A comparative laser-flash absorption spectroscopy study of Anabaena PCC 7119 plastocyanin and cytochrome c6 photooxidation by photosystem I particles

In vitro kinetics of P700+ reduction of Thermosynechococcus elongatus trimeric Photosystem I complexes by recombinant cytochrome c 6 using a Joliot-type LED spectrophotometer

The reduction rate of photo-oxidized Photosystem I (PSI) with various natural and artificial electron donors have been well studied by transient absorption spectroscopy. The electron transfer rate from various donors to P₇₀₀⁺ has been measured for a wide range of photosynthetic organisms encompassing cyanobacteria, algae, and plants. PSI can be a limiting component due to tedious extraction and purification methods required for this membrane protein. In this report, we have determined the in vivo, intracellular cytochrome c ₆ (cyt c ₆)/PSI ratio in Thermosynechococcus elongatus (T.e.) using quantitative Western blot analysis. This information permitted the determination of P₇₀₀⁺ reduction kinetics via recombinant cyt c ₆ in a physiologically relevant ratio (cyt c ₆: PSI) with a Joliot-type, LED-driven, pump-probe spectrophotometer. Dilute PSI samples were tested under varying cyt c ₆ concentration, temperature, pH, and ionic strength, each of which shows similar trends to the reported literature utilizing much higher PSI concentrations with laser-based spectrophotometer. Our results do however indicate kinetic differences between actinic light sources (laser vs. LED), and we have attempted to resolve these effects by varying our LED light intensity and duration. The standardized configuration of this spectrophotometer will also allow a more uniform kinetic analysis of samples in different laboratories. We can conclude that our findings from the LED-based system display an added total protein concentration effect due to multiple turnover events of P₇₀₀⁺ reduction by cyt c ₆ during the longer illumination regime.

Purification of plastocyanin and cytochrome c6 from plants, green algae, and cyanobacteria

Plastocyanin and cytochrome c6 are widely distributed over the oxygen-evolving photosynthetic organisms. The two proteins are functionally equivalent, but strongly differ in their global electrostatic charge. In fact, they are acidic in eukaryotes, but either neutral or basic in cyanobacteria. Such a difference in their electrostatic features is a critical factor in designing the purification procedure, which must thus be modified and adapted accordingly. This chapter reports the methods for producing (including cell cultures), isolating, and purifying plastocyanin and cytochrome c6--which greatly differ in their isoelectric point--from a number of eukaryotic and prokaryotic organisms.

Purification and characterization of cytochrome c(6) from Acaryochloris marina

Cytochrome c(6), (cyt c(6)) a soluble monoheme electron transport protein, was isolated and characterized from the chlorophyll d-containing cyanobacterium Acaryochoris marina, the type strain MBIC11017. The protein was purified using ammonium sulfate precipitation, ion exchange and gel filtration column chromatography, and fast performance liquid chromatography. Its molecular mass and pI have been determined to be 8.87 kDa and less than 4.2, respectively, by mass spectrometry and isoelectrofocusing (IEF). The protein has an alpha helical structure as indicated by CD (circular dichroism) spectroscopy and a reduction midpoint potential (E(m)) of +327 mV versus the normal hydrogen electrode (NHE) as determined by redox potentiometry. Its potential role in electron transfer processes is discussed.

Transient binding of plastocyanin to its physiological redox partners modifies the copper site geometry

The transient complexes of plastocyanin with cytochrome f and photosystem I are herein used as excellent model systems to investigate how the metal sites adapt to the changes in the protein matrix in transient complexes that are involved in redox reactions. Thus, both complexes from the cyanobacterium Nostoc sp. PCC 7119 (former Anabaena sp. PCC 7119) have been analysed by X-ray absorption spectroscopy. Our data are consistent with a significant distortion of the trigonal pyramidal geometry of the Cu coordination sphere when plastocyanin binds to cytochrome f, no matter their redox states are. The resulting tetrahedral geometry shows a shortening of the distance between Cu and the S(delta) atom of its ligand Met-97, with respect to the crystallographic structure of free plastocyanin. On the other hand, when plastocyanin binds to photosystem I instead of cytochrome f, the geometric changes are not significant but a displacement in charge distribution around the metal centre can be observed. Noteworthy, the electronic density around the Cu atom increases or decreases when oxidised plastocyanin binds to cytochrome f or photosystem I, respectively, thus indicating that the protein matrix affects the electron transfer between the two partners during their transient interaction.

