Mammalian de novo pyrimidine biosynthesis is accomplished by six enzymatic activities, encoded by only three genes - two encode multifunctional proteins. In the de novo synthesis of pyrimidines, the pyrimidine ring is added to the ribose-5-phosphate after the ring has been generated, unlike purines. The first reaction of the six-step pathway to generate uridine monophosphate (UMP) is catalyzed by the carbamoyl phosphate synthetase II activity of the trifunctional cytosolic mammalian enzyme Cad,
generating carbamoyl phosphate from carbonate and the amide nitrogen of glutamine. Carbamoyl phosphate, which is highlighted along with the other central pathway products, can also be supplied from arginine and proline metabolism. Carbonate contributes one carbon and glutamine supplies one of the two nitrogens to the pyrimidine ring. This reaction requires two molecules of ATP and is the rate-limiting step in pyrimidine biosynthesis, being feedback inhibited by uridine diphosphate and uridine triphosphate, and activated by phosphoribosyl pyrophosphate (PRPP) and ATP. Carbamoyl aspartate is then generated via the condensation of carbamoyl phosphate with aspartate, catalyzed by the second enzymatic activity of Cad, aspartate transcarbamoylase. Aspartate contributes three carbons and a nitrogen to the pyrimidine ring. Closure of the ring to generate dihydroorotate is performed by the last activity of Cad, dihydroorotase. Dihydroorotate is oxidized to orotate by dihydroorotate dehydrogenase (Dhodh), the only pathway enzyme located in the mitochondrion. Quinones supply the oxidizing power. This link to the respiratory chain makes pyrimidine biosynthesis a pacemaker for cell growth and division under limited oxygen conditions. Orotate is combined with PRPP, from the pentose phosphate pathway, to generate orotidine-5'-monophosphate (OMP), catalyzed by the first enzyme activity of the bifunctional enzyme Umps, orotate phosphoribosyl transferase. OMP is then decarboxylated to form uridine monophosphate (UMP) by OMP decarboxylase, the other activity of Umps. A second level of control is exerted here, with UMP and cytididine monophosphate (CMP; not shown) negatively regulating Umps, and PRPP positively regulating both activities. Increased expression of Cad and Dhodh are associated with certain cancers, while diabetes is associated with decreased expression of Cad, Dhodh and Umps. UMP is the precursor for all pyrimidine nucleotides. The di- and triphosphorylated forms are generated by cytidine monophosphate (UMP-CMP) kinase (Cmpk1 and 2), and isoforms of nucleoside diphosphate kinases (Nme1-6). Uridine triphosphate (UTP) is aminated to form cytosine triphosphate (CTP) by CTP synthase isoforms (Ctps, Ctps2). The diphosphorylated nucleotides are used to form deoxypyrimidine nucleotides. CTP and UTP are directly incorporated into RNA, and CTP is important in one of the pathways of phospholipid biosynthesis. Uracil nucleotides are essential in the formation of nucleotide sugars such as UDP-glucose, used for glycosylation reactions, and UDP-N-acetylglucosamine, used in protein glycosylation. UDP and UTP also serve as signaling molecules operating through metabotropic receptors. To see the Ontology Report for annotations, GViewer and download click here [Click here to see the ontology report for associated GO term - GO:0044205, KEGG map - map00240 and entry at Reactome - REACT_73848]...(less)
