Electrostatic and Hydrophobic Interactions during Complex Formation and Electron Transfer in the Ferredoxin/Ferredoxin:NADP+ Reductase System from Anabaena

A Molecular Interaction Analysis Reveals the Possible Roles of Graphene Oxide in a Glucose Biosensor

In this paper, we report the molecular docking study of graphene oxide and glucose oxidase (GOx) enzyme for a potential glucose biosensing application. The large surface area and good electrical properties have made graphene oxide as one of the best candidates for an enzyme immobilizer and transducer in the biosensing system. Our molecular docking results revealed that graphene oxide plays a role as a GOx enzyme immobilizer in the glucose biosensor system since it can spontaneously bind with GOx at specific regions separated from the active sites of glucose and not interfering or blocking the glucose sensing by GOx in an enzyme-assisted biosensor system. The strongest binding affinity of GOx-graphene oxide interaction is -11.6 kCal/mol and dominated by hydrophobic interaction. Other modes of interactions with a lower binding affinity have shown the existence of some hydrogen bonds (H-bonds). A possibility of direct sensing (interaction) model of glucose by graphene oxide (non-enzymatic sensing mechanism) was also studied in this paper, and showed a possible direct glucose sensing by graphene oxide through the H-bond interaction, even though with a much lower binding affinity of -4.2 kCal/mol. It was also found that in a direct glucose sensing mechanism, the sensing interaction can take place anywhere on the graphene oxide surface with almost similar binding affinity.

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

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

Pre-steady-state kinetic studies of redox reactions catalysed by Bacillus subtilis ferredoxin-NADP(+) oxidoreductase with NADP(+)/NADPH and ferredoxin

Ferredoxin-NADP(+) oxidoreductase ([EC1.18.1.2], FNR) from Bacillus subtilis (BsFNR) is a homodimeric flavoprotein sharing structural homology with bacterial NADPH-thioredoxin reductase. Pre-steady-state kinetics of the reactions of BsFNR with NADP(+), NADPH, NADPD (deuterated form) and B. subtilis ferredoxin (BsFd) using stopped-flow spectrophotometry were studied. Mixing BsFNR with NADP(+) and NADPH yielded two types of charge-transfer (CT) complexes, oxidized FNR (FNR(ox))-NADPH and reduced FNR (FNR(red))-NADP(+), both having CT absorption bands centered at approximately 600n m. After mixing BsFNR(ox) with about a 10-fold molar excess of NADPH (forward reaction), BsFNR was almost completely reduced at equilibrium. When BsFNR(red) was mixed with NADP(+), the amount of BsFNR(ox) increased with increasing NADP(+) concentration, but BsFNR(red) remained as the major species at equilibrium even with about 50-fold molar excess NADP(+). In both directions, the hydride-transfer was the rate-determining step, where the forward direction rate constant (~500 s(-1)) was much higher than the reverse one (<10 s(-1)). Mixing BsFd(red) with BsFNR(ox) induced rapid formation of a neutral semiquinone form. This process was almost completed within 1 ms. Subsequently the neutral semiquinone form was reduced to the hydroquinone form with an apparent rate constant of 50 to 70 s(-1) at 10°C, which increased as BsFd(red) increased from 40 to 120 μM. The reduction rate of BsFNR(ox) by BsFd(red) was markedly decreased by premixing BsFNR(ox) with BsFd(ox), indicating that the dissociation of BsFd(ox) from BsFNR(sq) is rate-limiting in the reaction. The characteristics of the BsFNR reactions with NADP(+)/NADPH were compared with those of other types of FNRs.

Thylakoid membrane function in heterocysts

Multicellular cyanobacteria form different cell types in response to environmental stimuli. Under nitrogen limiting conditions a fraction of the vegetative cells in the filament differentiate into heterocysts. Heterocysts are specialized in atmospheric nitrogen fixation and differentiation involves drastic morphological changes on the cellular level, such as reorganization of the thylakoid membranes and differential expression of thylakoid membrane proteins. Heterocysts uphold a microoxic environment to avoid inactivation of nitrogenase by developing an extra polysaccharide layer that limits air diffusion into the heterocyst and by upregulating heterocyst-specific respiratory enzymes. In this review article, we summarize what is known about the thylakoid membrane in heterocysts and compare its function with that of the vegetative cells. We emphasize the role of photosynthetic electron transport in providing the required amounts of ATP and reductants to the nitrogenase enzyme. In the light of recent high-throughput proteomic and transcriptomic data, as well as recently discovered electron transfer pathways in cyanobacteria, our aim is to broaden current views of the bioenergetics of heterocysts. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

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.

