Structure-activity characterization of sulfide:quinone oxidoreductase variants

Sulfide:quinone oxidoreductase (SQR) is a peripheral membrane protein that catalyzes the oxidation of sulfide species to elemental sulfur. The enzymatic reaction proceeds in two steps. The electrons from sulfides are transferred first to the enzyme cofactor, FAD, which, in turn, passes them onto the quinone pool in the membrane. Several wild-type SQR structures have been reported recently. However, the enzymatic mechanism of SQR has not been fully delineated. In order to understand the role of the catalytically essential residues in the enzymatic mechanism of SQR we produced a number of variants of the conserved residues in the catalytic site including the cysteine triad of SQR from the acidophilic, chemolithotrophic bacterium Acidithiobacillus ferrooxidans. These were structurally characterized and their activities for each reaction step were determined. In addition, the crystal structures of the wild-type SQR with sodium selenide and gold(I) cyanide have been determined. Previously we proposed a mechanism for the reduction of sulfides to elemental sulfur involving nucleophilic attack of Cys356 on C(4A) atom of FAD. Here we also consider an alternative anionic radical mechanism by direct electron transfer from Cys356 to the isoalloxazine ring of FAD.     

Cyanobacterial sulfide-quinone reductase: cloning and heterologous expression

The gene encoding sulfide-quinone reductase (SQR; E.C.1.8.5.'), the enzyme catalyzing the first step of anoxygenic photosynthesis in the filamentous cyanobacterium Oscillatoria limnetica, was cloned by use of amino acid sequences of tryptic peptides as well as sequences conserved in the Rhodobacter capsulatus SQR and in an open reading frame found in the genome of Aquifex aeolicus. SQR activity was also detected in the unicellular cyanobacterium Aphanothece halophytica following sulfide induction, with a V(max) of 180 micromol of plastoquinone-1 (PQ-1) reduced/mg of chlorophyll/h and apparent K(m) values of 20 and 40 microM for sulfide and quinone, respectively. Based on the conserved sequences, the gene encoding A. halophytica SQR was also cloned. The SQR polypeptides deduced from the two cyanobacterial genes consist of 436 amino acids for O. limnetica SQR and 437 amino acids for A. halophytica SQR and show 58% identity and 74% similarity. The calculated molecular mass is about 48 kDa for both proteins; the theoretical isoelectric points are 7.7 and 5.6 and the net charges at a neutral pH are 0 and -14 for O. limnetica SQR and A. halophytica SQR, respectively. A search of databases showed SQR homologs in the genomes of the cyanobacterium Anabaena PCC7120 as well as the chemolithotrophic bacteria Shewanella putrefaciens and Thiobacillus ferrooxidans. All SQR enzymes contain characteristic flavin adenine dinucleotide binding fingerprints. The cyanobacterial proteins were expressed in Escherichia coli under the control of the T7 promoter. Membranes isolated from E. coli cells expressing A. halophytica SQR performed sulfide-dependent PQ-1 reduction that was sensitive to the quinone analog inhibitor 2n-nonyl-4-hydroxyquinoline-N-oxide. The wide distribution of SQR genes emphasizes the important role of SQR in the sulfur cycle in nature.     

Sulfide oxidation in gram-negative bacteria by expression of the sulfide-quinone reductase gene of Rhodobacter capsulatus and by electron transport to ubiquinone.

The oxidation of sulfide was studied in recombinant bacteria expressing the sulfide-quinone reductase gene (sqr) from Rhodobacter capsulatus. Sulfide was oxidized by the Escherichia coli strain W3110 harboring the sqr construct (pKKSQ) under anaerobic conditions and nitrate was utilized as a terminal electron acceptor. Following the oxidation, elemental sulfur and nitrite were produced as the final reaction products. This activity was retained in the membrane preparation and was sensitive towards antimycin A, stigmatellin, and azide. As a consequence of the ubiquinone deficiency, this activity was markedly decreased. In additon, by recovery of ubiquinone, the oxidation was also restored to rates similar to those of the wild-type strain. These results indicate that sulfide oxidation in this strain occurs via the quinone pool in vivo, and that this sulfide-quinone reductase (SQR) in particular utilizes ubiquinone as a more appropriate electron acceptor than menaquinone or demetylmenaquinone. To our knowledge, this is the first study to show a direct interaction between SQR and ubiquinone in cells. When expressed in Pseudomonas putida and Rhizobium meliloti, the SQR conferred on these organisms the ability to oxidize sulfide as well as E. coli in vivo.     