CW-EPR and ENDOR study of cytochrome c6 from Anabaena PCC 7119

The detailed analysis of the continuous-wave electron paramagnetic resonance and electron nuclear double resonance measurements on cytochrome c(6) from Anabaena PCC7119 reveals several electronic and structural properties of this hemeprotein. The oxidized protein shows two forms that differ in the arrangement of the residues that act as heme axial ligands. Information about the orientation of these residues is obtained for one of the forms, which turns out to differ from that found in the reduced protein from x-ray experiments. The biological significance of these results is discussed.

Detecting transient protein-protein interactions by X-ray absorption spectroscopy: The cytochromec6-photosystem I complex

Reliable analysis of the functionality of metalloproteins demands a highly accurate description of both the redox state and geometry of the metal centre, not only in the isolated metalloprotein but also in the transient complex with its target. Here, we demonstrate that the transient interaction between soluble cytochrome c(6) and membrane-embedded photosystem I involves subtle changes in the heme iron, as inferred by X-ray absorption spectroscopy (XAS). A slight shift to lower energies of the absorption edge of Fe2+ in cytochrome c6 is observed upon interaction with photosystem I. This work constitutes a novel application of XAS to the analysis of weak complexes in solution.

Transient homodimer interactions studied using the electron self-exchange reaction

Transient homodimer protein interactions have been investigated by analyzing the influence of ionic strength (NaCl) on the electron self-exchange (the bimolecular reaction whereby the two oxidation states of a redox protein interconvert) rate constant (k(ese)) of four plastocyanins. The k(ese) values for the plastocyanins from spinach, Dryopteris crassirhizoma (a fern), and the green alga Ulva pertusa, which possess acidic patches of varying size and locations, increase 190-, 29-, and 21-fold, respectively, at elevated ionic strength (I = 2.03 M). In contrast, the k(ese) for the almost neutral cyanobacterial plastocyanin from Anabaena variabilis exhibits very little dependence on ionic strength. The temperature dependence of the k(ese) for spinach plastocyanin (I = 0.28 M) provides evidence for poor packing at the homodimer interface. Representative structures of the transient homodimers involved in electron self-exchange, which are consistent with fits of the ionic strength dependence of k(ese) to van Leeuwen theory, have been obtained from protein modeling and docking simulations. The Coulombic energy of the docked homodimers follows the order spinach > D. crassirhizoma > U. pertusa > A. variabilis, which matches that of the overall influence of ionic strength on k(ese). Analysis of the homodimer structures indicates that poor packing and high planarity are features of the interface that favor transient interactions. The physiologically relevant Mg2+ ion has a much more pronounced influence on the k(ese) of spinach plastocyanin, which along with the known properties of the thylakoid lumen suggests a biological role for electron self-exchange.

NMR Analysis of the Transient Complex between Membrane Photosystem I and Soluble Cytochrome c6

A structural analysis of the surface areas of cytochrome c(6), responsible for the transient interaction with photosystem I, was performed by NMR transverse relaxation-optimized spectroscopy. The hemeprotein was titrated by adding increasing amounts of the chlorophyllic photosystem, and the NMR spectra of the free and bound protein were analyzed in a comparative way. The NMR signals of cytochrome c(6) residues located at the hydrophobic and electrostatic patches, which both surround the heme cleft, were specifically modified by binding. The backbones of internal residues close to the hydrophobic patch of cytochrome c(6) were also affected, a fact that is ascribed to the conformational changes taking place inside the hemeprotein when interacting with photosystem I. To the best of our knowledge, this is the first structural analysis by NMR spectroscopy of a transient complex between soluble and membrane proteins.

Structure and function of photosystem I: interaction with its soluble electron carriers and external antenna systems

Photosystem I (PS I) is a large membrane protein complex that catalyzes the first step of solar conversion, the light-induced transmembrane electron transfer, and generates reductants for CO2 assimilation. It consists of 12 different proteins and 127 cofactors that perform light capturing and electron transfer. The function of PS I includes inter-protein electron transfer between PS I and smaller soluble electron transfer proteins. The structure of PS I is discussed with respect to the potential docking sites for the soluble electron acceptors, ferredoxin/flavodoxin, at the stromal side and the soluble electron donors, cytochrome c6/plastocyanin, at the luminal side of the PS I complex. Furthermore, the potential interaction sites with the peripheral antenna proteins are discussed.

