dNTP synthesis: ribonucleotide reductase, thymidylate synthase, methotrexate, 5-FU

General Characteristics of Deoxyribonucleotide Synthesis

Deoxyribonucleotides (dNTPs) serve as the building blocks of DNA and are not synthesized de novo independently, but rather through the reduction of preformed ribonucleotides. The key reaction—the conversion of the 2′-OH group of ribose to 2′-H of deoxyribose—is catalyzed by a single universal enzyme for all four bases. Thymidine nucleotide synthesis follows a separate pathway, since thymine is absent in RNA and its methyl group must be introduced specially.

Ribonucleotide Reductase Complex

The reduction of ribonucleoside diphosphates (ADP, GDP, CDP, UDP) to their respective deoxy forms is catalyzed by ribonucleotide reductase (RNR). The reaction requires a hydrogen donor, which is the protein cofactor thioredoxin, containing a pair of cysteine residues with SH groups.

The oxidized (disulfide) form of thioredoxin is regenerated by the system:

• thioredoxin reductase — an enzyme that reduces thioredoxin;
• NADPH — the ultimate electron and hydrogen donor.

Thus, the flow of reducing equivalents proceeds as follows: NADPH → thioredoxin reductase → thioredoxin → ribonucleotide reductase → the 2′-OH of ribose is converted to the 2′-H of deoxyribose.

Synthesis of dCMP and dUMP

Deoxycytidylate and deoxyuridylate nucleotides are synthesized in an interrelated manner. There are two pathways for the production of dUMP:

• via dUDP, obtained by the direct reduction of UDP by ribonucleotide reductase, followed by dephosphorylation;
• via dCDP → dCMP → deamination to dUMP.

The resulting dUMP then serves as a substrate for the synthesis of thymidine nucleotides.

Synthesis of dTMP and the Role of Folates

The key enzyme of the pathway is thymidylate synthase. It transfers a single-carbon fragment from N5,N10-methylene-H4-folate to dUMP, converting it into dTMP. A characteristic feature of the reaction is that the donor not only gives up the methyl group but is simultaneously oxidized: N5,N10-methylene-H4-folate is converted into dihydrofolate (H2-folate).

Continuous operation of thymidylate synthase requires the regeneration of the active form of the cofactor. Two enzymes participate in this process:

• dihydrofolate reductase (DHFR) — reduces H2-folate to tetrahydrofolate (H4-folate) using NADPH;
• serine hydroxymethyltransferase (pyridoxal-dependent) — transfers the β-carbon of serine to H4-folate, forming N5,N10-methylene-H4-folate; serine is thereby converted into glycine.

Purine Salvage Pathways

In parallel with de novo synthesis, the cell utilizes salvage pathways for free purine bases and nucleosides involving 5-phosphoribosyl-1-pyrophosphate (PRPP).

• Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) attaches guanine or hypoxanthine to PRPP to form GMP or IMP, respectively; pyrophosphate is released.
• Adenine phosphoribosyltransferase (APRT) forms AMP from adenine and PRPP.
• Adenosine kinase phosphorylates adenosine using ATP, yielding AMP.

Methotrexate: Dihydrofolate Reductase Inhibitor

Methotrexate is a structural analog of folic acid and a competitive inhibitor of dihydrofolate reductase. Blockade of the enzyme halts the reduction of H2-folate to H4-folate, depletes the pool of N5,N10-methylene-H4-folate, and thereby terminates dTMP synthesis. Without thymidine nucleotides, DNA replication is impossible. The drug selectively affects rapidly dividing cells, which is utilized in the treatment of leukemias, lymphomas, and a number of solid tumors, as well as certain autoimmune diseases.

5-Fluorouracil: Thymidylate Synthase Inhibitor

5-Fluorouracil (5-FU) is an uracil analog; within the cell, it is metabolized to 5-fluoro-dUMP, which forms a covalent ternary complex with thymidylate synthase and N5,N10-methylene-H4-folate. The enzyme is irreversibly blocked, dTMP synthesis ceases, and a "thymidine deficiency" and death of proliferating cells ensue. 5-FU is used in the chemotherapy of colorectal, gastric, breast, and other cancers.

Strategy of Antineoplastic Chemotherapy

Methotrexate and 5-fluorouracil illustrate the general principle of cytostatic therapy: targeting the enzymes of dNTP synthesis. Methotrexate blocks the regeneration of the cofactor (H4-folate), while 5-FU targets the thymidylate synthase enzyme itself. In both cases, dTMP formation is disrupted, DNA synthesis suffers, and cells with a high rate of division (tumor, hematopoietic, and mucosal epithelial cells) die first. This accounts for the typical toxicity of these drugs—myelosuppression and mucositis.

Key points

• Ribonucleotide reductase reduces all four ribonucleoside diphosphates to their deoxy forms.
• The hydrogen donor is thioredoxin, which is regenerated by thioredoxin reductase and NADPH.
• dUMP is formed via two pathways: through dUDP and via the deamination of dCMP.
• Thymidylate synthase converts dUMP to dTMP using N5,N10-methylene-H4-folate.
• Regeneration of the folate cofactor requires dihydrofolate reductase and serine hydroxymethyltransferase.
• Methotrexate inhibits dihydrofolate reductase, depleting the active folate pool.
• 5-Fluorouracil irreversibly blocks thymidylate synthase via 5-fluoro-dUMP.
• Purine salvage pathways operate via PRPP with the participation of HGPRT, APRT, and adenosine kinase.