Glutaredoxinsare small redox enzymes of approximately one hundred amino-acid residues that use glutathione as a cofactor. Glutaredoxins are oxidized by substrates, and reduced non-enzymatically by glutathione. In contrast to thioredoxins, which are reduced by thioredoxin reductase, no oxidoreductase exists that specifically reduces glutaredoxins. Instead, glutaredoxins are reduced by the oxidation of glutathione. Oxidized glutathione is then regenerated by glutathione reductase. Together these components compose the glutathione system. Like thioredoxin, which functions in a similar way, glutaredoxin possesses an active centre disulfide bond. It exists in either a reduced or an oxidized form where the two cysteine residues are linked in an intramolecular disulfide bond.
Glutaredoxins function as electron carriers in the glutathione-dependent synthesis of deoxyribonucleotides by the enzyme ribonucleotide reductase. Moreover, GRX/GLR act in antioxidant defence by reducing dehydroascorbate, peroxiredoxins, and methionine sulfoxide reductase.
Beside their function in antioxidant defence, bacterial and plant GRX/GLR were shown to bind iron-sulfur clusters and to deliver the cluster to enzymes on demand.
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