Single eubacterial origin of eukaryotic sulfide:quinone oxidoreductase,a mitochondrial enzyme conserved from the early evolution of eukaryotes during anoxic and sulfidic times

Mitochondria occur as aerobic, facultatively anaerobic, and, in the case of hydrogenosomes, strictly anaerobic forms. This physiological diversity of mitochondrial oxygen requirement is paralleled by that of free-living alpha-proteobacteria, the group of eubacteria from which mitochondria arose, many of which are facultative anaerobes. Although ATP synthesis in mitochondria usually involves the oxidation of reduced carbon compounds, many alpha-proteobacteria and some mitochondria are known to use sulfide (H2S) as an electron donor for the respiratory chain and its associated ATP synthesis. In many eubacteria, the oxidation of sulfide involves the enzyme sulfide:quinone oxidoreductase (SQR). Nuclear-encoded homologs of SQR are found in several eukaryotic genomes. Here we show that eukaryotic SQR genes characterized to date can be traced to a single acquisition from a eubacterial donor in the common ancestor of animals and fungi. Yet, SQR is not a well-conserved protein, and our analyses suggest that the SQR gene has furthermore undergone some lateral transfer among prokaryotes during evolution, leaving the precise eubacterial lineage from which eukaryotes obtained their SQR difficult to discern with phylogenetic methods. Newer geochemical data and microfossil evidence indicate that major phases of early eukaryotic diversification occurred during a period of the Earth's history from 1 to 2 billion years before present in which the subsurface ocean waters contained almost no oxygen but contained high concentrations of sulfide, suggesting that the ability to deal with sulfide was essential for prokaryotes and eukaryotes during that time. Notwithstanding poor resolution in deep SQR phylogeny and lack of a specifically alpha-protebacterial branch for the eukaryotic enzyme on the basis of current lineage sampling, a single eubacterial origin of eukaryotic SQR and the evident need of ancient eukaryotes to deal with sulfide, a process today germane to mitochondrial quinone reduction, are compatible with the view that eukaryotic SQR was an acquisition from the mitochondrial endosymbiont.     

Insights into the catalytic mechanism of type VI sulfide:quinone oxidoreductases

Sulfide oxidation is catalyzed by ancient membrane-bound sulfide:quinone oxidoreductases (SQR) which are classified into six different types. For catalysis of sulfide oxidation, all SQRs require FAD cofactor and a redox-active centre in the active site, usually formed between conserved essential cysteines. SQRs of different types have variation in the number and position of cysteines, highlighting the potential for diverse catalytic mechanisms. The photosynthetic purple sulfur bacterium, Thiocapsa roseopersicina contains a type VI SQR enzyme (TrSqrF) having unusual catalytic parameters and four cysteines likely involved in the catalysis. Site-directed mutagenesis was applied to identify the role of cysteines in the catalytic process of TrSqrF. Based on biochemical and kinetic characterization of these TrSqrF variants, Cys121 is identified as crucial for enzyme activity. The cofactor is covalently bound via a heterodisulfide bridge between Cys121 and the C8M group of FAD. Mutation of another cysteine present in all SQRs (Cys332) causes remarkably decreased enzyme activity (14.6% of wild type enzyme) proving important, but non-essential role of this residue in enzyme catalysis. The sulfhydril-blocking agent, iodoacetamide can irreversibly inactivate TrSqrF but only if substrates are present and the enzyme is actively catalyzing its reaction. When the enzyme is inhibited by iodoacetamide, the FAD cofactor is released. The inhibition studies support a mechanism that entails opening and reforming of the heterodisulfide bridge during the catalytic cycle of TrSqrF. Our study thus reports the first detailed structure-function analysis of a type VI SQR enzyme which enables the proposal of a distinct mechanism of sulfide oxidation for this class.     

Sulfide : quinone oxidoreductase (SQR) from the lugworm Arenicola marina shows cyanide- and thioredoxin-dependent activity

