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Karavaeva V, Sousa FL. Modular structure of complex II: An evolutionary perspective. BIOCHIMICA ET BIOPHYSICA ACTA. BIOENERGETICS 2023; 1864:148916. [PMID: 36084748 DOI: 10.1016/j.bbabio.2022.148916] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/15/2022] [Revised: 07/21/2022] [Accepted: 09/02/2022] [Indexed: 11/25/2022]
Abstract
Succinate dehydrogenases (SDHs) and fumarate reductases (FRDs) catalyse the interconversion of succinate and fumarate, a reaction highly conserved in all domains of life. The current classification of SDH/FRDs is based on the structure of the membrane anchor subunits and their cofactors. It is, however, unknown whether this classification would hold in the context of evolution. In this work, a large-scale comparative genomic analysis of complex II addresses the questions of its taxonomic distribution and phylogeny. Our findings report that for types C, D, and F, structural classification and phylogeny go hand in hand, while for types A, B and E the situation is more complex, highlighting the possibility for their classification into subgroups. Based on these findings, we proposed a revised version of the evolutionary scenario for these enzymes in which a primordial soluble module, corresponding to the cytoplasmatic subunits, would give rise to the current diversity via several independent membrane anchor attachment events.
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Affiliation(s)
- Val Karavaeva
- Department of Functional and Evolutionary Ecology, University of Vienna, Djerassiplatz 1, 1030 Wien, Austria
| | - Filipa L Sousa
- Department of Functional and Evolutionary Ecology, University of Vienna, Djerassiplatz 1, 1030 Wien, Austria.
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2
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Banerjee A, Lindenmair A, Hennerbichler S, Steindorf P, Steinborn R, Kozlov AV, Redl H, Wolbank S, Weidinger A. Cellular and Site-Specific Mitochondrial Characterization of Vital Human Amniotic Membrane. Cell Transplant 2019; 27:3-11. [PMID: 29562784 PMCID: PMC6434485 DOI: 10.1177/0963689717735332] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Over a century ago, clinicians started to use the human amniotic membrane for coverage of wounds and burn injuries. To date, literally thousands of different clinical applications exist for this biomaterial almost exclusively in a decellularized or denuded form. Recent reconsiderations for the use of vital human amniotic membrane for clinical applications would take advantage of the versatile cells of embryonic origin including the entirety of their cell organelles. Recently, more and more evidence was found, showing mitochondria to be involved in most fundamental cellular processes, such as differentiation and cell death. In this study, we focused on specific properties of mitochondria of vital human amniotic membrane and characterized bioenergetical parameters of 2 subregions of the human amniotic membrane, the placental and reflected amnion. We found significantly different levels of adenosine triphosphate (ATP) and extracellular reactive oxygen species, concentrations of succinate dehydrogenase, and lactate upon inhibition of ATP synthase in placental and reflected amnion. We also found significantly different rates of mitochondrial respiration in isolated human amniotic epithelial cells and human amniotic mesenchymal stromal cells, according to the subregions. Differences in metabolic activities were inversely related to mitochondrial DNA copy numbers in isolated cells of placental and reflected amnion. Based on significant differences of several key parameters of energy metabolism in 2 subregions of vital amnion, we propose that these metabolic differences of vital placental and reflected amnion could have critical impact on therapeutic applications. Inclusion of region-specific metabolic properties could optimize and fine-tune the clinical application of the human amniotic membrane and improve the outcome significantly.
