Notes · Biochemistry · 13 November 2026

Phenylketonuria (PKU) biochemistry

Definition

• Phenylketonuria (PKU) is an inherited autosomal‑recessive disorder of amino‑acid metabolism characterized by a deficiency of the hepatic enzyme phenylalanine hydroxylase (PAH) or its cofactor tetrahydrobiopterin (BH4), leading to accumulation of the essential amino‑acid phenylalanine (Phe) in blood and tissues.
• The excess Phe is converted into phenylpyruvate, phenylacetate and other “ketone” metabolites that are excreted in urine, hence the name “phenyl‑ketonuria”.

Epidemiology

• Global incidence varies from 1 in 10 000 to 1 in 15 000 live births in most populations; higher rates (≈1 in 2 500) are reported in Ireland, Turkey and parts of the Middle East due to founder mutations.
• Newborn screening programmes have reduced the prevalence of untreated PKU to <0.1 % of the population in countries with universal screening.

Aetiology (Genetic Basis)

• PAH gene mutations: > 1 000 different pathogenic variants have been identified; the most common are missense mutations (e.g., R408W, I65T).
• BH4‑deficient hyperphenylalaninemia: Mutations in genes encoding enzymes of BH4 synthesis (GCH1, PTS, QDPR) or regeneration (PCBD1) cause a secondary PAH deficiency.
• Inheritance: Autosomal‑recessive; both parents must be carriers for a child to be affected.

Pathophysiology (Biochemical Cascade)

1. Normal pathway

   • Phenylalanine + O₂ + tetrahydrobiopterin (BH4) → tyrosine + H₂O + quinonoid‑dihydrobiopterin (qBH₂)
   • Catalyzed by phenylalanine hydroxylase (PAH) in the liver cytosol.
   • qBH₂ is recycled to BH4 by dihydropteridine reductase (DHPR) using NADH as a co‑factor.

2. Defective PAH

   • Mutated PAH has reduced affinity for Phe or BH4, or is unstable, resulting in impaired conversion of Phe to tyrosine.
   • Blood Phe rises > 1200 µmol/L (normal < 120 µmol/L).

3. Alternative catabolism

   • Excess Phe undergoes transamination by aromatic amino‑acid aminotransferase → phenylpyruvate.
   • Phenylpyruvate is reduced to phenylacetate and phenyl lactate, which are excreted in urine as “phenylketones”.

4. Neurotoxicity mechanisms

   • Competitive inhibition of large neutral amino‑acid (LNAA) transport across the blood‑brain barrier (LAT1 transporter), reducing brain uptake of tyrosine, tryptophan and others.
   • Reduced synthesis of neurotransmitters: Tyrosine is a precursor for dopamine, norepinephrine; tryptophan for serotonin.
   • Direct toxicity of high Phe concentrations on oligodendrocytes, leading to hypomyelination.

5. BH4 deficiency (if present)

   • BH4 is also a co‑factor for tyrosine hydroxylase and tryptophan hydroxylase; its deficiency further impairs catecholamine and serotonin synthesis, worsening neurologic outcome.

Clinical Features (if untreated)

• Neurological: Severe intellectual disability, microcephaly, seizures, spasticity, ataxia.
• Dermatological: Eczematous rash, “musty” odor of urine (due to phenylacetate).
• Behavioral: Hyperactivity, autistic‑like features, poor social interaction.
• Growth: Failure to thrive, poor weight gain.
• Ophthalmic: Light‑sensitive retinopathy, optic atrophy (rare).

Note: Early detection via newborn screening prevents most of these manifestations.

Investigations

Management (Current Guidelines 2023‑2024)

1. Dietary Therapy

• Phe‑restricted diet: Aim for blood Phe 120‑360 µmol/L (2‑6 mg/dL) in infants; 120‑600 µmol/L (2‑10 mg/dL) in older children and adults.
• Protein substitute: Phe‑free amino‑acid formula providing all essential amino acids except Phe, plus vitamins and minerals.
• Natural protein allowance: Calculated individually (≈ 10‑20 mg Phe/kg/day in infants, up to 50 mg/kg/day in tolerant adolescents).
• Monitoring: Weekly Phe levels in the first year, then every 1‑3 months; adjust diet accordingly.

