Identification of core active site residues of the sulfur oxygenase reductase fromAcidianus ambivalensby site-directed mutagenesis

Cryo-electron structures of the extreme thermostable enzymes Sulfur Oxygenase Reductase and Lumazine Synthase

Thermostable enzymes have the potential for use in a wide variety of biotechnological applications. Cryo-electron microscopy (cryo-EM) enables the imaging of biomolecules in their native aqueous environment. Here, we present high resolution cryo-EM structures of two thermostable enzymes that exhibit multimeric cage-like structures arranged into two different point-group symmetries. First, we determined the structure of the Sulfur Oxygenase Reductase (SOR) enzyme that catalyzes both the oxygenation and disproportionation of elemental sulfur in Archea and is composed of 24 homomeric units each of MW ≃ 35 kDa arranged in octahedral symmetry. The structure of SOR from Acidianus ambivalens (7X9W) was determined at 2.78 Å resolution. The active site of each subunit inside the central nanocompartment is composed of Fe3+ coordinated to two water molecules and the three amino acids (H86, H90 and E114). Second, we determined the structure of Lumazine Synthase (LS) from Aquifex aeolicus (7X7M) at 2.33 Å resolution. LS forms a cage-like structure consisting of 60 identical subunits each of MW ≃ 15 kDa arranged in a strict icosahedral symmetry. The LS subunits are interconnected by ion-pair network. Due to their thermostability and relatively easy purification scheme, both SOR and LS can serve as a model for the catalytic and structural characterization of biocatalysts as well as a benchmark for cryo-EM sample preparation, optimization of the acquisition parameters and 3D reconstruction.

A review on bacterial and archaeal thermostable sulfur oxidoreductases (SORS)-an insight into the biochemical, molecular and in-silico structural comparative analysis of a neglected thermostable enzyme of industrial significance

Diverse thermophilic microorganisms with the potential to withstand extreme physiological conditions have long been investigated and explored for human commercial benefit. Thermozymes with distinct functional and structural properties isolated from these thermophiles are known to have high thermostability without significant loss of specific enzyme activity. Thermophiles isolated and characterised from the thermophilic ecological niche of India are well documented. There is a plethora of work in the literature emphasising its industrial significance. However, in-depth knowledge of the thermophilic oxidoreductase group of enzymes (Oxizymes) is restricted. Sulfur Oxygenase Reductases or Sulfur Oxygen-Reductases (SORs) are a group of thermophilic oxizymes reported predominantly from thermophilic and mesophilic archaea and bacteria, which catalyse oxygen-dependent disproportionation reactions of elemental sulfur, producing sulfite, thiosulfate, and sulphide. There have been few reports on isolated and characterised SORs from the Indian geothermal niche. The review article will highlight the SORs reported till date with a concise overview of different archaeal and bacterial species producing the enzymes. Based on the literature available till date, characteristics including physico-chemical properties, amino acid sequence homology, conserved motifs and their 3D structure comparison have been discussed. In-silico sequence and structure level preliminary comparative analysis of various SORs has also been discussed. However, a few SORs whose structural information is not reported in the protein data bank have been modelled to enrich our analysis.

Crystallographic and cryogenic electron microscopic structures and enzymatic characterization of sulfur oxygenase reductase from Sulfurisphaera tokodaii

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Sulfur oxygenase reductase (SOR) was biochemically and structurally characterized.

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High resolution structures of SOR were determined by crystallography and cryo-EM.

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Twenty-four identical subunits of SOR form a hollow sphere.

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Catalytic components exhibited different features in the crystal and cryo-EM structures.

Sulfur oxygenase reductases (SORs) are present in thermophilic and mesophilic archaea and bacteria, and catalyze oxygen-dependent oxygenation and disproportionation of elemental sulfur. SOR has a hollow, spherical homo-24-mer structure and reactions take place at active sites inside the chamber. The crystal structures of SORs from Acidianus species have been reported. However, the states of the active site components (mononuclear iron and cysteines) and the entry and exit paths of the substrate and products are still in dispute. Here, we report the biochemical and structural characterizations of SORs from the thermoacidophilic archaeon Sulfurisphaera tokodaii (StSOR) and present high-resolution structures determined by X-ray crystallography and cryogenic electron microscopy (cryo-EM). The crystal structure of StSOR was determined at 1.73 Å resolution. At the catalytic center, iron is ligated to His86, His90, Glu114, and two water molecules. Three conserved cysteines in the cavity are located 9.5–13 Å from the iron and were observed as free thiol forms. A mutational analysis indicated that the iron and one of the cysteines (Cys31) were essential for both activities. The cryo-EM structure was determined at 2.24 Å resolution using an instrument operating at 200 kV. The two structures determined by different methodologies showed similar main chain traces, but the maps exhibited different features at catalytically important components. A possible role of StSOR in the sulfur metabolism of S. tokodaii (an obligate aerobe) is discussed based on this study. Given the high resolution achieved in this study, StSOR was shown to be a good benchmark sample for cryo-EM.

