Catalytic subunit of mitochondrial acetolactate synthase, which catalyzes the first of a series of common steps in the biosynthesis of the branched-chain amino acids. Catalyzes the irreversible decarboxylation of pyruvate to a bound hydroxyethyl group that then condenses with either a second pyruvate molecule to form 2-acetolactate (AL) or with 2-ketobutyrate to form 2-aceto-2-hydroxybutyrate (AHB). The first product is the precursor for valine and leucine biosynthesis, while the second leads to isoleucine. H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 2-oxobutanoate + H(+) + pyruvate = (S)-2-ethyl-2-hydroxy-3-oxobutanoate + CO2 Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. The regulatory subunit ILV6 stimulates enzymatic activity seven- to tenfold and confers sensitivity to inhibition by valine and activation by ATP. Optimum pH is 7-7.5. 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. The acetolactate synthase complex contains the catalytic subunit ILV2 and the regulatory small subunit ILV6. Contains 1 molecule of FAD per monomer. The role of this cofactor is not clear considering that the reaction does not involve redox chemistry. Present with 31900 molecules/cell in log phase SD medium. Belongs to the TPP enzyme family.