Metal use in ribonucleotide reductase R2, di-iron, di-manganese and heterodinuclear--an intricate bioinorganic workaround to use different metals for the same reaction.

The ferritin-like superfamily comprises of several protein groups that utilize dinuclear metal sites for various functions, from iron storage to challenging oxidations of substrates. Ribonucleotide reductase R2 proteins use the metal site for the generation of a free radical required for the reduction of ribonucleotides to deoxyriboinucleotides, the building blocks of DNA. This ubiquitous and essential reaction has been studied for over four decades and the R2 proteins were, until recently, generally believed to employ the same cofactor and mechanism for radical generation. In this reaction, a stable tyrosyl radical is produced following activation and cleavage of molecular oxygen at a dinuclear iron site in the protein. Discoveries in the last few years have now firmly established that the radical generating reaction is not conserved among the R2 proteins but that different subgroups, that are structurally very similar, instead employ di-manganese or heterodinuclear Mn-Fe cofactors as radical generators. This is remarkable considering that the protein must exercise a strict control over oxygen activation, reactive metal-oxygen intermediate species and the resulting redox potential of the produced radical equivalent. Given the differences in redox properties between Mn and Fe, use of a different metal for this reaction requires associated adaptations of the R2 protein scaffold and the activation mechanism. Further analysis of the differences in protein sequence between R2 subgroups have also led to the discovery of new groups of R2-like proteins with completely different functions, expanding the chemical repertoire of the ferritin-like superfamily. This review describes the discoveries leading up to the identification of the different Mn-containing R2 protein groups and our current understanding of them. Hypotheses regarding the biochemical rationale to develop these chemically complex alternative solutions are also discussed.