Thioferrates as a source of Fe and S for cluster assembly

When the hyperothermophilic anaerobic archaeon Pyrococcus furiosus is grown using elemental sulfur as the terminal electron acceptor, an iron-sulfur storage protein, IssA, is the most highly upregulated protein. IssA stores iron and sulfide as an inorganic anionic polymer of linear ferric sulfide chains with two sulfides bridging each pair of ferric ions. Compounds with this structure are known as thioferrates. Thioferrates occur naturally as the mineral erdite. However, this is the first time they have been observed in a biological system¹. Thioferrates contain ferric and sulfide in anionic chains of edge-sharing FeS₄ tetrahedra separated by charge-balancing cations. P. furiosus has numerous Fe-S proteins, which almost exclusively contain cubane [4Fe-4S] clusters. In vitro, the stored thioferrate core of IssA has been shown to provide the iron and sulfide for assembly of the [4Fe-4S] cluster on apo-ferredoxin (Fd) from P. furiosus, an abundant electron donor in this organism. However, IssA-mediated reconstitution of apo-Fd was only observed in the presence of dithiothreitol, suggesting that a hitherto unidentified thiol reagent in P. furious disassembles the thioferrate core into smaller pieces by binding Fe, [2Fe-2S]²⁺, or linear [3Fe-4S]⁺ fragments, which can assemble into [4Fe-4S]²⁺ clusters on acceptor proteins. Furthermore, synthesized thioferrates have been explored as a source of Fe and S for cluster assembly on other Fe-S proteins. The in vitro studies revealed that thioferrates can reconstitute Fe-S clusters in Sc Grx5, Av Grx5, Pf Fd, Av IscU, and Av Fdx under anaerobic conditions. Furthermore, x-ray absorption spectroscopy was conducted to characterize the structure and properties of synthetic thioferrates. The Fe-EXAFS data for crystallographically defined synthetic thioferrate samples confirm the presence of linear chains of (FeS₂)^1-  units analogous to those found in thioferrate minerals in IssA.

In addition, the effect of 5- to 7-membered cyclic thiosulfinate crosslinkers² on Fe binding and cluster assembly in scaffold protein, IscU, will be discussed. This would serve as a mean to explore the mechanism of Fe binding and cluster formation in IscU.

Reference

1.  Vaccaro, B. J.; Clarkson, S. M.; Holden, J. F.; Lee, D.-W.; Wu, C.-H.; Poole Ii, F. L.; Cotelesage, J. J. H.; Hackett, M. J.; Mohebbi, S.; Sun, J.; Li, H.; Johnson, M. K.; George, G. N.; Adams, M. W. W., Biological iron-sulfur storage in a thioferrate-protein nanoparticle. Nat. Commun. 2017, 8 (1), 16110.

2.  Donnelly, D.; Dowgiallo, M.; Salisbury, J.; Aluri, K.; Iyengar, S.; Chaudhari, M.; Mathew, M.; Miele, I.; Auclair, J.; Lopez, S.; Manetsch, R.; Agar, J., Cyclic Thiosulfinates and Cyclic Disulfides Selectively Cross-Link Thiols While Avoiding Modification of Lone Thiols. J. Am. Chem. Soc. 2018, 140.