes. The antibody immunoprecipitated NADH dehydrogenase from P. denitrificans membranes biosynthetically labelled with 14C and solubilized with a wide range of detergents. All immunoprecipitates contained the two subunits of Mr 48,000 and 25,000, in an approximate 1:1 stoichiometry, that had previously been assigned to NADH dehydrogenase. A polypeptide of Mr 46,000 in P. denitrificans membranes, previously shown to cross-react with a subunit-specific antibody to mitochondrial NADH dehydrogenase (complex I), was not detected in any immunoprecipitate. Under some conditions a third polypeptide, of Mr 31,000, was also detected, but in variable and non-stoichiometric amounts relative to the two other subunits. It was concluded that this polypeptide was incorporated into the immunoprecipitates as an artefact and that the polypeptides of Mr 48,000 and 25,000 are the sole polypeptides firmly identified in the NADH dehydrogenase. Flavoproteins were specifically radiolabelled by growth of P. denitrificans in the presence of [14C]riboflavin. Crossed immunoelectrophoresis of membranes from such cells showed that succinate dehydrogenase contained flavin, but that there was no detectable flavin in NADH dehydrogenase under these conditions. Analysis of excised immunoprecipitates of succinate dehydrogenase showed that flavin was covalently bound to a polypeptide of Mr 56,000. Flavin was retained by NADH dehydrogenase under mild conditions of detergent solubilization. Subsequent immunoprecipitation, followed by analysis of the acid-extracted flavin, established that FMN is a cofactor, in common with mitochondrial NADH-ubiquinone oxidoreductase (complex I).

Processing of Bacillus subtilis succinate dehydrogenase and cytochrome b-558 polypeptides. Lack of covalently bound flavin in the Bacillus enzyme expressed in Escherichia coli

The DNA sequence of the Bacillus subtilis sdh operon coding for the two succinate dehydrogenase subunits and cytochrome b-558 (the membrane anchor protein) has recently been established. We have now determined the extent of N-terminal processing of each polypeptide by radiosequence analysis. At the same time, direct evidence for the correctness of the predicted reading frames has been obtained. The cytochrome showed a ragged N-terminus, with forms lacking one residue, and is inserted across the membrane without an N-terminal leader-peptide. Covalently bound flavin was not detectable in B. subtilis succinate dehydrogenase expressed in Escherichia coli despite normal N-terminal processing of the apoprotein. This provides an explanation to why the succinate dehydrogenase synthesized in E. coli is not functional and demonstrates that host-specific factors regulate the coenzyme attachment.

Electron-paramagnetic-resonance spectroscopy of Bacillus subtilis cytochrome b558 in Escherichia coli membranes and in succinate dehydrogenase complex from Bacillus subtilis membranes

Cytochrome b558 of the Bacillus subtilis succinate dehydrogenase complex was studied by electron-paramagnetic-resonance (EPR) spectroscopy. The cytochrome amplified in Escherichia coli membranes by expression of the cloned cytochrome gene and in the succinate dehydrogenase complex immunoprecipitated from solubilized B. subtilis membranes, respectively, is shown to be low spin with a highly anisotropic (gmax approximately equal to 3.5) EPR signal. The amino acid residues most likely forming fifth and sixth axial ligands to heme in cytochrome b558 are discussed on the basis of the EPR signal and the recently determined gene sequence (K. Magnusson, M. Philips, J.R. Guest, and L. Rutberg, J. Bacteriol. 166:1067-1071, 1986) and in comparison with other b-type cytochromes.

