The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

• Main Authors: Teigen Knut, Jensen Vidar R., Martinez Aurora
• Format: Article
• Language: English
• Published: De Gruyter 2005-02-01
• Series: Pteridines
• Subjects: phenylalanine hydroxylase. reaction mechanism; 5,6,7,8-tetrahydrobiopterin; density functional theory
• Online Access: https://doi.org/10.1515/pteridines.2005.16.1.27
• ISSN: 0933-4807; 2195-4720

Phenylalanine hydroxylase (PAH) is a non-heme iron and tetrahydrobiopterin-dependent enzyme that catalyzes the hydroxylation of L-phenylalanine to L-tyrosine using dioxygen as additional substrate. The cofactor tetrahydrobiopterin accepts one of the oxygen atoms of dioxygen during catalysis and also seems to be involved in prereduction of the active site iron from the ferric to the activated ferrous form. Structures of the truncated form of PAH in complex with substrate and cofactor are available, but the oxygen binding site and the actual mechanism of electron transfer are uncertain. It is believed that dioxygen binds directly to the metal, where it is activated, and several reaction mechanisms involving end-on binding of 0 2 have been proposed based on both experimental studies and quantum mechanical calculations. However, in this work we aimed to investigate the possibility of side-on binding of dioxygen to the iron. Furthermore, NMR and recent high-resolution crystallographic studies also place the cofactor in closer proximity to the iron, challenging the mechanistic conclusions from earlier crystallographic and computational studies. In this paper we report preliminary results from a density functional theory (DFT) study of the coordination of dioxygen to a structural model of PAH based on a recent crystallographic structure. These results are compared with existing computational and experimental data and their implications for the mechanism of the PAH-reaction are discussed. Particular attention is paid to the binding-mode of dioxygen and the iron-cofactor distance.