Control of metallation and active cofactor assembly in the class Ia and Ib ribonucleotide reductases: Diiron or dimanganese?

Ribonucleotide reductases (RNRs) convert nucleotides to deoxynucleotides in all organisms. Activity of the class Ia and Ib RNRs requires a stable tyrosyl radical (Y), which can be generated by the reaction of O₂ with a diferrous cluster on the β subunit to form active diferric-Y cofactor. Recent experiments have demonstrated, however, that in vivo the class Ib RNR contains an active dimanganese(III)-Y cofactor. The similar metal binding sites of the class Ia and Ib RNRs, their ability to bind both Mn^II and Fe^II, and the activity of the class Ib RNR with both diferric-Y and dimanganese(III)-Y cofactors raise the intriguing question of how the cell prevents mismetallation of these essential enzymes. The presence of the class Ib RNR in numerous pathogenic bacteria also highlights the importance of manganese for these organisms' growth and virulence.