Enzymes (EC 1.17.4.1-2), which catalyze the reduction of ribonucleotide diphosphates or triphosphates to the corresponding deoxyribonucleotides by a radical-dependent reaction. The enzyme of animal, yeast and aerobic E. coli cells contains an oxo-bridged dinuclear iron center and a tyrosyl radical cation, and uses thioredoxin, a thiol-containing protein, as reductant. At least three other ribonucleotide reductases are known from bacteria, containing, respectively, an iron-sulfur cluster with a glycyl radical, adenosyl cobalamin, and a dinuclear manganese cluster.
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