An NMR study of the 7Fe-8S ferredoxin from Rhodopseudomonas palustris and reinterpretation of data on similar systems

Protein Interactions in Rhodopseudomonas palustris TIE-1 Reveal the Molecular Basis for Resilient Photoferrotrophic Iron Oxidation

Rhodopseudomonas palustris is an alphaproteobacterium with impressive metabolic versatility, capable of oxidizing ferrous iron to fix carbon dioxide using light energy. Photoferrotrophic iron oxidation is one of the most ancient metabolisms, sustained by the pio operon coding for three proteins: PioB and PioA, which form an outer-membrane porin-cytochrome complex that oxidizes iron outside of the cell and transfers the electrons to the periplasmic high potential iron-sulfur protein (HIPIP) PioC, which delivers them to the light-harvesting reaction center (LH-RC). Previous studies have shown that PioA deletion is the most detrimental for iron oxidation, while, the deletion of PioC resulted in only a partial loss. The expression of another periplasmic HiPIP, designated Rpal_4085, is strongly upregulated in photoferrotrophic conditions, making it a strong candidate for a PioC substitute. However, it is unable to reduce the LH-RC. In this work we used NMR spectroscopy to map the interactions between PioC, PioA, and the LH-RC, identifying the key amino acid residues involved. We also observed that PioA directly reduces the LH-RC, and this is the most likely substitute upon PioC deletion. By contrast, Rpal_4085 demontrated significant electronic and structural differences from PioC. These differences likely explain its inability to reduce the LH-RC and highlight its distinct functional role. Overall, this work reveals the functional resilience of the pio operon pathway and further highlights the use of paramagnetic NMR for understanding key biological processes.

Draft genome sequence and overview of the purple non sulfur bacterium Rhodopseudomonas palustris 42OL

Rhodopseudomonas palustris strain 42OL was isolated in 1973 from a sugar refinery waste treatment pond. The strain has been prevalently used for hydrogen production processes using a wide variety of waste-derived substrates, and cultured both indoors and outdoors, either freely suspended or immobilized. R. palustris 42OL was suitable for many other applications and capable of growing in very different culturing conditions, revealing a wide metabolic versatility. The analysis of the genome sequence allowed to identify the metabolic pathways for hydrogen and poly-β-hydroxy-butyrate production, and confirmed the ability of using a wide range of organic acids as substrates.

Desulfovibrio gigas ferredoxin II: redox structural modulation of the [3Fe-4S] cluster

Desulfovibrio gigas ferredoxin II (DgFdII) is a small protein with a polypeptide chain composed of 58 amino acids, containing one Fe3S4 cluster per monomer. Upon studying the redox cycle of this protein, we detected a stable intermediate (FdIIint) with four 1H resonances at 24.1, 20.5, 20.8 and 13.7 ppm. The differences between FdIIox and FdIIint were attributed to conformational changes resulting from the breaking/formation of an internal disulfide bridge. The same 1H NMR methodology used to fully assign the three cysteinyl ligands of the [3Fe-4S] core in the oxidized state (DgFdIIox) was used here for the assignment of the same three ligands in the intermediate state (DgFdIIint). The spin-coupling model used for the oxidized form of DgFdII where magnetic exchange coupling constants of around 300 cm-1 and hyperfine coupling constants equal to 1 MHz for all the three iron centres were found, does not explain the isotropic shift temperature dependence for the three cysteinyl cluster ligands in DgFdIIint. This study, together with the spin delocalization mechanism proposed here for DgFdIIint, allows the detection of structural modifications at the [3Fe-4S] cluster in DgFdIIox and DgFdIIint.

Spectroscopic investigation of selective cluster conversion of archaeal zinc-containing ferredoxin from Sulfolobus sp. strain 7

Archaeal zinc-containing ferredoxin from Sulfolobus sp. strain 7 contains one [3Fe-4S] cluster (cluster I), one [4Fe-4S] cluster (cluster II), and one isolated zinc center. Oxidative degradation of this ferredoxin led to the formation of a stable intermediate with 1 zinc and approximately 6 iron atoms. The metal centers of this intermediate were analyzed by electron paramagnetic resonance (EPR), low temperature resonance Raman, x-ray absorption, and (1)H NMR spectroscopies. The spectroscopic data suggest that (i) cluster II was selectively converted to a cubane [3Fe-4S](1+) cluster in the intermediate, without forming a stable radical species, and that (ii) the local metric environments of cluster I and the isolated zinc site did not change significantly in the intermediate. It is concluded that the initial step of oxidative degradation of the archaeal zinc-containing ferredoxin is selective conversion of cluster II, generating a novel intermediate containing two [3Fe-4S] clusters and an isolated zinc center. At this stage, significant structural rearrangement of the protein does not occur. We propose a new scheme for oxidative degradation of dicluster ferredoxins in which each cluster converts in a stepwise manner, prior to apoprotein formation, and discuss its structural and evolutionary implications.

