Journal
FEBS JOURNAL
Volume 276, Issue 5, Pages 1282-1290Publisher
WILEY
DOI: 10.1111/j.1742-4658.2009.06863.x
Keywords
acetohydroxyacid synthase; branched-chain amino acids; crystal structure; herbicide; sulfonylurea
Categories
Funding
- Australian Synchrotron Research Program
- Commonwealth of Australia
- Department of Energy, Basic Energy Sciences, Office of Energy Research [W-31-109-Eng-38]
- Australian Research Council [DP0450275]
- National Basic Research Program of China [2003CB114406]
- National Natural Science Foundation Project of China [20432010, 20602021]
- Australian Research Council [DP0450275] Funding Source: Australian Research Council
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Acetohydroxyacid synthase (AHAS; EC 2.2.1.6) is the first enzyme in the biosynthetic pathway of the branched-chain amino acids. It catalyzes the conversion of two molecules of pyruvate into 2-acetolactate or one molecule of pyruvate and one molecule of 2-ketobutyrate into 2-aceto-2-hydroxybutyrate. AHAS requires the cofactors thiamine diphosphate (ThDP), Mg2+ and FAD for activity. The herbicides that target this enzyme are effective in protecting a broad range of crops from weed species. However, resistance in the field is now a serious problem worldwide. To address this, two new sulfonylureas, monosulfuron and monosulfuron ester, have been developed as commercial herbicides in China. These molecules differ from the traditional sulfonylureas in that the heterocyclic ring attached to the nitrogen atom of the sulfonylurea bridge is monosubstituted rather than disubstituted. The structures of these compounds in complex with the catalytic subunit of Arabidopsis thaliana AHAS have been determined to 3.0 and 2.8 angstrom, respectively. In both complexes, these molecules are bound in the tunnel leading to the active site, such that the sole substituent of the heterocyclic ring is buried deepest and oriented towards the ThDP. Unlike the structures of Arabidopsis thaliana AHAS in complex with the classic disubstituted sulfonylureas, where ThDP is broken, this cofactor is intact and present most likely as the hydroxylethyl intermediate.
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