Role of Cluster-Ligated Aspartate in Gating Electron Transfer in the Four-Iron Ferredoxin from the Hyperthermophilic Archaeon Pyrococcus furiosus

Exploiting Molecular Symmetry to Quantitatively Map the Excited-State Landscape of Iron-Sulfur Clusters

Cuboidal [Fe4S4] clusters are ubiquitous cofactors in biological redox chemistry. In the [Fe4S4]1+ state, pairwise spin coupling gives rise to six arrangements of the Fe valences ("valence isomers") among the four Fe centers. Because of the magnetic complexity of these systems, it has been challenging to understand how a protein's active site dictates both the arrangement of the valences in the ground state as well as the population of excited-state valence isomers. Here, we show that the ground-state valence isomer landscape can be simplified from a six-level system in an asymmetric protein environment to a two-level system by studying the problem in synthetic [Fe4S4]1+ clusters with solution C3v symmetry. This simplification allows for the energy differences between valence isomers to be quantified (in some cases with a resolution of <0.1 kcal/mol) by simultaneously fitting the VT NMR and solution magnetic moment data. Using this fitting protocol, we map the excited-state landscape for a range of clusters of the form [(SIMes)3Fe4S4-X/L]n, (SIMes = 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene; n = 0 for anionic, X-type ligands and n = +1 for neutral, L-type ligands) and find that a single ligand substitution can alter the relative ground-state energies of valence isomers by at least 103 cm-1. On this basis, we suggest that one result of "non-canonical" amino acid ligation in Fe-S proteins is the redistribution of the valence electrons in the manifold of thermally populated excited states.

Iron-sulfur flavoenzymes: the added value of making the most ancient redox cofactors and the versatile flavins work together

Iron-sulfur (Fe-S) flavoproteins form a broad and growing class of complex, multi-domain and often multi-subunit proteins coupling the most ancient cofactors (the Fe-S clusters) and the most versatile coenzymes (the flavin coenzymes, FMN and FAD). These enzymes catalyse oxidoreduction reactions usually acting as switches between donors of electron pairs and acceptors of single electrons, and vice versa. Through selected examples, the enzymes' structure−function relationships with respect to rate and directionality of the electron transfer steps, the role of the apoprotein and its dynamics in modulating the electron transfer process will be discussed.

Characterization of the [3Fe-4S](0/1+) cluster from the D14C variant of Pyrococcus furiosus ferredoxin via combined NRVS and DFT analyses

The D14C variant of Pyrococcus furiosus ferredoxin provides an extraordinary framework to investigate a [3Fe-4S] cluster at two oxidation levels and compare the results to its physiologic [4Fe-4S] counterpart in the very same protein. Our spectroscopic and computational study reveals vibrational property changes related to the electronic and structural aspects of both Fe-S clusters.

The dinitrosyliron complex [Fe₄(μ₃-S)₂(μ₂-NO)₂(NO)₆]²⁻ containing bridging nitroxyls: ¹⁵N (NO) NMR analysis of the bridging and terminal NO-coordinate ligands

The fluxional terminal and semibridging NO-coordinate ligands of DNIC [Fe4(μ3-S)2(μ2-NO)2(NO)6](2-), a precursor of Roussin's black salt (RBS), are characterized by IR ν(NO), (15)N (NO) NMR and single-crystal X-ray diffraction.

Heterometallic [AgFe(3)S (4)] ferredoxin variants: synthesis, characterization, and the first crystal structure of an engineered heterometallic iron-sulfur protein

