Ferredoxin:NADP+ Oxidoreductase (Photosynthesis …
This is strong evidence for the view thatferredoxin plays a key part in photosynthesis.
The Ferredoxin/Thioredoxin System of Oxygenic Photosynthesis
Two pathways of cyclic electron flow (CEF), which recycle electrons under conditions of oxidative stress to minimize the production of damaging reactive oxygen species (; ), are thought to require pFNR. CEF also pumps protons into the thylakoid lumen, powering the generation of ATP. Under normal physiological conditions CEF might have a role in adjusting the stoichiometry of ATP:NADPH generated by photosynthesis (, ). pFNR has been identified as a component of the cytochromeb6f complex () and implicated in ferredoxin plastoquinone reductase (FQR)-dependent CEF by measurements of FNR-dependent quinine reduction (). In this context, pFNR would bind ferredoxin molecules to allow the subsequent transfer of electrons to plastoquinone (by an unknown mechanism), but would not generate NADPH. pFNR is also bound to the NADPH dehydrogenase (NDH; ) complex, and NDH-dependent CEF can be inhibited using an FNR antibody (). It seems probable that pFNR generates a locally enriched pool of NADPH at the NDH complex that is then dehydrogenated in situ to release electrons back into the electron transport chain ().
The levels of pFNRISKKQ doubled between the greening middle section of the leaf (4–6cm from the base) and the leaf tip. Since photosynthetic rates at the leaf tip are one-third higher than at the leaf middle (), then the most obvious function for pFNRISKKQ would be to fulfil the proposed requirement for CEF to generate additional ATP at high rates of photosynthesis and carbon fixation (). However, such provision of ATP for the normal functioning of photosynthesis is linked to the FQR-dependent pathway (), requiring a pFNR with a high enzyme Km to bind ferredoxin for FQR without generating NADPH. This does not fit the low Km of the pFNRISKKQ isoform (). Instead, with half the catalytic efficiency of pFNRIKKVS (), pFNRISKKQ has catalytic properties more suitable for decreasing the tight coupling of PSI to LEF generation of NADPH. By increasing the amount of reduced ferredoxin that can escape to the stroma and enter CEF (), this fits the redox poise paradigm for electron flux partition expounded by Breyton and co-workers (2006).
Ferredoxin and Photosynthetic Phosphorylation
Comparisons of the location, expression pattern, and ferredoxin interaction of native isoforms, together with the analysis of recombinant enzymes and mutant lines, indicate that multiple forms of wheat (), maize (), and Arabidopsis (; ) pFNR provide plants with the flexibility to respond to changing reductant demands. In studies of Arabidopsis mutant lines both pFNR forms are necessary for optimal growth, with the absence of either form leading to low chlorophyll content, low accumulation of photosynthetic thylakoid proteins, and reduced carbon fixation rate (). In the results reported herein, different experimental conditions were used to provide an insight into potential roles of specific wheat pFNR isoforms.
Photosynthesis thus appears to require varying catalytic properties from the pFNR enzyme, in order to adjust the ratio of ferredoxin:NADPH reductant supplied and to function effectively in LEF and the two CEF pathways. These differing requirements for pFNR catalytic properties could be met by different isoforms, whether arising from different genes or from alternative translated protein products of the same gene. Plants possess two evolutionarily conserved classes of pfnr genes that encode more basic (pFNRI) and more acidic (pFNRII) forms of the protein. The two genes respond differently at the mRNA level to an altered supply of nitrate—high levels induce pfnrI (but not pfnrII) expression in both wheat () and Arabidopsis (). Precursor pFNR protein is translated in the cytosol from the nuclear pfnr genes and then imported into the chloroplast, with the removal of an N-terminal transit peptide domain. Multiple forms of pFNR varying at the N-terminus have been identified in the past () and suggested to arise as a result of N-terminal proteolytic degradation (). However, it has recently been demonstrated that the presence of mature wheat pFNR proteins with alternative N-terminal start points, differing by a three amino acid truncation in pFNRI and a two amino acid truncation in pFNRII, is not the result of N-terminal proteolytic degradation (). Recombinant versions of these four pFNR protein isoforms (pFNRIKKVS, pFNRISKKQ, pFNRIIISKK, and pFNRIIKKQD) were overexpressed in Escherichia coli. Each of the four recombinant pFNR proteins resolves to a unique isoelectric point (pI) with two-dimensional gel electrophoresis (). The purified enzymes have distinct reaction kinetics with leaf-type ferredoxin, as determined using an NADPH-dependent cytochrome c reduction assay (). The pFNRI isoforms have half the maximum activity (Vmax) of the pFNRII isoforms but higher affinity for ferredoxin, as indicated by a lower Michaelis–Menten constant (Km). Further differences were seen between alternative N-terminal truncations. pFNRISKKQ has lower activity, catalytic efficiency (Vmax/Km), and ferredoxin affinity when compared with pFNRIKKVS; whilst pFNRIIKKQD has 3-fold higher catalytic efficiency and ferredoxin affinity in comparison with pFNRIIISKK ().
