-!- The enzyme, which is expressed in the bacterium Escherichia coli during anaerobic growth, contains an iron sulfur center. -!- The active form of the enzyme contains an oxygen-sensitive glycyl (1-amino-2-oxoethan-1-yl) radical that is generated by the activating enzyme NrdG via chemistry involving S-adenosylmethionine (SAM) and a [4Fe-4S] cluster. -!- The glycyl radical is involved in generation of a transient thiyl (sulfanyl) radical on a cysteine residue, which attacks the substrate, forming a ribonucleotide 3'-radical, followed by water loss to form a ketyl (alpha-oxoalkyl) radical. -!- The ketyl radical gains an electron from a cysteine residue and a proton from formic acid, forming 3'-keto-deoxyribonucleotide and generating a thiosulfuranyl (1-lambda(4)-disulfan-1-yl) radical bridge between methionine and cysteine residues. -!- Oxidation of formate by the thiosulfuranyl radical results in the release of CO(2) and regeneration of the thiyl radical. -!- Cf. EC 1.17.4.1 and EC 1.17.4.2.