Abstract
Deoxyribonucleotides are DNA building blocks and are produced de novo by reduction of ribose to deoxyribose. This reduction is catalyzed by ribonucleotide reductase (RNR), a heterodimeric tetramer enzyme in mammalian cells, having one of two free radical-containing subunits called R2 and p53R2. R2 is S-phase specific and used for DNA replication, whereas p53R2 functions in DNA repair and mitochondrial DNA synthesis. The larger RNR subunit, R1, has catalytically active cysteine thiols in its buried active site and a C-terminal swinging arm, with a Cys-Leu-Met-Cys sequence suggested to act as a shuttle dithiol/disulfide for electron transport. After each catalytic cycle the active site contains a disulfide, which has to be reduced for turnover. Thioredoxin (Trx) and glutaredoxin (Grx) systems have been implicated as electron donors for the RNR disulfide reduction via the swinging arm. Using mouse R1-R2 and R1-p53R2 complexes, we found here that the catalytic efficiency of the GSH-Grx system is 4-6 times higher than that of the Trx1 system. For both complexes, the V(max) values for Grx are strongly depended on GSH concentrations. The GSH disulfide resulting from the Grx reaction was reduced by NADPH and GSH reductase and this enzyme was essential because reaction with GSH alone yielded only little activity. These results indicate that C-terminal shuttle dithiols of mammalian R1 have a crucial catalytic role and that the GSH-Grx system favors the R1-p53R2 enzyme for DNA replication in hypoxic conditions, mitochondrial DNA synthesis, and in DNA repair outside the S-phase.