Both human ferredoxins 1 and 2 and ferredoxin reductase are important for iron-sulfur cluster biogenesis

Abstract

Ferredoxins are iron–sulfur proteins that have been studied for decades because of their role in facilitating the monooxygenase reactions catalyzed by p450 enzymes. More recently, studies in bacteria and yeast have demonstrated important roles for ferredoxin and ferredoxin reductase in iron–sulfur cluster assembly. The human genome contains two homologous ferredoxins, ferredoxin 1 (FDX1) and ferredoxin 2 (FDX2 — formerly known as ferredoxin 1L). More recently, the roles of these two human ferredoxins in iron–sulfur cluster assembly were assessed, and it was concluded that FDX1 was important solely for its interaction with p450 enzymes to synthesize mitochondrial steroid precursors, whereas FDX2 was used for synthesis of iron–sulfur clusters, but not steroidogenesis. To further assess the role of the FDX–FDXR system in mammalian iron–sulfur cluster biogenesis, we performed siRNA studies on FDX1 and FDX2, on several human cell lines, using oligonucleotides identical to those previously used, along with new oligonucleotides that specifically targeted each gene. We concluded that both FDX1 and FDX2 were important in iron–sulfur cluster biogenesis. Loss of FDX1 activity disrupted activity of iron–sulfur cluster enzymes and cellular iron homeostasis, causing mitochondrial iron overload and cytosolic iron depletion. Moreover, knockdown of the sole human ferredoxin reductase, FDXR, diminished iron–sulfur cluster assembly and caused mitochondrial iron overload in conjunction with cytosolic depletion. Our studies suggest that interference with any of the three related genes, FDX1, FDX2 or FDXR, disrupts iron–sulfur cluster assembly and maintenance of normal cytosolic and mitochondrial iron homeostasis.

Deleterious mutation in FDX1L gene is associated with a novel mitochondrial muscle myopathy

“…A possible explanation may rely on the second ferredoxin Fdx1, which was recently demonstrated like Fdx2, to be important for Fe-S biogenesis. 13 We speculate that either Fdx1 or another, yet unidentified factor, may compensate for Fdx2 deficiency, but when exposed to extreme conditions such as physical exercise it may be exhausted leading to rhabdomyolysis. Then again, the initiation site at Met 14, although probably inefficient may serve as a partial alternative leading to a modified predominantly skeletal muscle phenotype.…”

“…13 Fe-S clusters are evolutionary conserved, ubiquitously expressed, cofactors which are composed of iron and sulfur. They usually ligate to prosthetic proteins by the sulfhydryl side chains of their cysteine and often function as electron acceptors or donors playing a major role in crucial cellular activities.…”

Functional reconstitution of mitochondrial Fe/S cluster synthesis on Isu1 reveals the involvement of ferredoxin

“…28). In contrast, RNA interference-mediated depletion of either FDX1 or FDX2 was reported to impair Fe/S protein maturation 29 . In our in vitro system, only FDX2, but not FDX1, could replace yeast Yah1.…”

Structure of human Fe–S assembly subcomplex reveals unexpected cysteine desulfurase architecture and acyl-ACP–ISD11 interactions

“…The NFS1-ISD11 complex catalyzes the conversion of L-cysteine to L-alanine and generates a persulfide intermediate on a cysteine of the mobile S-transfer loop of NFS1 (S-loop) (24,28). The terminal sulfur of this intermediate is transferred to the scaffold protein ISCU2, where it is combined with ferrous iron and electrons, from a ferredoxin (29,30), to form Fe-S clusters. In humans, a low-activity Fe-S assembly complex consisting of NFS1-ISD11 and the catalytic subunit ISCU2 can be activated by binding of the allosteric activator frataxin (FXN) (25).…”