Role of electrostatics in the interaction between plastocyanin and photosystem I of the cyanobacterium Phormidium laminosum

The interactions between photosystem I and five charge mutants of plastocyanin from the cyanobacterium Phormidium laminosum were investigated in vitro. The dependence of the overall rate constant of reaction, k2, on ionic strength was investigated using laser flash photolysis. The rate constant of the wild-type reaction increased with ionic strength, indicating repulsion between the reaction partners. Removing a negative charge on plastocyanin (D44A) accelerated the reaction and made it independent of ionic strength; removing a positive charge adjacent to D44 (K53A) had little effect. Neutralizing and inverting the charge on R93 slowed the reaction down and increased the repulsion. Specific effects of MgCl2 were observed for mutants K53A, R93Q and R93E. Thermodynamic analysis of the transition state revealed positive activation entropies, suggesting partial desolvation of the interface in the transition state. In comparison with plants, plastocyanin and photosystem I of Phormidium laminosum react slowly at low ionic strength, whereas the two systems have similar rates in the range of physiological salt concentrations. We conclude that in P. laminosum, in contrast with plants in vitro, hydrophobic interactions are more important than electrostatics for the reactions of plastocyanin, both with photosystem I (this paper) and with cytochrome f[Schlarb-Ridley, B.G., Bendall, D.S. & Howe, C.J. (2002) Biochemistry41, 3279-3285]. We discuss the implications of this conclusion for the divergent evolution of cyanobacterial and plant plastocyanins.

The location of plastocyanin in vascular plant photosystem I

We have studied the binding sites of the electron donor and acceptor proteins of vascular plant photosystem I by electron microscopy/crystallography. Previously, we identified the binding site for the electron acceptor (ferredoxin). In this paper we complete these studies with the characterization of the electron donor (plastocyanin) binding site. After cross-linking, plastocyanin is detected using Fourier difference analysis of two dimensionally ordered arrays of photosystem I located at the periphery of chloroplast grana. Plastocyanin binds in a small cavity on the lumenal surface of photosystem I, close to the center and with a slight bias toward the PsaL subunit of the complex. The recent release of the full coordinates for the cyanobacterial photosystem I reaction center has allowed a detailed comparison between the structures of the eukaryotic and prokaryotic systems. This reveals a very close homology, which is particularly striking for the lumenal side of photosystem I.

Daddy, where did (PS)I come from?

The reacton centre I (RCI)-type photosystems from plants, cyano-, helio- and green sulphur bacteria are compared and the essential properties of an archetypal RCI are deduced. Species containing RCI-type photosystems most probably cluster together on a common branch of the phylogenetic tree. The predicted branching order is green sulphur, helio- and cyanobacteria. Striking similarities between RCI- and RCII-type photosystems recently became apparent in the three-dimensional structures of photosystem I (PSI), PSII and RCII. The phylogenetic relationship between all presently known photosystems is analysed suggesting (a) RCI as the ancestral photosystem and (b) the descendence of PSII from RCI via gene duplication and gene splitting. An evolutionary model trying to rationalise available data is presented.

PHOTOSYSTEM I: Function and Physiology

Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. In recent years, sophisticated spectroscopy, molecular genetics, and biochemistry have been used to understand the light conversion and electron transport functions of photosystem I. The light-harvesting complexes and internal antenna of photosystem I absorb photons and transfer the excitation energy to P700, the primary electron donor. The subsequent charge separation and electron transport leads to the reduction of ferredoxin. The photosystem I proteins are responsible for the precise arrangement of cofactors and determine redox properties of the electron transfer centers. With the availability of genomic information and the structure of photosystem I, one can now probe the functions of photosystem I proteins and cofactors. The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem I has a critical role in the metabolic networks and physiological responses in plants.

