The Protein Matrix of Plastocyanin Supports Long-Distance Charge Transport with Photosystem I and the Copper Ion Regulates Its Spatial Span and Conductance

Charge exchange is the fundamental process that sustains cellular respiration and photosynthesis by shuttling electrons in a cascade of electron transfer (ET) steps between redox cofactors. While intraprotein charge exchange is well characterized in protein complexes bearing multiple redox sites, interprotein processes are less understood due to the lack of suitable experimental approaches and the dynamic nature of the interactions. Proteins constrained between electrodes are known to support electron transport (ETp) through the protein matrix even without redox cofactors, as the charges housed by the redox sites in ET are furnished by the electrodes. However, it is unknown whether protein ETp mechanisms apply to the interprotein medium present under physiological conditions. We study interprotein charge exchange between plant photosystem I (PSI) and its soluble redox partner plastocyanin (Pc) and address the role of the Pc copper center. Using electrochemical scanning tunneling spectroscopy (ECSTS) current-distance and blinking measurements, we quantify the spatial span of charge exchange between individual Pc/PSI pairs and ETp through transient Pc/PSI complexes. Pc devoid of the redox center (Pcapo) can exchange charge with PSI at longer distances than with the copper ion (Pcholo). Conductance bursts associated with Pcapo/PSI complex formation are higher than in Pcholo/PSI. Thus, copper ions are not required for long-distance Pc/PSI ETp but regulate its spatial span and conductance. Our results suggest that the redox center that carries the charge in Pc is not necessary to exchange it in interprotein ET through the aqueous solution and question the canonical view of tight complex binding between redox protein partners.

Light- and Redox-Dependent Force Spectroscopy Reveals that the Interaction between Plastocyanin and Plant Photosystem I Is Favored when One Partner Is Ready for Electron Transfer

Photosynthesis is a fundamental process that converts photons into chemical energy, driven by large protein complexes at the thylakoid membranes of plants, cyanobacteria, and algae. In plants, water-soluble plastocyanin (Pc) is responsible for shuttling electrons between cytochrome b6f complex and the photosystem I (PSI) complex in the photosynthetic electron transport chain (PETC). For an efficient turnover, a transient complex must form between PSI and Pc in the PETC, which implies a balance between specificity and binding strength. Here, we studied the binding frequency and the unbinding force between suitably oriented plant PSI and Pc under redox control using single molecule force spectroscopy (SMFS). The binding frequency (observation of binding-unbinding events) between PSI and Pc depends on their respective redox states. The interaction between PSI and Pc is independent of the redox state of PSI when Pc is reduced, and it is disfavored in the dark (reduced P700) when Pc is oxidized. The frequency of interaction between PSI and Pc is higher when at least one of the partners is in a redox state ready for electron transfer (ET), and the post-ET situation (PSI_Red-Pc_Ox) leads to lower binding. In addition, we show that the binding of ET-ready Pc_Red to PSI can be regulated externally by Mg²⁺ ions in solution.

The structure of a triple complex of plant photosystem I with ferredoxin and plastocyanin

The ability of photosynthetic organisms to use sunlight as a sole source of energy is endowed by two large membrane complexes-photosystem I (PSI) and photosystem II (PSII). PSI and PSII are the fundamental components of oxygenic photosynthesis, providing oxygen, food and an energy source for most living organisms on Earth. Currently, high-resolution crystal structures of these complexes from various organisms are available. The crystal structures of megadalton complexes have revealed excitation transfer and electron-transport pathways within the various complexes. PSI is defined as plastocyanin-ferredoxin oxidoreductase but a high-resolution structure of the entire triple supercomplex is not available. Here, using a new cryo-electron microscopy technique, we solve the structure of native plant PSI in complex with its electron donor plastocyanin and the electron acceptor ferredoxin. We reveal all of the contact sites and the modes of interaction between the interacting electron carriers and PSI.

The structure of a triple complex of plant photosystem I with ferredoxin and plastocyanin

The ability of photosynthetic organisms to use sunlight as a sole source of energy is endowed by two large membrane complexes-photosystem I (PSI) and photosystem II (PSII). PSI and PSII are the fundamental components of oxygenic photosynthesis, providing oxygen, food and an energy source for most living organisms on Earth. Currently, high-resolution crystal structures of these complexes from various organisms are available. The crystal structures of megadalton complexes have revealed excitation transfer and electron-transport pathways within the various complexes. PSI is defined as plastocyanin-ferredoxin oxidoreductase but a high-resolution structure of the entire triple supercomplex is not available. Here, using a new cryo-electron microscopy technique, we solve the structure of native plant PSI in complex with its electron donor plastocyanin and the electron acceptor ferredoxin. We reveal all of the contact sites and the modes of interaction between the interacting electron carriers and PSI.

Near-infrared in vitro measurements of photosystem I cofactors and electron-transfer partners with a recently developed spectrophotometer

A kinetic-LED-array-spectrophotometer (Klas) was recently developed for measuring in vivo redox changes of P700, plastocyanin (PCy), and ferredoxin (Fd) in the near-infrared (NIR). This spectrophotometer is used in the present work for in vitro light-induced measurements with various combinations of photosystem I (PSI) from tobacco and two different cyanobacteria, spinach plastocyanin, cyanobacterial cytochrome c6 (cyt. c6), and Fd. It is shown that cyt. c6 oxidation contributes to the NIR absorption changes. The reduction of (FAFB), the terminal electron acceptor of PSI, was also observed and the shape of the (FAFB) NIR difference spectrum is similar to that of Fd. The NIR difference spectra of the electron-transfer cofactors were compared between different organisms and to those previously measured in vivo, whereas the relative absorption coefficients of all cofactors were determined by using single PSI turnover conditions. Thus, the (840 nm minus 965 nm) extinction coefficients of the light-induced species (oxidized minus reduced for PC and cyt. c6, reduced minus oxidized for (FAFB), and Fd) were determined with values of 0.207 ± 0.004, - 0.033 ± 0.006, - 0.036 ± 0.008, and - 0.021 ± 0.005 for PCy, cyt. c6, (FAFB) (single reduction), and Fd, respectively, by taking a reference value of + 1 for P700+. The fact that the NIR P700 coefficient is larger than that of PCy and much larger than that of other contributing species, combined with the observed variability in the NIR P700 spectral shape, emphasizes that deconvolution of NIR signals into different components requires a very precise determination of the P700 spectrum.