Slow formation of [3Fe-4S](1+) clusters in mutant forms of Desulfovibrio africanus ferredoxin III

Desulfovibrio africanus ferredoxin III (Da FdIII) readily interconverts between a 7Fe and an 8Fe form with Asp-14 believed to provide a cluster ligand in the latter form. To investigate the factors important for cluster interconversion in Fe/S cluster-containing proteins we have studied two variants of Da FdIII produced by site-directed mutagenesis, Asp14Glu and Asp14His, with cluster incorporation performed in vitro. Characterisation of these proteins by UV/visible, EPR and (1)H NMR spectroscopies revealed that the formation of the stable 7Fe form of these proteins takes some time to occur. Evidence is presented which indicates the [4Fe-4S](2+) cluster is incorporated prior to the [3Fe-4S](1+) cluster.

Structural conservation of the isolated zinc site in archaeal zinc-containing ferredoxins as revealed by x-ray absorption spectroscopic analysis and its evolutionary implications

The zfx gene encoding a zinc-containing ferredoxin from Thermoplasma acidophilum strain HO-62 was cloned and sequenced. It is located upstream of two genes encoding an archaeal homolog of nascent polypeptide-associated complex alpha subunit and a tRNA nucleotidyltransferase. This gene organization is not conserved in several euryarchaeoteal genomes. The multiple sequence alignments of the zfx gene product suggest significant sequence similarity of the ferredoxin core fold to that of a low potential 8Fe-containing dicluster ferredoxin without a zinc center. The tightly bound zinc site of zinc-containing ferredoxins from two phylogenetically distantly related Archaea, T. acidophilum HO-62 and Sulfolobus sp. strain 7, was further investigated by x-ray absorption spectroscopy. The zinc K-edge x-ray absorption spectra of both archaeal ferredoxins are strikingly similar, demonstrating that the same zinc site is found in T. acidophilum ferredoxin as in Sulfolobus sp. ferredoxin, which suggests the structural conservation of isolated zinc binding sites among archaeal zinc-containing ferredoxins. The sequence and spectroscopic data provide the common structural features of the archaeal zinc-containing ferredoxin family.

Zinc and an N-terminal extra stretch of the ferredoxin from a thermoacidophilic archaeon stabilize the molecule at high temperature

Ferredoxin from the thermoacidophilic archaeon Sulfolobus sp. strain 7 has a 36-residue extra domain at its N-terminus and a 67-residue core domain carrying two iron-sulfur clusters. A zinc ion is held at the interface of the two domains through tetrahedral coordination of three histidine residues (-6, -19 and -34) and one aspartic acid residue (-76) [Fujii, T., Hata, Y., Oozeki, M., Moriyama, H., Wakagi, T., Tanaka, N. & Oshima, T. (1997) Biochemistry 36, 1505-1513]. To elucidate the roles of the novel zinc ion and the extra N-terminal domain, a series of truncated mutants was constructed: G1, V12, S17, G23, L31 and V38, which lack residues 0, 11, 16, 22, 30 and 37 starting from the N-terminus, respectively. A mutant with two histidine residues each replaced by an alanine residue, H16A/H19A, was also constructed. All the mutant ferredoxins had two iron-sulfur clusters, while zinc was retained only in G1 and V12. The thermal stability of the proteins was investigated by monitoring A408; the melting temperature (Tm) was approximately 109 degrees C for the natural ferredoxin, approximately 109 degrees C for G1, 97.6 degrees C for V12, 89.0 degrees C for S17, 89.2 degrees C for G23, 89.3 degrees C for L31, 82.1 degrees C for V38, and 89.4 degrees C for H16A/H19A. Km and Vmax values of 2-oxoglutarate:ferredoxin oxidoreductase for natural ferredoxin, G1, S17 and L31 were similar, suggesting that electron-accepting activities were not affected by the deletion. The combination of CD and fluorescent spectroscopic analyses with truncated mutant S17 indicated that not only the clusters but also the secondary and tertiary structures were simultaneously degraded at a Tm around 89 degrees C. These results unequivocally demonstrate that the zinc ion and certain parts, but not all, of the extra sequence stretch in the N-terminal domain are responsible not for function but for thermal stabilization of the molecule.

The structure of iron-sulfur proteins

Ferredoxins are a group of iron-sulfur proteins for which a wealth of structural and mutational data have recently become available. Previously unknown structures of ferredoxins which are adapted to halophilic, acidophilic or hyperthermophilic environments and new cysteine patterns for cluster ligation and non-cysteine cluster ligation have been described. Site-directed mutagenesis experiments have given insight into factors that influence the geometry, stability, redox potential, electronic properties and electron-transfer reactivity of iron-sulfur clusters.