2. Pharmacologic Therapy

• Sapropterin dihydrochloride (Kuvan®) – synthetic BH4; indicated for BH4‑responsive genotypes or as an adjunct to diet.

  • Dose: 10‑20 mg/kg/day orally, divided doses.
  • Re‑evaluate Phe after 2‑4 weeks; if reduction ≥ 30 %, continue and gradually liberalize diet.

• Pegvaliase (Palynziq®) – PEG‑modified phenylalanine ammonia lyase enzyme; approved for adults with uncontrolled Phe despite diet and sapropterin.

  • Induction: sub‑cutaneous injection weekly, titrated to maintain Phe < 600 µmol/L.

3. Neurodevelopmental Support

• Early intervention services: speech, occupational, physiotherapy.
• Regular neurocognitive assessment (IQ testing, school performance).

4. Pregnancy Management

• Strict Phe control (< 360 µmol/L) before conception and throughout pregnancy to prevent fetal PKU syndrome (microcephaly, congenital heart defects).

5. Follow‑up Schedule (per 2024 AAP/ACMG guideline)

• Infants (0‑12 mo): monthly clinic visits, growth parameters, Phe, diet review.
• Children (1‑12 yr): every 3 months; annual ophthalmology, bone density (DXA) if on long‑term low‑protein diet.
• Adolescents/Adults: every 6 months; monitor liver function, vitamin B12, iron status.

Complications

• Neurocognitive decline if Phe exceeds target for prolonged periods.
• Nutritional deficiencies: low intake of natural protein may cause zinc, iron, selenium deficits; supplement as needed.
• Osteopenia/osteoporosis: chronic low calcium and vitamin D intake; monitor bone mineral density.
• Maternal PKU syndrome: severe fetal malformations when maternal Phe is uncontrolled during pregnancy.
• Allergic reactions to sapropterin or pegvaliase (rare; anaphylaxis reported with pegvaliase).

High‑Yield Exam Points

• Key enzyme: Phenylalanine hydroxylase (PAH) – requires tetrahydrobiopterin (BH4) and O₂.
• Normal Phe range: 30‑120 µmol/L; diagnostic threshold for PKU ≈ 1200 µmol/L.
• Biochemical hallmark: Presence of phenylpyruvate and phenylacetate in urine (phenylketones).
• Inheritance: Autosomal recessive; both parents carriers.
• Newborn screening: Tandem mass spectrometry detects elevated Phe on dried blood spot.
• Dietary cornerstone: Phe‑restricted diet + Phe‑free amino‑acid formula; aim for blood Phe 120‑360 µmol/L in children.
• BH4‑responsive PKU: Identified by sapropterin loading test; allows liberalization of diet.
• Maternal PKU: Strict Phe control before conception prevents fetal anomalies.
• Pegvaliase: Enzyme substitution therapy for adults with refractory PKU; sub‑cutaneous injection.

Practice questions

1. A 6‑month‑old infant diagnosed with PKU has a blood phenylalanine level of 1500 µmol/L. Which of the following is the most appropriate immediate intervention?

   • A) Start high‑dose vitamin B12
   • B) Initiate a phenylalanine‑restricted diet with Phe‑free formula
   • C) Begin oral levodopa therapy
   • D) Give oral corticosteroids

   Answer: B — dietary restriction of phenylalanine is the primary treatment to lower blood Phe.

2. Which laboratory test best distinguishes a classic PAH‑deficient PKU from a BH4‑deficient hyperphenylalaninemia?

   • A) Serum ammonia level
   • B) Urine organic acid analysis for phenylpyruvate
   • C) Sapropterin loading test (≥ 30 % Phe reduction)
   • D) Liver function tests

   Answer: C — a significant Phe reduction after sapropterin indicates BH4 responsiveness, pointing to a cofactor deficiency.

3. A pregnant woman with PKU plans to conceive. Which blood phenylalanine target is recommended throughout pregnancy to avoid fetal PKU syndrome?

   • A) < 600 µmol/L
   • B) < 360 µmol/L
   • C) < 1200 µmol/L
   • D) < 900 µmol/L

   Answer: B — maintaining Phe below 360 µmol/L during pregnancy minimizes risk of fetal malformations.

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