Structure of the Succinate-Ubiquinone Oxidoreductase (Complex II)

Modular structure of complex II: An evolutionary perspective

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

Functions of ROS in Macrophages and Antimicrobial Immunity

Reactive oxygen species (ROS) are a chemically defined group of reactive molecules derived from molecular oxygen. ROS are involved in a plethora of processes in cells in all domains of life, ranging from bacteria, plants and animals, including humans. The importance of ROS for macrophage-mediated immunity is unquestioned. Their functions comprise direct antimicrobial activity against bacteria and parasites as well as redox-regulation of immune signaling and induction of inflammasome activation. However, only a few studies have performed in-depth ROS analyses and even fewer have identified the precise redox-regulated target molecules. In this review, we will give a brief introduction to ROS and their sources in macrophages, summarize the versatile roles of ROS in direct and indirect antimicrobial immune defense, and provide an overview of commonly used ROS probes, scavengers and inhibitors.

Investigation of exchange couplings in [Fe3S4]+ clusters by electron spin-lattice relaxation

We have studied four proteins containing oxidized 3Fe clusters ([Fe3S4]+, S=1/2, composed of three, antiferromagnetically coupled high-spin ferric ions) by continuous wave (CW) and pulsed EPR techniques: Azotobacter vinelandii ferredoxin I, Desulfovibrio gigas ferredoxin II, and the 3Fe forms of Pyrococcus furiosus ferredoxin and aconitase. The 35 GHz (Q-band) CW EPR signals are simulated to yield experimental g tensors, which either had not been reported, or had been reported only at X-band microwave frequency. Pulsed X- and Q-band EPR techniques are used to determine electron spin-lattice (T1, longitudinal) relaxation times at several positions on the samples' EPR envelope over the temperature range 2-4.2 K. The T1, values vary sharply across the EPR envelope, a reflection of the fact that the envelope results from a distribution in cluster properties, as seen earlier as a distribution in g3 values and in 57 Fe hyperfine interactions, as detected by electron nuclear double resonance spectroscopy. The temperature dependence of 1/T1 is analyzed in terms of the Orbach mechanism, with relaxation dominated by resonant two-phonon transitions to a doublet excited state at approximately 20 cm(-1) above the doublet ground state for all four of these 3Fe proteins. The experimental EPR data are combined with previously reported 57Fe hyperfine data to determine electronic spin exchange-coupling within the clusters, following the model of Kent et al. Their model defines the coupling parameters as follows: J13=J, J12=J(1+epsilon'), J23=J(1+epsilon), where Jij is the isotropic exchange coupling between ferric ions i and j, and epsilon' and epsilon' are measures of coupling inequivalence. We have extended their theory to include the effects of epsilon' not equal to 0 and thus derived an exact expression for the energy of the doublet excited state for any epsilon, epsilon'. This excited state energy corresponds roughly to epsilonJ and is in the range 5-10 cm(-1) for each of these four 3Fe proteins. This magnitude of the product epsilonJ, determined by our time-domain relaxation studies in the temperature range 2-4 K, is the same as that obtained from three other distinct types of study: CW EPR studies of spin relaxation in the range 5.5-50 K, NMR studies in the range 293-303 K, and static susceptibility measurements in the range 1.8-200 K. We suggest that an apparent disagreement as to the individual values of J and epsilon be resolved in favor of the values obtained by susceptibility and NMR (J > or approximately 200 cm(-1) and epsilon> or =0.02 cm(-1)). as opposed to a smaller J and larger r as suggested in CW EPR studies. However, we note that this resolution casts doubt on the accepted theoretical model for describing the distribution in magnetic properties of 3Fe clusters.

