title: Engineering the xenobiotic substrate specificity of maize glutathione S-transferase I.
abstract: Glutathione S-transferases (GSTs) are a heterogeneous family of enzymes that catalyse the conjugation of glutathione (GSH) to electrophilic sites on a variety of hydrophobic substrates. In the present study three amino acid residues (Trp12, Phe35 and Ile118) of the xenobiotic binding site (H-site) of maize GST I were altered in order to evaluate their contribution to substrate binding and catalysis. These residues are not conserved and hence may affect substrate specificity and/or product dissociation. The results demonstrate that these residues are important structural moieties that modulate an enzyme's catalytic efficiency and specificity. Phe35 and Ile118 also participate in k(cat) regulation by affecting the rate-limiting step of the catalytic reaction. The effect of temperature on the catalytic activity of the wild-type and mutant enzymes was also investigated. Biphasic Arrhenius and Eyring plots for the wild-type enzyme showed an apparent transition temperature at 35 degrees C, which seems to be the result of a change in the rate-limiting step of the catalytic reaction. Thermodynamic analysis of the activity data showed that the activation energy increases at low temperatures, whereas the entropy change seems to be the main determinant that contributes to the rate-limiting step at high temperatures.
authors.affiliation: Laboratory of Enzyme Technology, Department of Agricultural Biotechnology, Agricultural University of Athens, 75 Iera Odos Street, 11855 Athens, Greece. lambrou@aua.gr
authors.forename: Nikolaos E
authors.initials: NE
authors.lastname: Labrou
authors.forename: Georgia A
authors.initials: GA
authors.lastname: Kotzia
authors.forename: Yannis D
authors.initials: YD
authors.lastname: Clonis
mesh_terms: D019943:Amino Acid Substitution; D001280:Atrazine; D001483:Base Sequence; D020134:Catalytic Domain / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D004976:Ethacrynic Acid; D005982:Glutathione Transferase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D015202:Protein Engineering / Q000379:methods; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D013696:Temperature; D013816:Thermodynamics; D015262:Xenobiotics / Q000378:metabolism; D003313:Zea mays / Q000201:enzymology* / Q000235:genetics
chemical_list: D018744:DNA, Plant; D011994:Recombinant Proteins; D015262:Xenobiotics; D005982:Glutathione Transferase; D004976:Ethacrynic Acid; D001280:Atrazine