Photoreduction of the ferredoxin/ferredoxin-NADP(+)-reductase complex by a linked ruthenium polypyridyl chromophore.

Photosynthetic ferredoxin and its main partner ferredoxin-NADP(+)-reductase (FNR) are key proteins during the photoproduction of reductive power involved in photosynthetic growth. In this work, we used covalent attachment of ruthenium derivatives to different cysteine mutants of ferredoxin to trigger by laser-flash excitation both ferredoxin reduction and subsequent electron transfer from reduced ferredoxin to FNR. Rates and yields of reduction of the ferredoxin [2Fe-2S] cluster by reductively quenched Ru* could be measured for the first time for such a low redox potential protein whereas ferredoxin-FNR electron transfer was characterized in detail for one particular Ru-ferredoxin covalent adduct. For this adduct, the efficiency of FNR single reduction by reduced ferredoxin was close to 100% under both first-order and diffusion-limited second-order conditions. Interprotein intracomplex electron transfer was measured unambiguously for the first time with a fast rate of c. 6500s(-1). Our measurements point out that Ru photosensitizing is a powerful approach to study the functional interactions of ferredoxin with its numerous partners besides FNR.

Fast and direct extraction of cell-associated hepatotoxins from toxic cyanobacteria.

Microcystins are an important group of toxins produced by cyanobacteria of different genera. An increasing number of water contaminations by this class of toxins have been reported that are susceptible to generate important public health problems. We designed an efficient method for extracting these toxins on-site for a rapid testing of potentially contaminated water. The extraction parameters have been optimized using Microcystis aeruginosa and the technique successfully applied to different laboratory cultures and field samples. The procedure employs a simple and stable reagent mix of propanol, Tween 20, and trifluoroacetic acid. It is directly active on crude cell suspensions without any pre-treatment. Extraction yields measured by immunological quantification were at least equal to the best values obtained with the most commonly used laboratory techniques. An additional simple concentration/extraction step is also described that allows measurements on samples too dilute for direct detection by immunochromatographic techniques.

Photo-induced electron transfer from photosystem I to NADP(+): characterization and tentative simulation of the in vivo environment.

The photoproduction of NADPH in photosynthetic organisms requires the successive or concomitant interaction of at least three proteins: photosystem I (PSI), ferredoxin (Fd) and ferredoxin:NADP(+) oxidoreductase (FNR). These proteins and their surrounding medium have been carefully analysed in the cyanobacterium Synechocystis sp. PCC 6803. A high value of 550mg/ml was determined for the overall solute content of the cell soluble compartment. PSI and Fd are present at similar concentrations, around 500µM, whereas the FNR associated to phycobilisome is about 4 fold less concentrated. Membrane densities of FNR and trimeric PSI have been estimated to 2000 and 2550 per µm(2), respectively. An artificial confinement of Fd to PSI was designed using fused constructs between Fd and PsaE, a peripheral and stroma located PSI subunit. The best covalent system in terms of photocatalysed NADPH synthesis can be equivalent to the free system in a dilute medium. In a macrosolute crowded medium (375mg/ml), this optimized PSI/Fd covalent complex exhibited a huge superiority compared to the free system. This is a likely consequence of restrained diffusion constraints due to the vicinity of two out of the three protein partners. In vivo, Fd is the free partner, but the constant proximity between PSI and the phycobilisome associated FNR creates a similar situation, with two closely associated partners. This organization seems well adapted for an efficient in vivo production of the stable and fast diffusing NADPH.

Detection of the photosystem I:ferredoxin complex by backscattering interferometry.

The dissociation constant K(d) of the photosystem I (PSI):ferredoxin complex has been measured by backscattering interferometry (BSI) with cyanobacterial PSI (350 kDa) and ferredoxin (10.5 kDa). The BSI signal, consisting of shifts for interference fringes resulting from a change in refractive index due to complex formation, was monitored as ferredoxin concentration was titrated. K(d) values of 0.14-0.38 microM were obtained with wild-type PSI whereas no complex was detectable with a PSI mutant containing a single mutation (R39Q) in the PsaE extrinsic subunit. These results are in quantitative agreement with previous functional determinations consisting in the detection of fast electron transfer within the complex. They provide evidence that the main contribution for the high affinity binding of ferredoxin to PSI is due to a single region of PsaE comprising arginine 39. They do not support the existence of a secondary binding site that could have escaped functional detection.

A fluorescent immunochromatographic test using immunoliposomes for detecting microcystins and nodularins.

