Combined preparation of ferredoxin, ferredoxin-NADP+ reductase and plastocyanin from spinach leaves

Ferredoxin:NADP+ oxidoreductase in junction with CdSe/ZnS quantum dots: characteristics of an enzymatically active nanohybrid

Ferredoxin:NADP(+) oxidoreductase (FNR) is a plant and cyanobacterial photosynthetic enzyme, also found in non-photosynthetic tissues, where it is involved in redox reactions of biosynthetic pathways. In vivo it transfers electrons to nicotinamide adenine dinucleotide phosphate (NADP(+)), forming its reduced version, NADPH, while in vitro it can also use NADPH to reduce several substrates, such as ferricyanide, various quinones and nitriles. As an oxidoreductase catalyzing reaction of a broad range of substrates, FNR may be used in biotechnological processes. Quantum dots are semiconductor nanocrystals of a few to several nanometers diameter, having very useful luminescent properties. We present the spectroscopic and functional characteristics of a covalent conjugation of FNR and CdSe/ZnS quantum dots. Two types of quantum dots, of different diameter and emission maximum (550 and 650 nm), were used for comparison. Steady-state fluorescence and gel electrophoresis confirmed efficient conjugation, while fluorescence correlation spectroscopy (FCS) allowed for determination of the conjugates' radii. The nanohybrids sustained enzymatic activity; however, changes in maximal reaction rates and Michaelis constant were found. Detailed analysis of the kinetic parameters showed that the changes in the enzyme activity depend on the substrate used for activity measurement but also on the size of the quantum dots. The presented nanohybrids, as the first example using plant and photosynthetic enzyme as a protein partner, may became a tool to study photosynthesis as well as other biosynthetic and biotechnological processes, involving enzymatically catalyzed electron transfer.

Spectrally distinct cytochrome b-563 components in a chloroplast cytochrome b-f complex: Interaction with a hydroxyquinoline N-oxide

The two heme equivalents of cytochrome b-563 in the photosynthetic cytochrome b-f complex can be distinguished by their rate of reduction with dithionite at 25 degrees C and by their optical absorption spectra at 77 K. The cytochrome b component that is rapidly reduced after addition of dithionite or reduced ferredoxin possesses an alpha band that splits at 77 K into two peaks, at 557 and 561 nm. Prolonged incubation with reductant reveals a second, approximately equimolar cytochrome b component that has at 77 K an unsplit alpha-band maximum at 561 nm. The designations cytochrome b-563(H) and cytochrome b-563(L), respectively, are proposed for the rapidly and more slowly reduced cytochrome b-563 components. Potentiometric titration establishes a midpoint potential, E(m), of -30 mV (electron change n approximately 2) for cytochrome b-563(H) and -150 mV (n = 1) for cytochrome b-563(L) at pH 7.5. The reduction potential of these components is raised by 2-heptyl-4-hydroxyquinoline N-oxide, giving E(m) values of +57 and -34 mV, respectively, with each titration slope approximating n = 2.

Electron spin relaxation measurements on the blue-copper protein plastocyanin: Deviations from a power law temperature dependence

Electron spin relaxation rates over the temperatue range 1.41-15.6 K are presented for the copper-containing protein plastocyanin. Measurements are described for two samples, each derived from a different preparation of equivalent purity, for which the ionic, redox, and protein compositions varied slightly. X-band data are analyzed in terms of a phonon-limited direct process and a Raman relaxation process, where the index of the Raman transport integral is treated as a fitting parameter. Both samples yield rate data at the highest temperatures that are characterized by small deviations from a simple T(n) power law dependence, with n in the range 4.8-5.2. These deviations are most easily quantified when the T(n) power law fits are compared with similar functions that allow for a finite cutoff in the phonon density of states corresponding to Debye temperatures between 90 and 100 K with n in the range 5.0-5.5.