The Saccharomyces cerevisiae succinate dehydrogenase anchor subunit, Sdh4p: mutations at the C-terminal lys-132 perturb the hydrophobic domain

The yeast succinate dehydrogenase (SDH) is a tetramer of non-equivalent subunits, Sdh1p-Sdh4p, that couples the oxidation of succinate to the transfer of electrons to ubiquinone. One of the membrane anchor subunits, Sdh4p, has an unusual 30 amino acid extension at the C-terminus that is not present in SDH anchor subunits of other organisms. We identify Lys-132 in the Sdh4p C-terminal region as necessary for enzyme stability, ubiquinone reduction, and cytochrome b562 assembly in SDH. Five Lys-132 substituted SDH4 genes were constructed by site-directed mutagenesis and introduced into an SDH4 knockout strain. The mutants, K132E, K132G, K132Q, K132R, and K132V were characterized in vivo for respiratory growth and in vitro for ubiquinone reduction, enzyme stability, and cytochrome b562 assembly. Only the K132R substitution, which conserves the positive charge of Lys-132, produces a wild-type enzyme. The remaining four mutants do not affect the ability of SDH to oxidize succinate in the presence of the artificial electron acceptor, phenazine methosulfate, but impair quinone reductase activity, enzyme stability, and heme insertion. Our results suggest that the presence of a positive charge on residue 132 in the C-terminus of Sdh4p is critical for establishing a stable conformation in the SDH hydrophobic domain that is compatible with ubiquinone reduction and cytochrome b562 assembly. In addition, our data suggest that heme does not play an essential role in quinone reduction.

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

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

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

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

Inactivation of succinate-ubiquinone reductase in substrate mixture

Succinate-ubiquinone reductase plays an important role in the respiratory chain. Previous work showed that preparation of succinate-ubiquinone reductase was relatively stable. Though the enzyme catalysis has been extensively studied, the inactivation of succinate-ubiquinone reductase has never been reported. In the present study, the kinetic theory of the substrate reaction of irreversible inhibition described by Tsou (Adv. Enzymol. Relat. Areas Mol. Biol. 61 (1988) 381-436) was applied to study the course of an unexpected slow inactivation of succinate-ubiquinone reductase in the substrate assay mixture containing different concentrations of substrates, succinate and 2,6-dichloroindophenol. The results showed that the inactivation of succinate-ubiquinone reductase in the substrate mixture is a first order reaction. The inactivation rate decreased with increasing concentration of succinate. The values of the micro rate constants for free and succinate bound enzyme were 0.22 +/- 0.01 and 0.052 +/- 0.002 min-1, respectively. Binding with 2-thenoyl-trifluroacetone, a inhibitor specially for the quinone binding site, slowed down the inactivation. However, the rate of inactivation did not change with increasing 2,6-dichloroindophenol concentration. The study showed that succinate-ubiquinone reductase was irreversibly inactivated in the substrate mixture. The results suggest that the inactivation was not due to dilution or dissociation of the enzyme, nor to complete usage of the substrate, inhibition of the yielded product or some possible trace component in the substrate mixture, nor to modification of the essential thiol group in the succinate binding site of succinate-ubiquinone reductase. The enzyme became more stable after binding with succinate.

