Plastocyanin Molecule Research Articles

Comparative analysis of plastocyanin–cytochrome f complex formation in higher plants, green algae and cyanobacteria

Mechanisms of the complex formation between plastocyanin and cytochrome f in higher plants (Spinacia oleracea and Brassica rapa), green microalgae Chlamydomonas reinhardtii and two species of cyanobacteria (Phormidium laminosum and Nostoc sp.) were investigated using combined Brownian and molecular dynamics simulations and hierarchical cluster analysis. In higher plants and green algae, electrostatic interactions force plastocyanin molecule close to the heme of cytochrome f. In the subsequent rotation of plastocyanin molecule around the point of electrostatic contact in the vicinity of cytochrome f, copper (Cu) atom approaches cytochrome heme forming a stable configuration where cytochrome f molecule behaves as a rather rigid body without conformational changes. In Nostoc plastocyanin molecule approaches cytochrome f in a different orientation (head-on) where the stabilization of the plastocyanin-cytochrome f complex is accompanied by the conformational changes of the G188E189D190 loop that stabilizes the whole complex. In cyanobacterium P. laminosum, electrostatic preorientation of the approaching molecules was not detected, thus indicating that random motions rather than long-range electrostatic interactions are responsible for the proper mutual orientation. We demonstrated that despite the structural similarity of the investigated electron transport proteins in different photosynthetic organisms, the complexity of molecular mechanisms of the complex formation increases in the following sequence: non-heterocystous cyanobacteria-heterocystous cyanobacteria-green algae-flowering plants.

Regulation of Protein Function: Crystal Packing Interfaces and Conformational Dimerization

The accepted view of interprotein electron transport involves molecules diffusing between donor and acceptor redox sites. An emerging alternative hypothesis is that efficient long-range electron transport can be achieved through proteins arranged in supramolecular assemblies. In this study, we have investigated the crystal packing interfaces in three crystal forms of plastocyanin, an integral component of the photosynthetic electron transport chain, and discuss their potential relevance to in vivo supramolecular assemblies. Symmetry-related protein chains within these crystals have Cu-Cu separations of <25 A, a distance that readily supports electron transfer. In one structure, the plastocyanin molecule exists in two forms in which a backbone displacement coupled with side chain rearrangements enables the modulation of protein-protein interfaces.

Protein-coated β-ferric hydrous oxide particles: An electrokinetic and electrooptic study

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 β-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 β-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.

A Brownian Dynamics Study of the Interaction of Phormidium Cytochrome f with Various Cyanobacterial Plastocyanins

Brownian dynamics simulations were used to study the role of electrostatic forces in the interactions of cytochrome f from the cyanobacterium Phormidium laminosum with various cyanobacterial plastocyanins. Both the net charge on the plastocyanin molecule and the charge configuration around H92 (H87 in higher plants) are important in determining the interactions. Those plastocyanins (PCs) with a net charge more negative than −2.0, including those from Synechococcus sp. PCC7942, Synechocystis sp. 6803, and P. laminosum showed very little complex formation. On the other hand, complex formation for those with a net charge more positive than −2.0 (including Nostoc sp. PCC7119 and Prochlorothrix hollandica) as well as Nostoc plastocyanin mutants showed a linear dependence of complex formation upon the net charge on the plastocyanin molecule. Mutation of charged residues on the surface of the PC molecules also affected complex formation. Simulations involving plastocyanin mutants K35A, R93A, and K11A (when present) showed inhibition of complex formation. In contrast, D10A and E17A mutants showed an increase in complex formation. All of these residues surround the H92 (H87 in higher plant plastocyanins) ligand to the copper. An examination of the closest electrostatic contacts shows that these residues interact with D63, E123, R157, D188, and the heme on Phormidium cytochrome f. In the complexes formed, the long axis of the PC molecule lies perpendicular to the long axis of cytochrome f. There is considerable heterogeneity in the orientation of plastocyanin in the complexes formed.