Integrated Kinetics for the Production of Glucose in Plant Cells and the Effect of Temperature

We prepare a temperature-dependent formulation of the integrated kinetics for the overall process of photosynthesis in eukaryotic cells. To avoid complexity, the C4 plants are chosen because their rate of photosynthesis is independent of the partial pressure of O2. A systematically simplified but comprehensive scheme for both light and dark reactions is considered. The reaction rate per reaction center in the thylakoid membrane is related to the rate of exciton transfer between chlorophyll neighbors. An expression is formulated for the light reaction rate (R1'). The NADPH formation rate is related to R1' and the survival probability of the membrane. Rates of different steps in the simplified scheme can be related to each other by applying a few steady state conditions. The saturation probability of CO2 in a bundle sheath is also considered. The photochemical efficiency (phi) appears in terms of these probabilities. We find the glucose production rate as R(glucose) = (8/3) upsilon L: [corrected] R1'phi g(T)([G3P]/[P(i)]2) exp(-deltaG(E)S/RT), where g(T) is the activation quotient of the involved enzymes, G3P and P(i) represent glyceraldehyde-3-phosphate and inorganic phosphates, and deltaG(E)S is the free energy for the apparent equilibrium between G3P and glucose. This is the first time that such a comprehensive expression for R(glucose) has been derived. The probabilities are generally given by sigmoid curves. The corresponding parameters can be easily determined. The quotient g(T) incorporates a Gaussian distribution for temperature dependence and a sigmoid function describing deactivation. The theoretical plots of photochemical efficiency and glucose production rate versus temperature are in excellent agreement with the experimental ones, thereby validating the formalism.

Identification of the N- and C-terminal substrate binding segments of ferredoxin-NADP+ reductase by NMR

Ferredoxin-NADP(+) reductase (FNR) catalyzes the reduction of NADP(+) through the formation of an electron transfer complex with ferredoxin. To gain insight into the interaction of this enzyme with substrates at both ends of the polypeptide chain, we performed NMR analyses of a 314-residue maize leaf FNR with a nearly complete assignment of the backbone resonances. The chemical shift perturbation upon formation of the complex indicated that a flexible N-terminal region of FNR contributed to the interaction with maize ferredoxin, and an analysis of N-terminally truncated mutants of FNR confirmed the importance of this region for the binding of ferredoxin. Comparison between the spectra of FNR in the NADP(+)- and inhibitor-bound states also revealed that the nicotinamide moiety of NADP(+) was accessible to the C-terminal Tyr314. We propose that the formation of the catalytic competent complex of FNR and substrates is achieved through the interaction of the N- and C-terminal segments with ferredoxin and NADP(+), respectively. Since the ends of the polypeptide chain act as flexible regions of proteins, they may contribute to the search of a larger space for a binding partner and to the opening of active sites.

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

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

Glutamate synthase: structural, mechanistic and regulatory properties, and role in the amino acid metabolism

Ammonium ion assimilation constitutes a central metabolic pathway in many organisms, and glutamate synthase, in concert with glutamine synthetase (GS, EC 6.3.1.2), plays the primary role of ammonium ion incorporation into glutamine and glutamate. Glutamate synthase occurs in three forms that can be distinguished based on whether they use NADPH (NADPH-GOGAT, EC 1.4.1.13), NADH (NADH-GOGAT, EC 1.4.1.14) or reduced ferredoxin (Fd-GOGAT, EC 1.4.7.1) as the electron donor for the (two-electron) conversion of L-glutamine plus 2-oxoglutarate to L-glutamate. The distribution of these three forms of glutamate synthase in different tissues is quite specific to the organism in question. Gene structures have been determined for Fd-, NADH- and NADPH-dependent glutamate synthases from different organisms, as shown by searches in nucleic acid sequence data banks. Fd-glutamate synthase contains two electron-carrying prosthetic groups, the redox properties of which are discussed. A description of the ferredoxin binding by Fd-glutamate synthase is also presented. In plants, including nitrogen-fixing legumes, Fd-glutamate synthase and NADH-glutamate synthase supply glutamate during the nitrogen assimilation and translocation. The biological functions of Fd-glutamate synthase and NADH-glutamate synthase, which show a highly tissue-specific distribution pattern, are tightly related to the regulation by the light and metabolite sensing systems. Analysis of mutants and transgenic studies have provided insights into the primary individual functions of Fd-glutamate synthase and NADH-glutamate synthase. These studies also provided evidence that glutamate dehydrogenase (NADH-GDH, EC 1.4.1.2) does not represent a significant alternate route for glutamate formation in plants. Taken together, biochemical analysis and genetic and molecular data imply that Fd-glutamate synthase incorporates photorespiratory and non-photorespiratory ammonium and provides nitrogen for transport to maintain nitrogen status in plants. Fd-glutamate synthase also plays a role that is redundant, in several important aspects, to that played by NADH-glutamate synthase in ammonium assimilation and nitrogen transport.

