Formation of Deoxyribonucleotides
The typical cell contains 5 to10 times as much RNA (mRNAs, rRNAs and tRNAs) as DNA. Therefore, the majority of nucleotide biosynthesis has as its purpose the production of rNTPs. However, because proliferating cells need to replicate their genomes, the production of dNTPs is also necessary.
This process begins with the reduction of rNDPs, followed by phosphorylation to yield the dNTPs. The phosphorylation of dNDPs to dNTPs is catalyzed by the same nucleoside diphosphate kinases that phosphorylates rNDPs to rNTPs, using ATP as the phosphate donor.
Ribonucleotide reductase (RR) is a multifunctional enzyme that contains redox-active thiol groups for the transfer of electrons during the reduction reactions. In the process of reducing the rNDP to a dNDP, RR becomes oxidized.
RR is reduced in turn, by either thioredoxin or glutaredoxin. The ultimate source of the electrons is NADPH. The electrons are shuttled through a complex series of steps involving enzymes that regenerate the reduced forms of thioredoxin or glutaredoxin. These enzymes are thioredoxin reductase and glutathione reductase respectively.
Regulation of dNTP Formation
Ribonucleotide reductase is the only enzyme used in the generation of all the deoxyribonucleotides. Therefore, its activity and substrate specificity must be tightly regulated to ensure balanced production of all four of the dNTPs required for DNA replication. Such regulation occurs by binding of nucleoside triphosphate effectors to either the activity sites or the specificity sites of the enzyme complex. The activity sites bind either ATP or dATP with low affinity, whereas the specificity sites bind ATP, dATP, dGTP, or dTTP with high affinity. The binding of ATP at activity sites leads to increased enzyme activity, while the binding of dATP inhibits the enzyme. The binding of nucleotides at specificity sites effectively allows the enzyme to detect the relative abundance of the four dNTPs and to adjust its affinity for the less abundant dNTPs, in order to achieve a balance of production. thioredoxin reductase and glutathione reductase respectively.