Analysis of reductant supply systems for ferredoxin-dependent sulfite reductase in photosynthetic and nonphotosynthetic organs of maize

Sulfite reductase (SiR) catalyzes the reduction of sulfite to sulfide in chloroplasts and root plastids using ferredoxin (Fd) as an electron donor. Using purified maize (Zea mays L.) SiR and isoproteins of Fd and Fd-NADP(+) reductase (FNR), we reconstituted illuminated thylakoid membrane- and NADPH-dependent sulfite reduction systems. Fd I and L-FNR were distributed in leaves and Fd III and R-FNR in roots. The stromal concentrations of SiR and Fd I were estimated at 1.2 and 37 microM, respectively. The molar ratio of Fd III to SiR in root plastids was approximately 3:1. Photoreduced Fd I and Fd III showed a comparable ability to donate electrons to SiR. In contrast, when being reduced with NADPH via FNRs, Fd III showed a several-fold higher activity than Fd I. Fd III and R-FNR showed the highest rate of sulfite reduction among all combinations tested. NADP(+) decreased the rate of sulfite reduction in a dose-dependent manner. These results demonstrate that the participation of Fd III and high NADPH/NADP(+) ratio are crucial for non-photosynthetic sulfite reduction. In accordance with this view, a cysteine-auxotrophic Escherichia coli mutant defective for NADPH-dependent SiR was rescued by co-expression of maize SiR with Fd III but not with Fd I.     

Electron acceptor specificity of ferredoxin (flavodoxin):NADP+ oxidoreductase from Escherichia coli

Reduced flavodoxin I (Fld1) is required in Escherichia coli for reductive radical generation in AdoMet-dependent radical enzymes and reductive activation of cobalamin-dependent methionine synthase. Ferredoxin (Fd) and flavodoxin II (Fld2) are also present, although their precise roles have not been ascertained. Ferredoxin (flavodoxin):NADP+ oxidoreductase (FNR) was discovered in E. coli as an NADPH-dependent reductant of Fld1 that facilitated generation of active methionine synthase in vitro; FNR and Fld1 will also supply electrons for the reductive cleavage of AdoMet essential for generating protein or substrate radicals in pyruvate formate-lyase, class III ribonucleotide reductase, biotin synthase, and, potentially, lipoyl synthase. As part of ongoing efforts to understand the various redox pathways that will support AdoMet-dependent radical enzymes in E. coli, we have examined the relative specificity of E. coli FNR for Fd, Fld1, and Fld2. While FNR will reduce all three proteins, Fd is the kinetically and thermodynamically preferred partner. Fd binds to FNR with high affinity (K(d)相似文献   

Highly nonproductive complexes with Anabaena ferredoxin at low ionic strength are induced by nonconservative amino acid substitutions at Glu139 in Anabaena ferredoxin:NADP+ reductase

Ferredoxin (Fd) and ferredoxin:NADP(+) reductase (FNR) from Anabaena function in photosynthetic electron transfer (et). The et interaction between the FNR charge-reversal mutant E139K and Fd at 12 mM ionic strength (mu) is extremely impaired relative to the reaction with wt FNR, and the dependency of k(obs) on E139K concentration shows strong upward curvature at protein concentrations > or = 10 microM. However, at values of mu > or = 200 mM, reaction rates approach those of wild-type FNR, and normal saturation kinetics are observed. For the E139Q mutant, which is also significantly impaired in its et interaction with Fd at low FNR concentrations and low mu values, the dependency of k(obs) on E139Q concentration shows a smaller degree of upward curvature at mu = 12 and 100 mM and shows saturation kinetics at higher values of mu. wt FNR and the E139D mutant both show a slight amount of upward curvature at FNR concentrations >30 microM at mu = 12 mM but show the expected saturation kinetics at higher values of mu. These results are explained by a mechanism in which the mutual orientation of the proteins in the complex formed at low ionic strength with the E139K mutant is so far from optimal that it is almost unreactive. At increased E139K concentrations, the added mutant FNR reacts via a collisional interaction with the reduced Fd present in the unreactive complex. The et reactivity of the low ionic strength complexes depends on the particular amino acid substitution, which via electrostatic interactions alters the specific geometry of the interface between the two proteins. The presence of a negative charge at position 139 of FNR allows the most optimal orientations for et at ionic strengths below 200 mM.     