Role of the C-terminal tyrosine of ferredoxin-nicotinamide adenine dinucleotide phosphate reductase in the electron transfer processes with its protein partners ferredoxin and flavodoxin

The catalytic mechanism proposed for ferredoxin-NADP(+) reductase (FNR) is initiated by reduction of its flavin adenine dinucleotide (FAD) cofactor by the obligatory one-electron carriers ferredoxin (Fd) or flavodoxin (Fld) in the presence of oxidized nicotinamide adenine dinucleotide phosphate (NADP(+)). The C-terminal tyrosine of FNR, which stacks onto its flavin ring, modulates the enzyme affinity for NADP(+)/H, being removed from this stacking position during turnover to allow productive docking of the nicotinamide and hydride transfer. Due to its location at the substrate-binding site, this residue might also affect electron transfer between FNR and its protein partners. We therefore studied the interactions and electron-transfer properties of FNR proteins mutated at their C-termini. The results obtained with the homologous reductases from pea and Anabaena PCC7119 indicate that interactions with Fd or Fld are hardly affected by replacement of this tyrosine by tryptophan, phenylalanine, or serine. In contrast, electron exchange is impaired in all mutants, especially in the nonconservative substitutions, without major differences between the eukaryotic and the bacterial FNR. Introduction of a serine residue shifts the flavin reduction potential to less negative values, whereas semiquinone stabilization is severely hampered, introducing further constraints to the one-electron-transfer processes. Thus, the C-terminal tyrosine of FNR plays distinct and complementary roles during the catalytic cycle, (i) by lowering the affinity for NADP(+)/H to levels compatible with steady-state turnover, (ii) by contributing to the flavin semiquinone stabilization required for electron splitting, and (iii) by modulating the rates of electron exchange with the protein partners.

The architecture of the binding site in redox protein complexes: Implications for fast dissociation

Interprotein electron transfer is characterized by protein interactions on the millisecond time scale. Such transient encounters are ensured by extremely high rates of complex dissociation. Computational analysis of the available crystal structures of redox protein complexes reveals features of the binding site that favor fast dissociation. In particular, the complex interface is shown to have low geometric complementarity and poor packing. These features are consistent with the necessity for fast dissociation since the absence of close packing facilitates solvation of the interface and disruption of the complex.

Involvement of the pyrophosphate and the 2'-phosphate binding regions of ferredoxin-NADP+ reductase in coenzyme specificity

Previous studies indicated that the determinants of coenzyme specificity in ferredoxin-NADP+ reductase (FNR) from Anabaena are situated in the 2'-phosphate (2'-P) NADP+ binding region, and also suggested that other regions must undergo structural rearrangements of the protein backbone during coenzyme binding. Among the residues involved in such specificity could be those located in regions where interaction with the pyrophosphate group of the coenzyme takes place, namely loops 155-160 and 261-268 in Anabaena FNR. In order to learn more about the coenzyme specificity determinants, and to better define the structural basis of coenzyme binding, mutations in the pyrophosphate and 2'-P binding regions of FNR have been introduced. Modification of the pyrophosphate binding region, involving residues Thr-155, Ala-160, and Leu-263, indicates that this region is involved in determining coenzyme specificity and that selected alterations of these positions produce FNR enzymes that are able to bind NAD+. Thus, our results suggest that slightly different structural rearrangements of the backbone chain in the pyrophosphate binding region might determine FNR specificity for the coenzyme. Combined mutations at the 2'-P binding region, involving residues Ser-223, Arg-224, Arg-233, and Tyr-235, in combination with the residues mentioned above in the pyrophosphate binding region have also been carried out in an attempt to increase the FNR affinity for NAD+/H. However, in most cases the analyzed mutants lost the ability for NADP+/H binding and electron transfer, and no major improvements were observed with regard to the efficiency of the reactions with NAD+/H. Therefore, our results confirm that determinants for coenzyme specificity in FNR are also situated in the pyrophosphate binding region and not only in the 2'-P binding region. Such observations also suggest that other regions of the protein, yet to be identified, might also be involved in this process.

