Ferredoxin chloroplast

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

The 2Fe2S (S, acid-labile sulfur) ferredoxins have a redox active binuclear center, with each of the two iron atoms attached to the protein by two cysteinyl sulfur ligands and connected by two inorganic acid-labile sulfur ligands. At cty-ogenic temperatures these clusters are EPR detectable, with characteristic features in the vicinity of g = 1.94. Spinach ferredoxin has principal g values of 2.03, 1.96, and 1.88 and a broad absorbance spectrum with a weak maximum around 420 nm, giving these proteins a reddish brown color which bleaches on reduction. Ferredoxins are low potential electron carriers chloroplast ferredoxins function in photosynthetic electron transfer, but related proteins such as adrenal ferredoxin are involved in steroidogenic electron transfer in mitochondria in tissues which produce steroid hormones. 

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... 

Smillie, R. M. Isolation of two proteins with chloroplast ferredoxin activity from a blue-green alga. Biochem. Biophys. Res. Commun. 20, 621—629 (1965). 

Plant "Chloroplast ferredoxin or "PPNR (green plants, algae,... 

Packer, L. and Cullingford, W. 1977. Stoichiometry of H2 production by an in vitro chloroplast, ferredoxin, hydrogenase reconstituted system. Z. Naturforsch. 33c, 113-115. 

The [Fe2S2] + core, (2) (Figure 2), appears in plant-type (chloroplast) ferredoxins (Fd) and in Rieske proteins. In ferredoxins, the [Fe2S2] + core is ligated with thiolate from Cys residues, while in Rieske proteins, the thiolates ligating one of the iron atoms are substituted with imidazole ligands from His residues. 

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

Chloroplast ferredoxin containing the [(2Fe-2S)-(S-Cys)4] cluster is one common type of iron-sulfur protein. Another [2Fe-2S]-type protein is the Rieske iron-sulfur protein, present in the Cyt >6/complex as well as the Cyt Ac, complex. The pair of iron atoms in the cluster ofthe Rieske iron-sulfur protein are bound to two cysteine and two histidine residues, in addition to two sulfur atoms. The three-dimensional structures of ferredoxins and that of the Rieske iron-sulfur protein have been determined by X-ray crystallography (see Chapters 34 and 35, respectively, for the structure ofthe chloroplast ferredoxin and the Rieske iron-sulfur protein). The sulfide ions in iron-sulfur proteins urt acid-labile this provides a simple means for detecting the iron-sulfur proteins, as the sulfide is released as H2S upon acidification. The oxidized and reduced states of iron-sulfur clusters differ by just one unit of formal charge, corresponding to and Fe. Iron-sulfurproteins are commonly characterized by optical absorption, circular-dichro-... 

R Malkin and AJ Bearden (1971) Primary reactions of photosynthesis photoreduction of a bound chloroplast ferredoxin at low temperature as detected by EPR spectroscopy. Proc Nat Acad Sci USA 68 16-19... 

Rodriguez, R.E., Lodeyro, A., Poli, H.O., Zurbriggen, M., Peisker, M., Palatnik, J.R, Tognetti, V.B., Tschiersch, H., Hajirezaei, M.R., Valle, E.M., and Carrillo, N. (2007). Transgenic tobacco plants overexpressing chloroplastic ferredoxin-NADP(H) reductase display normal rates of photosynthesis and increased tolerance to oxidative stress. Plant Physiol. 143, 639-649. 

Most of the proteins listed have been isolated from bacteria but a very important example, the chloroplast ferredoxin, is found in green plants, and adrenodoxin is found in mammalian tissue. 

The absorption spectra of the main types of the iron-sulfur proteins is shown in Figure 1. There are very significant differences that have been overlooked with regard to the nomenclature of these proteins. All the proteins examined— the chloroplast ferredoxin, the clostridial ferredoxin, and rubredoxin—show absorption at 280 nm in the oxidized form shown here, but the other absorption peaks differ. The clostridial ferredoxin has an absorption peak at 390 nm but that peak is missing in the plant type ferredoxin. Rubredoxin shows an absorption maximum at 390 nm, but it shows other absorption peaks at 500 and 580 nm which are absent from the clostridial type protein. 

On either enzymatic or chemical reduction, these peaks disappear but the absorption does not go to zero. However, absorption at 280 nm remains unchanged in all cases. This absorption peak at 280 nm must be a function of the chromophore since removal of the iron and sulfide in a way to be described shortly causes this absorption to fall very markedly, whereas reduction has fittle effect on this absorption at 280 nm. I would like to emphasize the difference that exists between the spectra of the chloroplast ferredoxin and the clostridial ferredoxin. Even though they are both called ferredoxins, I think it remains to be determined how closely these compounds are related. 

The reaction catalyzed by the chloroplast ferredoxin is shown in Table II. It has been assumed that chloroplast ferredoxin acts as the primary electron acceptor in the photoreduction catalyzed by chloroplasts (42), However, there is some question as to whether ferredoxin is the... 

The sulfur atoms of the iron-sulfur cluster of chloroplast ferredoxin are derived from cysteine and a soluble stromal enzyme system is involved in the cluster formation (43). No information is available on the mechanisms by which the [4Fe-4S] clusters are inserted into the PSI-C apoprotein. 

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