Fe.S centers

Three protein complexes have been isolated, including the flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP). FP contains three peptides (of mass 51, 24, and 10 kD) and bound FMN and has 2 Fe-S centers (a 2Fe-2S center and a 4Fe-4S center). IP contains six peptides and at least 3 Fe-S centers. HP contains at least seven peptides and one Fe-S center. 

Complex II is perhaps better known by its other name—succinate dehydrogenase, the only TCA cycle enzyme that is an integral membrane protein in the inner mitochondrial membrane. This enzyme has a mass of approximately 100 to 140 kD and is composed of four subunits two Fe-S proteins of masses 70 kD and 27 kD, and two other peptides of masses 15 kD and 13 kD. Also known as flavoprotein 2 (FP2), it contains an FAD covalently bound to a histidine residue (see Figure 20.15), and three Fe-S centers a 4Fe-4S cluster, a 3Fe-4S cluster, and a 2Fe-2S cluster. When succinate is converted to fumarate in the TCA cycle, concomitant reduction of bound FAD to FADHg occurs in succinate dehydrogenase. This FADHg transfers its electrons immediately to Fe-S centers, which pass them on to UQ. Electron flow from succinate to UQ,... 

FIGURE 21.8 A probable scheme for electron flow in Complex II. Oxidation of succinate occurs with rednction of [FAD]. Electrons are then passed to Fe-S centers and then to coenzyme Q (UQ). Proton transport does not occur in this complex. 

FIGURE 22.20 The molecular architecture of PSI. PsaA and PsaB constitute the reaction center dimer, an integral membrane complex P700 is located at the lumenal side of this dimer. PsaC, which bears Fe-S centers and Fb, and PsaD, the interaction site for ferre-doxin, are on the stromal side of the thylakoid membrane. PsaF, which provides the plasto-cyaiiin interaction site, is on the lumenal side. (Adapted from Golbeck, J. H., 1992. Amiual Review of Plant Physiology and. Plant Molecular Biology 43 293-324.)... 

Complex III 280 kDa 11 28 type hemes (b and bg) bound to same mitochondrially coded peptide 1 C heme (cytochrome c,) 1 Fe-S center Rieske factor Spans membrane, cytochrome b, and b in membrane, cytochrome c, and Fe-S center on outer face 0.25-0.53 Pumps protons out of matrix during electron transport/2e"... 

A second unusual EPR spectrum was observed in the oxidized (as-isolated) protein (Fig. 3). This spectrum, which was assigned to an S = z system, was not reminiscent of any Fe-S cluster. Indeed, with g-values of 1.968, 1.953, and 1.903, it looked more like a molybdenum or tungsten spectrum. However, chemical analysis ruled out the possibility that this EPR spectrum arose from Mo or W, and the spectrum was assigned to an Fe-S center instead. The spin concentration, however, was sub stoichiometric and sample-dependent. Furthermore, when the as-isolated protein was oxidized with ferricyanide, it became EPR silent. This, together with the iron determination and the fingerprint of the reduced protein, led Hagen and colleagues to the... 

Fig. 10. The structure ofZJ. gigas Eildehyde oxidoreductase (AOR) monomer, showing the Mo-MCD site, the two [2Fe-2S] centers, and the tracing of the pol3ipeptide chain. The Fe-S center most exposed is included in a protein domain whose folding is quite similar to the one found in plant-type ferredoxins (199).

Mo(V) paramagnetic species is also an argument to exclude an interaction between the Mo site and Fe-S center I. These studies were further complemented by detailed study of the observable splitting and its temperature dependence, EPR saturation, and the effect of differential reduction of the Fe-S centers. A magnetic interaction was also seen in xanthine oxidase, between various Mo(V) EPR species and one of the Fe-S centers. A study on the... 

The oxidoreductase from Pseudomonas diminuta strain 7 that carries out hydroxylation of isoquinoline at C2 is a molybdenum enzyme containing [Fe-S] centers, which is comparable to the aldehyde oxidoreductase from Desulfovibrio gigas (Lehmann et al. 1994). 

The methyl gronp is snbseqnently transferred to a tetrahydropterin and coenzyme M. The p-snbnnit contains Ni and an Fe/S center, and an NijKFe-dS] arrangement at the active site has been proposed (Gencic and Grahame 2003). 

For example, Fig. 9.40 shows the NIS spectra of the oxidized and reduced FeS4 centers of a rubredoxin mutant from Pyrococcus abyssi obtained at 25 K together with DFT simulations using different models for the Fe-S center [103]. The spectrum from the oxidized protein Fe S4 (S = 5/2) reveals broad bands around 15-25 meV (121-202 cm ) and 42-48 meV (339-387 cm ) consistent with the results on rubredoxin from Pyrococcus furiosus [104]. 

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. 

Fe-hydrogenase, which usually functions in the direction of hydrogen evolution, has been known for over 30 years. This enzyme contains a highly reactive complex Fe-S center in which one of the Fe atoms is complexed with CO and CN [5], The Fe hydrogenases have extremely high turnover numbers 6,000 s 1 for C. pasteuriamm and 9,000 s 1 for Desulfovibrio spp. Note that this is a thousand times faster than the turnover number of nitrogenase ... 