Open questions in ferredoxin-NADP+ reductase catalytic mechanism

Ferredoxin (flavodoxin)-NADP(H) reductases (FNR) are ubiquitous flavoenzymes that deliver NADPH or low potential one-electron donors (ferredoxin, flavodoxin) to redox-based metabolisms in plastids, mitochondria and bacteria. The plant-type reductase is also the basic prototype for one of the major families of flavin-containing electron transferases that display common functional and structural properties. Many aspects of FNR biochemistry have been extensively characterized in recent years using a combination of site-directed mutagenesis, steady-state and transient kinetic experiments, spectroscopy and X-ray crystallography. Despite these considerable advances, various key features in the enzymology of these important reductases remain yet to be explained in molecular terms. This article reviews the current status of these open questions. Measurements of electron transfer rates and binding equilibria indicate that NADP(H) and ferredoxin interactions with FNR result in a reciprocal decrease of affinity, and that this induced-fit step is a mandatory requisite for catalytic turnover. However, the expected conformational movements are not apparent in the reported atomic structures of these flavoenzymes in the free state or in complex with their substrates. The overall reaction catalysed by FNR is freely reversible, but the pathways leading to NADP+ or ferredoxin reduction proceed through entirely different kinetic mechanisms. Also, the reductases isolated from various sources undergo inactivating denaturation on exposure to NADPH and other electron donors that reduce the FAD prosthetic group, a phenomenon that might have profound consequences for FNR function in vivo. The mechanisms underlying this reductive inhibition are so far unknown. Finally, we provide here a rationale to interpret FNR evolution in terms of catalytic efficiency. Using the formalism of the Albery-Knowles theory, we identified which parameter(s) have to be modified to make these reductases even more proficient under a variety of conditions, natural or artificial. Flavoenzymes with FNR activity catalyse a number of reactions with potential importance for biotechnological processes, so that modification of their catalytic competence is relevant on both scientific and technical grounds.

Role of critical charged residues in reduction potential modulation of ferredoxin-NADP+ reductase

Reduction potential determinations of K75E, E139K and E301A ferredoxin-NADP+ reductases provide valuable information concerning the factors that contribute to tune the flavin reduction potential. Thus, while E139 is not involved in such modulation, the K75 side-chain tunes the flavin potential by creating a defined environment that modulates the FAD conformation. Finally, the E301 side-chain influences not only the flavin reduction potential, but also the electron transfer mechanism, as suggested from the values determined for the E301A mutant, where E(ox/rd) and E(sq/rd) shifted +41 and +102 mV, respectively, with regard to wild-type. Reduction potentials allowed estimation of binding energies differences of the FAD cofactor upon reduction.

Flavin photochemistry in the analysis of electron transfer reactions: role of charged and hydrophobic residues at the carboxyl terminus of ferredoxin-NADP(+) reductase in the interaction with its substrates

The enzyme Ferredoxin-NADP(+) reductase participates in the reductive side of the photosynthetic chain transferring electrons from reduced Ferredoxin (Fd) (or Flavodoxin (Fld)) to NADP(+), a process that yields NADPH that can be used in many biosynthetic dark reactions. The involvement of specific amino acids in the interaction between the two proteins has been studied using site-directed mutagenesis. In the present study, the participation of charged (H299), polar (T302) or hydrophobic (V300) amino acid residues that are in the NADP(+)-binding domain of the reductase have been examined by analyzing its C-terminal region, which is located close to the active site. Stopped-flow and laser flash photolysis results of the reaction in which these mutant proteins participate show very little differences with respect to the wild-type protein. These results suggest that the NADPH-binding domain of the reductase has little effect on the processes of recognition and electron transfer to (and from) Fd or Fld, according to the recently reported crystallographic structure of the FNR/Fd complex.

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.

