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. Belongs to the TPP enzyme family.