Molecular genetics of succinate:quinone oxidoreductase in eukaryotes

Succinate:quinone oxidoreductase is a membrane-associated complex in mitochondria, often referred to as complex II, based on the fractionation scheme developed by Y. Hatefi and colleagues. It consists of four peptides, two of which are integral membrane proteins (15 and 12-13 kDa, respectively) and two others that are peripheral membrane proteins, i.e., a flavoprotein (Fp, 70 kDa) and an iron-protein (Ip, 27 kDa). The mature, functional complex contains a cytochrome in association with the membrane proteins, a flavin linked covalently to the largest peptide, and three iron-sulfur clusters in the 27-kDa subunit. The present review touches only briefly on the biochemical and biophysical properties of this complex. Instead, the focus is on the molecular-genetic studies that have become possible since the first genes from eukaryotes were cloned in 1989. The evolutionary conservation of the amino acid sequence of both the Fp and the Ip peptides has facilitated the cloning of these genes from a large variety of eukaryotic organisms by PCR-based methods. The review addresses questions related to the regulation of the expression of these genes, with an emphasis on mammals and yeast, for which most of the information is available. Four different genes have to be co-ordinately regulated. Transcriptional as well as posttranscriptional regulatory mechanisms have been observed in diverse organisms. Intriguing observations have been made in studies of this enzyme during the life cycle of organisms existing alternately under aerobic and anaerobic conditions. Naturally occurring or induced mutations in these genes have shed light on several questions related to the assembly of this complex, and on the relationship between structure and function. Four different peptides are imported into the mitochondria. They have to be modified, folded, and assembled. The stage is set for the exploration of highly specific changes introduced by site-directed mutagenesis. Until recently the genes were believed to be exclusively nuclear in all eukaryotes, but exceptions have since been found. This finding has relevance in the discussion of the evolution of mitochondria from prokaryotes. A highly conserved set of genes is found in prokaryotes, and some informative comparisons on gene organization and expression in prokaryotes and eukaryotes have been included.

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.

Kinetics of modification of the mitochondrial succinate-ubiquinone reductase by 5,5'-dithiobis-(2-nitro-benzoic acid)

The kinetic theory of the substrate reaction during modification of enzyme activity previously described by Tsou [Tsou (1988), Adv. Enzymol. Relat. Areas Mol. Biol. 61, 381-436] has been applied to a study of the kinetics of the course of inactivation of the mitochondrial succinate-ubiquinone reductase by 5,5'-dithiobis-(2-nitro-benzoic acid) (DTNB). The results show that the inactivation of this enzyme by DTNB is a conformation-change-type inhibition which involves a conformational change of the enzyme before inactivation. The microscopic rate constants were determined for the reaction of the inactivator with the enzyme. The presence of the substrate provides marked protection of this enzyme against inactivation by DTNB. The modification reaction of the enzyme using DTNB was shown to follow a triphasic course by following the absorption at 412 nm. Among these reactive thiol groups, the fast-reaction thiol group is essential for the enzyme activity. The results suggest that the essential thiol group is situated at the succinate-binding site of the mitochondrial succinate-ubiquinone reductase.

Rhodoquinone and complex II of the electron transport chain in anaerobically functioning eukaryotes

Many anaerobically functioning eukaryotes have an anaerobic energy metabolism in which fumarate is reduced to succinate. This reduction of fumarate is the opposite reaction to succinate oxidation catalyzed by succinate-ubiquinone oxidoreductase, complex II of the aerobic respiratory chain. Prokaryotes are known to contain two distinct enzyme complexes and distinct quinones, menaquinone and ubiquinone (Q), for the reduction of fumarate and the oxidation of succinate, respectively. Parasitic helminths are also known to contain two different quinones, Q and rhodoquinone (RQ). This report demonstrates that RQ was present in all examined eukaryotes that reduce fumarate during anoxia, not only in parasitic helminths, but also in freshwater snails, mussels, lugworms, and oysters. It was shown that the measured RQ/Q ratio correlated with the importance of fumarate reduction in vivo. This is the first demonstration of the role of RQ in eukaryotes, other than parasitic helminths. Furthermore, throughout the development of the liver fluke Fasciola hepatica, a strong correlation was found between the quinone composition and the type of metabolism: the amount of Q was correlated with the use of the aerobic respiratory chain, and the amount of RQ with the use of fumarate reduction. It can be concluded that RQ is an essential component for fumarate reduction in eukaryotes, in contrast to prokaryotes, which use menaquinone in this process. Analyses of enzyme kinetics, as well as the known differences in primary structures of prokaryotic and eukaryotic complexes that reduce fumarate, support the idea that fumarate-reducing eukaryotes possess an enzyme complex for the reduction of fumarate, structurally related to the succinate dehydrogenase-type complex II, but with the functional characteristics of the prokaryotic fumarate reductases.