Thermodynamics of the acid transition in blue copper proteins

The thermodynamic parameters of the conformational transition occurring at low pH (acid transition, AT) in blue copper proteins, involving protonation and detachment from the Cu(I) ion of one histidine ligand, have been determined electrochemically for spinach and cucumber plastocyanins, Rhus vernicifera stellacyanin, cucumber basic protein (CBP), and Paracoccus versutus amicyanin. These data were obtained from direct protein electrochemistry experiments carried out at varying pH and temperature. For all species but CBP, the overall conformational change turns out to be exothermic. The entropy change is remarkably species-dependent. In particular, we found that (i) the balance of bond breaking/formation favors the acid transition in plastocyanins, which show remarkably negative DeltaH degrees '(AT) values, and (ii) the transition enthalpy turns out to be much less negative (or even positive) for the two phytocyanins (stellacyanin and CBP): for these species, the transition turns out to be observable thanks to the favorable (positive) entropy change. Thus, it is apparent that the thermodynamic "driving force" for this transition is enthalpic for the plastocyanins and entropic for the phytocyanins. Amicyanin is an intermediate case in which both enthalpic and entropic terms favor the transition. Under the assumption that the transition entropy originates from solvent reorganization effects, which are known to involve compensative enthalpy and entropy changes, the free energy change of the transition would also correspond to the enthalpy change due to bond breaking/formation in the first coordination sphere of the metal and in its immediate environment. Indeed, this term turns out to be very similar for the proteins investigated, in line with the conservation of the Cu(I)-His bond strengths in these species, except for amicyanin, for which the greater exothermicity of the transition can be ascribed to peculiar features of the active site.

Isolation and purification of plastocyanin from spinach stored frozen using hydrophobic interaction and ion-exchange chromatography

A new simple three-day procedure for preparative isolation and purification of plastocyanin from spinach stored in the frozen state is described. This procedure is based on batch adsorption on ion-exchange resin, ammonium sulphate precipitation, and purification on a Phenyl-Sepharose hydrophobic interaction column and a single Q Sepharose High Performance ion-exchange column. Approximately 100 mg of plastocyanin with an absorbance ratio A278/A597 of 1.10±0.02 in the oxidized state was typically obtained from 12 kg of spinach leaves. The purified spinach plastocyanin is shown to be homogeneous to the resolution of free solution capillary electrophoresis.

Evidence for fast and discriminatory electron transfer of proteins at modified gold electrodes

The electrochemistry of the redox proteins, cytochrome c, cytochrome b5, plastocyanin and ferredoxin at modified gold electrodes has been examined on the basis that electron transfer takes place at electroactive sites which are microscopic in size. Using this model, it is now proposed that electrochemistry of these proteins occurs at suitably modified sites with fast rates at potentials near the standard redox potential. The microscopic model implies that redox proteins and enzymes take part in fast electron transfer at specific sites on the electrode, other sites being completely ineffective. This form of molecular recognition, i.e. the ability to discriminate between the different sites on an electrode surface, mimics homogeneous redox reactions wherein redox active proteins 'recognize' their biological partners in a very specific sense. Previously, protein electrochemistry has been interpreted via use of a macroscopic model in which the proteins are transported to the electrode surface by linear diffusion followed by quasi-reversible or irreversible electron transfer to the electrode surface. The microscopic model, which assumes that the movement of the protein occurs predominantly by radial diffusion to very small sites, would appear to explain the data more satisfactorily and be consistent with biologically important, homogeneous redox reactions which are known to be fast.

Electron transfer reactions of metalloproteins at peptide-modified gold electrodes

The electron transfer reactions of four small redox proteins, cytochrome c. ferredoxin, plastocyanin and azurin, have been investigated at novel peptide-modified gold electrodes. These proved to be effective and selective in facilitating electron transfer. Good, quasi-reversible electron transfer was achieved selectively at different peptide-protein configurations by changing the pH or the ionic strength of the solution. The use of peptides as promoters for protein electrochemistry opens up the possibility of designing very specific electrode surfaces for larger molecules like enzymes.

