Molecular basis for the enzymatic inactivity of class III glutaredoxins on standard glutathionylated substrates

Glutaredoxins (GRXs) are small proteins that interact with the atypical tripeptide glutathione (GSH), a redox active metabolite that forms a disulfide (GSSG) upon oxidation. GRXs encoding variants of a CPYC motif at a conserved active site act as GSH-dependent thiol-disulfide oxidoreductases (class I). GRXs with a CGFS site (class II) bind GSH in a way that is non-permissive for thiol-disulfide oxidoreductase reactions and favours transient iron-sulfur cluster binding. The biochemical functions of CCxC/S-type (class III) GRXs, which are found only in land plants, have remained largely unexplored. In this study, we characterized the in vitro properties of one of the Arabidopsis thaliana class III GRXs, namely ROXY9.