PetH is rate-controlling in the interaction between PetH, a component of the supramolecular complex with photosystem II, and PetF, a light-dependent electron transfer protein

Cyanobacterial PetH is similar to ferredoxin-NADP⁺ oxidoreductase (FNR) of higher plants and comprises 2 components, CpcD-like rod linker and FNR proteins. Here, I show that PetH controls the rate of the interaction with PetF (ferredoxin [Fd1]). Purified recombinant PetH protein, which cut off a CpcD-like rod linker domain, and Fd1 were used in detailed surface plasmon resonance analyses. The interaction between FNR and Fd1 chiefly involved extremely fast binding and dissociation reactions and the FNR affinity for Fd1 was stronger than the Fd1 affinity for FNR. The dissociation constant values were determined as approximately 93.65 μM (FNR) for Fd1 and 1.469 mM (Fd1) for FNR.     

Comparison of the electrostatic binding sites on the surface of ferredoxin for two ferredoxin-dependent enzymes, ferredoxin-NADP(+) reductase and sulfite reductase.

Plant-type ferredoxin (Fd), a [2Fe-2S] iron-sulfur protein, functions as an one-electron donor to Fd-NADP(+) reductase (FNR) or sulfite reductase (SiR), interacting electrostatically with them. In order to understand the protein-protein interaction between Fd and these two different enzymes, 10 acidic surface residues in maize Fd (isoform III), Asp-27, Glu-30, Asp-58, Asp-61, Asp-66/Asp-67, Glu-71/Glu-72, Asp-85, and Glu-93, were substituted with the corresponding amide residues by site-directed mutagenesis. The redox potentials of the mutated Fds were not markedly changed, except for E93Q, the redox potential of which was more positive by 67 mV than that of the wild type. Kinetic experiments showed that the mutations at Asp-66/Asp-67 and Glu-93 significantly affected electron transfer to the two enzymes. Interestingly, D66N/D67N was less efficient in the reaction with FNR than E93Q, whereas this relationship was reversed in the reaction with SiR. The static interaction of the mutant Fds with each the two enzymes was analyzed by gel filtration of a mixture of Fd and each enzyme, and by affinity chromatography on Fd-immobilized resins. The contributions of Asp-66/Asp-67 and Glu-93 were found to be most important for the binding to FNR and SiR, respectively, in accordance with the kinetic data. These results allowed us to map the acidic regions of Fd required for electron transfer and for binding to FNR and SiR and demonstrate that the interaction sites for the two enzymes are at least partly distinct.     

Roles of the species-specific subdomain and the N-terminal peptide of Toxoplasma gondii ferredoxin-NADP+ reductase in ferredoxin binding

The plant-type ferredoxin/ferredoxin-NADP(+) reductase (Fd/FNR) redox system found in parasites of the phylum Apicomplexa has been proposed as a target for novel drugs used against life-threatening diseases such as malaria and toxoplasmosis. Like many proteins from these protists, apicomplexan FNRs are characterized by the presence of unique peptide insertions of variable length and yet unknown function. Since three-dimensional data are not available for any of the parasite FNRs, we used limited proteolysis to carry out an extensive study of the conformation of Toxoplasma gondii FNR. This led to identification of 11 peptide bonds susceptible to the action of four different proteases. Cleavage sites are clustered in four regions of the enzyme, which include two of its three species-specific insertions. Such regions are thus predicted to form flexible surface loops. The protein substrate Fd protected FNR against cleavage both at its N-terminal peptide and at its largest sequence insertion (28 residues). Deletion by protein engineering of the species-specific subdomain containing the latter insertion resulted in an enzyme form that, although catalytically active, displayed a 10-fold decreased affinity for Fd. In contrast, removal of the first 15 residues of the enzyme unexpectedly enhanced its interaction with Fd. Thus, two flexible polypeptide regions of T. gondii FNR are involved in Fd interaction but have opposite roles in modulating the binding affinity for the protein ligand. In this respect, T. gondii FNR differs from plant FNRs, where the N-terminal peptide contributes to the stabilization of their complex with Fd.     

