How does light regulate chloroplast enzymes? Structure-function studies of the ferredoxin/thioredoxin system

Investigating the effect of drought stress and methanol spraying on the influential genes in the Calvin cycle and photorespiration of rapeseed (Brassica napus)

Due to global warming and changes in precipitation patterns, many regions are prone to permanent drought. Rapeseed (Brassica napus ) is one of the main sources of edible oils worldwide, and its production and yield are affected by drought. In this study, gene expression alterations under drought stress are investigated with bioinformatics studies to examine evolutionary relations of conserved motifs structure and interactions among Calvin cycle and photorespiration pathways key genes in drought-tolerant (SLM046) and drought-sensitive (Hayola308) genotypes of rapeseed. Investigating the conservation and evolutionary relationships revealed high conservation in motifs of FBPase, PRK, GlyK and NADP-ME enzymes. The analysis of protein interactions showed the correlation between FTRC, FBPase1, PRKX1, GlyKX2 and NADP-ME4 genes. Furthermore, in rapeseed, for the GlyKX2 and NADP-ME4 genes, four microRNAs of the miR172 family and four members of the miR167 family were identified as post-transcriptional regulators, respectively. The expression of ferredoxin thioredoxin reductase, fructose-1,6-bisphosphatase genes, phosphoribulokinase, glycerate kinase and malic enzyme 4 genes in the two rapeseed genotypes were evaluated by real-time qPCR method under 72h of drought stress and methanol foliar application. As a result, the highest expression levels of FTRC, PRKX1, GlyKX2, NADP-ME4 and FBPase1 were observed in methanol foliar application on the SLM046 genotype at 24h. In contrast, in methanol foliar application on the Hayola308 genotype, the highest expression levels of FTRC, PRKX1, GlyKX2, NADP-ME4 and FBPase1 were observed 8h after the treatment. Our study illustrated that methanol foliar application enhanced plant tolerance under drought stress.

ZmFdC2 Encoding a Ferredoxin Protein With C-Terminus Extension Is Indispensable for Maize Growth

As important electron carriers, ferredoxin (Fd) proteins play important roles in photosynthesis, and the assimilation of CO₂, nitrate, sulfate, and other metabolites. In addition to the well-studied Fds, plant genome encodes two Fd-like protein members named FdC1 and FdC2, which have extension regions at the C-terminus of the 2Fe-2S cluster. Mutation or overexpression of FdC genes caused alterations in photosynthetic electron transfer rate in rice and Arabidopsis. Maize genome contains one copy of each FdC gene. However, the functions of these genes have not been reported. In this study, we identified the ZmFdC2 gene by forward genetics approach. Mutation of this gene causes impaired photosynthetic electron transport and collapsed chloroplasts. The mutant plant is seedling-lethal, indicating the indispensable function of ZmFdC2 gene in maize development. The ZmFdC2 gene is specifically expressed in photosynthetic tissues and induced by light treatment, and the encoded protein is localized on chloroplast, implying its specialized function in photosynthesis. Furthermore, ZmFdC2 expression was detected in both mesophyll cells and bundle sheath cells, the two cell types specialized for C4 and C3 photosynthesis pathways in maize. Epigenomic analyses showed that ZmFdC2 locus was enriched for active histone modifications. Our results demonstrate that ZmFdC2 is a key component of the photosynthesis pathway and is crucial for the development of maize.

Crystal structure of thioredoxin 1 from Cryptococcus neoformans at 1.8 Å resolution shows unexpected plasticity of the loop preceding the catalytic site

