Glutathione (γ-glutamyl-cysteinyl-glycine, GSH) is a major thiol-containing peptide with cellular levels of up to 10 mM.1 Several recent reports have demonstrated glutaredoxins (Grx) to form [Fe2S2] cluster-bridged dimers, where glutathione provides two exogenous thiol ligands, and have implicated such species in cellular iron sulfur cluster biosynthesis. We report the finding that glutathione alone can coordinate and stabilize an [Fe2S2] cluster under physiological conditions, with optical, redox, Mössbauer and NMR characteristics that are consistent with a [Fe2S2](GS)4 composition. The Fe-S assembly protein ISU catalyzes formation of [Fe2S2](GS)4 from iron and sulfide ions in the presence of glutathione, and the [Fe2S2] core undergoes reversible exchange between apo ISU and free glutathione.

Exchange of [2Fe-2S] centers between Grx2 and the cluster scaffold protein ISU, and characterization of two mutually exclusive Grx2 binding sites on ISU by isothermal titration calorimetry supports a direct link for Grx and glutathione involvement in ISU promoted Fe-S cluster biosynthesis.

Glutathione-complexed [2Fe-2S] cluster is shown to significantly stimulate the ATPase activity of an ABCB7-type transporter in both solution and proteoliposome-bound forms (KD ~ 68 μM). The cluster is a likely natural substrate for this transporter, which has been implicated in cytosolic Fe-S cluster protein maturation. A possible substrate-binding site is identified on a new structural model for the active transporter.

Human NFU (also known as HIRIP5) has been implicated in cellular iron-sulfur cluster biosynthesis. Bacterial and yeast forms are smaller than the human protein and are homologous to the C-terminal domain of the latter. This C-terminal domain contains a pair of redox active cysteines and demonstrates thioredoxin-like activity by mediating persulfide bond cleavage of sulfur-loaded NifS (an IscS-type protein), the sulfide donor for [2Fe-2S] cluster assembly on ISU-type scaffold proteins. Herein, the affinity of full-length human NFU and the individual N- and C-terminal domains for sulfide donor and cluster scaffold proteins is assessed. The influence of the N-terminal domain on C-terminal NFU binding to NifS and persulfide reductase activity is also examined. Only the C-terminal domain is required for persulfide reductase activity, while complex formation of NifS with full-length NFU is similar to that of the C-terminal domain alone (K(D) approximately 9.7 +/- 0.7 and 10.1 +/- 0.6 microM, respectively). There is negligible affinity between the isolated C- and N-terminal domains, while the N-terminal domain has negligible affinity for either sulfide donor or cluster scaffold proteins. The temperature dependence of the binding enthalpy for formation of the complex between NifS and the C-terminal domain of NFU yields a change in molar heat capacity (DeltaC(p) approximately 138 cal mol(-1) K(-1)) that suggests bonding at the protein-protein interface is dominated by electrostatic interactions. This is consistent with electrostatic potential maps for bacterial homologues of the N- and C-terminal domains of human NFU, which most likely reflect the structural characteristics expected for full-length human NFU.