Ferredoxins Fe2S2

Figure 25.9 Some non-haem iron proteins (a) rubredoxin in which the single Fe is coordinated, almost tetra-hedrally, to 4 cysteine-sulfurs, (b) plant ferredoxin, [Fe2S2(S-Cys)4], (c) [Fe4S4(S-Cys)4] cube of bacterial ferredoxins. (This is in fact distorted, the Fe4 and S4 making up the two interpenetrating tetrahedra, of which the latter is larger than the former).

EPR spectra of various Fe-S proteins (A) oxidized Desulfovibrio gigas rubredoxin (B) reduced spinach ferredoxin [Fe2S2] (C) reduced Bacillus stearothermophilus ferredoxin [Fe4S4] (D) oxidized Thermus aquaticus ferredoxin [Fe3S4] ... 

Fig. 2. H NMR spectra of (A) oxidized spinach Fe2S2 ferredoxin (33) (B) reduced spinach Fe2S2 ferredoxin (5f) (C) oxidized Desulfovibrio gigas Fe3S4 ferredoxin (138) (D) oxidized ectothiorhodospira halophila HiPIP iso-II (23) (E) reduced Chromatium vinosum HiPIP (14) (F) fully reduced Clostridium pasteurianum 2(Fe4S4) ferredoxin (139). Chemical shift values are in ppm.

Fig. 4. Top Theoretical temperature dependence of the hyperfine shift of the H/3 protons of reduced spinach [Fe2S2] ferredoxin 151). The solid line corresponds to the situation where only one species exists in solution, whereas the dashed line corresponds to a situation where there is fast equilibrium between two species (in a 20/80 ratio) differing for the location of the extra electron 151). Bottom.-. Experimental temperature dependence of the H NMR shifts. The signals are labeled as in Fig. 2B.

Crouse BR, Meyer J, Johnson MJ. 1995. Spectroscopic evidence for a reduced Fe2S2 cluster with a S = 9/2 ground state in mutant forms of Clostridium pasteurianum 2Fe ferredoxin. J Am Chem Soc 117 9612-13. 

Han S, Czernuszewicz RS, Kimura T, et al. 1989a. Fe2S2 protein resonance Raman revisited structural variations among adrenodoxin, ferredoxin, and red paramagnetic protein. J Am Chem Soc 111 3505-11. 

Oxidized Fe2S2 ferredoxins, containing two equivalent iron atoms, with J = 400 cm , show sharper NMR lines with respect to the monomeric iron model provided by oxidized rubredoxin (107-109), due to the decreased Boltzmann population of the paramagnetic excited states. For reduced ferredoxins (Si = 5/2, S2 = 2), with J = 200 cm , the ground state is paramagnetic (S = 1/2) (110). A smaller decrease in linewidth is expected. However, the fast electron relaxation rates of the iron(II) ion cause both ions to relax faster, and the linewidths in the dimer are sharp. 

The literature on double exchange is not very extensive. The main papers dealing with the problem discussed here are the original work by Anderson and Hasegawa (10), and papers by Karpenko (33), Borshch et al. (30), Belinskii et al. (34) and Girend (31). Particularly noteworthy is the analysis by Noodleman and Baerends (35) in their theoretical study of the electronic structure of ferredoxins with Fe2S2 cores. Quite recently, Belinskii (32) has published an extensive theoretical study for mixed-valence trimers with D3h symmetry. 

Polynuclear Fe-M-S Complexes from "spontaneous self assembly" reactions. Synthetic analog clusters for the Fe2S2 and Fe4S4 centers in the Fe/S proteins (ferredoxins) have been obtained by procedures that are based on the concept of "spontaneous self assembly". The latter (30) assumes that the cores of the Fe/S centers are thermodynamically stable units that should be accessible fiom appropriate reagents even in the absence of a protein environment. 

