Meta-Biol

• Pathways

Ribonucleotide reductase (RNR (M1M2B)) catalyzes the reduction of adenine, guanine, cytidine, and uridine ribonucleoside 5'-diphosphates (NDPs) to form the corresponding deoxyribonucleoside 5'-diphosphates, coupled to the oxidation of glutaredoxin (Eklund et al. 2001). The enzyme complex is cytosolic (Pontarin et al. 2008). The form of ribonucleotide reductase annotated here is a tetramer of two large (M1) and two small (M2B) subunits (Shao et al. 2004; Zhou et al. 2005). M2B protein is stable throughout the cell cycle, unlike M2, and is induced by TP53 (Guittet et al. 2001; Tanaka et al. 2000). The RNR (M1M2B) complex can thus provide dNDPs for DNA repair in interphase and quiescent cells. Studies of mitochondrial instability in cells from patients deficient in M2 protein indicate that RNR (M1M2B) likewise provides dNDPs for mitochondrial DNA replication (Pontarin et al. 2012). The overall activity of the enzyme is regulated allosterically: ATP binding is stimulatory while dATP binding is inhibitory (Reichard et al. 2000).

The reducing equivalents needed for ribonucleotide reductase activity can be provided by either of two small proteins, glutaredoxin or thioredoxin (Holmgren 1989; Sun et al. 1998; Zahedi Avval & Holmgren 2009). Both are re-reduced with NADPH as the donor of reducing equivalents. The relative contributions of glutaredoxin and thioredoxin in vivo are unknown.