Microcystins (MCs), a group of cyclic heptapeptides produced by common cyanobacteria (blue green algae), cause both acute and chronic toxicity. Due to their toxicity, constant monitoring in drinking water, recreational waters as well as other potential exposure through ingestion of contaminated sea food, is very important. In this context, an immunochromatographic test (ICT) using a monoclonal antibody labeled with fluorescent liposomes (immunoliposomes) as tracer was developed, allowing a rapid and simple detection of a large number of MC and nodularin variants in field samples. The present ICT using immunoliposomes proved to be ten times more sensitive than the ICT using colloidal gold for labeling. To achieve quantitative measurement, this ICT was improved by including a stable signal on the control band allowing the expression of the results as a ratio of the fluorescence signals of the specific band versus the control band (SB/CB). Very low concentrations of MC-LR were detected in the analysis buffer (0.06 ng/ml), well below the guideline value of 1 ng/ml proposed by the World Health Organization (WHO), with a dynamic range from 0.06 to 1.5 ng/ml of MC-LR. This method was also validated using a hand-held commercial fluorometer (from ESE), providing the same performances obtained via the analysis station (from Kodak) used in our laboratory. Repeatability tests performed with both devices showed good accuracy (CV < 13%). Furthermore, quantification of MCs in natural samples (water bloom and Microcystis culture) was achieved using ICT, leading to similar results obtained via an EIA previously described. All these results demonstrate that this new fluorescent ICT could be used not only as a sensitive detection tool but also to quantify MCs in field samples.

Ferredoxin:NADP+ oxidoreductase association with phycocyanin modulates its properties.

In photosynthetic organisms, ferredoxin:NADP(+) oxidoreductase (FNR) is known to provide NADPH for CO(2) assimilation, but it also utilizes NADPH to provide reduced ferredoxin. The cyanobacterium Synechocystis sp. strain PCC6803 produces two FNR isoforms, a small one (FNR(S)) similar to the one found in plant plastids and a large one (FNR(L)) that is associated with the phycobilisome, a light-harvesting complex. Here we show that a mutant lacking FNR(L) exhibits a higher NADP(+)/NADPH ratio. We also purified to homogeneity a phycobilisome subcomplex comprising FNR(L,) named FNR(L)-PC. The enzymatic activities of FNR(L)-PC were compared with those of FNR(S). During NADPH oxidation, FNR(L)-PC exhibits a 30% decrease in the Michaelis constant K(m)((NADPH)), and a 70% increase in K(m)((ferredoxin)), which is in agreement with its predicted lower activity of ferredoxin reduction. During NADP(+) reduction, the FNR(L)-PC shows a 29/43% decrease in the rate of single electron transfer from reduced ferredoxin in the presence/absence of NADP(+). The increase in K(m)((ferredoxin)) and the rate decrease of single reduction are attributed to steric hindrance by the phycocyanin moiety of FNR(L)-PC. Both isoforms are capable of catalyzing the NADP(+) reduction under multiple turnover conditions. Furthermore, we obtained evidence that, under high ionic strength conditions, electron transfer from reduced ferredoxin is rate limiting during this process. The differences that we observe might not fully explain the in vivo properties of the Synechocystis mutants expressing only one of the isoforms. Therefore, we advocate that FNR localization and/or substrates availability are essential in vivo.

New insights into the catalytic cycle of plant nitrite reductase. Electron transfer kinetics and charge storage.

Nitrite reductase, which reduces nitrite to ammonium in a six-electron reaction, was characterized through kinetic analysis of an electron transfer cascade involving photoexcited Photosystem I and ferredoxin. This cascade was studied at physiological pH by flash-absorption spectroscopy. Two different forms of the enzyme were studied: one isolated from spinach leaf and one histidine-tagged recombinant form. When the enzyme is oxidized in the absence of nitrite, single-enzyme reduction leads mostly to siroheme reduction with the leaf enzyme, whereas the siroheme and the [4Fe-4S] cluster are both reduced in equivalent amounts in the recombinant enzyme. When combined with the results of deazaflavin/EDTA photoreduction experiments, these data support a 50 mV negative shift of the siroheme midpoint potential in the recombinant enzyme. Despite this difference, the two forms of the enzyme exhibit similar values for the rate constant of single reduction by reduced ferredoxin (1200 s(-1)) and for k(cat) (420-450 electrons per second and per nitrite reductase). When nitrite reductase is initially pre-reduced to the state ferrous siroheme-NO(*), the fast kinetics of reduction by ferredoxin and the thermodynamics of ferredoxin binding are equivalent to those observed with oxidized nitrite reductase without nitrite. Spectral and kinetic analyses of single reduction of the recombinant enzyme in the ferrous siroheme-NO(*) state by photoreduced ferredoxin reveal that this process leads to reduction of the [4Fe-4S] cluster with little, if any, NO(*) reduction. These data show that the enzyme must wait for the next reduction step before NO(*) undergoes substantial reduction.

