A HOMOLOGY MODEL FOR RAT MU-CLASS GLUTATHIONE-S-TRANSFERASE-4-4

Glutathione S-transferases (GSTs) are an important class of phase II ( de)toxifying enzymes, catalyzing the conjugation of glutathione (GSH) to electrophilic species. Recently, a number of cytosolic GSTs was cry stallized. In the present study, molecular modeling techniques have be en used to derive a three-dimensional homology model for rat GST 4-4 b ased upon the crystal structure of rat GST 3-3, both members of the mu class. GST 3-3 and GST 4-4 isoenzymes share a sequence homology of 88 %. GST 4-4 distinguishes itself from GST 3-3 in being much more effici ent and stereoselective in the nucleophilic addition of GSH to epoxide s and alpha,beta-unsaturated ketones. GST 3-3, however, is much more e fficient in catalyzing nucleophilic aromatic substitution reactions. I n this study, several known substrates of GST 4-4 were selected and th eir GSH conjugates docked into the active site of GST 4-4. GSH conjuga tes of phenanthrene 9(S),10(R)-oxide and 4,5-diazaphenanthrene 9(S),10 (R)-oxide were docked into the active site of both GST 3-3 and GST 4-4 . From these homology modeling and docking data, the difference in ste reoselectivity between GST 3-3 and GST 4-4 for the R- and S-configured carbons of the oxirane moiety could be rationalized. The data acquire d from a recently derived small molecule model for GST 4-4 substrates were compared with the results of the present protein homology model o f GST 4-4. The energy optimized positions of the conjugates in the pro tein model agreed very well with the original relative positions of th e substrates within the substrate model, confirming the usefulness of small molecule models in the absence of structural protein data. The p rotein homology model, together with the substrate model, will be usef ul to further rationalize the substrate selectivity of GST 4-4, and to identify new potential GST 4-4 substrates.