Thioredoxin or glutaredoxin

Ribonucleotide reductase uses a protein cofactor—thioredoxin or glutaredoxin—to provide electrons for reduction of the ribonucleotide substrate.

However, the ultimate electron donor is NADPH.

X-ray crystal structures of thioredoxin from E. coli and glutaredoxin from the yeast S. cerevisiae show the locations of the redox-active cysteine residues near their surfaces.

In the thioredoxin structure, Cys32 and Cys35 are in the oxidized disulfide state.

In the glutaredoxin structure, Cys27 and Cys30 are reduced, and a molecule of glutathione (GSH) is bound with its cysteinyl-SH adjacent to the two cysteine residues.