The [2Fe–2S] Ferredoxins

Abstract

The 2Fe ferredoxins (Fds) are simple iron–sulfur proteins, containing one 2Fe–2S cluster, that are involved mainly as electron carriers of low oxidation–reduction potential in fundamental metabolic processes. They can be subdivided into three families: class I, plant‐type; class II, adrenodoxin‐type; and class III, Clostridium ‐type; according to the spacing in the sequence of the cysteinyl residues that bind the iron atoms of the cluster. All the Fds are monomers of 11–14 kDa, except for class III Fds that are dimer, with one cluster per subunit. The cluster of 2Fe Fds contains two atoms of iron, bridged by two acid‐labile inorganic sulfide atoms, and coordinated to four cysteinyl sulfurs of the polypeptide chain. Fds absorb light in the 300–500 nm visible region, mainly due to sulfur‐Fe 3+ charge‐transfer transitions. The cluster in the oxidized protein contains two antiferromagnetically coupled, high‐spin (S = 5/2) ferric irons and is, therefore, electron paramagnetic resonance (EPR) silent. Upon one‐electron reduction, Fds become paramagnetic. The three‐dimensional (3D) structure of several 2Fe Fds have been obtained either by X‐ray crystallography or by n uclear magnetic resonance (NMR) spectroscopy. The solution structures are very similar to the 3D crystal structures. The protein structure motif characteristic of Fds has been named β‐grasp – a five‐stranded antiparallel β‐sheet with an α‐helix lying on top of the sheet. This central core (β1–β5 and α1) is surrounded by two short β‐strands, an α‐helix (α2), and a short C‐terminal helical turn (α3). Class II Fds have an additional interaction domain. Comparison of the structures of the different proteins reveals conformational differences especially between class I and II ferredoxins. The binuclear iron cluster lies in a solvent‐exposed pocket generated by the loop connecting helix α1 and strand β3 and comprising three of the cysteinyl ligands. This loop is anchored through several hydrogen bonds to different parts of the protein molecule. An extensive survey of the literature on the interactions of several 2Fe Fds with their respective partners is presented.

Characterization of the accessory protein ClpT1 from Arabidopsis thaliana: oligomerization status and interaction with Hsp100 chaperones

“…For that reason, cross-linking assays were carried out to stabilize the ClpT1 dimers. As controls, GST (54 kDa dimer protein) [ 27 ] and E. coli ferredoxin (12 kDa monomeric protein) [ 28 ] were used. By this technique, dimer formation was clearly seen, reaching a 60% of the total protein population at the highest cross-linker concentration used (Figure 4 ).…”

Redox Characterization of the FeS Protein MitoNEET and Impact of Thiazolidinedione Drug Binding

“…While articulating the details of the mechanism of proton coupling will remain a future challenge, it is clear that His87 is critically important. Replacement of His87 with Cys results in a dramatic decrease in the midpoint potential by more than 300 mV (Figure C) to a value of −320 mV at pH 7, suggestive of two-Fe ferredoxins . The overall pH dependence becomes much more nondescript with an apparent linear slope of −15 to −20 mV/pH unit [also mimicked by solution redox titrations (Figure S1)].…”

A Novel Ferredoxin-Dependent Glutamate Synthase from the Hydrogen-Oxidizing Chemoautotrophic BacteriumHydrogenobacter thermophilusTK-6

“…5). The g values are analogous to the rhombic resonances observed for plant-type ferredoxins and distinct from those for the axial EPR spectra that are characteristic of the mammalian adrenodoxin-like, bacterial putidaredoxin-like, and bacterial Isc ferredoxins (32). These results show that purified Fd3 retained the functional conformation of its [2Fe-2S] cluster.…”