Rhodanese (thiosulfate sulfurtransferase) (EC 2.8.1.1) [1,2] is an enzyme which catalyzes the transfer of the sulfane atom of thiosulfate to cyanide, to form sulfite and thiocyanate. Rhodanese (from the german word for thioyanate, 'rhodanid') is a widespread enzyme, rhodanese activity having been detected in all major phyla, including eubacteria and mammals. In vertebrates, rhodanese is a mitochondrial enzyme of about 300 amino-acid residues involved in forming iron-sulfur complexes and cyanide detoxification. In the course of catalysis, rhodanese cycles between a sulfur-free form and a persulfurated intermediate, hosting the persulfide sulfur atom on the catalytic cysteine residue.

Some bacterial proteins closely related to rhodanese are also thought to express a sulfotransferase activity. These are:

• 3-mercaptopyruvate sulfurtransferases (MST) (EC 2.8.1.1). They catalyze the same sulfane sulfur transfer reaction as rhodanese, but use 3- mercaptopyruvate as a sulfur donor.
• Azotobacter vinelandii rhdA.
• Escherichia coli sseA [3].
• Escherichia coli, Salmonella typhimurium and Haemophilus influenzae thiI [4]. ThiI is an enzyme common to the biosynthetic pathways leading to both thiamin and 4-thiouridine in bacterial tRNA.
• Escherichia coli glpE [5].
• Saccharopolyspora erythraea cysA [6].
• Synechococcus strain PCC 7942 rhdA [7]. RhdA is a periplasmic protein probably involved in the transport of sulfur compounds.
• Wolinella succinogenes periplasmic sulfide dehydrogenase (sud). Sud has been characterized as a polysulfide:cyanide sulfurtransferase.

The tertiary structure of rhodanese (see <PDB:1E0C>) is composed of two domains which, in spite of a negligible sequence homology, are characterized by very similar three dimensional folds. Each domain displays α/β topology, with a central parallel five-stranded β-sheet surrounded by α-helices on both sides [8]. Rhodanese homology domains are structural modules of about 120 amino acids, which occur in the three major evolutionary phyla [9]. Rhodanese-like proteins are either composed of a single catlytic rhodanese domain, as found in glpE, or composed of two rhodanese domains, with the C-terminal domain displaying the putative catalytic Cys as observed in Rhobov and rhdA. Rhodanese domains, either catalytic or inactive (i.e. where the active-site Cys is replaced by another residue), are also found associated with other protein domains such as MAPK-phosphatases or thiL, an Escherichia coli enzyme involved in thiamin and thiouridine biosynthesis. Catalytically active rhodanese domains are supposed to be versatile sulfur carriers that have adapted their function to fulfill the need for reactive sulfane sulfur in distinct metabolic and regulatory pathways, whereas the frequent association of catalytically inactive rhodanese domains with other protein domains suggests a distinct regulatory role for these inactive domains, possibly in connection with signaling [5,10].

Some proteins known to contain a rhodanese homology domain are listed below [5,9,E1]:

• The Cdc25 family of protein dual specificity phosphatases (EC 3.1.3.48).
• The MKP1/PAC1 family of MAP-kinase phosphatases (EC 3.1.3.48 / EC 3.1.3.16).
• The Pyp1/Pyp2 family of MAP-kinase phosphatases (EC 3.1.3.48).
• Several ubiquitin hydrolases (yeast UBP4,5,7; human UBPY) (EC 3.1.2.15).
• Various stress response proteins (heat shock, phage shock, cold shock) from all phyla.
• Archaeoglobus fulgidus NADH oxidase (NoxA-3).

We developed two patterns for the rhodanese family. They are based on highly conserved regions, one which is located in the N-terminal region, the other at the C-terminal extremity of the enzyme. We also developed a profile which covers the entire rhodanese homology domain.