Table 1.
The RNR classes
| 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 |
*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).