An elevated prevalence of cryptococcal infection is a tendency in low-income countries and constitutes a global public health problem due to factors such as the limited efficacy of antifungal therapy and the AIDS/transplant immunocompromised patients. The fungus Cryptococcus neoformans, implicated in this burden, has had several genes validated as drug targets. Among them, the thioredoxin system is one of the major regulators of redox homeostasis and antioxidant defense acting on protein disulfide bonds. Thioredoxin 1 from C. neoformans (CnTrx1) was cloned and expressed in E. coli and the recombinant protein was purified and crystallized. Functional assay shows that CnTrx1 catalyzes the reduction of insulin disulfide bonds using dithiothreitol, while acting as a monomer in solution. The crystal structure of oxidized CnTrx1 at 1.80 Å resolution presents a dimer in the asymmetric unit with typical Trx-fold. Differences between the monomers in the asymmetric unit are found specially in the loop leading to the Cys-Gly-Pro-Cys active-site motif, being even larger when compared to those found between reduced and oxidized states of other thioredoxins. Although the thioredoxins have been isolated and characterized from many organisms, this new structural report provides important clues for understanding the binding and specificity of CnTrx1 to its targets.

Small Molecules Govern Thiol Redox Switches

Oxygenic photosynthesis gave rise to a regulatory mechanism based on reversible redox-modifications of enzymes. In chloroplasts, such on-off switches separate metabolic pathways to avoid futile cycles. During illumination, the redox interconversions allow for rapidly and finely adjusting activation states of redox-regulated enzymes. Noncovalent effects by metabolites binding to these enzymes, here addressed as 'small molecules', affect the rates of reduction and oxidation. The chloroplast enzymes provide an example for a versatile regulatory principle where small molecules govern thiol switches to integrate redox state and metabolism for an appropriate response to environmental challenges. In general, this principle can be transferred to reactive thiols involved in redox signaling, oxidative stress responses, and in disease of all organisms.

FdC1 and Leaf-Type Ferredoxins Channel Electrons From Photosystem I to Different Downstream Electron Acceptors

Plant-type ferredoxins in Arabidopsis transfer electrons from the photosystem I to multiple redox-driven enzymes involved in the assimilation of carbon, nitrogen, and sulfur. Leaf-type ferredoxins also modulate the switch between the linear and cyclic electron routes of the photosystems. Recently, two novel ferredoxin homologs with extra C-termini were identified in the Arabidopsis genome (AtFdC1, AT4G14890; AtFdC2, AT1G32550). FdC1 was considered as an alternative electron acceptor of PSI under extreme ferredoxin-deficient conditions. Here, we showed that FdC1 could interact with some, but not all, electron acceptors of leaf-type Fds, including the ferredoxin-thioredoxin reductase (FTR), sulfite reductase (SiR), and nitrite reductase (NiR). Photoreduction assay on cytochrome c and enzyme assays confirmed its capability to receive electrons from PSI and donate electrons to the Fd-dependent SiR and NiR but not to the ferredoxin-NADP⁺ oxidoreductase (FNR). Hence, FdC1 and leaf-type Fds may play differential roles by channeling electrons from photosystem I to different downstream electron acceptors in photosynthetic tissues. In addition, the median redox potential of FdC1 may allow it to receive electrons from FNR in non-photosynthetic plastids.

Regulation by S-nitrosylation of the Calvin-Benson cycle fructose-1,6-bisphosphatase in Pisum sativum

Redox regulation is of great importance in chloroplasts. Many chloroplast enzymes, such as those belonging to the Calvin-Benson cycle (CBC), have conserved regulatory cysteines which form inhibitory disulphide bridges when physiological conditions become unfavourable. Amongst these enzymes, cFBP1, the CBC fructose-1,6-bisphosphatase (FBPase) isoform, is well known to be redox activated by thioredoxin f through the reduction of a disulphide bridge involving Cys153 and Cys173. Moreover, data obtained during recent years point to S-nitrosylation as another redox post-translational modification putatively regulating an increasing number of plant enzymes, including cFBP1. In this study we have shown that the Pisum sativum cFBP1 can be efficiently S-nitrosylated by GSNO and SNAP, triggering the formation of the regulatory disulphide. Using in vivo experiments with P. sativum we have established that cFBP1 S-nitrosylation only occurs during the light period and we have elucidated by activity assays with Cys-to-Ser mutants that this enzyme may be inactivated through the S-nitrosylation of Cys153. Finally, in the light of the new data, we have proposed an extended redox-regulation model by integrating the S-nitrosylation and the TRX f-mediated regulation of cFBP1.