Heterometallic [AgFe(3)S(4)] iron-sulfur clusters assembled in wild-type Pyrococcus furiosus ferredoxin and two variants, D14C and D14H, are characterized. The crystal structure of the [AgFe(3)S(4)] D14C variant shows that the silver(I) ion is indeed part of the cluster and is coordinated to the thiolate group of residue 14. Cyclic voltammetry shows one redox pair with a reduction potential of +220 mV versus the standard hydrogen electrode which is assigned to the [AgFe(3)S(4)](2+/+) couple. The oxidized form of the [AgFe(3)S(4)] D14C variant is stable in the presence of dioxygen, whereas the oxidized forms of the [AgFe(3)S(4)] wild type and D14H variants convert to the [Fe(3)S(4)] ferredoxin form. The monovalent d (10) silver(I) ion stabilizes the [Fe(3)S(4)](+/0) cluster fragment, as opposed to divalent d (10) metal ions, resulting in more than 0.4 V difference in reduction potentials between the silver(I) and, e.g., zinc(II) heterometallic [MFe(3)S(4)] ferredoxins. The trend in reduction potentials for the variants containing the [AgFe(3)S(4)] cluster is wild type ≤ D14C < D14H and shows the same trend as reported for the variants containing the [Fe(3)S(4)] cluster, but is different from the D14C < D14H < wild type trend reported for the [Fe(4)S(4)] ferredoxin. The similarity in the reduction potential trend for the variants containing the heterometallic [AgFe(3)S(4)] cluster and the [Fe(3)S(4)] cluster can be rationalized in terms of the electrostatic influence of the residue 14 side chains, rather than the dissociation constant of this residue, as is the case for [Fe(4)S(4)] ferredoxins. The trends in reduction potentials are in line with there being no electronic coupling between the silver(I) ion and the Fe(3)S(4) fragment.

Dynamics of the [4Fe-4S] cluster in Pyrococcus furiosus D14C ferredoxin via nuclear resonance vibrational and resonance Raman spectroscopies, force field simulations, and density functional theory calculations

We have used (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to study oxidized and reduced forms of the [4Fe-4S] cluster in the D14C variant ferredoxin from Pyrococcus furiosus (Pf D14C Fd). To assist the normal-mode assignments, we conducted NRVS with D14C ferredoxin samples with (36)S substituted into the [4Fe-4S] cluster bridging sulfide positions, and a model compound without ligand side chains, (Ph(4)P)(2)[Fe(4)S(4)Cl(4)]. Several distinct regions of NRVS intensity are identified, ranging from "protein" and torsional modes below 100 cm(-1), through bending and breathing modes near 150 cm(-1), to strong bands from Fe-S stretching modes between 250 and ∼400 cm(-1). The oxidized ferredoxin samples were also investigated by resonance Raman (RR) spectroscopy. We found good agreement between NRVS and RR frequencies, but because of different selection rules, the intensities vary dramatically between the two types of spectra. The (57)Fe partial vibrational densities of states for the oxidized samples were interpreted by normal-mode analysis with optimization of Urey-Bradley force fields for local models of the [4Fe-4S] clusters. Full protein model calculations were also conducted using a supplemented CHARMM force field, and these calculations revealed low-frequency modes that may be relevant to electron transfer with Pf Fd partners. Density functional theory (DFT) calculations complemented these empirical analyses, and DFT was used to estimate the reorganization energy associated with the [Fe(4)S(4)](2+/+) redox cycle. Overall, the NRVS technique demonstrates great promise for the observation and quantitative interpretation of the dynamical properties of Fe-S proteins.

Transformation of dinitrosyl iron complexes [(NO)2Fe(SR)2]- (R = Et, Ph) into [4Fe-4S] Clusters [Fe4S4(SPh)4]2-: relevance to the repair of the nitric oxide-modified ferredoxin [4Fe-4S] clusters