Variations in microbial community structure and functional gene expression in bio-treatment processes with odorous pollutants

Engineered microbial ecosystems in biofilters have been widely applied to treat odorous gases from industrial emissions. Variations in microbial community structure and function associated with the removal of odorous gases by biofilters are largely unknown. This study performed a metagenomic analysis to discover shifts in microbial community structures in a commercial scale biofilter after treating odorous gas. Our study identified 175,675 functional genes assigned into 43 functional KEGG pathways. Based on the unigene sequences, there were significant changes in microbial community structures in the biofilter after treating odorous gas. The dominant genera were Thiobacillus and Oceanicaulis before the treatment, and were Acidithiobacillus and Ferroplasma after the treatment. A clustering analysis showed that the number of down-regulated microbes exceeded the number of up-regulated microbes, suggesting that odorous gas treatment reduced in microbial community structures. A differential expression analysis identified 29,975 up- and 452,599 down-regulated genes. An enrichment analysis showed 17 classic types of xenobiotic biodegradation pathways. The results identified 16 and 15 genes involved in ammonia and sulfite metabolism, respectively; an analysis of their relative abundance identified several up-regulated genes, which may be efficient genes involved in removing odorous gases. The data provided in this study demonstrate the changes in microbial communities and help identify the dominant microflora and genes that play key roles in treating odorous gases.

A Sulfur Oxygenase from the Haloalkaliphilic Bacterium Thioalkalivibrio paradoxus with Atypically Low Reductase Activity

Sequence comparisons showed that the sulfur oxygenase reductase (SOR) of the haloalkaliphilic bacterium Thioalkalivibrio paradoxus Arh 1 (TpSOR) is branching deeply within dendrograms of these proteins (29 to 34% identity). A synthetic gene encoding TpSOR expressed in Escherichia coli resulted in a protein 14.7 ± 0.9 nm in diameter and an apparent molecular mass of 556 kDa. Sulfite and thiosulfate were formed from elemental sulfur in a temperature range of 10 to 98°C (optimum temperature ≈ 80°C) and a pH range of 6 to 11.5 (optimum pH ≈ 9; 308 ± 78 U/mg of protein). Sulfide formation had a maximum specific activity of 0.03 U/mg, or <1% of the corresponding activity of other SORs. Hence, reductase activity seems not to be an integral part of the reaction mechanism. TpSOR was most active at NaCl or glycine betaine concentrations of 0 to 1 M, although 0.2% of the maximal activity was detected even at 5 M NaCl and 4 M betaine. The melting point of TpSOR was close to 80°C, when monitored by circular dichroism spectroscopy or differential scanning fluorimetry; however, the denaturation kinetics were slow: 55% of the residual activity remained after 25 min of incubation at 80°C. Site-directed mutagenesis showed that the active-site residue Cys₄₄ is essential for activity, whereas alanine mutants of the two other conserved cysteines retained about 0.5% residual activity. A model of the sulfur metabolism in T. paradoxus is discussed.

IMPORTANCE

Sulfur oxygenase reductases (SORs) are the only enzymes catalyzing an oxygen-dependent disproportionation of elemental sulfur and/or polysulfides to sulfite, thiosulfate, and hydrogen sulfide. SORs are known from mesophilic and extremophilic archaea and bacteria. All SORs seem to form highly thermostable 24-subunit hollow spheres. They carry a low-potential mononuclear nonheme iron in the active site and an indispensable cysteine; however, their exact reaction mechanisms are unknown. Typically, the reductase activity of SORs is in the range of 5 to 50% of the oxygenase activity, but mutagenesis studies had so far failed to identify residues crucial for the reductase reaction. We describe here the first SOR, which is almost devoid of the reductase reaction and which comes from a haloalkaliphilic bacterium.