Direct electrochemistry of protein-protein complexes involving cytochrome c, cytochrome b5, and plastocyanin

The direct electrochemistry of the cytochrome c/cytochrome b5 and cytochrome c/plastocyanin complexes has been investigated at edge-plane graphite and modified gold electrode surfaces, which are selective for one of the two components of the complex. Electrochemical response of one protein at an otherwise electrostatically unfavorable electrode surface was achieved in the presence of the other protein, and the calculated heterogeneous electron-transfer rate constant and diffusion coefficient were found to be in good agreement with the values determined previously from the electrochemistry of the individual proteins [Armstrong, F. A., Hill, H. A. O., & Walton, N. J. (1988) Acc. Chem. Res. 21, 407 and references therein]. A dynamic model of the protein-protein-electrode ternary complex is proposed to explain the promotion effect, and this model is supported by a study comparing the electrochemical responses of covalent and electrostatic cytochrome c/plastocyanin complexes. It is also suggested that the behavior of protein-protein complexes at electrode surfaces could be related to that of the complexes associated with biological membranes.

The complex formed between plastocyanin and cytochrome c. Investigation by NMR spectroscopy

Spinach plastocyanin and horse heart cytochrome c have been shown, by monitoring the behaviour of the hyperfine-shifted heme resonances of Fe(III) cytochrome c on titration with Cu(II) plastocyanin, to form a 1:1 complex with a dissociation constant of 67 mM (D2O, pH* 7.0, 300 K). The interaction sites on the plastocyanin surface have been investigated in one- and two-dimensional NMR experiments involving competition for plastocyanin between cytochrome c and the paramagnetic cation Cr(NH3)(3+)6. The plastocyanin resonances which are paramagnetically broadened in the spectrum of plastocyanin alone are also broadened in the spectrum of the mixture of the two proteins. This shows that, on the NMR time scale, no plastocyanin residues are hidden from Cr(NH3)(3+)6 by complexation with cytochrome c. [It has been shown that Cr(NH3)(3+)6 does not disrupt formation of the complex between the two proteins.] It appears that initial complexation of cytochrome c takes place at the acidic east site of plastocyanin, and that the extensive negative electrostatic surface of plastocyanin accommodates the paramagnetic probe and cytochrome c simultaneously in a dynamic ternary complex. The location of the electron transfer site on plastocyanin is discussed.

1H-NMR sequential assignments and cation-binding studies of spinach plastocyanin

The essentially complete assignment of the 1H-NMR spectrum of the Cu(i) form of spinach plastocyanin has been achieved using two-dimensional NMR techniques and sequence-specific resonance assignment procedures. A variety of pH and temperature conditions was utilised to overcome the problems of resonance overlap in the spectrum, degeneracy of C alpha H and solvent H2O chemical shifts, and cross-saturation of labile NH resonances. A qualitative analysis of the long-range nuclear Overhauser effects observed indicates that the backbone fold of spinach plastocyanin is very similar to that of poplar plastocyanin, whose structure has been solved by X-ray crystallography and differs in 22 of its 99 amino acid residues. The assignments provide a basis for further investigations into the structural and ion- and protein-binding properties of plastocyanin in solution.

Inhibition of ferredoxin: NADP+ reductase activity by the hexacyanochromate (III) ion

The small inorganic complex Cr(CN)6(3-) is a clean inhibitor of the ferredoxin: NADP+ reductase-catalysed oxidation of reduced spinach ferredoxin by NADP+. Independent spectrophotometric measurements show that millimolar additions of Cr(CN)6(3-) to mixtures of ferredoxin and ferredoxin NADP+ reductase give a marked attenuation of the difference spectrum characteristic of ferredoxin-ferredoxin: NADP+ reductase complex formation. Since there is no evidence, from NMR studies, for significant binding of Cr(CN)6(3-) to ferredoxin, these results indicate that Cr(CN)6(3-) binds to ferredoxin: NADP+ reductase at a site which is crucial to its interaction with the electron-transfer protein. The effective kinetic binding constant for Cr(CN)6(3-), measured at low ferredoxin concentration, is 445 M-1 (ie Kdiss congruent to 2 mM) at 25 degrees, pH7.5, I = 0.10 M. With assumption of a simple electrostatic interaction, an enzyme domain with an effective charge of 3+/4+ is proposed.