A single arginyl residue in plastocyanin and in cytochrome c(6) from the cyanobacterium Anabaena sp. PCC 7119 is required for efficient reduction of photosystem I

Positively charged plastocyanin from Anabaena sp. PCC 7119 was investigated by site-directed mutagenesis. The reactivity of its mutants toward photosystem I was analyzed by laser flash spectroscopy. Replacement of arginine at position 88, which is adjacent to the copper ligand His-87, by glutamine and, in particular, by glutamate makes plastocyanin reduce its availability for transferring electrons to photosystem I. Such a residue in the copper protein thus appears to be isofunctional with Arg-64 (which is close to the heme group) in cytochrome c(6) from Anabaena (Molina-Heredia, F. P., Diaz-Quintana, A., Hervás, M., Navarro, J. A., and De la Rosa, M. A. (1999) J. Biol. Chem. 274, 33565-33570) and Synechocystis (De la Cerda, B., Diaz-Quintana, A., Navarro, J. A. , Hervás, M., and De la Rosa, M. A. (1999) J. Biol. Chem. 274, 13292-13297). Other mutations concern specific residues of plastocyanin either at its positively charged east face (D49K, H57A, H57E, K58A, K58E, Y83A, and Y83F) or at its north hydrophobic pole (L12A, K33A, and K33E). Mutations altering the surface electrostatic potential distribution allow the copper protein to modulate its kinetic efficiency: the more positively charged the interaction site, the higher the rate constant. Whereas replacement of Tyr-83 by either alanine or phenylalanine has no effect on the kinetics of photosystem I reduction, Leu-12 and Lys-33 are essential for the reactivity of plastocyanin.

Electron transfers amongst cytochrome f, plastocyanin and photosystem I: kinetics and mechanisms

The review covers the theory and practice of the determination of kinetic constants for the electron transfer reactions in chloroplast thylakoid membranes between plastocyanin and cytochrome f in cytochrome bf complexes, and between plastocyanin and the reaction centre of photosystem I. Effects of ionic strength and pH are featured. The contribution of mutant studies is included. It is concluded that nearly all data from in vitro experiments can be interpreted with a reaction scheme in which an encounter complex between donor and acceptor is formed by long-range electrostatic attraction, followed by rearrangement during which metal centres become close enough for rapid intra-complex electron transfer. In vivo experiments so far cast doubt on this particular sequence, but their interpretation is not straightforward. Means of modelling the bimolecular complex between cytochrome f and plastocyanin are outlined, and two likely structures are illustrated. The complex formed by plastocyanin and photosystem I in higher plants involves the PsaF subunit, but its structure has not been fully determined.

Site-directed mutagenesis of cytochrome c(6) from Anabaena species PCC 7119. Identification of surface residues of the hemeprotein involved in photosystem I reduction

A number of surface residues of cytochrome c(6) from the cyanobacterium Anabaena sp. PCC 7119 have been modified by site-directed mutagenesis. Changes were made in six amino acids, two near the heme group (Val-25 and Lys-29) and four in the positively charged patch (Lys-62, Arg-64, Lys-66, and Asp-72). The reactivity of mutants toward the membrane-anchored complex photosystem I was analyzed by laser flash absorption spectroscopy. The experimental results indicate that cytochrome c(6) possesses two areas involved in the redox interaction with photosystem I: 1) a positively charged patch that may drive its electrostatic attractive movement toward photosystem I to form a transient complex and 2) a hydrophobic region at the edge of the heme pocket that may provide the contact surface for the transfer of electrons to P(700). The isofunctionality of these two areas with those found in plastocyanin (which acts as an alternative electron carrier playing the same role as cytochrome c(6)) are evident.

Extinction coefficients and midpoint potentials of cytochrome c(6) from the cyanobacteria Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803

Cytochrome c(6) is a soluble heme protein that serves as a photosynthetic electron transport component in cyanobacteria and algae, carrying electrons from the cytochrome bf complex to photosystem I. The rapid accumulation of cytochrome c(6) sequence data from a wide range of species, combined with significant advances in determining high resolution three-dimensional structures, provides a powerful database for investigating the relationship between structure and function. The fact that the gene encoding cytochrome c(6) can be readily modified in a number of species adds to the usefulness of cytochrome c(6) as a tool for comparative analysis. Efforts to relate cytochrome c(6) sequence information to structure, and structural information to function depend on knowledge of the physical and thermodynamic properties of the cytochrome from different species. To this end we have determined the optical extinction coefficient, the oxidation/reduction midpoint potential, and the pH dependence of the midpoint potential of cytochrome c(6) isolated from three cyanobacteria, Arthrospira maxima, Microcystis aeruginosa, and Synechocystis 6803.

