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Phenylketonuria (PKU)


Phenylketonuria (PKU) Sanketh Proddutur What is PKU? Phenylketonuria (PKU) is an inherited error of metabolism caused by a deficiency in the enzyme phenylalanine ... – PowerPoint PPT presentation

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Title: Phenylketonuria (PKU)

Phenylketonuria (PKU)
  • Sanketh Proddutur

What is PKU?
  • Phenylketonuria (PKU) is an inherited error of
    metabolism caused by a deficiency in the enzyme
    phenylalanine hydroxylase (PAH).
  • phenylalanine hydroxylase converts the amino acid
    phenylalanine to tyrosine, another amino acid.

  • mutations in the human gene (chromosome
    12.q22-24.2), encoding PAH result in the
    autosomal recessively inherited disease
    hyperphenylalaninemia (HPA)
  • The resulting phenotypes can range in severity
    from mild hyperphenylalaninemia (HPA)
  • classic PKU which inevitably leads to mental
    retardation if left untreated.

  • Newborn infants develop signs of PKU within a few
    months of birth if untreated
  • Mental retardation
  • Behavioral or social problems
  • Seizures, tremors or jerking movements in the
    arms and legs
  • Rocking
  • Hyperactivity
  • Stunted growth

Symptoms (contnd)
  • Skin rashes (eczema)
  • Small head size (microcephaly)
  • Vomiting
  • A musty odor in the child's breath, skin or urine
  • Fair skin and blue eyes

  • Human phenylalanine hydroxylase converts the
    essential amino acid L-phenylalanine (L-Phe) into
    L-tyrosine using the co-factor (6R)-L-erythro-5,6,
    7,8-tetrahydrobiopterin (BH4) and molecular
  • Any disruption in this mechanism can lead to

Etiology (contnd)
  • 99 of the mutations that disrupt this mechanism
    can be traced back to the PAH gene, 1 of the
    mutations are in the genes encoding the enzymes
    which are involved in the regeneration of the
    cofactor BH4, which is a vital co-substrate of

BH4 regeneration
  • BH4 is a cofactor of PAH and its regeneration
    is vital for the ongoing reaction.

  • Liver PAH is reported to exist in solution as
    a pH dependent equilibrium between homo-dimers
    and the active homo-tetramers. Four identical
    molecules of phenylalanine hydroxylase interact
    to form the tetramer, which is the functional
    unit for this enzyme.

PAH (contnd)
  • Each monomer polypeptide is composed of three
  • An N-terminal regulatory domain (residues 1-142)
  • A central catalytic domain (residues 143-410)
    which includes the active site iron
  • A C-terminal tetramerization domain (residues

The regulatory domain consists of a four stranded
anti-parallel ß-sheet flanked on side by two
short a-helices and on the other side by the
catalytic domain
The catalytic domain houses the active site iron
in a novel basket like arrangement of 14
a-helices and 8-ß strands
the short tetramerization domain is formed by a
C-terminal "arm" of two -strands forming a
-ribbon, and a long -helix and is responsible for
assembling the four chains into the tetramer.
Catalytic domain
  • The active site iron is ligated in an octahedral
    manner and bound to His285, His290, and 1 oxygen
    atom in Glu330 as well as three water molecules

Fe(II) ion
His 290
Glu 330
His 285
Catalytic domain (contnd)
  • A tunnel adjacent to the active site might be
    responsible for directing the substrate.

Catalytic mechanism
  • In order to hydroxylate the unactivated
    Phenylalanine to Tyrosine, PAH incorporates one
    atom of oxygen from molecular oxygen into the
    substrate and reduces the other atom to water.
  • In the process, the BH4 cofactor undergoes a
    two-electron oxidation to form quinonoid
    dihydropterin (q-BH2) which is then regenerated
    back to BH4.

How exactly does this take place?
We dont know!
It is however speculated that the binding of the
substrate is cooperative, that is the tetramer
undergoes synergistic conformational changes to
bind the substrates subsequently hydroxylating
the Phe residue.
Binding of the cofactor
  • The crystal structure of the dimeric catalytic
    domain (residues 118-424) of human PAH
    cocrystallized with the oxidized form of the
    cofactor (7,8-dihydro-L-biopterin, BH2), has been
    determined at 2.0 Å resolution.

The pterin binds close to the catalytic iron and
forms an extensive H-bond network with Ala322,
Gly247, Leu249 and Glu286. The phenyl ring of
Tyr325 establishes hydrophobic contacts with the
pterin, and thus contributes to the correct
positioning of the pterin cofactor for catalysis,
furthermore this also ensures that the pterin
ring forms an aromatic pi-stacking interaction
with Phe254.
(No Transcript)
  • Guthrie card bloodspot is the standard test for
    PKU. Blood taken from the infants heel after
    first breast feeding is assayed for high levels
    of Phe.
  • tandem mass spectrometry (often abbreviated as
    MS/MS). This technology can detect the blood
    components that are elevated in certain
    disorders, and is capable of screening for more
    than 20 inherited metabolic disorders with a
    single test.
  • Amniocentesis is a form of prenatal diagnosis in
    which DNA extracted from fetal cells obtained
    from the amniotic fluid at about 15-18 weeks'
    gestation period.

  • Restriction of dietary phenylalanine is the
    most obvious treatment for this disorder and also
    the most prevalent. Phe is ubiquitous across a
    variety of food groups but is especially
    prevalent in proteins. A typical Phe-restricted
    diet would be rich in fruits and some amounts of
    potato and milk to maintain a phe concentration
    of about 2-6 mg/dL supplemented by a phe-free
    protein supplement.All kinds of meat, cheese and
    flour products are forbidden.
  • Prognosis is fairly good if the diet is strictly
    followed into adulthood. Patients who
    discontinued after late adolescence.
  • Other potential treatments being investigated are
    gene therapy, BH4 and phenylalanine lyase

  • Erlandsen, H. and Stevens, R.C. (1999) The
    structural basis of phenylketonuria. Molecular
    Genetics and Metabolism 68, 103-25.
  • F.Fusetti, H.Erlandsen, T.Flatmark, R.C.Stevens.
    (1998) Structure of Tetrameric Human
    Phenylalanine Hydroxylase and its Implications
    for Phenylketonuria. J.Biol.Chem. 273, 16962
  • O.A.Andersen, T.Flatmark, E.Hough. (2001) High
    Resolution Crystal Structures of the Catalytic
    Domain of Human Phenylalanine Hydroxylase in its
    Catalytically Active Fe(II) Form and Binary
    Complex with Tetrahydrobiopterin. J.Mol.Biol.
    314 266
  • H.Erlandsen, E.Bjorgo, T.Flatmark,
    R.C.Stevens(2000). Crystal Structure and
    Site-Specific Mutagenesis of Pterin-Bound Human
    Phenylalanine Hydroxylase. Biochemistry 2208
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