Ribonucleotide reductase (EC: 1.17.4.1) [PMID:3286319, PMID:8511586] catalyzes the reductive synthesis of deoxyribonucleotides from their corresponding ribonucleotides: 2'-deoxyribonucleoside diphosphate + oxidized thioredoxin + H₂O = ribonucleoside diphosphate + reduced thioredoxin It provides the precursors necessary for DNA synthesis. RNRs divide into three classes on the basis of their metallocofactor usage. Class I RNRs, found in eukaryotes, bacteria, bacteriophage and viruses, use a diiron-tyrosyl radical, Class II RNRs, found in bacteria, bacteriophage, algae and archaea, use coenzyme B12 (adenosylcobalamin, AdoCbl). Class III RNRs, found in anaerobic bacteria and bacteriophage, use an FeS cluster and S-adenosylmethionine to generate a glycyl radical. Many organisms have more than one class of RNR present in their genomes.

Ribonucleotide reductase is an oligomeric enzyme composed of a large subunit (700 to 1000 residues) and a small subunit (300 to 400 residues) - class II RNRs are less complex, using the small molecule B12 in place of the small chain [PMID:11875520]. The small chain binds two iron atoms [PMID:2190093] (three Glu, one Asp, and two His are involved in metal binding) and contains an active site tyrosine radical. The regions of the sequence that contain the metal-binding residues and the active site tyrosine are conserved in ribonucleotide reductase small chain from prokaryotes, eukaryotes and viruses. We have selected one of these regions as a signature pattern. It contains the active site residue as well as a glutamate and a histidine involved in the binding of iron.