H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Polyol metabolism; (R,R)-butane-2,3-diol biosynthesis; (R,R)-butane-2,3-diol from pyruvate: step 1/3. Strongly induced under anaerobic conditions. Activated by ResDE, Fnr and ArfM. Belongs to the TPP enzyme family.