H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Sensitive to valine inhibition. 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. Dimer of chain H and chain I. S.typhimurium contains genes for 3 AHAS isozymes: ilvBN, ilvGM and ilvIH. Contains 1 molecule of FAD per monomer. The role of this cofactor is not clear considering that the reaction does not involve redox chemistry (By similarity). Belongs to the TPP enzyme family. Truncated C-terminus.