I have my university exam of 1 st year of subject biochemistry These are the important question give me answer of this "Phenylketonuria (PKU)" and remeber it's a university examination The answer should be easy to memorize and follow the pattern of answer writing as in University paper

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Phenylketonuria (PKU)

Definition

Phenylketonuria (PKU) is an autosomal recessive inborn error of amino acid metabolism characterized by a deficiency of the enzyme phenylalanine hydroxylase (PAH), resulting in the inability to convert phenylalanine to tyrosine, leading to accumulation of phenylalanine and its toxic metabolites in the body.

Incidence / Epidemiology

Occurs in approximately 1 in 10,000 live-born infants
More common in persons of Scandinavian/Northern European descent
Less common in African American and Jewish populations
Equally affects both sexes (autosomal recessive)

Biochemical Basis / Pathogenesis

Normal Pathway:

Phenylalanine → (PAH + BH4) → Tyrosine → Melanin / other products

In a healthy individual, the enzyme Phenylalanine Hydroxylase (PAH), using the cofactor Tetrahydrobiopterin (BH4), converts phenylalanine to tyrosine. The cofactor BH4 is regenerated from dihydrobiopterin (BH2) by the enzyme Dihydropteridine Reductase (DHPR).

In PKU:

PAH system in healthy vs PKU - Robbins Pathology

When PAH is deficient:

Phenylalanine accumulates in blood (hyperphenylalaninemia)
It is shunted into minor pathways producing abnormal metabolites:
- Phenylpyruvate (phenylketone - excreted in urine - gives the disease its name)
- Phenylacetate (gives musty/mousy odor to urine and sweat)
- Phenyllactate
Tyrosine is deficient - since phenylalanine cannot be converted to tyrosine
Tyrosine is a precursor for melanin - hence reduced pigmentation

These excess metabolites and phenylalanine itself cause brain damage by interfering with neurotransmitter synthesis and transport across the blood-brain barrier.

(Source: Robbins, Cotran & Kumar Pathologic Basis of Disease)

Types of PKU

Type	Cause	Frequency
Classic PKU	Deficiency of PAH	~98% of cases
Variant (Non-classic) PKU	Deficiency of DHPR OR inability to synthesize BH4	~2% of cases
Benign Hyperphenylalaninemia	Partial PAH deficiency - modest Phe elevation, no neurological damage	Milder form

Clinical Features

In Untreated Infants / Children:

Feature	Mechanism
Intellectual disability (severe)	Toxic effect of phenylalanine/metabolites on developing brain
Seizures	Neurological damage
Fair skin and hair (hypopigmentation)	Tyrosine deficiency → decreased melanin
Blue eyes	Reduced melanin
Musty/mousy odor	Phenylacetate in urine and sweat
Eczema	Skin manifestation
Microcephaly	Impaired brain development
Hyperreflexia, motor deficits	Neurological involvement

"Fewer than 4% of untreated phenylketonuric children have IQs greater than 50 or 60; about one-third never walk and two-thirds cannot talk." - Robbins, Cotran & Kumar

Genetics

Autosomal recessive inheritance
Gene: PAH gene (chromosome 12q23)
Almost 1000 mutant alleles of the PAH gene have been identified
Mutations in both alleles are required (homozygous or compound heterozygous) to develop the disease
Heterozygous carriers - phenotypically normal

Diagnosis

Neonatal Screening (Most important):

Guthrie Test (Bacterial Inhibition Assay) - blood spot on filter paper
Tandem Mass Spectrometry (modern method) - measures blood phenylalanine
Done within 48-72 hours of birth (after protein feeding has begun)
Normal blood phenylalanine: < 2 mg/dL; PKU: > 20 mg/dL (5-fold or more above normal)

Other Tests:

Urine FeCl3 test - green color with phenylpyruvate (phenylketones)
Serum phenylalanine level - elevated
Molecular genetic testing - identifies specific PAH mutation (used for carrier testing)

Treatment

1. Dietary Restriction (Main treatment):

Low-phenylalanine diet - restrict phenylalanine intake
- Avoid high-protein foods: meat, fish, dairy, eggs, nuts
- Use special phenylalanine-free amino acid formula
Must be started early in life (within first few weeks of birth) - damage is irreversible
Continue at least until adolescence; ideally lifelong

2. BH4 Supplementation:

Sapropterin (synthetic BH4/tetrahydrobiopterin)
Effective in variant PKU (BH4-deficient forms) and some classic PKU patients
Cannot be used as sole therapy in classic PKU

3. Enzyme Substitution Therapy (Adults):

Phenylalanine Ammonia-Lyase (PAL) - enzyme that metabolizes excess phenylalanine
FDA-approved for adults with uncontrolled PKU

4. Gene Therapy:

Under investigation

Maternal PKU (Important complication)

Females with PKU who discontinue dietary treatment reach childbearing age with high phenylalanine levels
Phenylalanine crosses the placenta and exerts teratogenic effects on the fetus
Even though the infant is heterozygous (carrier), it is severely affected
Outcomes: 75-90% of infants have intellectual disability, microcephaly, 15% have congenital heart disease
This syndrome is called Maternal PKU syndrome
Management: Maternal dietary restriction must be started before conception and continued throughout pregnancy

Summary Table (Quick Revision)

Feature	Details
Enzyme deficient	Phenylalanine Hydroxylase (PAH)
Cofactor involved	Tetrahydrobiopterin (BH4)
Inheritance	Autosomal Recessive
Incidence	1 in 10,000
Key metabolite	Phenylpyruvate (in urine)
Characteristic smell	Musty/mousy odor
Screening test	Guthrie test / Tandem MS
Key clinical finding	Intellectual disability + hypopigmentation
Treatment	Low-phenylalanine diet

Memory Tip (Mnemonics)

"PKU SMELLS":

P - PAH enzyme deficient
K - Ketones (phenylketones) in urine
U - Urine smells musty
S - Seizures
M - Mental (intellectual) disability
E - Eczema
L - Light skin/hair
L - Low tyrosine
S - Screening (Guthrie test at birth)

Sources: Robbins, Cotran & Kumar - Pathologic Basis of Disease | Lippincott Biochemistry 8th ed | Robbins & Kumar Basic Pathology

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