HUMAN THETA-CLASS GLUTATHIONE TRANSFERASE - THE CRYSTAL-STRUCTURE REVEALS A SULFATE-BINDING POCKET WITHIN A BURIED ACTIVE-SITE

Background: Glutathione S-transferases (GSTs) comprise a multifunction al group of enzymes that play a critical role in the cellular detoxifi cation process. These enzymes reduce the reactivity of toxic compounds by catalyzing their conjugation with glutathione. As a result of thei r role in detoxification, GSTs have been implicated in the development of cellular resistance to antibiotics, herbicides and clinical drugs and their study is therefore of much interest. In mammals, the cytosol ic GSTs can be divided into five distinct classes termed alpha, mu, pi , sigma and theta, The human theta class GST, hGST T2-2, possesses sev eral distinctive features compared to GSTs of other classes, including a long C-terminal extension and a specific sulfatase,activity. It was hoped that the determination of the structure of hGST T2-2 may help u s to understand more about this unusual class of enzymes. Results: Her e we present the crystal structures of hGST T2-2 in the apo form and i n complex with the substrates glutathione and 1-menaphthyl sulfate. Th e enzyme adopts the canonical GST fold with a 40-residue C-terminal ex tension comprising two helices connected by a long loop. The extension completely buries the substrate-binding pocket and occludes most of t he glutathione-binding site. The enzyme has a purpose-built novel sulf ate-binding site. The crystals were shown to be catalytically active: soaks with 1-menaphthyl sulfate result in the production of the glutat hione conjugate and cleavage of the sulfate group. Conclusions: hGST T 2-2 shares less than 15% sequence identity with other GST classes, yet adopts a similar three-dimensional fold, The C-terminal extension tha t blocks the active site is not disordered in either the apo or comple xed forms of the enzyme, but nevertheless catalysis occurs in the crys talline state. A narrow tunnel leading from the active site to the sur face may provide a pathway for the entry of substrates and the release of products. The results suggest a molecular basis for the unique sul fatase activity of this GST.