L-threonine = 2-oxobutanoate + NH4(+) Isoleucine allosterically inhibits while valine allosterically activates this enzyme. Amino-acid biosynthesis; L-isoleucine biosynthesis; 2-oxobutanoate from L-threonine: step 1/1. Homotetramer. In response to starvation for tryptophan and branched-chain amino acid imbalance. Present with 11900 molecules/cell in log phase SD medium. Belongs to the serine/threonine dehydratase family.