Synthesis of deoxythymidine nucleotides occurs differently from that of the other dNTPs, which are derived directly from a ribonucleotide reductase-catalyzed conversion of ribonucleoside diphosphates to deoxyribonucleoside diphosphates (Figure 22.12). The terms thymidine and deoxythymidine (or dTTP and TTP) generally refer to the deoxyribonucleotide, because the ribonucleotide is not a normal metabolite. In the rare instances where the ribonucleotide of thymidine occurs, it is usually designated with an 'r' preceding it, as in rTTP.

Figure 22.17 shows the complicated de novo and salvage pathways for the synthesis of dTTP. The de novo pathways start at the top with either UDP or CDP and lead to dTTP. The salvage pathways begin with deoxycytidine, deoxyuridine, or deoxythymidine nucleosides, which are each converted to nucleoside monophosphates in the first step by appropriate kinases.

There are several points of regulation in the synthesis. For example, dCTP inhibits the salvage reaction catalyzed by deoxycytidine kinase and activates the reaction catalyzed by dCMP deaminase. On the other hand, dTTP inhibits the dCMP deaminase reaction and the enzyme thymidine kinase.

Note that conversion of dUMP to dTMP requires transfer of a single carbon from 5,10-methylenetetrahydrofolate in the reaction catalyzed by thymidylate synthase. The relationship between thymidylate synthase and the enzymes of tetrahydrofolate metabolism is shown in Figure 22.18. The reaction catalyzed by thymidylate synthase is the only one known in the cell in which tetrahydrofolate is not regenerated. Dihyrofolate reductase, thus, plays an essential role in the ultimate regeneration of 5,10-methylene tetrahydrofolate. This enzyme, which can be inhibited by the drug methotrexate, is a target of some anticancer treatments.