Electrochemical study of a reconstituted photosynthetic electron-transfer chain.

A multi-enzyme electron-transfer chain involving solubilized photosystem I (PSI) as photocatalytic unit, cytochrome c6 and ferredoxin as electron carriers and ferredoxin/NADPH oxidoreductase (FNR) as electron acceptor was reconstituted in an electrochemical cell and studied by cyclic voltammetry. The working gold electrodes were modified to react selectively with cytochrome c6. Quantitative analysis of the photocatalytic current under continuous illumination allowed the determination of the values kon and koff for the ferredoxin/PSI interaction. An efficient recycling system for NADPH was established, and the dissociation constant of the oxidized ferredoxin/semiquinone FNR complex was extracted by modeling the catalytic efficiency of the chain as a function of ferredoxin concentration. The value determined hereby is consistent with a shift of -50 to -100 mV of the reduction potential of ferredoxin when complexed with FNR.

A second isoform of the ferredoxin:NADP oxidoreductase generated by an in-frame initiation of translation.

Ferredoxin:NADP oxidoreductases (FNRs) constitute a family of flavoenzymes that catalyze the exchange of reducing equivalents between one-electron carriers and the two-electron-carrying NADP(H). The main role of FNRs in cyanobacteria and leaf plastids is to provide the NADPH for photoautotrophic metabolism. In root plastids, a distinct FNR isoform is found that has been postulated to function in the opposite direction, providing electrons for nitrogen assimilation at the expense of NADPH generated by heterotrophic metabolism. A multiple gene family encodes FNR isoenzymes in plants, whereas there is only one FNR gene (petH) in cyanobacteria. Nevertheless, we detected two FNR isoforms in the cyanobacterium Synechocystis sp. strain PCC6803. One of them (FNR(S) approximately 34 kDa) is similar in size to the plastid FNR and specifically accumulates under heterotrophic conditions, whereas the other one (FNR(L) approximately 46 kDa) contains an extra N-terminal domain that allows its association with the phycobilisome. Site-directed mutants allowed us to conclude that the smaller isoform, FNR(S), is produced from an internal ribosome entry site within the petH ORF. Thus we have uncovered a mechanism by which two isoforms are produced from a single gene, which is, to our knowledge, novel in photosynthetic bacteria. Our results strongly suggest that FNR(L) is an NADP(+) reductase, whereas FNR(S) is an NADPH oxidase.

Synechocystis ferredoxin/ferredoxin-NADP(+)-reductase/NADP+ complex: Structural model obtained by NMR-restrained docking.

Ferredoxin (Fd) and ferredoxin-NADP(+)-reductase (FNR) are two terminal physiological partners of the photosynthetic electron transport chain. Based on a nuclear magnetic resonance (NMR)-restrained-docking approach, two alternative structural models of the Fd-FNR complex in the presence of NADP+ are proposed. The protein docking simulations were performed with the software BiGGER. NMR titration revealed a 1:1 stoichiometry for the complex and allowed the mapping of the interacting residues at the surface of Fd. The NMR chemical shifts were encoded into distance constraints and used with theoretically calculated electronic coupling between the redox cofactors to propose experimentally validated docked complexes.

Ferredoxin-NADP+ reductase. Kinetics of electron transfer, transient intermediates, and catalytic activities studied by flash-absorption spectroscopy with isolated photosystem I and ferredoxin.

