A group of heterocyclic aromatic organic compound, pyrimidines are similar in structure to benzene and pyridine and count the nucleic acids cytosine, thymine, and uracil as structural derivatives. The following pathway illustrates a many pyrimidine-associated processes such as nucleotide biosynthesis, degradation, and salvage. This pathway depicts a number of pyrimidine-related processes such as nucleotide biosynthesis, degradation, and salvage. For pyrimidine nucleotide biosynthesis, carbamoyl phosphate derived from the action of carbamoyl phosphate synthetase II (CPS-II) on glutamine and bicarbonate is converted into carbamoyl aspartate by aspartate transcarbamoylase, ATCase. Dihydroorotic acid is subsequently generated by the action of carbamoyl aspartate dehydrogenase on carbamoyl aspartate. Dihydroorotate dehydrogenase then converts dihydroorotic acid to orotic acid. From this point, orotate phosphoribosyltransferase incorporates phosphoribosyl pyrophosphate into (PRPP) to produce orotidine monophosphate. Orotidine-5’-phosphate carboxylase subsequently converts orotidine monophosphate into uridine monophosphate (UMP). UMP is further phosphorylated twice to form UTP; the first instance by uridylate kinase and the second instance by ubiquitous nucleoside diphosphate kinase. UTP moves into the CTP synthesis pathway with the action of CTP synthase which aminates the molecule. The uridine nucleotides are also feedstock for the de novo thymine nucleotides synthesis pathway. DeoxyUMP which is derived from UDP or CDP metabolism is transformed by the action of thymidylate synthase into deoxyTMP of which the methyl group is sourced from N5,N10-methylene THF. THF is subsequently regenerated from DHF via dihydrofolate reductase (DHFR) which is essential for the continuation of thymidylate synthase activity. Serine hydroxymethyl transferase then acts on THF to regenerate N5,N10-THF. Pyrimidine synthesis is a comparatively simpler process than purine synthesis due to a couple of factors; pyrimidine ring structure is assembled as a free base rather being derived from PRPP and there is no branch in the pyrimidine synthesis pathway as opposed to the purine synthesis pathway. For thymidine, the action of thymidine kinase on it (or alternatively deoxyuridine) plays an important role in what is referred to as the salvage pathway to dTTP synthesis. However to form dTMP, the action of thymine phosphorylase and thymidine kinase is required. For deoxycytidine, deoxycytidine kinase is required (deoxycytidine also acts on deoxyadenosine and deoxyguanosine). For uracil, UMP can be formed by the action of uridine phosphorylase and uridine kinase on uracil. Pyrimidine catabolism ultimately results in the formation of the waste products of urea, H2O, and CO2. The product of cytosine breakdown, uracil, can be broken down to N-carbamoyl-β-alanine which can be catabolized into β-alanine. The product of thymine breakdown is β-aminoisobutyrate. The transamination of α-ketoglutarate to glutamate requires both of these breakdown products (β-alanine and β-aminoisobutyrate) to act as amine group donors. The products of this transamination can move through a further reaction that produces malonyl-CoA or methylmalonyl-CoA, a precursor for succinyl-CoA which is used in the Krebs cycle.

Pyrimidine Metabolism References