In aerobic organisms, from Escherichia coli to humans, deoxyribonucleotides are synthesized from the corresponding ribonucleotides in a reaction catalyzed by class I RNR (ribonucleotide reductase). The same enzyme is responsible for the synthesis of the four deoxyribonucleotides. This enzyme consists of two subunits. The R1 subunit is the site of the reaction. It binds the substrates, ribonucleotides, which are reduced thanks to a redox pair of cysteines present nearby in the active site, and also the deoxyribonucleotides, products of the reaction, at distant sites which are the actors of an extremely complex and as yet poorly characterized allosteric regulation. This regulation is essential to control the balanced production of the four deoxyribonucleotides in the cell. The second subunit, called R2, contains a binuclear iron center and a radical on a tyrosine, easily detected by electron paramagnetic resonance. This radical is absolutely essential for enzymatic activity.
In this lecture, we take stock of all the studies, particularly those developed by J. Stubbe (MIT, USA). Stubbe (MIT, USA), using substrate analogues, directed mutants and photochemical radical precursors, to understand how the radical is transferred, by complex electron transfer processes coupled to proton transfers, over a very long distance (around 35 Å) from R2 to the active site of R1, where it can finally pull a hydrogen atom off the substrate ribose, making it reactive for its transformation into deoxyribose. This system is particularly well-suited to the study of long-distance electron and radical transfer, so important in biology, whose theory and analytical methods are discussed. Also discussed in this lecture are: (i) the post-translational synthesis of the tyrosinyl radical of R2 and the essential role of the iron center and oxygen in this synthesis; (ii) the inhibition of this enzyme, since it is a target for anticancer drugs, the best-known of which is gemcitabine, a natural ribonucleoside analog used clinically.
