Questions: Aromatic Amino Acid Metabolism

Question 1

A patient has a loss-of-function mutation in phenylalanine hydroxylase (PKU). Which set of metabolic consequences would you expect?

• A. Elevated blood tyrosine and reduced phenylalanine, because the blocked enzyme normally degrades phenylalanine
• B. Reduced catecholamine synthesis and elevated serotonin, as the tryptophan pathway compensates
• C. Accumulation of phenylalanine in blood and reduced tyrosine production, because the hydroxylation step converting phenylalanine to tyrosine is blocked
• D. Deficiency of NAD+ because phenylalanine normally feeds into the kynurenine pathway

Answer: (C)

Phenylalanine hydroxylase converts phenylalanine to tyrosine; when this enzyme fails, phenylalanine cannot enter the tyrosine pathway and accumulates. Because phenylalanine must be converted to tyrosine before further catabolism, tyrosine must now be obtained entirely from the diet — it becomes conditionally essential. The toxic accumulation of phenylalanine and its transamination products causes neurological damage. NAD+ synthesis comes from tryptophan (kynurenine pathway), not phenylalanine.

Question 2

Which molecule is the direct biosynthetic precursor to both dopamine (a neurotransmitter) and thyroid hormones (T3/T4)?

• A. Phenylalanine — it is converted directly to L-DOPA in catecholamine synthesis
• B. Tryptophan — it serves as the universal aromatic amino acid precursor for signaling molecules
• C. Tyrosine — it is hydroxylated to L-DOPA for catecholamine synthesis and iodinated for thyroid hormone synthesis
• D. L-DOPA — it is the central hub molecule for all catecholamine and thyroid hormone biosynthesis

Answer: (C)

Tyrosine is the true metabolic hub. For catecholamines: tyrosine → L-DOPA → dopamine → norepinephrine → epinephrine. For thyroid hormones: tyrosine residues within thyroglobulin are iodinated and coupled to form T3 and T4. Phenylalanine must first be converted to tyrosine before it can enter any of these pathways — it is not a direct precursor. L-DOPA is an intermediate, not a hub upstream of thyroid hormones.

Question 3

Tryptophan serves as a dietary source for de novo NAD+ biosynthesis because the kynurenine pathway produces a pyridine nucleotide precursor.

• T. True
• F. False

Answer: True

True — the major catabolic route for tryptophan is the kynurenine pathway, which opens the indole ring and produces several intermediates, including quinolinate. Quinolinate is a direct precursor for NAD+ synthesis. This is why tryptophan is considered a niacin equivalent in nutrition: adequate dietary tryptophan can partially compensate for niacin (vitamin B3) deficiency, since NAD+ can be synthesized from either.

Question 4

Phenylalanine and tyrosine are catabolized through largely separate biochemical pathways with no shared intermediates or enzymatic steps.

• T. True
• F. False

Answer: False

False — phenylalanine is first converted to tyrosine by phenylalanine hydroxylase, after which both amino acids enter the same downstream degradation pathway. This convergence is why tyrosine is described as the metabolic hub: whether the aromatic carbon enters as phenylalanine or tyrosine, it passes through the same sequence leading to fumarate (glucogenic) and acetoacetate (ketogenic). The two pathways are not separate; they merge at tyrosine.

Question 5

Why is tyrosine described as a 'metabolic hub' among the aromatic amino acids? Name two distinct physiological systems that depend on tyrosine as their biosynthetic starting point.

Model answer: Tyrosine is the convergence point for phenylalanine catabolism and the upstream precursor for multiple major biosynthetic pathways. Two distinct systems: (1) the catecholamine neurotransmitter/hormone system — tyrosine → L-DOPA → dopamine → norepinephrine → epinephrine, which governs motor control, reward, and stress response; and (2) thyroid hormone synthesis — tyrosine residues in thyroglobulin are iodinated and coupled to form T3 and T4, which regulate metabolism and thermogenesis. Melanin synthesis in skin is a third pathway.

The term 'hub' is warranted because multiple upstream pathways (phenylalanine input) and multiple downstream biosynthetic branches all pass through tyrosine. This means defects in tyrosine availability — whether from PKU or dietary restriction — have cascading effects across unrelated physiological systems.