The role of ferredoxin:thioredoxin reductase/thioredoxin m in seed germination and the connection between this system and copper ion toxicity

Seed germination is highly sensitive to changes in the surrounding environment. This work examined the impact of imbibition with copper solution on the germination rate and behavior of some enzyme capacities involved in stress response. Chickpea (Cicer arietinum L.) seeds were germinated at 25°C in the dark for 7 days of imbibition with distilled water or an aqueous solution of chloride salt of 100 or 500μM CuCl2. The exposure of seeds to copper (Cu(2+)) induced changes in the antioxidant status. In Cu-treated seeds, the non-protein thiols (—SHNP) pool and ferredoxin:thioredoxin reductase (FTR) expression and activity increased. Cysteinyl sulfurs in the thioredoxin (Trx) function as ligands for metal ions. The accumulation of Cu(2+) inhibited seed germination and embryo growth. It appears that the FTR system mediates a novel form of redox signaling in plants under copper excess.

Plant type ferredoxins and ferredoxin-dependent metabolism

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.

Thioredoxin redox regulates ATPase activity of magnesium chelatase CHLI subunit and modulates redox-mediated signaling in tetrapyrrole biosynthesis and homeostasis of reactive oxygen species in pea plants

The chloroplast thioredoxins (TRXs) function as messengers of redox signals from ferredoxin to target enzymes. In this work, we studied the regulatory impact of pea (Pisum sativum) TRX-F on the magnesium (Mg) chelatase CHLI subunit and the enzymatic activation of Mg chelatase in vitro and in vivo. In vitro, reduced TRX-F activated the ATPase activity of pea CHLI and enhanced the activity of Mg chelatase reconstituted from the three recombinant subunits CHLI, CHLD, and CHLH in combination with the regulator protein GENOMES UNCOUPLED4 (GUN4). Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that TRX-F physically interacts with CHLI but not with either of the other two subunits or GUN4. In vivo, virus-induced TRX-F gene silencing (VIGS-TRX-F) in pea plants did not result in an altered redox state of CHLI. However, simultaneous silencing of the pea TRX-F and TRX-M genes (VIGS-TRX-F/TRX-M) resulted in partially and fully oxidized CHLI in vivo. VIGS-TRX-F/TRX-M plants demonstrated a significant reduction in Mg chelatase activity and 5-aminolevulinic acid synthesizing capacity as well as reduced pigment content and lower photosynthetic capacity. These results suggest that, in vivo, TRX-M can compensate for a lack of TRX-F and that both TRXs act as important redox regulators of Mg chelatase. Furthermore, the silencing of TRX-F and TRX-M expression also affects gene expression in the tetrapyrrole biosynthesis pathway and leads to the accumulation of reactive oxygen species, which may also serve as an additional signal for the transcriptional regulation of photosynthesis-associated nuclear genes.

Structural and evolutionary aspects of thioredoxin reductases in photosynthetic organisms

Thioredoxins (Trxs) are small oxidoreductases that are involved in redox homeostasis and are found in large numbers in the subcellular compartments of eukaryotic plant cells, including the chloroplasts. Also present in chloroplasts are two forms of thioredoxin reductase (TR), which use either NADPH or ferredoxin as an electron donor. In other compartments, two additional TR forms also use NADPH: one is distributed in all photosynthetic organisms and is similar to prokaryotic enzymes, whereas the other is restricted to algae and is similar to mammalian selenoproteins. Here, we review current knowledge of the different forms of TRs across organisms and discuss the possible evolutionary fate of this class of enzymes, which provide an example of convergent functional evolution.

Changing sugar partitioning in FBPase-manipulated plants

This review offers an overview of the current state of our knowledge concerning the role of fructose-1,6-bisphosphatase (FBPase) in sugar partitioning and biosynthesis, through the analysis of genetically manipulated plants. The existence of two well-characterized isoforms is a consequence of the subcellular compartmentalization of photosynthetic eukaryotes, conditioning their respective regulatory mechanisms and their influence over plant metabolism and photosynthesis. Both isoforms are important, as has been deduced from previous work with different plant species, although there is still much to be done in order to gain a definitive vision of this issue. Despite that, alteration of the FBPase content follows a general pattern, there are some differences that could be considered species-specific. Modifications lead to profound changes in the carbohydrate content and carbon allocation, raising questions as to whether flux of some sugars or sugar precursors from one side to the other of the chloroplast envelope occurs to rebalance carbohydrate metabolism or whether other compensatory, though not fully efficient, enzymatic activities come into play. Due to the pleiotropic nature of modifying the core carbon metabolism, an answer to the above questions would require an exhaustive study involving diverse aspects of plant physiology.

