Fe ferredoxins

Biotin synthase (BioB) is a Fe S protein that catalyses a presumed radical-mediated insertion of a sulfur atom between the saturated C-6 and C-9 carbon atoms of dethiobiotin. Ugulava et aV have measured equilibrium reduction potentials and monitored cluster conversions by UV/vis, and by X-band EPR spectroscopy between 5 and 50 K. [Fe2S2] and [Fe4S4] clusters and free ferric iron were detected. Overall the authors suggested that the dominant stable cluster state for BioB was a dimer containing two [Fe2S2] and two [Fc4S4] + clusters. 

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If the rate constants for parallel reactions are to be resolved, then analysis of the products is essential (Sec. 1.4.2). This is vital for understanding, for example, the various modes of deactivation of the excited state (Sec. 1.4.2), Only careful analysis of the products of the reactions of Co(NH3)jH20 + with SCN, at various times after initiation, has allowed the full characterization of the reaction (1.95) and the detection of linkage isomers. Kinetic analysis by a number of groups failed to show other than a single second-order reaction.As a third instance, the oxidation of 8-Fe ferredoxin with Fe(CN)g produces a 3Fe-cluster, thus casting some doubt on the reaction being a simple electron transfer. 

The [4 Fe-4 S] cores have been one of the most intriguing inorganic structures involved in biological systems. Carter et al. (1977) 191 demonstrated that the same basic structure is present in the two [4 Fe—4 S] centers of the 8 Fe ferredoxin of Peptococcus aerogenes (E 0 = - 400 mV)2) and in the high potential iron protein (HiPIP) isolated from the purple photosynthetic bacterium Chromatium vinosum... 

The structures of the active sites of 4-Fe and 8-Fe Ferredoxins has been univocally established by X-ray diffraction methods. Proteins of the 4-Fe type contain the [Fe4S4(S-Cys)4] site with a cubane-like structure as that illustrated schematically in Fig. 5.2. The structure of HP from Chromatium vinosum has been determined for two oxidation levels, HP ed and HPox- In both cases the active sites are tetranuclear clusters as shown in Fig. 5.2. Characteristic structural parameters for such centers can be observed in Fig. 5.3 where they are reproduced beside those of some synthetic analogues. As illustrated in Fig. 5.4, in the 8-Fe Fdox from Peptococcus aerogenes there are-two identical tetranuclear active centers which are separated by about 12 A. They are dimensionally very similar to the site in HP ed-... 

The equivalence of the oxidation state in both protein active sites and tetranuclear synthetic analogues is also clearly established by comparing the isomer shifts Fe observed in the corresponding Mossbauer spectra. Isomer shifts may be indeed considered to be a measure of the electron density in vicinity of the iron atom in a given species relative to that in a reference compound. For iron compounds with equivalent coordination spheres, the isomer shifts correlate with iron formal oxidation states. Isomers shifts observed for both proteins and model compounds are consistent with the hypothesis that synthetic species [Fe4S4(SR)4] are isoelectronic with the active sites in the 4Fe and 8-Fe-Ferredoxins and HP protein. Mossbauer spectra confirm moreover that all Fe-atoms in Fe4S4 units in synthetic and natural species are strictly equivalent in the time-scale of this technique (ca. 10 s) so they are, as mentioned above, electronically delocalized systems. 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

In this work we examine the low energy UV-visible absorption spectrum of the [Fe2 ft - S2) P o- 61148) )2] complex, Figure 1, whose synthesis, structure, and properties have recently been reported. The complex contains a [Fe — S — S - Fe] core and is a structural isomer of the 2-Fe [Fe — ill — 8)2 — Fe ferredoxin. The electronic structure of the disulfide complex is, however, unknown, and can be associated with either an antifer-romagnetically (AF) coupled [Fe d ) - - Fe d )] system, or with a... 

The bacterial-type iron-sulfur proteins all contain larger amounts of iron and labile sulfide than the plant-type iron-sulfur proteins best estimates for the iron and labile sulfide content being 8 Fe and 8 S per protein molecule (172, 173) for these ferredoxins from Clostridium and from Chromatium. Although these proteins have large amounts of Fe and S, the molecular weights are less than the molecular weights of the... 

The 3Fe center was first recognized " in the protein ferredoxin I from the anaerobic nitrogen-fixing h ct uava Azotobacter vinelandii. The protein is called Av Fdl for short. It is instructive to sketch historically the evolution of our understanding of this protein. Av ferredoxin I was reported to have 6 to 8 Fe... 

