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Table 1 The RNR classes

From: Ribonucleotide reduction - horizontal transfer of a required function spans all three domains

  Class I* Class II Class III
Operation Aerobic O2 independent,
B12 dependent
Anaerobic
SAM dependent
Structure α2β2 α or α2 α2, activated by β2
Component names α:NrdA or NrdE
β: NrdB or NrdF
NrdJ Enzyme proper: NrdD
Activase: NrdG
Radical generating metal site Fe-O-Fe Co (in AdoCbl) β: 4Fe-4S and AdoMet
Distribution Common in bacteria universal in eukaryotes, rare in archaea Common in bacteria and archaea, rare in eukaryotes Common in bacteria and archaea, rare in eukaryotes
  1. *Class I is traditionally subdivided into Ia (NrdA and NrdB) and Ib (NrdE and NrdF) respectively. Class Ib lacks activity regulation, and Ib operons contain two Ib-specific genes, a flavodoxin (NrdI) and a reductant (NrdH).