PKU mutation p.G46S prevents the stereospecific binding of L-phenylalanine to the dimer of human phenylalanine hydroxylase regulatory domain

Mammalian phenylalanine hydroxylase (PAH) has a potential allosteric regulatory binding site for L-phenylalanine (L-Phe), in addition to its catalytic site. This arrangement is supported by a crystal structure of a homodimeric truncated form of the regulatory domain of human PAH (hPAH-RD1-118/19-118) [Patel D et al. (2016) Sci Rep doi: 10.1038/srep23748]. In this study, a fusion protein of the domain (MBP-(pep(Xa))-hPAH-RD1-120) was overexpressed and recovered in a metastable and soluble state, which allowed the isolation of a dimeric and a monomeric fusion protein. When cleaved from MBP, hPAH-RD forms aggregates which are stereospecifically inhibited by L-Phe (> 95%) at low physiological concentrations. Aggregation of the cleaved dimer of the mutant form hPAH-G46S-RD was not inhibited by L-Phe, which is compatible with structurally/conformationally changed babbab ACT domain folds in the mutant.