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Can Ferredoxin and Ferredoxin NADP(H) Oxidoreductase Determine the Fate of Photosynthetic Electrons?

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Can Ferredoxin and Ferredoxin NADP(H) Oxidoreductase Determine the Fate of Photosynthetic Electrons? Send Orders for Reprints to [email protected] Current Protein and Peptide Science, 2014, 15, 385-393 385 The End of the Line: Can Ferredoxin and Ferredoxin NADP(H) Oxidoreductase Determine the Fate of Photosynthetic Electrons? Tatjana Goss and Guy Hanke* Department of Plant Physiology, Faculty of Biology and Chemistry, University of Osnabrück,11 Barbara Strasse, Osnabrueck, DE-49076, Germany Abstract: At the end of the linear photosynthetic electron transfer (PET) chain, the small soluble protein ferredoxin (Fd) transfers electrons to Fd:NADP(H) oxidoreductase (FNR), which can then reduce NADP+ to support C assimilation. In addition to this linear electron flow (LEF), Fd is also thought to mediate electron flow back to the membrane complexes by different cyclic electron flow (CEF) pathways: either antimycin A sensitive, NAD(P)H complex dependent, or through FNR located at the cytochrome b6f complex. Both Fd and FNR are present in higher plant genomes as multiple gene cop- ies, and it is now known that specific Fd iso-proteins can promote CEF. In addition, FNR iso-proteins vary in their ability to dynamically interact with thylakoid membrane complexes, and it has been suggested that this may also play a role in CEF. We will highlight work on the different Fd-isoproteins and FNR-membrane association found in the bundle sheath (BSC) and mesophyll (MC) cell chloroplasts of the C4 plant maize. These two cell types perform predominantly CEF and LEF, and the properties and activities of Fd and FNR in the BSC and MC are therefore specialized for CEF and LEF re- spectively. A diversity of Fd isoproteins and dynamic FNR location has also been recorded in C3 plants, algae and cyano- bacteria. This indicates that the principles learned from the extreme electron transport situations in the BSC and MC of maize might be usefully applied to understanding the dynamic transition between these states in other systems. Keywords: Bundle sheath, cyclic electron flow, ferredoxin, FNR, mesophyll. THE END OF THE LINE – POSSIBLE FATES OF therefore be used to reduce NADP+ to NADPH, a reaction PHOTOSYNTHETIC ELECTRONS catalyzed by the enzyme Fd:NADP(H) oxidoreductase (FNR) [7]. The relatively high affinity of FNR for reduced In photosynthetic electron transfer (PET), electrons are Fd [8] therefore reflects its status as the premier conduit of originally generated by the splitting of water at photosystem electrons from reduced Fd into soluble metabolism, and the II (PSII), transferred to the cytochrome b f complex (Cytb f) 6 6 high priority placed on carbon fixation. In addition to facili- by plastoquinone (PQ), and from there through the thylakoid tating the flux of electrons from the thylakoid membrane into lumen to photosystem I (PSI), via plastocyanin (PC). The soluble metabolism, Fd is an integral part of their return to final events in PET involve the transfer of excited electrons the PQ pool in cyclic electron flow (CEF) [9]. CEF allows from PSI to the small, soluble [2Fe-2S] protein ferredoxin the generation of pH without the net release of electrons (Fd) [1]. Fd was first identified in studies on nitrogen fixa- from the membrane, and is essential for balancing the pro- tion in Clostridium pasteuranium [2] as a soluble, iron con- portion of ATP:NADPH produced by PET, and therefore for taining electron transfer factor, but in work on plant proteins normal plant growth [10]. Alternative mechanisms by which this term has come to be almost exclusively associated with photosynthetic organisms can modulate the balance between the ~10 kDa [2Fe-2S] proteins involved in photosynthesis. In NADPH and ATP production, in order to respond to the de- linear electron flow (LEF) Fd is capable of transferring pho- mands on their metabolism caused by changing environ- tosynthetically derived electrons to diverse Fd-dependent mental conditions and environment are well reviewed else- enzymes [3], and shows variable interaction with these dif- where [11], and include the malate valve [12] and the water- ferent proteins [4, 5]. In this way, the physical properties of water cycle [13]. Fd help determine the flux of reducing power into different metabolic pathways [6]. In the water-water cycle, an absence of oxidized Fd at PSI results in electron donation to O2, resulting in generation The carbon fixing Calvin Benson cycle contains one re- - of superoxide (O2 ) [14]. Because of the damaging nature of action that requires photosynthetically generated electrons, - O , it is rapidly converted to H O by superoxide dismutase the NADPH-dependent reduction of 1,3-diphospholycerate 2 2 2 enzymes. Peroxidase enzymes then reductively convert H O to glyceraldehyde 3-P, catalyzed by glyceraldehyde-3-P de- 2 2 to H O and O , using ascorbate as the electron donor [15, hydrogenase. Electrons destined for carbon fixation must 2 2 16]. Ascorbate is regenerated using photosynthetic electrons, either directly via Fd, or by NADPH via reduced glutathione *Address correspondence to this author at the Department of Plant Physiol- [17]. The water-water cycle therefore combines the con- ogy, Faculty of Biology and Chemistry, University of Osnabrück,11 Barbara Strasse, Osnabrueck, DE-49076, Germany; Tel: 0049 541 969 2281; sumption of electrons with electron flow through PET, and Fax: 0049 541 969 2285; E-mail: [email protected] therefore proton pumping and ATP synthesis. 