Glutaredoxins are thioltransferases (disulfide reductases) which utilize glutathione and NADPH as cofactors. Oxidized glutathione is regenerated by glutathione reductase. Together these components compose the glutathione system. Glutaredoxins utilize the CXXC motif common to thioredoxins and are involved in multiple cellular processes including protection from redox stress, reduction of critical enzymes such as ribonucleotide reductase and the generation of reduced sulfur for iron sulfur cluster formation. Glutaredoxins are capable of reduction of mixed disulfides of glutathione as well as the formation of glutathione mixed disulfides. This model includes the E. coli glyutaredoxin GrxA which appears to have primary responsibility for the reduction of ribonucleotide reductase.