Theoretical Model of the Structure and the Reaction Mechanisms of Sulfur Oxygenase Reductase in Acidithiobacillus thiooxidans

Sulfur oxygenase reductase (SOR), which is thought to be an important enzyme involved in sulfur oxidation in many microorganisms, may play a key role in sulfur oxidation in Acidithiobacillusthiooxidans. Draft genome sequence of A. thiooxidans A01 indicated the presence of sulfur oxygenase reductase gene (sor). The complementary DNA fragment was speculated to encode a putative 311-aa full-length protein SOR. Structural analysis of SOR revealed that three cysteines located in the two conserved domains, C32 at V-G-P-K-V-C32 as well as C102 and C105 at C102-X-X-C105, might form the substrate activation and binding site. It was proposed that conserved motif H87-X3-H91-X23-E115 acted as ligands might combine with iron atom to constitute a mononuclear non-heme iron center, catalyzing the oxidation reaction of substrate.

Sulfur Oxygenase Reductase (Sor) in the Moderately Thermoacidophilic Leaching Bacteria: Studies in Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus

The sulfur oxygenase reductase (Sor) catalyzes the oxygen dependent disproportionation of elemental sulfur, producing sulfite, thiosulfate and sulfide. Being considered an “archaeal like” enzyme, it is also encoded in the genomes of some acidophilic leaching bacteria such as Acidithiobacillus caldus, Acidithiobacillus thiooxidans, Acidithiobacillus ferrivorans and Sulfobacillus thermosulfidooxidans, among others. We measured Sor activity in crude extracts from Sb. thermosulfidooxidans DSM 9293^T. The optimum temperature for its oxygenase activity was achieved at 75 °C, confirming the “thermophilic” nature of this enzyme. Additionally, a search for genes probably involved in sulfur metabolism in the genome sequence of Sb. thermosulfidooxidans DSM 9293^T was done. Interestingly, no sox genes were found. Two sor genes, a complete heterodisulfidereductase (hdr) gene cluster, three tetrathionate hydrolase (tth) genes, three sulfide quinonereductase (sqr), as well as the doxD component of a thiosulfate quinonereductase (tqo) were found. Seven At. caldus strains were tested for Sor activity, which was not detected in any of them. We provide evidence that an earlier reported Sor activity from At. caldus S1 and S2 strains most likely was due to the presence of a Sulfobacillus contaminant.

Whole-genome sequencing reveals novel insights into sulfur oxidation in the extremophile Acidithiobacillus thiooxidans

BACKGROUND

Acidithiobacillus thiooxidans (A. thiooxidans), a chemolithoautotrophic extremophile, is widely used in the industrial recovery of copper (bioleaching or biomining). The organism grows and survives by autotrophically utilizing energy derived from the oxidation of elemental sulfur and reduced inorganic sulfur compounds (RISCs). However, the lack of genetic manipulation systems has restricted our exploration of its physiology. With the development of high-throughput sequencing technology, the whole genome sequence analysis of A. thiooxidans has allowed preliminary models to be built for genes/enzymes involved in key energy pathways like sulfur oxidation.

RESULTS

The genome of A. thiooxidans A01 was sequenced and annotated. It contains key sulfur oxidation enzymes involved in the oxidation of elemental sulfur and RISCs, such as sulfur dioxygenase (SDO), sulfide quinone reductase (SQR), thiosulfate:quinone oxidoreductase (TQO), tetrathionate hydrolase (TetH), sulfur oxidizing protein (Sox) system and their associated electron transport components. Also, the sulfur oxygenase reductase (SOR) gene was detected in the draft genome sequence of A. thiooxidans A01, and multiple sequence alignment was performed to explore the function of groups of related protein sequences. In addition, another putative pathway was found in the cytoplasm of A. thiooxidans, which catalyzes sulfite to sulfate as the final product by phosphoadenosine phosphosulfate (PAPS) reductase and adenylylsulfate (APS) kinase. This differs from its closest relative Acidithiobacillus caldus, which is performed by sulfate adenylyltransferase (SAT). Furthermore, real-time quantitative PCR analysis showed that most of sulfur oxidation genes were more strongly expressed in the S0 medium than that in the Na2S2O3 medium at the mid-log phase.