Isolation and biochemical characterization of maleic-acid hydratase, an iron-requiring hydro-lyase

A procedure for the isolation of maleic acid hydratase (D-malate hydro-lyase, EC 4.2.1.31) of about 95% purity from rabbit kidneys is described. The enzyme consists of a single polypeptide chain of 582 amino-acid residues with an approximate molecular mass of 68 kDa. The enzyme is very unstable and has an absolute requirement for chloride ions. Addition of sodium sulphide during the purification process was essential to maintain the enzyme in an activatable state. The pure preparation has low activity but responds to activation with Fe2+ ions, Na2S and a thiol. The sequence of adding the activating reagents is critical to achieve optimal activity. Ni2+ and to a lesser extent Co2+ can replace iron in the activation process. The enzyme incorporates 4-5 mol iron/mol and 4.5-6 mol sulphide/mol during activation. In this process an [Fe-S] cluster appears to be built up, as indicated by optical and electron paramagnetic resonance (EPR) spectroscopy. In activated samples exposed to air the [Fe-S] cluster is EPR-detectable through an axial signal with g = 2.01 and g = 2.029 whose temperature and power saturation characteristics were similar to those of other [3Fe-xS] clusters. The activated enzyme, however, is readily inactivated even upon minor manipulation with destruction of the iron-sulfur core.

Analysis of strain-induced EPR-line shapes and anisotropic spin-lattice relaxation in a [2Fe-2S] ferredoxin

The electron paramagnetic resonance spectrum of the [2Fe-2S]1+(2+;1+) cluster in spinach-leaf ferredoxin has been measured at four microwave frequencies from 1 to 35 GHz. Using a modified g-strain formula, the asymmetrical spectrum has been simulated in detail without the assumption of signal multiplicity. In all but the lowest frequency bands the line width is dominated by an extremely anisotropic g-shift distribution, caused by a statistical distribution in dislocation strains. The crossover point of domination by unresolved proton splittings is around 2 GHz. The angle-dependent elasticity of the cluster can be related to an anisotropy in the spin-lattice relaxation rate. Intensity behaviour under continuous saturation, at temperatures in the two-phonon region, is in qualitative agreement with elementary theory. On the basis of these results it is argued that biochemists should be aware of the questionable nature of some ad hoc assumptions commonly made to interpret EPR of metalloproteins. Specifically, a physically meaningful determination of the number and stoicheiometry of distinguishable compounds, represented in a complex spectrum, may well require more advanced theoretical tools than the frequently employed deconvolution in symmetrical Gaussians with associated unique relaxation times.

Isoenzymes of ferredoxin-NADBΔ oxidoreductase from the cyanobacterium Nostoc strain MAC

Ferredoxin-NADP(+) oxidoreductase from the cyanobacterium Nostoc strain MAC was separated into two fractions by ion-exchange chromatography. Both were purified to electrophoretic homogeneity and exhibited diaphorase and ferredoxin-dependent cytochrome c reductase activity. The activities with three different electron carriers in this latter assay were similar for the two fractions, as were the pH optima in both assays. Each fraction, however, could be resolved into several active components by isoelectric focusing, both after initial separation and following apparent purification by gel filtration on Sephadex G-150, further chromatography on DEAE-cellulose, and use of hydroxylapatite columns.

Complex-forming properties of spinach NADP+ reductase with ferredoxin, ferrocyanide and NADP+

The flavoprotein NADP+ reductase from spinach chloroplasts may form a ternary complex with one molecule of NADP+ and one molecule of ferredoxin. Spectroscopic titration studies show that the NADP+ binding site and the ferredoxin binding site are totally independent, that is previous binding of ferredoxin does not modify binding of NADP+, and conversely. Since NADP+ reductase conditions the diaphorase reaction, that is an electron transfer between NADPH and various acceptors such as ferricyanide, the binding of ferrocyanide and its possible interaction with NADP+ and ferredoxin has been studied. Ferrocyanide behaves as a competitive inhibitor with respect to both NADP+ and ferredoxin. This seems paradoxical since NADP+ and ferredoxin are independently bound at two different non-overlapping sites of the flavoprotein. This apparent paradox may be resolved by a theoretical analysis of the interactions between either ferrocyanide and NADP+, or ferrocyanide and ferredoxin. Theory shows that if ferrocyanide is non-specifically bound at two independent sites, namely the NADP+ and the ferredoxin binding sites, it appears competitive with respect to both NADP+ and ferredoxin, although ternary flavoprotein-ferredoxin-ferrocyanide and flavoprotein-NADP+-ferrocyanide complexes are formed. The binding constants of NADP+, ferredoxin and ferrocyanide for the enzyme have been determined. These results are discussed in connection with the possible mechanism of the diaphorase reaction.

