113-Cd NMR was used to probe the Fe3+ metal ion binding site of Desulfovibrio gigas desulforedoxin. Using recombinant protein expressed in Escherichia coli, the ferric ion was removed and replaced with Cd2+. The Cd-113 NMR spectrum exhibited a single resonance at 746 ppm, indicating tetrahedral cysteinyl sulfur ligation of the metal.