3-DIMENSIONAL STRUCTURE OF GLUTATHIONE-S-TRANSFERASE FROM ARABIDOPSIS-THALIANA AT 2.2-ANGSTROM RESOLUTION - STRUCTURAL CHARACTERIZATION OF HERBICIDE-CONJUGATING PLANT GLUTATHIONE S-TRANSFERASES AND A NOVEL ACTIVE-SITE ARCHITECTURE

Glutathione S-transferases (GST) are a family of multifunctional enzym es involved in the metabolization of a broad variety of xenobiotics an d reactive endogenous compounds. The interest in plant glutathione S-t ransferases may be attributed to their agronomic value, since it has b een demonstrated that glutathione conjugation for a variety of herbici des is the major resistance and selectivity factor in plants. The thre e-dimensional structure of glutathione S-transferase from the plant Ar abidopsis thaliana has been solved by multiple isomorphous replacement and multiwavelength anomalous dispersion techniques at 3 Angstrom res olution and refined to a final crystallographic R-factor of 17.5% usin g data from 8 to 2.2 Angstrom resolution. The enzyme forms a dimer of two identical subunits each consisting of 211 residues. Each subunit i s characterized by the GST-typical modular structure with two spatiall y distinct domains. Domain I consists of a central four-stranded beta- sheet flanked on one side by two alpha-helices and on the other side b y an irregular segment containing three short 3(10)-helices, while dom ain II is entirely helical. The dimeric molecule is globular with a pr ominent large cavity formed between the two subunits. The active site is located in a cleft situated between domains I and II and each subun it binds two molecules of a competitive inhibitor S-hexylglutathione. Both hexyl moieties are oriented parallel and fill the H-subsite of th e enzyme's active site. The glutathione peptide of one inhibitor, term ed productive binding, occupies the G-subsite with multiple interactio ns similar to those observed for other glutathione S-transferases, whi le the glutathione backbone of the second inhibitor, termed unproducti ve binding, exhibits only weak interactions mediated by two polar cont acts. A most striking difference from the mammalian glutathione S-tran sferases, which share a conserved catalytic tyrosine residue, is the l ack of this tyrosine in the active site of the plant glutathione S-tra nsferase. (C) 1996 Academic Press Limited