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= Phenylalanine Hydroxylase = | = Phenylalanine Hydroxylase = | ||
<Structure load='2PAH_tetramer3.pdb' size='500' frame='true' align='right' caption='This is a model of the pheylalanine hydroxylase dimer as found in humans. The green ball in within each subunit represents the iron ion in the catalytic domains.' scene='Insert optional scene name here' /> | |||
'''Phenylalanine Hydroxylase'''(PheOH), otherwise known as phenylalaine-4-monooxygenase, is an enzyme produced by the ''PAH'' gene found on the twelfth chromosome in the human genome, but it is also found in some bacteria. This enzyme functions as a catalyst in the conversion of the amino acids phenylalanine to tyrosine by adding a hydroxyl group (-OH) to the benzene ring of the amino acid. This is why this protein is therefore classified as a hydroxylase. In most organisms, this hydroxylation process is the first step in phenylalanine degradation. A faulty ''PAH'' gene can cause an increase in phenylalanine level in the plasma, resulting in the genetic disorder Phenylketonuria (PKU).<ref> College, Davidson. The Structure-Function of Phenylalanine Hydroxylase [http://www.bio.davidson.edu/courses/molbio/molstudents/spring2005/castle/assign1home.html]</ref> Other proteins in this classification include Tryptophan and Tyrosine Hydroxylase. These three amino acid hydroxylases share a highly conserved region of 27 amino acids from His263 to His289. <ref> Erlandsen H., DirSci; Marianne G. Patch, PhD; Alejandra Gamez, PhD; Mary Straub; and Raymond C. Stevens, PhD. Structural Studies on Phenylalanine Hydroxylase and Implications Toward Understanding and Treating Phenylketonuria [http://www.pkuworld.org/home/docs/literature/erlandsen_2003_p.pdf]</ref> | |||
''' | == '''Structure''' == | ||
PheOH can exist as a dimer or tetramer with identical subunits. Each subunit is organized to have a regulatory, a catalytic and a tetramerization domain. The native form of human PheOH has an estimated secondary structure composed 48% alpha-helices, 28% extended structures, 12% beta-turns, and 12% non-structured conformations. The more structured elements are usually concentrated in the catalytic C-terminal domain of the protein, while the more flexible and unstructured elements are grouped in the regulatory N-terminal domain.<ref> Chehin, R., M. Thorolfsson, PM. Knappskog, A. Martinez, T. Flatmark, JL. Arrondo, and A. Muga,Domain structure and stability of human phenylalanine hydroxylase inferred from infrared spectroscopy[http://www.ncbi.nlm.nih.gov/pubmed/9490012]</ref> The PheOH model protein was generated via x-ray crystallography.<ref> Erlandsen H., DirSci; Marianne G. Patch, PhD; Alejandra Gamez, PhD; Mary Straub; and Raymond C. Stevens, PhD. Structural Studies on Phenylalanine Hydroxylase and Implications Toward Understanding and Treating Phenylketonuria [http://www.pkuworld.org/home/docs/literature/erlandsen_2003_p.pdf]</ref> | |||
'''Catalytic Domain''' | |||
The <scene name='Sandbox_Reserved_642/Catalytic_domain/1'>catalytic domain</scene> of phenylalanine hydroxylase includes resides 143-410. This region has a basket-like arrangement consisting of 13 alpha-helices and 8 beta-strands. This region of the protein also includes the active site. The active site of PheOH can be found in the center of the catalytic domain and is characterized by a 13 Angstroms deep and 10 Angstroms wide hydrophobic pocket. Lining the active site are 3 glutamates, 2 histadines and 1 tyrosine residue along with hydrophobic residues for a total of 34 amino acids. Covering the entrance of the active site is a short loop consisting or residues 378-381. | |||
