Figure 4.

Schematic representation of the main steps of pyrimidine biosynthesis. The initial step is catalyzed by Carbamoyl Phosphate Synthetase catalyzing the generation of carbamoyl phosphate (CP), starting from one molecule of glutamine and bicarbonate and two molecules of ATP. The second step involves the generation of carbamoyl aspartate (CA) from one molecule of aspartate and CP, catalyzed by carbamoyl-transferase (CA). In the third step, dihydroorotase catalyzes the conversion of CA to diihydrooratate (DHO). In the fourth step, DHO is converted to oratate by the enzyme diihydrooratate dehydrogenase (DHODH). In the following step, orotate phosphoribosyltranferase converts OA to AMP (orothidine 5′ monophosphate). Finally, in the last step, OMP is converted to UMP and OMP decarboxylase (uridine monophosphate).