Transformation of dinitrosyl iron complexes (DNICs) [(NO)(2)Fe(SR)(2)](-) (R = Et, Ph) into [4Fe-4S] clusters [Fe(4)S(4)(SPh)(4)](2-) in the presence of [Fe(SPh)(4)](2-/1-) and S-donor species S(8) via the reassembling process ([(NO)(2)Fe(SR)(2)](-) --> [Fe(4)S(3)(NO)(7)](-) (1)/[Fe(4)S(3)(NO)(7)](2-) (2) --> [Fe(4)S(4)(NO)(4)](2-) (3) --> [Fe(4)S(4)(SPh)(4)](2-) (5)) was demonstrated. Reaction of [(NO)(2)Fe(SR)(2)](-) (R = Et, Ph) with S(8) in THF, followed by the addition of HBF(4) into the mixture solution, yielded complex [Fe(4)S(3)(NO)(7)](-) (1). Complex [Fe(4)S(3)(NO)(7)](2-) (2), obtained from reduction of complex 1 by [Na][biphenyl], was converted into complex [Fe(4)S(4)(NO)(4)](2-) (3) along with byproduct [(NO)(2)Fe(SR)(2)](-) via the proposed [Fe(4)S(3)(SPh)(NO)(4)](2-) intermediate upon treating complex 2 with 1.5 equiv of [Fe(SPh)(4)](2-) and the subsequent addition of 1/8 equiv of S(8) in CH(3)CN at ambient temperature. Complex 3 was characterized by IR, UV-vis, and single-crystal X-ray diffraction. Upon addition of complex 3 to the CH(3)CN solution of [Fe(SPh)(4)](-) in a 1:2 molar ratio at ambient temperature, the rapid NO radical-thiyl radical exchange reaction between complex 3 and the biomimetic oxidized form of rubredoxin [Fe(SPh)(4)](-) occurred, leading to the simultaneous formation of [4Fe-4S] cluster [Fe(4)S(4)(SPh)(4)](2-) (5) and DNIC [(NO)(2)Fe(SPh)(2)](-). This result demonstrates a successful biomimetic reassembly of [4Fe-4S] cluster [Fe(4)S(4)(SPh)(4)](2-) from NO-modified [Fe-S] clusters, relevant to the repair of DNICs derived from nitrosylation of [4Fe-4S] clusters of endonuclease III back to [4Fe-4S] clusters upon addition of ferrous ion, cysteine, and IscS.

Accurate Computation of Reduction Potentials of 4Fe−4S Clusters Indicates a Carboxylate Shift in Pyrococcus furiosus Ferredoxin

This work describes the computation and accurate reproduction of subtle shifts in reduction potentials for two mutants of the iron-sulfur protein Pyrococcus furiosus ferredoxin. The computational models involved only first-sphere ligands and differed with respect to one ligand, either acetate (aspartate), thiolate (cysteine), or methoxide (serine). Standard procedures using vacuum optimization gave qualitatively wrong results and errors up to 0.07 V. Using electrostatically screened geometries and large basis sets for expanding the wave functions gave quantitatively correct results, with errors of only 0.03 V. Correspondingly, only this approach predicted a change in the coordination mode of aspartate (i.e., a carboxylate shift) accompanying the reduction of the wild-type cluster, confirming results from synthetic models and explaining why electrostatic screening is necessary. Hence, the carboxylate shift appears to occur in the proteins from which data were collected. The results represent the most accurate predictions of shifts in reduction potentials for modified proteins, the success in part being due to the similar nature of the three amino acid ligands involved. The predicted carboxylate shift is expected to tune aspartate's degree of electron donation to the cluster's two oxidation states, thus making the reversible redox reaction feasible.

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.

STRUCTURE, FUNCTION, AND FORMATION OF BIOLOGICAL IRON-SULFUR CLUSTERS

Iron-sulfur [Fe-S] clusters are ubiquitous and evolutionary ancient prosthetic groups that are required to sustain fundamental life processes. Owing to their remarkable structural plasticity and versatile chemical/electronic features [Fe-S] clusters participate in electron transfer, substrate binding/activation, iron/sulfur storage, regulation of gene expression, and enzyme activity. Formation of intracellular [Fe-S] clusters does not occur spontaneously but requires a complex biosynthetic machinery. Three different types of [Fe-S] cluster biosynthetic systems have been discovered, and all of them are mechanistically unified by the requirement for a cysteine desulfurase and the participation of an [Fe-S] cluster scaffolding protein. Important mechanistic questions related to [Fe-S] cluster biosynthesis involve the molecular details of how [Fe-S] clusters are assembled on scaffold proteins, how [Fe-S] clusters are transferred from scaffolds to target proteins, how various accessory proteins participate in [Fe-S] protein maturation, and how the biosynthetic process is regulated.

The 1.5 A resolution crystal structure of [Fe3S4]-ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus

The structure of [Fe(3)S(4)]-ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus has been determined to 1.5 A resolution from a crystal belonging to space group P2(1) with two molecules in the asymmetric unit. The structure has been solved with molecular replacement by use of the ferredoxin from Thermotoga maritima. The fold is similar to that of related monocluster ferredoxins and contains two double-stranded antiparallel beta-sheets and two alpha-helices. The hydrophobic interaction between Trp2 and Tyr46 is confirmed, linking the C-terminus to the longer alpha-helix. The structure contains a double-conformation disulfide bond existing in a left-handed and a right-handed spiral conformation. The crystal packing reveals a beta-sheet interaction, which supports the suggestion that P. furiosus ferredoxin is a functional dimer. The extraordinary thermostability of P. furiosus ferredoxin is further discussed.