Robust and Accurate Computational Estimation of the Polarizability Tensors of Macromolecules

Alignment of molecules through electric fields minimizes the averaging over orientations, e.g., in single-particle-imaging experiments. The response of molecules to external ac electric fields is governed by their polarizability tensor, which is usually calculated using quantum chemistry methods. These methods are not feasible for large molecules. Here, we calculate the polarizability tensor of proteins using a regression model that correlates the polarizabilities of the 20 amino acids with perfect conductors of the same shape. The dielectric constant of the molecules could be estimated from the slope of the regression line based on the Clausius-Mossotti equation. We benchmark our predictions against the quantum chemistry results for the Trp cagemini protein and the measured dielectric constants of larger proteins. Our method has applications in computing laser alignment of macromolecules, for instance, benefiting single-particle imaging, as well as for estimation of the optical and electrostatic characteristics of proteins and other macromolecules.

Structure of the green algal photosystem I supercomplex with a decameric light-harvesting complex I

In plants and green algae, the core of photosystem I (PSI) is surrounded by a peripheral antenna system consisting of light-harvesting complex I (LHCI). Here we report the cryo-electron microscopic structure of the PSI-LHCI supercomplex from the green alga Chlamydomonas reinhardtii. The structure reveals that eight Lhca proteins form two tetrameric LHCI belts attached to the PsaF side while the other two Lhca proteins form an additional Lhca2/Lhca9 heterodimer attached to the opposite side. The spatial arrangement of light-harvesting pigments reveals that Chlorophylls b are more abundant in the outer LHCI belt than in the inner LHCI belt and are absent from the core, thereby providing the downhill energy transfer pathways to the PSI core. PSI-LHCI is complexed with a plastocyanin on the patch of lysine residues of PsaF at the luminal side. The assembly provides a structural basis for understanding the mechanism of light-harvesting, excitation energy transfer of the PSI-LHCI supercomplex and electron transfer with plastocyanin.

Modular origins of biological electron transfer chains

Oxidoreductases catalyze electron transfer reactions that ultimately provide the energy for life. A limited set of ancestral protein-metal modules are presumably the building blocks that evolved into this diverse protein family. However, the identity of these modules and their path to modern oxidoreductases is unknown. Using a comparative structural analysis approach, we identify a set of fundamental electron transfer modules that have evolved to form the extant oxidoreductases. Using transition metal-containing cofactors as fiducial markers, it is possible to cluster cofactor microenvironments into as few as four major modules: bacterial ferredoxin, cytochrome c, symerythrin, and plastocyanin-type folds. From structural alignments, it is challenging to ascertain whether modules evolved from a single common ancestor (homology) or arose by independent convergence on a limited set of structural forms (analogy). Additional insight into common origins is contained in the spatial adjacency network (SPAN), which is based on proximity of modules in oxidoreductases containing multiple cofactor electron transfer chains. Electron transfer chains within complex modern oxidoreductases likely evolved through repeated duplication and diversification of ancient modular units that arose in the Archean eon.

Shedding new light on viral photosynthesis

Viruses infecting the environmentally important marine cyanobacteria Prochlorococcus and Synechococcus encode 'auxiliary metabolic genes' (AMGs) involved in the light and dark reactions of photosynthesis. Here, we discuss progress on the inventory of such AMGs in the ever-increasing number of viral genome sequences as well as in metagenomic datasets. We contextualise these gene acquisitions with reference to a hypothesised fitness gain to the phage. We also report new evidence with regard to the sequence and predicted structural properties of viral petE genes encoding the soluble electron carrier plastocyanin. Viral copies of PetE exhibit extensive modifications to the N-terminal signal peptide and possess several novel residues in a region responsible for interaction with redox partners. We also highlight potential knowledge gaps in this field and discuss future opportunities to discover novel phage-host interactions involved in the photosynthetic process.

[Identification of Intermediate States of Electron Transfer Proteins Plastocyanin and Cytochrome f in the Process of Diffusional Encounter]

The Brownian dynamics method is used for qualitative analysis of events leading to formation of a functionally active plastocyanin-cytochrome f complex. Intermediate states of this process are identified by density-based hierarchical clustering. Diffusive entrapment of plastocyanin by cytochrome f is a key point of the suggested putative scenario of protein-protein approaching. Mobility of plastocyanin is characterized for different values of protein-protein electrostatic interaction energy.

Development of a method for reconstruction of crowded NMR spectra from undersampled time-domain data

NMR is a unique methodology for obtaining information about the conformational dynamics of proteins in heterogeneous biomolecular systems. In various NMR methods, such as transferred cross-saturation, relaxation dispersion, and paramagnetic relaxation enhancement experiments, fast determination of the signal intensity ratios in the NMR spectra with high accuracy is required for analyses of targets with low yields and stabilities. However, conventional methods for the reconstruction of spectra from undersampled time-domain data, such as linear prediction, spectroscopy with integration of frequency and time domain, and analysis of Fourier, and compressed sensing were not effective for the accurate determination of the signal intensity ratios of the crowded two-dimensional spectra of proteins. Here, we developed an NMR spectra reconstruction method, "conservation of experimental data in analysis of Fourier" (Co-ANAFOR), to reconstruct the crowded spectra from the undersampled time-domain data. The number of sampling points required for the transferred cross-saturation experiments between membrane proteins, photosystem I and cytochrome b 6 f, and their ligand, plastocyanin, with Co-ANAFOR was half of that needed for linear prediction, and the peak height reduction ratios of the spectra reconstructed from truncated time-domain data by Co-ANAFOR were more accurate than those reconstructed from non-uniformly sampled data by compressed sensing.

HMA6 and HMA8 are two chloroplast Cu+-ATPases with different enzymatic properties

Copper (Cu) plays a key role in the photosynthetic process as cofactor of the plastocyanin (PC), an essential component of the chloroplast photosynthetic electron transfer chain. Encoded by the nuclear genome, PC is translocated in its apo-form into the chloroplast and the lumen of thylakoids where it is processed to its mature form and acquires Cu. In Arabidopsis, Cu delivery into the thylakoids involves two transporters of the PIB-1 ATPases family, heavy metal associated protein 6 (HMA6) located at the chloroplast envelope and HMA8 at the thylakoid membrane. To gain further insight into the way Cu is delivered to PC, we analysed the enzymatic properties of HMA8 and compared them with HMA6 ones using in vitro phosphorylation assays and phenotypic tests in yeast. These experiments reveal that HMA6 and HMA8 display different enzymatic properties: HMA8 has a higher apparent affinity for Cu(+) but a slower dephosphorylation kinetics than HMA6. Modelling experiments suggest that these differences could be explained by the electrostatic properties of the Cu(+) releasing cavities of the two transporters and/or by the different nature of their cognate Cu(+) acceptors (metallochaperone/PC).

