Library Core Topics
Foundational Science Series

Core Topics: Nucleotide Metabolism

High-yield reference for purine/pyrimidine synthesis, degradation, salvage pathways, ribonucleotide reduction, and chemotherapy agents — mapped to USMLE Step 1/2 CK foundational science objectives.

How to Use This Resource

Each pathway section includes: Core Concepts (location, net reaction), Key Enzymes (regulatory steps), and High-Yield Facts (clinical correlations, drug mechanisms, deficiencies). Use for active recall, spaced repetition, and integration with clinical cases.

Purine Synthesis (De Novo)

Cytoplasmic assembly of purine ring on ribose-5-phosphate backbone

Location & Starting Molecule
  • Location: Cytoplasm (all cells, especially liver)
  • Starting Molecule: PRPP (phosphoribosyl pyrophosphate)
  • PRPP Synthesis: Ribose-5-phosphate (from pentose phosphate pathway) + ATP → PRPP
  • Enzyme: PRPP synthetase
  • Key Concept: Purine ring is built atom-by-atom on ribose (vs. pyrimidines: ring first, then attach ribose)
Rate-Limiting Step & Regulation
Glutamine-PRPP Amidotransferase (GPAT)
Rate-limiting; PRPP → phosphoribosylamine
Inhibition
AMP, GMP, IMP (feedback inhibition by end products)
Activation
PRPP (substrate activation)
Atom Sources for Purine Ring
Atom PositionSource
C4, C5, N7Glycine (entire molecule incorporated)
N3, N9Glutamine (amide nitrogen)
N1Aspartate (amino nitrogen)
C6CO₂ (bicarbonate)
C2, C8N¹⁰-formyl-THF (folate derivative)

Mnemonic: "CAG" = C2/C8 from formyl-THF, A from aspartate, G from glycine/glutamine

High-Yield: Purine Synthesis
First Complete Nucleotide
IMP (inosine monophosphate) is the first complete purine nucleotide; branch point to AMP and GMP
IMP Branch Point
IMP → AMP (requires GTP + aspartate); IMP → GMP (requires ATP + NAD⁺); cross-regulation ensures balance
Energy Cost
Requires 6 ATP per purine nucleotide; glutamine, glycine, aspartate, CO₂, THF derivatives all required
Key Cofactors
Folate (THF) for C2/C8; Vitamin B6 for glycine metabolism; deficiency → impaired purine synthesis

Pyrimidine Synthesis (De Novo)

Cytoplasmic synthesis with ring assembly before ribose attachment

Location & Key Difference
  • Location: Cytoplasm (mostly); one mitochondrial step
  • Key Difference from Purines: Pyrimidine ring assembled BEFORE attachment to ribose-5-phosphate
  • First Complete Nucleotide: UMP (uridine monophosphate)
  • Trifunctional Enzyme (CAD): CPS-II, ATCase, and dihydroorotase on single polypeptide in humans
  • UMP Synthase: Orotate phosphoribosyltransferase + OMP decarboxylase activities
Rate-Limiting Step & Pathway
1
CPS-II (Carbamoyl Phosphate Synthetase II)
Glutamine + CO₂ + 2 ATP → carbamoyl phosphate (rate-limiting, cytoplasm)
2
ATCase
Carbamoyl phosphate + aspartate → carbamoyl aspartate
3
Dihydroorotase
Ring closure → dihydroorotate
4
Dihydroorotate Dehydrogenase
Dihydroorotate → orotate (mitochondrial; requires FMN)
5
OMP Synthase
Orotate + PRPP → OMP → UMP (ribose attached here)
Thymidylate Synthesis
  • Pathway: UMP → UDP → UTP → CTP (CTP synthetase, requires glutamine)
  • dUMP → dTMP: Thymidylate synthase converts dUMP to dTMP
  • Cofactor: Methylene-THF (oxidized to DHF in reaction)
  • DHF → THF: Dihydrofolate reductase (DHFR) regenerates THF
  • Regulation: CPS-II inhibited by UTP (feedback); activated by ATP, PRPP
High-Yield: Pyrimidine Synthesis
CPS-I vs CPS-II
CPS-I: Mitochondrial, urea cycle, uses NH₄⁺, activated by N-acetylglutamate; CPS-II: Cytoplasmic, pyrimidine synthesis, uses glutamine, activated by ATP/PRPP
Thymidylate Synthase
Critical for DNA synthesis; target of 5-fluorouracil (5-FU); requires methylene-THF
DHFR Inhibitors
Methotrexate, trimethoprim, pyrimethamine inhibit DHFR → ↓THF → ↓purine/thymidylate synthesis
Orotic Aciduria
UMP synthase deficiency → orotic acid in urine, megaloblastic anemia (no B12/folate deficiency), failure to thrive