Xanthine oxidase (XO) was the first enzyme studied from the family of enzymes now known as the molybdenum hydroxylases (HiUe 1999). XO, which catalyzes the hydroxylation of xanthine to uric acid is abundant in cow s milk and contains several cofactors, including FAD, two Fe-S centers, and a molybdenum cofactor, all of which are required for activity (Massey and Harris 1997). Purified XO has been shown to use xanthine, hypoxan-thine, and several aldehydes as substrates in the reduction of methylene blue (Booth 1938), used as an electron acceptor. Early studies also noted that cyanide was inhibitory but could only inactivate XO during preincubation, not during the reaction with xanthine (Dixon 1927). The target of cyanide inactivation was identified to be a labile sulfur atom, termed the cyanolyzable sulfur (Wahl and Rajagopalan 1982), which is also required for enzyme activity. 

Four distinct types of Fe-S center have now been found in proteins, ranging from mono- to tetranuclear in addition, a novel Mo-Fe-S cluster is present in the enzyme nitrogenase. Synthetic analogs of most of these have been prepared and used to provide insight into the intrinsic properties of the metal-sulfur centers in the absence of protein-imposed constraints. The strategies used to prepare both Fe-S and Mo-Fe-S clusters are described they range from spontaneous self-assembly to the designed synthesis of clusters with specific structural features. 

Figure 1. Schematic views of the structurally characterized Fe-S centers found in proteins to date. Core oxidation states are indicated by superscripts underlines indicate those structures and oxidation states for which structurally characterized synthetic analogs are available.

Efforts toward developing synthetic models for the Rieske Fe-S centers focussed initially on preparing Fe2S2 cores with non-thlolate ligands, and have centered on nitrogenous ligands since the realization of their probable occurrence in the Rieske protein. In addition to the [Fe2S2(0Ar) ] " ions (Ar = aryl)... 

Figure 9. A schematic view of the probable structure of the Rieske Fe-S center (based on the data in ref. 47.)...

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. 

A brief historical note on the structure of the iron-sulfur clusters in ferredoxins is relevant. After the first analytical results revealed the presence of (nearly) equimolar iron and acid-labile sulfur, it was clear that the metal center in ferredoxins did not resemble any previously characterized cofactor type. The early proposals for the Fe S center structure were based on a linear chain of iron atoms coordinated by bridging cysteines and inorganic sulfur (Blomstrom et al., 1964 Rabino-witz, 1971). While the later crystallographic analyses of HiPIP, PaFd, and model compounds (Herskovitz et al., 1972) demonstrated the cubane-type structure of the 4Fe 4S cluster, the original proposals have turned out to be somewhat prophetic. Linear chains of sulfide-linked irons are observed in 2Fe 2S ferredoxins and in the high-pH form of aconitase. Cysteines linked to several metal atoms are present in metallothionein. The chemistry of iron-sulfur clusters is rich and varied, and undoubtedly many other surprises await in the future. 

This field lists all metals or ions that have activating effects. The commentary explains the role each of the cited metal has, being either bound e.g. as Fe-S centers or being required in solution. If an ion plays a dual role, activating at a certain concentration but inhibiting at a higher or lower concentration, this will be given in the commentary. 

FIGURE 19-5 Iron-sulfur centers. The Fe-S centers of iron-sulfur proteins may be as simple as (a), with a single Fe ion surrounded by the S atoms of four Cys residues. Other centers include both inorganic and Cys S atoms, as in (b) 2Fe-2S or (c) 4Fe-4S centers, (d) The ferredoxin of the cyanobacterium Anabaena 7120 has one 2Fe-2S center (PDB ID 1 FRD) Fe is red, inorganic S2 is yellow, and the S of Cys is orange. (Note that in these designations only the inorganic S atoms are counted. For example, in the 2Fe-2S center (b), each Fe ion is actually surrounded by four S atoms.) The exact standard reduction potential of the iron in these centers depends on the type of center and its interaction with the associated protein. 

FIGURE 19-9 IMADH ubiquinone oxidoreductase (Complex I). Complex I catalyzes the transfer of a hydride ion from NADH to FMN, from which two electrons pass through a series of Fe-S centers to the iron-sulfur protein N-2 in the matrix arm of the complex. Electron transfer from N-2 to ubiquinone on the membrane arm forms QH2, which diffuses into the lipid bilayer. This electron transfer also drives the expulsion from the matrix of four protons per pair of electrons. The detailed mechanism that couples electron and proton transfer in Complex I is not yet known, but probably involves a Q cycle similar to that in Complex III in which QH2 participates twice per electron pair (see Fig. 19-12). Proton flux produces an electrochemical potential across the inner mitochondrial membrane (N side negative, P side positive), which conserves some of the energy released by the electron-transfer reactions. This electrochemical potential drives ATP synthesis. 

Amytal (a barbiturate drug), rotenone (a plant product commonly used as an insecticide), and piericidin A (an antibiotic) inhibit electron flow from the Fe-S centers of Complex I to ubiquinone (Table 19-4) and therefore block the overall process of oxidative phosphorylation. 

Myxothiazol Rotenone Amytal Prevent electron transfer from Fe-S center to ubiquinone... 

Complex III crystallizes in two distinct conformations (not shown). In one, the Rieske Fe-S center is close to its electron acceptor, the heme of cytochrome c, but relatively distant from cytochrome b and the QH2-binding site at which the Rieske Fe-S center receives electrons. In the other, the Fe-S center has moved away from cytochrome c, and toward cytochrome b. The Rieske protein is thought to oscillate between these two conformations as it is reduced, then oxidized. 

QH2 donates one electron (via the Rieske Fe-S center) to cytochrome c, and one electron (via cytochrome b) to a molecule of Q near the n side, reducing it in two steps to QH2. This reduction also uses two protons per Q, which are taken up from the matrix. 

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