Two isoforms of ferredoxin:NADP+ oxidoreductase from wheat leaves: purification and initial biochemical characterization

Ferredoxin:NADP⁺ oxidoreductase is an enzyme associated with the stromal side of the thylakoid membrane in the chloroplast. It is involved in photosynthetic linear electron transport to produce NADPH and is supposed to play a role in cyclic electron transfer, generating a transmembrane pH gradient allowing ATP production, if photosystem II is non-functional or no NADP⁺ is available for reduction. Different FNR isoforms have been described in non-photosynthetic tissues, where the enzyme catalyses the NADPH-dependent reduction of ferredoxin (Fd), necessary for some biosynthetic pathways. Here, we report the isolation and purification of two FNR isoproteins from wheat leaves, called FNR-A and FNR-B. These forms of the enzyme were identified as products of two different genes, as confirmed by mass spectrometry. The molecular masses of FNR-A and FNR-B were 34.3 kDa and 35.5 kDa, respectively. The isoelectric point of both FNR-A and FNR-B was about 5, but FNR-B appeared more acidic (of about 0.2 pH unit) than FNR-A. Both isoenzymes were able to catalyse a NADPH-dependent reduction of dibromothymoquinone and the mixture of isoforms catalysed reduction of cytochrome c in the presence of Fd. For the first time, the pH- and ionic strength dependent oligomerization of FNRs is observed. No other protein was necessary for complex formation. The putative role of the two FNR isoforms in photosynthesis is discussed based on current knowledge of electron transport in chloroplasts.     

A single in vivo-selected point mutation in the active center of Toxoplasma gondii ferredoxin-NADP+ reductase leads to an inactive enzyme with greatly enhanced affinity for ferredoxin

Electron transfer between plant-type [2Fe-2S] ferredoxin (Fd) and ferredoxin-NADP+ reductase (FNR) depends on the physical interaction between both proteins. We have applied a random mutagenesis approach with subsequent in vivo selection using the yeast two-hybrid system to obtain mutants of Toxoplasma gondii FNR with higher affinity for Fd. One mutant showed a 10-fold enhanced binding using affinity chromatography on immobilized Fd. A single serine-to-arginine exchange in the active site was responsible for its increased affinity. The mutant reductase was also enzymatically inactive. Homology modeling of the mutant FNR-Fd complex predicts substantial alterations of protein-FAD interactions in the active site of the enzyme with subsequent structural changes. Collectively, for the first time a point mutation in this important class of enzymes is described which leads to greatly enhanced affinity for its protein ligand.     

Identification of N-terminal regions of wheat leaf ferredoxin NADP+ oxidoreductase important for interactions with ferredoxin

Wheat leaves contain two isoproteins of the photosynthetic ferredoxin:NADP(+) reductase (pFNRI and pFNRII). Truncated forms of both enzymes have been detected in vivo, but only pFNRII displays N-terminal length-dependent changes in activity. To investigate the impact of N-terminal truncation on interaction with ferredoxin (Fd), recombinant pFNRII proteins, differing by deletions of up to 25 amino acids, were generated. During purification of the isoproteins found in vivo, the longer forms of pFNRII bound more strongly to a Fd affinity column than did the shorter forms, pFNRII(ISKK) and pFNRII[N-2](KKQD). Further truncation of the N-termini resulted in a pFNRII protein which failed to bind to a Fd column. Similar k(cat) values (104-140 s(-1)) for cytochrome c reduction were measured for all but the most truncated pFNRII[N-5](DEGV), which had a k(cat) of 38 s(-1). Stopped-flow kinetic studies, examining the impact of truncation on electron flow between mutant pFNRII proteins and Fd, showed there was a variation in k(obs) from 76 to 265 s(-1) dependent on the pFNRII partner. To analyze the sites which contribute to Fd binding at the pFNRII N-terminal, three mutants were generated, in which a single or double lysine residue was changed to glutamine within the in vivo N-terminal truncation region. The mutations affected binding of pFNRII to the Fd column. Based on activity measurements, the double lysine residue change resulted in a pFNRII enzyme with decreased Fd affinity. The results highlight the importance of this flexible N-terminal region of the pFNRII protein in binding the Fd partner.     

Binding of ferredoxin to ferredoxin:NADP+ oxidoreductase: the role of carboxyl groups, electrostatic surface potential, and molecular dipole moment.

The small, soluble, (2Fe-2S)-containing protein ferredoxin (Fd) mediates electron transfer from the chloroplast photosystem I to ferredoxin: NADP+ oxidoreductase (FNR), a flavoenzyme located on the stromal side of the thylakoid membrane. Ferredoxin and FNR form a 1:1 complex, which is stabilized by electrostatic interactions between acidic residues of Fd and basic residues of FNR. We have used differential chemical modification of Fd to locate aspartic and glutamic acid residues at the intermolecular interface of the Fd:FNR complex (both proteins from spinach). Carboxyl groups of free and FNR-bound Fd were amidated with carbodiimide/2-aminoethane sulfonic acid (taurine). The differential reactivity of carboxyl groups was assessed by double isotope labeling. Residues protected in the Fd:FNR complex were D-26, E-29, E-30, D-34, D-65, and D-66. The protected residues belong to two domains of negative electrostatic surface potential on either side of the iron-sulfur cluster. The negative end of the molecular dipole moment vector of Fd (377 Debye) is close to the iron-sulfur cluster, in the center of the area demarcated by the protected carboxyl groups. The molecular dipole moment and the asymmetric surface potential may help to orient Fd in the reaction with FNR. In support, we find complementary domains of positive electrostatic potential on either side of the FAD redox center of FNR. The results allow a binding model for the Fd:FNR complex to be constructed.     