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Affiliation(s)
- Asmita Banerjee
- 1 Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, AUVA Research Center, Vienna, Austria.,2 Austrian Cluster for Tissue Regeneration, Vienna, Austria
| | - Andrea Lindenmair
- 2 Austrian Cluster for Tissue Regeneration, Vienna, Austria.,3 Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, AUVA, Linz, Austria
| | - Simone Hennerbichler
- 2 Austrian Cluster for Tissue Regeneration, Vienna, Austria.,4 Red Cross Blood Transfusion Service for Upper Austria, Linz, Austria
| | - Philipp Steindorf
- 5 Genomics Core Facility, VetCore, University of Veterinary Medicine, Vienna, Austria
| | - Ralf Steinborn
- 5 Genomics Core Facility, VetCore, University of Veterinary Medicine, Vienna, Austria
| | - Andrey V Kozlov
- 1 Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, AUVA Research Center, Vienna, Austria.,2 Austrian Cluster for Tissue Regeneration, Vienna, Austria
| | - Heinz Redl
- 1 Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, AUVA Research Center, Vienna, Austria.,2 Austrian Cluster for Tissue Regeneration, Vienna, Austria
| | - Susanne Wolbank
- 1 Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, AUVA Research Center, Vienna, Austria.,2 Austrian Cluster for Tissue Regeneration, Vienna, Austria
| | - Adelheid Weidinger
- 1 Ludwig Boltzmann Institute for Experimental and Clinical Traumatology, AUVA Research Center, Vienna, Austria.,2 Austrian Cluster for Tissue Regeneration, Vienna, Austria
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Hägerhäll C, Magnitsky S, Sled VD, Schröder I, Gunsalus RP, Cecchini G, Ohnishi T. An Escherichia coli mutant quinol:fumarate reductase contains an EPR-detectable semiquinone stabilized at the proximal quinone-binding site. J Biol Chem 1999; 274:26157-64. [PMID: 10473567 DOI: 10.1074/jbc.274.37.26157] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The EPR and thermodynamic properties of semiquinone (SQ) species stabilized by mammalian succinate:quinone reductase (SQR) in situ in the mitochondrial membrane and in the isolated enzyme have been well documented. The equivalent semiquinones in bacterial membranes have not yet been characterized, either in SQR or quinol:fumarate reductase (QFR) in situ. In this work, we describe an EPR-detectable QFR semiquinone using Escherichia coli mutant QFR (FrdC E29L) and the wild-type enzyme. The SQ exhibits a g = 2.005 signal with a peak-to-peak line width of approximately 1.1 milliteslas at 150 K, has a midpoint potential (E(m(pH 7.2))) of -56.6 mV, and has a stability constant of approximately 1.2 x 10(-2) at pH 7.2. It shows extremely fast spin relaxation behavior with a P(1/2) value of >>500 milliwatts at 150 K, which closely resembles the previously described SQ species (SQ(s)) in mitochondrial SQR. This SQ species seems to be present also in wild-type QFR, but its stability constant is much lower, and its signal intensity is near the EPR detection limit around neutral pH. In contrast to mammalian SQR, the membrane anchor of E. coli QFR lacks heme; thus, this prosthetic group can be excluded as a spin relaxation enhancer. The trinuclear iron-sulfur cluster FR3 in the [3Fe-4S](1+) state is suggested as the dominant spin relaxation enhancer of the SQ(FR) spins in this enzyme. E. coli QFR activity and the fast relaxing SQ species observed in the mutant enzyme are sensitive to the inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO). In wild-type E. coli QFR, HQNO causes EPR spectral line shape perturbations of the iron-sulfur cluster FR3. Similar spectral line shape changes of FR3 are caused by the FrdC E29L mutation, without addition of HQNO. This indicates that the SQ and the inhibitor-binding sites are located in close proximity to the trinuclear iron-sulfur cluster FR3. The data further suggest that this site corresponds to the proximal quinone-binding site in E. coli QFR.
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Affiliation(s)
- C Hägerhäll
- Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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4
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Waldeck AR, Stowell MH, Lee HK, Hung SC, Matsson M, Hederstedt L, Ackrell BA, Chan SI. Electron paramagnetic resonance studies of succinate:ubiquinone oxidoreductase from Paracoccus denitrificans. Evidence for a magnetic interaction between the 3Fe-4S cluster and cytochrome b. J Biol Chem 1997; 272:19373-82. [PMID: 9235936 DOI: 10.1074/jbc.272.31.19373] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
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.
