Catalyzes the formation of acetolactate from pyruvate, the first step in valine and isoleucine biosynthesis. H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Inhibited by asymmetric aryl disulfides, triazolopyrimidine sulfonanilide compounds, isatin derivatives, and sulfonylurea and imidazolinone herbicides. Insensitive to feed-back inhibition by branched-chain amino acids. Optimum pH is 7.0-7.5. 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. Homodimer or homotetramer. The acetolactate synthase complex contains both large catalytic subunits and small regulatory subunits. Lethal when homozygous. Introduced by genetic manipulation and expressed in sulfonylurea resistant plants. Belongs to the TPP enzyme family.