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.

Resolution of the aerobic respiratory system of the thermoacidophilic archaeon, Sulfolobus sp. strain 7. I. The archaeal terminal oxidase supercomplex is a functional fusion of respiratory complexes III and IV with no c-type cytochromes

The aerobic respiratory system of the thermoacidophilic archaeon, Sulfolobus sp. strain 7, is unusual in that it consists of only a- and b-type cytochromes but no c-type cytochromes. In previous studies, a novel cytochrome oxidase a583-aa3 subcomplex has been purified, which showed a ferrocytochrome c oxidase but no caldariellaquinol oxidase activity (Wakagi, T., Yamauchi, T., Oshima, T., Müller, M., Azzi, A., and Sone, N. (1989) Biochem. Biophys. Res. Commun. 165, 1110-1114). We show here that the cytochrome subcomplex could be copurified with a non-CO-reactive cytochrome b562 as a novel terminal oxidase "supercomplex," which also contained a Rieske-type FeS cluster at gy = 1.89. It contained one copper and all four heme centers detectable in the archaeal membranes by the low temperature spectrophotometry and the potentiometric titration: cytochromes b562 (+146 mV), a583 (+270 mV), and aa3 (+117 and +325 mV). The presence of one copper atom indicates that it contains the conventional heme a3-CuB binuclear center for reducing molecular oxygen. In conjunction with the presence of a Rieske-type FeS center, inhibitor studies suggest that the terminal oxidase segment of the respiratory chain of Sulfolobus sp. strain 7 is a functional fusion of respiratory complexes III and IV, where cytochrome b562 and the Rieske-type FeS center probably play a central role in the oxidation of caldariellaquinol. This archaeal terminal oxidase supercomplex reconstitutes the in vitro succinate oxidase respiratory chain for the first time together with caldariellaquinone and the purified cognate succinate:caldariellaquinone oxidoreductase complex. The reconstitution system requires caldariellaquinone for the activity, and is highly sensitive to cyanide and 2-heptyl-4-hydroxy-quinoline-N-oxide. These results are also discussed in terms of the evolutionary considerations.

A Chinese hamster mutant cell line with a defect in the integral membrane protein CII-3 of complex II of the mitochondrial electron transport chain

In this study, a respiration-deficient Chinese hamster cell line with a defect in succinate dehydrogenase activity is shown to result from a single base change in a codon in the coding sequence for the membrane anchor protein CII-3 (also referred to as QPs-1). A premature translation stop results in the truncation of 33 amino acids from the C terminus. Bovine cDNA encoding this peptide complements the mutation. There is about 82% identity between these two mammalian proteins. The gene for CII-3 was mapped on human chromosome 1, and because it is also found on minichromosomes characterized by our laboratory, we can localize it on the short arm within 1-2 megabases from the centromere.

Structural organization of the gene encoding the human iron-sulfur subunit of succinate dehydrogenase

The iron-sulfur protein (Ip) subunit of succinate dehydrogenase (SDH and complex II) of the respiratory chain is highly conserved in evolution [Gould et al., Proc. Natl. Acad. Sci. USA 86 (1989) 1934-1938]. We have cloned the entire human Ip cDNA, as well as the Ip-encoding gene (SDH-B) from two genomic human libraries. The cDNA contains a coding sequence of 840 nt, flanked by a 5'-UTR of 133 nt and a 3'-UTR of 123 nt. The entire transcript is encoded by eight exons within approx. 40 kb. The seven introns range in size from 0.75 kb to > 11 kb, and they appear to be of the 'late' intron class. Approx. 5 kb of upstream sequence was also cloned, and approx. 2.4 kb of the promoter region were sequenced and analyzed for consensus elements binding potential transcription factors and transcriptional activators.