The lugworm Arenicola marina inhabits marine sediments in which sulfide concentrations can reach up to 2 mM. Although sulfide is a potent toxin for humans and most animals, because it inhibits mitochondrial cytochrome c oxidase at micromolar concentrations, A. marina can use electrons from sulfide for mitochondrial ATP production. In bacteria, electron transfer from sulfide to quinone is catalyzed by the membrane-bound flavoprotein sulfide : quinone oxidoreductase (SQR). A cDNA from A. marina was isolated and expressed in Saccharomyces cerevisiae, which lacks endogenous SQR. The heterologous enzyme was active in mitochondrial membranes. After affinity purification, Arenicola SQR isolated from yeast mitochondria reduced decyl-ubiquinone (K(m) = 6.4 microm) after the addition of sulfide (K(m) = 23 microm) only in the presence of cyanide (K(m) = 2.6 mM). The end product of the reaction was thiocyanate. When cyanide was substituted by Escherichia coli thioredoxin and sulfite, SQR exhibited one-tenth of the cyanide-dependent activity. Six amino acids known to be essential for bacterial SQR were exchanged by site-directed mutagenesis. None of the mutant enzymes was active after expression in yeast, implicating these amino acids in the catalytic mechanism of the eukaryotic enzyme.     

Organization of the Human Mitochondrial Hydrogen Sulfide Oxidation Pathway

Sulfide oxidation is expected to play an important role in cellular switching between low steady-state intracellular hydrogen sulfide levels and the higher concentrations where the physiological effects are elicited. Yet despite its significance, fundamental questions regarding how the sulfide oxidation pathway is wired remain unanswered, and competing proposals exist that diverge at the very first step catalyzed by sulfide quinone oxidoreductase (SQR). We demonstrate that, in addition to sulfite, glutathione functions as a persulfide acceptor for human SQR and that rhodanese preferentially synthesizes rather than utilizes thiosulfate. The kinetic behavior of these enzymes provides compelling evidence for the flow of sulfide via SQR to glutathione persulfide, which is then partitioned to thiosulfate or sulfite. Kinetic simulations at physiologically relevant metabolite concentrations provide additional support for the organizational logic of the sulfide oxidation pathway in which glutathione persulfide is the first intermediate formed.     

Oxidation of hydrogen sulfide remains a priority in mammalian cells and causes reverse electron transfer in colonocytes

Sulfide (H₂S) is an inhibitor of mitochondrial cytochrome oxidase comparable to cyanide. In this study, poisoning of cells was observed with sulfide concentrations above 20 µM. Sulfide oxidation has been shown to take place in organisms/cells naturally exposed to sulfide. Sulfide is released as a result of metabolism of sulfur containing amino acids. Although in mammals sulfide exposure is not thought to be quantitatively important outside the colonic mucosa, our study shows that a majority of mammalian cells, by means of the mitochondrial sulfide quinone reductase (SQR), avidly consume sulfide as a fuel. The SQR activity was found in mitochondria isolated from mouse kidneys, liver, and heart. We demonstrate the precedence of the SQR over the mitochondrial complex I. This explains why the oxidation of the mineral substrate sulfide takes precedence over the oxidation of other (carbon-based) mitochondrial substrates. Consequently, if sulfide delivery rate remains lower than the SQR activity, cells maintain a non-toxic sulfide concentration (< 1 µM) in their external environment. In the colonocyte cell line HT-29, sulfide oxidation provided the first example of reverse electron transfer in living cells, such a transfer increasing sulfide tolerance. However, SQR activity was not detected in brain mitochondria and neuroblastoma cells. Consequently, the neural tissue would be more sensitive to sulfide poisoning. Our data disclose new constraints concerning the emerging signaling role of sulfide.     

Identification of the heme axial ligands in the cytochrome b562 of the Saccharomyces cerevisiae succinate dehydrogenase

Succinate dehydrogenase (SDH) plays a key role in energy generation by coupling the oxidation of succinate to the reduction of ubiquinone in the mitochondrial electron transport chain. The Saccharomyces cerevisiae SDH is composed of a catalytic dimer of the Sdh1p and Sdh2p subunits containing flavin adenine dinucleotide (FAD) and iron-sulfur clusters and a heme b-containing membrane-anchoring domain comprised of the Sdh3p and Sdh4p subunits. We systematically mutated all the histidine and cysteine residues in Sdh3p and Sdh4p to identify the residues involved in axial heme ligation. The mutants were characterized for growth on a non-fermentable carbon source, for enzyme assembly, for succinate-dependent quinone reduction, for heme b content, and for heme spectral properties. Mutation of Sdh3p His-46 or His-113 leads to a marked reduction in the catalytic efficiency of the enzyme for quinone reduction, suggesting that these residues form part of a quinone-binding site. We identified Sdh3p His-106 and Sdh4p Cys-78 as the most probable axial ligands for cytochrome b(562). Replacement of His-106 or Cys-78 with an alanine residue leads to a marked reduction in cytochrome b(562) content and to altered heme spectral characteristics that are consistent with a direct perturbation of heme b environment. This is the first identification of a cysteine residue serving as an axial ligand for heme b in the SDH family of enzymes. Loss of cytochrome b(562) has no effect on enzyme assembly and quinone reduction; the role of the heme in enzyme structure and function is discussed.     

