Saccharomyces cerevisiae cells have three Omega class glutathione S-transferases acting as 1-Cys thiol transferases
Résumé
The Saccharomyces cerevisiae genome codes for three proteins that display homology with human Omega class glutathione transferases (GST) hGSTO1-1 and hGSTO2-2. The three yeast proteins have been named Gto1, Gto2 and Gto3, and their purified recombinant forms are active as thiol transferases (glutaredoxins) against {beta}-hydroxyethyl disulfide (HED), as dehydroascorbate reductases and as dimethylarsinic acid reductases, while they are not active against the standard GST substrate 1-chloro-2,4-dinitrobenzene (CDNB). Their glutaredoxin activity is also detectable in yeast cell extracts. The enzyme activity characteristics of the Gto proteins contrast with those of another yeast GST, Gtt1. The latter is active against CDNB and also displays glutathione peroxidase activity against organic hydroperoxides such as cumene hydroperoxide, but is not active as a thiol transferase. Analysis of point mutants derived from wild type Gto2 indicates that among the three cysteine residues of the molecule, only the residue at position 46 is required for the glutaredoxin activity. This indicates that the thiol transferase acts through a monothiol mechanism. Substitution of the active site of the yeast monothiol glutaredoxin Grx5 by the proposed Gto2 active site containing Cys46 makes Grx5 to recover some activity against HED. Therefore, the residues adjacent to the respective active cysteine residues in Gto2 and Grx5 are important determinants for the thiol transferase activity against small disulphide-containing molecules.
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