Purine & Pyrimidine Metabolism

Purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil) are the nitrogenous bases of nucleic acids. NEET PG loves this topic for its tight enzyme-deficiency → disease pairings (HGPRT → Lesch-Nyhan, ADA → SCID) and the pharmacology of gout (xanthine oxidase, allopurinol). Master the rate-limiting steps, the salvage enzymes, the end-products, and the drug targets.

Big picture: two bases, two fates

Feature	Purines	Pyrimidines
Bases	Adenine, Guanine (+ hypoxanthine, xanthine)	Cytosine, Uracil, Thymine
Ring synthesis	Built on the ribose-5-phosphate (PRPP) — ring assembled stepwise on sugar	Ring built first, then attached to PRPP
Committed/regulated step	PRPP → phosphoribosylamine by glutamine-PRPP amidotransferase	Carbamoyl phosphate (cytosolic) by CPS-II
Carbon/nitrogen donors	Glycine, aspartate, glutamine, CO₂, N¹⁰-formyl-THF (×2)	Aspartate, glutamine, CO₂
Final degradation product	Uric acid (poorly soluble)	CO₂, NH₃, β-alanine, β-aminoisobutyrate (water-soluble)
Clinical degradation issue	Gout, stones	Usually none (orotic aciduria is a synthesis defect)

High-yield: Purine ring is synthesised on the sugar (PRPP first); pyrimidine ring is synthesised before the sugar is added. This single fact discriminates many MCQs.

A useful purine-source mnemonic — atoms of the purine ring come from: Glycine (C4, C5, N7), Aspartate (N1), Glutamine (N3, N9), CO₂ (C6), N¹⁰-formyl-THF (C2, C8). "Glycine sits in the middle of the ring."

De novo purine synthesis

The pathway begins with ribose-5-phosphate (from the HMP shunt).

Ribose-5-P → (PRPP synthetase, + ATP) → PRPP → (glutamine-PRPP amidotransferase) → 5-phosphoribosylamine → … (9 steps) → IMP (inosine monophosphate)

• PRPP synthetase generates PRPP (5-phosphoribosyl-1-pyrophosphate), the activated ribose used by both purine and pyrimidine pathways and by salvage.
• Glutamine-PRPP amidotransferase is the committed, rate-limiting enzyme. It is feedback-inhibited by IMP, AMP, GMP (end-products) and activated by PRPP.
• The first fully-formed purine nucleotide is IMP (base = hypoxanthine), which is the branch point.

From IMP:

• IMP → AMP needs GTP (aspartate donor; adenylosuccinate intermediate).
• IMP → GMP needs ATP (NAD⁺-dependent oxidation to XMP, then glutamine).

High-yield: Synthesis of AMP needs GTP, and synthesis of GMP needs ATP — a reciprocal cross-regulation that balances the adenine/guanine pools. Frequently asked, easily confused.

Note: the de novo pathway is energetically expensive (multiple ATP) and requires two folate-dependent (N¹⁰-formyl-THF) steps — the basis for antifolate cytotoxic drugs.

Purine salvage pathway

Salvage recycles free bases (released from nucleic-acid turnover) back into nucleotides, sparing energy. Two key enzymes:

Enzyme	Substrate base	Product	Clinical link
HGPRT (hypoxanthine-guanine phosphoribosyltransferase)	Hypoxanthine, Guanine	IMP, GMP	Deficiency → Lesch-Nyhan
APRT (adenine phosphoribosyltransferase)	Adenine	AMP	Deficiency → 2,8-DHA stones (rare)

Both transfer the base onto PRPP. Adenosine is salvaged separately by adenosine kinase.

High-yield: Loss of HGPRT means hypoxanthine/guanine cannot be re-salvaged → they are shunted to uric acid, AND unused PRPP accelerates de novo synthesis → massive overproduction of uric acid. This explains the hyperuricaemia of Lesch-Nyhan.

Purine degradation & uric acid

AMP/GMP → nucleosides → free bases → hypoxanthine/xanthine → (xanthine oxidase) → uric acid

Stepwise:

1. Nucleotides are dephosphorylated to nucleosides (adenosine, guanosine, inosine).
2. Adenosine deaminase (ADA) converts adenosine → inosine (deficiency → SCID, below).
3. Purine nucleoside phosphorylase (PNP) liberates the free base (hypoxanthine, guanine).
4. Guanine → xanthine (guanase); hypoxanthine → xanthine → uric acid by xanthine oxidase.