[Brownian dynamics simulations of protein-protein interactions in photosynthetic electron transport chain]

The application of Brownian dynamics for simulation of transient protein-protein interactions is reviewed. The review focuses on theoretical basics of Brownian dynamics method, its particular implementations, advantages and drawbacks of the method. The outlook for future development of Brownian dynamics-based simulation techniques is discussed. Special attention is given to analysis of Brownian dynamics trajectories. The second part of the review is dedicated to the role of Brownian dynamics simulations in studying photosynthetic electron transport. Interactions of mobile electron carriers (plastocyanin, cytochrome c6, and ferredoxin) with their reaction partners (cytochrome b6f complex, photosystem I, ferredoxin:NADP-reductase, and hydrogenase) are considered.

Structural basis of efficient electron transport between photosynthetic membrane proteins and plastocyanin in spinach revealed using nuclear magnetic resonance

In the photosynthetic light reactions of plants and cyanobacteria, plastocyanin (Pc) plays a crucial role as an electron carrier and shuttle protein between two membrane protein complexes: cytochrome b(6)f (cyt b(6)f) and photosystem I (PSI). The rapid turnover of Pc between cyt b(6)f and PSI enables the efficient use of light energy. In the Pc-cyt b(6)f and Pc-PSI electron transfer complexes, the electron transfer reactions are accomplished within <10(-4) s. However, the mechanisms enabling the rapid association and dissociation of Pc are still unclear because of the lack of an appropriate method to study huge complexes with short lifetimes. Here, using the transferred cross-saturation method, we investigated the residues of spinach (Spinacia oleracea) Pc in close proximity to spinach PSI and cyt b(6)f, in both the thylakoid vesicle-embedded and solubilized states. We demonstrated that the hydrophobic patch residues of Pc are in close proximity to PSI and cyt b(6)f, whereas the acidic patch residues of Pc do not form stable salt bridges with either PSI or cyt b(6)f, in the electron transfer complexes. The transient characteristics of the interactions on the acidic patch facilitate the rapid association and dissociation of Pc.

Purification and characterization of anti-HIV-1 protein from Canna indica L. leaves

A novel 10 kDa protein with anti-HIV-1 reverse transcriptase (RT) inhibitory activity was isolated from leaves of Canna indica L. using a combination of native-PAGE and ammonium sulfate precipitation. HIV-1 and RT inhibitory activity was measured using a syncytium forming (deltaTat/Rev) MC99 virus in Tat/Rev transfected 1A2 cell line and ELISA technique, respectively. Edman N-terminal and internal amino acid sequence (using LC-MS-MS) determination revealed the 10 kDa Canna indica L. leaf protein as a putative plastocyanin. This is the first report of a plant plastocyanin with HIV-1 RT inhibitory property.

On the calculation of ³Jαβ-coupling constants for side chains in proteins

Structural knowledge about proteins is mainly derived from values of observables, measurable in NMR spectroscopic or X-ray diffraction experiments, i.e. absorbed or scattered intensities, through theoretically derived relationships between structural quantities such as atom positions or torsional angles on the one hand and observable quantities such as squared structure factor amplitudes, NOE intensities or (3) J-coupling constants on the other. The standardly used relation connecting (3) J-couplings to torsional angles is the Karplus relation, which is used in protein structure refinement as well as in the evaluation of simulated properties of proteins. The accuracy of the simple and generalised Karplus relations is investigated using side-chain structural and (3) J (αβ)-coupling data for three different proteins, Plastocyanin, Lysozyme, and FKBP, for which such data are available. The results show that the widely used Karplus relations are only a rough estimate for the relation between (3) J (αβ)-couplings and the corresponding χ(1)-angle in proteins.

The complex of cytochrome f and plastocyanin from Nostoc sp. PCC 7119 is highly dynamic

Cytochrome f (Cyt f) and plastocyanin (Pc) form a highly transient complex as part of the photosynthetic redox chain. The complex from Nostoc sp. PCC 7119 was studied by NMR relaxation spectroscopy with the aim of determining the orientation of Pc relative to Cyt f. Chemical-shift-perturbation analysis showed that the presence of spin labels on the surface of Cyt f does not significantly affect the binding of Pc. The paramagnetic relaxation enhancement results are not consistent with a single orientation of Pc, thus indicating that multiple orientations must occur and suggesting that an encounter state represents a large fraction of the complex.

Amphiphilic α-helical potential: a putative folding motif adding few constraints to protein evolution

Evidence from a number of studies indicates that protein folding is dictated not only by factors stabilizing the native state, but also by potentially independent factors that create folding pathways. How natural selection might cope simultaneously with two independent factors was addressed in this study within the framework of the "Lim-model" of protein folding, which postulates that the early stages of folding of all globular proteins, regardless of their native structure, are directed at least in part by potential to form amphiphilic α-helices. For this purpose, the amphiphilic α-helical potential in randomly ordered amino acid sequences and the conservation in phylogeny of amphiphilic α-helical potential within various proteins were assessed. These analyses revealed that amphiphilic α-helical potential is a common occurrence in random sequences, and that the presence of amphiphilic α-helical potential is present but not conserved in phylogeny within a given protein. The results suggest that the rapid formation of molten globules and the variable behavior of those globules depending on the protein may be a fundamental property of polymers of naturally occurring amino acids more so than a trait that must be derived or maintained by natural selection. Further, the results point toward the utility of randomly occurring process in protein function and evolution, and suggest that the formation of efficient pathways that determine early processes in protein folding, unlike the formation of stable, native protein structure, does not present a substantial hurdle during the evolution of amino acid sequences.

[Multiparticle computer simulation of protein interactions in the photosynthetic membrane]

The basic principles of the design of direct multiparticle models and the results of multiparticle computer simulation of electron transfer by mobile protein carriers in the photosynthetic membrane of a chloroplast thylakoid are presented. The reactions of complex formation of the protein plastocyanin with the protein cytochrome f and the pigment-protein complex of photosystem I, as well as of the protein ferredoxin with the protein FNR and photosystem 1 are considered. The role of diffusion and electrostatic interactions is discussed, and the effect of the shape of the reaction volume and ionic strength on the rate of electron transport are discussed.

