InterPro domain: IPR033909

The beta (small) subunit of ribonucleotide reductase (RNR) is a member of a broad superfamily of ferritin-like diiron-carboxylate proteins.

The RNR protein catalyzes the conversion of ribonucleotides to deoxyribonucleotides and is found in all eukaryotes, many prokaryotes, several viruses, and few archaea. The catalytically active form of RNR is a proposed alpha2-beta2 tetramer. The homodimeric alpha subunit (R1) contains the active site and redox active cysteines as well as the allosteric binding sites. The beta subunit (R2) contains a di-iron cluster that, in its reduced state, reacts with dioxygen to form a stable tyrosyl radical and a di-iron(III) cluster. This essential tyrosyl radical is proposed to generate a thiyl radical, located on a cysteine residue in the R1 active site that initiates ribonucleotide reduction. The beta subunit is composed of 10-13 helices, the eight longest helices form an alpha-helical bundle; some have two addition beta strands [ 1 , 2 , 3 , 4 ].

The beta-herpesvirus RNR R1 subunit homologues are catalytically inactive; the enzyme seem to function by inhibiting cellular adaptor protein RIP1 to block cellular signaling pathways involved in innate immunity and inflammation [ 5 ].

Yeast is unique in that it assembles both homodimers and heterodimers of RNR. The yeast heterodimer, Y2Y4, contains R2 (Y2) and a R2 homologue (Y4) that lacks the diiron centre and is proposed to only assist in cofactor assembly, and perhaps stabilize R1 (Y1) in its active conformation [ 6 , 7 ]. In mammals, the active form of the enzyme is composed of two identical large subunits (RRM1) and two identical small subunits (RRM2 or its homologue RRM2B). RRM1 is the catalytic subunit, and RRM2 and RRM2B the regulatory subunits. RRM2B (also called p53R2) can be induced by p53 [ 8 , 9 ].

1. Structure of Salmonella typhimurium nrdF ribonucleotide reductase in its oxidized and reduced forms. Biochemistry 37, 13359-69
2. Crystal structure of the di-iron/radical protein of ribonucleotide reductase from Corynebacterium ammoniagenes. Biochemistry 41, 1381-9
3. Structure and function of the Escherichia coli ribonucleotide reductase protein R2. J. Mol. Biol. 232, 123-64
4. The three-dimensional structure of mammalian ribonucleotide reductase protein R2 reveals a more-accessible iron-radical site than Escherichia coli R2. J. Mol. Biol. 262, 706-20
5. Tinkering with a viral ribonucleotide reductase. Trends Biochem. Sci. 34, 25-32
6. Structure of the yeast ribonucleotide reductase Y2Y4 heterodimer. Proc. Natl. Acad. Sci. U.S.A. 98, 10073-8
7. Characterization of Y122F R2 of Escherichia coli ribonucleotide reductase by time-resolved physical biochemical methods and X-ray crystallography. Biochemistry 37, 5840-8
8. A ribonucleotide reductase gene is a transcriptional target of p53 and p73. Oncogene 19, 4283-9
9. RRM2B-Mediated Regulation of Mitochondrial Activity and Inflammation under Oxidative Stress. Mediators Inflamm. 2015, 287345