Fe/S cluster proteins are key players in diverse pathways, such as mitochondrial respiration, gene regulation, and DNA/RNA metabolism. It is generally believed that an Fe/S cluster is synthesized inside the mitochondria before it is exported out to the cytosol. But the details of this export pathway are still unclear; especially what is the substrate that is exported. This substance should be essentially small enough to go through the exporter Atm1p, stable enough to survive the transport process and labile enough to deliver the cluster to cytosolic proteins. It has been shown that glutathione (GSH) or its derivatives might be involved in this pathway. Several Glutaredoxin protein (Grx) structures revealed that Grx is able to coordinate a Fe/S cluster along with two molecules of GSH. The cluster transfer between Grx and scaffold protein ISU was found to be reversible, which indicates Grx may deliver the cluster to the export pathway and plays important role in Fe/S cluster biosynthesis. However, the role of this cluster is still not well understood.
A complex of four GSH-coordinated Fe/S cluster was successfully synthesized and characterized and found to be stable under physiological conditions, but undergoes a reversible exchange with scaffold protein ISU. Considering the high cellular concentration of GSH and the stability of this complex under physiological conditions, this complex may contribute to a labile cellular Fe/S cluster pool. The GSH cluster complex is a very intriguing candidate for the substrate of mitochondrial Fe/S cluster exporter. A liposome system was constructed onto which transporter was successfully reconstituted. The stimulation of proteoliposome ATPase activity by GSH cluster complex indicates that the GSH cluster complex is very likely to be the exported substance.
Evidence indicates frataxin to be the general iron donor protein in Fe/S cluster biosynthesis. However, a thorough search of the genomic database shows that based on sequence similarity, there is no homolog in some species. By utilizing a novel structural similarity search, a previously uncharacterized protein - Bacillus subtilis ydhG with very similar structure but no sequence similarity to frataxin was identified. B. subtilis yhdG shows the same function as human frataxin and is identified as the first structural homolog of this protein. This finding greatly expands the frataxin family and supports the role of frataxin being the general iron donor.
Human ferredoxin-1 (hFd1) and human ferredoxin-2 (hFd2) share high sequence similarity but have distinct roles in different cellular pathways. HFd2 was found to undergo a unique conformational change upon heating to around 60 °C before cluster degradation is observed. Transition to the second conformation is entropy driven and enthalpy unfavorable. The transition is not reversible; hFd2 retains conformation when cooling from high temperature. This transition to a second stable conformation of hFd2 is a not observed for hFd1 and may help to explain their distinct cellular functions.