Ferredoxin: thioredoxin reductase: disulfide reduction catalyzed via novel site-specific [4Fe-4S] cluster chemistry

Thioredoxin-mediated light regulation in plant chloroplasts involves a unique class of disulfide reductases that catalyze disulfide reduction in two one-electron steps using a [2Fe-2S] ferredoxin as the electron donor and an active site comprising a [4Fe-4S] cluster and a redox-active disulfide. This review summarizes structural and spectroscopic studies of ferredoxin: thioredoxin reductase (FTR) and a chemically modified form, termed NEM-FTR, which provides a stable analog of the one-electron reduced catalytic intermediate. Detailed spectroscopic characterization of FTR and NEM-FTR using absorption, EPR, electron-nuclear double resonance, variable-temperature magnetic circular dichroism, resonance Raman and Mossbauer spectroscopies indicate that the one-electron reduced catalytic intermediate involves two-electron disulfide reduction coupled with one-electron cluster oxidation of a [4Fe-4S](2+) cluster to yield a unique type of S=1/2 [4Fe-4S](3+) cluster with two cysteine residues ligated at a specific Fe site. The results provide the basis for a novel mechanism for disulfide cleavage in two one-electron steps involving site-specific [4Fe-4S] cluster chemistry. A similar mechanism is proposed for direct [4Fe-4S]-mediated cleavage of the CoM-S-S-CoB heterodisulfide in methanogenic archaea by heterodisulfide reductases.