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Affiliation(s)
- A R Waldeck
- Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA
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5
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Hägerhäll C. Succinate: quinone oxidoreductases. Variations on a conserved theme. BIOCHIMICA ET BIOPHYSICA ACTA 1997; 1320:107-41. [PMID: 9210286 DOI: 10.1016/s0005-2728(97)00019-4] [Citation(s) in RCA: 304] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Affiliation(s)
- C Hägerhäll
- Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia 19104, USA.
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Anemuller S, Hettmann T, Moll R, Teixeira M, Schafer G. EPR Characterization of an Archaeal Succinate Dehydrogenase in the Membrane-Bound State. ACTA ACUST UNITED AC 1995. [DOI: 10.1111/j.1432-1033.1995.tb20845.x] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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7
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Underwood-Lemons T, Moura I, Yue KT. Resonance Raman study of sirohydrochlorin and siroheme in sulfite reductases from sulfate reducing bacteria. BIOCHIMICA ET BIOPHYSICA ACTA 1993; 1157:275-84. [PMID: 8323957 DOI: 10.1016/0304-4165(93)90110-t] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
Soret-excited resonance Raman (RR) spectra are reported for the sirohemes in the oxidized and Cr11(EDTA)-reduced forms of both desulforubidin from D. baculatus (DSR) and the low molecular weight sulfite reductase from D. vulgaris (1SIR) and for sirohydrochlorin in the oxidized form of desulfoviridin from D. gigas (DSV). Several patterns in the RR spectra of these enzymes can be utilized as signatures for the siroheme/sirohydrochlorin moiety. The active site for DSR and 1SIR consists of a siroheme exchange-coupled to a [4Fe-4S]2+ cluster. Upon addition of Cr11(EDTA), the active center of DSR and 1SIR undergoes a one-electron and two-electron reduction, respectively. The RR spectra of DSR suggest that the siroheme iron is high spin and 5-coordinate in the oxidized enzyme and probably remains high spin and 5-coordinate upon reduction. The iron in the siroheme of oxidized 1SIR changes from a low spin and probably 6-coordinate configuration to a high spin, 5-coordinate complex upon two-electron reduction of the active site. Close similarities between the RR spectral features of the two-electron-reduced assimilatory sulfite reductases from E. coli and from D. vulgaris (1SIR) are discussed.
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Matsubara H, Saeki K. Structural and Functional Diversity of Ferredoxins and Related Proteins. ADVANCES IN INORGANIC CHEMISTRY 1992. [DOI: 10.1016/s0898-8838(08)60065-3] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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9
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Chapter 7 Progress in succinate:quinone oxidoreductase research. ACTA ACUST UNITED AC 1992. [DOI: 10.1016/s0167-7306(08)60175-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/08/2023]
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10
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Tan J, Helms LR, Swenson RP, Cowan JA. Primary structure of the assimilatory-type sulfite reductase from Desulfovibrio vulgaris (Hildenborough): cloning and nucleotide sequence of the reductase gene. Biochemistry 1991; 30:9900-7. [PMID: 1911781 DOI: 10.1021/bi00105a013] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
The nucleotide sequence encoding the structural gene (651 bp) and flanking regions for the assimilatory-type sulfite reductase from the sulfate-reducing bacterium Desulfovibrio vulgaris (Hildenborough) was determined after cloning a 1.4 kb HindIII/SalI genomic fragment possessing the gene into Bluescript pBS(+)KS. The primary structure of the protein was deduced, and the molecular mass of the apoprotein was estimated as 24 kDa. The amino acid sequence of the polypeptide shows some similarities at putative [Fe4S4] cluster binding sites in comparison with the heme protein subunit of the larger Escherichia coli and Salmonella typhimurium sulfite reductases and spinach nitrite reductase. This is the first reported sequence of a member of a new class of low molecular weight assimilatory sulfite-reducing enzymes recently identified in a number of anaerobic bacteria [Moura, I., Lina, A. R., Moura, J. J. G., Xavier, A. V., Fauque, G., Peck, H. D., & Le Gall, J. (1986) Biochem. Biophys. Res. Commun. 141, 1032-1041].