Binding of ferredoxin NADP+ oxidoreductase (FNR) to plant photosystem I

The binding of FNR to PSI has been postulated long ago, however, a clear evidence is still missing. In this work, using isothermal titration calorimetry (ITC), we found that FNR binds to photosystem I with its light harvesting complex I (PSI-LHCI) from C. reinhardtii with a 1:1 stoichiometry, a Kd of ~0.8 μM and ?H of ?20.7 kcal/mol. Titrations at different temperatures were used to determine the heat capacity change, ?CP, of the binding, through which the size of the interface area between the proteins was assessed as ~3000 Ų. In a different set of ITC experiments, introduction of various sucrose concentrations was used to estimate that ~95 water molecules are released to the solvent. These observations support the notion of a binding site shared by few of the photosystem I - light harvesting complex I (PSI-LHCI) subunits in addition to PsaE. Based on these results, a hypothetical model was built for the binding site of FNR at PSI, using known crystallographic structures of: cyanobacterial PSI in complex with ferredoxin (Fd), plant PSI-LHCI and Fd:FNR complex from cyanobacteria. FNR binding site location is proposed to be at the foot of the stromal ridge and above the inner LHCI belt. It is expected to form contacts with PsaE, PsaB, PsaF and at least one of the LHCI. In addition, a ~4.5-fold increased affinity between FNR and PSI-LHCI under crowded 1 M sucrose environment led us to conclude that in C. reinhardtii FNR also functions as a subunit of PSI-LHCI.     

Molecular interaction of ferredoxin and ferredoxin-NADP+ reductase from human malaria parasite

The malaria parasite possesses plant-type ferredoxin (Fd) and ferredoxin-NADP(+) reductase (FNR) in a plastid-derived organelle called the apicoplast. This Fd/FNR redox system, which potentially provides reducing power for essential biosynthetic pathways in the apicoplast, has been proposed as a target for the development of specific new anti-malarial agents. We studied the molecular interaction of Fd and FNR of human malaria parasite (Plasmodium falciparum), which were produced as recombinant proteins in Escherichia coli. NMR chemical shift perturbation analysis mapped the location of the possible FNR interaction sites on the surface of P. falciparum Fd. Site-specific mutation of acidic Fd residues in these regions and the resulting analyses of electron transfer activity and affinity chromatography of those mutants revealed that two acidic regions (a region including Asp26, Glu29 and Glu34, and the other including Asp65 and Glu66) dominantly contribute to the electrostatic interaction with P. falciparum FNR. The combination of Asp26/Glu29/Glu34 conferred a larger contribution than that of Asp65/Glu66, and among Asp26, Glu29 and Glu34, Glu29 was shown to be the most important residue for the interaction with P. falciparum FNR. These findings provide the basis for understanding molecular recognition between Fd and FNR of the malaria parasite.     

Structure of the electron transfer complex between ferredoxin and ferredoxin-NADP(+) reductase

All oxygenic photosynthetically derived reducing equivalents are utilized by combinations of a single multifuctional electron carrier protein, ferredoxin (Fd), and several Fd-dependent oxidoreductases. We report the first crystal structure of the complex between maize leaf Fd and Fd-NADP(+) oxidoreductase (FNR). The redox centers in the complex--the 2Fe-2S cluster of Fd and flavin adenine dinucleotide (FAD) of FNR--are in close proximity; the shortest distance is 6.0 A. The intermolecular interactions in the complex are mainly electrostatic, occurring through salt bridges, and the interface near the prosthetic groups is hydrophobic. NMR experiments on the complex in solution confirmed the FNR recognition sites on Fd that are identified in the crystal structure. Interestingly, the structures of Fd and FNR in the complex and in the free state differ in several ways. For example, in the active site of FNR, Fd binding induces the formation of a new hydrogen bond between side chains of Glu 312 and Ser 96 of FNR. We propose that this type of molecular communication not only determines the optimal orientation of the two proteins for electron transfer, but also contributes to the modulation of the enzymatic properties of FNR.     

