H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Amino-acid biosynthesis; L-isoleucine biosynthesis; L-isoleucine from 2-oxobutanoate: step 1/4. Amino-acid biosynthesis; L-valine biosynthesis; L-valine from pyruvate: step 1/4. There are two distinct ALS genes in tobacco. The enzyme shown here is derived from the ALS gene on locus SuRB. Acetolactate synthase is the target enzyme for sulfonylurea and imidazolinone herbicides. Belongs to the TPP enzyme family.