Table 1.
RNRs are classified into three main classes (I-III) and a few subclasses differing in oxygen dependency, subunit composition, free radical chemistry, and allosteric regulation.
| Classes | Genes (subunits) | Free radical chemistry | Allosteric sites | Active form: | Inhibited form |
|---|---|---|---|---|---|
| Class I (Ia): aerobic | NrdA (R1, α), NrdB (R2, β) | NrdB: Tyrosyl radical and Fe-O-Fe centre. | s-site + a-site (majority) | α2β2 (general) α6β2-6 (ATP, eukar.) | α4β4 (E. coli) α6β2 (eukar.) |
| Ib | NrdE (R1E, α), NrdF (R2F, β) | NrdF: Tyrosyl radical and Mn-O-Mn (or Fe-O-Fe) centre. | s-site | α2β2† | None |
| NrdI needed for the generation of the Mn-O-Mn centre. | |||||
| Ic | NrdA(R1, α), NrdB* (R2, β) | NrdB: Lacks tyrosyl radical. Unpaired e in Fe-O-Mn centre | α subunit associated features are equivalent to canonical class I (Ia) enzymes | ||
| Class II: oxygen independent | NrdJ(α) | 5'-deoxyadenosyl radical generated from AdoCbl | s-site (rarely a-site) | α (monomeric class II) α2 (dimeric class II) | ? |
| Class III: anaerobic | NrdD (α) | NrdD: Glycyl radical. NrdG (activase) needed for the generation of the glycyl radical. | s-site + a-site (majority) | α2† | α2† |
*
Named as NrdBphe, NrdBLeu, NrdBVal depending on which amino acid the tyrosyl radical is replaced by.
†
Additional complexes formed by the association of RNR subunits with activating components such as NrdI and NrdG are excluded from the table.