Pyrimidine metabolism covers the de novo synthesis, salvage, and degradation of pyrimidine nucleotides. Unlike purine synthesis, the pyrimidine ring is assembled first and then attached to ribose-5-phosphate.
De Novo Pyrimidine Synthesis
The pyrimidine pathway begins in the cytoplasm with the formation of carbamoyl phosphate from glutamine and bicarbonate, catalyzed by carbamoyl phosphate synthetase II. This is distinct from the mitochondrial CPS I enzyme involved in the urea cycle, and uses glutamine rather than ammonia as the nitrogen donor. CPS-II is the committed and rate-limiting step of pyrimidine synthesis.
Aspartate transcarbamoylase catalyzes the second step, condensing carbamoyl phosphate with aspartate to form N-carbamoylaspartate. In bacteria, ATCase is a classic model of allosteric regulation, but in mammals the balance is simpler with CPS-II being the primary regulatory point.
Dihydroorotase cyclizes N-carbamoylaspartate to form dihydroorotate, which is then oxidized to orotate by dihydroorotate dehydrogenase. This enzyme is located on the outer surface of the inner mitochondrial membrane and uses ubiquinone as the electron acceptor, linking pyrimidine synthesis to the electron transport chain. Orotate phosphoribosyltransferase adds PRPP to orotate to form orotidine monophosphate, which is then decarboxylated by OMP decarboxylase to form UMP.
Conversion to UTP and CTP
UMP is sequentially phosphorylated by two kinases. UMP-CMP kinase phosphorylates UMP to UDP using ATP, and nucleoside diphosphate kinase converts UDP to UTP. CTP synthetase then aminates UTP to form CTP, using glutamine as the nitrogen donor and ATP as the energy source. CTP synthetase is inhibited by CTP and activated by GTP, providing a regulatory connection between purine and pyrimidine pools.
Thymidylate Synthesis
Thymidylate, dTMP, is synthesized from dUMP by thymidylate synthase. The reaction transfers a methylene group from methylenetetrahydrofolate to dUMP and reduces it to a methyl group, generating dTMP and dihydrofolate. This is the only de novo source of thymidine nucleotides and is essential for DNA replication.
Dihydrofolate is reduced back to tetrahydrofolate by dihydrofolate reductase, which is the target of the anticancer drug methotrexate. The thymidylate synthase reaction is also targeted by 5-fluorouracil, which forms a stable covalent complex with the enzyme. These drugs selectively kill rapidly dividing cells by blocking DNA synthesis.
Pyrimidine Salvage
Pyrimidine salvage is less efficient than purine salvage. Uridine and cytidine are phosphorylated by uridine-cytidine kinase to form UMP and CMP. Thymidine is phosphorylated by thymidine kinase. Deoxynucleosides are salvaged by deoxycytidine kinase and thymidine kinase 2. Unlike HGPRT for purines, there are no efficient salvage enzymes for free pyrimidine bases, though some interconversion occurs through nucleoside phosphorylases.
Pyrimidine Degradation
Pyrimidine degradation produces soluble end products suited for excretion. Cytosine is deaminated to uracil by cytidine deaminase. Uracil is reduced to dihydrouracil by dihydropyrimidine dehydrogenase, the rate-limiting step of catabolism. Dihydropyrimidinase opens the ring to produce beta-alanine, ammonia, and carbon dioxide. Thymine follows the same pathway, producing beta-aminoisobutyrate, ammonia, and carbon dioxide. beta-Aminoisobutyrate is excreted in urine, and its levels vary with thymine turnover, including from DNA degradation.
Regulation
Like many metabolic pathways, pyrimidine synthesis is regulated at CPS-II, which is activated by PRPP and ATP and inhibited by UTP and CTP. The liver has high CPS-II activity and also expresses CPS-I for the urea cycle, allowing independent regulation of pyrimidine synthesis and nitrogen disposal. OMP decarboxylase is inhibited by UMP and CMP through product inhibition. The interconversion of nucleotides is regulated by the energy status of the cell, with ATP required for CTP synthesis and dTMP synthesis linked to the folate cycle.
