Abstract:

<p><b>AIMS: </b>New drugs are needed to treat flatworm infections that cause severe human diseases such as schistosomiasis. The unique flatworm enzyme thioredoxin glutathione reductase (TGR), structurally different from the human enzyme, is a key drug target. Structural studies of the flatworm Echinococcus granulosus TGR, free and complexed with AuI-MPO, a novel gold inhibitor, together with inhibition assays were performed.</p><p><b>RESULTS: </b>AuI-MPO is a potent TGR inhibitor that achieves 75% inhibition at a 1:1 TGR:Au ratio and efficiently kills E. granulosus in vitro. The structures revealed salient insights: i) unique monomer-monomer interactions, ii) distinct binding sites for thioredoxin and the glutaredoxin domain, iii) a single glutathione disulfide reduction site in the glutaredoxin domain, iv) rotation of the glutaredoxin domain towards the Sec-containing redox active site, v) a single gold atom bound to Cys519 and Cys573 in the AuI-TGR complex. Structural modeling suggests that these residues are involved in the stabilization of the Sec-containing C-terminus. Consistently, Cys&rarr;Ser mutations in these residues decreased TGR activities. Mass spectroscopy confirmed these cysteines are the primary binding site.</p><p><b>INNOVATION: </b>The identification of a primary site for gold binding and the structural model provide a basis for gold compound optimization through scaffold adjustments.</p><p><b>CONCLUSIONS: </b>The structural study revealed that TGR functions are achieved not only through a mobile Sec-containing redox center, but also by rotation of the glutaredoxin domain and distinct binding sites for glutaredoxin domain and thioredoxin. The conserved Cys519 and Cys573 residues targeted by gold assist catalysis through stabilization of the Sec-containing redox center.</p>