The center of each catalytic domain consists of an iron ion which is vital to the enzyme activity. The iron atom binds in the active site to <scene name='Sandbox_Reserved_642/Iron_binding/2'>histadine residues 285 and 290, 1 oxygen atom in glutamate 330</scene>. Histadine 285 and 290 were found to be required for the binding of iron through site directed mutagenisis studies. The iron ions are coordinated to three water molecules and arrange in an octahedral geometry. The active site also binds the | |||
<scene name='Sandbox_Reserved_642/Cofactor/1'>cofactor tetrahydrobiopterin</scene>. This cofactor binds closely to the iron ion and forms hydrogen bonds with two of the three water molecules. The cofactor also forms hydrogen bonds with the carbonyl oxygen of the protein residues including Ala322, Gly247, and Leu249 and the amide of Leu249.<ref> Erlandsen H., DirSci; Marianne G. Patch, PhD; Alejandra Gamez, PhD; Mary Straub; and Raymond C. Stevens, PhD. Structural Studies on Phenylalanine Hydroxylase and Implications Toward Understanding and Treating Phenylketonuria [http://www.pkuworld.org/home/docs/literature/erlandsen_2003_p.pdf]</ref> | |||
'''Tetramerization Domain''' | |||
Phenylalanine Hydroxylase exists in equilibrium between a homodimer and a homotetramer. The region responsible for the tertamerization is the <scene name='Sandbox_Reserved_642/Tetramerization_domain/1'>tetramerization domain</scene> located at the C terminal end of the protein. It consists of residues 411-452. The tetramerization domain consists of 2 beta-strands forming a beta-ribbon and an alpha-helix that is 40 angstroms long. The four alpha helices, consisting of one from each monomer, pack into a coil coil motif with the helices arranged in an anti-parallel manner.<ref> Erlandsen H., DirSci; Marianne G. Patch, PhD; Alejandra Gamez, PhD; Mary Straub; and Raymond C. Stevens, PhD. Structural Studies on Phenylalanine Hydroxylase and Implications Toward Understanding and Treating Phenylketonuria [http://www.pkuworld.org/home/docs/literature/erlandsen_2003_p.pdf]</ref> | |||
'''Regulatory Domain''' | |||
Housed in the N-terminus, the regulatory domain contains residues 19-142 and is more flexible than the other domains. The core of this domain contains an alpha beta sandwich and a beta alpha beta double motif. <ref> Bostjan Kobe, Ian G. Jennings, Colin M. House1, Belinda J. Michell, Kenneth E. Goodwill, Bernard D. Santarsiero, Raymond C. Stevens, Richard G. H. Cotton and Bruce E. Kemp. Nature Structural Biology 6, 442 - 448 (1999), Structural basis of autoregulation of phenylalanine hydroxylase, [http://http://www.nature.com/nsmb/journal/v6/n5/full/nsb0599_442.html]</ref> | |||
== '''Mechanism''' == | == '''Mechanism''' == | ||
Although the exact mechanism of phenylalanine degradation is still not fully understood, the main reaction requires the addition of an hydroxyl group to the benzene ring of the phenylalanine residue. In order for this process to occur, the cofactor tetrahydrobiopterin(BH4) loses two hydrogen atoms to become dihydrobiopterin. BH4 acts as a reductant by reducing one of the diatomic oxygens while the other is added to the 6-membered ring. In order to stabilize the substrate- enzyme complex as this reaction occurs, an iron atom within the protein is necessary. It is within the active site that the hydrogen atom from phenylalanine is stripped off and replaced with a hydroxyl group. | Although the exact mechanism of phenylalanine degradation is still not fully understood, the main reaction requires the addition of an hydroxyl group to the benzene ring of the phenylalanine residue. In order for this process to occur, the cofactor tetrahydrobiopterin(BH4) loses two hydrogen atoms to become dihydrobiopterin. BH4 acts as a reductant by reducing one of the diatomic oxygens while the other is added to the 6-membered ring. In order to stabilize the substrate- enzyme complex as this reaction occurs, an iron atom within the protein is necessary. It is within the active site that the hydrogen atom from phenylalanine is stripped off and replaced with a hydroxyl group.<ref> College, Davidson. Phenylalanine Hydroxylase [http://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2010/Piper/Protein.html] </ref> | ||
