Deoxyribonucleotide Formation (Ribonucleotide Reductase pathway and thymidylate synthesis)

1. 1

   Nucleoside diphosphates → deoxyribonucleoside diphosphates

   Ribonucleotide reductase (RNR, class Ia) 1.17.4.1 ST-0252 Irreversible/directional rate-limiting
   In ADP GDP CDP UDP

   Out dADP dGDP dCDP dUDP
   › Notes

   Class Ia RNR catalyzes ADP, GDP, CDP, or UDP + reducing equivalents → dADP, dGDP, dCDP, or dUDP + oxidized electron-transfer proteins. The human enzyme is an alpha2beta2 complex: the RRM2/RRM2B small subunit uses a diiron center to generate a stable tyrosyl radical, and the RRM1 large subunit contains catalytic cysteines that reduce the 2′-hydroxyl group. Reducing equivalents are transferred from NADPH through thioredoxin reductase and reduced thioredoxin (or through NADPH, glutathione reductase, glutathione, and glutaredoxin); the chemical reduction of each NDP requires one NADPH-derived pair of electrons. This is the rate-limiting and committed step for de novo deoxyribonucleotide production.

2. 2

   Oxidized thioredoxin → reduced thioredoxin

   Thioredoxin reductase (TXNRD) ST-0253 Irreversible/directional NADP+ NADPH Thioredoxin
   In Oxidized thioredoxin

   Out Reduced thioredoxin
   › Notes

   Thioredoxin reductase catalyzes oxidized thioredoxin + NADPH + H+ → reduced thioredoxin + NADP+. This FAD- and selenocysteine-containing flavoprotein restores the RNR electron donor after each catalytic cycle. The glutaredoxin alternative is sustained by glutathione reductase, which uses NADPH to regenerate reduced glutathione.

3. 3

   dADP, dGDP, dCDP, or dUDP → corresponding dNTP

   Nucleoside diphosphate kinase (NDPK/NME) 2.7.4.6 ST-0254 Irreversible/directional ATP
   In dADP dGDP dCDP dUDP

   Out dATP dGTP dCTP dUTP
   › Notes

   NDPK catalyzes dNDP + ATP (or another NTP) ⇌ dNTP + ADP. This near-equilibrium phosphotransfer uses a phosphohistidine intermediate and supplies dATP, dGTP, dCTP, and dUTP from the RNR products.

4. 4

   dUDP → dUTP → dUMP

   ↪ dUTP → dUMP

   Nucleoside diphosphate kinase (NDPK) + dUTPase 2.7.4.6 / 3.6.1.23 ST-0255 Irreversible/directional H2O PPi
   In dUDP

   Out dUTP
   › Notes

   NDPK phosphorylates dUDP to dUTP, and dUTPase hydrolyzes dUTP + H2O → dUMP + PPi. dUTPase is functionally crucial: it creates dUMP while minimizing dUTP incorporation into DNA, which would otherwise initiate repeated uracil-excision repair.

5. 5

   dCMP → dUMP

   dCMP deaminase 3.5.4.12 ST-0256 Irreversible/directional H2O
   In dCMP

   Out dUMP
   › Notes

   dCMP deaminase catalyzes dCMP + H2O → dUMP + NH3. The enzyme provides a second dUMP source and uses allosteric control to coordinate dCTP and dTTP pools; dCTP generally activates and dTTP inhibits the enzyme.

6. 6

   dUMP → dTMP

   Thymidylate synthase (TYMS) 2.1.1.45 ST-0257 Irreversible THF (folate)
   In dUMP

   Out dTMP
   › Notes

   Thymidylate synthase (TYMS) catalyzes dUMP + 5,10-methylene-THF → dTMP + DHF. An active-site cysteine forms a covalent intermediate with dUMP, and 5,10-methylene-THF provides the methylene group and the reducing hydride equivalent; this is the committed, effectively irreversible step of de novo dTMP synthesis.

7. 7

   DHF → THF

   Dihydrofolate reductase (DHFR) 1.5.1.3 ST-0258 Irreversible/directional NADP+ NADPH THF (folate)
   In Dihydrofolate (DHF)

   Out Tetrahydrofolate (THF)
   › Notes

   DHFR catalyzes DHF + NADPH + H+ → THF + NADP+. This reaction regenerates reduced folate after every thymidylate synthase turnover and is essential to maintain dTMP synthesis.

8. 8

   THF → 5,10-methylene-THF

   Serine hydroxymethyltransferase (SHMT1/SHMT2) 2.1.2.1 ST-0259 Irreversible/directional PLP (B6) THF (folate)
   In Tetrahydrofolate (THF)

   Out 5,10-Methylene-THF
   › Notes

   Serine hydroxymethyltransferase (SHMT) catalyzes serine + THF ⇌ glycine + 5,10-methylene-THF, requiring pyridoxal phosphate. The folate-bound one-carbon unit can also be supplied through the cytosolic mitochondrial one-carbon network; adequate folate metabolism is required to sustain this step.

