Ferredoxins iron-sulfur center

Fig. 6. Representative EPR spectra displayed by trinuclear and tetranucleEir iron-sulfur centers, (a) and (b) [3Fe-4S] + center in the NarH subunit of Escherichia coli nitrate reductase and the Ni-Fe hydrogenase fromD. gigas, respectively, (c) [4Fe-4S] + center in D. desulfuricans Norway ferredoxin I. (d) [4Fe-4S] center in Thiobacillus ferrooxidans ferredoxin. Experimental conditions temperature, 15 K microwave frequency, 9.330 GHz microwave power, (a) 100 mW, (b) 0.04 mW, (c) smd (d) 0.5 mW modulation amplitude (a), (c), (d) 0.5 mT, (b) 0.1 mT.

Fe-4S] + + clusters are certainly the most ubiquitous iron-sulfur centers in biological systems. They play the role of low potential redox centers in ferredoxins, membrane-bound complexes of the respiratory... 

The conversion of a [3Fe-4S] into a [4Fe-4S] center was achieved by restoring the second residue of the consensus motif in E. coli fu-marate reductase (181) and in D. africanus ferredoxin III (161). However, the coordination scheme of the iron-sulfur centers of A. vinelan-... 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

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. 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

Xanthine oxidase catalyzes the oxidation of hypox-anthine and xanthine to uric acid. Xanthine oxidase is a complex metalloflavoprotein containing one molybdenum, one FAD and two iron-sulfur centers of the ferredoxine type in each of its two independent subunits. Usually, the enzyme is isolated from cow s milk. The enzyme is inhibited by allopurinol and related compounds. The production of uric acid from the substrate (xanthine) can be determined by measuring the change in optical density in the UV range. 

Azotobactor vinelandii ferredoxin I has two Fe/S clusters, [4Fe-4S] and a [3Fe-xS], with redox potentials at —0.42 and +0.32 V versus NHE, respectively (36). The existence of a normal Fe4S42+ core and a [3Fe-3S](S-cys)s(oxo) core has been considered. In the ferredoxin I the unique sequence Cys-Val-Glu-Val-Cys has been suggested as a tridentate ligand for the [3Fe-3S] cluster (37). Recently, the structure has been crystallographically (38) corrected, and the iron-sulfur centers are now believed to consist of Fe4S42+ and [3Fe-4S] clusters, as proposed by EXAFS (39). One possibility is that the above-mentioned [3Fe jcS] structure is formed during isolation by oxidative removal of an Fe ion from the [4Fe-4S] cluster (40). 

As indicated in Sections 1 and 2, succinate is an electron donor widely utilized for NAD(P) reduction by phototrophic purple bacteria. The membrane-bound enzyme responsible for succinate oxidation has been solubilized and partially characterized in the purple non-sulfur bacteria R. rubrum [73,74] and Rhodopseudo-monas sphaeroides (recently renamed Rhodobacter sphaeroides) [57]. In situ characterization of the iron-sulfur centers likely to be associated with succinate dehydrogenase has been accomplished for Rps. capsulata [59] and C. vinosum [51]. Of particular interest is the presence of a succinate-reducible [51,57,58,73] and fu-marate-oxidizable [51] iron-sulfur cluster with near +50 mV that, like center S-3 [60,61,75,76] of mitochondrial succinic dehydrogenase (Complex II), is paramagnetic in the oxidized state. The enzyme in phototrophic bacteria also appears to have one or two ferredoxin-like (i.e., paramagnetic in the reduced state) iron-sulfur centers that correspond to centers S-1 (succinate-reducible, EJ ranging from... 

TheP7007P700 couple has a redox potential of+0.45 V [c/. redox-potential scale in Fig. 2]. The of the Aq/Aq" couple is probably less than -1 V if it is consistent with the in vitro redox-potential value of Chl/Chl of < -1.0 V. The initial charge separation into P700 and Ao would store approximately 1.5 eV out of 1.8 eV of energy of the absorbed 700-nm photon. The redox potential ofthe A,/A," couple is probably -0.8 V. The redox potentials ofthe iron-sulfur centers FeS-X, FeS-B and FeS-A have been experimentally determined to be -0.73, -0.58 and -0.53 V , respectively. The final electron acceptor in photosystem 1 is the [2Fe-2S]-type ferredoxin (Fd) present in the stroma region of the chloroplasts and having a redox potential of -0.4 V. Under iron-deficient growth conditions, a flavoprotein called flavodoxin is synthesized as a replacement acceptor for ferredoxin. 

The membrane-bound iron-sulfiir centers were discovered by Dick Malkin and Alan Bearden in 1971 in spinach chloroplasts using EPR spectroscopy. Since the EPR spectrum was found to resemble that of the iron-sulfur protein ferredoxin and since the soluble ferredoxin had already been removed from the chloroplast sample used in the measurement, the substance represented by the newly found EPR spectrum was initially called membrane-bound ferredoxin. And since the iron-sulfur center was also found to be photo-reducible at cryogenic temperature, it was therefore suggested that it was the primary electron acceptor of photosystem I. 

The EPR results for thePsaC Cysl4->Asp andPsaC Cys-51->Asp mutant proteins reconstituted with the PS-I core complex, combined with knowledge available on cysteine coordination patterns in bacterial ferredoxins containing two [4Fe 4S] clusters, as discussed below, permitted Zhao et a/. to conclude that cysteine coordination to the two iron-sulfur centers in PsaC assumes the same pattern as in bacterial ferredoxin. 

The initial 6 A-resolution X-ray work showed one of the [4Fe 4S] clusters is 15 A and the other 22 A from the preceding iron-sulfur electron acceptor, FeS-X. Discussion of the iron-sulfur center FeS-X itself will be deferred to Chapter 31, where the interaction of the FeS-X domain with PsaC (FeS-A/FeS-B) and its involvement in electron transfer to FeS-A/B will also be addressed. More recent work by the Berlin group at 4 A resolution has refined the distance measurements to 15.4 and 22.2 A, respectively. Fig. 12 (B) shows a three-dimensional model of PsaC based on low-temperature EPR measurements by Kamlowski and coworkers on PS-I single crystals. Their interpretation was made by using the bacterial ferredoxin model as a PsaC substitute in the PS-I reaction center matrix, and adjusting its orientation for the best agreement with the EPR data. The g-tensor orientation of EeS-A and FeS-B in the PS-I single crystals determines the PsaC orientation relative to the two clusters. 

IR Vassiliev Y-S Jung Pi ang and JH Golbeck (1998) PsaC subunit of photosystem I is oriented with iron-sulfur dusterpB as the immediate electron donor to ferredoxin and flavodoxin. Biophys J 74 2029-2035 Y-S Jung L Yu and JH Golbeck (1995) Reconstitution of iron-sulfur center Fb results in complete restoration of NADP photoreduction in Hg-treated photosystem I complexes from Synechococcus sp PCC 6301. Photosynthesis Res 46 249-255... 

J Rawlings, 0 Siiman and HB Gray (1974) Low temperature electronic absorption spectra of oxidized and reduced spinach ferredoxins. Evidence for nonequivalent iron (III) sites. Proc Nat Acad Sci, USA 71 125-127 K Brettel (1988) Electron transfer from Af to an iron-sulfur center with t% = 200 ns at room temperature in photosystem I. FEBS Lett 239 93-98. 

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