Transient Kinetic Analysis of Hydrogen Sulfide Oxidation Catalyzed by Human Sulfide Quinone Oxidoreductase

The first step in the mitochondrial sulfide oxidation pathway is catalyzed by sulfide quinone oxidoreductase (SQR), which belongs to the family of flavoprotein disulfide oxidoreductases. During the catalytic cycle, the flavin cofactor is intermittently reduced by sulfide and oxidized by ubiquinone, linking H₂S oxidation to the electron transfer chain and to energy metabolism. Human SQR can use multiple thiophilic acceptors, including sulfide, sulfite, and glutathione, to form as products, hydrodisulfide, thiosulfate, and glutathione persulfide, respectively. In this study, we have used transient kinetics to examine the mechanism of the flavin reductive half-reaction and have determined the redox potential of the bound flavin to be −123 ± 7 mV. We observe formation of an unusually intense charge-transfer (CT) complex when the enzyme is exposed to sulfide and unexpectedly, when it is exposed to sulfite. In the canonical reaction, sulfide serves as the sulfur donor and sulfite serves as the acceptor, forming thiosulfate. We show that thiosulfate is also formed when sulfide is added to the sulfite-induced CT intermediate, representing a new mechanism for thiosulfate formation. The CT complex is formed at a kinetically competent rate by reaction with sulfide but not with sulfite. Our study indicates that sulfide addition to the active site disulfide is preferred under normal turnover conditions. However, under pathological conditions when sulfite concentrations are high, sulfite could compete with sulfide for addition to the active site disulfide, leading to attenuation of SQR activity and to an alternate route for thiosulfate formation.     

Response of Sulfide:Quinone Oxidoreductase to Sulfide Exposure in the Echiuran Worm Urechis unicinctus

Sulfide is a natural, widely distributed, poisonous substance, and sulfide:quinone oxidoreductase (SQR) is responsible for the initial oxidation of sulfide in mitochondria. In this study, we examined the response of SQR to sulfide exposure (25, 50, and 150 μM) at mRNA, protein, and enzyme activity levels in the body wall and hindgut of the echiuran worm Urechis unicinctus, a benthic organism living in marine sediments. The results revealed SQR mRNA expression during sulfide exposure in the body wall and hindgut increased in a time- and concentration-dependent manner that increased significantly at 12 h and continuously increased with time. At the protein level, SQR expression in the two tissues showed a time-dependent relationship that increased significantly at 12 h in 50 μM sulfide and 6 h in 150 μM, and then continued to increase with time while no significant increase appeared after 25 μM sulfide exposure. SQR enzyme activity in both tissues increased significantly in a time-dependent manner after 50 μM sulfide exposure. We concluded that SQR expression could be induced by sulfide exposure and that the two tissues studied have dissimilar sulfide metabolic patterns. A U. unicinctus sulfide-induced detoxification mechanism was also discussed.     

Comparison of catalytic activity and inhibitors of quinone reactions of succinate dehydrogenase (Succinate-ubiquinone oxidoreductase) and fumarate reductase (Menaquinol-fumarate oxidoreductase) from Escherichia coli.

Escherichia coli succinate-ubiquinone oxidoreductase (SQR) and menaquinol-fumarate reductase (QFR) are excellent model systems to understand the function of eukaryotic Complex II. They have structural and catalytic properties similar to their eukaryotic counterpart. An exception is that potent inhibitors of mammalian Complex II, such as thenoyltrifluoroacetone and carboxanilides, only weakly inhibit their bacterial counterparts. This lack of good inhibitors of quinone reactions and the higher level of side reactions in the prokaryotic enzymes has hampered the elucidation of the mechanism of quinone oxidation/reduction in E. coli Complex II. In this communication DT-diaphorase and an appropriate quinone are used to measure quinol-fumarate reductase activity and E. coli bo-oxidase and quinones are used to determine succinate-quinone reductase activity. Simple Michaelis kinetics are observed for both enzymes with ubiquinones and menaquinones in the succinate oxidase (forward) and fumarate reductase (reverse) reactions. The comparison of E. coli SQR and QFR demonstrates that 2-n-heptyl 4-hydroxyquinoline-N-oxide (HQNO) is a potent inhibitor of QFR in both assays; however, SQR is not sensitive to HQNO. A series of 2-alkyl-4,6-dinitrophenols and pentachlorophenol were found to be potent competitive inhibitors of both SQR and QFR. In addition, the isolated E. coli SQR complex demonstrates a mixed-type inhibition with carboxanilides, whereas the QFR complex is resistant to this inhibitor. The kinetic properties of SQR and QFR suggest that either ubiquinone or menaquinone operates at a single exchangeable site working in forward or reverse reactions. The pH activity profiles for E. coli QFR and SQR are similar showing maximal activity between pH 7.4 and 7.8, suggesting the importance of similar catalytic groups in quinol deprotonation and oxidation.     

