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The quaternary structure of PAH is a homotetramer, dimer of dimers. It is a multidomain, homo-oligomeric protein with dihedral (D2) symmetry (1 & 5).

<Structure load='2PHM' size='250' frame='true' align='left' caption='Human Tetramer' scene='Insert optional scene name here' />

The substrate of phenylalanine hydroxylase is the amino acid L-phenylalanine. Phenylalanine binds between the regulatory domain and the interacting catalytic domain, near the sequence binding motif. The activation of phenylalanine hydroxylase by L-phenylalanine induces a large conformational change, but a slow global conformational change. Full activation of PAH involves the shift and dimerization of the regulatory domains (1).

The genetic information that codes for the production of phenylalanine hydroxylase is found on the long arm of chromosome 12 and contains 13 exons (7, 9, & 11). PAH is a metabolic enzyme contained in liver cells that catalyzes the hydroxylation reaction of the amino acid L-Phenylalanine to L-Tyrosine (2, 6, & 10). This protein specifically catalyzes the rate-limiting step in the phenylalanine catabolism, which is the para-hydroxylation step of the aromatic side chain (2 & 6). This catalysis is done by hydroxylation of its substrate by incorporation of one oxygen atom into the aromatic ring, and the final reaction includes the reduction of the second oxygen atom to water using electrons supplied by tetrahydrobiopterin (BH4). BH4 functions as a co-substrate that is hydroxylated at each turnover to pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzyme (6).  

<Structure load='1KWD' size='250' frame='true' align='right' caption='Catalytic Domain of PAH' scene='Insert optional scene name here' />

<Structure load='6HYC' size='250' frame='true' align='right' caption='Cofactor and Negative Regulator BH4='Insert optional scene name here' />

L-Tyrosine is the precursor to neurotransmitters such as epinephrine, dopamine, and serotonin (11). It is essential that L-phenylalanine is converted into L-tyrosine by the hydroxylation reaction. In order for this conversion to be successful, the enzyme phenylalanine hydroxylase needs to be able to function properly. PAH depletion or mutation leads to excessive accumulation of toxic L-Phe levels. However, normal physiological plasmatic levels of L-phenylalanine are less than 120 micromolar (6). Not only does dysfunctional PAH lead to the increased phenylalanine levels in the blood, but it also has the appearance of urine metabolites that arise from the transamination of L-Phe to phenylpyruvate (6). When the enzyme PAH doesn’t function correctly, the  autosomal recessive metabolic disorder Phenylketonuria (PKU) occurs. PKU is a congenital disorder characterized by excessive amounts of L-phenylalanine that buildup to neurotoxic amounts leading to cognitive disability and neurological impairment, including profound mental retardation, seizures, microcephaly, and delayed development (1, 5, & 10). The severity of PKU is dependent upon the severity of the enzyme’s mutation (4 & 5). PAH mutations result in reduced enzyme activity and stability and some alter its oligomeric state (1). These mutations spread throughout the 3D structure, but most are located in the catalytic domain (4). Loss of enzymatic function is caused mainly by folding defects that lead to decreased protein stability (1). The mutation leads to a truncated form of the final 52 amino acids. These C-terminal amino acids are a part of the tetramerization domain. Another frequent mutation is a CGG-to-TGG transition on exon 12. This mutation leads to a substitution of Arginine for Tryptophan on position 408. This missense mutation results in undetectable levels of phenylalanine and the severe PKU phenotype (4).

Treatments for phenylketonuria include a lifelong diet avoiding foods containing phenylalanine and supplementation of synthetic formations of the cofactor tetrahydrobiopterin (BH4) (8). Testing for PKU can be done early on in the lifespan to determine if the disease is present and to start avoiding foods containing phenylalanine (5).