Arabidopsis thaliana (thale cress, or mouse-ear cress)
Herbicides are chemicals that kill weeds which are harmful to grass, field, and road. Sulfonylureas and imidazolinones are popular commercial herbicides because they are nontoxic to animals and highly selective to plants. These herbicides inhibit branched-chain amino acid biosynthesis in plants by targeting acetohydroxyacid synthase (AHAS), a thiamin diphosphate (ThDP)-dependent enzyme. However, herbicide-resistant weeds with mutant AHAS emerged in recent years. To understand how the mutant weeds have developed herbicide resistance, crystal structures of commercial herbicide-plant AHAS complexes are needed.
The structure described here is AHAS derived from a plant, Arabidopsis thaliana. AHAS forms a 4-fold symmetric homotetramer. Each subunit contains three domains, the alpha, beta, and gamma domain. With the exception of an additional small two-stranded antiparallel beta sheet in the alpha domain, each domain consists of a six-stranded parallel beta sheet surrounded by six to nine alpha helices. These domains and the C-terminal tail form the catalytic active site where not only substrates and ThDP, but also Mg2+ and FAD bind. Sulfonylureas and imidazolinones inhibit AHAS by binding within and obstructing the channel leading to the active site. It was expected that the two classes of the herbicides within the channel would overlap because both herbicides have two rings. Although, it is observed that they share some part of the binding site, no pair of the rings coincides. Of the 12 amino acid residues involved in imidazolinone-enzyme interactions, G654 and R199 are the only two residues that do not make contact with the sulfonylureas. This is because the side chains of the amino acids in the active site change their conformation in order to recognize the herbicides optimally; R199, M200, and W574 are located closer to imidazolinones, while D376 and R377 are located closer to sulfonylureas. Sulfonylurea- or imidazolinone-resistant weeds have mutations in these amino acids; the most common ones are substitutions for A122, P197, W574, or S653. These mutations are highly conserved across species and do not change the activity of the enzyme. The best characterized mutations are those of W574, which results in tolerance to both classes of herbicide. Not only is this residue important for defining the shape of the active site channel, but it also serves to anchor both classes of herbicide to the enzyme. Eight different amino acid substitutions for P197 are known to confer herbicide resistance. Whereas only P197L has been implicated in strong resistance to imidazolinones, all known mutations of this residue impart resistance to sulfonylureas. P197 is observed at one end of an alpha-helix at the entrance to the active site access channel. Therefore, it is likely that only a bulky amino acid substitution for P197 will impede the entry of imidazolines, whereas almost any substitution will prevent sulfonylurea access. Both S653N and A122T are known to induce strong resistance to the imidazolinones but not to the sulfonylureas. The structure of commercial herbicide-AHAS complexes provide a molecular basis for understanding the mutations, thus allowing more sophisticated AHAS inhibitors to be developed.
Protein Data Bank (PDB)
author: Daisuke Ino