PHOTOSYNTHETIC CYTOCHROMES c IN CYANOBACTERIA, ALGAE, AND PLANTS

The cytochromes that function in photosynthesis in cyanobacteria, algae, and higher plants have, like the other photosynthetic catalysts, been largely conserved in their structure and function during evolution. Cyanobacteria and algae contain cytochrome c6, which is not found in higher plants and which may enhance survival in their planktonic mode of life. Cyanobacteria and algae contain another cytochrome, low-potential c549, which is not found in higher plants. This cytochrome has a structural role in PSII and may contribute to anaerobic survival. There is a third unique cytochrome, cytochrome M, in the planktonic photosynthesizers, and its function is unknown. New evidence is appearing to indicate evolution of cytochrome interaction mechanisms during the evolution of photosynthesis. The ease of cytochrome gene manipulation in cyanobacteria and in Chlamydomonas reinhardtii now provides great advantages in understanding of photosynthesis. The solution of tertiary and quaternary structures of cytochromes and cytochrome complexes will provide structural and functional detail at atomic resolution.

Cloning and correct expression in Escherichia coli of the petE and petJ genes respectively encoding plastocyanin and cytochrome c6 from the cyanobacterium Anabaena sp. PCC 7119

The genes coding for plastocyanin (petE) and cytochrome c6 (petJ) from Anabaena sp. PCC 7119 have been cloned and properly expressed in Escherichia coli. The recombinant proteins are identical to those purified from the cyanobacterial cells. The products of both the petE and petJ genes are correctly processed in E. coli, as deduced from their identical N-terminal amino acid sequences as compared with those of the metalloproteins isolated from the cyanobacterium. Physicochemical and functional properties of the native and recombinant protein preparations are also identical, thereby confirming that expression of petE and petJ genes in E. coli is an adequate tool to address the study of the structure/function relationships in plastocyanin and cytochrome c6 from Anabaena by site-directed mutagenesis.

Changes in the reaction mechanism of electron transfer from plastocyanin to photosystem I in the cyanobacterium Synechocystis sp. PCC 6803 as induced by site-directed mutagenesis of the copper protein

The kinetic mechanism of plastocyanin oxidation by photosystem I in the cyanobacterium Synechocystis sp. PCC 6803 is drastically changed by modifying the metalloprotein by site-directed mutagenesis. The mutations herein considered concern four specific residues, two in the east face and the other two in the hydrophobic patch of plastocyanin. The first set of mutants include D44A, D44K, D47A, and D47R, as well as the double mutants D44A/D47A and D44R/D47R; the second set consists of L12A and K33E. The kinetic efficiency of all these mutant plastocyanins has been analyzed by laser-flash absorption spectroscopy. The plastocyanin concentration dependence of the observed electron transfer rate constant (kobs) is linear with most mutant plastocyanins, as with wild-type plastocyanin, but exhibits a saturation plateau at high protein concentration with the double mutant D44R/D47R, which suggests the formation of a plastocyanin-PSI transient complex. The effect of ionic strength on kobs varies from the wild-type plastocyanin to some of the mutants, for instance D44K, for which the salt concentration dependence of kobs is just the reverse as compared to the wild-type protein. The ionic strength dependence of kobs with D44R/D47R exhibits a bell-shaped profile, which is similar to that of green algae and higher plants. These findings indicate that the double mutant D44R/D47R follows a reaction mechanism involving not only complex formation with PSI but also further reorientation to properly accommodate the redox centers prior to electron transfer, as is the case in most evolved species, whereas the wild-type copper protein reacts with PSI by following a simple collisional kinetic model.

