Sandbox Reserved 768

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This Sandbox is Reserved from Sep 25, 2013, through Mar 31, 2014 for use in the course "BCH455/555 Proteins and Molecular Mechanisms" taught by Michael B. Goshe at the North Carolina State University. This reservation includes Sandbox Reserved 299, Sandbox Reserved 300 and Sandbox Reserved 760 through Sandbox Reserved 779.

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spinqualitylabelsall models Dimeric Unit of Phenylalanine Hydroxylase with the iron atom in the active sites in brown. Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Phenylalanine Hydroxylase (also known as Phenylalanine-4-monooxygenase or simply PAH) is the enzyme that catalyzes the conversion of L-phenylalanine into L-tyrosine by hydroxylation (addition of an -OH group) of the aromatic side chain of phenyalanine. This reaction is the initial and rate-limiting step in the phenylalanine catabolism pathway. The tyrosine product (a non-essential amino acid) can then serve as a precursor to the synthesis of important neurotransmitters.^[1]. PAH uses tetrahydrobiopterin (BH4)as a cofactor and has a nonheme iron atom bound to its active site. PAH is classified as an oxidoreductase, specifically enzyme class EC 1.14 since its mechanism of action involves the oxidation/reduction of its substrate. ^[2]. PAH enzyme is present in the liver cells of humans and other mammals. It is also present in non-mammalian eukaryote organisms and some bacteria such as E.coli. ^[2]. Recently, it has been identified in some protozoans and slime molds, and even in nonflowering plants such as spinach from which it has been extracted and studied. ^[3]. Mammalian PAH is a homo-tetrameric enzyme of 50 kDa subunits composed of two asymmetric dimeric units. The four subunits are connected to each other via a coiled-coil motif as shown in the diagram below.

Each monomeric subunit is composed of three sites: the N-terminal, the catalytic site, and the C-terminal. ^[1]. PAH enzyme has been studied extensively because of its correlation with the genetic defective condition phenylketonuria (PKU). Errors in the function or stability of PAH lead to its malfunction which causes a buildup of phenylalanine resulting in numerous health detriments.

Phenylalanine Hydroxylase Mechanism of Action

PAH, belonging to the oxidoreductase enzyme class, acts on paired donors with pteridine being a donor and first incorporates one atom of molecular oxygen into the aromatic ring of phenylalanine. Then, it reduces the second oxygen atom to water using the two electrons that are supplied by the BH4 cofactor. BH4 is also hydroxylated at each turnover to produce pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzyme. 4a-OHBH4 is dehydrated and reduced back to BH4 by the action of the enzyme pterin carbinolamine dehydratase. ^[1].