. "Cytosolic ribonucleotide reductase catalyzes the reduction of adenine, guanine, cytidine, and uridine ribonucleoside 5'-diphosphates to form the corresponding deoxyribonucleoside 5'-diphosphates, coupled to the oxidation of thioredoxin (Holmgren 1989; Eklund et al. 2001). Ribonucleotide reductase is a tetramer of two large and two small subunits (Shao et al. 2004; Zhou et al. 2005). 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 and thioredoxin (Holmgren 1989; Sun et al. 1998). Both are re-reduced with NADPH as the donor of reducing equivalents, and so are active in catalytic amounts. The relative contributions of glutaredoxin and thioredoxin in vivo are unknown."^^ . . . . . . . . . . . "NDP + reduced thioredoxin => dNDP + oxidized thioredoxin + H2O"^^ . "Reduction of cytosolic ribonucleoside 5'-diphosphates to deoxyribonucleoside 5'-diphosphates (thioredoxin)"^^ . . . . . .