The Active Site in Class Alpha Glutathione Transferases

Electrophilic groups of a wide range of compounds are targets for nucleophilic attack by the sulfur of activated glutathione 1 . This reaction is catalyzed by glutathione transferases (GSTs), a family of isoenzymes that plays a key role in the detoxification of xenobiotics and in oxidative metabolism. Increased GST activity causes resistance toward chemotherapeutic agents, herbicides and pesticides. In addition to their enzymatic role, GSTs also serve as carrier or binding proteins for organic molecules such as heme, steroids and bilirubin. Mammalian cytosolic GTSs can be grouped into four classes: Alpha, Mu, Pi and Theta 2 , 3 . The three-dimensional structures of a porcine and a human class Pi enzyme 4 , 5 , a rat class Mu 6 and a human class Alpha 7 enzyme have recently been determined by X-ray crystallography, which enables a detailed comparison of these structures 7 . The active site was previously assumed to consist of a G-site (or glutathione-binding site) and an H-site (or hydrophobic or electrophile binding site) 8 . However, once the X-ray structure of the class Pi GST from pig lung had been determined in complex with glutathione sulphonate 4 and that of the rat class Mu GST in complex with glutathione 6 , the location of the H-site was still not clear. The structure of the human Pi GST in complex with S-hexylglutathione 5 and, more recently, that of the human Alpha GST Al-1 in complex with S-benzyl-glutathione 7 , provided detailed information on the interaction of those electrophiles with the protein. In this paper we present an overview of the overall structure of human Al-1 and a more detailed description of the active site.