BCHM 551
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Liberty University
Biochemistry
BCHM 551
ASSIGNMENT INSTRUCTIONS
Assignment 2: Module/Week 8
You should read chapter 19 (Phenylketonuria) from your Clinical Studies in Medical
Biochemistry textbook (by Rosenthal and Glew) then answer the following questions;
1. How would you justify the observation that two different patients possessing the same
mutation in the PAH enzyme may exhibit very different phenotypes with respect to PA
metabolism ?
2. How can you explain the observation that in some individuals with a mutation to the
PAH enzyme outside the BH4-binding site show a decrease in serum PA levels when
given dietary BH4 ?
3. What would you propose as the least expensive and most rapid method to identify the
existence of a defect in BH4 metabolism as the cause of a patient’s PKU or HPA?
4. Based on metabolic pathways, what phenotypic differences would you expect to observe
between individuals suffering from either a defect in BH4 metabolism or a defect in the
PAH enzyme?
5. Based on biochemical and physiological mechanisms, how can you explain impaired
brain development as a consequence of uncontrolled PKU?
Answers:
1. PAH enzyme referes to phenylalanine hydroxylase. This enzyme is responsible for the
metabolism of phenylalanine which is the building block of proteins. PAH metabolises
phenylalanine to other amino acids that can be used by the body in producing
neurotransmitter of brain. Mutation in PAH gene happens in chromosome 12 where
arginine is replaced by tryptophan. This mutation causes the enzyme to be defetive and
becomes not able to metabolize phenylalanine resulting in a condition named
Phenylketonuria (PKU). Accumulation of phenylalanine may cause brain damage.
However the amount of phenylalanine in body may be restricted by the intake of the
amount of dietary phenylalanine. Thus, two patients may have same PAH mutation but
show different phenotypes depending on the intake and metabolism rate of
phenylalanine.
In addition, PAH gene has pleiotrophic effect which means a single gene controlling one
or more unrelated phenotypes. Thus, pleotrophic effect may produce unrelated response
in each individual. Also the homozygosity and heterozygosity of the mutated allele
determine the PAH amount produced in each individual.
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2. In order for the conversion of phenylalanine to tyrosine, the PAH enzyme needs the
cofactor BH4. In the PAH enzyme there is a BH4-binding site that allows the enzyme to
convert phenylalanine to tyrosine. However, if there is a mutation at the BH4-binding site
of the PAH enzyme, then there will be a decrease in affinity for BH4 binding. As a result,
PAH activity will be decreased and there will be an accumulation of phenylalanine in the
blood.
If a patient is given dietary BH4, then PAH enzyme will be able to bind BH4 and as a
result phenylalanine levels will be normal again. Increasing BH4 concentration will make
BH4 more likely to bind and therefore increase PAH enzyme activity. Should there be a
circumstance in which there is a mutation outside of the BH4-binding site then there will
most likely be conformational changes and as a result, affect the folding of the enzyme.
This would lead to a decrease in the affinity of BH4 and possibly prevent BH4 from
binding and therefore making the binding of BH4 unfavorable.
Therefore, giving a patient dietary BH4 can help the concentration of BH4 high enough
to stabilize the enzyme and encourage proper folding. In doing so, this would explain the
decrease in serum phenylalanine when given dietary BH4.
3. PKU is a condition where the phenylalanine is not converted into Tyrosine thus it
accumulates in the body. The enzyme responsible for this conversion is called PAH.
Tetrahydrobiopterin known as BH4, binds to the catalytic site of the enzyme to initiate its
activity. There are several enzymes resposible for the synthesis of BH4 including GTP
cyclohydrolase I (GTPCH) which is involved in the conversion of Guanosine
Triphosphate to an intermediate and ultimately to BH4. The most rapid and least
expensive method for diagnosis of PKU or HPA is to conduct the neonatal heel prick test
in which blood sample is collected from the heel of the infant followed by collection of the
blood sample on Guthrie cards which are specifically used to detect phenylalanine
levels. This can be associate with checking the levels of the enzymes that are
associated with the BH4 pathway (enzymatic assay). For the disc diffusion test, blood
sample can be used to detect levels of phenylalanine for enzyme associated defects.
4. The individuals suffering from a defect in BH4 metabolism symptoms range from mild to
severe. Mild complications can be temporary low muscle tone. Severe symptoms include
intellectual disability, movement disorders, swallowing problems, seizures, behavioral
problems, progressive problems associated with development and reduced ability to
control body temperature.
The individuals suffering from defect in the PAH enzyme would show abnormality in
growth, IQ loss, microcephaly, seizures, eczema ,and severe intellectual disabilities. In
addition, Phenylalanine usually turns into melanin which gives color to hair and skin
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tone. PKU prevents phenylalanine from turning into melanin which causes a phenotype
of pale skin and blue eyes.
5. PKU is a congenital error of the metabolism of phenylalanine caused by the deficiency of
the enzyme PAH. People suffering from PKU, accumulate phenylalanine in their cells
and therefore have high levels of phenylalanine in their brains. This results in the
alteration of the synthesis of neurotransmitters. The high amounts of phenylalanine in
the brain causes toxicity to neurons and glial cells, and disrupts the growth and density
of dendrites in the brain. This can also affect the white cortical and subcortical
substance.
This disruption produces the neuropsychological compromise that affects executive
functions, interhemispheric transfer, sustained attention and processing speed, disorders
that negatively impact cognitive functions. The neurotoxicity mechanisms of PKU have
not been fully identified but it is known that hyperphenylalaninemia and the consequent
decrease in the transport of neutral amino acids to the brain decreases at the same time
the synthesis of brain proteins and neurotransmitter proteins.
