Questions: Nucleotide Synthesis Pathways (De Novo and Salvage)

Question 1

A patient has a complete deficiency of HGPRT. Which statement best explains why this causes neurological damage despite cells retaining intact de novo synthesis?

• A. HGPRT is required for de novo purine synthesis, so its loss blocks all purine production in neurons
• B. The brain relies heavily on salvage pathways to recycle purines because neurons have very low de novo synthesis capacity; without HGPRT, hypoxanthine and guanine cannot be recycled and neurons are starved of purines
• C. HGPRT deficiency causes excess pyrimidines to accumulate, which are neurotoxic at high concentrations
• D. De novo synthesis is only active during cell division; non-dividing neurons depend entirely on salvage for all nucleotide production

Answer: (B)

The key insight is tissue-specific dependence on salvage. The brain has unusually high purine demand (for ATP, cAMP, signaling) but low de novo synthesis activity, making neurons disproportionately dependent on HGPRT to recycle hypoxanthine and guanine from nucleic acid turnover. When HGPRT is absent, the brain cannot compensate through de novo synthesis at the required rate, leading to purine depletion in neurons and the devastating neurological symptoms of Lesch-Nyhan syndrome. Option D overstates the case — non-dividing cells can perform de novo synthesis, just at rates insufficient to compensate in the brain.

Question 2

Which statement correctly describes the fundamental architectural difference between de novo purine and de novo pyrimidine synthesis?

• A. Purines are synthesized from a single amino acid precursor; pyrimidines require three different amino acids
• B. Purine synthesis builds the ring step-by-step while attached to PRPP (ribose-first); pyrimidine synthesis builds the ring as a free base (orotate) and only then attaches to PRPP
• C. Purines are assembled as free bases and then attached to ribose, while pyrimidines are assembled directly on ribose from the start
• D. Both pathways build the base as a free molecule first, but purines require GTP for the final attachment while pyrimidines require ATP

Answer: (B)

This architectural reversal is the central organizational fact of nucleotide synthesis. In purine de novo synthesis, PRPP serves as the starting scaffold — atoms from glutamine, glycine, aspartate, CO₂, and formyl-THF are added piece by piece to the ribose-phosphate backbone, ultimately building IMP. In pyrimidine synthesis, the ring is completed first as orotate (from carbamoyl phosphate and aspartate), and only afterward is it attached to PRPP to form orotidylate, which is then decarboxylated to UMP. Option C has the two pathways exactly reversed.

Question 3

Salvage pathways produce nucleotides by assembling purines or pyrimidines from small precursor molecules like CO₂ and amino acids, making them the energetically preferred alternative to de novo synthesis.

• T. True
• F. False

Answer: False

Salvage pathways do not build bases from small precursors — that is de novo synthesis. Salvage pathways *recycle* preformed bases (hypoxanthine, guanine, adenine) that have been released during normal nucleic acid degradation, reattaching them to PRPP in a single enzymatic step. This is energetically cheaper than de novo synthesis precisely because the complex ring structures have already been built. The distinction is critical: de novo synthesis = build from scratch using amino acids, CO₂, folate; salvage = recycle existing bases using PRPP.

Question 4

The de novo synthesis of purines requires folate derivatives (formyl-THF) as carbon donors, which is why drugs blocking folate metabolism (like methotrexate) preferentially kill rapidly dividing cells.

• T. True
• F. False

Answer: True

Formyl-THF donates carbons at two steps in purine de novo synthesis, and thymidylate synthase requires a folate coenzyme to convert dUMP to dTMP (the specific thymidine nucleotide needed for DNA). Rapidly dividing cells (cancer cells, immune cells) require enormous amounts of new nucleotides for DNA replication and cannot rely solely on recycled bases — they are highly dependent on de novo synthesis pathways that require folate. Methotrexate blocks dihydrofolate reductase, depleting active folate cofactors and starving dividing cells of the purines and thymidylate they need. Normal slow-dividing cells are much less affected because their demand for de novo synthesis is lower.

Question 5

Why do chemotherapy drugs like 5-fluorouracil and methotrexate preferentially kill rapidly dividing cancer cells rather than non-dividing normal cells?

Model answer: Rapidly dividing cells require massive amounts of new nucleotides to replicate their DNA during each cell cycle. They cannot meet this demand through salvage alone and depend heavily on de novo synthesis pathways. 5-Fluorouracil inhibits thymidylate synthase (blocking dTMP production needed for DNA), and methotrexate blocks dihydrofolate reductase (depleting the folate coenzymes required for purine synthesis and dTMP synthesis). Without these nucleotides, cells cannot replicate their DNA and are forced into arrest or apoptosis. Non-dividing normal cells have low nucleotide demands that can be partially met through salvage pathways and have time to compensate, making them less sensitive to these targeted blocks.

The mechanistic logic applies broadly: any drug that blocks a committed step in de novo nucleotide synthesis will disproportionately harm cells that divide rapidly and therefore have the highest demand for newly synthesized nucleotides. This selectivity is partial — some normal dividing tissues (gut epithelium, bone marrow) are also affected, explaining common chemotherapy side effects like mucositis and myelosuppression.