Design of Glutathione Transferase Variants for Novel Activities with Alternative Substrates

Glutathione transferases (GSTs) play a pivotal role in cellular defense, since they are main contributors to the inactivation of genotoxic compounds of exogenous and endogenous origins. Directed evolution was used to improve the catalytic activities of Theta class GST T1-1 toward different substrates. The library was constructed by recombination of cDNA coding for human GST T1-1 and rodent Theta class GSTs, resulting in the F2-F5 generations. The clones were heterologously expressed in Escherichia coli and screened for variants with enhanced alkyltransferase activity. A mutant, F2:1215, with a 70-fold increased catalytic efficiency with 4-nitrophenethyl bromide (NPB) compared to human GST T1-1, was isolated from the second generation. NPB was used as a surrogate substrate of the anticancer drug 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in order to facilitate the screening process. The catalytic efficiency of the F2:1215 with BCNU had improved 170-fold compared to wild-type human GST T1-1, suggesting that NPB is a suitable model substrate for the anticancer drug BCNU. The sequence of the F2:1215 mutant differs from wild-type human GST T1-1 by three residues; one of these differences is Arg234, which corresponds to Trp in the human enzyme. Upon replacing the Trp234 in the human GST T1-1 with Arg, the resulting mutant (hTrp234Arg) showed enhanced alkyltransferase activity with a wide range of substrates (e.g. haloalkanes and other typical GSTs substrates). The three-dimensional structures of both wild-type human GST T1-1 and hTrp234Arg mutant help to explain the higher activity showed by of hTrp234Arg mutant compared to wild-type enzyme. The reciprocal mutation of the residue 234 in mouse GST T1-1 to that found in human, mArg234Trp, caused a dramatic decrease in the activity of the mouse enzyme to be similar to human GST T1-1. This indicates that residue 234 can be considered as a master switch of activities between human and rodent GST T1-1. Another important residue in the C-terminal helix of GST T1-1 is Met232. Although residue 232 points away from the H-site, it influences the catalytic activity and substrate selectivity of the mouse GST T1-1. A minor modification of Met232 induces major changes in the substrate-activity profile of the mouse GST T1-1 to favor novel substrates such as isothiocyanates and hydroperoxides and decreases the activity toward substrates that catalyzed by the wild-type enzyme.