Solubilization, purification, and characterization of succinate dehydrogenase from membranes of Mycobacterium phlei

Succinate dehydrogenase (SDH) was solubilized from membranes of Mycobacterium phlei by Triton X-100 with a recovery of about 90%. The solubilized SDH was purified about 90-fold by Sephacryl S-300, DEAE-cellulose, hydroxylapatite, and isoelectric focusing in the presence of Triton X-100 with a 20% recovery. SDH was homogeneous, as determined by polyacrylamide gel electrophoresis in nondenaturing gels containing Triton X-100. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the enzyme revealed two subunits with molecular weights of 62,000 and 26,000. SDH is a flavoprotein containing 1 mol of flavin adenine dinucleotide, 7 to 8 mol of nonheme iron, and 7 to 8 mol of acid-labile sulfide per mol of protein. Using phenazine methosulfate and 2,6-dichloroindophenol as electron acceptors, the enzyme had an apparent Km of 0.12 mM succinate. SDH exhibited a sigmoidal relationship of rate to succinate concentration, indicating cooperativity. The enzyme was competitively inhibited by fumarate with a Ki of 0.15 mM. In the absence of Triton X-100, the enzyme aggregated, retained 50% of the activity, and could be resolubilized with Triton X-100 with full restoration of activity. Cardiolipin had no effect on the enzyme activity in the absence of Triton X-100, but it stimulated the activity by about 30% in the presence of 0.1% Triton X-100 in the assay mixture. Menaquinone-9(2H), isolated from M. phlei, had no effect on the enzyme activity either in the presence or absence of Triton X-100.

Nucleotide sequence of the gene for cytochrome b558 of the Bacillus subtilis succinate dehydrogenase complex

The nucleotide sequence was determined for the first part of the Bacillus subtilis sdh operon. An open reading frame corresponding to the structural gene, sdhA, for cytochrome b558 was identified. The predicted molecular weight of the cytochrome (excluding the N-terminal methionine) is 22,770. It is a very hydrophobic protein with five probable membrane-spanning segments. There is little homology between the B. subtilis cytochrome b558 and cytochrome b of mitochondrial complex III from different organisms or between cytochrome b558 and the hydrophobic sdhC and sdhD peptides of the Escherichia coli sdh operon. About 30 bases downstream of the sdhA stop codon, a new open reading frame starts. The nucleotide sequence predicts the presence of a typical flavin-binding peptide which identifies this reading frame as part of the sdhB gene. Seven bases upstream of the sdhA initiation codon ATG there is a typical B. subtilis ribosome binding site (free energy of interaction, -63 kJ), and further upstream, tentative sigma 55 and sigma 32 promoter sequences were found. The upstream region also contains two 12-base-long direct repeats; their significance is unknown.

Electron-transfer complexes of Ascaris suum muscle mitochondria. II. Succinate-coenzyme Q reductase (complex II) associated with substrate-reducible cytochrome b-558

A succinate-coenzyme Q reductase (complex II) was isolated in highly purified form from Ascaris muscle mitochondria by detergent solubilization, ammonium sulfate fractionation and gel filtration on a Sephadex G-200 column. The enzyme preparation catalyzes electron transfer from succinate to coenzyme Q1 with a specific activity of 1.2 mumol coenzyme Q1 reduced per min per mg protein at 25 degrees C. The isolated complex II is essentially free of NADH-ferricyanide reductase, reduced CoQ2-cytochrome c reductase and cytochrome c oxidase and consists of four major polypeptides with apparent molecular weights of 66 000, 27 000, 12 000 and 11 000 and two minor ones with Mr of 36 000 and 16 000. The complex II contained cytochrome b-558, a major constituent cytochrome of Ascaris mitochondria, at a concentration of 3.6 nmol per mg protein, but neither other cytochromes nor quinone. The cytochrome b-558 in the complex II was reduced with succinate. In the presence of Ascaris NADH-cytochrome c reductase (complex I-III) (Takamiya, S., Furushima, R. and Oya, H. (1984) Mol. Biochem. Parasitol. 13, 121-134), the cytochrome b-558 in complex II was also reduced with NADH and reoxidized with fumarate. These results suggest the cytochrome b-558 to function as an electron carrier between NADH dehydrogenase and succinate dehydrogenase in the Ascaris NADH-fumarate reductase system.

