Ferredoxin (Fd) is a small [2Fe-2S] cluster-containing protein found in all organisms performing oxygenic photosynthesis.Fd is the first soluble acceptor of electrons on the stromal side of the chloroplast electron transport chain, and as such is pivotal to determining the distribution of these electrons to different metabolic reactions. In chloroplasts, the principle sink for electrons is in the production of NADPH, which is mostly consumed during the assimilation of CO2. In addition to this primary function in photosynthesis, Fds are also involved in a number of other essential metabolic reactions, including biosynthesis of chlorophyll, phytochrome and fatty acids, several steps in the assimilation of sulphur and nitrogen, as well as redox signalling and maintenance of redox balance via the thioredoxin system and Halliwell-Asada cycle. This makes Fds crucial determinants of the electron transfer between the thylakoid membrane and a variety of soluble enzymes dependent on these electrons. In this article, we will first describe the current knowledge on the structure and function of the various Fd isoforms present in chloroplasts of higher plants and then discuss the processes involved in oxidation of Fd, introducing the corresponding enzymes and discussing what is known about their relative interaction with Fd.

In higher plant plastids, ferredoxin (Fd) is the unique soluble electron carrier protein located in the stroma. Consequently, a wide variety of essential metabolic and signaling processes depend upon reduction by Fd. The currently available plant genomes of Arabidopsis and rice (Oryza sativa) contain several genes encoding putative Fds, although little is known about the proteins themselves. To establish whether this variety represents redundancy or specialized function, we have recombinantly expressed and purified the four conventional [2Fe-2S] Fd proteins encoded in the Arabidopsis genome and analyzed their physical and functional properties. Two proteins are leaf type Fds, having relatively low redox potentials and supporting a higher photosynthetic activity. One protein is a root type Fd, being more efficiently reduced under nonphotosynthetic conditions and supporting a higher activity of sulfite reduction. A further Fd has a remarkably positive redox potential and so, although redox active, is limited in redox partners to which it can donate electrons. Immunological analysis indicates that all four proteins are expressed in mature leaves. This holistic view demonstrates how varied and essential soluble electron transfer functions in higher plants are fulfilled through a diversity of Fd proteins.Ferredoxin (Fd) is a soluble, low M r protein that mediates transfer of one electron from a donor to an acceptor. The redox active center is a [2Fe-2S] cluster that confers a highly negative redox potential on the protein (Ϫ350 to Ϫ450 mV), making Fd a powerful reductant. The [2Fe-2S] cluster is ligated by four highly conserved Cys residues.A broad spectrum of redox metabolism in higher plant plastids involves Fd. Although the name Fd was first used to describe a non-photosynthetic bacterial protein involved in nitrogen fixation (Mortenson et al., 1962), Fd is best known for a photosynthetic role: accepting electrons from photosystem I (PSI) and donating them to the enzyme Fd:NADP ϩ oxidoreductase (FNR) for photoreduction of NADP ϩ (Arnon, 1989). Donation of electrons by Fd has now been demonstrated to many other plastid enzymes essential for cellular processes, including nitrogen assimilation (nitrite reductase), sulfur assimilation (sulfite reductase [SiR]), amino acid synthesis (Glnoxoglutarate amino transferase), fatty acid synthesis (fatty acid desaturase), and redox regulation (Fd: thioredoxin reductase) (Knaff, 1996). In addition to PSI, Fd may be reduced by NADPH oxidation in a reversal of the FNR reaction (Suzuki et al., 1985). This enables Fd-dependent metabolism to continue under non-photosynthetic conditions, such as in root plastids.Fds are present as multiple isoforms in many plants and algae (Bertini et al., 2002). In higher plants, those predominantly expressed in photosynthetic tissues can be crudely divided from those that are not on the basis of primary sequence (Wada et al., 1986). Work using maize (Zea mays) has exposed functional differences between these Fd types; the rate of light-dependent NA...

