Structure and Functional Effect of Tryptophan Mutants of Nicotiana tabacum

The branched chain amino acids (BCAAs) are synthesized by plants, algae, fungi, bacteria and archaea, but not by animals. Therefore, the enzymes of the BCAA biosynthetic pathway are potential targets for the development of herbicides, fungicides and antimicrobial compounds. Most interest has focused on the first enzyme in this pathway, acetohydroxyacid synthase (AHAS, EC 2.2.16) because it is the target of several herbicides including the sulfonylureas (SUs),1 imidazolinones (IMs),2 triazolopyrimidines3 and pyri-midinyl oxybenzoates.4,5 AHAS catalyzes the conden-sation of two molecules of pyruvate to give rise to 2-aceto-lactate in the first step of valine and leucine biosynthesis, and in parallel, it also catalyzes the condensation of pyruvate and 2-ketobutyrate to yield 2-aceto-2-hydroxybutyrate in the second step of isoleucine biosynthesis.6 AHAS require