The electron transfer cascade from photosystem I to NADP+ was studied at physiological pH by flash-absorption spectroscopy in a Synechocystis PCC6803 reconstituted system comprised of purified photosystem I, ferredoxin, and ferredoxin-NADP+ reductase. Experiments were conducted with a 34-kDa ferredoxin-NADP+ reductase homologous to the chloroplast enzyme and a 38-kDa N-terminal extended form. Small differences in kinetic and catalytic properties were found for these two forms, although the largest one has a 3-fold decreased affinity for ferredoxin. The dissociation rate of reduced ferredoxin from photosystem I (800 s(-1)) and the redox potential of the first reduction of ferredoxin-NADP+ reductase (-380 mV) were determined. In the absence of NADP+, differential absorption spectra support the existence of a high affinity complex between oxidized ferredoxin and semireduced ferredoxin-NADP+ reductase. An effective rate of 140-170 s(-1) was also measured for the second reduction of ferredoxin-NADP+ reductase, this process having a rate constant similar to that of the first reduction. In the presence of NADP+, the second-order rate constant for the first reduction of ferredoxin-NADP+ reductase was 20% slower than in its absence, in line with the existence of ternary complexes (ferredoxin-NADP+ reductase)-NADP+-ferredoxin. A single catalytic turnover was monitored, with 50% NADP+ being reduced in 8-10 ms using 1.6 microM photosystem I. In conditions of multiple turnover, we determined initial rates of 360-410 electrons per s and per ferredox-in-NADP+ reductase for the reoxidation of 3.5 microM photoreduced ferredoxin. Identical rates were found with photosystem I lacking the PsaE subunit and wild type photosystem I. This suggests that, in contrast with previous proposals, the PsaE subunit is not involved in NADP+ photoreduction.

Mechanism of spinach chloroplast ferredoxin-dependent nitrite reductase: spectroscopic evidence for intermediate states.

Nitrite reductases found in plants, algae, and cyanobacteria catalyze the six-electron reduction of nitrite to ammonia with reduced ferredoxin serving as the electron donor. They contain one siroheme and one [4Fe-4S] cluster, acting as separate one-electron carriers. Nitrite is thought to bind to the siroheme and to remain bound until its complete reduction to ammonia. In the present work the enzyme catalytic cycle, with ferredoxin reduced by photosystem 1 as an electron donor, has been studied by EPR and laser flash absorption spectroscopy. Substrate depletion during enzyme turnover, driven by a series of laser flashes, has been demonstrated. A complex of ferrous siroheme with NO, formed by two-electron reduction of the enzyme complex with nitrite, has been shown to be an intermediate in the enzyme catalytic cycle. The same complex can be formed by incubation of free oxidized nitrite reductase with an excess of nitrite and ascorbate. Hydroxylamine, another putative intermediate in the reduction of nitrite catalyzed by nitrite reductase, was found to react with oxidized nitrite reductase to produce the same ferrous siroheme-NO complex, with a characteristic formation time of about 13 min. The rate-limiting step for this reaction is probably hydroxylamine binding to the enzyme, with the conversion of hydroxylamine to NO at the enzyme active site likely being much faster.

Solution NMR structure and backbone dynamics of the PsaE subunit of photosystem I from Synechocystis sp. PCC 6803.

PsaE is a small peripheral subunit of photosystem I (PSI) that is very accessible to the surrounding medium. It plays an essential role in optimizing the interactions with the soluble electron acceptors of PSI, ferredoxin and flavodoxin. The solution structure of PsaE from the cyanobacterium Synechocystis sp. PCC 6803 has been investigated by NMR with a special emphasis on its protein dynamic properties. PsaE is characterized by a well-defined central core that consists of a five-stranded beta-sheet (+1, +1, +1, -4x). Four loops (designated the A-B, B-C, C-D, and D-E loops) connect these beta-strands, the overall resulting structure being that of an SH3-like domain. As compared to previously determined PsaE structures, conformational differences are observed in the first three loops. The flexibility of the loops was investigated using (15)N relaxation experiments. This flexibility is small in amplitude for the A-B and B-C loops, but is large for the C-D loop, particularly in the region corresponding to the missing sequence of Nostoc sp. PCC 8009. The plasticity of the connecting loops in the free subunit is compared to that when bound to the PSI and discussed in relation to the insertion process and the function(s) of PsaE.

The ferredoxin docking site of photosystem I.

The reaction center of photosystem I (PSI) reduces soluble ferredoxin on the stromal side of the photosynthetic membranes of cyanobacteria and chloroplasts. The X-ray structure of PSI from the cyanobacterium Synechococcus elongatus has been recently established at a 2.5 A resolution [Nature 411 (2001) 909]. The kinetics of ferredoxin photoreduction has been studied in recent years in many mutants of the stromal subunits PsaC, PsaD and PsaE of PSI. We discuss the ferredoxin docking site of PSI using the X-ray structure and the effects brought by the PSI mutations to the ferredoxin affinity.