InterPro domain: IPR000358

Ribonucleotide reductase (RNR), also known as ribonucleoside diphosphate reductase, ( 1.17.4.1 ) [ 1 , 2 ] catalyses the reductive synthesisof deoxyribonucleotides from their corresponding ribonucleotides: 2'-deoxyribonucleoside diphosphate + oxidized thioredoxin + H ₂ O = ribonucleoside diphosphate + reduced thioredoxin

RNR 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.

Class I 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 [ 3 ]. The small chain binds two iron atoms [ 4 ] (three Glu, one Asp, and two His areinvolved in metal binding) and contains an active site tyrosine radical. Theregions of the sequence that contain the metal-binding residues and the activesite tyrosine are conserved in ribonucleotide reductase small chain fromprokaryotes, eukaryotes and viruses.

This family consist of the small subunit of class I ribonucleotide reductases. It also includes R2-like ligand-binding oxidase, which is homologous to the ribonucleotide reductase small subunit (R2), but whose function is still unknown [ 5 , 6 ].

1. Structure-function studies of the large subunit of ribonucleotide reductase from Escherichia coli. Biochem. Soc. Trans. 16, 91-4
2. From RNA to DNA, why so many ribonucleotide reductases? Science 260, 1773-7
3. The crystal structure of class II ribonucleotide reductase reveals how an allosterically regulated monomer mimics a dimer. Nat. Struct. Biol. 9, 293-300
4. Three-dimensional structure of the free radical protein of ribonucleotide reductase. Nature 345, 593-8
5. A Mycobacterium tuberculosis ligand-binding Mn/Fe protein reveals a new cofactor in a remodeled R2-protein scaffold. Proc. Natl. Acad. Sci. U.S.A. 106, 5633-8
6. A Dynamic C-Terminal Segment in the Mycobacterium tuberculosis Mn/Fe R2lox Protein Can Adopt a Helical Structure with Possible Functional Consequences. Chem. Biodivers. 9, 1981-8