Amino Acid Metabolism & Urea Cycle

Amino acid metabolism links protein turnover to energy, gluconeogenesis and nitrogen excretion. This is among the densest, highest-yield areas of NEET PG Biochemistry — expect direct questions on urea cycle enzymes, hyperammonemia, and the classic inborn errors (PKU, alkaptonuria, MSUD). Master the enzyme defects, accumulated substrates, and the biochemical "smell/colour" clues.

Overview: fate of amino acid nitrogen

Unlike carbohydrate and fat, the body cannot store excess amino acids. Surplus amino acids are deaminated; the carbon skeleton enters energy/glucose pathways and the nitrogen is converted to urea (the major non-toxic excretory form in humans — ureotelic).

The nitrogen flow is a two-step funnel:

Amino acid → (transamination) → glutamate → (oxidative deamination) → free NH₃ → (urea cycle) → urea → urine

• Transamination collects amino groups onto α-ketoglutarate, forming glutamate.
• Oxidative deamination of glutamate (by glutamate dehydrogenase) liberates free ammonia.
• The urea cycle detoxifies ammonia into urea.

Transamination

Transfer of an α-amino group from an amino acid to an α-keto acid (usually α-ketoglutarate), catalysed by aminotransferases (transaminases). The reaction is freely reversible and requires pyridoxal phosphate (PLP, vitamin B6) as coenzyme.

• ALT (alanine aminotransferase / SGPT): alanine + α-KG ⇌ pyruvate + glutamate. Liver-specific marker.
• AST (aspartate aminotransferase / SGOT): aspartate + α-KG ⇌ oxaloacetate + glutamate. Found in liver, heart, muscle.

High-yield: All transaminases require PLP (vitamin B6). The coenzyme forms a Schiff base (aldimine) intermediate. Three amino acids do NOT undergo transamination: lysine, threonine, proline (and hydroxyproline) — useful negative fact.

High-yield: A high AST:ALT (De Ritis) ratio >2 suggests alcoholic liver disease; ratio <1 favours viral hepatitis. AST is also raised in MI and muscle injury.

Deamination

Removal of the amino group as free ammonia.

• Oxidative deamination — Glutamate dehydrogenase (GDH): the key enzyme. Glutamate + NAD(P)⁺ → α-ketoglutarate + NH₃. It is unique in using either NAD⁺ or NADP⁺, is mitochondrial, allosterically activated by ADP/GDP and inhibited by ATP/GTP. This is the chief route generating ammonia for the urea cycle.
• Non-oxidative deamination: serine and threonine via serine/threonine dehydratase (PLP-dependent); histidine via histidase (deficiency → histidinemia).

The Urea Cycle (Krebs–Henseleit cycle)

Operates only in the liver. Two reactions are mitochondrial, three are cytosolic. One urea molecule disposes of two nitrogen atoms (one from ammonia, one from aspartate) and incorporates carbon from CO₂ (as bicarbonate).

Step	Enzyme	Location	Substrate → Product	Notes
1	Carbamoyl phosphate synthetase I (CPS-I)	Mitochondria	NH₃ + CO₂ + 2ATP → carbamoyl phosphate	Rate-limiting; needs N-acetylglutamate (NAG) as allosteric activator
2	Ornithine transcarbamoylase (OTC)	Mitochondria	carbamoyl-P + ornithine → citrulline	X-linked; commonest urea cycle defect
3	Argininosuccinate synthetase (ASS)	Cytosol	citrulline + aspartate + ATP → argininosuccinate	Aspartate donates 2nd nitrogen
4	Argininosuccinate lyase (ASL)	Cytosol	argininosuccinate → arginine + fumarate	Fumarate links to TCA
5	Arginase	Cytosol	arginine → urea + ornithine	Ornithine recycled

Flow: NH₃ + CO₂ → carbamoyl phosphate → citrulline → argininosuccinate → arginine → urea + ornithine → (ornithine re-enters mitochondria).

High-yield: CPS-I is the rate-limiting enzyme of the urea cycle and is activated by N-acetylglutamate (NAG). (Do not confuse with CPS-II, which is cytosolic and used in pyrimidine synthesis — a favourite distractor.)

High-yield: The two nitrogens of urea come from ammonia (via carbamoyl phosphate) and aspartate. The carbon comes from CO₂/bicarbonate.

High-yield: Energy cost = 3 ATP consumed (4 high-energy phosphate bonds) per urea molecule.

Mnemonic for the cycle order: *"Ordinarily, Careless Crappers Are Also Frivolous About Urination"* → Ornithine → Carbamoyl phosphate → Citrulline → Argininosuccinate → Arginine → Fumarate → Arginine → Urea. (Many use the simpler: Ornithine, Citrulline, Argininosuccinate, Arginine, Urea.)

