The ACT domain is a structural motif of 70-90 amino acids that functions in the control of metabolism, solute transport and signal transduction. They are thus found in a variety of different proteins in a variety of different arrangements [1]. In mammalian phenylalanine hydroxylase the domain forms no contacts but promotes an allosteric effect despite the apparent lack of ligand binding [2].

1: The ACT domain: a small molecule binding domain and its role as a common regulatory element. Grant GA; J Biol Chem. 2006;281:33825-33829. PMID:16987805

2: Searching distant homologs of the regulatory ACT domain in phenylalanine hydroxylase. Siltberg-Liberles J, Martinez A; Amino Acids. 2009;36:235-249. PMID:18368466