Expression of Maize Ferredoxin cDNA in Escherichia coli: Comparison of Photosynthetic and Nonphotosynthetic Ferredoxin Isoproteins and their Chimeric Molecule

Maize (Zea mays L.) has two types of ferredoxin (Fd) differentially expressed in photosynthetic and nonphotosynthetic organs. A cDNA fragment encoding the mature polypeptide of Fd III, an Fd isoprotein of the nonphotosynthetic type, was expressed in Escherichia coli, and the Fd was synthesized as a holo-form assembled with the [2Fe-2S] cluster, which was completely identical with authentic Fd III prepared from maize roots. This expression system made it possible to prepare Fd present at fairly low levels in plants in amounts sufficient for functional and structural studies. Comparison of electron transfer activity of Fd III with that of Fd I, an Fd isoprotein of the photosynthetic type, showed that Fd III was superior as an electron acceptor from NADPH, and Fd I was superior as an electron donor for NADP⁺, in reactions catalyzed by Fd-NADP⁺ reductase from maize leaf. The circular dichronism spectra of the two Fds also indicated a subtle difference in the geometry of their iron-sulfur clusters. These results are consistent with the view that photosynthetic and nonphotosynthetic Fds may be functionally differentiated. An artificial chimeric Fd, Fd III/Fd I, whose amino-terminal and carboxylterminal halves are derived from the corresponding regions of Fd III and Fd I, respectively, showed an activity and CD spectrum significantly similar to those of Fd I. This suggests that 18 amino acid substitutions between Fd III and Fd III/Fd I alter the properties of Fd III so that they resemble those of Fd I.     

Electron transfer of site-specifically cross-linked complexes between ferredoxin and ferredoxin-NADP(+) reductase

Ferredoxin (Fd) and Fd-NADP(+) reductase (FNR) are redox partners responsible for the conversion between NADP(+) and NADPH in the plastids of photosynthetic organisms. Introduction of specific disulfide bonds between Fd and FNR by engineering cysteines into the two proteins resulted in 13 different Fd-FNR cross-linked complexes displaying a broad range of activity to catalyze the NADPH-dependent cytochrome c reduction. This variability in activity was thought to be mainly due to different levels of intramolecular electron transfer activity between the FNR and Fd domains. Stopped-flow analysis revealed such differences in the rate of electron transfer from the FNR to Fd domains in some of the cross-linked complexes. A group of the cross-linked complexes with high cytochrome c reduction activity comparable to dissociable wild-type Fd/FNR was shown to assume a similar Fd-FNR interaction mode as in the native Fd:FNR complex by analyses of NMR chemical shift perturbation and absorption spectroscopy. However, the intermolecular electron transfer of these cross-linked complexes with two Fd-binding proteins, nitrite reductase and photosystem I, was largely inhibited, most probably due to steric hindrance by the FNR moiety linked near the redox center of the Fd domain. In contrast, another group of the cross-linked complexes with low cytochrome c reduction activity tends to mediate higher intermolecular electron transfer activity. Therefore, reciprocal relationship of intramolecular and intermolecular electron transfer abilities was conferred by the linkage of Fd and FNR, which may explain the physiological significance of the separate forms of Fd and FNR in chloroplasts.     

Ferredoxin-NADP+ reductase uses the same site for the interaction with ferredoxin and flavodoxin

The enzyme ferredoxin-NADP(+) reductase (FNR) forms a 1 : 1 complex with ferredoxin (Fd) or flavodoxin (Fld) that is stabilised by both electrostatic and hydrophobic interactions. The electrostatic interactions occur between acidic residues of the electron transfer (ET) protein and basic residues on the FNR surface. In the present study, several charge-reversal mutants of FNR have been prepared at the proposed site of interaction of the ET protein: R16E, K72E, K75E, K138E, R264E, K290E and K294E. All of these mutants have been assayed for reactivity with Fd and Fld using steady-state and stopped-flow kinetics. Their abilities for complex formation with the ET proteins have also been tested. The data presented here indicate that the mutated residues situated within the FNR FAD-binding domain are more important for achieving maximal ET rates, either with Fd or Fld, than those situated within the NADP(+)-binding domain, and that both ET proteins occupy the same region for the interaction with the reductase. In addition, each individual residue does not appear to participate to the same extent in the different processes with Fd and Fld.