In humans (lacking uricase), the end-product is uric acid, which is poorly soluble → predisposes to gout and stones. Most other mammals convert it further to soluble allantoin via uricase — the rationale for rasburicase (recombinant urate oxidase) in tumour lysis.

High-yield: Xanthine oxidase is the dual-purpose enzyme catalysing both hypoxanthine→xanthine and xanthine→uric acid — the single target of allopurinol/febuxostat.

Disorders of purine metabolism

Lesch-Nyhan syndrome

• Defect: complete deficiency of HGPRT (salvage enzyme).
• Inheritance: X-linked recessive (almost exclusively boys).
• Biochemistry: ↑ uric acid (overproduction + reduced salvage), ↑ PRPP driving de novo synthesis.
• Clinical tetrad: hyperuricaemia, self-mutilation (lip/finger biting), choreoathetosis/dystonia, intellectual disability, with gouty tophi and orange "sand" (urate crystals) in nappies.
• Mnemonic — "He's Got Purine Recovery Trouble" (HGPRT). Or recall: Lesch-Nyhan = Lacks Nucleotide salvage.
• Management: allopurinol for hyperuricaemia (does NOT correct neurological features); supportive/behavioural.

ADA deficiency → SCID

• Defect: Adenosine deaminase deficiency → accumulation of deoxyadenosine → conversion to dATP, which inhibits ribonucleotide reductase → no dNTPs → no DNA synthesis. Lymphocytes are exquisitely sensitive.
• Result: Severe Combined Immunodeficiency (SCID) — both B and T cells fail ("bubble boy"). Autosomal recessive.
• Treatment: bone-marrow/HSC transplant; PEG-ADA enzyme replacement; ADA was the first disease treated by gene therapy.

High-yield: ADA deficiency → dATP accumulation → inhibits ribonucleotide reductase → SCID. PNP deficiency also causes immunodeficiency but predominantly a T-cell defect.

PNP deficiency

• Purine nucleoside phosphorylase loss → ↑ dGTP → also inhibits ribonucleotide reductase but selectively poisons T cells → cellular (T-cell) immunodeficiency with low uric acid (a clue distinguishing it).

Gout (the big one)

• Hyperuricaemia → monosodium urate (MSU) crystal deposition in joints (classically first MTP — podagra).
• Synovial fluid: needle-shaped, negatively birefringent MSU crystals (yellow when parallel to the compensator axis) — contrast with pseudogout (CPPD: rhomboid, positively birefringent).
• Causes of overproduction: PRPP synthetase overactivity, HGPRT deficiency, ↑ cell turnover (tumour lysis, psoriasis, haemolysis). Underexcretion (90% of primary gout) — reduced renal clearance, thiazides, alcohol, lead nephropathy ("saturnine gout").
• Acute attack: NSAIDs, colchicine, or corticosteroids. Do NOT start/stop urate-lowering therapy during an acute flare (can worsen it).

	Gout (MSU)	Pseudogout (CPPD)
Crystal shape	Needle	Rhomboid
Birefringence	Negative	Positive
Classic joint	1st MTP	Knee
Association	Hyperuricaemia	Haemochromatosis, hyperPTH

De novo pyrimidine synthesis

The ring is built first as orotic acid, then joined to PRPP.

Glutamine + CO₂ + 2ATP → (CPS-II, cytosol) → carbamoyl phosphate → carbamoyl aspartate → … → orotate → (+ PRPP) → OMP → (decarboxylation) → UMP

• CPS-II (cytosolic) is the regulated/committed step — contrast with CPS-I (mitochondrial) of the urea cycle.
• UMP is the parent pyrimidine nucleotide → phosphorylated to UTP → aminated to CTP (glutamine donor).
• Thymidylate (dTMP) is made from dUMP by thymidylate synthase, using N⁵,N¹⁰-methylene-THF as the methyl donor — target of 5-fluorouracil (5-FU). Dihydrofolate reductase (DHFR) regenerates THF — target of methotrexate, trimethoprim, pyrimethamine.