Succinate dehydrogenase in Rhodopseudomonas sphaeroides: subunit composition and immunocross-reactivity with other related bacteria

Antibodies were raised against the succinate dehydrogenase (SDH) present in the chromatophores of phototrophically grown Rhodopseudomonas sphaeroides. Crossed immunoelectrophoresis experiments indicated that the SDH present in the cytoplasmic membranes of heterotrophically grown R. sphaeroides is probably the same enzyme observed in the chromatophores. The enzyme was extracted by Triton X-100 in a form which consisted of only two subunits (molecular weight, 68,000 and 30,000) and was not associated with a cytochrome b. The antibodies directed against SDH from R. sphaeroides showed no immunocross-reactivity with SDH from phylogenetically related bacterial species, including Rhodopseudomonas capsulata, Paracoccus denitrificans, Rhodopseudomonas palustris, Rhodospirillum rubrum, and Rhodospirillum fulvum.

Cloning and expression in Escherichia coli of sdhA, the structural gene for cytochrome b558 of the Bacillus subtilis succinate dehydrogenase complex

Bacillus subtilis cytochrome b558 is a transmembrane protein which anchors succinate dehydrogenase (SDH) to the cytoplasmic membrane and is reduced by succinate. The structural gene for this cytochrome was cloned and expressed in Escherichia coli. Random BamHI or BglII fragments of B. subtilis 168 DNA were cloned in the BamHI site of plasmid pHV32. The derived plasmids were used to transform B. subtilis SDH mutants to chloramphenicol resistance by integration of the plasmid via DNA homology. Of some 3,000 transformants tested, 6 were SDH positive and had pHV32 integrated close to the sdh operon. Two plasmids, pKIM2 and pKIM4, with an insert of B. subtilis DNA of 5.7 and 3.4 kilobases, respectively, were generated by transforming E. coli with DNA from the SDH-positive transformants after cleavage with EcoRI or BglII and ligation. In E. coli carrying either of the two plasmids, about 4% of total membrane protein was B. subtilis cytochrome b558. E. coli (pKIM2) also contained antigen which reacted with antibodies specific for the flavoprotein and the iron-sulfur protein subunit of B. subtilis SDH. Enzymatically active, membrane-bound B. subtilis SDH could not be demonstrated in E. coli (pKIM2). The B. subtilis DNA insert in pKIM2 could transform B. subtilis sdhA (cytochrome b558), sdhB (flavoprotein), and sdhC (iron-sulfur protein) mutants to the wild type. The results suggest that pKIM2 carries the whole B. subtilis sdh operon. The data confirm the gene order and the proposed direction of transcription of the B. subtilis sdh operon. Most likely the sdh genes in E. coli(pKIM2) are controlled by their natural promoter.

Succinate dehydrogenase mutants of Bacillus subtilis lacking covalently bound flavin in the flavoprotein subunit

Succinate dehydrogenase consists of two unequal subunits; Fp and Ip. An FAD group is covalently linked to a histidyl residue in the Fp subunit. The mechanism by which flavin is attached to protein is not known. Covalently bound flavin was studied in wild-type and succinate-dehydrogenase-negative Bacillus subtilis. The Fp subunit of succinate dehydrogenase was found to be the only (major) flavinylated protein in the cell. Mutants lacking covalently bound flavin and still containing the Fp polypeptide are described. It is shown that the flavin is not essential for assembly and membrane binding of succinate dehydrogenase in B. subtilis.

Regulation of alternate peripheral pathways of glucose catabolism during aerobic and anaerobic growth of Pseudomonas aeruginosa

Glucose may be converted to 6-phosphogluconate by alternate pathways in Pseudomonas aeruginosa. Glucose is phosphorylated to glucose-6-phosphate, which is oxidized to 6-phosphogluconate during anaerobic growth when nitrate is used as respiratory electron acceptor. Mutant cells lacking glucose-6-phosphate dehydrogenase are unable to catabolize glucose under these conditions. The mutant cells utilize glucose as effectively as do wild-type cells in the presence of oxygen; under these conditions, glucose is utilized via direct oxidation to gluconate, which is converted to 6-phosphogluconate. The membrane-associated glucose dehydrogenase activity was not formed during anaerobic growth with glucose. Gluconate, the product of the enzyme, appeared to be the inducer of the gluconate transport system, gluconokinase, and membrane-associated gluconate dehydrogenase. 6-Phosphogluconate is probably the physiological inducer of glucokinase, glucose-6-phosphate dehydrogenase, and the dehydratase and aldolase of the Entner-Doudoroff pathway. Nitrate-linked respiration is required for the anaerobic uptake of glucose and gluconate by independently regulated transport systems in cells grown under denitrifying conditions.