Modulation of copper site properties by remote residues determines the stability of plastocyanins

The metal cofactor determines the thermal stability in cupredoxins, but how the redox state of copper modulates their melting points remains unknown. The metal coordination environment is highly conserved in cyanobacterial plastocyanins. However, the oxidised form is more stable than the reduced one in thermophilic Phormidium, but the opposite occurs in mesophilic Synechocystis. We have performed neutral amino-acid substitutions at loops of Phormidium plastocyanin far from the copper site. Notably, mutation P49G/G50P confers a redox-dependent thermal stability similar to that of the mesophilic plastocyanin. Moreover, X-ray absorption spectroscopy reveals that P49G/G50P mutation makes the electron density distribution at the oxidised copper site shift towards that of Synechocystis plastocyanin.

[Multiparticle computer simulation of diffusion and interaction of plastocyanin with cytochrome f in the electrostatic field of the thylakoid membrane]

A multiparticle computer model of plastocyanin-cytochrome f complex formation in the thylakoid lumen has been designed, which takes electrostatic interactions of proteins and the thylakoid membrane into account. The Poisson-Boltzmann formalism was used to determine the electrostatic potential field generated by the electrical charges of the proteins and the thylakoid membrane for different ionic strength values. The role of electrostatic field of the thylakoid membrane in plastocyanin-cytochrome f complex formation was determined. Using the model, the rate constant of plastocyanin-cytochrome f reaction for different values of ionic strength and membrane surface charge were calculated.

Terahertz response of dipolar impurities in polar liquids: on anomalous dielectric absorption of protein solutions

A theory of radiation absorption by dielectric mixtures is presented. The coarse-grained formulation is based on the wave-vector-dependent correlation functions of molecular dipoles of the host polar liquid and a density structure factor of the solutes. A nonlinear dependence of the dielectric absorption coefficient on the solute concentration is predicted and originates from the mutual polarization of the liquid surrounding the solutes by the collective field of the solute dipoles aligned along the radiation field. The theory is applied to terahertz absorption of hydrated saccharides and proteins. While the theory gives an excellent account of the observations for saccharides, without additional assumptions and fitting parameters, experimental absorption coefficient of protein solutions significantly exceeds theoretical calculations with dipole moment of the bare protein assigned to the solute and shows a peak against the protein concentration. A substantial polarization of protein's hydration shell, resulting in a net dipole moment, is required to explain the disagreement between theory and experiment. When the correlation function of the total dipole moment of the protein with its hydration shell from numerical simulations is used in the analytical model, an absorption peak, qualitatively similar to that seen in experiment, is obtained. The existence and position of the peak are sensitive to the specifics of the protein-protein interactions. Numerical testing of the theory requires the combination of dielectric and small-angle scattering measurements. The calculations confirm that "elastic ferroelectric bag" of water shells observed in previous numerical simulations is required to explain terahertz dielectric measurements.

Histidine side-chain dynamics and protonation monitored by 13C CPMG NMR relaxation dispersion

The use of 13C NMR relaxation dispersion experiments to monitor micro-millisecond fluctuations in the protonation states of histidine residues in proteins is investigated. To illustrate the approach, measurements on three specifically 13C labeled histidine residues in plastocyanin (PCu) from Anabaena variabilis (A.v.) are presented. Significant Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion is observed for 13C(epsilon1) nuclei in the histidine imidazole rings of A.v. PCu. The chemical shift changes obtained from the CPMG dispersion data are in good agreement with those obtained from the chemical shift titration experiments, and the CPMG derived exchange rates agree with those obtained previously from 15N backbone relaxation measurements. Compared to measurements of backbone nuclei, 13C(epsilon1) dispersion provides a more direct method to monitor interchanging protonation states or other kinds of conformational changes of histidine side chains or their environment. Advantages and shortcomings of using the 13C(epsilon1) dispersion experiments in combination with chemical shift titration experiments to obtain information on exchange dynamics of the histidine side chains are discussed.

Importance of translational entropy of water in biological self-assembly processes like protein folding

We briefly review our studies on the folding/unfolding mechanisms of proteins. In biological self-assembly processes such as protein folding, the number of accessible translational configurations of water in the system increases greatly, leading to a large gain in the water entropy. The usual view looking at only the water in the close vicinity of the protein surface is capable of elucidating neither the large entropic gain upon apoplastocyanin folding, which has recently been found in a novel experimental study, nor the pressure and cold denaturation. With the emphasis on the translational entropy of water, we are presently constructing a reliable method for predicting the native structure of a protein from its amino-acid sequence.

Mutants, overexpressors, and interactors of Arabidopsis plastocyanin isoforms: revised roles of plastocyanin in photosynthetic electron flow and thylakoid redox state

Two homologous plastocyanin isoforms are encoded by the genes PETE1 and PETE2 in the nuclear genome of Arabidopsis thaliana. The PETE2 transcript is expressed at considerably higher levels and the PETE2 protein is the more abundant isoform. Null mutations in the PETE genes resulted in plants, designated pete1 and pete2, with decreased plastocyanin contents. However, despite reducing plastocyanin levels by over approximately 90%, a pete2 null mutation on its own affects rates of photosynthesis and growth only slightly, whereas pete1 knockout plants, with about 60-80% of the wild-type plastocyanin level, did not show any alteration. Hence, plastocyanin concentration is not limiting for photosynthetic electron flow under optimal growth conditions, perhaps implying other possible physiological roles for the protein. Indeed, plastocyanin has been proposed previously to cooperate with cytochrome c(6A) (Cyt c(6A)) in thylakoid redox reactions, but we find no evidence for a physical interaction between the two proteins, using interaction assays in yeast. We observed homodimerization of Cyt c(6A) in yeast interaction assays, but also Cyt c(6A) homodimers failed to interact with plastocyanin. Moreover, phenotypic analysis of atc6-1 pete1 and atc6-1 pete2 double mutants, each lacking Cyt c(6A) and one of the two plastocyanin-encoding genes, failed to reveal any genetic interaction. Overexpression of either PETE1 or PETE2 in the pete1 pete2 double knockout mutant background results in essentially wild-type photosynthetic performance, excluding the possibility that the two plastocyanin isoforms could have distinct functions in thylakoid electron flow.

Dynamical transition, hydrophobic interface, and the temperature dependence of electrostatic fluctuations in proteins

Molecular dynamics simulations have revealed a dramatic increase, with increasing temperature, of the amplitude of electrostatic fluctuations caused by water at the active site of metalloprotein plastocyanin. The increased breadth of electrostatic fluctuations, expressed in terms of the reorganization energy of changing the redox state of the protein, is related to the formation of the hydrophobic protein-water interface, allowing large-amplitude collective fluctuations of the water density in the protein's first solvation shell. On top of the monotonic increase of the reorganization energy with increasing temperature, we have observed a spike at approximately 220 K also accompanied by a significant slowing of the exponential collective Stokes shift dynamics. In contrast to the local density fluctuations of the hydration-shell waters, these spikes might be related to the global property of the water solvent crossing the Widom line or undergoing a weak first-order transition.

