Phenylalanine and Tyrosine Pathway
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Direct answer
Phenylalanine is an essential amino acid whose hydroxylation to tyrosine by phenylalanine hydroxylase — a tetrahydrobiopterin (BH4)-dependent, iron-containing liver enzyme — makes tyrosine conditionally dispensable and constitutes the rate-limiting step of the whole pathway. Tyrosine then feeds four destinations: catecholamines (rate-limited by tyrosine hydroxylase, also BH4-dependent), thyroid hormones (iodination of tyrosine residues on thyroglobulin), melanin (tyrosinase), and energy through homogentisate to fumarate and acetoacetate. Blocks at successive steps produce the classic inborn errors: phenylketonuria, tyrosinaemia types I and II, alkaptonuria, and albinism, each with a distinct urine clue and inheritance.
What you must remember
- Phenylalanine hydroxylase converts phenylalanine to tyrosine; cofactor BH4 is regenerated by dihydropteridine reductase. BH4 deficiency (about 1-2% of PKU) causes low neurotransmitters despite normal enzyme, so severe cases need BH4 plus L-DOPA and 5-hydroxytryptophan.
- Normal plasma phenylalanine is under 2 mg/dL; classical PKU shows values above 20 mg/dL with a phenylalanine-to-tyrosine ratio well above 2, the pattern newborn screening flags on tandem mass spectrometry.
- Phenylketonuria: autosomal recessive, musty mousy odour (phenylacetate in urine), fair skin and hair (tyrosine diversion fails, melanin falls), eczema, seizures, and intellectual disability unless a phenylalanine-restricted diet starts in the first weeks of life.
- Tyrosinaemia type I from fumarylacetoacetate hydrolase deficiency: cabbage-like odour, succinylacetone in urine (diagnostic, inhibits ALA dehydratase causing porphyria-like crises), liver failure, renal Fanconi features; treated with nitisinone (NTBC), which blocks 4-hydroxyphenylpyruvate dioxygenase upstream.
- Alkaptonuria from homogentisate oxidase deficiency: urine darkens on standing (homogentisate oxidises to benzoquinone acetate), ochronotic pigmentation of sclera and ear cartilage, early degenerative arthritis of spine and large joints.
- Oculocutaneous albinism type 1 is tyrosinase-negative: no melanin at all, nystagmus, white hair; the tyrosinase-positive type retains some pigment.
- Aspartame warning: it releases phenylalanine, so it is contraindicated in PKU — a viva favourite and a statutory label line on Indian diet drinks.
A newborn screening result walked through
A day-3 dried blood spot reports phenylalanine 28 mg/dL with tyrosine 1.5 mg/dL. The ratio, not the absolute value, convinces you: a primary phenylalanine hydroxylase block starves the tyrosine pool, whereas liver disease would raise both. Next steps run in order — repeat sample, stop breast milk and shift to a phenylalanine-free formula such as phenylalanine-free L-amino acid mixture while checking BH4 metabolites (neopterin and biopterin) in urine to exclude the treatable cofactor variant. If the loading BH4 test normalises the value, you have dihydropteridine reductase or synthesis deficiency, and diet alone will not prevent neurological damage because catecholamine and serotonin synthesis remain blocked. Management thereafter is lifelong: diet keeping phenylalanine roughly 2-6 mg/dL in early childhood, tyrosine supplementation, and monitoring through the school years. A girl with PKU counselled later must know about maternal PKU — a phenylalanine-rich intrauterine environment causes microcephaly and congenital heart disease in an otherwise normal foetus, so strict pre-conception diet control is essential.
How the examiner frames it
The trap is the word "tyrosine, non-essential." In PKU it becomes essential — the single most quoted one-mark statement of this chapter. The second trap is Guthrie versus tandem mass spectrometry: the Guthrie test is a bacterial inhibition assay using Bacillus subtilis whose growth around the disc is inhibited by β-2-thienylalanine and relieved by phenylalanine, while modern Indian screening laboratories use tandem mass spectrometry on the same heel-prick card. Examiners also enjoy the cofactor overlap: phenylalanine hydroxylase, tyrosine hydroxylase and tryptophan hydroxylase all demand BH4, which links PKU to neurotransmitter biochemistry in one sentence.
Frequently asked questions
Which enzyme is rate-limiting in the phenylalanine-tyrosine pathway?
Phenylalanine hydroxylase, which converts phenylalanine to tyrosine using BH4 and molecular oxygen; for catecholamine synthesis the rate-limiting step shifts to tyrosine hydroxylase.
Why does classical PKU cause fair skin and a musty odour?
Diversion of excess phenylalanine to phenylpyruvate, phenylacetate and phenyllactate limits melanin substrate and gives urine its mousy smell from phenylacetate.
What distinguishes tyrosinaemia type I from PKU biochemically?
Succinylacetone in urine is pathognomonic of fumarylacetoacetate hydrolase deficiency, and treatment adds nitisinone to a tyrosine-restricted diet.
How is BH4-dependent hyperphenylalaninaemia treated?
With sapropterin (synthetic BH4) plus L-DOPA/carbidopa and 5-hydroxytryptophan, because diet alone cannot replace the missing neurotransmitters.
Why must phenylketonuric mothers plan pregnancy carefully?
Maternal phenylalanine above about 6 mg/dL is teratogenic, causing microcephaly, cardiac defects and growth retardation, so dietary control must begin before conception.