Role of histidine-86 in the catalytic mechanism of ferredoxin:thioredoxin reductase

Ferredoxin:thioredoxin reductase catalyzes the reduction of thioredoxins in plant chloroplasts using the [Fe2S2] ferredoxin as a one-electron donor and as such plays a central role in light regulation of oxygenic photosynthesis. The active-site comprises a [Fe4S4] cluster next to a redox-active disulfide that is cleaved in sequential one-electron steps and the combination of spectroscopic and crystallographic studies have revealed a catalytic mechanism involving novel site specific cluster chemistry in the oxidized, one-electron- and two-electron-reduced redox states. Histidine-86 has emerged as a potential proton donor/acceptor in the catalytic mechanism based on redox-related changes in the positioning of the imidazole ring during redox cycling and greatly decreased activity for the H86Y variant. Here we report on spectroscopic and redox characterization of the [Fe4S4] center in Synechocystis sp. PCC 6803 H86Y ferredoxin:thoredoxin reductase in the accessible redox states of both the as purified and N-ethylmaleimide-modified forms, using the combination of UV-visible absorption and variable-temperature magnetic circular dichroism, EPR, resonance Raman and Mössbauer spectroscopies. The results demonstrate that His86 is required for formation of the partially valence-localized [Fe4S4]2+ cluster that is the hallmark of two-electron-reduced intermediate. Taken together with the available structural data, the spectroscopic results indicate a functional role for His86 in protonation/deprotonation of the cluster-interacting thiol and anchoring the cluster interacting thiol in close proximity to the cluster in the two-electron-reduced intermediate.

The ferredoxin/thioredoxin system of oxygenic photosynthesis

Forty years ago, ferredoxin (Fdx) was shown to activate fructose 1,6-bisphosphatase in illuminated chloroplast preparations, thereby laying the foundation for the field now known as "redox biology." Enzyme activation was later shown to require the ubiquitous protein thioredoxin (Trx), reduced photosynthetically by Fdx via an enzyme then unknown-ferredoxin:thioredoxin reductase (FTR). These proteins, Fdx, FTR, and Trx, constitute a regulatory ensemble, the "Fdx/Trx system." The redox biology field has since grown beyond all expectations and now embraces a spectrum of processes throughout biology. Progress has been notable with plants that possess not only the plastid Fdx/Trx system, but also the earlier known NADP/Trx system in the cytosol, endoplasmic reticulum, and mitochondria. Plants contain at least 19 types of Trx (nine in chloroplasts). In this review, we focus on the structure and mechanism of action of members of the photosynthetic Fdx/Trx system and on biochemical processes linked to Trx. We also summarize recent evidence that extends the Fdx/Trx system to amyloplasts-heterotrophic plastids functional in the biosynthesis of starch and other cell components. The review highlights the plant as a model system to uncover principles of redox biology that apply to other organisms.

Transcript profiling indicates that the absence of PsbO affects the coordination of C and N metabolism in Synechocystis sp. PCC 6803