Ferredoxins, Fat low-molecular- mass iron-sulfur proteins which transfer electrons from one enzyme system to another, without possessing any enzyme activity themselves. The name was coined by Mortenson for iron-containing proteins from Clostridium pasteu-ranum. The 8-Fe-Fd take part in many election transport processes in organisms like Clostridia and photosynthetic bacteria (Table). The primary structures of many of these proteins have been determined. They consist of about SS amino adds, including 8 cysteines, which occupy the same positions in each Fd. The molecule contains two identical 4Fe-4S clusters, each one forming a cube and covalently bonded to 4 cysteine residues in the peptide chain. Each 4Fe-4S center can transfer one electron. 

The mechanism of electron transfer reactions in metal complexes has been elucidated by Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, cytochrome (Fe /Fe ), ferredoxin (Fe /Fe ), nicotine-adenine-dinucleotide (NAD /NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

In a typical nitrogenase C pasteurianum , both proteins, MoFe-protein and Fe-protein, share one [Fe-4S] cluster of the ferredoxin type. Later on, it was shown that all molybdenum, and partly sulfur, can be isolated as an insoluble substance of the relative molecular mass from 1000 to 1500. This substance was named the iron-molybdenum cofactor (FeMoco) it contains 2Mo, 6 to 8 Fe and 6 or more S atoms. 

Figure 8.39 Fourier transformed Fe extended X-ray absorption fine structure (EXAFS) and retransformation, after applying a 0.9-3.5 A filter window, of (a) a rubredoxin, (b) a plant ferredoxin and (c) a bacterial ferredoxin, whose structures are also shown. (Reproduced, with permission, Ifom Teo, B. K. and Joy, D. C. (Eds), EXAFS Spectroscopy, p. 15, Plenum, New York, 1981)...

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

V = -0.431 V. Thus, the value of E1/2 changes from -0.371 to -0.460 V as the pH is increased. In the pH range between the pKa values of 7.4 and 8.9 reduction of the protein will lead to binding of a proton from the medium and oxidation to loss of a proton. Human and other vertebrate ferredoxins also show pH-depen-dent redox potentials.282 This suggests, as with the cytochromes, a possible role of Fe-S centers in the operation of proton pumps in membranes. Nevertheless, many ferredoxins, such as that of C. pasteurianum, show a constant value of E0 from pH 6.3 to 10296 and appear to be purely electron carriers. 

Fig. 8. Mossbauer spectra of oxidized plant-type iron-sulfur proteins in high applied magnetic field. Abbreviations Ad. = Pig Adrenodoxin, 4.2 °K, 46 kG PPNR = Spinach Ferredoxin, 4.5 °K, 50 kG Clos. = Clostridial Paramagnetic Protein, 4.2 °K, 46 kG AZI = Azotobacter Fe-S Protein I, 4.6°K, 46 kG AZII = Azotobacter Fe-S Protein II, 4.2 °K, 46 kG. Applied magnetic field is parallel to gamma-ray direction...

Figure 1. The Nitrogenase Reaction. The electron transfer proteins ferredoxin (Fd) and flavodoxin (Fid) serve to couple the nitrogenase reaction to metabolically generated reducing equivalents. Ammonia synthesis requires 8 electrons 6 for the reduction of dinitrogen and 2 for the coupled, obligatory synthesis of H2. These reactions are catalyzed by the terminal component in the complex, the MoFe-protein. The electrons are transferred to the MoFe-protein from the Fe-protein in a process coupled to the hydrolysis of 2ATP/electron (Howard and Rees, 1994,1996).

The [Fc4S4] cluster, (5), appears in ferredoxins, in high-potential ferredoxin proteins (HP or HiPiP), and in a variety of enzymes. As shown in Table 8, the cluster itself is of mixed-valency (Fe and Fe°) for all forms except the most reduced, [FC4S4] . 

The iron-sulphur centre is probably of the 2Fe-2S type [191,223]. It is a one-electron donor/acceptor with of approx. 280 mV in mitochondria (pH independent below pH 8 [224,225]). It exhibits an EPR spectrum in the reduced state that is somewhat anomalous for 2Fe-2S clusters (see Ref. 221). This, as well as the high midpoint redox potential, suggest that the iron ligands may be less electronegative than the four cysteine sulphurs of the plant ferredoxin model (see Ref. 226). The EPR spectrum of the FeS cluster is affected by the redox state of ubiquinone... 

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