Lysine residues on ferredoxin-NADP+ reductase from Anabaena sp. PCC 7119 involved in substrate binding

Interaction and electron transfer between ferredoxin-NADP+ oxidoreductase and its partners: structural, functional, and physiological implications

Ferredoxin-NADP⁺ reductase (FNR) catalyzes the last step of linear electron transfer in photosynthetic light reactions. The FAD cofactor of FNR accepts two electrons from two independent reduced ferredoxin molecules (Fd) in two sequential steps, first producing neutral semiquinone and then the fully anionic reduced, or hydroquinone, form of the enzyme (FNR_hq). FNR_hq transfers then both electrons in a single hydride transfer step to NADP⁺. We are presenting the recent progress in studies focusing on Fd:FNR interaction and subsequent electron transfer processes as well as on interaction of FNR with NADP⁺/H followed by hydride transfer, both from the structural and functional point of views. We also present the current knowledge about the physiological role(s) of various FNR isoforms present in the chloroplasts of higher plants and the functional impact of subchloroplastic location of FNR. Moreover, open questions and current challenges about the structure, function, and physiology of FNR are discussed.

Electron transferases

The flavin isoalloxazine ring in electron transferases functions in a redox capacity, being able to take up electrons from a donor to subsequently deliver them to an acceptor. The main characteristics of these flavoproteins, including their unique ability to mediate obligatory processes of two-electron transfers with those involving single-electron transfer, are here described. To illustrate the versatility of these proteins, the acquired knowledge of the function of the two electron transferases involved in the cyanobacterial photosynthetic electron transfer from photosystem I to NADP(+) is presented. Many aspects of their biochemistry and biophysics have been extensively characterized using site-directed mutagenesis, steady-state and transient kinetics, spectroscopy, calorimetry, X-ray crystallography, electron paramagnetic resonance, and computational methods.

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.

Structural analysis of interactions for complex formation between Ferredoxin-NADP+ reductase and its protein partners

The three-dimensional structures of K72E, K75R, K75S, K75Q, and K75E Anabaena Ferredoxin-NADP+ reductase (FNR) mutants have been solved, and particular structural details of these mutants have been used to assess the role played by residues 72 and 75 in optimal complex formation and electron transfer (ET) between FNR and its protein redox partners Ferredoxin (Fd) and Flavodoxin (Fld). Additionally, because there is no structural information available on the interaction between FNR and Fld, a model for the FNR:Fld complex has also been produced based on the previously reported crystal structures and on that of the rat Cytochrome P450 reductase (CPR), onto which FNR and Fld have been structurally aligned, and those reported for the Anabaena and maize FNR:Fd complexes. The model suggests putative electrostatic and hydrophobic interactions between residues on the FNR and Fld surfaces at the complex interface and provides an adequate orientation and distance between the FAD and FMN redox centers for efficient ET without the presence of any other molecule as electron carrier. Thus, the models now available for the FNR:Fd and FNR:Fld interactions and the structures presented here for the mutants at K72 and K75 in Anabaena FNR have been evaluated in light of previous biochemical data. These structures confirm the key participation of residue K75 and K72 in complex formation with both Fd and Fld. The drastic effect in FNR activity produced by replacement of K75 by Glu in the K75E FNR variant is explained not only by the observed changes in the charge distribution on the surface of the K75E FNR mutant, but also by the formation of a salt bridge interaction between E75 and K72 that simultaneously "neutralizes" two essential positive charged side chains for Fld/Fd recognition.

Structure-function relationships in Anabaena ferredoxin/ferredoxin:NADP(+) reductase electron transfer: insights from site-directed mutagenesis, transient absorption spectroscopy and X-ray crystallography