Plastocyanin microheterogeneity in Scenedesmus acutus MT8

Two total plastocyanin (PC) fractions - loosely bound (lPC) and strongly bound (sPC) were extracted (84% and 16%, respectively) from the homogenate of Scenedesmus acutus MT8. Two-fold isolation-purification procedure including DE-52 chromatography separated IPC into a smaller oxidized [IPC (II)] and a larger reduced [IPC(I)] fractions, in contrast to sPC, where sPC(ll) greatly dominated over sPC(I). Analytical isoelectric focusing (IEF) separated IPC(II) into two main fractions only in the presence of 8 M urea, implying microheterogeneity. Preparative IEF in immobiline pH-gradient of 3.2-4.1 separated IPC(II) into two blue fractions - a more alkaline IPC(II) and a more acidic IPC"(II), which were probably stereoisomers. Their UV-Vis spectra exhibited rarely observed tryptophane (291.5 nm) and some differences at 270 and 287 nm. The exact molecular masses of apo-/holo-lPC (10131 Da/10194 Da) were determined by mass spectrometry. The number of -SH groups was determined from the mass difference between alkylated with 4-vinylpyridine (4-VP) and non-alkylated protein. Additionally, a simple procedure for simultaneous separation of both primary structure and stereoisomers of PC was developed.

[Computer simulation of complex formation between plastocyanine and cytochrome f in thylakoid lumen]

The diffusion of the protein plastocyanine and complex formation between plastocyanine and cytochrome f (a subunit of a cytochrome b6/f complex) in the chloroplast thylakoid lumen has been studied. A 3D computer simulation model of diffusion and binding of plastocyanine and cytochrome f has been constructed, which considers their electrostatic interaction. Based on the experimental data, the parameters of the model for complex formation between plastocyanine and cytochrome f in solution have been estimated. The dependence of the rate of plastocyanine-cytochrome f reaction on the size of the luminal space has been studied. It was shown that the contraction of the luminal space leads to a decrease in the reaction rate, which is in agreement with the experimental data on the inhibition of the reaction under hyperosmotic stress.

A Brownian Dynamics computational study of the interaction of spinach plastocyanin with turnip cytochrome f: the importance of plastocyanin conformational changes

Brownian Dynamics (BD) computer simulations were used to study electrostatic interactions between turnip cytochrome f (cyt f) and spinach plastocyanin (PC). Three different spinach PC structures were studied: The X-ray crystal structure of Xue and coworkers [(1998) Protein Sci 7:2099-2105] and the NMR structure of Musiani et al. [(2005) J Biol Chem 280:18833-18841] and Ubbink and co-workers [(1998) Structure 6:323-335]. Significant differences exist in the backbone conformation between the PC taken from Ubbink and coworkers and the other two PC structures particularly the regions surrounding G10, E59-E60, and D51. Complexes formed in BD simulations using the PC of Ubbink and colleagues had a smaller Cu-Fe distance than the other two. These results suggest that different PC conformations may exist in solution with different capabilities of forming electron-transfer-active docks. All three types of complexes show electrostatic contacts between D42, E43, and D44 on PC and K187 on cyt f as well as between E59 on PC and K58 on cyt f. However, the PC of Ubbink and coworkers reveals additional contacts between D51 and cyt f as a result of the difference in backbone configuration. A second minor complex component was observed for the PC of Ubbink and co-workers and Xue and co-workers which had contacts between K187 on cyt f and E59 and E60 on PC rather than between K187 on cyt f and D42-D44 on PC as observed for the major components. This second type of complex may represent an earlier complex which rearranges to form a final complex capable of electron transfer.

QM/MM calculations with DFT for taking into account protein effects on the EPR and optical spectra of metalloproteins. Plastocyanin as a case study

A detailed study of the influence of the surrounding protein on magnetic and optical spectra of metalloproteins is presented using the quantum-mechanical/molecular mechanical (QM/MM) approach. The well-studied type I copper site in plastocyanin in the cupric oxidation state is taken as a test case because its spectroscopic properties have been extensively studied and are well understood. The calculations have been performed using nonrelativistic and scalar relativistic (at the level of the zeroth order regular approximation, ZORA) calculations (B3LYP functional). Linear response theory has been used to calculate first- and second-order properties, namely the EPR g-tensor, the central metal hyperfine couplings (HFCs), the HFCs of the directly coordinating ligands, as well as superhyperfine couplings (1H, 14N) from remote nuclei, transition energies, and oscillator strengths. Two different model systems have been defined that do not and do include important amino acids from the second coordination sphere, respectively. For comparison, calculations have been carried out in the gas phase and in a dielectric continuum (conductor like screening model, COSMO) with a dielectric constant of four. The best results were obtained at the scalar relativistic ZORA level for the largest model in conjunction with explicit modeling of the protein environment through the QM/MM procedure, which is also considered to be the highest level of theory used in this work. The protein effects beyond the second coordination sphere were found to be quite substantial (up to 30% changes on some properties), and were found to require an explicit treatment of the protein beyond the second coordination sphere. In addition, the embedding water cage was found to have a nonnegligible influence on the calculated spectroscopic data, which is of the same order as the influence of the protein backbone charges. However, while qualitatively satisfactory, the errors in the calculated spectroscopic parameters are still substantial, and can all be traced back to the fact that the linear-response of the presently available functionals is "too stiff" with respect to the external perturbations at least for the model systems studied here. Ligand field-based approaches are used to correct for systematic errors in the DFT procedures. As a consequence, we propose a new breakdown of the copper hyperfine interaction into Fermi-contact, spin-dipolar and spin-orbit contributions.

