sor encodes the sulfur oxygenase/reductase (SOR, EC 1.13.11.55) of the thermoacidophilic, chemolithoautotrophic and facultatively anaerobic crenarchaeon Acidianus ambivalens (Sulfolobales). SOR is a soluble cytoplasmic enzyme, induced under aerobic growth, that catalyses an oxygen-dependent disproportionation of elemental sulfur: one part of the sulfur is oxygenated to sulfite while another part is reduced to hydrogen sulfide, with thiosulfate arising mainly non-enzymatically from sulfite and sulfur. It is considered the initial enzyme of sulfur oxidation in Acidianus, supplying sulfite and thiosulfate to downstream membrane-bound oxidoreductases, although an ancillary "recycling" role has also been proposed. Twenty-four identical ~35 kDa subunits assemble into a thermostable hollow sphere (432 symmetry, ~871 kDa) enclosing a positively charged nanocompartment; each subunit has an inward-facing active-site pocket containing a low-potential mononuclear non-haem iron bound by a 2-His-1-carboxylate facial triad (His86, His90, Glu114) and an essential cysteine persulfide (Cys31). Substrate enters through apolar chimney-like channels and pores; Zn2+ inhibits the enzyme by binding in a side channel.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Cytoplasm, from the UniProt subcellular location mapping. SOR was purified as a soluble enzyme and is described as a cytoplasmic hollow 24-mer; its substrate chemistry (polysulfide stability) is consistent with the near-neutral cytoplasm. Reason: SOR has no signal peptide or transmembrane segment, was purified as a soluble protein from A. ambivalens, and is consistently described as cytoplasmic. A partial membrane association reported for the A. tengchongensis ortholog (PMID:15691910) has not been shown for A. ambivalens and does not contradict the cytoplasmic location. Supporting Evidence: PMID:21747782 The spherical, hollow, cytoplasmic enzyme is composed of 24 identical subunits PMID:2493451 a soluble oxygenase reductase (SOR) was purified |
| GO:0033755 sulfur oxygenase/reductase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Sulfur oxygenase/reductase activity, from the EC/Rhea mapping. This is the defining activity of this protein, which is the type enzyme for EC 1.13.11.55; the GO term definition itself cites the A. ambivalens sor gene paper (PMID:1522063). Activity was demonstrated on the native enzyme, the recombinant enzyme and mutants. Reason: Directly demonstrated by enzyme assays on purified native and recombinant A. ambivalens SOR (oxygen-dependent production of sulfite and H2S from elemental sulfur), with iron-ligand and Cys31 mutants abolishing both partial reactions. The IEA row is correct and represents the core function; experimental (IDA) support exists but has not been captured in GOA. Supporting Evidence: PMID:2493451 In the presence of oxygen but not under a hydrogen atmosphere, the enzyme simultaneously produced sulfite, thiosulfate, and hydrogen sulfide from sulfur. PMID:16484493 which catalyzes an oxygen-dependent disproportionation of elemental sulfur PMID:15970399 In all cases the oxygenase and reductase partial reactions were equally affected. |
| GO:0019417 sulfur oxidation | IDA PMID:15030315 The sulphur oxygenase reductase from Acidianus ambivalens is... | NEW | Summary: Proposed new process annotation. SOR itself catalyses the oxygenation of elemental sulfur to sulfite, which is exactly the GO:0019417 definition ("addition of oxygen to elemental sulfur"), and it is described as the initial enzyme of the sulfur-oxidation pathway of A. ambivalens. Reason: Participation test: SOR performs the step (the oxygenase half-reaction on S0), it is not a substrate or regulator. Comparator check: GO:0019417 is used in GOA for enzymes that catalyse sulfur-oxidation steps (over 2000 rows, e.g. SoxA family proteins), so annotating the enzyme that oxygenates S0 follows the existing convention. GO:0070220 aerobic sulfur oxidation was deliberately not used because its definition specifies the sulfur dioxygenase reaction, a different enzyme. The in vivo debate (initial step vs. a recycling role; PMID:31188531) concerns pathway position, not whether SOR oxidises sulfur. Supporting Evidence: PMID:15030315 The SOR (sulphur oxygenase reductase) is the initial enzyme in the sulphur-oxidation pathway of Acidianus ambivalens. PMID:21747782 The sulfur oxygenase reductase (SOR) is the initial enzyme of the sulfur oxidation pathway in the thermoacidophilic Archaeon Acidianus ambivalens. PMID:31188531 Here, heterologous expression of both SOR and membrane-bound thiosulphate-quinone oxidoreductase (TQO) from Sulfolobus tokodaii 'restored' sulphur oxidation capacity in Sulfolobus acidocaldarius DSM639 |
| GO:0005506 iron ion binding | IDA PMID:16484493 X-ray Structure of a self-compartmentalizing sulfur cycle me... | NEW | Summary: Proposed new function annotation. Each subunit binds one mononuclear non-haem iron, shown by chemical analysis and EPR, mapped by mutagenesis (H86A, H90A, E114A abolish iron incorporation) and seen in the 1.7 A crystal structure as a 2-His-1-carboxylate facial triad. Reason: Iron is the catalytic cofactor (the likely site of both sulfur oxidation and reduction), so iron ion binding is a mechanistic feature of the enzyme supported by three independent experimental approaches on the A. ambivalens protein. UniProt records Fe BINDING sites and the Iron/Metal-binding keywords, but no corresponding GO row exists in GOA. Kept non-core: it is a cofactor-binding feature subsidiary to the sulfur oxygenase/reductase activity. Supporting Evidence: PMID:16484493 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 PMID:15030315 Iron was found in the wild-type enzyme at a stoichiometry of one iron atom/subunit. PMID:15970399 Mutations of the suspected iron-binding residues H86, H90 and E114 to alanine resulted in inactive enzyme with no iron incorporated |
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Download this section (compressed HTML)Q: Is SOR the obligatory first step of elemental sulfur oxidation in A. ambivalens in vivo, or mainly a recycling enzyme acting on sulfur produced by other pathways (e.g. sulfide:quinone or thiosulfate oxidation)?
Q: How are the reactive products (sulfite, H2S, thiosulfate) channelled from the SOR nanocompartment to downstream membrane enzymes (sulfite:acceptor oxidoreductase, TQO), and does A. ambivalens SOR associate with the membrane as reported for the A. tengchongensis ortholog?
Q: Should GO's "aerobic sulfur oxidation" (GO:0070220), currently defined by the sulfur dioxygenase reaction, be broadened or given a sibling so that O2-dependent SOR-initiated sulfur oxidation in archaea and bacteria can be captured precisely?
Experiment: Construct an sor deletion (or CRISPR knockdown) in a genetically tractable sulfur-oxidizing Sulfolobales host and measure growth and sulfate production on S0 under aerobic chemolithoautotrophic conditions.
Hypothesis: If SOR initiates sulfur oxidation, its loss will abolish or severely reduce aerobic growth on elemental sulfur and sulfate formation.
Experiment: Immunogold electron microscopy and cell fractionation of aerobically grown A. ambivalens, combined with crosslinking/pull-down, to detect SOR association with the cytoplasmic membrane and with TQO or sulfite oxidoreductase.
Hypothesis: A fraction of SOR is membrane-associated in proximity to downstream oxidoreductases, enabling product channelling.
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