This final lesson introduces the class II ribonucleotide reductase (RNR). Found in bacteria and archaea, RNR is characterized by the use of adenosylcobalamin as a radical precursor. This organometallic cofactor produces the 5'-deoxyadenosyl radical by breaking the cobalt-carbon bond. This analysis shows that Class II RNR shares many similarities with other classes of RNR, notably the radical enzymatic mechanism, using the 5'-deoxyadenosyl radical to create a cysteinyl radical that activates the ribose substrate, structural organization and allosteric regulation.
At the end of this lecture, on the basis of knowledge of the 3 classes of RNR present in living organisms today, it is possible to discuss the primitive RNR and the type of chemistry responsible for the "invention of DNA" necessary for the transition from the RNA to the DNA world. The great similarities between the 3 classes of RNR point to the existence of a common ancestor, an initial RNR that subsequently evolved by essentially modifying the radical-creation mechanism: tyrosinyl radical thanks to the combination of oxygen and binuclear iron center for class I, adenosylcobalamin for class II, glycinyl radical thanks to the combination of S-adenosylmethionine (SAM) and iron-sulfur cluster , for class III. While class I is undoubtedly a late product of evolution, notably due to its dependence on oxygen from the air, which arrived on earth after DNA, it is proposed that the primitive RNR is rather close to class III, due to its anaerobic character, the use of simple cofactors such as SAM and an iron-sulfur cluster , and a reductant, present very early at the origin of life, formate.