硫醌氧化还原酶研究进展

硫化物是一种广泛分布的有毒物质。硫醌氧化还原酶是生物体硫化物代谢的一种关键酶。对该酶的发现与分布、序列特征、催化机制、催化特性、三维结构和生理功能等几个方面进行概述,并对该酶未来研究方向进行展望。     

Taxonomic distribution,structure/function relationship and metabolic context of the two families of sulfide dehydrogenases: SQR and FCSD

Hydrogen sulfide (H₂S) is a versatile molecule with different functions in living organisms: it can work as a metabolite of sulfur and energetic metabolism or as a signaling molecule in higher Eukaryotes. H₂S is also highly toxic since it is able to inhibit heme cooper oxygen reductases, preventing oxidative phosphorylation. Due to the fact that it can both inhibit and feed the respiratory chain, the immediate role of H₂S on energy metabolism crucially relies on its bioavailability, meaning that studying the central players involved in the H₂S homeostasis is key for understanding sulfide metabolism.Two different enzymes with sulfide oxidation activity (sulfide dehydrogenases) are known: flavocytochrome c sulfide dehydrogenase (FCSD), a sulfide:cytochrome c oxidoreductase; and sulfide:quinone oxidoreductase (SQR).In this work we performed a thorough bioinformatic study of SQRs and FCSDs and integrated all published data. We systematized several properties of these proteins: (i) nature of flavin binding, (ii) capping loops and (iii) presence of key amino acid residues. We also propose an update to the SQR classification system and discuss the role of these proteins in sulfur metabolism.     

Structural and computational analysis of the quinone-binding site of complex II (succinate-ubiquinone oxidoreductase): a mechanism of electron transfer and proton conduction during ubiquinone reduction

The transfer of electrons and protons between membrane-bound respiratory complexes is facilitated by lipid-soluble redox-active quinone molecules (Q). This work presents a structural analysis of the quinone-binding site (Q-site) identified in succinate:ubiquinone oxidoreductase (SQR) from Escherichia coli. SQR, often referred to as Complex II or succinate dehydrogenase, is a functional member of the Krebs cycle and the aerobic respiratory chain and couples the oxidation of succinate to fumarate with the reduction of quinone to quinol (QH(2)). The interaction between ubiquinone and the Q-site of the protein appears to be mediated solely by hydrogen bonding between the O1 carbonyl group of the quinone and the side chain of a conserved tyrosine residue. In this work, SQR was co-crystallized with the ubiquinone binding-site inhibitor Atpenin A5 (AA5) to confirm the binding position of the inhibitor and reveal additional structural details of the Q-site. The electron density for AA5 was located within the same hydrophobic pocket as ubiquinone at, however, a different position within the pocket. AA5 was bound deeper into the site prompting further assessment using protein-ligand docking experiments in silico. The initial interpretation of the Q-site was re-evaluated in the light of the new SQR-AA5 structure and protein-ligand docking data. Two binding positions, the Q(1)-site and Q(2)-site, are proposed for the E. coli SQR quinone-binding site to explain these data. At the Q(2)-site, the side chains of a serine and histidine residue are suitably positioned to provide hydrogen bonding partners to the O4 carbonyl and methoxy groups of ubiquinone, respectively. This allows us to propose a mechanism for the reduction of ubiquinone during the catalytic turnover of the enzyme.     

