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 large and small chains. E.coli 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.