Thymidylate Synthesis

Abstract

Thymidylate is a component of DNA (deoxyribonucleic acid), and is synthesised from the de novo or nucleotide salvage pathways. Many cell types rely on the salvage pathway, but often this is insufficient. Thymidylate is synthesised de novo from deoxyuridylate and methylenetetrahydrofolate by thymidylate synthase (TYMS), with the enzymes dihydrofolate reductase (DHFR) and serine hydroxymethyltransferase (SHMT) or methylenetetrahydrofolate dehydrogenase 1 (MTHFD1) which are required to regenerate methylenetetrahydrofolate. The de novo synthesis pathway forms a nuclear complex at the nuclear lamina at sites of DNA replication. DNA polymerases do not distinguish between deoxyuridylate and thymidylate, and when thymidylate synthesis is insufficient, uracil is misincorporated into DNA. This can lead to futile cycles of DNA repair and subsequent DNA single‐ and double‐strand breaks. Impaired de novo thymidylate synthesis can result in neural tube defects, megaloblastic anaemia and severe combined immunodeficiency (SCID). Inhibitors of TYMS and DHFR impair cell replication and have been used in the treatment of cancer.

Key Concepts

  • Thymidylate (deoxythymidine 5′‐monophosphate, 5‐methyluracil‐2′‐deoxyriboside‐5′‐phosphate, dTMP) (Figure 1) is an essential metabolite required for deoxyribonucleic acid (DNA) synthesis and repair.
  • Thymidylate can be synthesised through a salvage pathway or de novo through a folate‐dependent pathway.
  • Thymidylate synthesis through both the salvage and de novo pathways occurs in the nucleus and mitochondria.
  • Thymidylate biosynthesis in the nucleus functions during DNA replication and repair.
  • Thymidylate synthesis occurs at a greater rate during DNA replication when cells are in S‐phase of the cell cycle.
  • De novo thymidylate biosynthesis is impaired by folate or vitamin B12 deficiency.
  • Impaired de novo thymidylate biosynthesis causes uracil misincorporation into DNA and DNA instability.
  • Thymidylate synthesis enzymes have been successfully targeted by chemotherapeutic agents for treatment of several types of cancers.
  • Common polymorphisms in the TYMS gene are associated with cancer risk.
  • Inhibition of thymidylate biosynthesis can cause a class of common birth defect known as neural tube defects, as well as severe combined immune deficiency (SCID) and megaloblastic anaemia.

Keywords: methylenetetrahydrofolate; deoxyuridylate; methotrexate; fluorouracil; thymidylate; neural tube defects; megaloblastic anaemia; severe combined immunodeficiency

Figure 1. Thymidylate synthesis and incorporation into DNA.
Figure 2. The chemical structure of 5,10‐methylene‐5,6,7,8,‐tetrahydrofolic acid (CH2H4PteGlu). (PteGlu = pteroylglutamic acid).
Figure 3. Thymidylate synthesis in mammalian cells. The enzymes of the thymidylate cycle (SHMT1/SHMT2, DHFR/DHFRL1 and TYMS) are shown in both the nucleus and the mitochondria. In the nucleus, these enzymes are translocated from the cytoplasm following covalent modification by the small ubiquitin‐like modifier (SUMO) protein. Serine and formate are one‐carbon sources for folate‐dependent de novo thymidylate synthesis. THF, tetrahydrofolate; DHR, dihydrofolate; dTMP, thymidylate; dUMP, deoxyuridylate and MTHFD1, methylenetetrahydrofolate dehydrogenase.
Figure 4. (a–d) Inhibitors of thymidylate synthesis.
Figure 5. Degradation of thymidine and thymine arising from DNA breakdown, thymidylate and the diet.
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Further Reading

Carreras CW and Santi DV (1995) The catalytic mechanism and structure of thymidylate synthase. Annual Review of Biochemistry 64: 721–762.

Stover PJ and Field MS (2011) Trafficking of intracellular folates. Advances in Nutrition 2: 325–331.

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Field, Martha S, Stover, Patrick J, and Kisliuk, Roy(Apr 2016) Thymidylate Synthesis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001397.pub3]