Molecule of the Month: Acetohydroxyacid Synthase
In plants, AHAS performs the first step in synthesis of three essential amino acids, making it an effective target for herbicides.
Building Amino Acids
Exploring the Structure
AHAS in Action
Several crystallographic structures of yeast AHAS reveal steps of the catalytic reaction, and how herbicides block it. The active site uses several cofactors to assist with the reaction. A thiamine cofactor (TPP, shown in yellow with the reactive ring in orange) performs the central reaction that removes carbon dioxide. A magnesium ion (Mg, turquoise) positions the cofactor. AHAS also binds to FAD (magenta), which must be in the reduced state in the active enzyme, and several oxygen molecules of unknown function (not shown here). PDB ID 6bd9 includes two pyruvate molecules (red and pink) entering the active site and PDB ID 6bd3 captures the enzyme after one of these has attached to the thiamine cofactor and carbon dioxide has been removed. PDB ID 5wkc has a herbicide molecule (penoxsulam, in green) that blocks the entryway into the active site. It also leads to additional problems: in this structure, the attached substrate molecule has been converted into a reactive oxygenated molecule. In another subunit in this PDB entry, the reactive molecule (ethaneperoxoic acid) has been released, leaving a damaged thiamine cofactor with a broken ring. Ethaneperoxoic acid can also oxidize the FAD, further inhibiting the action of the enzyme. To explore these structures in more detail, click on the image for an interactive JSmol.
Topics for Further Discussion
- Several different classes of herbicides block the action of AHAS. They all block entry of substrates into the active site, but have different structures and bind in slightly different ways. Try searching for “AHAS” at the main RCSB site to explore these structures.
- Note that when you’re exploring the many AHAS structures in the PDB archive, you’ll encounter complexes with many shapes and sizes. For example, many of these structures include only a dimer of the catalytic subunits, which often associate to form tetramers when herbicides are bound, so you won’t see the characteristic cross-shaped assembly found in the structures in this article.
November 2021, David Goodselldoi:10.2210/rcsb_pdb/mom_2021_11