Sensitive intracavity photoacoustic measurements with a CO2 waveguide laser

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Correction of kinetic and stability defects by tetrahydrobiopterin in phenylketonuria patients with certain phenylalanine hydroxylase mutations

Phenylketonuria patients harboring a subset of phenylalanine hydroxylase (PAH) mutations have recently shown normalization of blood phenylalanine levels upon oral administration of the PAH cofactor tetrahydrobiopterin [(6R)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH(4))]. Several hypotheses have been put forward to explain BH(4) responsiveness, but the molecular basis for the corrective effect(s) of BH(4) has not been understood. We have investigated the biochemical, kinetic, and structural changes associated with BH(4)-responsive mutations (F39L, I65T, R68S, H170D, E178G, V190A, R261Q, A300S, L308F, A313T, A373T, V388M, E390G, P407S, and Y414C). The biochemical and kinetic characterization of the 15 mutants studied points toward a multifactorial basis for the BH(4) responsiveness; the mutants show residual activity (>30% of WT) and display various kinetic defects, including increased K(m) (BH(4)) and reduced cooperativity of substrate binding, but no decoupling of cofactor (BH(4)) oxidation. For some, BH(4) seems to function through stabilization and protection of the enzyme from inactivation and proteolytic degradation. In the crystal structures of a phenylketonuria mutant, A313T, minor changes were seen when compared with the WT PAH structures, consistent with the mild effects the mutant has upon activity of the enzyme both in vitro and in vivo. Truncations made in the A313T mutant PAH form revealed that the N and C termini of the enzyme influence active site binding. Of fundamental importance is the observation that BH(4) appears to increase Phe catabolism if at least one of the two heterozygous mutations has any residual activity remaining