The interaction between reduced Anabaena ferredoxin and oxidized ferredoxin:NADP(+) reductase (FNR), which occurs during photosynthetic electron transfer (ET), has been investigated extensively in the authors' laboratories using transient and steady-state kinetic measurements and X-ray crystallography. The effect of a large number of site-specific mutations in both proteins has been assessed. Many of the mutations had little or no effect on ET kinetics. However, non-conservative mutations at three highly conserved surface sites in ferredoxin (F65, E94 and S47) caused ET rate constants to decrease by four orders of magnitude, and non-conservative mutations at three highly conserved surface sites in FNR (L76, K75 and E301) caused ET rate constants to decrease by factors of 25-150. These residues were deemed to be critical for ET. Similar mutations at several other conserved sites in the two proteins (D67 in Fd; E139, L78, K72, and R16 in FNR) caused smaller but still appreciable effects on ET rate constants. A strong correlation exists between these results and the X-ray crystal structure of an Anabaena ferredoxin/FNR complex. Thus, mutations at sites that are within the protein-protein interface or are directly involved in interprotein contacts generally show the largest kinetic effects. The implications of these results for the ET mechanism are discussed.

Role of a cluster of hydrophobic residues near the FAD cofactor in Anabaena PCC 7119 ferredoxin-NADP+ reductase for optimal complex formation and electron transfer to ferredoxin

In the ferredoxin-NADP(+) reductase (FNR)/ferredoxin (Fd) system, an aromatic amino acid residue on the surface of Anabaena Fd, Phe-65, has been shown to be essential for the electron transfer (ET) reaction. We have investigated further the role of hydrophobic interactions in complex stabilization and ET between these proteins by replacing three hydrophobic residues, Leu-76, Leu-78, and Val-136, situated on the FNR surface in the vicinity of its FAD cofactor. Whereas neither the ability of FNR to accept electrons from NADPH nor its structure appears to be affected by the introduced mutations, different behaviors with Fd are observed. Thus, the ET interaction with Fd is almost completely lost upon introduction of negatively charged side chains. In contrast, only subtle changes are observed upon conservative replacement. Introduction of Ser residues produces relatively sizable alterations of the FAD redox potential, which can explain the modified behavior of these mutants. The introduction of bulky aromatic side chains appears to produce rearrangements of the side chains at the FNR/Fd interaction surface. Thus, subtle changes in the hydrophobic patch influence the rates of ET to and from Fd by altering the binding constants and the FAD redox potentials, indicating that these residues are especially important in the binding and orientation of Fd for efficient ET. These results are consistent with the structure reported for the Anabaena FNR.Fd complex.

Probing the determinants of coenzyme specificity in ferredoxin-NADP+ reductase by site-directed mutagenesis

On the basis of sequence and three-dimensional structure comparison between Anabaena PCC7119 ferredoxin-NADP(+) reductase (FNR) and other reductases from its structurally related family that bind either NADP(+)/H or NAD(+)/H, a set of amino acid residues that might determine the FNR coenzyme specificity can be assigned. These residues include Thr-155, Ser-223, Arg-224, Arg-233 and Tyr-235. Systematic replacement of these amino acids was done to identify which of them are the main determinants of coenzyme specificity. Our data indicate that all of the residues interacting with the 2'-phosphate of NADP(+)/H in Anabaena FNR are not involved to the same extent in determining coenzyme specificity and affinity. Thus, it is found that Ser-223 and Tyr-235 are important for determining NADP(+)/H specificity and orientation with respect to the protein, whereas Arg-224 and Arg-233 provide only secondary interactions in Anabaena FNR. The analysis of the T155G FNR form also indicates that the determinants of coenzyme specificity are not only situated in the 2'-phosphate NADP(+)/H interacting region but that other regions of the protein must be involved. These regions, although not interacting directly with the coenzyme, must produce specific structural arrangements of the backbone chain that determine coenzyme specificity. The loop formed by residues 261-268 in Anabaena FNR must be one of these regions.

Crystal structure of paprika ferredoxin-NADP+ reductase. Implications for the electron transfer pathway

