LOCATION OF A POTENTIAL TRANSPORT BINDING-SITE IN A SIGMA-CLASS GLUTATHIONE TRANSFERASE BY X-RAY CRYSTALLOGRAPHY

The crystal structure of the sigma class glutathione transferase from squid digestive gland ire complex with S-(3-iodobenzyl)glutathione rev eals a third binding site for the glutathione conjugate besides the tw o in the active sites of the dimer, The additional binding site is nea r the crystallographic two-fold axis between the two alpha 4-turn-alph a 5 motifs. The principal binding interactions with the conjugate incl ude specific electrostatic interactions between the peptide and tile t wo subunits and a hydrophobic cavity found across the two-fold axis th at accommodates the 3-iodobenzyl group. Thus, two identical, symmetry- related but mutually exclusive binding modes for the third conjugate a re observed, The hydrophobic pocket is about 14 Angstrom from the hydr oxyl group of Tyr-7 in the active site. This site is a potential trans port binding site for hydrophobic molecules or their glutathione conju gates.