Ribonucleotide reductase (RNR), also known as
ribonucleoside diphosphate reductase, is an
enzyme that catalyzes the formation of
deoxyribonucleotides from
ribonucleotides. Deoxyribonucleotides in turn are used in the synthesis of
DNA. The reaction catalyzed by RNR is strictly conserved in all living organisms. Furthermore RNR plays a critical role in regulating the total rate of DNA synthesis so that DNA to cell mass is maintained at a constant ratio during
cell division and
DNA repair. A somewhat unusual feature of the RNR enzyme is that it catalyzes a reaction that proceeds via a
free radical mechanism of action. The substrates for RNR are
ADP,
GDP,
CDP and
UDP. dTDP (deoxythymidine diphosphate) is synthesized by another enzyme (
thymidylate kinase) from dTMP (deoxythymidine monophosphate).