Ferredoxins photosynthetic reactions

Studies of ferredoxin [152] and a photosynthetic reaction center [151] have analyzed further the protein s dielectric response to electron transfer, and the protein s role in reducing the reorganization free energy so as to accelerate electron transfer [152], Different force fields were compared, including a polarizable and a non-polarizable force field [151]. One very recent study considered the effect of point mutations on the redox potential of the protein azurin [56]. Structural relaxation along the simulated reaction pathway was analyzed in detail. Similar to the Cyt c study above, several slow relaxation channels were found, which limited the ability to obtain very precise free energy estimates. Only semiquantitative values were... 

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. ( ). 

A representative sampling of non-heme iron proteins is presented in Fig. 3. Evident from this atlas is the diversity of structural folds exhibited by non-heme iron proteins it may be safely concluded that there is no unique structural motif associated with non-heme iron proteins in general, or even for specific types of non-heme iron centers. Protein folds may be generally classified into several categories (i.e., all a, parallel a/)3, or antiparallel /8) on the basis of the types and interactions of secondary structures (a helix and sheet) present (Richardson, 1981). Non-heme iron proteins are found in all three classes (all a myohemerythrin, ribonucleotide reductase, and photosynthetic reaction center parallel a/)8 iron superoxide dismutase, lactoferrin, and aconitase antiparallel )3 protocatechuate dioxygenase, rubredoxins, and ferredoxins). This structural diversity is another reflection of the wide variety of functional roles exhibited by non-heme iron centers. 

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.39 Green sulfur bacteria RCs are of the type I or the Fe-S-type (photosystem I). Here the electron acceptor is not the quinine instead, chlorophyll molecules (BChl 663, 81 -OII-Chi a, or Chi a) serve as primary electron acceptors, and three Fe4S4 centers (ferredoxins) serve as secondary acceptors. A quinone molecule may or may not serve as an intermediate carrier between the primary electron acceptor (Chi) and the secondary acceptor (Fe-S centers).40 The process sequence leading to the energy conversion in RCI is shown in Figure 21. 

The role of ferredoxin in reactions of photosynthetic bacteria is summarized in Fig. 11. Reactions which should now be possible to show, but so far have not been observed in cell-free systems, are the fcrredoxin-dependent photoproduction of hydrogen gas and photoreduction of pyridine nucleotide. Hood (56) reported a two-fold stimulation by ferredoxin in the photoreduction of DPN by chlorophyll-containing particles from Chromatium. Hood s results were inconsistent and Hinkson (54) found that the ferredoxin requirement is not specific and may be satisfied by serum albumin. In addition, there is still no evidence for a role of ferredoxin in photophosphorylation by photosynthetic bacteria, similar to that in chloroplasts. 

Following the initial work of Salemme, models for other electron transfer complexes have been proposed for cytochrome c-cytochrome-c peroxidase (98), cytochrome c-flavodoxin (99), cytochrome Z>s-hemoglobin (100), cytochrome (75-myoglobin (101), cytochrome c-photosynthetic reaction center (102, 103), and ferredoxin-cytochrome C3 (104). The same general approach of identifying potential salt bridges that could stabilize a complex with approximately coplanar cofactor rings has been utilized in many of these studies. 

The 2[4Fe-4S] and [3Fe-4S][4Fe-4S] ferredoxins are components of virtually all eubacteria and archaebacteria (3). Several comprehensive reviews dealing with these small metalloproteins have appeared (3, 8-12), but only those participating directly in the photosynthetic light reactions will be addressed here. 

Figure 12.2a. Photosynthetic Z-scheme for green plants. Abbreviations not included in the text are PQ, plastiquinone Cyt bse, a form of cytochrome b absorbing at 564 nm FD, ferredoxin FP a flavoprotein. Long vertical arrows indicate steps arising from photoactivation of pigment reaction centers dashed arrows indicate uncertain pathways.0185...

It can be seen from the normal potentials E° (see p. 18) of the most important redox systems involved in the light reactions why two excitation processes are needed in order to transfer electrons from H2O to NADP"". After excitation in PS II, E° rises from around -IV back to positive values in plastocyanin (PC)—i. e., the energy of the electrons has to be increased again in PS I. If there is no NADP" available, photosynthetic electron transport can still be used for ATP synthesis. During cyclic photophosphorylation, electrons return from ferredoxin (Fd) via the plastoquinone pool to the b/f complex. This type of electron transport does not produce any NADPH, but does lead to the formation of an gradient and thus to ATP synthesis. 

