Catalyzes the first step in the biosynthesis of branched-chain amino acids. H(+) + 2 pyruvate = (2S)-2-acetolactate + CO2 Binds 1 FAD per subunit. The role of this cofactor is not clear considering that the reaction does not involve redox chemistry. However, after removal of the FAD no AHAS activity can be detected (PubMed:9581571), indicating that the cofactor is essential. The large subunit alone does not bind FAD, and FAD is not necessary for association of the subunits. Binds 1 Mg(2+) ion per subunit. Is not necessary for subunit association. Binds 1 thiamine pyrophosphate per subunit. Is not necessary for subunit association. Inhibited by the herbicides chlorimuron ethyl, chlorsulfuron and imazapyr. 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. Tetramer of two large (IlvG) and two small (IlvM) chains. E.coli contains genes for 3 AHAS isozymes: ilvBN, ilvGM and ilvIH. In strain K12 only the Val-inhibitable ilvBN and ilvIH are expressed; ilvGM has a frameshift that disturbs the ilvG reading frame. IlvG (this protein) is Val-resistant and is expressed in K12 strains having what used to be referred to as ilv0 mutations. The ilv02096 mutation (shown here, an insertion of 2 bp, PubMed:7015336) causes a frameshift which restores the open reading frame, permitting the expression of this isozyme. The inactive N-terminal pseudogene fragment is found here (AC P0DP89). Belongs to the TPP enzyme family.