The synthesis of purine and pyrimidine deoxyribonucleotides occurs from ribonucleotides by a reduction of the C₂ of ribose moiety as shown in Figure 11.5. This reaction is catalysed by a multi-subunit (two B₁ and two B₂ subunits) enzyme ribonucleotide reductase.

The ribonucleotide reductase itself provides the hydrogen atoms needed for the reduction from its sulfhydryl groups. The reducing equivalents, in turn, are supplied by thioredoxin, a monomeric proteins with two cysteine residues. NADPH-dependent thioredoxin reductase converts the oxidised thioredoxin to reduced form, which can be recycled again and again. Thioredoxin thus serves as a cofactor in an enzymatic reaction.

Figure 11.5 Conversion of Ribonucleotides to Deoxyribonucleotides