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Affiliation(s)
- J Tan
- Evans Laboratory of Chemistry, Ohio State University, Columbus 43210
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OHNISHI TOMOKO. Structure of the Succinate-Ubiquinone Oxidoreductase (Complex II). CURRENT TOPICS IN BIOENERGETICS - STRUCTURE, BIOGENESIS, AND ASSEMBLY OF ENERGY TRANSDUCING ENZYME SYSTEMS 1987. [DOI: 10.1016/b978-0-12-152515-6.50006-0] [Citation(s) in RCA: 56] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
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12
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Maguire JJ, Magnusson K, Hederstedt L. Bacillus subtilis mutant succinate dehydrogenase lacking covalently bound flavin: identification of the primary defect and studies on the iron-sulfur clusters in mutated and wild-type enzyme. Biochemistry 1986; 25:5202-8. [PMID: 3021212 DOI: 10.1021/bi00366a033] [Citation(s) in RCA: 35] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Succinate dehydrogenase consists of two protein subunits and contains one FAD and three iron-sulfur clusters. The flavin is covalently bound to a histidine in the larger, Fp, subunit. The reduction oxidation midpoint potentials of the clusters designated S-1, S-2, and S-3 in Bacillus subtilis wild-type membrane-bound enzyme were determined as +80, -240, and -25 mV, respectively. Magnetic spin interactions between clusters S-1 and S-2 and between S-1 and S-3 were detected by using EPR spectroscopy. The point mutations of four B. subtilis mutants with defective Fp subunits were mapped. The gene of the mutant specifically lacking covalently bound flavin in the enzyme was cloned. The mutation was determined from the DNA sequence as a glycine to aspartate substitution at a conserved site seven residues downstream from the histidine that binds the flavin in wild-type enzyme. The redox midpoint potential of the iron-sulfur clusters and the magnetic spin interactions in mutated succinate dehydrogenases were indistinguishable from the those of the wild type. This shows that flavin has no role in the measured magnetic spin interactions or in the structure and stability of the iron-sulfur clusters. It is concluded from sequence and mutant studies that conserved amino acid residues around the histidyl-FAD are important for FAD binding; however, amino acids located more than 100 residues downstream from the histidyl in the Fp subunit can also effect flavinylation.
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Cammack R, Patil DS, Weiner JH. Evidence that centre 2 in Escherichia coli fumarate reductase is a [4Fe-4S]cluster. BIOCHIMICA ET BIOPHYSICA ACTA 1986; 870:545-51. [PMID: 3008846 DOI: 10.1016/0167-4838(86)90264-5] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Redox titrations of the iron-sulphur clusters in fumarate reductase purified from Escherichia coli, monitored by ESR spectroscopy, identified three redox events, similar to those observed in other fumarate reductases and succinate dehydrogenases: Centre 1, a [2Fe-2S] cluster, at g = 2.03, 1.93, appeared on reduction with Em = -20 mV. Centre 3, probably a [3Fe-xS] cluster, at g = 2.02 appeared in the oxidized state with Em = -70 mV. Centre 2 has been observed as an increase in the electron-spin relaxation of Centre 1. It titrates as an n = 1 species with Em = -320 mV, but in our hands did not appear to contribute significant intensity to the g = 2.03, 1.93 signal. It therefore appears to be an additional centre which undergoes spin-spin interaction with Centre 1. The reduction of Centre 2 coincided with the appearance of an extremely broad ESR spectrum, observed at temperatures below 20 K, with features at g = 2.17, 1.9, 1.68. The broad signal was observed in both soluble and membrane-bound preparations. Its midpoint potential was -320 mV. Its integrated intensity was approximately equal to that of Centre 1, if its broad outer wings were taken into account. Consideration of the ESR properties of this signal, together with the amino acid sequence of the frdB subunit of the enzyme, indicates that Centre 2 is a [4Fe-4S] cluster which, in its reduced state, enhances the spin relaxation of the [2Fe-2S] Centre 1.