cDNA of Capsicum annuum Yolo Wonder (paprika) has been prepared from total cellular RNA, and the complete gene encoding paprika ferredoxin-NADP(+) reductase (pFNR) precursor was sequenced and cloned from this cDNA. Fusion to a T7 promoter allowed expression in Escherichia coli. Both native and recombinant pFNR were purified to homogeneity and crystallized. The crystal structure of pFNR has been solved by Patterson search techniques using the structure of spinach ferredoxin-NADP(+) reductase as search model. The structure was refined at 2.5-A resolution to a crystallographic R-factor of 19.8% (R(free) = 26.5%). The overall structure of pFNR is similar to other members of the ferredoxin-NADP(+) reductase family, the major differences concern a long loop (residues 167-177) that forms part of the FAD binding site and some of the variable loops in surface regions. The different orientation of the FAD binding loop leads to a tighter interaction between pFNR and the adenine moiety of FAD. The physiological redox partners [2Fe-2S]-ferredoxin I and NADP(+) were modeled into the native structure of pFNR. The complexes reveal a protein-protein interaction site that is consistent with existing biochemical data and imply possible orientations for the side chain of tyrosine 362, which has to be displaced by the nicotinamide moiety of NADP(+) upon binding. A reasonable electron transfer pathway could be deduced from the modeled structures of the complexes.

Electrostatic forces involved in orienting Anabaena ferredoxin during binding to Anabaena ferredoxin:NADP+ reductase: site-specific mutagenesis, transient kinetic measurements, and electrostatic surface potentials

Transient absorbance measurements following laser flash photolysis have been used to measure the rate constants for electron transfer (et) from reduced Anabaena ferredoxin (Fd) to wild-type and seven site-specific charge-reversal mutants of Anabaena ferredoxin:NADP+ reductase (FNR). These mutations have been designed to probe the importance of specific positively charged amino acid residues on the surface of the FNR molecule near the exposed edge of the FAD cofactor in the protein-protein interaction during et with Fd. The mutant proteins fall into two groups: overall, the K75E, R16E, and K72E mutants are most severely impaired in et, and the K138E, R264E, K290E, and K294E mutants are impaired to a lesser extent, although the degree of impairment varies with ionic strength. Binding constants for complex formation between the oxidized proteins and for the transient et complexes show that the severity of the alterations in et kinetics for the mutants correlate with decreased stabilities of the protein-protein complexes. Those mutated residues, which show the largest effects, are located in a region of the protein in which positive charge predominates, and charge reversals have large effects on the calculated local surface electrostatic potential. In contrast, K138, R264, K290, and K294 are located within or close to regions of intense negative potential, and therefore the introduction of additional negative charges have considerably smaller effects on the calculated surface potential. We attribute the relative changes in et kinetics and complex binding constants for these mutants to these characteristics of the surface charge distribution in FNR and conclude that the positively charged region of the FNR surface located in the vicinity of K75, R16, and K72 is especially important in the binding and orientation of Fd during electron transfer.

Complex formation between Azotobacter vinelandii ferredoxin I and its physiological electron donor NADPH-ferredoxin reductase

In Azotobacter vinelandii, deletion of the fdxA gene, which encodes ferredoxin I (FdI), leads to activation of the expression of the fpr gene, which encodes NADPH-ferredoxin reductase (FPR). In order to investigate the relationship of these two proteins further, the interactions of the two purified proteins have been examined. AvFdI forms a specific 1:1 cross-linked complex with AvFPR through ionic interactions formed between the Lys residues of FPR and Asp/Glu residues of FdI. The Lys in FPR has been identified as Lys258, a residue that forms a salt bridge with one of the phosphate oxygens of FAD in the absence of FdI. UV-Vis and circular dichroism data show that on binding FdI, the spectrum of the FPR flavin is hyperchromatic and red-shifted, confirming the interaction region close to the FAD. Cytochrome c reductase assays and electron paramagnetic resonance data show that electron transfer between the two proteins is pH-dependent and that the [3Fe-4S]+ cluster of FdI is specifically reduced by NADPH via FPR, suggesting that the [3Fe-4S] cluster is near FAD in the complex. To further investigate the FPR:FdI interaction, the electrostatic potentials for each protein were calculated. Strongly negative regions around the [3Fe-4S] cluster of FdI are electrostatically complementary with a strongly positive region overlaying the FAD of FPR, centered on Lys258. These proposed interactions of FdI with FPR are consistent with cross-linking, peptide mapping, spectroscopic, and electron transfer data and strongly support the suggestion that the two proteins are physiological redox partners.

