Exploring the Functional Plasticity of Human Glutathione Transferases Allelic Variants, Novel Isoenzyme and Enzyme Redesign

Abstract: Glutathione transferases (GSTs) make up a superfamily that is involved in the cellular defense against various reactive compounds by catalyzing the conjugation of glutathione to electrophilic centra. Members of this family have also been implicated in different facets of biological signaling. The gene encoding human GST P1-1 is polymorphic, resulting in variant amino acid residues in positions 105 and 114. The role of the polymorphism in the active-site residue 105 on enzyme stability and activity with various substrates was investigated. A valine instead of an isoleucine in position 105 decreased the thermal stability of the enzyme. The effect on enzyme activity was dependent on the substrate and reaction studied. With some substrates tested, such as carcinogenic diolepoxides derived from polyaromatic hydrocarbons, GST P1-1/Val105 displayed the highest catalytic efficiency. In contrast, with 1-chloro-2,4-dinitrobenzene, the GST P1-1/Ile105 showed higher activity. Residue 105 was mutated to alanine and tryptophan to investigate the role of size and hydrophobicity of residue 105 on enzyme properties. Generally, a smaller amino acid in position 105 gave increased activity with large substrates. Clearly, residue 105 of GST P1-1 helps to determine the substrate selectivity of the enzyme. In addition, more voluminous amino acids in position 105 increase the thermal stability of the enzyme. GST P1-1 is believed to contribute to the development of drug resistance in cancer cells. The affinity of GST P1-1 for TER 117, designed to inhibit GST P1-1 in tumors, was not affected by the variability in position 105. TER 117 was found to be a potent inhibitor of glyoxalase I as well.The cDNA encoding GST A3-3 was isolated from a placental cDNA library. GST A3-3 was heterologously expressed, purified and found to catalyze efficiently the double-bond isomerization of ?5-androstene-3,17-dione and ?5-pregnene-3,20-dione, reactions taking place in the biosynthesis of the steroid hormones testosterone and progesterone, respectively. GST A3-3 was found to be selectively expressed in steroidogenic tissues, suggesting that this enzyme is involved in the production of steroid hormones. The presence of both the hydroxyl group of the active-site tyrosine 9 and the thiolate form of glutathione, acting as a cofactor, is important for high double-bond isomerase activity. A leucine in position 111 appears to have a major role in productive binding of the steroid substrate but also residues F10 and A216 are determinants for the high isomerase activity. GST A2-2 is a poor catalyst of the steroid double-bond isomerization of ?5-androstene-3,17-dione as compared to GST A3-3, despite 88% sequence identity. GST A2-2 was redesigned to a highly efficient double-bond isomerase by mutating five active-site residues to the corresponding residues of GST A3-3. This demonstrates the functional plasticity of GSTs and the power of a rational approach to redesign of these enzymes.