Bacterial ferredoxins function primarily as electron carriers in ferredoxin-mediated oxidation reduction reactions. Some examples are reduction of NAD, NADP, FMN, FAD, sulfite and protons in anaerobic bacteria, CO -fixation cycles in photosynthetic bacteria, nitrogen fixation in anaerobic nitrogen fixing bacteria, and reductive carboxylation of substrates in fermentative bacteria. The roles of bacterial ferredoxins in these reactions have been summarized by Orme-Johnson (2), Buchanan and Arnon (3), and Mortenson and Nakos (31). 

Rieke proteins, 47 337, 347-355 superoxide dismutases and, 45 129 Photosynthetic bacteria, 2[4Fe-4S] and [4Fe-4S] [3Fe-4S] ferredoxins, 38 255-257 Photosystem 1, 38 303-304 Pa/Fb proteins, 38 262-263 reaction center X proteins, single [4Fe-4S] ferredoxins cluster bridging two subunits, 38 251-252 Photosystem II, 46 328 interatomic separations, 33 228 mechanisms for water oxidation, 33 244-247... 

The photosynthetic process involves photochemical reactions followed by sequential dark chemical transformations (Fig. 3). The photochemical processes occur in two photoactive sites, photosystem I and photosystem II (PS-I and PS-II, respectively), where chlorophyll a and chlorophyll b act as light-active compounds [6, 8]. Photoinduced excitation of photosystem I results in an electron transfer (ET) process to ferredoxin, acting as primary electron acceptor. This ET process converts light energy to chemical potential stored in the reduced ferredoxin and oxidized chlorophyll. Photoexcitation of PS-II results in a similar ET process where plastoquinone acts as electron acceptor. The reduced photoproduct generated in PS-II transfers the electron across a chain of acceptors to the oxidized chlorophyll of PS-I and, consequently, the light harnessing component of PS-I is recycled. Reduced ferredoxin formed in PS-I induces a series of ET processes,... 

Bennett and Fuller (21, 22) reported that cell-free extracts of the photosynthetic bacterium Chromatium catalyzed a reaction similar to the anaerobic breakdown of pyruvate shown above in Eq. 7, but they detected neither acetyl phosphate nor acetyl-CoA as a product. Ferredoxin itself was not tested in the reaction, although viologen dyes were found to be stimulatory. 

Buchanan, Bachofen, and Arnon (29) showed that the conversion of pyruvate to the above amino acids required DPN and NHs- They found a ferredoxin-dependent reduction of DPN with hydrogen gas was catalyzed by Ckromalium extracts, and concluded that DPNH2, formed from reduced ferredoxin, and NH3 were used in reductive amination reactions leading to the synthesis of amino acids. A ferredoxin-dependent reduction of pyridine nucleotides was shown first for spinach chloroplasts (Tagawa and Arnon (99) it was shown later in other photosynthetic bacteria, when ferredoxin was reduced either with dithionite (Yamanaka and Kamen (115)) or with hydrogen gas and hydrogenase (Weaver, Tinker, and Valentine (112)). 

Evans, Buchanan, and Arnon (41a) have recently found that the ferredoxin-dependent pyruvate and a-ketoglutarate synthesizing reactions function in a new carbon cycle for the photosynthetic fixation of C02. The new cycle was named the reductive carboxylic acid cycle, and apart from pyruvate and a-ketoglutarate synthases, it includes certain of the enzymes associated with Krebs citric acid cycle, operating in the synthetic direction. Photoreduced ferredoxin and ATP, formed by photo-... 

Bennett, R. and R. C. Fuller The pyruvate phosphoclastic reaction in Chromatium, a probable role for ferredoxin in a photosynthetic bacterium. Biochem. Biophys. Res. Commun. 16, 300—307 (1964). 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

Fig. 1. A simplified scheme of the photosynthetic membrane, illustrating electron transfer from water to ferredoxin, which involves three protein complexes (the PS II reaction centre, the Cyl complex, the PS I reaction centre) and two diffusible components, plastoquinone (PO pool) and plastocyanin (Pc),...

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