The sulphur oxygenase reductase from Acidianus ambivalens is a multimeric protein containing a low-potential mononuclear non-haem iron centre

Biochem J. 2004 Jul 1;381(Pt 1):137-46. doi: 10.1042/BJ20040003.


The SOR (sulphur oxygenase reductase) is the initial enzyme in the sulphur-oxidation pathway of Acidianus ambivalens. Expression of the sor gene in Escherichia coli resulted in active, soluble SOR and in inclusion bodies from which active SOR could be refolded as long as ferric ions were present in the refolding solution. Wild-type, recombinant and refolded SOR possessed indistinguishable properties. Conformational stability studies showed that the apparent unfolding free energy in water is approx. 5 kcal x mol(-1) (1 kcal=4.184 kJ), at pH 7. The analysis of the quaternary structures showed a ball-shaped assembly with a central hollow core probably consisting of 24 subunits in a 432 symmetry. The subunits form homodimers as the building blocks of the holoenzyme. Iron was found in the wild-type enzyme at a stoichiometry of one iron atom/subunit. EPR spectroscopy of the colourless SOR resulted in a single isotropic signal at g=4.3, characteristic of high-spin ferric iron. The signal disappeared upon reduction with dithionite or incubation with sulphur at elevated temperature. Thus both EPR and chemical analysis indicate the presence of a mononuclear iron centre, which has a reduction potential of -268 mV at pH 6.5. Protein database inspection identified four SOR protein homologues, but no other significant similarities. The spectroscopic data and the sequence comparison led to the proposal that the Acidianus ambivalens SOR typifies a new type of non-haem iron enzyme containing a mononuclear iron centre co-ordinated by carboxylate and/or histidine ligands.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acidianus / enzymology
  • Amino Acid Sequence / genetics
  • Archaeal Proteins / chemistry
  • Archaeal Proteins / genetics
  • Archaeal Proteins / metabolism
  • Archaeal Proteins / ultrastructure
  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Bacterial Proteins / ultrastructure
  • Dimerization
  • Electron Spin Resonance Spectroscopy / methods
  • Escherichia coli / genetics
  • Gene Expression Regulation, Enzymologic / genetics
  • Heme*
  • Iron / metabolism*
  • Microscopy, Electron / methods
  • Molecular Sequence Data
  • Multienzyme Complexes / chemistry
  • Multienzyme Complexes / genetics
  • Multienzyme Complexes / metabolism
  • Multienzyme Complexes / ultrastructure
  • Oxidation-Reduction
  • Oxidoreductases / chemistry*
  • Oxidoreductases / genetics
  • Oxidoreductases / metabolism
  • Oxidoreductases / ultrastructure
  • Oxidoreductases Acting on Sulfur Group Donors
  • Protein Conformation
  • Protein Denaturation / genetics
  • Protein Structure, Secondary
  • Recombinant Fusion Proteins / chemistry
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / ultrastructure
  • Substrate Specificity
  • Sulfur / metabolism


  • Archaeal Proteins
  • Bacterial Proteins
  • Multienzyme Complexes
  • Recombinant Fusion Proteins
  • Heme
  • Sulfur
  • Iron
  • Oxidoreductases
  • Oxidoreductases Acting on Sulfur Group Donors
  • soR protein, Acidianus ambivalens