Molecular properties of succinate dehydrogenase isolated from Micrococcus luteus (lysodeikticus)

Succinate dehydrogenase (EC 1.3.99.1) of Micrococcus luteus was selectively precipitated from Triton X-100-solubilized membranes by using specific antiserum. The precipitated enzyme contained equimolar amounts of four polypeptides with apparent molecular weights of 72,000, 30,000, 17,000, and 15,000. The 72,000 polypeptide possessed a covalently bound flavin prosthetic group and appeared to be strongly antigenic as judged by immunoprinting experiments. Low-temperature absorption spectroscopy revealed the presence of cytochrome b556 in the antigen complex. By analogy with succinate dehydrogenase purified from other sources, the 72,000 and 30,000 polypeptides were considered to represent subunits of the succinate dehydrogenase enzyme, whereas one (or both) of the low-molecular-weight polypeptides was attributed to the apoprotein of the b-type cytochrome. A succinate dehydrogenase antigen cross-reacting with the M. luteus enzyme complex could be demonstrated in membranes of Micrococcus roseus, Micrococcus flavus, and Sarcina lutea, but not in the membranes isolated from a wide variety of other gram-positive and gram-negative bacteria.

Orientation of succinate dehydrogenase and cytochrome b558 in the Bacillus subtilis cytoplasmic membrane

The orientation of the three subunits of the membrane-bound succinate dehydrogenase (SDH)-cytochrome b558 complex in Bacillus subtilis was studied in protoplasts ("right side out") and isolated membranes (random orientation), using immunoadsorption and surface labeling with [35S]diazobenzenesulfonate. Anti-SDH antibodies were adsorbed by isolated membranes but not by protoplasts. The SDH Mr 65,000 flavoprotein subunit was labeled with [35S]diazobenzenesulfonate in isolated membranes but not in protoplasts. The flavoprotein subunit is thus located on the cytoplasmic side of the membrane. The location of the SDH Mr 28,000 iron-protein subunit was not definitely established, but most probably the iron-protein subunit also is located on the cytoplasmic side of the membrane. Antibodies were not obtained to the hydrophobic cytochrome b558. The cytochrome was strongly labeled with [35S]diazobenzenesulfonate in protoplasts, and labeling was also obtained with isolated membranes. Cytochrome b558 is thus exposed on the outside of the membrane. In B. subtilis SDH binds specifically to cytochrome b558, which suggests that the cytochrome is exposed also on the cytoplasmic side of the membrane. The results obtained suggest that the B. subtilis SDH is exclusively located on the cytoplasmic side of the membrane where it is bound to cytochrome b558, which spans the membrane.

Immunochemical analysis of respiratory-chain components of micrococcus luteus (lysodeikticus)

Membrane-bound antigens of the respiratory chain of Micrococcus luteus were analyzed by crossed immunoelectrophoresis after growth of the organism in the presence of 59Fe, the flavin adenine dinucleotide-flavin mononucleotide precursor D-[2-14C]riboflavin, or the heme precursor 5-amino-[4-(14)C]levulinic acid. Using zymograms and procedures of selective extraction in conjunction with autoradiography, it was possible to resolve and partially characterize a number of antigens. Succinate dehydrogenase (EC 1.3.99.1) was shown to possess covalently bound flavin and nonheme iron and was possibly present as a complex with cytochrome. Three other dehydrogenases, namely, NADH dehydrogenase, NAD(P)H dehydrogenase (EC 1.6.99.3), and malate dehydrogenase (EC 1.1.1.37), contained flavin in noncovalent linkage, the NAD(P)H dehydrogenase also possessing nonheme iron. Four other discrete antigens (or antigen complexes) containing both iron and heme centers also resolved, as were two minor immunogens possessing iron as the sole detectable prosthetic group.

