Homoglutathione Selectivity by Soybean GlutathioneS-Transferases

Abstract

Soybeans (Glycine max) have nearly undetectable levels of glutathione, substituting the tripeptide homoglutathione for the same functions. Some herbicides are detoxified in soybeans by homoglutathione conjugation catalyzed by glutathioneS-transferase (GST) enzyme(s). We have cloned and overexpressed a new soybean GST (GSTa), the previously described soybean GST (GH2/4), and two maize GSTs. Their ability to utilize homoglutathione and glutathione in several nucleophilic substitution reactions was measured. In most cases conjugation to subsaturating concentrations of electrophilic substrate was faster with glutathione. However homoglutathione conjugation was faster with some combinations of enzyme and substrate, notably, GH2/4 and the herbicide chlorimuron ethyl. Steady-state kinetic evaluations revealed that a ternary complex is part of the reaction mechanism, and the binding of substrates takes place in random order. A random order rapid equilibrium model was used to compare the GH2/4-catalyzed reaction of both thiols with chlorimuron ethyl and alachlor. This revealed that catalytic rate constants do not differ significantly between the thiols. Conjugation rates with homoglutathione exceed those with glutathione when a high dissociation constant for second substrate makes formation of a glutathione containing ternary complex unfavorable. In GH2/4 this occurs with chlorimuron ethyl but not with alachlor.