The structural roles of a conserved small hydrophobic core in the active site and an ionic bridge in domain I of Delta class glutathione S-transferase

Abstract

GSTs (glutathione S-transferases; E.C.2.5.1.18) are a supergene family of dimeric multifunctional enzymes that have a major role in detoxification pathways. Using a GST from the mosquito Anopheles dirus (adGSTD4-4), we have characterized the enzymatic and physical properties of Leu-6, Thr-31, Leu-33, Ala-35, Glu-37, Lys-40 and Glu-42. These residues generate two motifs located in the N-terminal domain (domain I) that are functionally conserved across GST classes. The aim of this study was to understand the function of these two motifs. The first motif is a small hydrophobic core in the G-site (glutathione-binding site) wall, and the second motif contains an ionic bridge at the N-terminus of the alpha2 helix and is also part of the G-site. The mutations in the small hydrophobic core appear to have structural effects, as shown by the thermal stability, refolding rate and intrinsic fluorescence differences. In the Delta class GST, interactions form an ionic bridge motif located at the beginning of the alpha2 helix. The data suggest that electrostatic interactions in the alpha2 helix are involved in alpha-helix stabilization, and disruption of this ionic bridge interaction changes the movement of the alpha2-helix region, thereby modulating the interaction of the enzyme with substrates. These results show that the small hydrophobic core and ionic bridge have a major impact on structural stabilization, as well as being required to maintain structural conformation of the enzyme. These structural effects are also transmitted to the active site to influence substrate binding and specificity. Therefore changes in the conformation of the G-site wall in the active site appear to be capable of exerting influences on the tertiary structural organization of the whole GST protein.