Toward a mechanistic and physiological understanding of a ferredoxin:disulfide reductase from the domains Archaea and Bacteria.

Disulfide reductases reduce other proteins and are critically important for cellular redox signaling and homeostasis. Methanosarcina acetivorans is a methane-producing microbe from the domain Archaea that produces a ferredoxin:disulfide reductase (FDR) for which the crystal structure has been reported, yet its biochemical mechanism and physiological substrates are unknown. FDR and the extensively characterized plant-type ferredoxin:thioredoxin reductase (FTR) belong to a distinct class of disulfide reductases that contain a unique active-site [4Fe-4S] cluster. The results reported here support a mechanism for FDR similar to that reported for FTR with notable exceptions. Unlike FTR, FDR contains a rubredoxin [1Fe-0S] center postulated to mediate electron transfer from ferredoxin to the active-site [4Fe-4S] cluster. UV-visible, EPR, and Mössbauer spectroscopic data indicated that two-electron reduction of the active-site disulfide in FDR involves a one-electron-reduced [4Fe-4S]1+ intermediate previously hypothesized for FTR. Our results support a role for an active-site tyrosine in FDR that occupies the equivalent position of an essential histidine in the active site of FTR. Of note, one of seven Trxs encoded in the genome (Trx5) and methanoredoxin, a glutaredoxin-like enzyme from M. acetivorans, were reduced by FDR, advancing the physiological understanding of FDR's role in the redox metabolism of methanoarchaea. Finally, bioinformatics analyses show that FDR homologs are widespread in diverse microbes from the domain Bacteria.