This is the alpha helical domain of ribonucleotide reductases. Family members include Ribonucleotide reductase (RNR, EC:1.17.4.1) [1, 2] which catalyse the reductive synthesis of deoxyribonucleotides from their corresponding ribonucleotides. It provides the precursors necessary for DNA synthesis. RNRs divide into three classes on the basis of their metallocofactor usage. This domain is found in Class II. Class II RNRs, found in bacteria, bacteriophage, algae and archaea, use coenzyme B12 (adenosylcobalamin, AdoCbl). Many organisms have more than one class of RNR present in their genomes. Ribonucleotide reductase is an oligomeric enzyme composed of a large sub-unit (700 to 1000 residues) and a small sub-unit (300 to 400 residues) - class II RNRs are less complex, using the small molecule B12 in place of the small chain [3]. Some family members carry ATP cone domain which acts as a functional regulator. Competitive binding of ATP and dATP to an N-terminal ATP-cone domain determines enzyme activity. As the ratio of dATP to ATP increases above a certain threshold, the enzyme activity is turned off. Substrate nucleotides are recognised by relatively simple H-bonding interactions at the N-terminus of one or more alpha helices. In the monomeric class II RNR, the effector binds in a pocket formed by helices in a 130 amino acid insertion which constitutes this domain [4].

1: Structure-function studies of the large subunit of ribonucleotide reductase from Escherichia coli. Nilsson O, Lundqvist T, Hahne S, Sjoberg BM; Biochem Soc Trans. 1988;16:91-94. PMID:3286319

2: From RNA to DNA, why so many ribonucleotide reductases?. Reichard P; Science. 1993;260:1773-1777. PMID:8511586

3: The crystal structure of class II ribonucleotide reductase reveals how an allosterically regulated monomer mimics a dimer. Sintchak MD, Arjara G, Kellogg BA, Stubbe J, Drennan CL; Nat Struct Biol. 2002;9:293-300. PMID:11875520

4: The origin and evolution of ribonucleotide reduction. Lundin D, Berggren G, Logan DT, Sjoberg BM; Life (Basel). 2015;5:604-636. PMID:25734234