[1][2] CTP (cytidine triphosphate) synthetase catalyzes the last committed step in pyrimidine nucleotide biosynthesis:[3] ATP + UTP + glutamine → ADP + Pi + CTP + glutamate It is the rate-limiting enzyme for the synthesis of cytosine nucleotides from both the de novo and uridine salvage pathways.

At low enzyme concentrations and in the absence of ATP and UTP, CTP synthase exists as inactive monomer.

[12] One major example of this is phosphorylation of the Ser-571 residue by glycogen synthase kinase 3 (GSK3) in response to low serum conditions.

GTP acts as an allosteric activator that strongly promotes the hydrolysis of glutamine, but is also inhibiting to glutamine-dependent CTP formation at high concentrations.

These include bacteria (C. crescentus),[18] yeast (S. cerevisiae),[19] fruit flies (D. melanogaster)[20] and human cells.

It has been shown that filamentation downregulates or upregulates CTP synthase activity depending on the species.

Mutations in the CTP synthase have been seen to confer resistance to cytotoxic drugs such as cytosine arabinoside (ara-C) in a Chinese hamster ovary (CHO) cell model of leukemia though such mutations were not found in human patients with ara-C resistance.