A solution NMR molecular model for the aspartate-ligated, cubane cluster containing ferredoxin from the hyperthermophilic archeaon Pyrococcus furiosus

A solution molecular model for the conformationally dynamically heterogeneous Pyrococcus furiosus ferredoxin with an intact disulfide bond has been constructed on the basis of reported (1)H NMR spectral parameters using distance geometry and simulated annealing protocols. Conventional long-mixing time NOESY and H-bonding constraints have been augmented by previously reported short-mixing time NOESY, steady-state NOE, and cluster paramagnetism-induced relaxation. The family of 15 structures with inconsequential violations exhibited low rms deviations for backbone atoms for the overwhelming majority of the residues, including the cluster ligating loop with the unprecedented ligated Asp14. Larger rms deviations were observed across the disulfide bond, but closer inspection revealed that the 15 structures can be factored into 10 substructures exhibiting an "S" or right-handed disulfide orientation and 5 exhibiting an "R" or left-handed disulfide orientation. The remainder of the structure is indistinguishable for the two disulfide orientations but confirms stabilizing extensions of secondary structural elements in the lengthening of the long helix and both the lengthening and incorporation of a third strand into the beta-sheet involving the termini, with these extensions interacting strongly in a modular fashion through the rings of Tyr46 and Trp2. These extensions of stabilizing interactions in Pyrococcus furiosus Fd, however, lead to strong destabilization of the disulfide bond and destabilization of the highly conserved first and last beta-turns in the sequence. It is concluded that the structural alternations in Pyrococcus Fd relative to other hyperthermostable Fds are not to increase thermostability but to place "stress" on the disulfide bond and render it more reducible. The possible physiological implications of this unique reducible disulfide bond are discussed.

Solution NMR characterization of the thermodynamics of the disulfide bond orientational isomerism and its effect of cluster electronic properties for the hyperthermostable three-iron cluster ferredoxin from the archaeon Pyrococcus furiosus

The thermodynamics and dynamics of the Cys21-Cys48 disulfide "S" if "R" conformational isomerism in the three-iron, single cubane cluster ferredoxin (Fd) from the hyperthermophilic archaeon Pyrococcus furiosus (Pf) have been characterized by (1)H NMR spectroscopy in both water and water/methanol mixed solvents. The mean interconversion rate at 25 degrees C is 3 x 10(3) s(-1) and DeltaG(298) = -0.2 kcal/mol [DeltaH = 4.0 kcal/mol; DeltaS = 14 cal/(mol.K)], with the S orientation as the more stable form at low temperature (< 0 degrees C) but the R orientation predominating at >100 degrees C, where the organism thrives. The distinct pattern of ligated Cys beta-proton contact shifts for the resolved signals and their characteristic temperature behavior for the forms of the 3Fe Fd with alternate disulfide orientations have been analyzed to determine the influences of disulfide orientation and methanol cosolvent on the topology of the inter-iron spin coupling in the 3Fe cluster. The Cys21-Cys48 disulfide orientation influences primarily the spin couplings involving the iron ligated to Cys17, whose carbonyl oxygen is a hydrogen bond acceptor to the Cys21 peptide proton. Comparison of the Cys beta-proton contact shift pattern for the alternate disulfide orientations with the pattern exhibited upon cleaving the disulfide bridge confirms an earlier [Wang, P.-L., Calzolai, L., Bren, K. L., Teng, Q., Jenney, F. E., Jr., Brereton, P. S., Howard, J. B., Adams, M. W. W., and La Mar, G. N. (1999) Biochemistry 38, 8167-8178] proposal that the structure of the same Fd with the R disulfide orientation resembles that of the Fd upon cleaving the disulfide bond.

