Abstract
The lectures 2 and 3 describe the successive discoveries of new classes of ribonucleotide reductases (RNRs). These enzymes are essential in all living organisms, as they ensure the biosynthesis of deoxyribonucleotides, the precursors of DNA, and thus DNA synthesis and replication. The enzymatic reaction, the reduction of ribonucleotides to deoxyribonucleotides, is a highly specific one, using protein radicals - tyrosinyl, cysteinyl or dihydroxyphenylalanyl - to activate the substrate. The originality of this chemistry lies in the variety of mechanisms that nature has discovered and put in place to synthesize and control these highly reactive radicals. In particular, this synthesis involves oxidation of the amino acid, the oxidant being oxygen from the air in anaerobic organisms, S-adenosylmethionine in anaerobic organisms and vitamin B12 in certain organisms. The five families of RNRs, four of which use metal centers for radical synthesis, based on iron or manganese, are described both from the point of view of their molecular structure and the mechanism implemented to form radicals and reduce ribonucleotides.
