Contorted catalysis. Active sites and proposed reaction mechanisms in DNA biosynthesis. The orange lines indicate the enzyme, with important residues shown in yellow. Chemical structures represent substrates. Red dots indicate free radicals. (A) RNR class I enzymes. (B) RNR class III enzymes. Only two cysteine residues are used, the cysteine radical is generated from a glycyl radical further along the protein, and formate is reduced to carbon dioxide and water during the reaction.