Structure of C42D Azotobacter vinelandii FdI. A Cys-X-X-Asp-X-X-Cys motif ligates an air-stable [4Fe-4S]2+/+ cluster

All naturally occurring ferredoxins that have Cys-X-X-Asp-X-X-Cys motifs contain [4Fe-4S](2+/+) clusters that can be easily and reversibly converted to [3Fe-4S](+/0) clusters. In contrast, ferredoxins with unmodified Cys-X-X-Cys-X-X-Cys motifs assemble [4Fe-4S](2+/+) clusters that cannot be easily interconverted with [3Fe-4S](+/0) clusters. In this study we changed the central cysteine of the Cys(39)-X-X-Cys(42)-X-X-Cys(45) of Azotobacter vinelandii FdI, which coordinates its [4Fe-4S](2+/+) cluster, into an aspartate. UV-visible, EPR, and CD spectroscopies, metal analysis, and x-ray crystallography show that, like native FdI, aerobically purified C42D FdI is a seven-iron protein retaining its [4Fe-4S](2+/+) cluster with monodentate aspartate ligation to one iron. Unlike known clusters of this type the reduced [4Fe-4S](+) cluster of C42D FdI exhibits only an S = 1/2 EPR with no higher spin signals detected. The cluster shows only a minor change in reduction potential relative to the native protein. All attempts to convert the cluster to a 3Fe cluster using conventional methods of oxygen or ferricyanide oxidation or thiol exchange were not successful. The cluster conversion was ultimately accomplished using a new electrochemical method. Hydrophobic and electrostatic interaction and the lack of Gly residues adjacent to the Asp ligand explain the remarkable stability of this cluster.

Effect of serinate ligation at each of the iron sites of the [Fe4S4] cluster of Pyrococcus furiosus ferredoxin on the redox, spectroscopic, and biological properties

Pyrococcus furiosus ferredoxin (Fd) contains a single [Fe(4)S(4)] cluster coordinated by three cysteine (at positions 11, 17, and 56) and one aspartate ligand (at position 14). In this study, the spectroscopic, redox, and functional consequences of D14C, D14C/C11S, D14S, D14C/C17S, and D14C/C56S mutations have been investigated. The four serine variants each contain a potential cluster coordination sphere of one serine and three cysteine residues, with serine ligation at each of the four Fe sites of the [Fe(4)S(4)] cluster. All five variants were expressed in Escherichia coli, and each contained a [Fe(4)S(4)](2+,+) cluster as shown by UV-visible absorption and resonance Raman studies of the oxidized protein and EPR and variable-temperature magnetic circular dichroism (VTMCD) studies of the as-prepared, dithionite-reduced protein. Changes in both the absorption and resonance Raman spectra are consistent with changing from complete cysteinyl cluster ligation in the D14C variant to three cysteines and one oxygenic ligand in each of the four serine variants. EPR and VTMCD studies show distinctive ground and excited state properties for the paramagnetic [Fe(4)S(4)](+) centers in each of these variant proteins, with the D14C and D14C/C11S variants having homogeneous S = (1)/(2) ground states and the D14S, D14C/C17S, and D14C/C56S variants having mixed-spin, S = (1)/(2) and (3)/(2) ground states. The midpoint potentials (pH 7.0, 23 degrees C) of the D14C/C11S and D14C/C17S variants were unchanged compared to that of the D14C variant (E(m) = -427 mV) within experimental error, but the potentials of D14C/C56S and D14S variants were more negative by 49 and 78 mV, respectively. Since the VTMCD spectra indicate the presence of a valence-delocalized Fe(2. 5+)Fe(2.5+) pair in all five variants, the midpoint potentials are interpreted in terms of Cys11 and Cys17 ligating the nonreducible valence-delocalized pair in D14C. Only the D14S variant exhibited a pH-dependent redox potential over the range of 3.5-10, and this is attributed to protonation of the serinate ligand to the reduced cluster (pK(a) = 4.75). All five variants had similar K(m) and V(m) values in a coupled assay in which Fd was reduced by pyruvate ferredoxin oxidoreductase (POR) and oxidized by ferredoxin NADP oxidoreductase (FNOR), both purified from P. furiosus. Hence, the mode of ligation at each Fe atom in the [Fe(4)S(4)] cluster appears to have little effect on the interaction and the electron transfer between Fd and FNOR.

