1L1L | pdb_00001l1l

Ribonucleotide reductase (RNR) is the enzyme responsible for the conversion of the four standard ribonucleotides, 5V -di(or tri)-phospho -adenosine, -cytidine, -guanosine, and -uridine, to their 2V -deoxyribonucleotide counterparts and thereby provides the precursors needed for both synthesis and repair of DNA. Class II enzymes are found in bacteria that can live under both aerobic and anaerobic conditions, and also in some of their phages. The prototype from this class is the monomeric enzyme from Lactobacillus leichmannii , but as more and more members of this class have been found that are predominately homodimers, the Lactobacillus enzyme seems to be rather an extreme of the class. They all, however, utilize a cobaltous cofactor, adenosylcobalamin, a vitamin B12 derivative, that interacts directly with an active site cysteine to form the reactive cysteine radical needed for ribonucleotide reduction.