Role of Arg100 and Arg264 from Anabaena PCC 7119 ferredoxin-NADP+ reductase for optimal NADP+ binding and electron transfer

Previous studies and the crystal structure of Anabaena PCC 7119 FNR suggest that the side chains of Arg100 and Arg264 may be directly involved in the proper NADP+/NADPH orientation for an efficient electron-transfer reaction. Protein engineering on Arg100 and Arg264 from Anabaena PCC 7119 FNR has been carried out to investigate their roles in complex formation and electron transfer to NADP+ and to ferredoxin/flavodoxin. Arg100 has been replaced with an alanine, which removes the positive charge, the long side chain, as well as the ability to form hydrogen bonds, while a charge reversal mutation has been made at Arg264 by replacing it with a glutamic acid. Results with various spectroscopic techniques indicate that the mutated proteins folded properly and that significant protein structural rearrangements did not occur. Both mutants have been kinetically characterized by steady-state as well as fast transient kinetic techniques, and the three-dimensional structure of Arg264Glu FNR has been solved. The results reported herein reveal important conceptual information about the interaction of FNR with its substrates. A critical role is confirmed for the long, positively charged side chain of Arg100. Studies on the Arg264Glu FNR mutant demonstrate that the Arg264 side chain is not critical for the nicotinamide orientation or for nicotinamide interaction with the isoalloxazine FAD moiety. However, this mutant showed altered behavior in its interaction and electron transfer with its protein partners, ferredoxin and flavodoxin.

Protein-protein interaction in electron transfer reactions: the ferredoxin/flavodoxin/ferredoxin:NADP+ reductase system from Anabaena

Electron transfer reactions involving protein-protein interactions require the formation of a transient complex which brings together the two redox centres exchanging electrons. This is the case for the flavoprotein ferredoxin:NADP+ reductase (FNR) from the cyanobacterium Anabaena, an enzyme which interacts with ferredoxin in the photosynthetic pathway to receive the electrons required for NADP+ reduction. The reductase shows a concave cavity in its structure into which small proteins such as ferredoxin can fit. Flavodoxin, an FMN-containing protein that is synthesised in cyanobacteria under iron-deficient conditions, plays the same role as ferredoxin in its interaction with FNR in spite of its different structure, size and redox cofactor. There are a number of negatively charged amino acid residues on the surface of ferredoxin and flavodoxin that play a role in the electron transfer reaction with the reductase. Thus far, in only one case has charge replacement of one of the acidic residues produced an increase in the rate of electron transfer, whereas in several other cases a decrease in the rate is observed. In the most dramatic example, replacement of Glu at position 94 of Anabaena ferredoxin results in virtually the complete loss of ability to transfer electrons. Charge-reversal of positively charged amino acid residues in the reductase also produces strong effects on the rate of electron transfer. Several degrees of impairment have been observed, the most significant involving a positively charged Lys at position 75 which appears to be essential for the stability of the complex between the reductase and ferredoxin. The results presented in this paper provide a clear demonstration of the importance of electrostatic interactions on the stability of the transient complex formed during electron transfer by the proteins presently under study.

Lys75 of Anabaena ferredoxin-NADP+ reductase is a critical residue for binding ferredoxin and flavodoxin during electron transfer

Previous studies, and the three-dimensional structure of Anabaena PCC 7119 ferredoxin-NADP+ reductase (FNR), indicate that the positive charge of Lys75 might be directly involved in the interaction between FNR and its protein partners, ferredoxin (Fd) and flavodoxin (Fld). To assess this possibility, this residue has been replaced by another positively charged residue, Arg, by two uncharged residues, Gln and Ser, and by a negatively charged residue, Glu. UV-vis absorption, fluorescence, and CD spectroscopies of these FNR mutants (Lys75Arg, Lys75Gln, Lys75Ser, and Lys75Glu) indicate that all the mutated proteins folded properly and that significant protein structural rearrangements did not occur. Steady-state kinetic parameters for these FNR mutants, utilizing the diaphorase activity with DCPIP, indicate that Lys75 is not a critical residue for complex formation and electron transfer (ET) between FNR and NADP+ or NADPH. However, steady-state kinetic activities requiring complex formation and ET between FNR and Fd or Fld were appreciably affected when the positive charge at position of Lys75 was removed, and the ET reaction was not even measurable if a negatively charged residue was placed at this position. These kinetic parameters also suggest that it is complex formation that is affected by mutation. Consistent with this, when dissociation constants (Kd) for FNRox-Fdox (differential spectroscopy) and FNRox-Fdrd (laser flash photolysis) were measured, it was found that neutralization of the positive charge at position 75 increased the Kd values by 50-100-fold, and that no complex formation could be detected upon introduction of a negative charge at this position. Fast transient kinetic studies also corroborated the fact that removal of the positive charge at position 75 of FNR appreciably affects the complex formation process with its protein partners but indicates that ET is still achieved in all the reactions. This study thus clearly establishes the requirement of a positive charge at position Lys75 for complex formation during ET between FNR and its physiological protein partners. The results also suggest that the interaction of this residue with its protein partners is not structurally specific, since Lys75 can still be efficiently substituted by an arginine, but is definitely charge specific.