Density functional study of EPR parameters and spin-density distribution of azurin and other blue copper proteins

Modern density functional methods have been used to study spin-density distribution, g tensors, as well as Cu and ligand hyperfine tensors for azurin models, for two more blue copper proteins plastocyanin and stellacyanin, and for small model complexes. The aim was to establish a consistent computational protocol that provides a realistic description of the EPR parameters as probes of the spin-density distribution between metal and coordinated ligands in copper proteins. In agreement with earlier conclusions for plastocyanin, hybrid functionals with appreciable exact-exchange admixtures, roughly around 50%, provide the best overall agreement with all parameters. Then the bulk of the spin density is almost equally shared by the copper atom and the sulfur atom of the equatorial cysteine ligand, and the best values are obtained for copper, histidine nitrogen, and cysteine beta-proton hyperfine couplings, as well as for g(parallel). Spin-orbit effects on the EPR parameters may be appreciable and have to be treated carefully to obtain agreement with experiment. Most notably, spin-orbit effects on the (65)Cu hyperfine coupling tensors in blue copper sites are unusually large compared to more regularly coordinated Cu(II) complexes with similar spin density on copper. In addition to the often emphasized high covalency of the Cu-S(Cys) bond, the characteristically small A(parallel) component of blue copper proteins is shown to derive to a large part from a near-cancellation between negative first-order (Fermi contact and dipolar) and unusually large positive second-order (spin-orbital) contributions. The large spin-orbit effects relate to the distorted tetrahedral structures. Square planar dithiolene complexes with similar spin density on copper exhibit much more negative A(parallel) values, as the cancellation between nonrelativistic and spin-orbit contributions is less complete. Calculations on a selenocysteine-substituted variant of azurin have provided further insight into the relations between bonding and EPR parameters.

Engineering copper sites in proteins: loops confer native structures and properties to chimeric cupredoxins

The ligand-containing loops of two copper-binding electron-transfer proteins (cupredoxins) have been swapped. In the azurin (AZ) variant in which the plastocyanin (PC) sequence is introduced (AZPC), the loop adopts a conformation identical to that in PC. The reduction potential of AZPC is raised as compared to AZ and matches that of PC. In the previously published AZAMI variant (AMI = amicyanin), the shorter introduced loop adopts the same conformation as in AMI, and the reduction potential is lowered to equal that of AMI (Yanagisawa, S.; Dennison, C. J. Am. Chem. Soc. 2004, 126, 15711-15719. Li, C.; et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 7258-7263). Thus, the loop structure plays an important role in tuning the reduction potential of a type 1 copper site with contributions from protein dipoles in this region probably the most important feature. The structure of the loop also seems to be a major factor in controlling dissociation and protonation of the C-terminal His ligand, which can act as a switch to regulate electron-transfer reactivity. The PCAZ variant (PC with the AZ loop) possesses an active site, which is different from those of both PC and AZ, and it is assumed that the introduced loop does not adopt a structure as in AZ. This contributes to the observed instability of PCAZ and highlights that loop-scaffold interactions are important for stabilizing the active site of a cupredoxin.

Protein-coated β-ferric hydrous oxide particles

Using microelectrophoresis and electric light scattering techniques, we investigated the adsorption characteristics, surface coverage and surface electric parameters of superstructures from two isoforms of plastocyanin, PCa and PCb, in an oxidized state adsorbed on beta-ferric hydrous oxide particles. The surface electric charge and electric dipole moments of the composite particles and the thickness of the protein adsorption layer are determined in a wide pH range, at different ionic strengths and concentration ratios of PC to beta-FeOOH. The adsorption of the two proteins was found to shift the particles' isoelectric point and to alter the total electric charge and the electric dipole moments of the oxide particles to different extent. A "reversal" in the direction of the permanent dipole moment is observed at lower pH for PCb- than for PCa-coated oxide particles. Strict correlation is found between the changes in the electrokinetic charge of the composite particles and the variation in their "permanent" dipole moments. Data suggest that the adsorption of the proteins is driven by electrostatic and/or hydrophobic interactions with the oxide surfaces dependent on pH. The adsorption behaviour is consistent with the involvement of the "eastern" and "northern" patches of the plastocyanin molecules in their adsorption on the oxide surfaces that are differently charged depending on pH.

Protonation of a Histidine Copper Ligand in Fern Plastocyanin

Plastocyanin is a small blue copper protein that shuttles electrons as part of the photosynthetic redox chain. Its redox behavior is changed at low pH as a result of protonation of the solvent-exposed copper-coordinating histidine. Protonation and subsequent redox inactivation could have a role in the down regulation of photosynthesis. As opposed to plastocyanin from other sources, in fern plastocyanin His90 protonation at low pH has been reported not to occur. Two possible reasons for that have been proposed: pi-pi stacking between Phe12 and His90 and lack of a hydrogen bond with the backbone oxygen of Gly36. We have produced this fern plastocyanin recombinantly and examined the properties of wild-type protein and mutants Phe12Leu, Gly36Pro, and the double mutant with NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The results demonstrate that, contrary to earlier reports, protonation of His90 in the wild-type protein does occur in solution with a pKa of 4.4 (+/-0.1). Neither the single mutants nor the double mutant exhibit a change in protonation behavior, indicating that the suggested interactions have no influence. The crystal structure at low pH of the Gly36Pro variant does not show His90 protonation, similar to what was found for the wild-type protein. The structure suggests that movement of the imidazole ring is hindered by crystal contacts. This study illustrates a significant difference between results obtained in solution by NMR and by crystallography.

The Specificity in the Interaction between Cytochrome f and Plastocyanin from the Cyanobacterium Nostoc sp. PCC 7119 Is Mainly Determined by the Copper Protein

The plastocyanin-cytochrome f complex from Nostoc exhibits relevant structural differences when compared with the homologous complexes from other cyanobacteria and plants, with electrostatic and hydrophobic interactions being differently involved in each case. Here, five negatively charged residues of a recombinant form of cytochrome f from Nostoc have been replaced with either neutral or positively charged residues, and the effects of mutations on the kinetics of electron transfer to wild-type and mutant forms of plastocyanin have been measured by laser flash absorption spectroscopy. Cytochrome f mutants with some negative charges replaced with neutral residues exhibit an apparent electron transfer rate constant with wild-type plastocyanin similar to or slightly higher than that of the wild-type species, whereas the mutants with negative charges replaced with positive residues exhibit a significantly lower reactivity. Taken together, these results indicate that the effects of neutralizing residues at the electrostatically charged patch of cytochrome f are smaller than those previously observed for mutants of plastocyanin, thus suggesting that it is the copper protein which determines the specificity of the electrostatic interaction with the heme protein. Moreover, cross reactions between mutants of both proteins reveal the presence of some short-range specific electrostatic interactions. Our findings also make evident the fact that in Nostoc the main contribution to the electrostatic nature of the complex is provided by the small domain of cytochrome f.

