EVIDENCE FOR AN INDUCED-FIT MECHANISM OPERATING IN PI CLASS GLUTATHIONE TRANSFERASES

Three-dimensional structures of the apo form of human pi class glutath ione transferase have been determined by X-ray crystallography. The st ructures suggest the enzyme recognizes its substrate, glutathione, by an induced-fit mechanism. Compared to complexed forms of the enzyme, t he environment around the catalytic residue, Tyr 7, remains unchanged in the apoenzyme, This observation supports the view that Tyr 7 does n ot act as a general base in the reaction mechanism. The observed coope rativity of the dimeric enzyme may be due to the movements of a helix that forms one wall of the active site and, in particular, to movement s of a tyrosine residue that is located in the subunit interface.