Fast electron transfer from cytochrome c6 and plastocyanin to photosystem I of Chlamydomonas reinhardtii requires PsaF

To study the function of the PsaF subunit of photosystem I (PSI), the interactions between plastocyanin or cytochrome c6 and PSI isolated from wild-type and a PsaF-deficient mutant of Chlamydomonas reinhardtii have been examined using cross-linking techniques and flash absorption spectroscopy. We show that efficient electron transfer from both plastocyanin and cytochrome c6 to PSI depends on PsaF. A remarkable feature of the PSI complex of C. reinhardtii is that both plastocyanin and cytochrome c6 reduce P700+ with first-order kinetics and a half-time of 3 micros, which is unique among photosynthetic organisms examined.

A comparative thermodynamic analysis by laser-flash absorption spectroscopy of photosystem I reduction by plastocyanin and cytochrome c6 in Anabaena PCC 7119, Synechocystis PCC 6803 and Spinach

A comparative thermodynamic analysis of photosystem I (PSI) reduction by plastocyanin (Pc) and cytochrome c6 (Cyt) has been carried out by laser-flash absorption spectroscopy in the cyanobacteria Anabaena PCC 7119 and Synechocystis PCC 6803 as well as in spinach. These three organisms have been reported to exhibit different reaction mechanisms [Hervas, M., Navarro, J. A.. Díaz, A., Bottin, H., & De la Rosa, M. A. (1995) Biochemistry, 34, 11321-11326]. Whereas the activation free energy for the overall reaction is mainly enthalpic in nature, long-range electrostatic interactions appear to be attractive in Anabaena, but repulsive in Synechocystis and spinach. The net interaction between PSI and its two donor proteins in Anabaena is similarly affected by ionic strength (the rate constant decreases with increasing salt concentration), but the activation parameters delta H+/+ delta S+/+ show different dependencies on ionic strength. A compensation effect between entropy and enthalpy at varying ionic strength is found in all these Pc/PSI and Cyt/PSI systems, except with Cyt and PSI from Anabaena. Such a compensation effect is proposed to be mainly due to stabilization of the intermediate electrostatic complex by hydrophobic forces. The electron transfer step seems to be well optimized in the Anabaena Cyt/PSI couple, which exhibits a temperature-independent fast kinetic phase and, therefore, a low activation energy barrier. Short-distance forces appear to have gained relevancy in the reaction mechanism of PSI reduction by Cyt and Pc throughout evolution, whereas long-range interactions are prevalent in less evolved organisms.

Function and organization of Photosystem I polypeptides

Photosystem I functions as a plastocyanin:ferredoxin oxidoreductase in the thylakoid membranes of chloroplasts and cyanobacteria. The PS I complex contains the photosynthetic pigments, the reaction center P700, and five electron transfer centers (A0, A1, FX, FA, and FB) that are bound to the PsaA, PsaB, and PsaC proteins. In addition, PS I complex contains at least eight other polypeptides that are accessory in their functions. Recent use of cyanobacterial molecular genetics has revealed functions of the accessory subunits of PS I. Site-directed mutagenesis is now being used to explore structure-function relations in PS I. The overall architecture of PSI complex has been revealed by X-ray crystallography, electron microscopy, and biochemical methods. The information obtained by different techniques can be used to propose a model for the organization of PS I. Spectroscopic and molecular genetic techniques have deciphered interaction of PS I proteins with the soluble electron transfer partners. This review focuses on the recent structural, biochemical and molecular genetic studies that decipher topology and functions of PS I proteins, and their interactions with soluble electron carriers.

Cloning and correct expression in E. coli of the petJ gene encoding cytochrome c6 from Synechocystis 6803

Cytochrome c6 from the cyanobacterium Synechocystis 6803 has been isolated and purified to electrophoretic homogeneity. The gene coding for such a heme protein (petJ) has been cloned and properly expressed in E. coli. This procedure yields a protein preparation completely identical to that obtained from the cyanobacterial cells. The N-terminal amino acid sequences of cytochrome c6 synthesized in both organisms are the same, thus allowing us to conclude that the petJ gene product is correctly processed in E. coli. To the best of our knowledge, this is the first time that any cytochrome c6 is produced in the enterobacterium. The identical physicochemical and kinetic properties of the proteins isolated from both sources confirm that expression of the petJ gene in E. coli is an adequate tool to address the study of Synechocystis cytochrome c6 by site-directed mutagenesis in a parallel way to that carried out with plastocyanin from the same organism.