[[Image:Phenylalanine_Hydroxylase_mechanism.jpg |thumb| | [[Image:Phenylalanine_Hydroxylase_mechanism.jpg |thumb|200 px|center|Reaction catalyzed by PheOH]] | ||
== '''Implications or Possible Applications''' == | == '''Implications or Possible Applications''' == | ||
The first diagnosed cases of Phenylketonuria (PKU), otherwise known as Folling's Disease, were identified in 1934 by Norwegian doctor and biochemist Asbjorn Folling. Dr. Folling found that the urine of two of his young mentally handicapped patients contained a high level of phenylalanine. Follwing this discovery, it was found that the absence or malfunction of the phenylalanine hydroxylase enzyme is due to the mutation of the ''PAH'' gene and inherited autosomal recessively. This may result in a genetic disorder known as Phenylketonuria (PKU). This information was not utilized until the early 1950s when it was found that under a low phenylalanine diet, some of the symptoms found in children suffering from PKU could be reversed. Due to a diet rich in phenylalanine, this enzyme is vital in the regulation in phenylalanine plasma concentration by converting about 75% of the amino acid to tyrosine. Excessive amounts of phenylalanine has been shown to cause mental retardation in humans. Presently, it is regulation to screen newborns children for phenylketonuria with a simple blood or urine test. <ref> January 2005: Phenylalanine Hydroxylase [http://www.pdb.org/pdb/education_discussion/molecule_of_the_month/download/PhenylalanineHydroxylase.pdf]</ref> Due to his discovery and development of the PKU test, Dr. Folling is remembered as one of the most important medical scientists that has not received a Nobel Prize for Physiology or Medicine. <ref> Dr. Ivar Asbjorn Folling- The Man Who discovered PKU Disorder [http://http://www.pkutest.com/2012/07/12/dr-ivar-asbjorn-folling-discovered-pku-disorder/]</ref> | |||
==== Symptoms ==== | |||
[[Image:PKU diet.jpg|thumb|300 px|right|PKU diet]] | |||
Phenylalanine plays a variety of roles in the body among which is the production of melanin, the pigment responsible for hair and skin color. Infants with an overabundance of this residue may therefore have a lighter skin, hair and eye color than those who do not. <ref> A.D.A.M Medical Encyclopedia, Phenylketonuria. [http://http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002150/]</ref> | |||
Other symptoms may include: | |||
- Delayed mental and social skills | |||
- Head size significantly below normal | |||
- Hyperactivity | |||
- Jerking movements of the arms or legs | |||
- Mental retardation | |||
- Seizures | |||
- Skin rashes | |||
- Tremors | |||
- Unusual positioning of hands | |||
==== Treatment ==== | |||
Treatment for such a PKU is a low phenylalanine diet and early detection. Those who start the diet early and adhere to it will have better mental and physical health. Infants diagnosed with the disease can fed a specially made formula called Lofenalac while others should follow a diet plan as illustrated in the image to the left. The main rule to follow is to avoid protein sources rich in phenylalanine and sugars containing aspartame. Taking extra supplements like fish oil can replace the fatty acids missing from the phenylalanine free diet and may also improve neurological development. PKU can also be caused by a deficiency in or inability to regenerate tetrahydrobipternin, the cofactor essential to the function of PheOH. Although this is not usually the cause of PKU, patients can be treated by taking tetrahydrobiopterin supplements. If the diet is not strictly followed, mental retardation may result after the first year of life. <ref> A.D.A.M Medical Encyclopedia, Phenylketonuria. [http://http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002150/]</ref> | |||
== '''References''' == | |||
<references/> | |||