1875-5550/14 $58.00+.00 © 2014 Bentham Science Publishers 386 Current Protein and Peptide Science, 2014, Vol. 15, No. 4 Goss and Hanke In the absence of other acceptors, excited chlorophyll at plants (Fig. 1b). However, it has been demonstrated that the light harvesting complex of PSII can generate the ex- the enzyme can be present in distinct locations, as both tremely dangerous singlet oxygen radical [18]. Higher plants membrane bound and soluble pools in cyanobacteria [42] and algae prevent this through a variety of mechanisms re- and higher plants [41, 43, 44]. Higher plants often contain ferred to as non-photochemical quenching (NPQ) [19]. In multiple gene copies for both photosynthetic FNR (often this case, activity of PSII is down regulated through a com- termed leaf FNR, or LFNR) and non-photosynthetic FNR bination of photoinhibition at PSII [20], state transitions [21] (often termed root FNR, or RFNR) [41, 44-46]. In het- and photon quenching in the xanthophyll cycle [22]. During erotrophic plant tissues, essential Fd-dependent metabolism these circumstances, CEF takes on even greater importance. is supported by this root FNR in a reversal of the photosyn- For example in high light, or under conditions of drought thetic reaction, and also involves specific root type Fd [47]. [23], when stomata are closed and there is therefore a lack of These iso-proteins will not be discussed in this review, electron acceptors in CO2-assimilation, PSII activity is de- which focuses on photosynthetic electron channeling. We creased by NPQ, while PSI is still relatively active in CEF. will specifically focus on how different Fd and FNR forms could be differentially involved in partitioning electrons CEF pathways in photosynthetic organisms are well re- into either LEF or CEF. This would therefore allow the viewed elsewhere [11, 24], and we will focus here only on plant to control electron partitioning into LEF and CEF by the well characterized pathways of CEF involving Fd. The antimycin A sensitive pathway is regulated by the PGR5 regulating the abundance of specific Fd and FNR iso- proteins. In this article we will discuss the evidence sup- protein [25], and involves transfer of electrons from Fd to the porting this view, and highlight the remaining gaps in our PQ pool by the protein PGRL1 [26] in a calcium dependent understanding. manner [27]. The PGR5 and PGRL1 proteins are absent from cyanobacterial genomes, and this pathway appears to THE DIFFERENT FATE OF ELECTRONS IN MAIZE have evolved in the algae [28] and is present in higher plants [25]. The NAD(P)H dehydrogenase (NDH) complex de- MESOPHYLL AND BUNDLE SHEATH CHLORO- PLASTS pendent pathway involves a homologue of the respiratory complex I, comprising multiple subunits [29]. Although In C4 photosynthesis, different steps in carbon fixation genes for essential subunits appear to have been lost in some are divided between two different cell types, known as the algae [30] and gymnosperms [31] the complex is an impor- mesophyll (MC) and bundle sheath (BSC) [48, 49]. Initially tant part of angiosperm PET under high light conditions [32] carbon is fixed into a C4 acid in the mesophyll cytosol and has also recently been shown to require Fd as a direct (rather than a C3 sugar in the chloroplast, as occurs in stan- electron donor [33]. There is evidence that FNR may also be dard, C3 photosynthesis), and then transported into the BSC involved in CEF, based on biochemical and genetic data [34- for decarboxylation to yield a high CO2 environment around 36]. It has been suggested that FNR could either directly RuBisCO. C4 metabolism has arisen many different times in reduce the PQ pool using electrons from Fd [37], or act as a evolution, and plants vary in the form of carbon transported Fd binding site on the thylakoid membrane during CEF, as it and the specific enzymes involved in decarboxylation [50]. has been found to associate with both the NDH complex The more efficient C fixation in C4 plants comes at the cost [38], and the Cytb6f [39]. Cyanobacterial FNR contains a of two more ATP molecules per C fixed [49], and in order to phycobilisome linker protein for attachement to the thylakoid compensate for the increased ATP/NADPH demand, it has [40], and was not found associated with the Cytb6f [39], so been reported that these plants perform increased CEF [51, such a pathway must have evolved after the loss of phycobi- 52].
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