Conductive atomic force microscopy study of plastocyanin molecules adsorbed on gold electrode

The electrical conduction of mutant plastocyanin molecules self-assembled on gold was measured by conductive atomic force microscopy. The copper protein molecules, able to bind to gold through an engineered S–S group, were immobilised on Au(1 1 1) substrates to form a dense monolayer, which was contacted by a conductive atomic force microscopy tip at controlled force. Specifically, the electronic conduction of the gold coated tip–plastocyanin–Au(1 1 1) junction was well characterised between +1 and −1 V for increasing compressional forces. The conduction varies slightly within the force range of 3–9 nN, while it rapidly increases above these force values. The occurrence of a jump in mutant plastocyanin conduction at a critical force value, suggests that the current transport mechanism through the bioelectronic junction can be dominated by protein mechanical characteristics and/or by considerable variations in the protein conduction upon molecular deformation. By operating in imaging mode, we have obtained good topographic images of the protein self-assembled on Au(1 1 1) surface and for the first time simultaneous current images were recorded. Remarkably, a correspondence between the biomolecules observed in the topography images and the current spots was found for negative bias.

A Brownian Dynamics Study of the Interaction of Phormidium laminosum Plastocyanin with Phormidium laminosum Cytochrome f

The interaction of Phormidium laminosum plastocyanin (PC) with P. laminosum cytochrome f (cyt f) was studied using Brownian dynamics (BD) simulations. Few complexes and a low rate of electron transfer were observed for wild-type PC. Increasing the positive electrostatic field on PC by the addition of a Zn 2+ ion in the neighborhood of D44 and D45 on PC (as found in crystal structure of plastocyanin) increased the number of complexes formed and the calculated rates of electron transfer as did PC mutations D44A, D45A, E54A, and E57A. Mutations of charged residues on Phormidium PC and Phormidium cyt f were used to map binding sites on both proteins. In both the presence and absence of the Zn 2+ ion, the following residues on PC interact with cyt f: D44, D45, K6, D79, R93, and K100 that lie in a patch just below H92 and Y88 and D10, E17, and E70 located on the upper portion of the PC molecule. In the absence of the Zn 2+ ion, K6 and K35 on the top of the PC molecule also interact with cyt f. Cyt f residues involved in binding PC, in the absence of the Zn 2+ ion, include E165, D187, and D188 that are located on the small domain of cyt f. The orientation of PC in the complexes was quite random in accordance with NMR results. In the presence of the Zn 2+ ion, K53 and E54 in the lower patch of the PC molecule also interact with cyt f and PC interacts with E86, E95, and E123 on the large domain of cyt f. Also, the orientation of PC in the complexes was much more uniform than in the absence of the Zn 2+ ion. The difference may be due to both the larger electrostatic field and the greater asymmetry of the charge distribution on PC observed in the presence of the Zn 2+ ion. Hydrophobic interactions were also observed suggesting a model of cyt f-PC interactions in which electrostatic forces bring the two molecules together but hydrophobic interactions participate in stabilizing the final electron-transfer-active dock.

Intensity fluctuations of the copper site resonant vibrational modes as observed by MD simulation in single plastocyanin molecule

The bond distance of the copper–sulfur in the plastocyanin (PC) active site has been followed along the molecular dynamics (MD) simulation trajectory, in a single molecule configuration. The power spectrum of the bond distance fluctuations, calculated for different time intervals, qualitatively reproduces the main spectral features of the experimental resonance Raman spectra of PC. The peak intensities of the vibrational modes appear significantly fluctuate during the dynamical evolution of the system. Such a variability, even if it occurs at an enormously different time scale, is consistent with the fluctuations observed in the experimental Raman spectra of similar systems when the single molecule limit is approached.

Coupled Plasmon-Waveguide Resonance Spectroscopy Studies of the Cytochrome b6f/Plastocyanin System in Supported Lipid Bilayer Membranes

The incorporation of cytochrome (cyt) b 6 f into a solid-supported planar egg phosphatidylcholine (PC) bilayer membrane and complex formation with plastocyanin have been studied by a variant of surface plasmon resonance called coupled plasmon-waveguide resonance (CPWR) spectroscopy, developed in our laboratory. CPWR combines greatly enhanced sensitivity and spectral resolution with direct measurement of anisotropies in refractive index and optical extinction coefficient, and can therefore probe structural properties of lipid-protein and protein-protein interactions. Cyt b 6 f incorporation into the membrane proceeds in two stages. The first occurs at low protein concentration and is characterized by an increase in total proteolipid mass without significant changes in the molecular order of the system, as demonstrated by shifts of the resonance position to larger incident angles without changing the refractive index anisotropy. The second stage, occurring at higher protein concentrations, results in a decrease in both the mass density and the molecular order of the system, evidenced by shifts of the resonance position to smaller incident angles and a large decrease in the membrane refractive index anisotropy. Plastocyanin can bind to such a proteolipid system in three different ways. First, the addition of plastocyanin before the second stage of b 6 f incorporation begins results in complex formation between the two proteins with a K D of ∼10 μM and induces structural changes in the membrane that are similar to those occurring during the second stage of complex incorporation. The addition of larger amounts of plastocyanin under these conditions leads to nonspecific binding to the lipid phase with a K D of ∼180 μM. Finally, the addition of plastocyanin after the completion of the second phase of b 6 f incorporation results in tighter binding between the two proteins ( K D ≈ 1 μM). Quantitation of the binding stoichiometry indicates that two plastocyanin molecules bind tightly to the dimeric form of the cyt b 6 f complex, assuming random insertion of the cytochrome into the bilayer. The structural basis for these results and formation of the proteolipid membrane are discussed.

