Which of the following best describes how the purine ring is constructed in de novo biosynthesis?
AThe complete purine base is synthesized first in the cytosol, then attached to ribose-5-phosphate in a single condensation step
BAtoms are assembled one or a few at a time directly onto the ribose-5-phosphate scaffold beginning from PRPP, with every intermediate remaining a ribonucleotide
CPurine bases are assembled on a carrier protein and transferred en bloc to PRPP at the final step
DThe imidazole ring is formed first on ribose, exported from the cell, and the pyrimidine ring is added extracellularly
A defining feature of purine (not pyrimidine) biosynthesis is that construction occurs directly on the ribose-5-phosphate scaffold. PRPP is the starting material, and each of the ten steps adds atoms to the growing ring while it remains attached to ribose-phosphate. This contrasts with pyrimidine biosynthesis, where the ring IS assembled first and then attached to ribose. Every intermediate in the purine pathway is already a ribonucleotide.
Question 2 Multiple Choice
Methotrexate is a folate antagonist used in cancer chemotherapy. It depletes N10-formyl-THF in cells. Which specific steps in purine biosynthesis are directly blocked?
AThe committed step catalyzed by glutamine-PRPP amidotransferase, which requires folate as a direct cofactor
BThe steps incorporating carbons 2 and 8 into the purine ring, which require N10-formyl-THF as the one-carbon donor
CAll ten steps, because THF is required as an energy source throughout the pathway
DThe branch-point conversion of IMP to AMP, which requires a folate-derived methyl group
N10-formyl-THF donates carbons 2 and 8 of the purine ring in two specific steps. Without sufficient folate, these one-carbon transfer reactions stall and the ring cannot be completed. This is the direct mechanistic link between one-carbon metabolism (a prerequisite topic) and purine biosynthesis. Rapidly dividing cancer cells rely heavily on de novo purine synthesis for DNA replication, making them selectively vulnerable to folate antagonism.
Question 3 True / False
The conversion of IMP to AMP requires GTP as an energy source, while the conversion of IMP to GMP requires ATP — and this reciprocal arrangement helps the cell maintain a balanced ratio of adenine and guanine nucleotides.
TTrue
FFalse
Answer: True
This cross-regulation is elegant: when adenine nucleotides are abundant, the plentiful ATP drives GMP synthesis; when guanine nucleotides are abundant, the plentiful GTP drives AMP synthesis. Each purine type uses the other's surplus to fuel its own production of the opposite nucleotide, creating a self-balancing system that maintains the approximately 1:1 ratio of adenine to guanine nucleotides the cell needs for nucleic acid synthesis.
Question 4 True / False
The committed step in purine biosynthesis is the synthesis of PRPP from ribose-5-phosphate, because without PRPP no purine can be made.
TTrue
FFalse
Answer: False
PRPP is used in multiple biosynthetic pathways — pyrimidine synthesis, amino acid synthesis (histidine, tryptophan), and NAD+ synthesis — so its production is not committed to purines. The true committed step is the glutamine-PRPP amidotransferase reaction, which replaces the pyrophosphate of PRPP with an amino group from glutamine, producing phosphoribosylamine — a compound used only in purine synthesis. Once this step fires, the cell is irreversibly committed to making a purine, which is why this enzyme is the primary feedback-regulated control point.
Question 5 Short Answer
Explain why methotrexate (a folate antagonist) blocks purine biosynthesis, and why rapidly dividing cancer cells are selectively vulnerable to this block.
Think about your answer, then reveal below.
Model answer: Methotrexate inhibits dihydrofolate reductase, depleting the active pool of tetrahydrofolate (THF) and its derivative N10-formyl-THF. N10-formyl-THF is required to donate carbons 2 and 8 to the growing purine ring in two steps of the ten-step pathway. Without these one-carbon donors, purine ring assembly stalls. Rapidly dividing cancer cells depend heavily on de novo purine synthesis to supply the massive nucleotide demand created by continuous DNA replication; normal resting cells can often meet their modest needs via purine salvage pathways that recycle existing purines. This differential reliance on de novo synthesis makes cancer cells selectively vulnerable.
The same logic applies to pyrimidine synthesis: N5,N10-methylene-THF is required for thymidylate synthesis. Folate antagonists therefore hit both purine and pyrimidine pathways simultaneously, creating a broad blockade of nucleotide production that is especially lethal to cells in rapid division.