Domain structure and stability of human phenylalanine hydroxylase inferred from infrared spectroscopy

We have studied the conformation and thermal stability of recombinant human phenylalanine hydroxylase (hPAH) and selected truncated forms, corresponding to distinct functional domains, by infrared spectroscopy. The secondary structure of wild-type hPAH was estimated to be 48% alpha-helix, 28% extended structures, 12% beta-turns and 12% non-structured conformations. The catalytic C-terminal domain (residues 112-452) holds most of the regular secondary structure elements, whereas the regulatory N-terminal domain (residues 2-110) adopts mainly an extended and disordered, flexible conformation. Thermal stability studies of the enzyme forms indicate the existence of interactions between the two domains. Our results also demonstrate that the conformational events involved in the activation of hPAH by its substrate (L-Phe) are mainly related to changes in the tertiary/quaternary structure. The activating effect of phosphorylation, however, affects the secondary structure of the N-terminal domain of the protein.