Secondary structure extensions in Pyrococcus furiosus ferredoxin destabilize the disulfide bond relative to that in other hyperthermostable ferredoxins. Global consequences for the disulfide orientational heterogeneity

The single cubane cluster ferredoxin (Fd) from the hyperthermophilic archaeon Pyrococcus furiosus (Pf) possesses several unique properties when compared even to Fds from other hyperthermophilic archaea or bacteria. These include an equilibrium molecular heterogeneity, a six- to seven-residue increase in size, an Asp rather than the Cys as one cluster ligand, and a readily reducible disulfide bond. NMR assignments and determination of both secondary structure and tertiary contacts remote from the paramagnetic oxidized cluster of Pf 3Fe Fd with an intact disulfide bond reported previously (Teng Q., Zhou, Z. H., Smith, E. T., Busse, S. C., Howard, J. B. Adams, M. W. W., and La Mar, G. (1994) Biochemistry 33, 6316-6328) are extended here to the 4Fe oxidized cluster WT (1H and 15N) and D14C (1H only) Fds with an intact disulfide bond and to the 4Fe oxidized WT Fd (1H and 15N) with a cleaved disulfide bond. All forms are shown to possess a long (13-member) alpha-helix, two beta-sheets (one double-, one triple-stranded), and three turns outside the cluster vicinity, each with tertiary contacts among themselves as found in other Fds. While the same secondary structural elements, with similar tertiary contacts, are found in other hyperthermostable Fds, Pf Fd has two elements, the long helix and the triple-stranded beta-sheet, that exhibit extensions and form multiple tertiary contacts. All Pf Fd forms with an intact disulfide bond exhibit a dynamic equilibrium heterogeneity which is shown to modulate a hydrogen-bonding network in the hydrophobic core that radiates from the Cys21-Cys48 disulfide bond and encompasses residues Lys36, Val24, Cys21, and Cys17 and the majority of the long helix. The heterogeneity is attributed to population of the alternate S and R chiralities of the disulfide bond, each destabilized by steric interactions with the extended alpha-helix. Comparison of the chemical shifts and their temperature gradients reveals that the molecular structure of the protein with the less stable R disulfide resembles that of the Fd with a cleaved disulfide bond. Both cluster architecture (3Fe vs 4Fe) and ligand mutation (Cys for Asp14) leave the disulfide orientational heterogeneity largely unperturbed. It is concluded that the six- to seven-residue extension that results in a longer helix and larger beta-sheet in Pf Fd, relative to other hyperthermostable Fds, more likely serves to destabilize the disulfide bond, and hence make it more readily reducible, than to significantly increase protein thermostability.

A T14C variant of Azotobacter vinelandii ferredoxin I undergoes facile [3Fe-4S]0 to [4Fe-4S]2+ conversion in vitro but not in vivo

[4Fe-4S]2+/+ clusters that are ligated by Cys-X-X-Cys-X-X-Cys sequence motifs share the general feature of being hard to convert to [3Fe-4S]+/0 clusters, whereas those that contain a Cys-X-X-Asp-X-X-Cys motif undergo facile and reversible cluster interconversion. Little is known about the factors that control the in vivo assembly and conversion of these clusters. In this study we have designed and constructed a 3Fe to 4Fe cluster conversion variant of Azotobacter vinelandii ferredoxin I (FdI) in which the sequence that ligates the [3Fe-4S] cluster in native FdI was altered by converting a nearby residue, Thr-14, to Cys. Spectroscopic and electrochemical characterization shows that when purified in the presence of dithionite, T14C FdI is an O2-sensitive 8Fe protein. Both the new and the indigenous clusters have reduction potentials that are significantly shifted compared with those in native FdI, strongly suggesting a significantly altered environment around the clusters. Interestingly, whole cell EPR have revealed that T14C FdI exists as a 7Fe protein in vivo. This 7Fe form of T14C FdI is extremely similar to native FdI in its spectroscopic, electrochemical, and structural features. However, unlike native FdI which does not undergo facile cluster conversion, the 7Fe form T14C FdI quickly converts to the 8Fe form with a high efficiency under reducing conditions.

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.

