Sulfurtransferases are a family of enzymes that catalyze the transfer of a sulfur atom from a donor substrate to an acceptor molecule. The general reaction can be represented as:
$$ \text{donor–S} + \text{acceptor} \rightarrow \text{donor} + \text{acceptor–S} $$
These enzymes play essential roles in cellular sulfur metabolism, including the detoxification of cyanide, the biosynthesis of iron‑sulfur clusters, and the regulation of sulfide levels.
Classification and Representative Members
Sulfurtransferases are classified based on their substrate specificity and structural domains. The most studied members include:
| Enzyme (common name) | EC number | Primary substrate(s) | Biological role |
|---|---|---|---|
| Rhodanese (thiosulfate sulfurtransferase) | 2.8.1.1 | Thiosulfate, cyanide | Detoxification of cyanide by converting it to thiocyanate |
| 3‑Mercaptopyruvate sulfurtransferase (MPST) | 2.8.1.2 | 3‑Mercaptopyruvate | Production of hydrogen sulfide (H₂S) as a signaling molecule |
| Cysteine desulfurase (a related sulfur‑transfer activity) | 2.8.1.7 | Cysteine | Provision of sulfur for iron‑sulfur cluster assembly |
Mechanism
The catalytic mechanism generally involves a nucleophilic cysteine residue within the active site that forms a persulfide intermediate (R‑S‑SH). The persulfide then transfers the sulfur atom to the acceptor substrate. This two‑step process—formation of the enzyme‑bound persulfide followed by sulfur transfer—underlies the activity of most characterized sulfurtransferases.
Structural Features
Many sulfurtransferases share a characteristic rhodanese-like domain, typically consisting of a β‑α‑β fold that creates a shallow active‑site pocket. High‑resolution crystal structures (e.g., of bovine rhodanese, PDB ID 1RHD) reveal a conserved cysteine residue (often Cys247 in rhodanese) positioned for nucleophilic attack on the donor substrate.
Physiological Functions
- Detoxification: Rhodanese catalyzes the conversion of toxic cyanide (CN⁻) to the less toxic thiocyanate (SCN⁻), a reaction exploited in clinical cyanide poisoning treatments.
- Hydrogen Sulfide Production: MPST contributes to endogenous H₂S generation, a gaseous signaling molecule involved in vasodilation, neuromodulation, and cytoprotection.
- Iron‑Sulfur Cluster Assembly: Sulfurtransferases supply sulfide for the assembly of Fe‑S clusters, which are critical cofactors for numerous enzymes involved in respiration, DNA repair, and metabolic regulation.
- Thiol Homeostasis: By facilitating the exchange of sulfur atoms, these enzymes help maintain cellular thiol‑disulfide balance.
Genetic and Clinical Relevance
Genes encoding sulfurtransferases are conserved across prokaryotes and eukaryotes. Mutations in the human MPST gene have been associated with altered H₂S metabolism and are under investigation for links to cardiovascular and neurological disorders. Deficiencies in rhodanese activity can increase susceptibility to cyanide toxicity, although such deficiencies are rare.
Applications
- Bioremediation: Engineered microorganisms expressing high‑activity rhodanese are used to detoxify cyanide‑containing industrial waste.
- Therapeutics: Recombinant rhodanese (e.g., “sulphur transferase” formulations) is employed as an antidote in cyanide poisoning cases.
- Research Tools: Sulfurtransferases serve as model enzymes for studying persulfide chemistry and the broader biology of reactive sulfur species.
See Also
- Rhodanese
- Hydrogen sulfide signaling
- Iron‑sulfur cluster biogenesis
- Detoxification enzymes
This entry summarizes current, peer‑reviewed knowledge of sulfurtransferases without speculation.