Mitochondrial biogenesis in early mouse embryos: expression of the mRNAs for subunits IV, Vb, and VIIc of cytochrome c oxidase and subunit 9 (P1) of H(+)-ATP synthase

The mouse egg contains about 90,000 mitochondria which undergo a buildup of mitochondrial cristae and increase in respiratory activity during cleavage. The mitochondrial DNA does not replicate during preimplantation development but is transcribed actively from the two-cell stage onward (Pikó and Taylor, 1987: Dev Biol 123:364-374). To gain further insight into mitochondrial biogenesis, we have now determined the steady state amounts of the mRNAs for the cytochrome c oxidase (COX) subunits IV, Vb and VIIc and the H(+)-ATPase subunit 9 (P1) (all encoded by nuclear genes) in slot hybridization experiments of total RNA from oocytes and early embryos. All four mRNAs showed a similar developmental pattern of prevalence, characterized by a steady decline in mRNA copy numbers from the late growth-phase oocyte through the two-cell embryo, and an about 30-fold rise during cleavage through the blastocyst stage. However, the ATPase subunit 9 (P1) mRNA was about three times more prevalent in cleavage-stage embryos than the COX mRNAs. A similar pattern was obtained previously for the mitochondrial-encoded COX I and II mRNAs, but the latter accumulate at a 30-50-fold excess over the nuclear-encoded COX subunit mRNAs during the cleavage stages. The results suggest a coordinated activation and transcription of the mitochondrial and nuclear genes for the components of the respiratory apparatus beginning with the two-cell stage. It is estimated that new respiratory chains are produced at a rate of 50-100 chains hr-1/mitochondrion in the early blastocyst, accounting for 3.5-7% of the total protein synthetic activity at this stage.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the proton-translocating NADH:ubiquinone oxidoreductases of mitochondria and Escherichia coli using the inhibitor 1,10-phenanthroline

Mitochondrial NADH:ubiquinone oxidoreductase (complex I) is uncompetitively inhibited by 1,10-phenanthroline (OP). EPR spectroscopy of submitochondrial particles indicates that OP, similarly to rotenone, inhibits electron transfer between the Fe-S clusters of complex I and the ubiquinone pool. The proton-translocating NADH dehydrogenase (NDH1) of E. coli is more sensitive to OP than is NDH1 of Paracoccus. EPR spectroscopy of membranous E. coli NDH1 shows that two slow- and one fast-relaxing Fe-S clusters become detectable upon reduction by NADH in the presence of OP. However, none of them resembles the mitochondrial cluster 2.

The proton-translocating NADH: ubiquinone oxidoreductase: a discussion of selected topics

The proton-translocating NADH:ubiquinone oxidoreductase (complex I) is a large, multi-subunit and multi-redox centre enzyme which is found in the mitochondrial inner membrane and plasma membrane of some bacteria. In this minireview an attempt has been made to critically discuss selected topics in the structure and function of this enzyme. A special emphasis is given to the iron-sulphur cluster and to the proteins that may bind them. Previous suggestions for the mechanism of proton-translocation by complex I are discussed. Subcomplexes that contain several but not all of the subunits of the intact mitochondrial enzyme are described and analysed in order to identify the functional core of the enzyme. The data on the trans-membrane organisation of several subunits is examined. It is hoped that most of the suggestions as well as the questions raised here could be experimentally tested in the near future.

cDNA sequence of three cysteine-rich clusters in the iron-sulfur subunit of complex II (succinate-ubiquinone oxidoreductase) from Caenorhabditis elegans determined by automated DNA sequencer