9. 9

   dTMP → dTDP

   Thymidylate kinase ST-0260 Irreversible/directional ATP
   In dTMP

   Out dTDP
   › Notes

   Thymidylate kinase catalyzes dTMP + ATP → dTDP + ADP. This kinase is specific for thymidylate and is required before dTTP can be generated.

10. 10

    dTDP → dTTP

    Nucleoside diphosphate kinase (NDPK/NME) 2.7.4.6 ST-0261 Irreversible/directional ATP
    In dTDP

    Out dTTP
    › Notes

    NDPK catalyzes dTDP + ATP (or another NTP) ⇌ dTTP + ADP. dTTP then participates in DNA polymerization and also serves as an allosteric specificity effector for RNR and an inhibitor of dCMP deaminase.

11. 11

    Ribonucleoside triphosphates → dNDP precursor pool

    Nucleoside diphosphate kinases/phosphatases (NTP<->NDP pool) 2.7.4.6 ST-0262 Irreversible/directional
    In ATP GTP CTP UTP

    Out ADP GDP CDP UDP
    › Notes

    Nucleoside diphosphate phosphatases/kinases maintain interconversion between NTP and NDP pools so that ADP, GDP, CDP, and UDP are available to RNR. RNR acts on diphosphates, not directly on NTPs, a feature central to its allosteric substrate-specificity system.

12. 12

    Deoxycytidine or thymidine → dCMP or dTMP (salvage support)

    Deoxycytidine kinase (dCK) 2.7.1.74 ST-0263 Irreversible/directional ATP
    In Deoxycytidine Thymidine

    Out dCMP dTMP
    › Notes

    Cytosolic deoxycytidine kinase phosphorylates deoxycytidine + ATP → dCMP + ADP, while cytosolic thymidine kinase 1 phosphorylates thymidine + ATP → dTMP + ADP in S phase; mitochondrial thymidine kinase 2 supports mitochondrial dTTP supply in nondividing cells. These are salvage reactions, not de novo formation, but they can bypass part of the RNR/thymidylate pathway and materially affect dNTP homeostasis.

Showing all 12 steps.

Overview

Deoxyribonucleotide formation converts ribonucleotide pools into the balanced deoxyribonucleoside triphosphate (dNTP) pools required for faithful DNA replication and repair. Ribonucleotide reductase (RNR) reduces ribonucleoside diphosphates, whereas thymidylate synthase uniquely creates the methylated pyrimidine nucleotide dTMP from dUMP. Tight allosteric and cell-cycle regulation is essential because either shortage or imbalance of dNTPs increases replication stress, mutagenesis, and genome instability.

Cellular location

RNR large and small subunits are predominantly cytosolic but are regulated by cell cycle and can redistribute to support nuclear DNA synthesis; dNTPs are transported or locally generated for nuclear and mitochondrial DNA replication. Thymidylate synthase, DHFR, and folate enzymes are cytosolic and can form a replication-associated nuclear complex during S phase. Mitochondria maintain distinct dNTP homeostasis using mitochondrial dGK, thymidine kinase 2, and dedicated transport/metabolic systems, especially in nondividing cells.

Net energetics

RNR reduction of one NDP to one dNDP consumes one NADPH-derived reducing equivalent through thioredoxin or glutaredoxin systems; RNR itself does not hydrolyze ATP in the reduction reaction. Phosphorylation of dNDP to dNTP costs one NTP → NDP equivalent. For each dTMP formed from dUMP, thymidylate synthase consumes one 5,10-methylene-THF molecule and produces DHF; DHFR consumes one NADPH to regenerate THF, while SHMT supplies the methylene group from serine. Conversion of dTMP to dTTP requires two ATP-equivalent phosphorylations (dTMP kinase and NDPK); the cost of forming dUMP additionally depends on whether it arises from dUDP/dUTP or dCMP deamination.

Clinical significance

Balanced dNTP pools are indispensable for DNA replication, base-excision repair, and mitochondrial genome maintenance; excessive dATP, for example, can inhibit RNR and disrupt overall pool balance. Hydroxyurea inhibits RNR by quenching its tyrosyl radical and is used in myeloproliferative disorders and to induce fetal hemoglobin in sickle cell disease. 5-fluorouracil inhibits thymidylate synthase after conversion to 5-fluoro-dUMP, whereas methotrexate inhibits DHFR and causes functional depletion of reduced folates; leucovorin can enhance 5-fluorouracil-mediated thymidylate synthase inhibition or rescue selected antifolate effects. Folate deficiency produces impaired dTMP synthesis, uracil misincorporation, megaloblastic anemia, and chromosomal instability. Energy metabolism and cellular respiration integrate the oxidative pathways that convert carbon fuels into ATP, culminating in the citric acid cycle, electron transport chain, and oxidative phosphorylation. These pathways represent the convergence point for carbohydrate, lipid, and amino acid catabolism and are the primary determinant of cellular ATP yield under aerobic and anaerobic conditions. Mitochondrial dysfunction in this category underlies a wide range of metabolic and neurodegenerative disease.