CONCLUSION

Sulfur oxidation model of A. thiooxidans A01 has been constructed based on previous studies from other sulfur oxidizing strains and its genome sequence analyses, providing insights into our understanding of its physiology and further analysis of potential functions of key sulfur oxidation genes.

Site-specific mutagenesis and functional analysis of active sites of sulfur oxygenase reductase from Gram-positive moderate thermophile Sulfobacillus acidophilus TPY

Sequence alignments revealed that the conserved motifs of SORSa which formed an independent branch between archaea and Gram-negative bacteria SORs according to the phylogenetic relationship were similar with the archaea and Gram-negative bacteria SORs. In order to investigate the active sites of SORSa, cysteines 31, 101 and 104 (C31, C101, C104), histidines 86 and 90 (H86 and H90) and glutamate 114 (E114) of SORSa were chosen as the target amino acid residues for site-specific mutagenesis. The wild type and six mutant SORs were expressed in E. coli BL21, purified and confirmed by SDS-PAGE and Western blotting analysis. Enzyme activity determination revealed that the active sites of SORSa were identical with the archaea and Gram-negative bacteria SORs reported. Replacement of any cysteine residues reduced SOR activity by 53-100%, while the mutants of H86A, H90A and E114A lost their enzyme activities largely, only remaining 20%, 19% and 32% activity of the wild type SOR respectively. This study will enrich our awareness for active sites of SOR in a Gram-positive bacterium.

Sulfur metabolism in archaea reveals novel processes

Studies on sulfur metabolism in archaea have revealed many novel enzymes and pathways and have advanced our understanding on metabolic processes, not only of the archaea, but of biology in general. A variety of dissimilatory sulfur metabolisms, i.e. reactions used for energy conservation, are found in archaea from both the Crenarchaeota and Euryarchaeota phyla. Although not yet fully characterized, major processes include aerobic elemental sulfur (S(0)) oxidation, anaerobic S(0) reduction, anaerobic sulfate/sulfite reduction and anaerobic respiration of organic sulfur. Assimilatory sulfur metabolism, i.e. reactions used for biosynthesis of sulfur-containing compounds, also possesses some novel features. Cysteine biosynthesis in some archaea uses a unique tRNA-dependent pathway. Fe-S cluster biogenesis in many archaea differs from that in bacteria and eukaryotes and requires unidentified components. The eukaryotic ubiquitin system is conserved in archaea and involved in both protein degradation and biosynthesis of sulfur-containing cofactors. Lastly, specific pathways are utilized for the biosynthesis of coenzyme M and coenzyme B, the sulfur-containing cofactors required for methanogenesis.

The sulfur oxygenase reductase from the mesophilic bacterium Halothiobacillus neapolitanus is a highly active thermozyme

A biochemical, biophysical, and phylogenetic study of the sulfur oxygenase reductase (SOR) from the mesophilic gammaproteobacterium Halothiobacillus neapolitanus (HnSOR) was performed in order to determine the structural and biochemical properties of the enzyme. SOR proteins from 14 predominantly chemolithoautotrophic bacterial and archaeal species are currently available in public databases. Sequence alignment and phylogenetic analysis showed that they form a coherent protein family. The HnSOR purified from Escherichia coli after heterologous gene expression had a temperature range of activity of 10 to 99°C with an optimum at 80°C (42 U/mg protein). Sulfite, thiosulfate, and hydrogen sulfide were formed at various stoichiometries in a range between pH 5.4 and 11 (optimum pH 8.4). Circular dichroism (CD) spectroscopy and dynamic light scattering showed that the HnSOR adopts secondary and quaternary structures similar to those of the 24-subunit enzyme from the hyperthermophile Acidianus ambivalens (AaSOR). The melting point of the HnSOR was ≈20°C lower than that of the AaSOR, when analyzed with CD-monitored thermal unfolding. Homology modeling showed that the secondary structure elements of single subunits are conserved. Subtle changes in the pores of the outer shell and increased flexibility might contribute to activity at low temperature. We concluded that the thermostability was the result of a rigid protein core together with the stabilizing effect of the 24-subunit hollow sphere.

