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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).