NADPH-flavodoxin reductase and flavodoxin from Escherichia coli: characteristics as a soluble microsomal P450 reductase

In addition to their endogenous roles as an activation system for various Escherichia coli metabolic pathways, the soluble flavoproteins flavodoxin (Fld) and NADPH-flavodoxin (ferredoxin) reductase (Fpr) can serve as an electron-transfer system for microsomal cytochrome P450s. Furthermore, since Fld and Fpr are structurally similar to the functional domains (FMN binding and NADPH/FAD binding domains, respectively) of NADPH-cytochrome P450 reductases (P450 reductases), these bacterial proteins represent a potentially useful model system for eukaryotic P450 reductases. Here we delineate similarities and differences between the E. coli Fpr-Fld system and rat P450 reductase as electron donors to bovine 17alpha-hydroxylase/17,20-lyase P450 (P450c17). Importantly, recombinant Fpr, in combination with recombinant Fld, supports both the hydroxylase and lyase activities of P450c17 to the same proportional extent (hydroxylase-to-lyase ratio) as does P450 reductase. Maximum P450c17 turnover [5-6 mol of 17alpha-OH-progesterone (mol of P450c17)-1 min-1] was achieved using a large molar excess (50-100-fold over P450c17) of a 1:1 ratio of Fpr-Fld, although this rate was an order of magnitude less than the maximal P450 reductase-supported activity. Using these conditions, we have examined the effects of increasing ionic strength and the presence of cytochrome b5 (b5) on these two systems. Critical Fld-P450c17 electrostatic interactions are disrupted at moderate ionic strength (>100 mM NaCl) as evidenced by significant inhibition (>50%) of Fpr-Fld-supported P450c17 activity while much higher ionic strength (300 mM NaCl) is required to disrupt P450 reductase-P450c17 interactions to the same extent. Interestingly, cytochrome b5 was found to dramatically inhibit both P450 reductase- and Fpr-Fld-supported P450c17 progesterone 17alpha-hydroxylase activity while in contrast 17alpha-OH-pregnenolone lyase activity was stimulated by b5. Investigation of the fate of reducing equivalents from NADPH added to Fpr under aerobic conditions revealed that the majority of the protein-bound FAD of Fpr is converted to the hydroquinone form. In constrast, the FMN of Fld is reduced by Fpr to a stable blue, neutral semiquinone which serves as the predominant electron donor to P450c17 in reconstitution assays. Thus, while the Fpr-Fld system and P450 reductase are fundamentally different with respect to their electrostatic interactions with P450c17, their ability to support maximal P450c17 turnover, and the FMN redox states (one-electron-reduced for Fld and two-electron-reduced for P450 reductase) capable of transferring electrons to microsomal cytochrome P450s, these differences do not appear to influence the relative catalytic efficiency of the P450c17 hydroxylase and lyase reactions.

