Edited by Ruma Banerjee Dysfunction of human phenylalanine hydroxylase (hPAH, EC 1.14.16.1) is the primary cause of phenylketonuria, the most common inborn error of amino acid metabolism. The dynamic domain rearrangements of this multimeric protein have thwarted structural study of the full-length form for decades, until now. In this study, a tractable C29S variant of hPAH (C29S) yielded a 3.06 A ˚resolution crystal structure of the tetrameric restingstate conformation. We used size-exclusion chromatography in line with small-angle X-ray scattering (SEC-SAXS) to analyze the full-length hPAH solution structure both in the presence and absence of Phe, which serves as both substrate and allosteric activators. Allosteric Phe binding favors accumulation of an activated PAH tetramer conformation, which is biophysically distinct in solution. Protein characterization with enzyme kinetics and intrinsic fluorescence revealed that the C29S variant and hPAH are otherwise equivalent in their response to Phe, further supported by their behavior on various chromatography resins and by analytical ultracentrifugation. Modeling of resting-state and activated forms of C29S against SAXS data with available structural data created and evaluated several new models for the transition between the architecturally distinct conformations of PAH and highlighted unique intra-and intersubunit interactions. Three best-fitting alternative models all placed the allosteric Phe-binding module 8 -10 A ˚farther from the tetramer center than do all previous models. The structural insights into allosteric activation of hPAH reported here may help inform ongoing efforts to treat phenylketonuria with novel therapeutic approaches.

No abstract

Phenylalanine hydroxylase (PAH) is an allosteric enzyme responsible for maintaining phenylalanine (Phe) below neurotoxic levels; its failure results in phenylketonuria (PKU). PAH equilibrates among long‐lived conformations, including resting‐state (RS‐PAH) and activated (A‐PAH), whose equilibrium position depends upon allosteric Phe binding to the A‐PAH conformation. The RS‐PAH conformation contains a stabilizing cation‐pi sandwich between Phe80, Arg123, and Arg240 (PDB entry 5DEN), which cannot exist in the A‐PAH conformation. Intrinsic protein fluorescence, enzyme kinetic analysis, native PAGE, size exclusion chromatography, limited proteolysis, and behavior on ion exchange resin are reported for F80A, F80D, F80L, and F80R, many as a function of [Phe]. These data indicate that amino acid substitutions at Phe80 destabilizes both the RS‐PAH and A‐PAH conformations so that intermediate, on‐pathway conformations are longer lived. The addition of Phe allows stabilization of the A‐PAH conformation. Kinetic characterization of F80A and F80D reflects allosteric activation while F80L and F80R are constitutively active. The reaction rates of all Phe80 variants suggest relief of a rate determining conformational change present in the wild type protein. Limited proteolysis of WT rPAH in the absence of Phe reveals facile cleavage within a central C‐terminal 4‐helix bundle, reflecting dynamic dissociation of the PAH tetramer. Under these conditions, the Phe80 variants show proteolytic hypersensitity in a linker region that repositions in the RS‐PAH to A‐PAH conformational interchange; this is protected by addition of Phe. We conclude that manipulation of Phe80 dramatically affects the conformational space sampled by PAH, increasing the population of intermediates between RS‐PAH and A‐PAH. Support or Funding Information NIH 5R01‐NS100081; NIH P30 CA006927