Delta T 14/Delta D 15 Azotobacter vinelandii ferredoxin I: creation of a new CysXXCysXXCys motif that ligates a [4Fe-4S] cluster

In clostridial-type ferredoxins, each of the two [4Fe-4S]2+/+ clusters receives three of its four ligands from a CysXXCysXXCys motif. Azotobacter vinelandii ferredoxin I (AvFdI) is a seven-iron ferredoxin that contains one [4Fe-4S]2+/+ cluster and one [3Fe-4S]+/0 cluster. During the evolution of the 7Fe azotobacter-type ferredoxins from the 8Fe clostridial-type ferredoxins, one of the two motifs present changed to a CysXXCysXXXXCys motif, resulting in the inability to form a 4Fe cluster and the appearance of a 3Fe cluster in that position. In a previous study, we were unsuccessful in using structure as a guide in designing a 4Fe cluster in the 3Fe cluster position of AvFdI. In this study, we have reversed part of the evolutionary process by deleting two residues between the second and third cysteines. UV/Vis, CD, and EPR spectroscopies and direct electrochemical studies of the purified protein reveal that this DeltaT14/DeltaD15 FdI variant is an 8Fe protein containing two [4Fe-4S]2+/+ clusters with reduction potentials of -466 and -612 mV versus SHE. Whole-cell EPR shows that the protein is present as an 8Fe protein in vivo. These data strongly suggest that it is the sequence motif rather than the exact sequence or the structure that is critical for the assembly of a 4Fe cluster in that region of the protein. The new oxygen-sensitive 4Fe cluster was converted in partial yield to a 3Fe cluster. In known ferredoxins and enzymes that contain reversibly interconvertible [4Fe-4S]2+/+ and [3Fe-4S]+/0 clusters, the 3Fe form always has a reduction potential ca. 200 mV more positive than the 4Fe cluster in the same position. In contrast, for DeltaT14/DeltaD15 FdI, the 3Fe and 4Fe clusters in the same location have extremely similar reduction potentials.

Effect of iron-sulfur cluster environment in modulating the thermodynamic properties and biological function of ferredoxin from Pyrococcus furiosus

The ferredoxin (7.5 kDa) of the hyperthermophilic archaeon, Pyrococcus furiosus, contains a single [4Fe-4S]1+,2+ cluster that is coordinated by three Cys and one Asp residue rather than the expected four Cys. The role of this Asp residue was investigated using a series of mutants, D14X, where X = C, S, H, N, V, and Y, prepared by heterologous gene expression in Escherichia coli. While the recombinant form of the wild-type and the D14S and D14C mutants contained a [4Fe-4S]1+,2+ cluster, the D14V, D14H, D14Y, and D14N proteins contained a [3Fe-4S]0,+ center, as determined by visible spectroscopy and electrochemistry. The redox potentials (at pH 7.0, 23 degrees C) of the D14C and D14S mutants were decreased by 58 and 133 mV, respectively, compared to those of the wild-type 4Fe-ferredoxin (Em -368 mV), while those of the 3Fe-protein mutants (including the 3Fe-form of the D14S, generated by chemical oxidation) were between 15 and 118 mV more positive than that of wild-type 3Fe-form (obtained by chemical oxidation, Em -203 mV). The reduction potentials of all of the 3Fe-forms, except the D14S mutant, showed a pH response over the range 3.0-10.0 with a pK of 3.3-4.7, and this was assigned to cluster protonation. The D14H mutant and the wild-type 3Fe-proteins showed an additional pK (both at 5.9) assumed to arise from protonation of the amino acid side chain. With the 4Fe-proteins, there was no dramatic change in the potentials of the wild-type or D14C form, while the pH response of the D14S mutant (pK 4.75) was ascribed to protonation of the serinate. While the ferredoxin variants exhibited a range of thermal stabilities (measured at 80 degrees C, pH 2.5), none of them showed any temperature-dependent transitions (0-80 degrees C) in their reduction potentials, and there was no correlation between the calculated DeltaS degrees' values and the absorbance maximum, reduction potential, or hydrophobicity of residue 14. In contrast, there was a linear correlation between the DeltaH degrees' value and reduction potential. Kinetic analyses were carried out at 80 degrees C using the ferredoxin as either an electron acceptor to pyruvate oxidoreductase (POR) or as an electron donor to ferredoxin:NADP oxidoreductase (FNOR, both from P. furiosus). The data showed that the reduction potential of the ferredoxin, rather than cluster type or the nature of the residue at position 14, appears to be the predominant factor in determining efficiency of electron transfer in both systems. However, compared to all the variants, the reduction potential of WT Fd makes it the most appropriate protein to both accept electrons from POR and donate them to FNOR.