Involvement of glutamic acid 301 in the catalytic mechanism of ferredoxin-NADP+ reductase from Anabaena PCC 7119

The crystal structure of Anabaena PCC 7119 ferredoxin-NADP+ reductase (FNR) suggests that the carboxylate group of Glu301 may be directly involved in the catalytic process of electron and proton transfer between the isoalloxazine moiety of FAD and FNR substrates (NADPH, ferredoxin, and flavodoxin). To assess this possibility, the carboxylate of Glu301 was removed by mutating the residue to an alanine. Various spectroscopic techniques (UV-vis absorption, fluorescence, and CD) indicate that the mutant protein folded properly and that significant protein structural rearrangements did not occur. Additionally, complex formation of the mutant FNR with its substrates was almost unaltered. Nevertheless, no semiquinone formation was seen during photoreduction of Glu301Ala FNR. Furthermore, steady-state activities in which FNR semiquinone formation was required during the electron-transfer processes to ferredoxin were appreciably affected by the mutation. Fast transient kinetic studies corroborated that removal of the carboxylate at position 301 decreases the rate constant approximately 40-fold for the electron transfer process with ferredoxin without appreciably affecting complex formation, and thus interferes with the stabilization of the transition state during electron-transfer between the FAD and the iron-sulfur cluster. Moreover, the mutation also altered the nonspecific reaction of FNR with 5'-deazariboflavin semiquinone, the electron-transfer reactions with flavodoxin, and the reoxidation properties of the enzyme. These results clearly establish Glu301 as a critical residue for electron transfer in FNR.

Interaction of positively charged amino acid residues of recombinant, cyanobacterial ferredoxin:NADP+ reductase with ferredoxin probed by site directed mutagenesis

The petH genes encoding ferredoxin:NADP+ reductase (FNR) from two Anabaena species (PCC 7119 and ATCC 29413) were cloned and overexpressed in E. coli. Several positively charged residues (Arg, Lys) have been implicated to be involved in ferredoxin binding and electron transfer by cross-linking, chemical modification and protection experiments, and crystallographic studies. The following substitutions were introduced by site-directed mutagenesis: R153Q, K209Q, K212Q, R214Q, K275N, K430Q and K431Q in Anabaena 29413 FNR, and R153E, K209E, K212E, R214E, K275E, R401E, K427E, and K431E in Anabaena 7119 FNR. Comparison of the diaphorase activities, the specific rates of ferredoxin dependent NADP(+)-photoreduction and cytochrome c reduction catalyzed by FNR showed that all these amino acid residues were required for efficient electron transfer between FNR and ferredoxin. Replacement of any one of these basic residues produced a much more pronounced effect on the cytochrome c reductase activity, where FNR, reduced by NADPH, acted as electron donor, than in the reduction of NADP+ by photosystem I via FNR. A mutation involving the replacement of positive charge by a neutral amide produced in all cases a smaller inhibitory effect on the activity than a charge reversal mutation. In addition, it has been found that R214 was necessary for stable integration of the non covalently bound FAD-cofactor.

Glutamate 94 of [2Fe-2S]-ferredoxins is important for efficient electron transfer in the 1:1 complex formed with ferredoxin-glutamate synthase (GltS) from Synechocystis sp. PCC 6803

We have analyzed the role of critical amino acid residues involved in the interaction between ferredoxin and ferredoxin-glutamate synthase (GOGAT) encoded by the gltS gene from the cyanobacterium Synechocystis sp. PCC 6803. Our results indicated that the glutamate 94 residue of Anabaena 7120 ferredoxin (= E92 of the Synechocystis 6803 ferredoxin) was necessary for an efficient electron transfer to GOGAT comparable to ferredoxin:NADP-reductase, nitrite reductase and nitrate reductase [Schmitz and Böhme (1995) Biochim. Biophys. Acta 1231, 335-341]. The K(m) value determined for wt-ferredoxins and mutant E94Q (and E92Q) was 1 muM, respectively, and activity loss of E94Q was due to a lowered Vmax. Exchange of residue F65 for aliphatic substitutions, which was crucial to electron transfer to ferredoxin:NADP-reductase and nitrite reductase, exhibited only small effects on glutamate synthase-dependent activity while heterocyst ferredoxin and flavodoxin were almost inactive as electron donors. In contrast to data reported for the spinach system, the stoichiometry of the cross-linked complex between ferredoxin and glutamate synthase was 1:1.

