THE 3-DIMENSIONAL STRUCTURE OF CLASS-PI GLUTATHIONE-S-TRANSFERASE IN COMPLEX WITH GLUTATHIONE SULFONATE AT 2.3 A RESOLUTION

The three-dimensional structure of class pi-glutathione S-transferase from pig lung, a homodimeric enzyme, has been solved by multiple isomorphous replacement at 3 angstrom resolution and preliminarily refined at 2.3 angstrom resolution (R = 0.24). Each subunit (207 residues) is folded into two domains of different structure. Domain I (residues 1 - 74) consists of a central four-stranded beta-sheet flanked on one side by two alpha-helices and on the other side, facing the solvent, by a bent, irregular helix structure. The topological pattern resembles the bacteriophage T4 thioredoxin fold, in spite of their dissimilar sequences. Domain 11 (residues 81 - 207) contains five alpha-helices. The dimeric molecule is globular with dimensions of about 55 angstrom x 52 angstrom x 45 angstrom. Between the subunits and along the local diad, is a large cavity which could possibly be involved in the transport of nonsubstrate ligands. The binding site of the competitive inhibitor, glutathione sulfonate, is located on domain 1, and is part of a cleft formed between intrasubunit domains. Glutathione sulfonate is bound in an extended conformation through multiple interactions. Only three contact residues, namely Tyr7, Gln62 and Asp96 are conserved within the family of cytosolic glutathione S-transferases. The exact location of the binding site(s) of the electrophilic substrate is not clear. Catalytic models are discussed on the basis of the molecular structure.