Browsing gene banks for Fe2S2 ferredoxins and structural modeling of 88 plant-type sequences: an analysis of fold and function

One-hundred-and-seventy-nine sequences of Fe2S2 ferredoxins and ferredoxin precursors were identified in and retrieved from currently available protein and cDNA databases. On the basis of their cluster-binding patterns, these sequences were divided into three groups: those containing the CX4CX2CXnC pattern (plant-type ferredoxins), those with the CX5CX2CXnC pattern (adrenodoxins), and those with a different pattern. These three groups contain, respectively, 139, 36, and 4 sequences. After excluding ferredoxin precursors in the first group, two subgroups were identified, again based on their cluster-binding patterns: 88 sequences had the CX4CX2CX29C pattern, and 29 had the CX4CX2CXmC (m not equal 29) pattern. The structures of the 88 ferredoxins with the CX4CX2CX29C pattern were modeled based on the available experimental structures of nine proteins within this same group. The modeling procedure was tested by building structural models for the ferredoxins with known structures. The models resulted, on average, in being within 1 A of the backbone root-mean-square deviation from the corresponding experimental structures. In addition, these structural models were shown to be of high quality by using assessment procedures based on energetic and stereochemical parameters. Thus, these models formed a reliable structural database for this group of ferredoxins, which is meaningful within the framework of current structural genomics efforts. From the analysis of the structural database generated it was observed that the secondary structural elements and the overall three-dimensional structures are maintained throughout the superfamily. In particular, the residues in the hydrophobic core of the protein were found to be either absolutely conserved or conservatively substituted. In addition, certain solvent-accessible charged groups, as well as hydrophobic groups, were found to be conserved to the same degree as the core residues. The patterns of conservation of exposed residues identified the regions of the protein that are critical for its function in electron transfer. An extensive analysis of protein-protein interactions is now possible. Some conserved interactions between residues have been identified and related to structural and/or functional features. All this information could not be obtained from the analyses of the primary sequences alone. Finally, the analysis of the sequences of the related subgroup featuring the CX4CX2CXmC (m not equal 29) cluster-binding pattern in the light of the structural and functional insights provided by the inspection of the mentioned structural database affords some hints on the functional features of ferredoxins belonging to this subgroup.

Laser flash induced electron transfer in P450cam monooxygenase: putidaredoxin reductase-putidaredoxin interaction

The P450cam monooxygenase from Pseudomonas putida consists of three redox proteins: NADH-putidaredoxin reductase (Pdr), putidaredoxin (Pdx), and cytochrome P450cam. The redox properties of the FAD-containing Pdr and the mechanism of Pdr-Pdx complex formation are the least studied aspects of this system. We have utilized laser flash photolysis techniques to produce the one-electron-reduced species of Pdr, to characterize its spectral and electron-transferring properties, and to investigate the mechanism of its interaction with Pdx. Upon flash-induced reduction by 5-deazariboflavin semiquinone, the flavoprotein forms a blue neutral FAD semiquinone (FADH(*)). The FAD semiquinone was unstable and partially disproportionated into fully oxidized and fully reduced flavin. The rate of FADH(*) decay was dependent on ionic strength and NAD(+). In the mixture of Pdr and Pdx, where the flavoprotein was present in excess, electron transfer (ET) from FADH(*) to the iron-sulfur cluster was observed. The Pdr-to-Pdx ET rates were maximal at an ionic strength of 0.35 where a kinetic dissociation constant (K(d)) for the transient Pdr-Pdx complex and a limiting k(obs) value were equal to 5 microM and 226 s(-1), respectively. This indicates that FADH(*) is a kinetically significant intermediate in the turnover of P450cam monooxygenase. Transient kinetics as a function of ionic strength suggest that, in contrast to the Pdx-P450cam redox couple where complex formation is predominantly electrostatic, the Pdx-Pdr association is driven by nonelectrostatic interactions.

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.

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.

Association of cytochromes P450 with their reductases: opposite sign of the electrostatic interactions in P450BM-3 as compared with the microsomal 2B4 system