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Hederstedt L. Molecular properties, genetics, and biosynthesis of Bacillus subtilis succinate dehydrogenase complex. Methods Enzymol 1986; 126:399-414. [PMID: 3152413 DOI: 10.1016/s0076-6879(86)26040-1] [Citation(s) in RCA: 71] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
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15
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Cole ST, Condon C, Lemire BD, Weiner JH. Molecular biology, biochemistry and bioenergetics of fumarate reductase, a complex membrane-bound iron-sulfur flavoenzyme of Escherichia coli. BIOCHIMICA ET BIOPHYSICA ACTA 1985; 811:381-403. [PMID: 3910107 DOI: 10.1016/0304-4173(85)90008-4] [Citation(s) in RCA: 139] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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16
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Condon C, Cammack R, Patil DS, Owen P. The succinate dehydrogenase of Escherichia coli. Immunochemical resolution and biophysical characterization of a 4-subunit enzyme complex. J Biol Chem 1985. [DOI: 10.1016/s0021-9258(17)39384-5] [Citation(s) in RCA: 57] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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17
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Barassi CA, Kranz RG, Gennis RB. Succinate dehydrogenase in Rhodopseudomonas sphaeroides: subunit composition and immunocross-reactivity with other related bacteria. J Bacteriol 1985; 163:778-82. [PMID: 3874866 PMCID: PMC219190 DOI: 10.1128/jb.163.2.778-782.1985] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
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.
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Characterization by electron paramagnetic resonance and studies on subunit location and assembly of the iron-sulfur clusters of Bacillus subtilis succinate dehydrogenase. J Biol Chem 1985. [DOI: 10.1016/s0021-9258(18)89058-5] [Citation(s) in RCA: 30] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022] Open
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19
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Crowe BA, Owen P, Cammack R. Study of the respiratory chain in Micrococcus luteus (lysodeikticus) by electron-spin-resonance spectroscopy. EUROPEAN JOURNAL OF BIOCHEMISTRY 1983; 137:185-90. [PMID: 6317382 DOI: 10.1111/j.1432-1033.1983.tb07813.x] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Low-temperature electron spin resonance spectroscopy was used to investigate the redox centres of Micrococcus luteus membranes. Three different types of iron-sulphur centres were distinguished. Two of these, a [4Fe-4S]3+-type cluster giving rise to a signal at g = 2.01 in the oxidized state and a [2Fe-2S] cluster with a spectrum at g = 2.03 and 1.93 in the reduced state, were attributable to succinate dehydrogenase. Another, generating signals in the reduced state at g = 2.027, 1.90 and 1.78 was identified as a 'Rieske' iron-sulphur centre. This latter cluster had a mid-point potential (pH 7.0) of +130 mV. In addition, signals characteristic of high-spin ferric haem (g = 6.20), low-spin ferric haem (g = 3.67, 3.36 and 3.01) and Cu2+ (g = 2.18 and 2.02) were also detected. The ferric-haem features, together with the Cu2+ and 'Rieske' centres, were enriched in membrane residues insoluble in Triton X-100, which are known from difference spectroscopy to contain cytochromes b-560, c-550 and a-601 (aa3 oxidase). The signals demonstrated by electron spin resonance for M. luteus membranes showed marked similarities to those documented for the complexes II, III, and IV of mitochondria. However, signals analogous to complex I (NADH-ubiquinone reductase) could not be demonstrated for M. luteus membranes.
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