Electronic, Magnetic, and Redox Properties of [MFe(3)S(4)] Clusters (M = Cd, Cu, Cr) in Pyrococcus furiosus Ferredoxin

The ground- and excited-state properties of heterometallic [CuFe(3)S(4)](2+,+), [CdFe(3)S(4)](2+,+), and [CrFe(3)S(4)](2+,+) cubane clusters assembled in Pyrococcus furiosus ferredoxin have been investigated by the combination of EPR and variable-temperature/variable-field magnetic circular dichroism (MCD) studies. The results indicate Cd(2+) incorporation into [Fe(3)S(4)](0,-) cluster fragments to yield S = 2 [CdFe(3)S(4)](2+) and S = (5)/(2) [CdFe(3)S(4)](+) clusters and Cu(+) incorporation into [Fe(3)S(4)](+,0) cluster fragments to yield S = (1)/(2) [CuFe(3)S(4)](2+) and S = 2 [CuFe(3)S(4)](+) clusters. This is the first report of the preparation of cubane type [CrFe(3)S(4)](2+,+) clusters, and the combination of EPR and MCD results indicates S = 0 and S = (3)/(2) ground states for the oxidized and reduced forms, respectively. Midpoint potentials for the [CdFe(3)S(4)](2+,+), [CrFe(3)S(4)](2+,+), and [CuFe(3)S(4)](2+,+) couples, E(m) = -470 +/- 15, -440 +/- 10, and +190 +/- 10 mV (vs NHE), respectively, were determined by EPR-monitored redox titrations or direct electrochemistry at a glassy carbon electrode. The trends in redox potential, ground-state spin, and electron delocalization of [MFe(3)S(4)](2+,+) clusters in P. furiosus ferredoxin are discussed as a function of heterometal (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Tl).

Site-directed mutations of the 4Fe-ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus: role of the cluster-coordinating aspartate in physiological electron transfer reactions

Ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus is a monomeric protein (7.5 kDa) that contains a single [4Fe-4S]1+, 2+ cluster. The protein is unusual in that its cluster is coordinated by three Cys and one Asp residue, rather than by the typical four Cys residues. Site-directed mutagenesis has been used to obtain mutant forms in which the cluster-coordinating Asp was replaced by Cys (D14C) and also by Ser (D14S), together with a third mutant (A1K) which contained N-Met-Lys at the N-terminus instead of N-Ala. Analyses using UV-visible absorption, far-UV circular dichroism, and EPR spectroscopy showed that there were no gross structural differences between the native and the three mutant forms and that they each contained a [4Fe-4S] cluster. The reduction potentials, determined by direct electrochemistry (at 23 degrees C, pH 8.0), of the D14S, D14C, and A1K mutants were -490, -422, and -382 mV, respectively, which compare with values of -375 mV for native [4Fe-4S]-containing ferredoxin and -160 mV for the [3Fe-4S]-containing form. The native, D14C, and A1K proteins functioned as electron acceptors in vitroat 80 degrees C for pyruvate ferredoxin oxidoreductase (POR) and aldehyde ferredoxin oxidoreductase (AOR) from P. furiosus using pyruvate and crotonaldehyde as substrates, respectively. The calculated kcat/Km values were similar for the three proteins when ferredoxin reduction was measured either directly by visible absorption or indirectly by coupling ferredoxin reoxidation to the reduction of metronidazole. In contrast, using the D14S mutant and the 3Fe-form of the native ferredoxin as electron acceptors, the activity with AOR was virtually undetectable, and with POR the calculated kcat/Km values were at least 3-fold lower than those obtained with the native (4Fe-), D14C, and A1K proteins. The ability of this 4Fe-ferredoxin to accept electrons from two oxidoreductases of the same organism is therefore not absolutely dependent upon Asp14, as this residue can be effectively replaced by Cys. However, the efficiency of electron transfer is compromised if Asp14 is replaced by Ser, or if the 4Fe-cluster is converted to the 3Fe-form, but Asp14 does not appear to offer any kinetic advantage over the expected Cys.