Reconstitution of succinate dehydrogenase in Bacillus subtilis by protoplast fusion

Bacillus subtilis succinate dehydrogenase (SDH) is composed of two unequal subunits designated Fp (Mr, 65,000) and Ip (Mr. 28,000). The enzyme is structurally and functionally complexed to cytochrome b 558 (Mr, 19,000) in the membrane. A total of 21 B. subtilis SDH-negative mutants were isolated. The mutants fall into five phenotypic classes with respect to the presence and localization of the subunits of the SDH-cytochrome b558 complex. One class contains mutants with an inactive membrane-bound complex. Membrane-bound enzymatically active SDH could be reconstituted in fused protoplasts of selected pairs of SDH-negative mutants. Most likely reconstitution is due to the assembly of preformed subunits in the fused cells. On the basis of the reconstitution data, the mutants tested could be divided into three complementation groups. The combined data of the present and previous work indicate that the complementation groups correspond to the structural genes for the three subunits of the membrane-bound SDH-cytochrome b558 complex. A total of 31 SDH-negative mutants of B. subtilis have now been characterized. The respective mutations all map in the citF locus at 255 degrees on the B. subtilis chromosomal map. In the present paper, we have revised the nomenclature for the genetics of SDH in B. subtilis. All mutations which give an SDH-negative phenotype will be called sdh followed by an isolation number. The designation citF will be omitted, and the citF locus will be divided into three genes: sdhA, sdhB, and sdhC. Mutations in sdhA affect cytochrome b558, mutations in sdhB affect Fp, and mutations in sdhC affect Ip.

Isoelectric focusing and crossed immunoelectrophoresis of heme proteins in the Escherichia coli cytoplasmic membrane

Isoelectric focusing (IEF), agarose electrophoresis, and crossed immunoelectrophoresis (CIE) were used to resolve the heme-containing proteins of the Escherichia coli cytoplasmic membrane after solubilization by Triton X-100. Two bands in IEF stained for heme with pI values of 4.7 and 5.3. One of the bands, with an isoelectric point of pH 5.3, was present only when the cells were grown to late log or stationary phase and possessed N,N,N,'N'-tetramethyl-p-phenylene-diamine (TMPD) oxidase activity. The pI 4.7 band was present in cells harvested in both mid-log and stationary phases. Agarose electrophoresis, using larger samples, revealed the same two components apparent by IEF, and, in addition, a third component. The heme-containing fractions were extracted after agarose electrophoresis and subjected to further study. The component which was present in cells grown to stationary phase contained hemes b, a1, and d. The other two fractions contained only b heme. One of these corresponded to the component with pI 4.7 in IEF and had catalase activity. Antisera were raised against Triton X-100-solubilized cytoplasmic membranes and against the focused TMPD oxidase complex. With these anti-sera, CIE in the presence of Triton X-100 revealed four precipitin complexes containing heme. Three of these corresponded to the components identified by IEF and agarose electrophoresis. We demonstrate that the combined use of IEF and CIE is valuable for analysis of membrane proteins. In particular, this work represents a substantial initial step toward a structural elucidation of the E. coli aerobic respiratory chain.

Nucleotide sequence coding for the flavoprotein subunit of the fumarate reductase of Escherichia coli

The nucleotide sequence of the frdA gene, which encodes the flavoprotein subunit of the fumarate reductase, of Escherichia coli, has been determined. A polypeptide of Mr = 66,052, containing 602 amino acid residues, is predicted. In composition the FrdA protein strongly resembles the flavoprotein subunits of two succinate dehydrogenases. Moreover, a sequence of nine consecutive residues is common to the flavoprotein subunits from fumarate reductase and the beef heart succinate dehydrogenase. This sequence contains a histidyl residue which probably services as the site for attachment of the FAD cofactor to the reductase.

