Possible Contributions of Labile Asparagine Residues to Differences in Regulatory Properties of Human and Rat Phenylalanine Hydroxylase

Abstract

Phenylalanine hydroxylase (PAH, phenylalanine 4-monooxygenase, EC 1.14.16.1) catalyzes a single hydroxylation by an intermolecular oxygenation reaction of an aromatic ring (L-phenylalanine, L-Phe) in an Fe(II)- and pterin-dependent fashion (reviewed in (1,2)). X-ray crystal structure analyses of human and rat PAH (hPAH and rPAH) have revealed that the monomer is organized into three main domains, i.e. a N-terminal regulatory domain, a catalytic domain and a small C-terminal dimerization/tetramerization domain (reviewed in (2)). PAH isolated from rat liver and as recombinant human enzyme are both in a tetramer dimer equilibrium (3,4). The tetrameric form is a dimer of dimers with defined interphases between the subunits in the dimers. The 41 C-terminal residues that comprise the tetramerization motif are organized into an “arm” consisting of two short ß strands, forming a ß ribbon, and a 40-Å long α helix (Gln428-Lys452)(5). The helices interact with each other, forming an anti-parallel coiled coil structure in the center of the tetramers and mutually switch their position to promote tetramerization (5). A comparison of the crystal structure of hPAH (5,6) and rPAH (7) have revealed the same overall fold for the highly conserved catalytic domain of the two enzyme forms. Thus, the human and rat enzyme share 96% sequence homology and have several molecular and kinetic properties in common (8).