Provide a brief overview of nucleic acid metabolism: de novo synthesis vs salvage pathways for purines and pyrimidines.

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Multiple Choice

Provide a brief overview of nucleic acid metabolism: de novo synthesis vs salvage pathways for purines and pyrimidines.

Nucleic acid metabolism relies on two parallel strategies to maintain nucleotide pools: de novo synthesis builds nucleotides from basic precursors, while salvage recycles bases and nucleosides back into nucleotides. For purines, the de novo route starts with ribose-5-phosphate being converted to PRPP, and the purine ring is built on that scaffold to produce IMP. From IMP, the two main branches lead to AMP and GMP. Salvage recycles purines by attaching them to PRPP: hypoxanthine or guanine are converted to IMP and GMP, respectively, via HGPRT, and adenine can be converted to AMP via APRT. This saves energy by reusing bases rather than rebuilding the whole ring. For pyrimidines, de novo synthesis proceeds through the carbamoyl phosphate pathway (carbamoyl phosphate formed by CPS II), followed by steps that build the ring on aspartate and ultimately yield UMP, which is phosphorylated to UDP and UTP, with CTP formed from UTP. Salvage for pyrimidines uses specific phosphoribosyltransferases to attach bases like uracil or cytosine to PRPP, forming UMP or CMP directly, thereby recycling bases into nucleotides. This combination—purines made de novo from PRPP then converted to AMP/GMP, with purine salvage via HGPRT; and pyrimidines made de novo via the CPS II–aspartate pathway to UDP/UTP (and CTP), with pyrimidine salvage via phosphoribosyltransferases—accurately captures how cells balance synthesis and reuse to maintain nucleotide supplies.

Nucleic acid metabolism relies on two parallel strategies to maintain nucleotide pools: de novo synthesis builds nucleotides from basic precursors, while salvage recycles bases and nucleosides back into nucleotides.

For purines, the de novo route starts with ribose-5-phosphate being converted to PRPP, and the purine ring is built on that scaffold to produce IMP. From IMP, the two main branches lead to AMP and GMP. Salvage recycles purines by attaching them to PRPP: hypoxanthine or guanine are converted to IMP and GMP, respectively, via HGPRT, and adenine can be converted to AMP via APRT. This saves energy by reusing bases rather than rebuilding the whole ring.

For pyrimidines, de novo synthesis proceeds through the carbamoyl phosphate pathway (carbamoyl phosphate formed by CPS II), followed by steps that build the ring on aspartate and ultimately yield UMP, which is phosphorylated to UDP and UTP, with CTP formed from UTP. Salvage for pyrimidines uses specific phosphoribosyltransferases to attach bases like uracil or cytosine to PRPP, forming UMP or CMP directly, thereby recycling bases into nucleotides.

This combination—purines made de novo from PRPP then converted to AMP/GMP, with purine salvage via HGPRT; and pyrimidines made de novo via the CPS II–aspartate pathway to UDP/UTP (and CTP), with pyrimidine salvage via phosphoribosyltransferases—accurately captures how cells balance synthesis and reuse to maintain nucleotide supplies.

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