What is Phenylketonuria

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What is Phenylketonuria?

• Autosomal Recessive Disorder.
• Inherited error of metabolism caused by
  deficiency in the enzyme phenylalanine
  hydroxylase (PAH).
• Mutation in both alleles of the gene for the
  enzyme.
• Chromosome 12.
• Recessive allele carried by 1 out of every 60
  individuals.

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Phenylketonuria

• By Ilyssa Ramos
• Biochemistry II

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Autosomal Recessive Disorders
http//upload.wikimedia.org/wikipedia/commons/thum
b/3/3e/Autorecessive.svg/250px-Autorecessive.svg.p
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• Symptoms Appear within the 1st year
• Vomiting/Convulsions
• Mental Retardation
• Albinism
• Light skin and hair color
• Two forms
• Common - defect in Phenylalanine Hydroxylase
• Rarer - defect in tetrahydrobiopterin synthesis
• Proposed Treatment
• Low intake of phenylalanine (synthetic diet)
• must contain Tyrosine
• Adult phenylketonurics can go off diet with only
  minor symptoms.
• But mothers-to-be should be on synthetic diet at
  least 3 months prior to conception

www.uhmc.sunysb.edu/som/courses/mgac/ANLect8AACa
tabC.ppt
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Phenylalanine

• L-phenylalanine-natural form found in proteins
  throughout the body.
• 1of 20 amino acids.
• Not manufactured in vivo.
• Obtained through consumption of foods that
  contain the amino acid.
• A vital component of proteins
• Chief precursor for numerous aromatic compounds
  needed for bodily function.

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http//www.newbornscreening.info/tools/GraphicsLib
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Normal Metabolism
www.uic.edu/.../PKU20biochem20intro.htm
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Degradation Pathway

• Enzymes Used
• Phenylalanine Hydroxylase
• Aminotransferase
• p-hydroxyphenylpyruvate
• Homogenistate dioxygenase
• Maleylacetoacetate isomerase
• Fumarylacetoacetase

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Final Products of the Pathway

• Breakdown of glucose is considered
• Glucogenic-converted to glucose by
  gluconeogenesis.
• Ketogenic-converted to a ketone body.
• Final products of 6-step degradation pathway
  include
• Fumarate-Citric Acid Cycle intermediate.
• Convert to oxaloacetate stimulating
  gluconeogenesis.
• Glucose exported into the blood.
• Acetoacetate-Ketone body.
• Used by neurons, and muscle tissue in oxidative
  degradation.

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The Initial Step of Degradation

• Rate limiting step in Phe catabolism.
• PKU is caused from problems with
• Phenylalanine hydroxylase.
• Dihydrobiopterin reductase.
• Tetrahydrobiopterin (BH4)

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Phenylalanine Degradation

• Phenylalanine is converted to tyrosine via PAH
  during the 1st degradation step.
• After initial step of phenylalanine catabolism
  the degradation pathway is the same as that of
  tyrosine.
• Tyrosine is a direct product of the breakdown of
  phenylalanine.

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http//www.med.ufl.edu/biochem/plaipis/html/Teachi
ng/VEM5131-Pku.pdf
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Abnormal Phenylalanine Metabolism
www.uic.edu/.../PKU20biochem20intro.htm
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Phenylalanine Hydroxylase

• Liver specific enzyme.
• Member of the monoxygenase group.
• Can be allosterically inhibited by its substrate,
  phenylalanine.
• Main function is to maintain a low yet steady
  amount of phenylalanine circulating in the blood.
• Catalyzes the formation of L-tyrosine from
  L-phenylalanine using
• (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4)
• Molecular Oxygen

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Phenylalanine Hydroxylase

• In its native environment PAH exists as
  homotetramers and homodimers
• Enzyme is composed of 3 domains
• N-terminal-regulatory domain (1-142)
• Catalytic domain (143-410)
• C-terminal-tetramerization domain (411-452)
• Note The C-terminal is said to contain the
  catalytic domain.

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N-terminal Regulatory Domain

• Used for auto-regulation.
• Extends over active site located within the
  catalytic domain.
• Regulatory domain of PAH consists of
• An a-ß sandwich
• A ßaß motif.
• The crystal structure of PAH tetramer showed that
  the enzyme contains
• 2 ß strands which form a ß ribbon.
• 40 Å long a-helix.
• Four a helices pack into tight antiparallel
  coiled coil located centrally in the tetramer
  structure.

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C-terminal/Catalytic Domain

• Basket like arrangement.
• Active site located in catalytic domain.
• 13-Å-deep and 10-Å-wide pocket.
• Amino acids lining active site mostly
  hydrophobic.
• Exception being 3 glutamates, 2 histidine, and
  1 tyrosine.
• Entrance covered by a short loop composed of
  residues 378-381.
• Composed of
• 13 a-helices
• 8 ß-strands

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Phenylalanine Hydroxylase
Monomer of PAH Red C-terminus/Catalytic
domain Cyan N-terminal/autoregulatory
domain. Green Hinge (111-117)
www.bio.davidson.edu/.../Castle/assign1home.html
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Phenylalanine Hydroxylase
www.pahdb.mcgill.ca/?TopicAboutSectionCura
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Phenylalanine Hydroxylase

• Phenylalanine Hydroxylase
• Non-heme iron at its active site.
• Ligands to the iron atom His nitrogen Glu
  oxygen Water oxygen atoms

www.rpi.edu/.../MBWeb/mb2/part1/aacarbon.htm
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Other Sources of the Disease

• Small percent of cases of PKU have risen from
  defects in
• Dihydropteridine reductase
• Biosynthetic route to tetrahydrobiopterin (BH4)
• Dihydropteridine reductase (DHPR) catalyzes the
  reduction of the quinoid dihydrobiopterin (q-BH2)
  into tetrahydrobiopterin.
• Tetrahydrobiopterin is a natural cofactor for the
  initial step in phenylalanine degradation.
• Facilitates monoxygenase activity
• Tetrahydrobiopterin is maintained in its reduced
  state by DHPR.

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Mechanism
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Dihydropteridine Reductase

• Dimeric protein
• Composed of two identical peptides.
• Molecular weights between 21-27 kDa.
• The enzyme is an aß protein.
• The ß sheet is composed of 7 parallel and 1
  antiparallel strand at the C-terminus.
• Motif referred to as dinucleotide or Rossman
  Fold.

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Tetrahydrobiopterin (BH4)

• co-factor that carries electrons for reduction
  reactions.
• Produced from dihydrobiopterin from dihydrofolate
  reductase.
• Cases of PKU stemming from BH4 deficiency termed
  malignant.
• Patients experience progressive deterioration of
  neurological function.

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How does this cause PKU?

• Lack of functional DHPR will result in the
  failure to convert BH4 to q-BH2
• This inhibits the recycling of BH4 because q-BH2
  is a substrate for DHPR.
• Results in a buildup of phenylalanine because
  this is the initial conversion from phenylalanine
  to tyrosine.

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Summary

• Phenylketonuria.
• Symptoms.
• Phenylalanine.
• Degradation Pathway of Phenylalanine.
• Initial step of phenylalanine degradation.
• Phenylalanine Hydroxylase.
• Other causes of PKU.
• DHPR.
• BH4.