Long lived intermediate metal site structure upon binding of cadmium to plastocyanin

Perturbed angular correlation of gamma-rays (PAC) spectroscopy of cadmium substituted plastocyanin shows one dominant metal site configuration at pH 7.5. Lowering the pH to 4.8 a fraction of the molecules undergoes structural change and loses the bound cadmium ion. At pH 4.4 all plastocyanin is in the apo-form. Increasing the pH back to neutral pH values two distinct metal site coordination geometries were observed. One of the two signals is the same as that found initially at pH 7.5; the other form is stable for hours at 1 degrees C, indicating the existence of a long lived intermediate metal site structure. The cadmium ion is surrounded by the same ligands (His37, Cys84, His87 and Met92) in both forms, however the metal center in the long lived intermediate metal site structure can be best described by a larger His-metal-His angle.

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.

A Brownian dynamics study of the interactions of the luminal domains of the cytochrome b6f complex with plastocyanin and cytochrome c6: the effects of the Rieske FeS protein on the interactions

The availability of the structures of the cytochrome b6f complex (cyt b6f), plastocyanin (PC), and cytochrome c6 (cyt c6) from Chlamydomonas reinhardtii allowed us, for the first time, to model electron transfer interactions between the luminal domains of this complex (including cyt f and the Rieske FeS protein) and its redox partners in the same species. We also generated a model structure in which the FeS center of the Rieske protein was positioned closer to the heme of cyt f than observed in the crystal structure and studied its interactions with both PC and cyt c6. Our data showed that the Rieske protein in both the original crystal structure and in our modeled structure of the cyt b6f complex did not physically interfere with binding position or orientation of PC or cyt c6 on cyt f. PC docked on cyt f with the same orientation in the presence or the absence of the Rieske protein, which matched well with the previously reported NMR structures of complexes between cyt f and PC. When the FeS center of the Rieske protein was moved close to the heme of cyt f, it even enhanced the interaction rates. Studies using a cyt f modified in the 184-191 loop showed that the cyt f structure is a more important factor in determining the rate of complex formations than is the presence or the absence of the Rieske protein or its position with respect to cyt f.

The atypical iron-coordination geometry of cytochrome f remains unchanged upon binding to plastocyanin, as inferred by XAS

The transient complex between cytochrome f and plastocyanin from the cyanobacterium Nostoc sp. PCC 7119 has been analysed by X-ray Absorption Spectroscopy in solution, using both proteins in their oxidized and reduced states. Fe K-edge data mainly shows that the atypical metal coordination geometry of cytochrome f, in which the N-terminal amino acid acts as an axial ligand of the heme group, remains unaltered upon binding to its redox partner, plastocyanin. This fact suggests that cytochrome f provides a stable binding site for plastocyanin and minimizes the reorganization energy required in the transient complex formation, which could facilitate the electron transfer between the two redox partners.

Structure of plastocyanin from the cyanobacterium Anabaena variabilis

Plastocyanin from the cyanobacterium Anabaena variabilis was heterologously produced in Escherichia coli and purified. Plate-like crystals were obtained by crystallization in 1.15 M trisodium citrate and 7.67 mM sodium borate buffer pH 8.5. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 67.85, b = 45.81, c = 63.41 Angstrom. The structure of the oxidized protein was solved to a resolution of 1.6 Angstrom using plastocyanin from Phormidium laminosum as a search model. Two molecules were found in the asymmetric unit. The electrostatic surface of the basic protein showed a large population of positively charged residues in the northern site, whereas the eastern site lacked the two strongly negatively charged patches. The copper ion was found to be relatively mobile and there were two distinct conformations of His61.

Spectroscopic Methods in Bioinorganic Chemistry: Blue to Green to Red Copper Sites

A wide variety of spectroscopic methods are now available that provide complimentary insights into the electronic structures of transition-metal complexes. Combined with calculations, these define key bonding interactions, enable the evaluation of reaction coordinates, and determine the origins of unique spectroscopic features/electronic structures that can activate metal centers for catalysis. This presentation will summarize the contributions of a range of spectroscopic methods combined with calculations in elucidating the electronic structure of an active site using the blue copper site as an example. The contribution of electronic structure to electron-transfer reactivity will be considered in terms of anisotropic covalency, electron-transfer pathways, reorganization energy, and protein contributions to the geometric and electronic structures of blue-copper-related active sites.

Evaluation of two simplified 15N-NMR methods for determining micros-ms dynamics of proteins

Two methods for estimating the microsecond-millisecond dynamics in proteins from only two 15N relaxation parameters at one magnetic field strength are investigated. Thus, the chemical exchange contribution, R(ex), to the transversal relaxation rate, which contains the dynamics information, is evaluated by two methods: (i) one in which the R(ex) term is derived from the 15N R1 and R2 relaxation rates alone, and (ii) one in which it is obtained from the transversal dipole-chemical shift anisotropy (CSA) cross-correlation rate, eta(xy), and the R2 rate. Since the R1, R2, and eta(xy) experiments are fast and sensitive, both methods are attractive in studies where large amounts of dynamical information are required. However, both methods are liable to effects that can compromise the estimation of the R(ex) terms. In the R2/R1 method, internal ps-ns dynamics and rotational anisotropy can interfere with the determination of R(ex), while in the R2/eta(xy) method it can be affected by variations in the 15N chemical shift anisotropy. Here, the applicability of the two methods is investigated using plastocyanin from Anabaena variabilis as an example, and the quality of the obtained R(ex) terms is evaluated both theoretically and experimentally. It is found that the R2/R1 method gives reliable R(ex) terms if the protein is relatively rigid and tumbles fast and nearly isotropically in solution, as for instance plastocyanin, and is preferable in such cases. In contrast, the R2/eta(xy) method gives better results if the protein is flexible or highly non-spherical and can be used for such proteins, if the sequential variation in the 15N chemical shift anisotropy is negligible. For exchange terms <1 s(-1) neither method is reliable.

Structure of Cytochrome c6A, a Novel Dithio-cytochrome of Arabidopsis thaliana, and its Reactivity with Plastocyanin: Implications for Function

Cytochrome c6A is a unique dithio-cytochrome present in land plants and some green algae. Its sequence and occurrence in the thylakoid lumen suggest that it is derived from cytochrome c6, which functions in photosynthetic electron transfer between the cytochrome b6f complex and photosystem I. Its known properties, however, and a strong indication that the disulfide group is not purely structural, indicate that it has a different, unidentified function. To help in the elucidation of this function the crystal structure of cytochrome c6A from Arabidopsis thaliana has been determined in the two redox states of the heme group, at resolutions of 1.2 A (ferric) and 1.4 A (ferrous). These two structures were virtually identical, leading to the functionally important conclusion that the heme and disulfide groups do not communicate by conformational change. They also show, however, that electron transfer between the reduced disulfide and the heme is feasible. We therefore suggest that the role of cytochrome c6A is to use its disulfide group to oxidize dithiol/disulfide groups of other proteins of the thylakoid lumen, followed by internal electron transfer from the dithiol to the heme, and re-oxidation of the heme by another thylakoid oxidant. Consistent with this model, we found a rapid electron transfer between ferro-cytochrome c6A and plastocyanin, with a second-order rate constant, k2=1.2 x 10(7) M(-1) s(-1).

