The purine ribonucleotide inosine 5'-monophosphate (IMP) is assembled on 5-phospho-alpha-D-ribose 1-diphosphate (PRPP), with atoms derived from aspartate, glutamine, glycine, N10-formyl-tetrahydrofolate, and carbon dioxide. Although several of the individual reactions in this sequence are reversible, as indicated by the double-headed arrows in the diagram, other irreversible steps drive the pathway in the direction of IMP synthesis in the normal cell. All of these reactions are thus annotated here only in the direction of IMP synthesis. Guanosine 5'-monophosphate (GMP) and adenosine 5'-monophosphate (AMP) are synthesized from IMP (Zalkin & Dixon 1992).

The pyrimidine orotate (orotic acid) is synthesized in a sequence of four reactions, deriving its atoms from glutamine, bicarbonate, and aspartate. A single multifunctional cytosolic enzyme catalyzes the first three of these reactions, while the last one is catalyzed by an enzyme associated with the inner mitochondrial membrane. In two further reactions, catalyzed by a bifunctional cytosolic enzyme, orotate reacts with 1-phosphoribosyl 5-pyrophosphate (PRPP) to yield orotidine 5'-monophosphate, which is decarboxylated to yield uridine 5'-monophosphate (UMP). While several individual reactions in this pathway are reversible, other irreversible reactions drive the pathway in the direction of UMP biosynthesis in the normal cell. All reactions are thus annotated here only in the forward direction.

This pathway has been most extensively analyzed at the genetic and biochemical level in hamster cell lines. All three enzymes have also been purified from human sources, however, and the key features of these reactions have been confirmed from studies of this human material (Jones 1980).

All other pyrimidines are synthesized from UMP. The reactions annotated here, catalyzed by dCMP deaminase and dUTP diphosphatase yield dUMP, which in turn is converted to TMP by thymidylate synthase.