EXPRESSION, PURIFICATION AND CHARACTERIZATION OF ARABIDOPSIS-THALIANAACETOHYDROXYACID SYNTHASE

Acetohydroxyacid synthase (EC 4.1.3.18) is the enzyme that catalyses t he first step in the synthesis of the branched-chain amino acids valin e, leucine and isoleucine. The AHAS gene from Arabidopsis thaliana wit h part of the chloroplast transit sequence removed was cloned into the bacterial expression vector pT7-7 and expressed in the Escherichia co li strain BL21(DE3). The expressed enzyme was purified by an extensive procedure involving (NK,),SO, fractionation followed by hydrophobic a nd anion-exchange chromatography. The purified enzyme appears as a sin gle band on SDS/PAGE with a molecular mass of about 61 kDa. On gel fil tration the enzyme is a dimer, migrating as a single peak with molecul ar masses of 109 and 113 kDa in the absence and presence of FAD respec tively. Ion spray MS analysis yielded a mass of 63 864 Da. The enzyme has optimum activity the pH range 6.5-8.5 and exhibits absolute depend ence on the three cofactors FAD, Mg2+ and thiamine diphosphate for act ivity. It displays negatively co-operative kinetics with respect to py ruvate concentration. A model was derived to explain the non-hyperboli c substrate-saturation curve, involving interaction between the active sites of the dimer. The K-m for the first active site was found to be 8.01 +/- 0.66 mM; the K-m for the second active site could not be acc urately determined but was estimated to be approx. 100 mM. The enzyme is insensitive to valine, leucine and isoleucine but is strongly inhib ited by the sulphonylurea herbicide, chlorsulphuron, and the imidazoli none herbicide, imazapyr. Inhibition by both herbicides exhibits slow- binding kinetics, whereas chlorsulphuron also shows tight-binding inhi bition.