Homology probing by using mixed primers for polymerase chain reaction (PCR) and a subsequent sequence analysis by automated DNA sequencer were applied to determine a partial cDNA sequence of the iron-sulfur subunit of complex II (succinate-ubiquinone oxidoreductase). Complex II is a membrane-bound flavoenzyme, which catalyzes the oxidation of succinate to fumarate in the tricarboxylic acid cycle, and it is a component of the mitochondrial and bacterial respiratory chains. In this study, the partial amino acid sequence of iron-sulfur subunits in Caenorhabditis elegans mitochondria was deduced from the DNA sequence obtained from cDNA-PCR. Mixed oligonucleotide primers corresponding to two conserved regions which appear to be the binding site for the prosthetic group were used. The product of PCR was cloned into plasmid vector pUC 119 and the sequence was determined from double strand plasmid DNA by the dideoxy method using of one-dye, four-lane type the automated DNA sequencer (DSQ-1, Shimadzu). The PCR product contained 483 nucleotides and its deduced amino acid sequence was highly homologous with that in human liver (68.9%) and that of Escherichia coli sdh B product (50.3%). As expected, striking sequence conservation was found around the three cysteine-rich clusters which have been thought to comprise the iron-sulfur centers of the enzyme.

Electron-transfer complexes of mitochondria in Ascaris suum

During the past ten years, studies on the respiratory chain of mitochondria in parasites have progressed to provide new insight into the structural organization and physiological significance of the mitochondrial respiratory chain. In this review, Kiyoshi Kita focuses on studies on the respiratory chain of Ascaris mitochondria in which major advances have recently been made. These include the identification of the unique features of anaerobic respiration, the elucidation of the molecular structures of the components involved and an understanding of the evolution of the energy transducing system and of the developmental changes that occur during the life cycle of this nematode.

Evidence for non-cysteinyl coordination of the [2Fe-2S] cluster in Escherichia coli succinate dehydrogenase

The consequences of replacing Cys65 in the FrdB subunit of Escherichia coli fumarate reductase by Asp or Ala have been investigated in terms of bacterial growth, enzymatic activity, and the ERP/redox properties of the [2Fe-2S] cluster. An aspartic acid residue occupies the equivalent position in E. coli succinate dehydrogenase, and the FrdBCys65Asp mutation has little effect on cell growth, enzyme activity or the physical properties of the Frd [2Fe-2S] cluster. In contrast, the [2Fe-2S] cluster was not observed in the FrdBCys65Ala mutant showing that a coordinating residue is required at this position for assembly of this cluster and significant levels of enzymatic activity. These results support the presence of one non-cysteinyl, oxygenic ligand for the [2Fe-2S] cluster in E. coli succinate dehydrogenase.

[3Fe-4S] to [4Fe-4S] cluster conversion in Escherichia coli fumarate reductase by site-directed mutagenesis

Site-directed mutants of Escherichia coli fumarate reductase in which FrdB Cys204, Cys210, and Cys214 were individually replaced by Ser and in which Val207 was replaced by Cys were constructed and overexpressed in a strain of E. coli lacking a wild-type copy of fumarate reductase and succinate dehydrogenase. The consequences of these mutations on bacterial growth, enzymatic activity, and the EPR properties of the constituent iron-sulfur clusters were investigated. The FrdB Cys204Ser, Cys210Ser, and Cys214Ser mutations result in enzymes with negligible activity that have dissociated from the membrane and consequently are incapable of supporting cell growth under conditions requiring a functional fumarate reductase. EPR studies indicate that these effects are associated with loss of both the [3Fe-4S] and [4Fe-4S] clusters, centers 3 and 2, respectively. In contrast, the FrdB Val207Cys mutation results in a functional membrane-bound enzyme that is able to support growth under anaerobic and aerobic conditions. However, EPR studies indicate that the indigenous [3Fe-4S]+,0 cluster (Em = -70 mV), center 3, has been replaced by a much lower potential [4Fe-4S]2+,+ cluster (Em = -350 mV), indicating that the primary sequence of the polypeptide determines the type of clusters assembled. The results of these studies afford new insights into the role of centers 2 and 3 in mediating electron transfer from menaquinol, the residues that ligate these clusters, and the intercluster magnetic interactions in the wild-type enzyme.

