Involvement of the Azotobacter vinelandii rhodanese-like protein RhdA in the glutathione regeneration pathway

An increasing number of studies correlated the expression of rhodanese-like proteins to the management of cellular redox omeostasis. In Azotobacter vinelandii, the absence of the rhodanese-like protein RhdA results in an impaired redox condition and in the activation of the OxyR-regulon mediated response. Moreover, the finding that in the RhdA-null mutant the glutathione pool was depleted and glutathione precursors were accumulated suggested an impairment in the regeneration pathway of reduced glutathione (GSH). In the presence of hydroxyl radical (OH*), GSH is oxidized to glutathione thiyl radical (GS*) which needs an enzymatic pathway to be recovered as glutathione disulfide (GSSG), a GSH-generable form. Proteins with a low-pKa cysteine residue (like in the case of RhdA) are good candidates to stabilize an adduct with GS* for enzymatic GSSG dependent recovery. In the present study, we found that RhdA was able to specifically bind both GSH and GS* in vitro with high affinity (Kd = μM range) and that RhdA-Cys230 residue was mandatory for the adduct formation. The RhdA/glutathione adduct was stable enough to be used as a substrate to produce GSSG (kcat = 628 s-1), despite MS analyses did not reveal the formation of a covalent bond. The RhdA scavenging activity of GS* was O2-dependent suggesting that O2 might be the final acceptor of the radical. The GS*-scavenging activity of RhdA supports the phenotype observed on the RhdA-null mutant defined in our previous studies, highlighting RhdA’s key role as ROS-level mediator by preserving glutathione from irreversible oxidations induced by OH*.