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. Dimer of large and small chains. 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.