An Extracellular Tetrathionate Hydrolase from the Thermoacidophilic Archaeon Acidianus Ambivalens with an Activity Optimum at pH 1

BACKGROUND

The thermoacidophilic and chemolithotrophic archaeon Acidianus ambivalens is routinely grown with sulfur and CO(2)-enriched air. We had described a membrane-bound, tetrathionate (TT) forming thiosulfate:quinone oxidoreductase. Here we describe the first TT hydrolase (TTH) from Archaea.

RESULTS

A. ambivalens cells grown aerobically with TT as sole sulfur source showed doubling times of 9 h and final cell densities of up to 8 × 10(8)/ml. TTH activity (≈0.28 U/mg protein) was found in cell-free extracts of TT-grown but not of sulfur-grown cells. Differential fractionation of freshly harvested cells involving a pH shock showed that about 92% of the TTH activity was located in the pseudo-periplasmic fraction associated with the surface layer, while 7.3% and 0.3% were present in the soluble and membrane fractions, respectively. The enzyme was enriched 54-fold from the cytoplasmic fraction and 2.1-fold from the pseudo-periplasmic fraction. The molecular mass of the single subunit was 54 kDa. The optimal activity was at or above 95°C at pH 1. Neither PQQ nor divalent cations had a significant effect on activity. The gene (tth1) was identified following N-terminal sequencing of the protein. Northern hybridization showed that tth1 was transcribed in TT-grown cells in contrast to a second paralogous tth2 gene. The deduced amino acid sequences showed similarity to the TTH from Acidithiobacillus and other proteins from the PQQ dehydrogenase superfamily. It displayed a β-propeller structure when being modeled, however, important residues from the PQQ-binding site were absent.

CONCLUSION

The soluble, extracellular, and acidophilic TTH identified in TT-grown A. ambivalens cells is essential for TT metabolism during growth but not for the downstream processing of the TQO reaction products in S°-grown cells. The liberation of TTH by pH shock from otherwise intact cells strongly supports the pseudo-periplasm hypothesis of the S-layer of Archaea.

Substrate pathways and mechanisms of inhibition in the sulfur oxygenase reductase of acidianus ambivalens

BACKGROUND

The sulfur oxygenase reductase (SOR) is the initial enzyme of the sulfur oxidation pathway in the thermoacidophilic Archaeon Acidianus ambivalens. The SOR catalyzes an oxygen-dependent sulfur disproportionation to H(2)S, sulfite and thiosulfate. The spherical, hollow, cytoplasmic enzyme is composed of 24 identical subunits with an active site pocket each comprising a mononuclear non-heme iron site and a cysteine persulfide. Substrate access and product exit occur via apolar chimney-like protrusions at the fourfold symmetry axes, via narrow polar pores at the threefold symmetry axes and via narrow apolar pores within in each subunit. In order to investigate the function of the pores we performed site-directed mutagenesis and inhibitor studies.

RESULTS

Truncation of the chimney-like protrusions resulted in an up to sevenfold increase in specific enzyme activity compared to the wild type. Replacement of the salt bridge-forming Arg(99) residue by Ala at the threefold symmetry axes doubled the activity and introduced a bias toward reduced reaction products. Replacement of Met(296) and Met(297), which form the active site pore, lowered the specific activities by 25-55% with the exception of an M(296)V mutant. X-ray crystallography of SOR wild type crystals soaked with inhibitors showed that Hg(2+) and iodoacetamide (IAA) bind to cysteines within the active site, whereas Zn(2+) binds to a histidine in a side channel of the enzyme. The Zn(2+) inhibition was partially alleviated by mutation of the His residue.

CONCLUSIONS

The expansion of the pores in the outer shell led to an increased enzyme activity while the integrity of the active site pore seems to be important. Hg(2+) and IAA block cysteines in the active site pocket, while Zn(2+) interferes over a distance, possibly by restriction of protein flexibility or substrate access or product exit.

Crystal structure studies on sulfur oxygenase reductase from Acidianus tengchongensis

Sulfur oxygenase reductase (SOR) simultaneously catalyzes oxidation and reduction of elemental sulfur to produce sulfite, thiosulfate, and sulfide in the presence of molecular oxygen. In this study, crystal structures of wild type and mutants of SOR from Acidianus tengchongensis (SOR-AT) in two different crystal forms were determined and it was observed that 24 identical SOR monomers form a hollow sphere. Within the icosatetramer sphere, the tetramer and trimer channels were proposed as the paths for the substrate and products, respectively. Moreover, a comparison of SOR-AT with SOR-AA (SOR from Acidianus ambivalens) structures showed that significant differences existed at the active site. Firstly, Cys31 is not persulfurated in SOR-AT structures. Secondly, the iron atom is five-coordinated rather than six-coordinated, since one of the water molecules ligated to the iron atom in the SOR-AA structure is lost. Consequently, the binding sites of substrates and a hypothetical catalytic process of SOR were proposed.