Alternative sites for proton entry from the cytoplasm to the quinone binding site in Escherichia coli succinate dehydrogenase

Escherichia coli succinate dehydrogenase (Sdh) belongs to the highly conserved complex II family of enzymes that reduce ubiquinone. These enzymes do not generate a protonmotive force during catalysis and are electroneutral. Because of its electroneutrality, the quinone reduction reaction must consume cytoplasmic protons which are released stoichiometrically during succinate oxidation. The X-ray crystal structure of E. coli Sdh shows that residues SdhB (G227), SdhC (D95), and SdhC (E101) are located at or near the entrance of a water channel that has been proposed to function as a proton wire connecting the cytoplasm to the quinone binding site. However, the pig and chicken Sdh enzymes show an alternative entrance to the water channel via the conserved SdhD (Q78) residue. In this study, site-directed mutants of these four residues were created and characterized by in vivo growth assays, in vitro activity assays, and electron paramagnetic resonance spectroscopy. We show that the observed water channel in the E. coli Sdh structure is the functional proton wire in vivo, while in vitro results indicate an alternative entrance for protons. In silico examination of the E. coli Sdh reveals a possible H-bonding network leading from the cytoplasm to the quinone binding site that involves SdhD (D15). On the basis of these results we propose an alternative proton pathway in E. coli Sdh that might be functional only in vitro.     

Limited reversibility of transmembrane proton transfer assisting transmembrane electron transfer in a dihaem-containing succinate:quinone oxidoreductase

Membrane protein complexes can support both the generation and utilisation of a transmembrane electrochemical proton potential (Δp), either by supporting transmembrane electron transfer coupled to protolytic reactions on opposite sides of the membrane or by supporting transmembrane proton transfer. The first mechanism has been unequivocally demonstrated to be operational for Δp-dependent catalysis of succinate oxidation by quinone in the case of the dihaem-containing succinate:menaquinone reductase (SQR) from the Gram-positive bacterium Bacillus licheniformis. This is physiologically relevant in that it allows the transmembrane potential Δp to drive the endergonic oxidation of succinate by menaquinone by the dihaem-containing SQR of Gram-positive bacteria. In the case of a related but different respiratory membrane protein complex, the dihaem-containing quinol:fumarate reductase (QFR) of the ?-proteobacterium Wolinella succinogenes, evidence has been obtained that both mechanisms are combined, so as to facilitate transmembrane electron transfer by proton transfer via a both novel and essential compensatory transmembrane proton transfer pathway (“E-pathway”). Although the reduction of fumarate by menaquinol is exergonic, it is obviously not exergonic enough to support the generation of a Δp. This compensatory “E-pathway” appears to be required by all dihaem-containing QFR enzymes and results in the overall reaction being electroneutral. However, here we show that the reverse reaction, the oxidation of succinate by quinone, as catalysed by W. succinogenes QFR, is not electroneutral. The implications for transmembrane proton transfer via the E-pathway are discussed.     

Purification and characterization of sulfide:quinone oxidoreductase from an acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans

Sulfide:quinone oxidoreductase (SQR) was purified from membrane of acidophilic chemolithotrophic bacterium Acidithiobacillus ferrooxidans NASF-1 cells grown on sulfur medium. It was composed of a single polypeptide with an apparent molecular mass of 47 kDa. The apparent K(m) values for sulfide and ubiquinone were 42 and 14 muM respectively. The apparent optimum pH for the SQR activity was about 7.0. A gene encoding a putative SQR of A. ferrooxidans NASF-1 was cloned and sequenced. The gene was expressed in Escherichia coli as a thioredoxin-fusion protein in inclusion bodies in an inactive form. A polyclonal antibody prepared against the recombinant protein reacted immunologically with the purified SQR. Western blotting analysis using the antibody revealed an increased level of SQR synthesis in sulfur-grown A. ferrooxidans NASF-1 cells, implying the involvement of SQR in elemental sulfur oxidation in sulfur-grown A. ferrooxidans NASF-1 cells.     

Three enzymatic activities catalyze the oxidation of sulfide to thiosulfate in mammalian and invertebrate mitochondria

Hydrogen sulfide is a potent toxin of aerobic respiration, but also has physiological functions as a signalling molecule and as a substrate for ATP production. A mitochondrial pathway catalyzing sulfide oxidation to thiosulfate in three consecutive reactions has been identified in rat liver as well as in the body-wall tissue of the lugworm, Arenicola marina. A membrane-bound sulfide : quinone oxidoreductase converts sulfide to persulfides and transfers the electrons to the ubiquinone pool. Subsequently, a putative sulfur dioxygenase in the mitochondrial matrix oxidizes one persulfide molecule to sulfite, consuming molecular oxygen. The final reaction is catalyzed by a sulfur transferase, which adds a second persulfide from the sulfide : quinone oxidoreductase to sulfite, resulting in the final product thiosulfate. This role in sulfide oxidation is an additional physiological function of the mitochondrial sulfur transferase, rhodanese.