The Z-Scheme Diagram of Photosynthesis
Ferredoxin NADP+ oxidoreductase (FNR) enzymes transfer electrons between the one-electron carrier ferredoxin and the two-electron carrier NADPH (). Leaf metabolism is dominated by photosynthesis. At the end of the photosynthetic electron transport chain, energized electrons are transferred from the photosystem I protein complex (PSI) to ferredoxin (; ). In linear electron flow (LEF), which is the dominant electron pathway, electrons are transferred from ferredoxin to NADPH by a photosynthetic FNR (pFNR) enzyme (). Most of the reductant (in the form of NADPH) and ATP generated by photosynthesis is used for carbon fixation in the Calvin cycle (). However, a number of additional electron sinks, grouped under the collective term ‘alternative electron flow’ (AEF; ), require reduced ferredoxin generated at PSI as their direct reductant supply. The ferredoxin:NADPH ratio demanded downstream of photosynthesis varies according to the relative activities (dependent on the physiological state of the chloroplast) of carbon fixation and these AEF sinks. Electron partition between NADPH supply (LEF) and reduced ferredoxin supply (AEF) depends, in part, on the catalytic properties of the pFNR enzyme that connects the two reductant pools.
Ferredoxin NADP+ oxidoreductase (FNR) enzymes catalyse electron transfer between ferredoxin and NADPH. In plants, a photosynthetic FNR (pFNR) transfers electrons from reduced ferredoxin to NADPH for the final step of linear electron flow, providing reductant for carbon fixation. pFNR is also thought to play important roles in two different mechanisms of cyclic electron flow around photosystem I; and photosynthetic reductant is itself partitioned between competing linear, cyclic, and alternative electron flow pathways. Four pFNR protein isoforms in wheat that display distinct reaction kinetics with leaf-type ferredoxin have previously been identified. It has been suggested that these isoforms may be crucial to the regulation of reductant partition between carbon fixation and other metabolic pathways. Here the 12cm primary wheat leaf has been used to show that the alternative N-terminal pFNRI and pFNRII protein isoforms have statistically significant differences in response to the physiological parameters of chloroplast maturity, nitrogen regime, and oxidative stress. More specifically, the results obtained suggest that the alternative N-terminal forms of pFNRI have distinct roles in the partitioning of photosynthetic reductant. The role of alternative N-terminal processing of pFNRI is also discussed in terms of its importance for thylakoid targeting. The results suggest that the four pFNR protein isoforms are each present in the chloroplast in phosphorylated and non-phosphorylated states. pFNR isoforms vary in putative phosphorylation responses to physiological parameters, but the physiological significance requires further investigation.
Electron Transfer Pathway from Water to NADP in photosynthesis
Light-dependent reactions - Wikipedia
This is strong evidence for the view that ferredoxin plays a key part in photosynthesis…
Photosystems I and II - Encyclopedia Britannica
During photosynthesis, electrons are removed from water and transferred to the single electron carrier ferredoxin
PHOTOSYNTHETIC REACTION CENTERS - Photobiology
| The iron-bearing protein ferredoxin is present in all photosynthetlc cells
Chapter 2 - Energy conversion by photosynthetic …
Depending on the type of microorganism, the reduced ferredoxin which supplies electrons for this process is generated byphotosynthesis, respiration or fermentation.
Diversity of Microbial Metabolism
It has now been shown that ferredoxin can catalyse, by twodistinct photochemical reactions, the production of ATP in cell-freephotosynthetic systems at rates comparable with the maximum rates ofphotosynthesis in vivo.
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