Ferredoxin from chloroplasts

Plant type ferredoxins. Tagawa and Arnon (16) described the isolation of a ferredoxin from spinach chloroplast. This ferredoxin is a protein of 12,000 molecular weight, and consists of 97 amino acids (17). Spinach ferredoxin has abosrbance maxima at 325, 420 and 465 nm (18). Ferredoxins of this type have been isolated from other sources of plants and algae, e.g., alfalfa (19), taro (20), Leuceana glauca (21) and Scenedesmus (22). The prot s of thes erredoxins are similar in their properties to ferredoxin from spinach. 

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. ( ). 

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.

The history of ferredoxin is outlined in its simplest form in Table 1. Prior to the isolation of ferredoxin from Clostridium pasteurianum, proteins now known to be functionally similar to the ferredoxin from this organism had been isolated from photosynthetic cells and were designated by a variety of names in the belief that each name described the protein s true function. In 1952, Davenport, Hill, and Whatley (39) isolated a soluble factor from chloroplasts which, upon illumination with chloroplast fragments, catalyzed the reduction of methemoglobin. This factor, which they named the "methaemoglobin reducing factor , had the characte... 

Brintzinger, H., G. Palmer, and R. H. Sands On the ligand field of iron in ferredoxin from spinach chloroplasts and related non-heme iron protein. Proc. Natl. Acad. Sci. U.S. 55, 397 (1966). 

The 2 Fe 2S plant type ferredoxins, MW 12,000 dal ton, Em = —430 mV, were first isolated from chloroplast and photosynthetic bacteria. Similar proteins have been purified from the bacteria E. coli (264) and Pseudomonas putida [ putidaredoxin , Em7 = —235 mV, (275)] and from mammalian adrenal cortex mitochondria [ adrenodoxin Em = — 367 mV, 13,100 dalton (165)] among other sources. 

The complete system contained in a total volume of 1.5 ml heated spinach chloroplast fragments (equivalent to 185 tug chlorophyll) 100 /itmoles ascorbate, 0.05 /imole DCIP, 10 moles MgCl2, 5 /tmoles ATP, /onoles creatine phosphate, 05 mg creatine phosphokinase, 200 tug ferredoxin from C. pasteurianum and 6.6 mg Chromatium extract. Gas phase was N2 or 0.1 atm acetylene and 0.9 atm argon, depending on type of nitrogenase assay used. Temp. 30 light intensity 20,000 lux. 

It is interesting to note that the UV spectrum for the ferredoxin from chromatium, a photosynthesizing bacteria, suggests that it is very similar, if not identical, to that of the ferredoxin from clostridia, which are nonphotosynthetic organisms. It was originally suggested that this is a form of ferredoxin intermediate between the plant type and the clostridial type, because chemical analysis suggested that the iron content was intermediate between the chloroplast and clostridial type ferredoxin. 

During the 1960s, research on proteins containing iron—sulfur clusters was closely related to the field of photosynthesis. Whereas the first ferredoxin, a 2[4Fe-4S] protein, was obtained in 1962 from the nonphotosynthetic bacterium Clostridium pasteurianum (1), in the same year, a plant-type [2Fe-2S] ferredoxin was isolated from spinach chloroplasts (2). Despite the fact that members of this latter class of protein have been reported for eubacteria and even archaebacteria (for a review, see Ref. (3)), the name plant-type ferredoxin is often used to denote this family of iron—sulfur proteins. The two decades... 

The prototype of this class of soluble ferredoxins was initially obtained from spinach chloroplasts and subsequently been shown to play a role in physiological electron shuttling between PSl and a number of redox proteins, most prominently ferredoxin-NADP-reduc-tase 13). Homologous proteins were purified from several cyanobac-... 

Artificial cell-free systems have been investigated, to test models of photosynthetic production of H2. Benemann et al. (1973) demonstrated that it was possible to produce H2 and O2 by combining chloroplasts from green plants and bacterial hydrogenase, with ferredoxin as the intermediate electron carrier ... 

In plants a similar enzyme catalyzes formation of the first double bond in a fatty acyl group converting stearoyl-ACP into oleoyl-ACP in the chloroplasts.72 753/105 108 The soluble A9 stearoyl-ACP desaturase has a diiron-oxo active site (Fig. 16-20, B, C).i°9 no Electrons are donated from light-generated reduced ferredoxin (see Chapter 23). In addition to the A9 desaturase both plants and cyanobacteria usually desaturate C18 acids also at the A12 and A15 positions and C16 acids at the A7, A20, and A13 (co3) positions.iii ii2 Desaturation of oleate occurs primari-... 

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... 

Photosystem I contains three iron-sulfur clusters firmly associated with the reaction center. These are designated Fe-Sx, Fe-SA, and Fe-SB in figure 15.17. The cysteines of Fe-Sx are provided by the two main polypeptides of the reaction center, which also bind P700 and its initial electron acceptors Fe-SA and Fe-SB are on a separate polypeptide. The quinone that is reduced in photosystem I probably transfers an electron to Fe-Sx, which in turn reduces Fe-SA and Fe-SB. From here, electrons move to ferredoxin, a soluble iron-sulfur protein found in the chloroplast stroma, then to a flavoprotein (ferredoxin-NADP oxidoreductase), and finally to NADP+. 

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