A new structure-based classification of sulfide:quinone oxidoreductases

Proteins. 2010 Apr;78(5):1073-83. doi: 10.1002/prot.22665.


Sulfide:quinone oxidoreductases (SQR) are ubiquitous membrane-bound flavoproteins involved in sulfide detoxification, in sulfide-dependent energy conservation processes and potenatially in the homeostasis of the neurotransmitter sulfide. The first 2 structures of SQRs from the bacterium Aquifex aeolicus (Marcia et al., Proc Natl Acad Sci USA 2009; 106:9625-9630) and the archaeon Acidianus ambivalens (Brito et al., Biochemistry 2009; 48:5613-5622) were determined recently by X-ray crystallography revealing unexpected differences in the active sites and in flavin adenine dinucleotide binding. Besides the reciprocal differences, they show a different conformation of the active site compared with another sulfide oxidizing enzyme, the flavocytochrome c:sulfide dehydrogenase (FCSD) from Allochromatium vinosum (protein data bank id: 1FCD). In addition to the new structural data, the number of available SQR-like protein sequences is continuously increasing (Pham et al., Microbiology 2008; 154:3112-3121) and the SQR activity of new members of this protein family was recently proven too (Chan et al., J Bacteriol 2009; 191:1026-1034). In the light of the new data, here we revisit the previously proposed contradictory SQR classification and we define new structure-based sequence fingerprints that support a subdivision of the SQR family into six groups. Our report summarizes the state-of-art knowledge about SQRs and highlights the questions that still remain unanswered. Despite two decades of work already done on these enzymes, new and most exciting discoveries can be expected in the future.

Publication types

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

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Archaeal Proteins / chemistry
  • Archaeal Proteins / classification
  • Archaeal Proteins / metabolism
  • Bacterial Proteins / chemistry
  • Bacterial Proteins / classification
  • Bacterial Proteins / metabolism
  • Binding Sites
  • Crystallography, X-Ray
  • Humans
  • Isoenzymes / chemistry*
  • Isoenzymes / classification*
  • Isoenzymes / metabolism
  • Models, Molecular
  • Molecular Sequence Data
  • Molecular Structure
  • Oxidation-Reduction
  • Phylogeny
  • Protein Structure, Tertiary*
  • Quinone Reductases / chemistry*
  • Quinone Reductases / classification*
  • Quinone Reductases / metabolism
  • Quinones / metabolism
  • Sequence Alignment


  • Archaeal Proteins
  • Bacterial Proteins
  • Isoenzymes
  • Quinones
  • Quinone Reductases
  • sulfide quinone reductase