Main Pathway: Reduction of Diphosphates
In rapidly dividing cells, the demand for DNA precursors is extremely high. The primary mechanism for their production is the conversion of ribonucleoside diphosphates (NDPs) into deoxyribonucleoside diphosphates (dNDPs). The reaction occurs exclusively at the diphosphate level.
Substrates include nucleotides containing adenine, guanine, cytosine, and uracil. The key enzyme of this process is ribonucleotide reductase. It catalyzes the replacement of the hydroxyl group at the C2' position of ribose with a hydrogen atom, yielding a molecule of water and deoxyribose. Subsequently, the resulting dNDPs are phosphorylated to triphosphates (dNTPs) utilizing ATP.
The Thioredoxin Cycle
Protons and electrons are required to reduce ribose to deoxyribose. This task is carried out by a specialized system:
- The direct hydrogen donor is the protein thioredoxin. During the reaction, its sulfhydryl groups (-SH) are oxidized to form a disulfide bond (-S-S-).
- To keep the cycle running, oxidized thioredoxin must be reduced. This is accomplished by the enzyme thioredoxin reductase.
- The ultimate source of reducing equivalents in this chain is the coenzyme NADPH + H⁺.
This entire set of reactions is tightly regulated: an excess of the final product, dATP, acts as a potent allosteric inhibitor, blocking the synthesis of all deoxyribonucleotides and preventing DNA overproduction.
Specificity of Thymidylate Synthesis
While adenine, guanine, and cytosine derivatives are formed directly from their respective diphosphates, the synthesis of the thymidylate nucleotide (dTMP) has distinct features.
The substrate for this pathway is deoxyuridine monophosphate (dUMP), and the reaction is catalyzed by thymidylate synthase. $N^5,N^{10}$-methylenetetrahydrofolate plays an active role in this process, serving a dual function: it provides the methyl group ($CH_3$) and acts as a hydrogen donor, ultimately being oxidized to dihydrofolate.
Salvage Pathway
In addition to the primary reduction mechanism, cells can utilize a salvage pathway. In this case, deoxyribonucleotides are formed via the direct phosphorylation of preformed nucleosides.
The process consumes ATP and involves specific kinases:
- Thymidine kinase phosphorylates thymidine to dTMP.
- Deoxycytidine kinase converts deoxycytidine to dCMP. This enzyme exhibits broad specificity and can also phosphorylate deoxyguanosine and deoxyadenosine.