The function of the subunits of the fumarate reductase complex of Vibrio succinogenes

The membrane-bound fumarate reductase complex of Vibrio succinogenes catalyzes the reduction of fumarate by 2,3-dimethyl-1,4-naphthohydroquinone (dimethylnaphthohydroquinone) and consists of three different peptides (Mr 79,000, Mr 31,000 and Mr 25,000), the smallest of which is cytochrome b [Unden, G., Hackenberg, H. and Kröger A. (1980) Biochem. Biophys. Acta 591, 275-288]. The complex was cleaved with guanidinium chloride, the resulting subunits characterized and their functions within the complex investigated by reconstitutional experiments. 1. The Mr-79,000 subunits catalyzed the reduction of fumarate by benzylviologen radicals as well as the oxidation of succinate by methylene blue, but not fumarate reduction by dimethylnaphthohydroquinone. 2. The spectral and the redox properties of the isolated cytochrome b (Mr 25,000) were equivalent to those of the high-potential cytochrome b of the bacteria. The isolated cytochrome b had a midpoint potential of -15 mV and was reducible by dimethylnaphthohydroquinone in the absence of the other subunits. 3. The Mr-31,000 subunit did not catalyze any of the reactions mentioned above. For the reduction of cytochrome b by succinate in the presence of the Mr-79,000 subunit, an amount of the Mr-31,000 subunit was required which was equimolar to cytochrome b. 4. The activity of fumarate reduction by dimethylnaphthohydroquinone could be restored by coprecipitation of the three subunits. It is concluded that the fumarate reductase complex has two different sites, which are essential for its function in the phosphorylative electron transport of the bacterium. The site reacting with the substrates fumarate and succinate is situated on the Mr-79,000 subunit, and that reacting with dimethylnaphthohydroquinone is cytochrome b. The Mr-31,000 subunit mediates the electron transport between cytochrome b and the Mr-79,000 subunit.

Iron-sulphur clusters in fumarate reductase from Vibrio succinogenes

(1) The fumarate reductase complex from Vibrio succinogenes contains one FAD molecule, one [4Fe-4S]3+(3+,2+) and one [2Fe-2S]2+(2+,1+) cluster per enzyme molecule. Both clusters can be partly reduced by succinate. In the presence of excess Na2S2O4 and fumarate, the [2Fe-2S] cluster is completely oxidized, whereas the other cluster is largely reduced. (2) The [2Fe-2S] cluster is localized in the Mr, 31,000 subunit. The EPR spectrum of the reduced cluster in the isolated subunit differs slightly in line width, but not in g-value, from the spectrum of reduced, intact enzyme complex. The demonstrates that the immediate environment of th cluster is little perturbed by dissociating this subunit from the FAD-containing Mr 79,000 subunit. The temperature dependence of the power-saturation behaviour has, however, greatly decreased in the isolated subunit, the saturation at 11 K of the paramagnetic cluster being much less than in the enzyme complex. Moreover, the temperature dependence of th power-saturation behaviour of this cluster in the enzyme is greater with succinate as reducing agent, than with dithionite. (3) The [4Fe-4S] cluster is located on the Mr 79,000 subunit. This cluster is unstable in air when the subunit has been dissociated from the enzyme complex.

Cytochrome b reducible by succinate in an isolated succinate dehydrogenase-cytochrome b complex from Bacillus subtilis membranes

In previous work with membranes of Bacillus subtilis, the succinate dehydrogenase complex was isolated by immunoprecipitation of Triton X-100-solubilized membranes. The complex included a polypeptide with an apparent molecular weight of 19,000, probably attributable to apocytochrome. This paper reports the further characterization of this cytochrome and its relation to the respiratory chain of B. subtilis. The cytochrome was identified as cytochrome b, and its difference absorption spectra showed maxima at 426, 529, and 558 nm at room temperature. The oxidized cytochrome had an absorption maximum at 413 nm. The cytochrome was reduced by succinate in the isolated succinate dehydrogenase complex and in Triton X-100-solubilized membranes. In whole membranes cytochromes b, c, and a were reduced by succinate. In membranes from a mutant containing normal cytochromes but lacking succinate dehydrogenase no reduction of cytochrome was seen with succinate. It was concluded that the isolated succinate dehydrogenase-cytochrome b complex is a functional unit in the intact B. subtilis membrane. An accompanying paper describes cytochrome b as a structural unit involved in the membrane binding of succinate dehydrogenase.

