{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nPhenylalanine hydroxylase (PAH) is a multidomain enzyme that plays a central role in amino‐acid metabolism by catalyzing the stereospecific hydroxylation of L‐phenylalanine to form L‐tyrosine. This reaction—mediated by an iron center and its essential cofactor tetrahydrobiopterin (BH₄)—represents the committed step in phenylalanine catabolism and is critical for preventing toxic buildup of phenylalanine. High‐resolution structural and biochemical studies have revealed that the PAH catalytic domain adopts a specific active‐site geometry, and that substrate binding induces extensive conformational rearrangements. These structure–function analyses, including X‑ray crystallography of both binary and ternary complexes and studies of regulatory phosphorylation events, have provided detailed insight into how PAH is organized to perform its catalytic function."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "5"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn addition to its intrinsic catalytic activity, the function of PAH is exquisitely regulated by dynamic conformational changes that are triggered by its substrate L‐phenylalanine and modulated by BH₄. Binding of L‐phenylalanine not only supplies the substrate for hydroxylation but also induces large-scale movements—such as repositioning of key residues (e.g., Tyr138)—that promote establishment of an active conformation. BH₄ serves a dual role as a cofactor and as a chemical chaperone that stabilizes the enzyme against inactivation; indeed, BH₄ binding can both attenuate inhibitory interactions at the active site and rescue certain destabilized (misfolded) PAH variants. Together, these features underscore the importance of allostery and protein dynamics in governing PAH’s catalytic efficiency."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "6", "end_ref": "14"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nDysfunction of PAH underlies the metabolic disorder phenylketonuria (PKU), and numerous studies have shown that missense mutations typically disrupt its normal folding and stability, thereby impairing enzymatic activity. In affected individuals, insufficient PAH activity results from both structural instability and defective allosteric regulation, and the residual activity observed in vitro is strongly linked to the severity of the metabolic phenotype. Detailed analyses of mutant PAH enzymes—including examination of subunit interactions, aggregation propensity, and the beneficial “chaperone‐like” effect of BH₄ supplementation—have refined our understanding of genotype–phenotype correlations. Moreover, recent work on the regulatory (ACT) domains and interactions with other cellular chaperones or even noncoding RNAs has illuminated additional layers by which PAH function may be modulated, opening avenues for targeted pharmacological rescue in BH₄‑responsive PKU patients."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "15", "end_ref": "30"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "O A Andersen, T Flatmark, E Hough "}, {"type": "b", "children": [{"type": "t", "text": "High resolution crystal structures of the catalytic domain of human phenylalanine hydroxylase in its catalytically active Fe(II) form and binary complex with tetrahydrobiopterin."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Mol Biol (2001)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1006/jmbi.2001.5061"}], "href": "https://doi.org/10.1006/jmbi.2001.5061"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "11718561"}], "href": "https://pubmed.ncbi.nlm.nih.gov/11718561"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Matthías Thórólfsson, Beatriz Ibarra-Molero, Peter Fojan, et al. 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"}, {"type": "b", "children": [{"type": "t", "text": "Structural comparison of bacterial and human iron-dependent phenylalanine hydroxylases: similar fold, different stability and reaction rates."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Mol Biol (2002)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/s0022-2836(02)00496-5"}], "href": "https://doi.org/10.1016/s0022-2836(02"}, {"type": "t", "text": "00496-5) PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12096915"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12096915"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Ole Andreas Andersen, Torgeir Flatmark, Edward Hough "}, {"type": "b", "children": [{"type": "t", "text": "Crystal structure of the ternary complex of the catalytic domain of human phenylalanine hydroxylase with tetrahydrobiopterin and 3-(2-thienyl)-L-alanine, and its implications for the mechanism of catalysis and substrate activation."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Mol Biol (2002)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/s0022-2836(02)00560-0"}], "href": "https://doi.org/10.1016/s0022-2836(02"}, {"type": "t", "text": "00560-0) PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12126628"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12126628"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "Frederico Faria Miranda, Knut Teigen, Matthías Thórólfsson, et al. 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