On the structure of the iron-sulfur complex in the two-iron ferredoxins

The 13C-resonances of the 2',6'-ring carbon atoms of both tyrosyl residues of Clostridium acidi-urici ferredoxin are shifted downfield in the oxidized and reduced protein relative to these resonance positions in free tyrosine. These results show that both tyrosyl residues in the oxidized and reduced protein are in magnetically equivalent environments, and suggest that both tyrosyl residues are close to the two iron-sulfur clusters in the reduced and oxidized proteins and that each cluster is equally accessible to one reducing electron.Ferredoxins are relatively small molecular weight iron-sulfur proteins that contain equal amounts of iron and acid-labile sulfur and show an ESR signal at g = 1.94 in the reduced state. They function as electron carriers in cell metabolism (1) and have low redox potentials (-230 mV to -430 mV) (2). The characteristic ESR resonances of reduced ferredoxins are broadened in 57Fe-and 33S-enriched ferredoxins (3). Thus, the reducing electron is associated with the iron and acidlabile sulfur atoms. One of the bacterial clostridial-type ferredoxins (a class of ferredoxins that has eight iron and eight acid-labile sulfur atoms) contains two identical iron-sulfur (Fe-S) clusters, each containing four iron and four acid-labile sulfur atoms forming two interpenetrating tetrahedra with a cysteine sulfur atom coordinated to each iron atom (4). Clostridial ferredoxins usually: contain only two residues of either phenylalanine or tyrosine or both, and the positions of these aromatic residues in five clostridial ferredoxins of known aminoacid sequence are conserved (5). This conservation suggests that the aromatic residues may be important to -the structure and function of these proteins. Mixed-valence organometallic compounds containing aromatic ligands have high electron-transfer rates and have been proposed as ferredoxin model compounds (6).For determination of the role of aromatic residues in clostridial ferredoxins, the '3C-NMR technique was used because: (i) aromatic carbon resonances can be easily assigned; (ii) carbon resonance positions, unlike those of protons, are not very sensitive to changes in protein conformation and are rarely shifted 1-2 ppm from the corresponding carbon resonance positions in free amino acids (7, 8); (iii) carbon resonances can be perturbed by nearby paramagnetic electrons, and the effect of oxidation and reduction of tyrosyl ring carbon atoms in ferredoxin can be monitored and correlated with that of tyrosyl ring protons by double resonance techniques (9). C. acidi-urici ferredoxin was chosen for this study because it contains two tyrosyl residues, and has a known sequence (10). Takano et al. (11) have observed that in cytochrome c a t In Clostridium tartarovorum ferredoxin, a histidyl and tyrosyl residue occupy the conserved aromatic positions and a histidyl residue occurs in a nonconserved position (5). tyrosyl residue changes orientation upon reduction, and have suggested that electron transfer occurs by way of a tyrosyl radical. Electron transport b...

Section: Discussionmentioning

confidence: 83%

The Possible Role of Aromatic Residues of Clostridium acidi-urici Ferredoxin in Electron Transport

Packer¹,

Sternlicht²,

Rabinowitz³

1972

“…Experimental values of J for Anabaena Fd are not available, but similar proteins like spinach Fd yield values between Ϫ183 cm Ϫ1 (34,35) and Ϫ145 cm Ϫ1 (36). Overestimation of the absolute value of J by about a factor of two is a well known problem in DFT when using generalized gradient approximations (7).…”