The role of electrostatic interactions in the association of P450s with their nicotinamide adenine dinucleotide phosphate- (NADPH) dependent flavoprotein reductases was studied by fluorescence resonance energy transfer. The fluorescent probe 7-(ethylamino)-3-(4'-maleimidylphenyl)-4-methylcoumarin maleimide (coumarylphenylmaleimide, CPM) was introduced into the flavoprotein molecule at a 1:1 molar ratio. The interaction of P450 2B4 and NADPH-P450 reductase (CPR) from rabbit liver microsomes was compared with that of the isolated heme domain (BMP) and the flavoprotein domain (BMR) of P450BM-3. The cross-pairs of the components were also studied. Increasing ionic strength (0.05-0.5 M) was shown to result in the dissociation of the CPR-P450 2B4 complex with the dissociation constant increasing from 0.01 to 0.09 microM. This behavior is consistent with the assumption that charge pairing between CPR and P450 2B4 is involved in their association. In contrast, the electrostatic component of the interaction of the partners in P450BM-3 was shown to have an opposite sign. The isolated BMP and BMR domains have very low affinity for each other and the dissociation constant of their complex decreases from 8 to 3 microM with increasing ionic strength (0.05-0.5 M). Importantly, the BMP-CPR and P450 2B4-BMR "mixed", heterogeneous pairs behave similarly to the pairs of BMP and P450 2B4 with their native electron donors. Therefore, the observed difference in the interaction mechanisms between these two systems is determined mainly by the different structure of the heme proteins rather than their flavoprotein counterparts. P450BM-3 is extremely efficient and highly coupled, with the reductase and the P450 domains tethered to one another. Therefore, in contrast to P450 2B4-CPR binding, very tight binding between the P450BM-3 redox partners would be of no value in the synchronization of complex formation during catalytic turnover.

Kinetic evidence for the PsaE-dependent transient ternary complex photosystem I/Ferredoxin/Ferredoxin:NADP(+) reductase in a cyanobacterium

A mutant of Synechocystis PCC 6803, deficient in psaE, assembles photosystem I reaction centers without the PsaE subunit. Under conditions of acceptor-side rate-limited photoreduction assays in vitro (with 15 microM plastocyanin included), using 100 nM ferredoxin:NADP(+) reductase (FNR) and either Synechocystis flavodoxin or spinach ferredoxin, lower rates of NADP(+) photoreduction were measured when PsaE-deficient membranes were used, as compared to the wild type. This effect of the psaE mutation proved to be due to a decrease of the apparent affinity of the photoreduction assay system for the reductase. In the psaE mutant, the relative petH (encoding FNR) expression level was found to be significantly increased, providing a possible explanation for the lack of a phenotype (i.e., a decrease in growth rate) that was expected from the lower rate of linear electron transport in the mutant. A kinetic model was constructed in order to simulate the electron transfer from reduced plastocyanin to NADP(+), and test for possible causes for the observed change in affinity for FNR. The numerical simulations predict that the altered reduction kinetics of ferredoxin, determined for the psaE mutant [Barth, P., et al., (1998) Biochemistry 37, 16233-16241], do not significantly influence the rate of linear electron transport to NADP(+). Rather, a change in the dissociation constant of ferredoxin for FNR does affect the saturation profile for FNR. We therefore propose that the PsaE-dependent transient ternary complex PSI/ferredoxin/FNR is formed during linear electron transport. Using the yeast two-hybrid system, however, no direct interaction could be demonstrated in vivo between FNR and PsaE fusion proteins.

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.

Investigation of the role of surface residues in the ferredoxin from Clostridium pasteurianum

Eleven mutant forms of the ferredoxin from Clostridium pasteurianum (CpFd; 2 Fe4S4; 6200 Da) have been isolated in which six surface carboxylates are changed systematically to their uncharged but stereochemically equivalent carboxamide analogues. Such changes provide molecules which vary in overall charge and its surface distribution but vary minimally in structure and reduction potential. Glu-17 and Asp-6, -27, -33, -35, and -39 were converted providing six single mutants, four double mutants and one triple mutant. The proteins were characterised by UV-visible spectroscopy, square-wave voltammetry and 1H NMR. Their ability to mediate electron transfer between spinach NADH:ferredoxin oxidoreductase and horse heart cytochrome c was assessed. Each mutant is 30-100% as active as the recombinant protein with the triple mutant D33,35,39N being least active. Second-order rate constants k2 for the oxidation of reduced mutant ferredoxins by [Co(NH3)6]3+ were measured at 25 degrees C and I = 0.1 M by stopped-flow techniques. Each mutant displayed saturation kinetics with k2 being 30-100% of that for the recombinant protein. The rates were moderately sensitive to ionic strength. Variation in association constant K could not be detected within the confidence limits of the data. Overall the effects of the mutations were minor. In contrast to human and Anabaena 7120 [Fe2S2]-ferredoxins, electron transfer does not appear to rely on the presence of one or two specific surface carboxylate residues. It may occur from multiple sites on the surface of CpFd with recognition processes for its many physiological redox partners being controlled by relative reduction potentials, in addition to unidentified criteria. The conclusions are consistent with previous results for another series of mutant CpFd proteins interacting with physiological redox partners pyruvate: Fd oxidoreductase and hydrogenase (J.M. Moulis, V. Davasse (1995) Biochemistry 34, 16781-16788).