Purine Degradation

Breakdown of purine nucleotides to uric acid for excretion

Degradation Pathway
1
Nucleotidases
Nucleotides → nucleosides (remove phosphate)
2
Purine Nucleoside Phosphorylase
Nucleosides → free bases (remove ribose)
3
Adenosine Deaminase (ADA)
Adenosine → inosine (AMP pathway)
4
Xanthine Oxidase
Hypoxanthine → xanthine → uric acid (both steps)
Uric Acid & Clinical
  • End Product: Uric acid (humans lack uricase/urate oxidase)
  • Solubility: Poorly soluble in water → crystallizes in joints (gout) and kidneys (stones)
  • Excretion: Primarily renal (2/3), some GI (1/3)
  • Gout: Monosodium urate crystal deposition in joints → inflammatory arthritis (podagra = 1st MTP)
  • Tumor Lysis Syndrome: Rapid cell death → ↑purine degradation → ↑uric acid → acute kidney injury
Purine Degradation Disorders
Lesch-Nyhan Syndrome
HGPRT deficiency → ↑uric acid, self-mutilation, dystonia, intellectual disability (X-linked)
ADA Deficiency
Severe combined immunodeficiency (SCID); adenosine/deoxyadenosine accumulation → lymphocyte toxicity
Xanthine Oxidase Deficiency
Xanthinuria; low uric acid, xanthine stones (rare)
High-Yield: Purine Degradation
Key Enzyme
Xanthine oxidase catalyzes both hypoxanthine→xanthine and xanthine→uric acid; inhibited by allopurinol, febuxostat
Lesch-Nyhan
HGPRT deficiency (purine salvage); self-mutilation (lip/finger biting), choreoathetosis, aggressive behavior, hyperuricemia
ADA-SCID
Adenosine deaminase deficiency → toxic dATP accumulation → inhibits ribonucleotide reductase → lymphocyte death
Gout Treatment
Acute: NSAIDs, colchicine, steroids; Chronic: allopurinol, febuxostat (↓production), probenecid (↑excretion), rasburicase (acute TLS)

Pyrimidine Degradation

Breakdown to water-soluble products without crystal formation

Degradation Pathway
  • Key Difference from Purines: Pyrimidines degraded to soluble products (no crystal deposition)
  • Cytosine Pathway: Cytosine → uracil → dihydrouracil → β-alanine + NH₃ + CO₂
  • Thymine Pathway: Thymine → dihydrothymine → β-aminoisobutyrate + NH₃ + CO₂
  • Key Enzyme: Dihydropyrimidine dehydrogenase (DPD) (rate-limiting)
  • Excretion: Products are water-soluble → excreted in urine without crystallization
DPD & 5-FU Toxicity
  • DPD Function: Degrades uracil, thymine, AND 5-fluorouracil (5-FU)
  • DPD Deficiency: Autosomal recessive; inability to clear 5-FU → severe toxicity
  • Clinical: Profound myelosuppression, mucositis, neurotoxicity, diarrhea after standard 5-FU dose
  • Testing: DPD activity or DPYD genotyping before 5-FU/capecitabine therapy
  • Management: Dose reduction or avoid fluoropyrimidines in deficient patients
Purine vs Pyrimidine Degradation
FeaturePurinesPyrimidines
End ProductUric acid (insoluble)β-alanine, β-aminoisobutyrate (soluble)
Crystal FormationYes (gout, stones)No
Key EnzymeXanthine oxidaseDihydropyrimidine dehydrogenase
Clinical DisorderGout, Lesch-NyhanDPD deficiency (5-FU toxicity)
High-Yield: Pyrimidine Degradation
Solubility Difference
Purines → insoluble uric acid (crystals); Pyrimidines → soluble products (no crystals)
DPD Testing
Screen for DPD deficiency before 5-FU/capecitabine; ~3-5% of population has partial deficiency
β-Aminoisobutyrate
Thymine degradation product; elevated in conditions with high cell turnover (leukemia, chemotherapy)
5-FU Toxicity Signs
Severe mucositis, neutropenia, diarrhea, neurotoxicity, hand-foot syndrome in DPD-deficient patients

