The PAH monomer (51.9 kDa) consists of three distinct domains: a regulatory N-terminal domain (residues 1-117), the catalytic domain (residues 118-427), and a C-terminal domain (residues 428-453) responsible for oligomerization of identical monomers.
The regulatory nature of the N-terminal domain (residues 1-117) is conferred by its structural flexibility [REF].