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.

The structure of iron-sulfur proteins

Ferredoxins are a group of iron-sulfur proteins for which a wealth of structural and mutational data have recently become available. Previously unknown structures of ferredoxins which are adapted to halophilic, acidophilic or hyperthermophilic environments and new cysteine patterns for cluster ligation and non-cysteine cluster ligation have been described. Site-directed mutagenesis experiments have given insight into factors that influence the geometry, stability, redox potential, electronic properties and electron-transfer reactivity of iron-sulfur clusters.

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.

An electrochemical, kinetic, and spectroscopic characterization of [2Fe-2S] vegetative and heterocyst ferredoxins from Anabaena 7120 with mutations in the cluster binding loop

Residues within the cluster binding loops of plant-type [2Fe-2S] ferredoxins are highly conserved and serve to structurally stabilize this unique region of the protein. We have investigated the influence of these residues on the thermodynamic reduction potentials and rate constants of electron transfer to ferredoxin:NADP+ reductase (FNR) by characterizing various single and multiple site-specific mutants of both the vegetative (VFd) and the heterocyst (HFd) [2Fe-2S] ferredoxins from Anabaena. Incorporation of residues from one isoform into the polypeptide backbone of the other created hybrid mutants whose reduction potentials either were not significantly altered or were shifted, but did not reconcile the 33-mV potential difference between VFd and HFd. The reduction potential of VFd appears relatively insensitive to mutations in the binding loop, excepting nonconservative variations at position 78 (T78A/I) which resulted in approximately 40- to 50-mV positive shifts compared to wild type. These perturbations may be linked to the role of the T78 side chain in stabilizing an ordered water channel between the iron-sulfur cluster and the surface of the wild-type protein. While no thermodynamic barrier to electron transfer to FNR is created by these potential shifts, the electron-transfer reactivities of mutants T78A/I (as well as T48A which has a wild-type-like potential) are reduced to approximately 55-75% that of wild type. These studies suggest that residues 48 and 78 are involved in the pathway of electron transfer between VFd and FNR and/or that mutations at these positions induce a unique, but unproductive orientation of the two proteins within the protein-protein complex.

The role of aromatic and acidic amino acids in the electron transfer reaction catalyzed by spinach ferredoxin-dependent glutamate synthase

Treatment of the ferredoxin-dependent, spinach glutamate synthase with N-bromosuccinimide (NBS) modifies 2 mol of tryptophan residues per mol of enzyme, without detectable modification of other amino acids, and inhibits enzyme activity by 85% with either reduced ferredoxin or reduced methyl viologen serving as the source of electrons. The inhibition of ferredoxin-dependent activity resulting from NBS treatment arises entirely from a decrease in the turnover number. Complex formation of glutamate synthase with ferredoxin prevented both the modification of tryptophan residues by NBS and inhibition of the enzyme. NBS treatment had no effect on the secondary structure of the enzyme, did not affect the Kms for 2-oxoglutarate and glutamine, did not affect the midpoint potentials of the enzyme's prosthetic groups and did not decrease the ability of the enzyme to bind ferredoxin. It thus appears that the ferredoxin-binding site(s) of glutamate synthase contains at least one, and possibly two, tryptophans. Replacement of either phenylalanine at position 65, in the ferredoxin from the cyanobacterium Anabaena PCC 7120, with a non-aromatic amino acid, or replacement of the glutamate at ferredoxin position 94, decreased the turnover number compared to that observed with wild-type Anabaena ferredoxin. The effect of the change at position 65 was quite modest compared to that at position 94, suggesting that an aromatic amino acid is not absolutely essential at position 65, but that glutamate 94 is essential for optimal electron transfer.

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.