Direct simulation of plastocyanin and cytochrome f interactions in solution

Most biological functions, including photosynthetic activity, are mediated by protein interactions. The proteins plastocyanin and cytochrome f are reaction partners in a photosynthetic electron transport chain. We designed a 3D computer simulation model of diffusion and interaction of spinach plastocyanin and turnip cytochrome f in solution. It is the first step in simulating the electron transfer from cytochrome f to photosystem 1 in the lumen of thylakoid. The model is multiparticle and it can describe the interaction of several hundreds of proteins. In our model the interacting proteins are represented as rigid bodies with spatial fixed charges. Translational and rotational motion of proteins is the result of the effect of stochastic Brownian force and electrostatic force. The Poisson-Boltzmann formalism is used to determine the electrostatic potential field generated around the proteins. Using this model we studied the kinetic characteristics of plastocyanin-cytochrome f complex formation for plastocyanin mutants at pH 7 and a variety of ionic strength values.

Thermal unfolding of plastocyanin from the mesophilic cyanobacterium Synechocystis sp. PCC 6803 and comparison with its thermophilic counterpart from Phormidium laminosum

The thermal unfolding of plastocyanin from the mesophilic cyanobacterium Synechocystis is described herein, and the results are compared with those obtained for the homologous thermophilic protein from Phormidium laminosum. The thermal unfolding is irreversible under all the conditions that were investigated. Plastocyanin from the thermophilic organism, both in the native state and in the apoprotein form, proved to be more thermostable than its mesophilic counterpart under all experimental conditions. Synechocystis reduced plastocyanin has been shown to be more stable than the oxidized species, both with respect to the required temperature for protein unfolding and with respect to the kinetics of the process. This behavior contrasts with that observed for Phormidium plastocyanin, in which the oxidized form is the more stable one. The unfolding pH dependence and kinetic studies indicate that around physiological pH, the most kinetically stable form is also the one more resistant to temperature variations, suggesting a close compromise between function and stability. Molecular dynamics simulations suggest that Phormidium and Synechocystis plastocyanins follow different unfolding pathways that affect different protein areas and which could be responsible for the observed dissimilar thermal resistance.

Conformational Changes during Apoplastocyanin Folding Observed by Photocleavable Modification and Transient Grating

A new method to investigate the initial protein folding dynamics is developed based on a pulsed laser light triggering method and a unique transient grating method. The side chain of the cysteine residue of apoplastocyanin (apoPC) was site-specifically modified with a 4,5-dimethoxy-2-nitrobenzyl derivative, where the CD and 2D NMR spectra showed that the modified apoPC was unfolded. The substituent was cleaved with a rate of about 400 ns by photoirradiation, which was monitored by the disappearance of the absorption band at 355 nm and the increase in the transient grating signal. After a sufficient time from the photocleavage reaction, the CD and NMR spectra showed that the native beta-sheet structure was recovered. Protein folding dynamics was monitored in the time domain with the transient grating method from a viewpoint of the molecular volume change and the diffusion coefficient, both of which reflect the global structural change, including the protein-water interaction. The observed volume decrease of apoPC with a time scale of 270 micros is ascribed to the initial hydrophobic collapse. The increase in the diffusion coefficient (23 ms) is considered to indicate a change from an intermolecular to an intramolecular hydrogen bonding network. The initial folding process of apoPC is discussed based on these observations.

[Purification of plastocynin from Ulva pertusa by column chromatography and analysis of its N-terminal amino acid sequence]

Protein plastocyanin from a green alga, Ulva pertusa, has been purified. Samples were homogenized in 0.02 mol/L phosphate buffer (pH 7.2) and then centrifuged to remove debris and subjected to ammonium sulfate fractionation (40%-80% saturation). Ion exchange column chromatography with DEAE-Sepharose Fast Flow and gel filtration column chromatography with Sephadex G-75 were then employed for further purification of plastocyanin. Three peaks, A, B and C, were eluted with 0.01 mol/L phosphate buffer, containing a NaCl linear gradient from 0 to 1.0 mol/L at the flow rate of 32 mL/h through DEAE-Sepharose chromatography. The protein fractions containing the plastocyanin were then purified further with Sephardex G-75 column chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophores (SDS-PAGE) is analysis indicates that the protein was purified to homogeneity and its relative molecular mass is 10,000. N-terminal amino acid sequence was used to identify the protein. The protein was transblotted to PVDF membrane and N-terminal amino acid sequence was performed via Edman degradation with an automated amino acid sequencer. The 20 N-terminal amino acid residues are AAIVKLGPDDGSLAFVPSKI, which share 85% homology with the 20 N-terminal amino acid sequence of U. prolifera and U. arasakii, and share 90% homology with the ones of U. pertusa formerly reported.

pH-dependent structural change of reduced spinach plastocyanin studied by perturbed angular correlation of γ-rays and dynamic light scattering

In this study the pH-dependent structural changes of reduced spinach plastocyanin were investigated using perturbed angular correlation (PAC) of gamma-rays and dynamic light scattering (DLS). PAC data of Ag-substituted plastocyanin indicated that the coordinating ligands are two histidine residues (His37, His87) and a cysteine residue (Cys84) in a planar configuration, whereas the methionine (Met92) found perpendicular to this plane is not a coordinating ligand at neutral pH. Two slightly different conformations with differences in the Cys-metal ion-His angles could be observed with PAC spectroscopy. At pH 5.3 a third coordination geometry appears which can be explained as the absence of the His87 residue and the coordination of Met92 as a ligand. With DLS the aggregation of reduced plastocyanin could be observed below pH 5.3, indicating that not only the metal binding site but also the aggregation properties of the protein change upon pH reduction. Both the structural changes at the metal binding site and the aggregation are shown to be reversible. These results support the hypothesis that the pH of the thylakoid lumen has to remain moderate during steady-state photosynthesis and indicate that low pH induced aggregation of plastocyanin might serve as a regulatory switch for photosynthesis.