Role of beta-carotene in the reaction centres of photosystems I and II of spinach chloroplasts prepared in non-polar solvents

Spinach chloroplasts have been prepared nonaqueously using non-polar solvents (n-hexane, CCl4, n-hepatane) and the beta-carotene content extracted in a controlled manner. This procedure is reproducible and does not result in large structural or spectral changes of the chloroplasts. The organisation of the chlorophyll-proteins is unaltered, as fragmentation with digitonin results in the appearance of the same fractions as found previously for aqueously-prepared chloroplasts, including the pink zone containing cytochromes f and b6 in the ratio 1 : 2. The chloroplasts possess both Photosystem I activity (P-700 photo-bleaching, and NADP+ photoreduction) and Photosystem II activity (parabenzoquinone reduction with Mn2+ as electron donor, and chlorophyll fluorescence induction). Use of moderate intensity red illumination has allowed a study of the role of beta-carotene in photochemistry separate from its roles in energy transfer and photoprotection. Removal of the fraction of beta-carotene closely associated with the Photosystem I reaction centre caused the rate of NADP+ photoreduction to fall to a low, but significantly non-zero level. Thus, in the complete absence of beta-carotene, photochemistry can still be observed, however the specific association of beta-carotene with the reaction centre is required for maximal rates. We propose that beta-carotene bound at the reaction centre decreases the rate of transfer of excitation energy away from the reaction centre, and increases the rate of photochemistry. It is possible that this occurs via formation of an exciplex between ground state beta-carotene and chlorophyll in the first excited state.

Polypeptide profiles of chlorophyll . protein complexes and thylakoid membranes of spinach chloroplasts

In addition to the major chlorophyll . protein complexes I and II, two minor chlorophyll proteins have been observed in sodium dodecyl sulfate (SDS))-polyacrylamide gels of spinach chloroplast membranes. These minor pigmented zones appeared to be derived from the light-harvesting chlorophyll a/b . protein and from the reaction centre complex of Photosystem II. Data are presented on the polypeptide profiles of purified digitonin-subschloroplast particles, with special regard to the effect of solubilization temperature and extraction of lipids. The results are compared with the SDS-polypeptide pattern of spinach thylakoids obtained under exactly the same conditions with respect to electrophoresis technique, solubilization method and presence of lipid. In addition, the effects of temperature and lipid extraction on the distinct chlorophyll . protein complexes appearing in SDS gel electrophoretograms of chloroplast membranes were studied by slicing the chlorophyll-containing regions and subjecting them to a second run with or without heating or extraction with acetone. By supplementing these data with an examination of the polypeptide composition of cytochrome f and coupling factor, it has been possible to identify most of the major chloroplast membrane polypeptides.

Molecular heterogeneity of ferredoxin-NADP+ reductase from spinach leaves

Ferredoxin-NADP+ reductase (NADPH: ferredoxin oxidoreductase, EC 1.6.7.1) from spinach leaves has been purified according to a new procedure. The enzyme shows the presence of five molecular forms as identified by isoelectric focusing, namely a, b, c, d and e with pI values of 6.0, 5.5, 5.2, 5.0 and 4.8, respectively. All the bands are catalytically active and are clearly identifiable after the first steps of the purification procedure. The basic pattern of the ferredoxin-NADP+ reductase forms is the same whether extracted from one or many spinach plants and is not affected by the different purification procedures used. Two distinct classes of molecular weight have been found for the isolated forms b, c and d as measured by sodium dodecyl sulphate electrophoresis, with values of 33 000-34 000 for the first and 36 000-38 000 for the later two forms. Gel electrophoresis in non-denaturing media at different gel concentrations gives the same order of molecular weight values, thus ruling out the possibility that the native enzyme is a dimer, as has been reported by Schneeman, R. and Krogmann, D.W. ((1975) J. Biol. Chem. 250, 4965-4971). No significant kinetic differences were detectable for the isolated forms of ferredoxin-NADP+ reductase.