Electrostatic properties deduced from refined structures of NADH-cytochrome b5 reductase and the other flavin-dependent reductases: pyridine nucleotide-binding and interaction with an electron-transfer partner

Electrostatic properties on the protein surface were examined on the basis of the crystal structure of NADH-cytochrome b5 reductase refined to a crystallographic R factor of 0.223 at 2.1 A resolution and of the other three flavin-dependent reductases. A structural comparison of NADH-cytochrome b5 reductase with the other flavin-dependent reductases, ferredoxin-NADP+ reductase, phthalate dioxygenase reductase, and nitrate reductase, showed that the alpha/beta structure is the common motif for binding pyridine nucleotide. Although the amino acid residues associated with pyridine nucleotide-binding are not conserved, the electrostatic properties and the location of the pyridine nucleotide-binding pockets of NADH-requiring reductases were similar to each other. The electrostatic potential of the surface near the flavin-protruding side (dimethylbenzene end of the flavin ring) of NADH-cytochrome b5 reductase was positive over a wide area while that of the surface near the heme-binding site of cytochrome b5 was negative. This implied that the flavin-protruding side of NADH-cytochrome b5 reductase is suitable for interacting with its electron-transfer partner, cytochrome b5. This positive potential area is conserved among four flavin-dependent reductases. A comparison of the electron-transfer partners of four flavin-dependent reductases showed that there are significant differences in the distribution of electrostatic potential between inter-molecular and inter-domain electron-transfer reactions.

The effect of N-bromosuccinimide on ferredoxin:NADP+ oxidoreductase

Treatment of spinach leaf ferredoxin:NADP+ oxidoreductase (FNR) with N-bromosuccinimide (NBS), under conditions where approximately one tryptophan residue per enzyme was modified, resulted in a loss of between 80 and 85% of the activity of the enzyme when electron transfer from NADPH to either ferredoxin or 2,6-dichlorophenol-indophenol was measured. Amino acid analysis revealed no detectable modification by NBS of any FNR amino acids other than tryptophan. Complex formation with ferredoxin, but not with NADP+, prevented both the inhibition of activity and the modification of tryptophan caused by the treatment with NBS. Modification of one FNR tryptophan residue had no significant effect on the Km values of the enzyme for either ferredoxin or NADPH or on the binding constants for the FNR complexes with either ferredoxin or NADP+. NBS treatment had only very small effects on the absorbance and circular dichroism spectra of FNR and did not significantly affect either the oxidation-reduction midpoint potential of the FAD prosthetic group of the enzyme or inhibit the reduction of the FAD group by NADPH. These results raise the possibility that a tryptophan residue may play a role in the electron transfer between the FAD of FNR and the enzyme substrate, ferredoxin.

Structure-function relationships in the ferredoxin/ferredoxin: NADP+ reductase system from Anabaena

We have used a combination of laser flash photolysis time-resolved spectrophotometry and site-specific mutagenesis of surface amino acid residues to investigate the structural factors which influence electron transfer from Anabaena ferredoxin to its physiological partner ferredoxin-NADP+ reductase. Two ferredoxin residues (E94 and F65) are found to be highly critical interaction sites, whereas other nearby residues are found to be either inconsequential or to have only moderate effects. Basic residues near the N-terminus of the reductase are also found to exert a significant influence on interprotein electron transfer. The mechanistic implications of these results are discussed.

Structure-function relations for ferredoxin reductase

Ferredoxin: NADP+ reductase is representative of a large family of flavoenzymes which catalyze the interchange of reducing equivalents between one-electron carriers and the two-electron-carrying nicotinamide dinucleotides. The structure of the enzyme from spinach is known at 1.7 A resolution and this structure, together with results of chemical modification and site-directed mutagenesis studies, gives insights into features of the structure that are important for function.

Amino acid residues in Anabaena ferredoxin crucial to interaction with ferredoxin-NADP+ reductase: site-directed mutagenesis and laser flash photolysis

