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== Function == Catalyzes the hydroxylation reaction to the amino acid L-Phenylalanine to L-Tyrosine. Catalyzes the rate-limiting step in the phenylalanine catabolism. Para-hydroxylation of the aromatic side-chain (rate-limiting step and the initial step). Catalyzes the hydroxylation of its substrate by incorporation of one oxygen atom into the aromatic ring, and the final reaction includes the reduction of the 2nd oxygen atom to water using electrons supplied by BH4. A metabolic enzyme involved in catabolism of L-Phe in the liver. This enzyme is responsible for the first step in processing phenylalanine, which is a building block of proteins obtained through the diet.

BH4 functions as a co-substrate that is hydroxylated at each turnover to pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzymes. The major regulatory mechanisms of PAH include activation of phenylalanine inhibition by BH4, and additional activation by phosphorylation. Substrate activation and positive cooperativity for Phe binding involves all 3 functional domains and all four subunits in the holoenzyme. Phe binds between the regulatory domain and the interacting catalytic domain, near the sequence binding motif. Hypothesized causes of Phe activation mechanism: Homotropic binding of Phe at active site and the regulatory domain is involved in cooperativity through the interactions with the catalytic and oligomerization domains. Phe binds to an allosteric site, besides the active site, on the regulatory domain, inducing large conformational changes. The allosteric. regulation is necessary to maintain Phe below neurotoxic levels. BH4 acts as a negative regulator by blocking Phe activation, however, BH4 binding to a Phe-activated form of PAH results in positive cooperativity. Phosphorylation acts as mediator of Phe activation by decreasing Phe concentration required to activate enzyme phosphorylation at Ser16.

==Function in the body/ Disease== L-Tyrosine is the precursor to neurotransmitters such as epinephrine, dopamine, and serotonin. PAH depletion or mutation leads to excessive accumulation of toxic L-Phe levels (physiological plasmatic levels <120 micromolar) This causes of the autosomal recessive metabolic disorder Phenylketonuria (PKU). PKU is a congenital disorder characterized by excessive amounts of L-Phenylalanine that buildup to neurotoxic amounts leading to cognitive disability and neurological impairment- profound mental retardation, seizures, microcephaly, and delayed development. The severity of mutation indicated the severity of PKU that leaded to increase the accumulation of phenylalanine in the patient blood with toxic effect. PAH secreted from the liver, and a variety of deficiency syndromes causing various levels of hyperphenylalaninemia have been observed in PKU patient. (Shebl 2019) finds, the most severe PKU was a 14-year-old female followed by a 6-month-old male and a 16-year-old male. Tyrosine biosynthesis as liver enzyme associated with melanin-associated physiological processes. Treatments include a lifelong diet avoiding foods containing phenylalanine and supplementation of synthetic formations of the cofactor tetrahydrobiopterin (BH4) PAH mutations result in reduced enzyme activity and stability and some alter its oligomeric state. Mutations spread throughout 3D structure, but most located in catalytic domain. Loss of enzymatic function caused mainly by folding defects leading to decreased stability. PAH proteins found in hepatocytes (liver cells). Found on chromosome 12 with 13 exons.

==Structure== Primary Structure:

452 residues/monomer Around 52kDa/monomer (Include image of entire amino acid sequence) Secondary Structure: Right-handed alpha helices Beta-strands that run antiparallel and parallel to one another Amino acids without secondary structure Loop containing regions (L42-V45), (D59-H69), (S70-D75), and (H82-V90) Tertiary Structure: Subunits/Monomers Have an alpha/beta structure Built from an N-terminal regulatory domain (residues 1-117), a catalytic domain (residues 118-410)-the catalytic domain includes the binding sites for iron, substrate and cofactor- and a tetramerization domain (residues 411-452) Structural motif 4 Beta strands and 2 alpha helices arranged in a BaBBaB fold Alpha-beta sandwich motif with a 4 stranded antiparallel Beta-sheet flanked on one side by 2 alpha-helices Binding sites at residues 285, 290, and 330 ACT domain: N-terminal regulatory domain where proposed enzyme binding to an allosteric site (residues 3-11) Quaternary Structure: Homotetramer Dimer of dimers Multidomain, homo-oligomeric Dihedral (D2) symmetry Iron containing enzyme (Fe+3) Iron binds to 2 histidines at active site Substrate: L-Phenylalanine The activation of PAH by L-phenylalanine induces a large conformational change Activation of PAH by L-Phe requires a slow global conformational change Activation rate is slower for the BH4-preincubated than for the unbound enzyme, associated to the negative regulation of PAH activation exerted by BH4 Full activation of PAH involves the shift and dimerization of the RDs Cofactors: 6R-L-erythro-tetrahydrobiopterin (BH4) and oxygen BH4 forms several hydrogen bonds with the N-terminal autoregulatory tail Prior to BH4 binding, (PAH unbound state) a polar and salt-bridge interaction network links the three PAH domains BH4 binding causes a limited conformational change (mostly constrained to the N-terminal tail) PAH lacking this tail is not regulated by either BH4 or L-phenylalanine and is constitutively active BH4 binding-site is flanked by the N-terminal (residues 21-32), the active-site lid (130-150), the Fe+2-coordinating residues, the Beta6-alpha7 loop (residues 245-251), and F254 BH4 is sandwiched between hydrophobic residues

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Structural highlights

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