Transcript profiling of nitrate-grown Synechocystis sp. PCC 6803 PsbO-free mutant cells in comparison to wild-type (WT) detected substantial deviations. Because we had previously observed phenotypical differences between Synechocystis sp. PCC 6803 WT and its corresponding PsbO-free mutant when cultivated with l-arginine as sole N source and a light intensity of 200 mumol photons m(-2) s(-1), we also performed transcript profiling for both strains grown either with nitrate or with l-arginine as sole N source. We observed a total number of 520 differentially regulated transcripts in Synechocystis WT because of a shift from nitrate- to l-arginine-containing BG11 medium, while we detected only 13 differentially regulated transcripts for the PsbO-free mutant. Thus, the PsbO-free Synechocystis mutant had already undergone a preconditioning process for growth with l-arginine in comparison to WT. While Synechocystis WT suffered from growth with l-arginine at a light intensity of 200 mumol photons m(-2) s(-1), the PsbO-free mutant developed only a minor stress phenotype. In summary, our results suggest that the absence of PsbO in Synechocystis affects the coordination of photosynthesis/respiration and l-arginine metabolism through complex probably redox-mediated regulatory pathways. In addition, we show that a comparison of the transcriptomes of nitrate-grown Synechococcus elongatus PCC 7942 WT cells and its corresponding PsbO-free mutant cells resulted in only a few differentially regulated transcripts between both strains. The absence of the manganese/calcium-stabilizing PsbO protein of PSII with an assigned regulatory function for photosynthetic water oxidation causes bigger changes in the transcriptome of the permissive photoheterotrophically growing Synechocystis sp. PCC 6803 than in the transcriptome of the obligate photoautotrophically growing S. elongatus PCC 7942.

Structural snapshots along the reaction pathway of ferredoxin–thioredoxin reductase

Oxygen-evolving photosynthetic organisms regulate carbon metabolism through a light-dependent redox signalling pathway. Electrons are shuttled from photosystem I by means of ferredoxin (Fdx) to ferredoxin-thioredoxin reductase (FTR), which catalyses the two-electron-reduction of chloroplast thioredoxins (Trxs). These modify target enzyme activities by reduction, regulating carbon flow. FTR is unique in its use of a [4Fe-4S] cluster and a proximal disulphide bridge in the conversion of a light signal into a thiol signal. We determined the structures of FTR in both its one- and its two-electron-reduced intermediate states and of four complexes in the pathway, including the ternary Fdx-FTR-Trx complex. Here we show that, in the first complex (Fdx-FTR) of the pathway, the Fdx [2Fe-2S] cluster is positioned suitably for electron transfer to the FTR [4Fe-4S] centre. After the transfer of one electron, an intermediate is formed in which one sulphur atom of the FTR active site is free to attack a disulphide bridge in Trx and the other sulphur atom forms a fifth ligand for an iron atom in the FTR [4Fe-4S] centre--a unique structure in biology. Fdx then delivers a second electron that cleaves the FTR-Trx heterodisulphide bond, which occurs in the Fdx-FTR-Trx complex. In this structure, the redox centres of the three proteins are aligned to maximize the efficiency of electron transfer from the Fdx [2Fe-2S] cluster to the active-site disulphide of Trxs. These results provide a structural framework for understanding the mechanism of disulphide reduction by an iron-sulphur enzyme and describe previously unknown interaction networks for both Fdx and Trx (refs 4-6).

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

Characterization of Ferredoxin:Thioredoxin Reductase Modified by Site-directed Mutagenesis

Ferredoxin:thioredoxin reductase (FTR) is a key regulatory enzyme of oxygenic photosynthetic cells involved in the reductive regulation of important target enzymes. It catalyzes the two-electron reduction of the disulfide of thioredoxins with electrons from ferredoxin involving a 4Fe-4S cluster and an adjacent active-site disulfide. We replaced Cys-57, Cys-87, and His-86 in the active site of Synechocystis FTR by site-directed mutagenesis and studied the properties of the mutated proteins. Mutation of either of the active-site cysteines yields inactive enzymes, which have different spectral properties, indicating a reduced Fe-S cluster when the inaccessible Cys-87 is replaced and an oxidized cluster when the accessible Cys-57 is replaced. The oxidized cluster in the latter mutant can be reversibly reduced with dithionite showing that it is functional. The C57S mutant is a very stable protein, whereas the C87A mutant is more labile because of the missing interaction with the cluster. The replacement of His-86 greatly reduces its catalytic activity supporting the proposal that His-86 increases the nucleophilicity of the neighboring cysteine. Ferredoxin forms non-covalent complexes with wild type (WT) and mutant FTRs, which are stable except with the C87A mutant. WT and mutant FTRs form stable covalent heteroduplexes with active-site modified thioredoxins. In particular, heteroduplexes formed with WT FTR represent interesting one-electron-reduced reaction intermediates, which can be split by reduction of the Fe-S cluster. Heteroduplexes form non-covalent complexes with ferredoxin demonstrating the ability of FTR to simultaneously dock thioredoxin and ferredoxin, which is in accord with the proposed reaction mechanism and the structural analyses.