In chloroplasts ferredoxin:NADP(H) oxidoreductase (FNR) enzymes oxidize the final reduced product of the photosynthetic electron transport chain, ferredoxin (Fd), to reduce NADP + , and play a role in cyclic electron transport. Oppositely, in non-photosynthetic plastids FNR oxidizes NADPH to provide reduced Fd for enzymes of bioassimilation and biosynthesis. These separate plastid types predominantly contain different iso-proteins, with distinct leaf FNR (LFNR) and root FNR (RFNR) features. Genomic and transcript information has identified multiple isoforms of both LFNR and RFNR in several species. We have used a technique for rapidly purifying Fd-interacting proteins from Arabidopsis thaliana to identify the two LFNR and two RFNR proteins encoded in the genome. Analysis of purified LFNRs revealed variation in pI and in abundance between stromal and thylakoid fractions of chloroplasts. Transcript and protein levels of the two LFNRs were similar in leaves, but varied in relative abundance between stems and siliques and in response to different nitrogen growth regimes. Relative transcript accumulation and protein abundance of the two RFNR isoforms varied between organs and in response to different nitrogen growth regimes. These results show that the multiple FNR isoproteins of A. thaliana have variable metabolic roles and contribute differentially to nitrogen assimilation.

In the absence of photosynthesis, ATP is imported into chloroplasts and non-green plastids by ATP/ADP transporters or formed during glycolysis, the latter requiring continuous regeneration of NAD(+), supplied by the plastidial isoform of NAD-MDH. During screening for T-DNA insertion mutants in the plNAD-MDH gene of Arabidopsis, only heterozygous plants could be isolated and homozygous knockout mutants grew only after complementation. These heterozygous plants show higher transcript levels of an alternative NAD(+)-regenerating enzyme, NADH-GOGAT, and, remarkably, improved growth when ammonium is the sole N-source. In situ hybridization and GUS-histochemical staining revealed that plNAD-MDH was particularly abundant in male and female gametophytes. Knockout plNAD-MDH pollen exhibit impaired tube growth in vitro, which can be overcome by adding the substrates of NADH-GOGAT. In vivo, knockout pollen is able to fertilize the egg cell. Young siliques of selfed heterozygous plants contain both green and white seeds corresponding to wild-type/heterozygous (green) and homozygous knockout mutants (white) in a (1:2):1 ratio. Embryos of the homozygous knockout seeds only reached the globular stage, did not green, and developed to tiny wrinkled seeds. Complementation with the gene under the native promoter rescued this defect, and all seeds developed as wild-type. This suggests that a blocked major physiological process in plNAD-MDH mutants stops both embryo and endosperm development, thus avoiding assimilate investment in compromised offspring.

In higher plants, ferredoxin (Fd):NADPH oxidoreductase (FNR) catalyzes reduction of NADP1 in the final step of linear photosynthetic electron transport and is also implicated in cyclic electron flow. We have identified three leaf FNR isoenzymes (LFNR1, LFNR2, and LFNR3) in maize (Zea mays) chloroplasts at approximately equivalent concentrations. Fractionation of chloroplasts showed that, while LFNR3 is an exclusively soluble enzyme, LFNR1 is only found at the thylakoid membrane and LFNR2 has a dual location. LFNR1 and LFNR2 were found to associate with the cytochrome b 6 f complex following its partial purification. We cloned LFNR3 and produced all three isoenzymes as stable, soluble proteins. Measurement of Fd reduction ability showed no significant differences between these recombinant enzymes. Column chromatography revealed variation between the interaction mechanisms of LFNR1 and LFNR2 with Fd, as detected by differential dependence on specific intermolecular salt bridges and variable sensitivity of interactions to changes in pH. A comparison of LFNR transcripts in leaves of plants grown on variable nitrogen regimes revealed that LFNR1 and LFNR2 transcripts are relatively more abundant under conditions of high demand for NADPH. These results are discussed in terms of the functional differentiation of maize LFNR isoenzymes.Ferredoxin (Fd):NADPH oxidoreductase (FNR; EC 1.18.1.2) is a flavoenzyme that catalyzes reduction of NADP 1 or oxidation of NADPH through electron transfer with Fd. In the final step of photosynthetic electron transport, FNR reduces NADP 1

