Ribonucleotide reductase large subunit, C-terminal
<p>Ribonucleotide reductase (<db_xref db="EC" dbkey="1.17.4.1"/>) [<cite idref="PUB00000559"/>, <cite idref="PUB00005164"/>] catalyzes the reductivesynthesis of deoxyribonucleotides from their corresponding ribonucleotides. It providesthe precursors necessary for DNA synthesis. RNRs divide into three classes on the basis of their metallocofactor usage. Class I RNRs, found in eukaryotes, bacteria, bacteriophage and viruses, use a diiron-tyrosyl radical, Class II RNRs, found in bacteria, bacteriophage, algae and archaea, use coenzyme B12 (adenosylcobalamin, AdoCbl). Class III RNRs, found in anaerobic bacteria and bacteriophage, use an FeS cluster and S-adenosylmethionine to generate a glycyl radical. Many organisms have more than one class of RNR present in their genomes. </p><p>Ribonucleotide reductase is an oligomericenzyme composed of a large subunit (700 to 1000 residues) and a small subunit (300 to400 residues) - class II RNRs are less complex, using the small molecule B12 in place of the small chain [<cite idref="PUB00007088"/>]. </p> <p>The reduction of ribonucleotides to deoxyribonucleotides involves the transfer of free radicals, the function of each metallocofactor is to generate an active site thiyl radical. This thiyl radical then initiates the nucleotide reduction process by hydrogen atom abstraction from the ribonucleotide [<cite idref="PUB00005953"/>]. The radical-based reaction involves five cysteines: two of these are located at adjacent anti-parallel strands in a new type of ten-stranded alpha/beta-barrel; two others reside at the carboxyl end in a flexible arm; and the fifth, in a loop in the centre of the barrel, is positioned to initiate the radical reaction [<cite idref="PUB00005954"/>]. There are several regions of similarity in the sequence of the large chain of prokaryotes, eukaryotes and viruses spread across 3 domains: an N-terminal domain common to the mammalian and bacterial enzymes; a C-terminal domain common to the mammalian and viral ribonucleotide reductases; and a central domain common to all three [<cite idref="PUB00005953"/>].</p>