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).

Lundin et al. BMC Evolutionary Biology 2010 10:383   doi:10.1186/1471-2148-10-383

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