Characterization of ubisemiquinone radicals in succinate-ubiquinone reductase

A thenoyl trifluoroacetone-sensitive and antimycin-insensitive ubisemiquinone radical (Qs) is readily detected in purified succinate-cytochrome c reductase. When this reductase is resolved into succinate-Q and ubiquinol-cytochrome c reductases, Qs was not detected in either reductase. The difficulty in detecting such a radical in purified succinate-Q reductase has puzzled investigators for years. A deficiency of Q in the isolated complex is the reason for the failure to detect Qs. Upon addition of exogenous Q, a thenoyl trifluoroacetone-sensitive Q-radical is readily detectable in isolated succinate-Q reductase under a controlled redox potential. Maximum radical concentration is observed when 5 mol of exogenous Q, per mole of flavin, is added. The radical gives an EPR signal with a g-value of 2.005 and a line-width of 12 G. The Em of Qs is 84 mV at pH 7.4, with half-potentials of E1 = 40 mV and E2 = 128 mV. The Qs-radical does not show power saturation, even at 200 mW.

Hypochlorous acid and myeloperoxidase-catalyzed oxidation of iron-sulfur clusters in bacterial respiratory dehydrogenases

Hypochlorous acid and related oxidants derived from myeloperoxidase-catalyzed reactions contribute to the microbicidal activities of phagocytosing neutrophils and monocytes. Microbial iron-sulfur (Fe/S) clusters have been suggested as general targets of myeloperoxidase-derived oxidations, but no susceptible Fe/S site has yet been identified. In this study, the effects of HOCl and myeloperoxidase-catalyzed peroxidation of chloride ion upon EPR-detectable Fe/S clusters in Escherichia coli and Pseudomonas aeruginosa were examined. Increasing amounts of oxidant produced progressive loss of signal amplitudes from the S-1 and S-3 Fe/S clusters of succinate:ubiquinone oxidoreductase in respiring membrane fragments. These changes were compared to loss of microbial viability, succinate uptake rates, succinate dehydrogenase activity and succinate-dependent respiration. The amounts of oxidant required to destroy Fe/S clusters exceeded the amounts required to kill organisms or inhibit respiratory function by factors of four or five. Power saturation characteristics of the S-1 signal indicated that the S-2 signal was also resistant to modification, even in highly oxidized membranes. Loss of succinate-dependent respiration was closely associated with HOCl and myeloperoxidase-mediated microbicidal activity against P. aeruginosa and was also an early event in the oxidant-mediated metabolic dysfunctions of E. coli. However, these effects were not caused by the destruction of the Fe/S clusters within the succinate:ubiquinone oxidoreductase. Rather, the major respiration-inhibiting lesion(s) appeared to reside at points in the respiratory chain between the Fe/S clusters and the ubiquinone reductase site.

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.

Tryptophan fluorescence in electron-transfer flavoprotein:ubiquinone oxidoreductase: fluorescence quenching by a brominated pseudosubstrate

We have studied the intrinsic fluorescence of the 12 tryptophan residues of electron-transfer flavoprotein:ubiquinone oxidoreductase (ETF:QO). The fluorescence emission spectrum (lambda ex 295 nm) showed that the fluorescence is due to the tryptophan residues and that the contribution of the 22 tyrosine residues is minor. The emission maximum (lambda m 334 nm) and the bandwidth (delta lambda 1/2 56 nm) suggest that the tryptophans lie in hydrophobic environments in the oxidized protein. Further, these tryptophans are inaccessible to a range of ionic and nonionic collisional quenching agents, indicating that they are buried in the protein. Enzymatic or chemical reduction of ETF:QO results in a 5% increase in fluorescence with no change of lambda m or delta lambda 1/2. This change is reversible upon reoxidation and is likely to reflect a conformational change in the protein. The ubiquinone analogue Q0(CH2)10Br, a pseudosubstrate of ETF:QO (Km = 2.6 microM; kcat = 210 s-1), specifically quenches the fluorescence of one tryptophan residue (Kd = 1.6-3.2 microM) in equilibrium fluorescence titrations. The ubiquinone homologue UQ-2 (Km = 2 microM; kcat = 162 s-1) and the analogue Q0(CH2)10OH (Km = 2 microM; kcat = 132 s-1) do not quench tryptophan fluorescence; thus the brominated analogue acts as a static heavy atom quencher. We also describe a rapid purification for ETF:QO based on extraction of liver submitochondrial particles with Triton X-100 and three chromatographic steps, which results in yields 3 times higher than previously published methods.