It has been pointed out 33) that synthesis of [Fe2S2(Ss)2] from [Fe(SPh)4] and dibenzyl trisulfide would have implications regarding the enzymatic biosynthesis of the metal clusters in Fe2 ferredoxins, since trisulfides seem to be present in biological systems. 

The cluster in the ferredoxin molecule associated with photosynthesis in higher plants is thought to have the bridged structure Fe2S2 as shown in figure. It is known as photo-synthetic ferredoxin. 

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

A Fe2S2 ferredoxin from the hypertherm ophilic bacterium Aquifex aeolicus, expressed in E. coli, studied by Meyer et al. was shown not to be closely similar to plant or mammalian Fe2S2ferrredoxins.83 The combined EPR, MCD, resonance Raman and Mossbauer study identified an S = state from dithionite reduction with ghvalues of 2.05,1.96 and 1.88. 

Figure 16-16 (A) Superimposed stereoscopic a-carbon traces of the peptide chain of rubredoxin from Clostridium pasteurianum with either Fe3+ (solid circles) or Zn2+ (open circles) bound by four cysteine side chains. From Dauter et al.27i (B) Alpha-carbon trace for ferredoxin from Clostridium, acidurici. The two Fe4S4 clusters attached to eight cysteine side chains are also shown. The open circles are water molecules. Based on a high-resolution X-ray structure by Duee et al.267 Courtesy of E. D. Duee. (C) Polypeptide chain of a chloroplast-type ferredoxin from the cyanobacterium Spirulina platensis. The Fe2S2 cluster is visible at the top of the molecule. From Fukuyama et al.276 Courtesy of K. Fukuyama.

Figure 16-18 Mossbauer X-ray absorption spectra of iron-sulfur clusters. (See Chapter 23 for a brief description of the method.) Quadrupole doublets are indicated by brackets and isomer shifts are marked by triangles. (A) [Fe2S2]1+ cluster of the Rieske protein from Pseudomonas mendocina, at temperature T = 200 K. (B) [Fe3S4]1+ state of D. gigas ferre-doxin II, T = 90 K. (C) [Fe3S4]° state of D. gigas ferredoxin II, T = 15 K. (D) [Fe4S4]2+ cluster of E. coli FNR protein, T = 4.2 K. (E) [Fe4S4]1+ cluster of E. coli sulfite reductase, T = 110 K. From Beinert et al.260...

Figure 16-31 (A) Structure of molybdopterin cytosine dinucleotide complexed with an atom of molybdenum. (B) Stereoscopic ribbon drawing of the structure of one subunit of the xanthine oxidase-related aldehyde oxidoreductase from Desulfo-vibrio gigas. Each 907-residue subunit of the homodimeric protein contains two Fe2S2 clusters visible at the top and the molybdenum-molybdopterin coenzyme buried in the center. (C) Alpha-carbon plot of portions of the protein surrounding the molybdenum-molybdopterin cytosine dinucleotide and (at the top) the two plant-ferredoxin-like Fe2S2 clusters. Each of these is held by a separate structural domain of the protein. Two additional domains bind the molybdopterin coenzyme and there is also an intermediate connecting domain. In xanthine oxidase the latter presumably has the FAD binding site which is lacking in the D. gigas enzyme. From Romao et al.633 Courtesy of R. Huber.

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

The soluble electron carriers released from the reaction centers into the cytoplasm of bacteria or into the stroma of chloroplasts are reduced single-electron carriers. Bacterial ferredoxin with two Fe4S4 clusters is formed by bacteria if enough iron is present. In its absence flavodoxin (Chapter 15), which may carry either one or two electrons, is used. In chloroplasts the carrier is the soluble chloroplast ferredoxin (Fig. 16-16,C), which contains one Fe2S2 center. Reduced ferredoxin transfers electrons to NADP+ (Eq. 15-28) via ferredoxin NADP oxidoreductase, a flavoprotein of known three-dimensional structure.367 369... 

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