Ferredoxin (Fd) functions in photosynthesis to transfer electrons from photosystem I to ferredoxin-NADP+ reductase (FNR). We have made several site-directed mutants of Anabaena 7120 Fd and have used laser flash photolysis to investigate the effects of these mutations on the kinetics of reduction of oxidized Fd by deazariboflavin semiquinone (dRfH.) and the reduction of oxidized Anabaena FNR by reduced Fd. None of the mutations influenced the second-order rate constant for dRfH. reduction by more than a factor of 2, suggesting that the ability of the [2Fe-2S] cluster to participate in electron transfer was not seriously affected. In contrast, a surface charge reversal mutation, E94K, resulted in a 20,000-fold decrease in the second-order rate constant for electron transfer from Fd to FNR, whereas a similar mutation at an adjacent site, E95K, produced little or no change in reaction rate constant compared to wild-type Fd. Such a dramatic difference between contiguous surface mutations suggests a very precise surface complementarity at the protein-protein interface. Mutations introduced at F65 (F65I and F65A) also decreased the rate constant for the Fd/FNR electron transfer reaction by more than 3 orders of magnitude. Spectroscopic and thermodynamic measurements with both the E94 and F65 mutants indicated that the kinetic differences cannot be ascribed to changes in gross conformation, redox potential, or FNR binding constant but rather reflect the protein-protein interactions that control electron transfer. Several mutations at other sites in the vicinity of E94 and F65 (R42, T48, D68, and D69) resulted in little or no perturbation of the Fd/FNR interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Ferredoxin binding site on ferredoxin: NADP+ reductase. Differential chemical modification of free and ferredoxin-bound enzyme

The chloroplast enzyme ferredoxin: NADP+ reductase (FNR) catalyzes the reduction of NADP+ by ferredoxin (Fd). FNR and Fd form a 1:1 complex that is stabilized by electrostatic interactions between acidic residues of Fd and basic residues of FNR. To localize lysine residues at the Fd binding site of FNR, the FNR:Fd complex (both proteins from spinach) was studied by differential chemical modification. In a first set of experiments, free FNR and the FNR:Fd complex were reacted with the N-hydroxysuccinimidyl ester of biotin. Biotinylated peptides and non-biotinylated peptides were separated on monovalent avidin-Sepharose and purified by high-performance liquid chromatography. Two peptides containing Lys18 and Lys153, respectively, were less biotinylated in complexed FNR than in free FNR. In a second set of experiments, free and complexed FNR were treated with 4-N,N-dimethylaminoazobenzene-4'-isothiocyano-2'-sulfonic acid (S-DABITC) to obtain coloured lysine-modified FNR. Protection of Lys153 was again found by modification with S-DABITC. In addition, Lys33 and Lys35 were less labelled in the S-DABITC-modified. Fd-bound enzyme. FNR modified in the presence, but not in the absence, of Fd was still able to bind Fd, indicating that the Fd-protected residues are involved in the formation of the Fd:FNR complex. The lysine residues disclosed by differential modification surround the positive end of the molecular dipole moment (558 Debye approximately 1.85 x 10(-27) Cm) and are located in a domain of strong positive potential on the surface of the FNR molecule. This domain we had proposed to belong to the binding site of FNR for Fd [De Pascalis, A. R., Jelesarov, I., Ackermann, F., Koppenol, W. H., Hirasawa, M., Knaff, D. B. & Bosshard, H. R. (1993) Protein Science 2. 1126-1135]. The prediction was based on the complementarity of shape between positive and negative potential domains of FNR and Fd, respectively.

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.

Identification of arginyl residues involved in the binding of ferredoxin-NADP+ reductase from Anabaena sp. PCC 7119 to its substrates

Ferredoxin-NADP+ reductase from the cyanobacterium Anabaena sp. PCC 7119 was chemically modified by the alpha-dicarbonyl reagent phenylglyoxal. The studies of the inactivation by this compound, which is specific for arginyl residues, of both the diaphorase and NADPH-cytochrome c reductase activities, characteristic of the enzyme, are indicative of the involvement of at least one group of this kind in the binding site of NADP+ and a second one implicated in the interaction with ferredoxin. After specific cleavage of a FNR sample incubated with [7-14C]phenylglyoxal, two major labeled peptides were identified. The peptide which exhibited the higher degree of modification corresponded to residues 208-242. It contained four arginine residues but only two of them were the target of the modification: Arg224 and Arg233. Protection studies with protein substrates and sequence comparison with other reductases allow us to propose that these residues in Anabaena sp. PCC 7119 FNR must be involved in the interaction with the pyridine nucleotide. The second peptide corresponds to residues 75-103 and although it contains three arginine residues, Arg77 is the only one that exhibits the modification. This residue seems to be a key one in the interaction of this reductase with ferredoxin.