Link between urea cycle and TCA — the "Krebs bicycle"

ASL releases fumarate, which enters the TCA cycle → malate → oxaloacetate. OAA is transaminated to aspartate, which re-enters the urea cycle at step 3. This elegant coupling is frequently tested.

Ammonia toxicity & hyperammonemia

Ammonia is neurotoxic. The brain detoxifies it by combining glutamate + NH₃ → glutamine (glutamine synthetase). Excess ammonia therefore depletes α-ketoglutarate and glutamate, impairing the TCA cycle and neurotransmission, and causes astrocyte (Alzheimer type II) swelling → cerebral oedema.

Feature	Acquired (hepatic) hyperammonemia	Inherited (urea cycle defect)
Cause	Cirrhosis, portosystemic shunt, Reye syndrome	Enzyme deficiency (OTC commonest)
Onset	Adults, chronic	Neonates/infants after protein feeds
Clue	Liver disease history	Family history, vomiting, lethargy

Distinguishing urea cycle defects

Key discriminators = blood citrulline and urinary orotic acid.

Defect	Blood citrulline	Urinary orotic acid	Comment
CPS-I deficiency	Low	Normal/low	No orotic aciduria
OTC deficiency	Low	High ↑↑	X-linked; carbamoyl-P spills into pyrimidine pathway → orotic aciduria; no acidosis (differentiates from orotic aciduria of pyrimidine synthesis defect, which has megaloblastic anaemia)
Citrullinemia (ASS def)	Very high	Mildly high	Citrulline accumulates
Argininosuccinic aciduria (ASL def)	High	High	Argininosuccinate in urine; trichorrhexis nodosa (brittle hair)
Arginase deficiency	Mild ↑	Mild ↑	Spastic diplegia, less hyperammonemia

High-yield: OTC deficiency is the most common urea cycle disorder, X-linked recessive, and causes increased urinary orotic acid WITHOUT acidosis — distinguishing it from hereditary orotic aciduria (a pyrimidine-synthesis defect with megaloblastic anaemia).

Management of hyperammonemia: restrict dietary protein; sodium benzoate (conjugates glycine → hippurate, excreted) and sodium phenylbutyrate/phenylacetate (conjugates glutamine → phenylacetylglutamine) provide alternative nitrogen-excretion routes; arginine/citrulline supplementation; lactulose and rifaximin in hepatic encephalopathy; haemodialysis in crisis.

Essential vs non-essential amino acids

Essential (must be in diet): PVT TIM HaLL → Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, Lysine. Arginine is semi-essential (conditionally essential in growth/pregnancy).

High-yield: Purely ketogenic amino acids = Leucine and Lysine ("Leu and Lys" — both start with L). Purely glucogenic are the majority; both glucogenic + ketogenic: isoleucine, phenylalanine, tyrosine, tryptophan, threonine (mnemonic "PITTT").

Carbon skeleton catabolism → TCA intermediates

Entry point	Amino acids feeding in
Pyruvate	Alanine, Glycine, Serine, Cysteine, Threonine, Tryptophan
Acetyl-CoA / acetoacetyl-CoA	Leucine, Lysine, (Ile, Trp, Phe, Tyr)
α-Ketoglutarate	Glutamate, Glutamine, Proline, Arginine, Histidine
Succinyl-CoA	Methionine, Valine, Isoleucine, Threonine
Fumarate	Phenylalanine, Tyrosine, Aspartate
Oxaloacetate	Aspartate, Asparagine

Inborn errors of amino acid metabolism (very high-yield)

Phenylketonuria (PKU)

• Defect: Phenylalanine hydroxylase (PAH) deficiency (classic) — cannot convert phenylalanine → tyrosine. Tetrahydrobiopterin (BH₄) is the cofactor; BH₄ deficiency = "malignant PKU."
• Accumulates: phenylalanine → phenylpyruvate, phenyllactate, phenylacetate.
• Features: musty/mousy odour, fair skin & hair, blue eyes (↓ melanin as tyrosine→melanin blocked), intellectual disability, seizures, eczema.
• Diagnosis: Guthrie bacterial inhibition test / tandem mass spectrometry on newborn screening; serum phenylalanine.
• Management: phenylalanine-restricted diet, tyrosine supplementation; sapropterin (BH₄) in responsive cases. Avoid aspartame. Maternal PKU → fetal damage, so strict control in pregnancy.

High-yield: PKU is autosomal recessive; the classic odour is mousy/musty; avoid aspartame (contains phenylalanine).

Alkaptonuria

• Defect: Homogentisate oxidase (homogentisic acid dioxidase) in tyrosine catabolism.
• Accumulates: homogentisic acid → urine darkens on standing (oxidises to a black pigment).
• Features: ochronosis (blue-black pigment in cartilage/sclera/ears), arthritis of spine and large joints in adults; usually benign in children.
• AR inheritance; generally no specific treatment (vitamin C may slow pigment deposition).

