sor (P29082, SOR_ACIAM) – Acidianus ambivalens sulfur oxygenase/reductase

Provenance note

Automated deep research was unavailable for this review (falcon and openai providers
both failed authentication). No -deep-research-*.md file exists. The notes below were
compiled manually from PubMed searches (NCBI E-utilities, query "sulfur oxygenase reductase
Acidianus ambivalens" / "... Desulfurolobus") and the cached publications in
publications/. Most cached papers are abstract-only; PMID:21747782 (Veith et al. 2011)
and PMID:36191023 (cryo-EM, 2022) have full text.

Identity

• 309 aa (308 after Met removal) homo-24-mer, ~871 kDa; Pfam PF07682 (SOR), dimeric
  alpha/beta-barrel fold. Organism formerly Desulfurolobus ambivalens; facultatively
  anaerobic, chemolithoautotrophic thermoacidophile (Sulfolobales).
• EC 1.13.11.55; Rhea:13957 (4 S + O2 + 4 H2O = 2 H2S + 2 sulfite + 6 H+).

Activity

• Purified from aerobically grown cells; absent from anaerobic cells. "In the presence of
  oxygen but not under a hydrogen atmosphere, the enzyme simultaneously produced sulfite,
  thiosulfate, and hydrogen sulfide from sulfur" PMID:2493451. Thiosulfate is mostly a
  non-enzymatic product: "thiosulfate was produced mainly in a chemical reaction between
  sulfite and sulfur" PMID:2493451. Optimum 85 C, range 50-108 C.
• sor transcript 38-42x higher in aerobically grown cells PMID:1522063.
• Described as "the initial enzyme in the sulphur-oxidation pathway of Acidianus
  ambivalens" PMID:15030315; same framing in PMID:21747782.
• Physiological role debate: PMID:31188531 Heterologous SOR + TQO restored sulfur
  oxidation in S. acidocaldarius DSM639 (but not autotrophy).

Iron centre and active site

• One Fe per subunit, high-spin ferric EPR g=4.3, Em -268 mV; "mononuclear iron centre"
  PMID:15030315.
• Mutagenesis: H86A, H90A, E114A inactive with no iron; C31A/S inactive with normal iron
  PMID:15970399.
• 1.7 A crystal structure PMID:16484493.
• Zn2+ inhibits by binding a His in a side channel, Hg2+/IAA bind active site cysteines
  PMID:21747782.

Quaternary structure / localization

• Icosatetramer, 432 symmetry, hollow sphere with internal nanocompartment
  [PMID:15698962; PMID:16484493 "Twenty-four monomers form a large hollow sphere enclosing
  a positively charged nanocompartment."].
• Cytoplasmic: PMID:21747782. Kletzin 1989 called it a "soluble" enzyme PMID:2493451. Note that
  in Acidianus tengchongensis the orthologous SOR was also found associated with the
  cytoplasmic membrane PMID:15691910; not shown for
  A. ambivalens.

GO considerations

• GO:0033755 sulfur oxygenase/reductase activity (IEA, EC mapping) – correct, core.
• GO:0005737 cytoplasm (IEA SubCell) – correct.
• Missing: GO:0019417 sulfur oxidation (definition "addition of oxygen to elemental
  sulfur" – exactly the oxygenase half-reaction SOR performs). Comparator check: GO:0019417
  is carried by >2000 sulfur-oxidizing enzymes (e.g. SoxA family, by IEA), so the term is
  used for the enzymes performing sulfur oxidation steps; SOR itself catalyses the step.
• NOT GO:0070220 aerobic sulfur oxidation: its definition ties it to the sulfur
  dioxygenase reaction (a distinct enzyme, EC 1.13.11.18), so although SOR-dependent sulfur
  oxidation is O2-dependent, the term does not fit. Ontology pitfall for archaeal SORs.
• Missing: GO:0005506 iron ion binding – crystal structure + mutagenesis (IDA). The UniProt
  entry carries KW Iron / Metal-binding and FT BINDING Fe (86, 90, 114) but no KW2GO or
  UniRule metal-binding row was propagated into GOA.
• Only 5 proteins in all of GOA carry GO:0033755 (all IEA); no experimental GO annotations
  for any SOR despite four decades of biochemistry and multiple structures – archaeal
  under-annotation pattern.