Enzyme	Pathway	Regulation/Drug
CPS-I (mitochondria, needs NAG)	Urea cycle	—
CPS-II (cytosol)	Pyrimidine synthesis	Committed step; inhibited by UTP, activated by ATP/PRPP
Thymidylate synthase	dUMP→dTMP	Inhibited by 5-FU (via FdUMP)
DHFR	THF regeneration	Inhibited by methotrexate
Ribonucleotide reductase	NDP→dNDP	Inhibited by hydroxyurea

High-yield: CPS-I = urea cycle (mitochondrial, activator N-acetylglutamate); CPS-II = pyrimidine synthesis (cytosolic). A perennial one-liner MCQ.

Pyrimidine degradation & orotic aciduria

Pyrimidines degrade to water-soluble products (β-alanine, β-aminoisobutyrate, CO₂, NH₃) — so they do not cause stones or gout.

Hereditary orotic aciduria

• Defect: UMP synthase (bifunctional: orotate phosphoribosyltransferase + OMP decarboxylase) → block in pyrimidine synthesis.
• Features: megaloblastic anaemia unresponsive to B12/folate, failure to thrive, and orotic acid in urine.
• Treatment: dietary uridine (bypasses the block, provides UMP, and feedback-inhibits CPS-II reducing orotate).

High-yield: Orotic aciduria distinguishing point — megaloblastic anaemia NOT corrected by folate/B12 + orotic crystalluria.

Orotic aciduria from urea-cycle defects

• Ornithine transcarbamoylase (OTC) deficiency (X-linked) → mitochondrial carbamoyl phosphate spills into the cytosol → drives pyrimidine synthesis → orotic aciduria WITH hyperammonaemia (but without megaloblastic anaemia). The presence/absence of hyperammonaemia separates OTC deficiency from hereditary orotic aciduria.

Drug targets — the pharmacology layer

• Allopurinol / febuxostat — xanthine oxidase inhibitors → ↓ uric acid (chronic gout, tumour lysis prophylaxis). Allopurinol's active metabolite is oxypurinol. Caution: blocks metabolism of 6-mercaptopurine/azathioprine (also XO substrates) → reduce their dose.
• Rasburicase — recombinant uricase, converts urate to soluble allantoin (acute tumour lysis); contraindicated in G6PD deficiency (H₂O₂ generation → haemolysis).
• Methotrexate — inhibits DHFR.
• 5-Fluorouracil — inhibits thymidylate synthase.
• Hydroxyurea — inhibits ribonucleotide reductase.
• 6-Mercaptopurine / 6-thioguanine — purine analogues; mycophenolate inhibits IMP dehydrogenase (IMP→GMP).

High-yield: Co-prescribing allopurinol with azathioprine/6-MP is a classic exam trap — leads to dangerous accumulation of the thiopurine.

Regulation — keeping the pools balanced

The pathways are tightly controlled at the first committed step of each branch and by reciprocal feedback:

• Purine: PRPP synthetase and glutamine-PRPP amidotransferase are inhibited by the end-products (AMP, GMP, IMP) and stimulated by PRPP. A gain-of-function (superactivity) of PRPP synthetase is itself an X-linked cause of purine overproduction → gout and uric-acid stones with sensorineural deafness — an exam-worthy variant.
• Cross-regulation: because AMP synthesis consumes GTP and GMP synthesis consumes ATP, an excess of one nucleotide channels IMP toward the deficient one, maintaining a roughly equal A:G ratio.
• Pyrimidine: CPS-II is inhibited by UTP and activated by ATP/PRPP, coordinating pyrimidine output with available ribose-phosphate and energy.
• Coordination: PRPP is the common currency linking sugar (HMP shunt), folate one-carbon metabolism, and energy charge to nucleotide output — disturb any of these and nucleotide balance shifts.

High-yield: PRPP synthetase superactivity = X-linked overproduction gout (often with deafness in children). Pair it mentally with HGPRT deficiency as the two inherited overproduction causes of hyperuricaemia.