Salvage Pathways

Energy-efficient recycling of free bases back to nucleotides

Purine Salvage
HGPRT
Hypoxanthine + PRPP → IMP; Guanine + PRPP → GMP
APRT
Adenine + PRPP → AMP
Energy Savings
Salvage uses 1 ATP vs. 6+ ATP for de novo synthesis
Pyrimidine Salvage
Thymidine Kinase
Thymidine → TMP (important for DNA synthesis)
Uridine Kinase
Uridine → UMP
Clinical Use
Salvage enzymes activate nucleoside analog chemotherapy (e.g., 6-MP, Ara-C)
HGPRT Deficiency
  • Lesch-Nyhan Syndrome: Complete HGPRT deficiency (X-linked recessive)
  • Pathophysiology: ↓Purine salvage → ↑de novo synthesis → ↑uric acid production
  • Clinical Triad: (1) Hyperuricemia/gout, (2) Severe neurologic dysfunction (dystonia, choreoathetosis), (3) Self-mutilation (lip/finger biting)
  • Behavioral: Aggressive behavior, intellectual disability, spasticity
  • Treatment: Allopurinol/febuxostat for hyperuricemia; no treatment for neurologic symptoms
  • Partial Deficiency: Kelley-Seegmiller syndrome (gout without neurologic symptoms)
High-Yield: Salvage Pathways
Tissue Specificity
Salvage: Primary pathway in most tissues (energy-efficient); De novo: Important in rapidly dividing cells (bone marrow, GI, tumors)
Lesch-Nyhan Mnemonic
"HGPRT" = Hyperuricemia, Gout, Pissed off (aggression), Retardation, Torticollis/dystonia
Chemotherapy Activation
6-Mercaptopurine activated by HGPRT → toxic nucleotides; TPMT metabolizes 6-MP (genetic variation affects dosing)
X-Linked
Lesch-Nyhan is X-linked recessive → males affected; females are carriers

Ribonucleotide Reduction

Conversion of ribonucleotides to deoxyribonucleotides for DNA synthesis

Key Enzyme & Mechanism
  • Enzyme: Ribonucleotide reductase (RNR)
  • Reaction: NDP → dNDP (at diphosphate level, not mono- or triphosphate)
  • Reducing Agent: Thioredoxin (reduced by thioredoxin reductase using NADPH)
  • Alternative: Glutaredoxin system (uses glutathione)
  • Location: Cytoplasm (all dividing cells)
  • dUMP → dTMP: Separate pathway via thymidylate synthase (requires methylene-THF)
Complex Regulation
Activity Site
ATP activates RNR (signals energy availability); dATP inhibits (prevents overproduction)
Specificity Site
Determines which NDP is reduced: ATP/dATP → CDP/UDP; dTTP → GDP; dGTP → ADP (balanced dNTP pool)
Hydroxyurea
  • Mechanism: Inhibits ribonucleotide reductase (quenches tyrosyl free radical)
  • Effect: ↓dNTP production → inhibits DNA synthesis → S-phase specific
  • Clinical Uses:
    • Chronic myeloid leukemia (CML)
    • Sickle cell disease (↑HbF production)
    • Polycythemia vera, essential thrombocythemia
    • Head and neck cancers (radiation sensitizer)
  • Toxicity: Myelosuppression, GI upset, skin hyperpigmentation, teratogenic
High-Yield: Ribonucleotide Reduction
Substrate Level
RNR acts on NDPs (not NMPs or NTPs); dNDPs then phosphorylated to dNTPs for DNA synthesis
dATP Inhibition
dATP is potent allosteric inhibitor of RNR; ADA deficiency → ↑dATP → RNR inhibition → SCID
Hydroxyurea in Sickle Cell
Increases fetal hemoglobin (HbF) → ↓HbS polymerization → ↓vaso-occlusive crises; also ↓WBC count (↓adhesion)
Thioredoxin System
Thioredoxin (reduced) → thioredoxin (oxidized) during RNR reaction; thioredoxin reductase + NADPH regenerates reduced form