Structure of thioredoxin from Trypanosoma brucei brucei

The three-dimensional structure of thioredoxin from Trypanosoma brucei brucei has been determined at 1.4 A resolution. The overall structure is more similar to that of human thioredoxin than to any other thioredoxin structure. The most striking difference to other thioredoxins is the absence of a buried carboxylate behind the active site cysteines. Instead of the common Asp, there is a Trp that binds an ordered water molecule probably involved in the protonation/deprotonation of the more buried cysteine during catalysis. The conserved Trp in the WCGPC sequence motif has an exposed position that can interact with target proteins.

N-terminal truncation of the variable subunit stabilizes spinach ferredoxin:thioredoxin reductase

The variable subunit of spinach ferredoxin:thioredoxin reductase (FTR) has an extended N-terminus compared to FTRs from other sources and this was proposed to contribute to the instability of the protein. We constructed two N-terminal truncation mutants of recombinant FTR by removing 16 or 24 residues from the variable subunit. The mutant proteins are readily expressed and show half-saturation values (S(0.5)) for ferredoxin and thioredoxin f comparable to WT. However, truncation increases significantly their stability. Using the stabilized FTR an exposed Cys on its thioredoxin contact surface could be substituted without altering its properties, whereas the replacement of an active site Cys by Ser completely destabilized the protein.

Redox signaling in the chloroplast: the ferredoxin/thioredoxin system

Chloroplasts have developed a light-dependent system for the control of the activities of key enzymes involved in assimilatory (photosynthetic) and dissimilatory pathways, which allows a switch between these opposing pathways to prevent futile cycling. This regulatory system, known as the ferredoxin/thioredoxin system, consists of several proteins constituting a redox cascade that transmits the light signal perceived by chlorophyll to selected target proteins, thereby influencing their activity. A central component of the redox cascade is a novel enzyme, the ferredoxin:thioredoxin reductase, which is capable of reducing a disulfide bridge with the help of an iron-sulfur cluster. Recent developments on the elucidation of the structures of several implicated proteins and on the mechanism of signal transfer have greatly improved our understanding of this regulatory mechanism. This review describes the components of the redox cascade, the principal target proteins, and the mechanism of action of the light-signal transfer.

Redox control by dithiol-disulfide exchange in plants: I. The chloroplastic systems

In plants, the photons of light are absorbed at the level of the photosystems in the chloroplasts. The functioning of the photosynthetic electron transfer chain linked to this process is required to generate NADPH and ATP. In addition, the light signal promotes a regulatory cascade, situated in the stroma, that involves ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxins. This redox-based signal transduction chain allows fine regulation of stromal enzymes and tight control of the photosynthetic process. The molecular properties and the functioning of this redox regulatory chain will be described in this review.

Thioredoxins and related proteins in photosynthetic organisms: molecular basis for thiol dependent regulation

Thioredoxins are small molecular weight disulfide oxidoreductases specialized in the reduction of disulfide bonds on other proteins. Generally, the enzymes which are selectively and reversibly reduced by these proteins oscillate between an oxidized and inactive conformation and a reduced and active conformation. Thioredoxin constitutes the archetype of a family of protein disulfide oxidoreductases which comprises glutaredoxin and protein disulfide isomerase. Thioredoxin and glutaredoxin serve many roles in the cell, including the redox regulation of target enzymes and transcription factors. They can also serve as hydrogen donors to peroxiredoxins, recently discovered heme free peroxidases, the function of which is to get rid of hydroperoxides in the cell. This review describes the molecular basis for the functioning and interaction between these enzymes in photosynthetic organisms.