Iron-sulfur cluster cysteine-to-serine mutants of Anabaena -2Fe-2S- ferredoxin exhibit unexpected redox properties and are competent in electron transfer to ferredoxin:NADP+ reductase

The reduction potentials and the rate constants for electron transfer (et) to ferredoxin:NADP+ reductase (FNR) are reported for site-directed mutants of the [2Fe-2S] vegetative cell ferredoxin (Fd) from Anabaena PCC 7120, each of which has a cluster ligating cysteine residue mutated to serine (C41S, C46S, and C49S). The X-ray crystal structure of the C49S mutant has also been determined. The UV-visible optical and CD spectra of the mutants differ from each other and from wild-type (wt) Fd. This is a consequence of oxygen replacing one of the ligating cysteine sulfur atoms, thus altering the ligand --> Fe charge transfer transition energies and the chiro-optical properties of the chromophore. Each mutant is able to rapidly accept an electron from deazariboflavin semiquinone (dRfH.) and to transfer an electron from its reduced form to oxidized FNR although all are somewhat less reactive (30-50%) toward FNR and are appreciably less stable in solution than is wt Fd. Whereas the reduction potential of C46S (-381 mV) is not significantly altered from that of wt Fd (-384 mV), the potential of the C49S mutant (-329 mV) is shifted positively by 55 mV, demonstrating that the cluster potential is sensitive to mutations made at the ferric iron in reduced [2Fe-2S] Fds with localized valences. Despite the decrease in thermodynamic driving force for et from C49S to FNR, the et rate constant is similar to that measured for C46S. Thus, the et reactivity of the mutants does not correlate with altered reduction potentials. The et rate constants of the mutants also do not correlate with the apparent binding constants of the intermediate (Fdred:FNRox) complexes or with the ability of the prosthetic group to be reduced by dRfH.. Furthermore, the X-ray crystal structure of the C49S mutant is virtually identical to that of wt Fd. We conclude from these data that cysteine sulfur d-orbitals are not essential for et into or out of the iron atoms of the cluster and that the decreased et reactivity of these Fd mutants toward FNR may be due to small changes in the mutual orientation of the proteins within the intermediate complex and/or alterations in the electronic structure of the [2Fe-2S] cluster.

Structure-function relationships in Anabaena ferredoxin: correlations between X-ray crystal structures, reduction potentials, and rate constants of electron transfer to ferredoxin:NADP+ reductase for site-specific ferredoxin mutants

A combination of structural, thermodynamic, and transient kinetic data on wild-type and mutant Anabaena vegetative cell ferredoxins has been used to investigate the nature of the protein-protein interactions leading to electron transfer from reduced ferredoxin to oxidized ferredoxin:NADP+ reductase (FNR). We have determined the reduction potentials of wild-type vegetative ferredoxin, heterocyst ferredoxin, and 12 site-specific mutants at seven surface residues of vegetative ferredoxin, as well as the one- and two-electron reduction potentials of FNR, both alone and in complexes with wild-type and three mutant ferredoxins. X-ray crystallographic structure determinations have been carried out for six of the ferredoxin mutants. None of the mutants showed significant structural changes in the immediate vicinity of the [2Fe-2S] cluster, despite large decreases in electron-transfer reactivity (for E94K and S47A) and sizable increases in reduction potential (80 mV for E94K and 47 mV for S47A). Furthermore, the relatively small changes in Calpha backbone atom positions which were observed in these mutants do not correlate with the kinetic and thermodynamic properties. In sharp contrast to the S47A mutant, S47T retains electron-transfer activity, and its reduction potential is 100 mV more negative than that of the S47A mutant, implicating the importance of the hydrogen bond which exists between the side chain hydroxyl group of S47 and the side chain carboxyl oxygen of E94. Other ferredoxin mutations that alter both reduction potential and electron-transfer reactivity are E94Q, F65A, and F65I, whereas D62K, D68K, Q70K, E94D, and F65Y have reduction potentials and electron-transfer reactivity that are similar to those of wild-type ferredoxin. In electrostatic complexes with recombinant FNR, three of the kinetically impaired ferredoxin mutants, as did wild-type ferredoxin, induced large (approximately 40 mV) positive shifts in the reduction potential of the flavoprotein, thereby making electron transfer thermodynamically feasible. On the basis of these observations, we conclude that nonconservative mutations of three critical residues (S47, F65, and E94) on the surface of ferredoxin have large parallel effects on both the reduction potential and the electron-transfer reactivity of the [2Fe-2S] cluster and that the reduction potential changes are not the principal factor governing electron-transfer reactivity. Rather, the kinetic properties are most likely controlled by the specific orientations of the proteins within the transient electron-transfer complex.