Abstract
Plants and animals have evolved various potential mechanisms to surmount the adverse effects of heavy metal toxicity. Plants possess low molecular weight compounds containing sulfhydryl groups (-SH) that actively react with toxic metals. For instance, glutathione (γ-Glu-Cys-Gly) is a sulfur-containing tripeptide thiol and a substrate of cysteine-rich phytochelatins (γ-Glu-Cys)2–11-Gly (PCs). Phytochelatins react with heavy metal ions by glutathione S-transferase in the cytosol and afterwards they are sequestered into the vacuole for degradation. Furthermore, heavy metals induce reactive oxygen species (ROS), which directly or indirectly influence metabolic processes. Reduced glutathione (GSH) attributes as an antioxidant and participates to control ROS during stress. Maintenance of the GSH/GSSG ratio is important for cellular redox balance, which is crucial for the survival of the plants. In this context, sulfurtransferases (Str), also called rhodaneses, comprise a group of enzymes widely distributed in all phyla, paving the way for the transfer of a sulfur atom from suitable sulfur donors to nucleophilic sulfur acceptors, at least in vitro. The best characterized in vitro reaction is the transfer of a sulfane sulfur atom from thiosulfate to cyanide, leading to the formation of sulfite and thiocyanate. Plants as well as other organisms have multi-protein families (MPF) of Str. Despite the presence of Str activities in many living organisms, their physiological role has not been clarified unambiguously. In mammals, these proteins are involved in the elimination of cyanide released from cyanogenic compounds. However, their ubiquity suggests additional physiological functions. Furthermore, it is speculated that a member of the Str family acts as arsenate reductase (AR) and is involved in arsenate detoxification. In summary, the role of Str in detoxification processes is still not well understood but seems to be a major function in the organism.
Highlights
Sulfurtransferases (Str), called rhodaneses, catalyzes the transfer of a sulfur atom from suitable sulfur donors to nucleophilic sulfur acceptors [1]
The most studied and best characterized Str is bovine liver rhodanese that catalyzes in vitro the transfer of a sulfane sulfur atom from thiosulfate (TS) to cyanide, leading to the formation of sulfite and thiocyanate by forming a Rhod-S intermediate, which is characterized by a persulfide bond at the sulfhydryl group of the essential cysteine residue 247 (Scheme 1) [2]: S2O32− + Rhd →SO32− + Rhd-S
Various heavy metal ions trigger the overproduction of reactive oxygen species (ROS) or free radicals in plants which are toxic and highly sensitive to proteins, lipids, carbohydrates and DNA, and results in oxidative stress
Summary
Sulfurtransferases (Str), called rhodaneses, catalyzes the transfer of a sulfur atom from suitable sulfur donors to nucleophilic sulfur acceptors [1]. It has been reported that single rhodanese domain proteins are involved in reactions to stress defense, such as the Drosophila melanogaster heat shock protein 67B2, the E. coli phage shock protein PspE [6] or the Vibrio cholerae shock protein q9KN65 [7]. Sulfurtransferases or Str-like proteins have been identified in different subcellular compartments. There are 20 different Strs or Str-like proteins in A. thaliana [18,19] These have been classified into six groups based on their amino acid sequence similarities [20]. The high abundance of Str proteins in A. thaliana and other plant species [1] in different cellular compartments is speculated to pave the way for several specific biological functions, especially in abiotic and biotic stress defense
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