Biosynthesis and membrane binding of succinate dehydrogenase in Bacillus subtilis

Antibodies specific for the Mr 65,000 (flavoprotein) and the Mr 28,000 subunits of the succinic dehydrogenase (SDH) of Bacillus subtilis were obtained. By using these antibodies it was shown that both subunits accumulated in the cytoplasm during 5-aminolevulinic acid starvation of a 5-aminolevulinic acid auxotroph. In the cytoplasm the subunits were not associated since they precipitated essentially independently of each other with subunit-specific antibody. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis the cytoplasmic subunits migrated identically with the corresponding subunits from the purified membrane-bound SDH complex. Cytoplasmic subunits were pulse-labeled with L-[35S]methionine during 5-aminolevulinic acid starvation. The labeled subunits bound to the membrane when heme synthesis was resumed and also when protein synthesis was blocked by chloramphenicol before readdition of 5-aminolevulinic acid. The experiments thus demonstrated a precursor relationship between cytoplasmic subunits and the subunits of the membrane-bound SDH complex. All SDH-negative mutants isolated so far carry mutations in the citF locus. None of the mutants was found to have either the Mr 65,000 or the Mr 28,000 SDH subunits in the membrane. Four citF mutants, however, contained both subunits in the cytoplasm. Three of these mutants lacked spectrally detectable cytochrome b558. The respective mutations mapped at one end of the citF locus. These results strongly support our previous suggestion that cytochrome b558 is (part of) a membrane binding site for SDH in B. subtilis.

Isolation and functional aspects of the fumarate reductase involved in the phosphorylative electron transport of Vibrio succinogenes

1. The fumarate reductase of Vibrio succinogenes, the terminal component of the electron transport chain of the bacterium, was extracted from the cytoplasmic membrane with Triton X=100 and purified to homogeneity (approx. 30-fold) by means of chromatograhy on hydroxyapatite and DEAE-Sephadex. The enzyme eluted from the ion-exchange column in two forms, one containing and the other lacking cytochrome b. 2. The enzyme lacking cytochrome b consisted of two peptides (Mr 79 000 and 31 000) which were present in a molar ratio of 1:1. The cytochrome-containing species contained an additional peptide (Mr 25 000) which was present in twice the molar amount of the others (2:1:1). 3. The hydrodynamic properties indicate that the functional enzymes consist of only one set of the corresponding peptides. 4. Each of the two enzyme molecules contains one protein-bound FAD which is linked to the Mr 79 000 peptide. Both enzyme species contain 9-10 mol iron-sulfur per mol of FAD which is associated with the Mr 79 000 and the Mr 31 000 peptide. Cytochrome b is present in an amount of 2 mol/mol of FAD, half of the cytochrome b has a midpoint potential of -20 mV. 5. The enzyme catalyzed two types of reaction. (a) Fumarate reduction by viologen radicals or anthrahydroquinonesulfonate, as well as succinate oxidation by ferricyanide or methylene blue, was independent of cytochrome b. (b) The activities of fumarate reduction by naphthohydroquinones and those of succinate oxidation by naphthoquinones were cytochrome b-dependent. This indicates that the electron transport from menaquinone to fumarate reductase in the membrane of the bacterium is mediated by a single component, cytochrome b (-20 mV). 6. The Km for fumarate was 0.35 mM with menadiol as the donor and that for succinate as 20 mM in the reverse reaction. Succinate oxidation was competitively inhibited by fumarate with a Ki of 0.35 mM.