High-yield: Black urine on standing + ochronosis + arthropathy = alkaptonuria (homogentisate oxidase deficiency).

Maple Syrup Urine Disease (MSUD)

• Defect: Branched-chain α-ketoacid dehydrogenase (BCKD) — fails to decarboxylate keto acids of leucine, isoleucine, valine (BCAAs).
• Accumulates: branched-chain amino/keto acids → sweet, maple-syrup/burnt-sugar urine odour.
• Features: neonatal feeding difficulty, vomiting, lethargy, seizures, ketoacidosis, neurodegeneration; leucine is most neurotoxic.
• Cofactor: BCKD uses thiamine (B1); thiamine-responsive variants exist.
• Management: restrict BCAAs; dialysis in acute crisis.

High-yield mnemonic: MSUD = "I Love Vermont maple syrup" → Isoleucine, Leucine, Valine; enzyme is BCKD needing thiamine.

Homocystinuria

• Defect: commonly cystathionine β-synthase (CBS) (PLP-dependent).
• Features: Marfanoid habitus, downward (inferonasal) lens dislocation (vs Marfan = upward), thromboembolism, intellectual disability.
• Management: high-dose vitamin B6 (pyridoxine) in responsive cases; betaine, folate, B12, methionine restriction.

High-yield: Homocystinuria → downward lens dislocation + thrombosis; Marfan → upward lens dislocation, no thrombosis.

Other named conditions

• Cystinuria: defective renal/intestinal transport of COLA (Cysteine, Ornithine, Lysine, Arginine) → hexagonal cystine crystals, recurrent radiolucent renal stones. Treat with hydration, urine alkalinisation, penicillamine.
• Hartnup disease: defective neutral amino acid (tryptophan) transport → pellagra-like rash + ataxia; treat with niacin.
• Tyrosinemia type I: fumarylacetoacetate hydrolase deficiency → liver failure, "boiled cabbage" odour; treat with nitisinone (NTBC).

Key differentials (orotic aciduria pitfall)

Condition	Orotic acid	Ammonia	Anaemia	Clue
OTC deficiency	↑	↑ (high)	No	Urea cycle defect, no megaloblastic anaemia
Hereditary orotic aciduria (UMP synthase defect)	↑↑	Normal	Megaloblastic anaemia (uridine-unresponsive to B12/folate)	Pyrimidine synthesis defect; treat with uridine

Recently asked / exam angle

• "Enzyme deficient with ↑urinary orotic acid + hyperammonemia + no acidosis" → OTC deficiency (X-linked).
• "Rate-limiting enzyme of urea cycle / activated by N-acetylglutamate" → CPS-I.
• "Source of the two nitrogen atoms of urea" → ammonia + aspartate.
• "Mousy odour + fair child + intellectual disability" → PKU (phenylalanine hydroxylase / BH₄).
• "Urine turns black on standing" → alkaptonuria (homogentisate oxidase).
• "Maple-syrup urine in a neonate" → MSUD (BCKD, thiamine cofactor).
• "Purely ketogenic amino acids" → Leucine, Lysine.
• "Downward lens dislocation + thromboembolism" → homocystinuria (CBS, B6-responsive).
• "Coenzyme common to all transaminases" → PLP (vitamin B6).
• "Drug that lowers ammonia by conjugating glycine" → sodium benzoate.

Rapid revision

1. Urea cycle runs only in the liver; 2 mitochondrial + 3 cytosolic steps.
2. CPS-I = rate-limiting, mitochondrial, activated by NAG; CPS-II = cytosolic, pyrimidine synthesis.
3. Urea's 2 N = ammonia + aspartate; carbon = CO₂; cost = 3 ATP.
4. OTC deficiency = commonest urea cycle defect, X-linked, ↑orotic acid, no acidosis.
5. Order: Ornithine → Citrulline → Argininosuccinate → Arginine → Urea (+ recycled ornithine).
6. All transaminases need PLP (B6); lysine, threonine, proline are not transaminated.
7. Glutamate dehydrogenase is the main route for free ammonia and uses NAD⁺ or NADP⁺.
8. Essential AAs = PVT TIM HaLL; arginine semi-essential; purely ketogenic = Leu + Lys.
9. PKU: phenylalanine hydroxylase / BH₄; mousy odour; avoid aspartame; Guthrie test.
10. Alkaptonuria: homogentisate oxidase; black urine; ochronosis; arthritis.
11. MSUD: BCKD; Ile/Leu/Val; thiamine cofactor; maple-syrup urine.
12. Homocystinuria: CBS; downward lens dislocation + thrombosis; B6-responsive. Hyperammonemia treated with sodium benzoate + phenylbutyrate + arginine.