First characterisation of the active oligomer form of sulfur oxygenase reductase from the bacterium Aquifex aeolicus

Sulfur oxygenase reductase (SOR) enzyme is responsible for the initial oxidation step of elemental sulfur in archaea. Curiously, Aquifex aeolicus, a hyperthermophilic, chemolithoautotrophic and microaerophilic bacterium, has the SOR-encoding gene in its genome. We showed, for the first time the presence of the SOR enzyme in A. aeolicus, its gene was cloned and recombinantly expressed in Escherichia coli and the protein was purified and characterised. It is a 16 homo-oligomer of approximately 600 kDa that contains iron atoms indispensable for the enzyme activity. The optimal temperature of SOR activity is 80 degrees C and it is inactive at 20 degrees C. Studies of the factors involved in getting the fully active molecule at high temperature show clearly that (1) incubation at high temperature induces more homogeneous form of the enzyme, (2) conformational changes observed at high temperature are required to get the fully active molecule and (3) acquisition of an active conformation induced by the temperature seems to be more important than the subunit number. Differences between A. aeolicus SOR and the archaea SORs are described.

Novel bacterial sulfur oxygenase reductases from bioreactors treating gold-bearing concentrates

The microbial community and sulfur oxygenase reductases of metagenomic DNA from bioreactors treating gold-bearing concentrates were studied by 16S rRNA library, real-time polymerase chain reaction (RT-PCR), conventional cultivation, and molecular cloning. Results indicated that major bacterial species were belonging to the genera Acidithiobacillus, Leptospirillum, Sulfobacillus, and Sphingomonas, accounting for 6.3, 66.7, 18.8, and 8.3%, respectively; the sole archaeal species was Ferroplasma sp. (100%). Quantitative RT-PCR revealed that the 16S rRNA gene copy numbers (per gram of concentrates) of bacteria and archaea were 4.59 x 10(9) and 6.68 x 10(5), respectively. Bacterial strains representing Acidithiobacillus, Leptospirillum, and Sulfobacillus were isolated from the bioreactors. To study sulfur oxidation in the reactors, pairs of new PCR primers were designed for the detection of sulfur oxygenase reductase (SOR) genes. Three sor-like genes, namely, sor (Fx), sor (SA), and sor (SB) were identified from metagenomic DNAs of the bioreactors. The sor (Fx) is an inactivated SOR gene and is identical to the pseudo-SOR gene of Ferroplasma acidarmanus. The sor (SA) and sor (SB) showed no significant identity to any genes in GenBank databases. The sor (SB) was cloned and expressed in Escherichia coli, and SOR activity was determined. Quantitative RT-PCR determination of the gene densities of sor (SA) and sor (SB) were 1,000 times higher than archaeal 16S rRNA gene copy numbers, indicating that these genes were mostly impossible from archaea. Furthermore, with primers specific to the sor (SB) gene, this gene was PCR-amplified from the newly isolated Acidithiobacillus sp. strain SM-1. So far as we know, this is the first time to determine SOR activity originating from bacteria and to document SOR gene in bioleaching reactors and Acidithiobacillus species.

X-ray Structure of a Self-Compartmentalizing Sulfur Cycle Metalloenzyme

Numerous microorganisms oxidize sulfur for energy conservation and contribute to the global biogeochemical sulfur cycle. We have determined the 1.7 angstrom-resolution structure of the sulfur oxygenase reductase from the thermoacidophilic archaeon Acidianus ambivalens, which catalyzes an oxygen-dependent disproportionation of elemental sulfur. Twenty-four monomers form a large hollow sphere enclosing a positively charged nanocompartment. Apolar channels provide access for linear sulfur species. A cysteine persulfide and a low-potential mononuclear non-heme iron site ligated by a 2-His-1-carboxylate facial triad in a pocket of each subunit constitute the active sites, accessible from the inside of the sphere. The iron is likely the site of both sulfur oxidation and sulfur reduction.