Structural studies on three flavin-interacting regions of the flavoprotein subunit of complex II in Ascaris suum mitochondria

The flavoprotein (Fp) subunit of mitochondrial complex II contains covalently bound FAD as a prosthetic group. In this study, the primary structure of the flavin-bound tryptic peptide from the Fp subunit of Ascaris complex II was determined and found to be highly similar to those of the corresponding flavin-binding regions of bovine heart and bacterial Fp subunits. Furthermore, the Ascaris Fp subunit was shown to contain two regions exhibiting striking sequence similarity to the segments that have been predicted to interact noncovalently with the AMP moiety of FAD in bacterial Fp subunits. The conservation of these two regions also in the mitochondrial Fp subunit suggests their functional importance.

Human complex II (succinate-ubiquinone oxidoreductase): cDNA cloning of iron sulfur (Ip) subunit of liver mitochondria

Complex II (succinate-ubiquinone oxidoreductase) is an important enzyme complex of both the tricarboxylic acid cycle and of the aerobic respiratory chains of mitochondria in eukaryotic cell and prokaryotic organisms. In this study, the amino acid sequence of iron sulfur-subunit in human liver mitochondria was deduced from cDNA which was isolated by immunoscreening a human liver lambda gtll cDNA library. An isolated clone contains an open reading frame of 786 nucleotides and encodes a mature protein of 252 amino acids with a molecular weight of 28,804. The amino acid sequence was highly homologous with that of bovine heart (94.1%) which has been determined from the purified peptide and that of Escherichia coli sdh B product (50.8%). Striking sequence conservation was found around the three cysteine-rich clusters which have been thought to comprise the iron-sulfur centers of the enzyme. This is the first report on the cDNA sequence of mitochondrial complex II.

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.

ESR studies on iron-sulfur clusters of complex II in Ascaris suum mitochondria which exhibits strong fumarate reductase activity

Complex II of Ascaris suum mitochondria, which functions as fumarate reductase in physiological conditions, contains three types of iron-sulfur clusters. These correspond to clusters S-1, S-2 and S-3 and are distinguishable by low-temperature ESR studies. Cluster S-1 is reduced by succinate, giving ESR signals with gz, gy and gx values at 2.033, 1.939 and 1.920. The existence of cluster S-2 is suggested by an enhancement of the S-1 spin relaxation induced upon reduction of S-2 by dithionite. Cluster S-3 is ESR detectable under air-oxidized conditions and gives a strong signal at g = 2.025. Cluster S-3 was only partially reduced even with an excess amount of sodium succinate, which is a common characteristic of fumarate reductase but this is not seen in the mitochondrial complex II.

Ligands to the 2Fe iron-sulfur center in succinate dehydrogenase

Membrane-bound succinate oxidoreductases are flavoenzymes containing one each of a 2Fe, a 3Fe and a 4Fe iron-sulfur center. Amino acid sequence homologies indicate that all three centers are located in the Ip (B) subunit. From polypeptide and gene analysis of Bacillus subtilis succinate dehydrogenase-defective mutants combined with earlier EPR spectroscopic data, we show that four conserved cysteine residues in the first half of Ip are the ligands to the [2Fe-2S] center. These four residues have previously been predicted to be the ligands. Our results also suggest that the N-terminal part of B. subtilis Ip constitutes a domain which can incorporate separately the 2Fe center and interact with Fp, the flavin-containing subunit of the dehydrogenase.