In higher plants, [2Fe-2S] ferredoxin (Fd) proteins are the unique electron acceptors from photosystem I (PSI).. Whereas FdC1 was capable of electron transfer with FNR, redox potentiometry showed that it had a more positive redox potential than photosynthetic Fds by around 220 mV. These results indicate that FdC1 electron donation to FNR is prevented because it is thermodynamically unfavorable. Based on our data, we speculate that FdC1 has a specific function in conditions of acceptor limitation at PSI, and channels electrons away from NADP ؉ photoreduction.Ferredoxins (Fds) 3 are small soluble electron carrier proteins. In the final reaction of photosynthetic electron transfer (PET), photosystem I (PSI) donates electrons to Fd (1), which acts as the soluble electron donor to various acceptors in the chloroplast stroma and can also return electrons to the thylakoid in cyclic electron flow (CET) (2). The electron cascade to supply carbon fixation requires photoreduction of NADP ϩ by Fd, catalyzed by Fd-NADP(H) oxidoreductase (FNR) (3). Many other plastid enzymes accept electrons directly from Fd for metabolic processes. These include, but are not limited to, nitrite reductase and sulfite reductase, which are essential for assimilation of inorganic nitrogen and sulfur, respectively, and Fd-dependent glutamine oxoglutarate aminotransferase and fatty acid desaturase, which catalyze key steps in amino acid and fatty acid metabolism, respectively (4). In addition, Fd donation to thioredoxin via the Fd:thioredoxin reductase translates the redox state of the electron transfer chain into a regulatory signal controlling the activity of many enzymes (5). Fds are also capable of accepting electrons from NADPH via FNR, in a reversal of the photosynthetic reaction (6), allowing electron donation from reduced Fd to different acceptors under non-photosynthetic conditions. Most higher plants studied possess genes for several different Fd isoproteins (7-9). There is always an isoprotein that is more abundant in non-photosynthetic tissues and has higher affinity than photosynthetic and PetF-type Fds for FNR in the non-photosynthetic (often called "root") cascade (9, 10), where electrons are transferred from NADPH to Fd. In all plants for which we possess significant EST and cDNA information at least 2 separate photosynthetic isoproteins have been identified (7,8). In the C4-plant maize, different functions have been identified for two of the leaf-type Fds (11). There is a higher demand for ATP (which is disproportionately produced in CET) in the bundle sheath cells of NADP ϩ malic enzyme type C4 plants, and maize FdI and FdII are differentially expressed in mesophyll and bundle sheath cells, respectively (12). FdII has decreased affinity for FNR (13) and demonstrates a higher activity in CET around the photosystems, whereas FdI drives linear electron flow (11). In C3 plants, this spatial distribution is not observed, but duplicate photosynthetic Fds are still present, and there is some evi-* This work was supported by Deutsche For...

Ferredoxin (Fd) is the soluble protein that accepts electrons from photosystem I (PSI) and makes them available to stromal enzymes in higher plant chloroplasts. In linear electron flow, Fd mainly donates electrons to Fd:NADPH reductase (FNR) which generates NADPH for use in the Calvin cycle, but Fd may also return electrons to the thylakoid plastoquinone pool, forming a cyclic electron flow. Many higher plants contain two different photosynthetic Fd proteins, but there are no conserved sequence differences that allow their division into evolutionary groups. In the model C3 photosynthesizing dicot, Arabidopsis thaliana, there are two such photosynthetic Fds, and we have exploited RNA interference (RNAi) techniques to specifically decrease transcript abundance of different Fds in this plant. Surprisingly, the perturbation of photosynthesis, as measured by cholorophyll fluorescence, in RNAi lines of the two different photosynthetic Fds shows opposite trends. Linear electron flow is retarded in lines with lower Fd2 (the most abundant Fd species) levels and under certain circumstances enhanced in lines with lower Fd1 (the minor isoprotein) levels. These data are evidences for at least partially differentiated roles of Fd1 and Fd2 in photosynthetic electron transfer, possibly in the partition of electrons into linear and cyclic electron flow.

This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc 1 and b 6 f (Cytbc 1 /b 6 f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc 1 /b 6 f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc 1 /b 6 f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc 1 /b 6 f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.