Why this matters clinically — chemotherapy & immunosuppression

Rapidly dividing cells (tumours, activated lymphocytes, bone marrow) depend on a steady dNTP supply, making nucleotide synthesis a prime drug target. Understanding where each agent acts is repeatedly tested:

• Antifolates (methotrexate, pemetrexed) deplete THF → block both purine synthesis (formyl-THF steps) and dTMP synthesis. Leucovorin (folinic acid) rescue bypasses DHFR to spare normal cells.
• 5-FU → FdUMP covalently traps thymidylate synthase ("thymineless death"); its toxicity is enhanced by leucovorin (stabilises the ternary complex) — note this is the opposite of MTX rescue.
• Thiopurines (6-MP, azathioprine) are purine antimetabolites metabolised by TPMT; low-TPMT patients risk severe myelosuppression — and allopurinol further raises levels.
• Tumour lysis syndrome (massive purine release) → hyperuricaemia, hyperkalaemia, hyperphosphataemia, hypocalcaemia, acute urate nephropathy; prophylaxis with hydration + allopurinol (low risk) or rasburicase (high risk).

Key differentials & discriminators

When a stem describes hyperuricaemia or a nucleotide disorder, separate the look-alikes by the highlighted clue:

Presentation	Most likely	Discriminator
Boy, self-mutilation, ↑ urate	Lesch-Nyhan (HGPRT)	Neuro features + hyperuricaemia, X-linked
Infant, recurrent infections, both B & T cells absent	ADA-SCID	dATP ↑, normal/high urate, gene therapy history
Recurrent infections, T cells low, low urate	PNP deficiency	Low uric acid is the tell
Megaloblastic anaemia, no B12/folate response, orotic crystals, no hyperammonaemia	Hereditary orotic aciduria (UMP synthase)	Treated with uridine
Hyperammonaemia + orotic aciduria, no megaloblastic anaemia	OTC deficiency (urea cycle)	X-linked, neonatal lethargy
Acute monoarthritis, needle crystals, negative birefringence	Gout	1st MTP, MSU

Recently asked / exam angle

• "Purine ring synthesised on the sugar; pyrimidine ring synthesised before adding sugar" — direct true/false.
• Enzyme-disease matching: HGPRT → Lesch-Nyhan, ADA → SCID, UMP synthase → orotic aciduria, PNP → T-cell immunodeficiency.
• Mechanism MCQ: ADA deficiency causes SCID via dATP inhibiting ribonucleotide reductase.
• CPS-I vs CPS-II location and pathway — repeated favourite.
• Allopurinol mechanism (xanthine oxidase) and interaction with 6-MP/azathioprine.
• Crystal identification: gout = needle, negatively birefringent; pseudogout = rhomboid, positively birefringent.
• Source atoms of the purine ring (glycine, glutamine, aspartate, CO₂, formyl-THF).
• "Megaloblastic anaemia not responding to B12/folate + orotic acid in urine" → hereditary orotic aciduria; add hyperammonaemia → think OTC deficiency.
• Rasburicase contraindicated in G6PD deficiency.

Rapid revision

1. PRPP is the activated ribose shared by de novo synthesis and salvage of both purines and pyrimidines.
2. Purine committed enzyme = glutamine-PRPP amidotransferase; first nucleotide = IMP.
3. AMP synthesis needs GTP; GMP synthesis needs ATP (reciprocal).
4. HGPRT salvages hypoxanthine + guanine; its loss = Lesch-Nyhan (X-linked: self-mutilation, hyperuricaemia, choreoathetosis, ID).
5. ADA deficiency → dATP ↑ → inhibits ribonucleotide reductase → SCID; first gene-therapy disease.
6. PNP deficiency → selective T-cell immunodeficiency with low uric acid.
7. Human purine end-product = uric acid (no uricase); rasburicase supplies uricase → allantoin.
8. Xanthine oxidase makes uric acid → inhibited by allopurinol/febuxostat; beware azathioprine/6-MP interaction.
9. Gout crystals = needle-shaped, negatively birefringent MSU; first MTP (podagra).
10. Pyrimidine ring built first (orotate) then joined to PRPP; committed step = cytosolic CPS-II.
11. CPS-I = urea cycle (mitochondrial); CPS-II = pyrimidine (cytosolic).
12. Hereditary orotic aciduria (UMP synthase defect): megaloblastic anaemia unresponsive to B12/folate + orotic crystalluria; treat with uridine. OTC deficiency = orotic aciduria with hyperammonaemia.