Crystal Structures of <i>Arabidopsis thaliana</i> Acetohydroxyacid Synthase in Complex with the Herbicide Triasulfuron and Two Analogues with Herbicidal Activity in Field Trials

Triasulfuron is a commercial herbicide of the sulfonylurea family. This compound targets acetohydroxyacid synthase (AHAS, E.C. 2.2.1.6), the first enzyme in the branched chain amino acid biosynthesis pathway. Here, we have determined crystal structures of <i>Arabidopsis thaliana</i> AHAS (<i>At</i>AHAS) in complex with triasulfuron and two newly designed herbicidal compounds, identified as FMO and CMO, showing that their binding modes are subtly different. Kinetic studies showed all three compounds exhibit varying <i>K</i>_i values, 0.192 ± 0.013 μM for triasulfuron, 0.086 ± 0.013 μM for FMO, and 1.448 ± 0.058 μM for CMO, but all are strong time-dependent accumulative inhibitors of <i>At</i>AHAS. Apart from triasulfuron being a powerful herbicide with application rates of 10-15 g/ha in wheat fields, CMO and FMO are also herbicidal at 7.5-30 g/ha for barnyard grass. This study emphasizes that accumulative inhibition is an important factor that contributes to herbicidal activity.