Catalyzes the conversion of 2 pyruvate molecules into acetolactate in the first common step of the biosynthetic pathway of the branched-amino acids such as leucine, isoleucine, and valine. Also involved in condensing pyruvate and 2-ketobutyrate to form 2-aceto-2-hydroxybutyrate. H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Inhibited by valine, sulfometuron methyl (SM), sulfonylureas (SU) and imidazolinones (IM). Pyrazosulfuron ethyl (PSE), promisulfuron methyl (PSM), sulfometuron methyl (SMM), metsulfuron methyl (MSM), and chlorimuron ethyl (CE) inhibited more than 80% of the activity. Optimum pH is between 6 and 7. Optimum temperature is between 35 and 40 degrees Celsius. 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. Heterodimer of large catalytic subunit and small regulatory subunit. The expression is high during the mid-exponential phase and low during the stationary phase. Auxotrophic for all of the 3 branched-chain amino acids (isoleucine, leucine and valine), when grown with either C6 or C2 carbon sources. Depletion of these branched chain amino acids in the medium led to loss of viability. Belongs to the TPP enzyme family.