The Accessibility of Iron at the Active Site of Recombinant Human Phenylalanine Hydroxylase to Water As Studied by 1H NMR Paramagnetic Relaxation

The high-spin ( S = 5/2) Fe(III) ion at the active site of recombinant human phenylalanine hydroxylase (PAH) has a paramagnetic effect on the longitudinal relaxation rate of water protons. This effect is proportional to the concentration of enzyme, with a paramagnetic molar-relaxivity value at 400 MHz and 25 °C of 1.3 (± 0.03) × 10 3 s −1 m −1 . The value of the Arrhenius activation energy ( E a ) for the relaxation rate was −14.4 ± 1.1 kJ/mol for the resting enzyme, indicating a fast exchange of water protons in the paramagnetic environment. The frequency dependence of the relaxation rate also supported this hypothesis. Thus, the recombinant human PAH appears to have a more solvent-accessible catalytic iron than the rat enzyme, in which the water coordinated to the metal is slowly exchanging with the solvent. These findings may be related to the level of basal activity before activation for these enzymes, which is higher for human than for rat PAH. In the presence of saturating (5 m m ) concentrations of the substrate l -Phe, the paramagnetic molar relaxivity for human PAH decreased to 0.72 (± 0.05) × 10 3 s −1 m −1 with no significant change in the E a . Effective correlation times (τ C ) of 1.8 (± 0.3) × 10 −10 and 1.25 (± 0.2) × 10 −10 s −1 were calculated for the enzyme and the enzyme-substrate complex, respectively, and most likely represent the electron spin relaxation rate (τ S ) for Fe(III) in each case. Together with the paramagnetic molar-relaxivity values, the τ C values were used to estimate Fe(III)-water distances. It seems that at least one of the three water molecules coordinated to the iron in the resting rat and human enzymes is displaced from coordination on the binding of l -Phe at the active site.