The enzyme, characterized from the bacterium Lactobacillus leichmannii, is similar to class II ribonucleoside-diphosphate reductase (cf. EC 126.96.36.199). However, it is specific for the triphosphate versions of its substrates. The enzyme contains an adenosylcobalamin cofactor that is involved in generation of a transient thiyl (sulfanyl) radical on a cysteine residue. This radical attacks the substrate, forming a ribonucleotide 3'-radical, followed by water loss to form a ketyl (alpha-oxoalkyl) radical. The ketyl radical is reduced to 3'-keto-deoxynucleotide concomitant with formation of a disulfide anion radical between two cysteine residues. A proton-coupled electron-transfer from the disulfide radical to the substrate generates a 3'-deoxynucleotide radical, and the final product is formed when the hydrogen atom that was initially removed from the 3'-position of the nucleotide by the thiyl radical is returned to the same position. The disulfide bridge is reduced by the action of thioredoxin. cf. EC 188.8.131.52, ribonucleoside-triphosphate reductase (formate).