| The elutathione S-transferases (GSTs, E.C. 2.5.1.18) together with their physiological substrate, glutathione (;Structural information about GSTs have been previously obtained by X-ray crystallography at atomic resolution, from which the geometrical details at the glutathione binding site (G-site) have been revealed. Location of the H-site has also been identified in crystal structures of the enzyme complexed with various product analogs. Mechanisms about the enzymatic activation of the bound glutathione have been proposed base on crystal structures, physicochemical data comparing wild-type and mutant enzymes and theoretical calculation of the local charge properties. However, it is not obvious from the crystal structures how the protein architecture at or near the active site actually active site actually stabilizes the intermediate states of the catalytic cycle. Structural elements that facilitate the binding of a range of H-site substrates need to be further defined.;In this study, the isoenzyme GST3-3 from rat liver has been crystallized in three different forms under the same conditions. One of the crystal forms is essentially the same as previously reported by another laboratory (Ji, et al., 1992). We have since solved the structures of all three crystals forms by molecular replacement methods, refined them and compared the structural variations in all three different crystal lattices. The physiological substrate, glutathione, has been found at its binding site in each crystal with slightly varying geometry of hydrogen bonding and ionic interaction. The key interatomic distance (Reinemer, et al., 1991; Liu, et al., 1992)--from GSH sulfur to Tyr6 hydroxyl--in different subunits and different crystal lattices, falls within the range typical for hydrogen bonding involving a sulfur atom. A main-chain amide is also in a favorable position to form a hydrogen bond to the hydroxyl of Tyr6 in all three crystal forms. A change of conformation of a flexible loop (Res. 35-40) was observed in one of the new crystal forms. The molecular packings in the three crystal forms have been analyzed; the proteins associate in such a way that they form similar repeating layers (dimensions: 70 x 81 A). The layers stack on top of each other in different ways to build the different crystal lattices. |
