In molecular biology, the ACT domain is a protein domain that is found in a variety of proteins involved in metabolism. ACT domains are linked to a wide range of metabolic enzymes that are regulated by amino acid concentration. The ACT domain is named after three of the proteins that contain it: aspartate kinase, chorismate mutase and TyrA. The archetypical ACT domain is the C-terminal regulatory domain of 3-phosphoglycerate dehydrogenase (3PGDH), which folds with a ferredoxin-like topology. A pair of ACT domains form an eight-stranded antiparallel sheet with two molecules of allosteric inhibitor serine bound in the interface. Biochemical exploration of a few other proteins containing ACT domains supports the suggestions that these domains contain the archetypical ACT structure.^[1]

The ACT domain was discovered by Aravind and Koonin using iterative sequence searches.^[2]