The molecular basis for catalytic differences between structurally closely related murine class alpha glutathione (GSH) transferases mGSTA1-1 and mGSTA2-2 in the GSH conjugation of anti-diol epoxide isomers of benzo[c]phenanthrene (anti-B[c]PDE) was investigated. GSH conjugation of both (-)- and (+)-enantiomers of anti-B[c]PDE was observed in the presence of mGSTA1-1 (60 and 40% GSH conjugation, respectively), whereas mGSTA2-2 exhibited a preference for the (-)-anti-isomer (>97%). In addition, the specific activity of mGSTA2-2 toward the (-)-anti-B[c]PDE isomer was relatively higher than that of mGSTA1-1. The amino acid sequences of mGSTA1-1 and mGSTA2-2 differ at 10 positions that are distributed in three sections. Section I contains amino acid residues in positions 65 and 95; section II contains residues in positions 157, 162, and 169, and section III contains residues in positions 207, 213, 218, 221, and 222. Enzyme activity measurements with chimeras of mGSTA1-1 and mGSTA2-2 revealed that amino acid substitutions in section III account for their differential enantioselectivity and catalytic activity toward anti-B[c]PDE. Site-directed mutagenesis of amino acid residues in section III of mGSTA2-2 with corresponding residues of mGSTA1-1 followed by activity measurements of the wild type and mutated enzymes indicates that leucine 207 and phenylalanine 221 may be critical for the high catalytic activity of mGSTA2-2 toward (-)-anti-B[c]PDE. Molecular modeling studies demonstrated that the active site of mGSTA1-1 accommodates both enantiomers of anti-B[c]PDE, whereas the (-)-anti-isomer interacts more favorably with active site residues in mGSTA2-2. The results of this study clearly indicate that amino acid substitutions in the C-terminal region contribute to catalytic differences between mGSTA1-1 and mGSTA2-2 with respect to anti-B[c]PDE.