A lysyl residue at the NADP binding site of ferredoxin-NADP reductase

Dansyl chloride, at low molar ratio, inactivates ferredoxin-NADP reductase (NADPH:ferredoxin oxidoreductase, EC 1.6.7.1). The complete protection afforded either by NADP or NADPH suggests a direct involvement of the active site. Experiments with [Me-14C] dansyl chloride showed that about 1.5 residues per flavin were dansylated: by differential labelling experiments using NADP, it has been proved that enzyme inactivation is due to dansylation of one residue. The group modified has been identified as the epsilon-amino group of a lysine. The pH-inactivation profile indicates that this essential group has an apparent pKa of 8.7. The dansylated flavoprotein seems to maintain its native conformation; it shows a fluorescent chromophore with a peak at 335 nm. The modified enzyme has lost the capacity to form a complex with NADP, nevertheless it interacts normally with ferredoxin. It is concluded that the loss of catalytic activity which parallels the dansylation of a lysyl residue occurs because this residue is essential for the binding of the pyridine nucleotide substrate. Protection experiments with a series of coenzyme analogs further indicate that this lysyl residue interacts, most likely, with the 2'-phosphate moiety of NADP(H).

The copper coordination group in "blue" copper proteins: evidence from resonance Raman spectra

Tunable dye laser excitation in the intense similar to 600-nm absorption band of azurin, plastocyanin, and ceruloplasmin provides resonance enhanced Raman spectra. They consist of a complex set of bands, at least three or four in number, between 350 and 473 cm-1, which are assignable to Cu-N or Cu-O bond stretching, and a weak band near 270 cm-1, which probably arises from Cu-S stretching. A weak band at 765 cm-1 found in plastocyanin may arise from C-S stretching. Analysis of the Raman intensity pattern, as well as of the nature of the resonant electronic transition, leads to a model of the "blue" copper site involving approximately trigonal-bipyramidal coordination, with a sulfur and two nitrogen ligands in the equatorial plane, and less strongly bound nitrogen or oxygen ligands at axial positions. This arrangement would be well poised for stabilization of Cu(I) upon reduction.

A combined procedure for preparation of plastocyanin, ferredoxin and CF1

Homogeneous preparations of ferredoxin, plastocyanin, and chloroplast coupling factor (CF1) have been isolated from spinach by a combined procedure in which supernatants from preparation of chloroplasts are used for isolation of ferredoxin and the chloroplasts serve as the source of plastocyanin. The proteins were purified by DEAE-cellulose chromatography and gel filtration, after precipitation with acetone in the case of ferredoxin or release from membranes in the case of plastocyanin. The proteins obtained by this procedure are pure, as evidenced by absorption ratios (ferredoxin, A420/A276 = 0.47-0.48; plastocyanin, A278/A597 = 1.2) and by the fact that both proteins migrate as single bands on polyacrylamide gels in the presence of sodium dodecyl sulfate.

EPR signals of oxidized plastocyanin in intact algae

(1) An electron paramagnetic resonance (EPR) signal was observed at g = 2.05 in the low temperature spectra of intact cells of green, red and blue-green algae and of spinach chloroplasts. The g-value and the shape of the signal were similar to that of purified, soluble plastocyanin. (2) The amount of the copper protein, determined from the EPR signal height, was estimated to be nearly the same in all the studied organisms on the basis of the concentration of chlorophyll. Furthermore, it was found that the amount of the copper protein, determined from the EPR signal height in spinach chloroplasts corresponds with that of plastocyanin as determined chemically by Katoh, S., Suga, I., Shiratori, I. and Takamiya, A. (1961) Arch. Biochem. Biophys. 94, 136-141. (3) Experiments with far-red and red illumination show that the site of the copper protein in vivo is in the electron transport pathway between Photosystems 1 and 2. Plastocyanin is not oxidized by illumination at 77 degrees K, indicating that no electron transfer occurs between the primary electron donor of Photosystem 1, P700, and plastocyanin at that temperature. Furthermore, the experiments suggest that in the intact cells of the studied algae, plastocyanin is not only reduced by Photosystem 2 but also by cyclic electron transport around Photosystem 1.