Nucleotide Analogs & Chemotherapy

Antimetabolite drugs targeting nucleotide synthesis for cancer and immunosuppression

Purine Analogs

  • 6-Mercaptopurine (6-MP): Inhibits de novo purine synthesis; activated by HGPRT; used in ALL; metabolized by xanthine oxidase (↓dose with allopurinol) and TPMT
  • 6-Thioguanine (6-TG): Similar to 6-MP; used in AML
  • Azathioprine: Prodrug → 6-MP; immunosuppressant (transplant, autoimmune); ↓dose with allopurinol
  • Mycophenolate mofetil: Inhibits IMP dehydrogenase → ↓GMP; selective for lymphocytes (de novo dependent)
  • Fludarabine: Purine analog; inhibits DNA polymerase; used in CLL
  • Cladribine (2-CdA): ADA-resistant; accumulates in lymphocytes; used in hairy cell leukemia

Pyrimidine Analogs

  • 5-Fluorouracil (5-FU): Inhibits thymidylate synthase; converted to 5-FdUMP → covalent complex with TS + methylene-THF; used in colorectal, breast, head/neck cancers
  • Leucovorin: Folinic acid; enhances 5-FU (stabilizes ternary complex)
  • Capecitabine: Oral prodrug of 5-FU; activated in tumor (thymidine phosphorylase)
  • Cytarabine (Ara-C): Inhibits DNA polymerase; used in AML
  • Gemcitabine: Inhibits RNR + DNA polymerase; used in pancreatic, lung, bladder cancers

Folate Antagonists & Other

  • Methotrexate (MTX): Inhibits DHFR → ↓THF → ↓purine/thymidylate synthesis; used in cancer, RA, ectopic pregnancy, psoriasis; leucovorin rescue bypasses block
  • Trimethoprim: Inhibits bacterial DHFR (selective); used in UTIs, PCP prophylaxis
  • Pyrimethamine: Inhibits protozoal DHFR; used in toxoplasmosis, malaria
  • Pemetrexed: Inhibits TS, DHFR, GARFT; used in mesothelioma, NSCLC
  • Hydroxyurea: Inhibits ribonucleotide reductase; used in CML, sickle cell, polycythemia vera

Uric Acid-Lowering Agents

  • Allopurinol: Xanthine oxidase inhibitor (competitive); ↓uric acid production; used in gout, TLS prophylaxis; ↑6-MP/azathioprine levels
  • Febuxostat: Xanthine oxidase inhibitor (non-competitive); alternative to allopurinol
  • Rasburicase: Recombinant urate oxidase; converts uric acid → allantoin (more soluble); used in acute TLS; contraindicated in G6PD deficiency (H₂O₂ production)
  • Probenecid: Uricosuric; ↑renal uric acid excretion; used in chronic gout
High-Yield: Chemotherapy Agents
6-MP Drug Interactions
Metabolized by xanthine oxidase and TPMT; allopurinol inhibits xanthine oxidase → ↑6-MP toxicity (↓dose by 75%); TPMT deficiency → severe myelosuppression
5-FU Mechanism
5-FdUMP forms covalent ternary complex with thymidylate synthase + methylene-THF → irreversible inhibition; leucovorin enhances
Methotrexate Toxicity
Myelosuppression, mucositis, hepatotoxicity, pneumonitis, nephrotoxicity; leucovorin rescue provides reduced folate to bypass DHFR block
Rasburicase Warning
Contraindicated in G6PD deficiency (produces H₂O₂ → hemolysis); used for acute tumor lysis syndrome (rapid uric acid reduction)
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Evidence & Further Reading