Section: Resultsmentioning

confidence: 99%

Dynamical magnetostructural properties of Anabaena ferredoxin

Schreiner

Nair

Pollet

et al. 2007

A mixed quantum/classical investigation of the dynamical magnetostructural properties, that is, ''magnetodynamics,'' of oxidized Anabaena PCC7119 ferredoxin is carried out at room temperature in two distinct conformational states. This protein hosts a [2Fe-2S] cluster in which two iron centers are antiferromagnetically coupled to an overall low-spin electronic ground state that has a genuine multireference character. To study the magnetodynamics of this prosthetic group, an approximate spin projection method is formulated in the framework of density functional theory that allows for multideterminant ab initio molecular dynamics simulations to be carried out efficiently. By using this scheme, the influence of both thermal fluctuations and conformational motion on the structure of the [2Fe-2S] cluster and on the dynamics of the antiferromagnetic coupling constant, J(t), has been investigated. In addition to demonstrating how sensitively the shape of the [2Fe-2S] core itself is affected by hydrogen bonding, the analyses reveal a complex dynamical coupling of J to both local vibrations and large-amplitude motion. It is shown that this interplay can be understood in terms of specific vibrational modes and distinct hydrogen-bonding patterns between the iron-sulfur cluster and the protein backbone, respectively. This implies going beyond the Goodenough-Kanamori rules for angular magnetostructural correlations of oxidized iron-sulfur prosthetic groups.exchange coupling ͉ extended broken-symmetry ͉ hydrogen bonding ͉ iron-sulfur protein ͉ multideterminant hybrid molecular dynamics I ron-sulfur clusters are among the most ubiquitous, functionally versatile, and ancient prosthetic groups in proteins (1-4). One of the major classes of mobile electron carriers in contemporary biology is ferredoxin (Fd). Its fourfold cysteine-coordinated [2Fe-2S] core mediates electron transfer from photosystem I to ferredoxin-NADP ϩ -oxidoreductase, which reduces NADP ϩ to NADPH. A particularly interesting system is Fd from cyanobacterium Anabaena PCC7119 whose crystal structure was recently obtained at 1.3 and 1.17 Å resolution in the oxidized and the reduced forms (5), respectively. Most interestingly, it was found that two alternative conformations of Cys-46 exist, which is one of the four cysteinyl ligands of the [2Fe-2S] cluster (5). In the oxidized state Cys-46 adopts the so-called CO-in conformation, where the peptide oxygen points in the direction of the cluster, whereas, on reduction, the backbone flips to the CO-out conformation.Spin Coupling in Iron-Sulfur Clusters. In the ground state of the oxidized form of bioinorganic [mFe-nS] clusters, the Fe 3ϩ metal centers are typically in a local high-spin d 5 configuration [i.e., S Fe ϭ 5/2 for ferric Fe(III) centers], whereas they themselves are antiferromagnetically coupled to yield an overall low-spin state due to dominant superexchange interactions (6). In particular, the electronic ground state of [2Fe-2S] clusters in Fd has genuine multireference character and makes computational ...

“…Physicochemical data summarized elsewhere (1,2,8) are concordant with formulation of the reduced proteins in terms of the mixed valence entity 5. The coordination sites in this oxidation level are apparently sufficiently differentiated by the protein environment that electron transfer between them is slow compared with the time scales of the spectroscopic methods used.…”

mentioning

confidence: 61%

“…The active-site structures of the 2Fe-2S* proteins, which appear to be the most widespread of the three types and are exemplified by spinach Fd, adrenodoxin, and putidaredoxin, have not been unequivocally defined by x-ray methods. However, a large body of spectroscopic and magnetic data is entirely consistent with the essential formulation 3 (1,2,(8)(9)(10), which as 1 and 2 contains tetrahedrally coordinated iron. The oxidized forms of these proteins are characterized by the presence of two antiferromagnetically coupled high-spin Fe(III) Abbreviations: HP, high potential iron protein; Fd, ferredoxin.…”

mentioning

confidence: 90%

Synthetic Analogs of the Active Sites of Iron-Sulfur Proteins. Structure and Properties of Bis[ o -xylyldithiolato-μ ₂ -sulfidoferrate(III)], an Analog of the 2Fe-2S Proteins

Mayerle

Frankel

Holm

et al. 1973

The synthetic analog approach has been applied to a clarification of the active sites of 2Fe-2S* proteins. The compound (Et4N)2[FeS(SCH2)2CJ4LJ2, derived from o-xylyl-a,a'-dithiol, has been prepared and its structure has been determined by x-ray diffraction. The centrosymmetric anion contains two tetrahedrally coordinated ferric ions bridged by two sulfide ions and separated by 2.70 A. Comparison of electronic, Mossbauer, and proton magnetic resonance spectra and magnetic susceptibility of the anion with the corresponding properties of the oxidized forms of the proteins reveals significant degrees of similarity. The anion also exhibits the essential redox capacity of the proteins. We conclude that [FeS(SCH2)2CJL1H22-possesses the same total oxidation level and electronic configuration as the active sites of the oxidized proteins, and that its structure provides a feasible representation of the minimal structure of the active site. [FeS(SCH2)2CIL]2J' is thus the first well-defined synthetic analog of the active sites of two-iron ferredoxins.