Crystallization and preliminary X-ray studies of plastocyanin from Silene expressed in E. coli.

Plastocyanin from Silene (SilPc) expressed in E. coli has been crystallized in a form suitable for X-ray diffraction analysis by a macroseeding method using ammonium sulfate as a precipitant in acetate buffer (pH = 5.5). These crystals belong to the trigonal space group P3(1)21 or P3(2)21 with lattice parameters a = b = 76.6, c = 65.5 A, indicating an asymmetric unit containing two plastocyanin molecules. The crystals diffracted to at least 2.0 A.

Computational Simulation and Analysis of Dynamic Association between Plastocyanin and Cytochromef. Consequences for the Electron-Transfer Reaction

This work is a theoretical analysis in four stages of association between the blue copper protein plastocyanin and the heme protein cytochrome f, which are physiological partners in the photosynthetic electron-transfer chain. In the first stage, 32 000 trajectories of approach by plastocyanin to cytochrome f were generated with implicit consideration of hydration and with gradual cooling of the system from 300 to 0 K. Approximately 2000 trajectories resulted in local minima of energy, i.e., in docking. The molecular configurations having relatively low energies were grouped, by structural similarity, into six families. In the second stage, six configurations having the lowest energies, one from each family, were subjected to thorough molecular dynamics simulation, for 260 ps. Extensive hydration of the proteins was treated explicitly. The whole plastocyanin molecule and the relevant parts of the cytochrome f molecule were given conformational freedom. In the third stage, the following three contributions ...

Binding dynamics and electron transfer between plastocyanin and photosystem I.

The mechanism of the electron transfer from the soluble protein plastocyanin to the multiprotein complex of photosystem I from spinach has been studied in detail. The two kinetic components of P700+ reduction by plastocyanin after a laser flash, showing a constant half-life of 11 microseconds and a variable half-life of the second-order reaction, respectively, are used to monitor the electron transfer from bound and soluble plastocyanin. The effect of increasing concentration of reduced plastocyanin on both of these kinetic components and the competition by oxidized plastocyanin is used to estimate the individual dissociation constants of the complex between the proteins in each of its oxidized and reduced state. The dissociation constant of oxidized plastocyanin is about six times larger than that of 7 microM found for reduced plastocyanin and purified PSI. Consistent with this result the midpoint redox potential of plastocyanin bound to photosystem I either in equilibrium with soluble plastocyanin or after cross-linking to photosystem I is found to be 50-60 mV higher than that of soluble plastocyanin. It is concluded that the driving force of the intracomplex electron transfer is decreased in favor of an optimized turnover of photosystem I. Double-flash excitation shows that oxidized plastocyanin has to leave the complex after the electron transfer before a new reduced plastocyanin molecule can bind to photosystem I. This release of oxidized plastocyanin with a half-life of about 60 microseconds limits the turnover of photosystem I. All data are consistently described by a model including the formation of a complex at a single binding site of photosystem I. Differences in the rate and binding constants are discussed with respect to the structure and the electrostatic and hydrophobic interactions stabilizing the complex as well as their modification by the membrane environment in situ.

Role of transit peptide sequence of plastocyanin for its expression, processing, and copper-binding activity in Escherichia coli.

Plastocyanin is a copper protein that functions as an electron carrier in the thylakoid lumen of the chloroplast. To characterize the transit peptide of plastocyanin and develop expression systems for it in Escherichia coli, three kinds of expression vectors which encode different size precursor plastocyanin molecules were constructed. Their expression, processing, and copper-binding activity have been examined. When the full-length cDNA encoding the precursor plastocyanin from Silene pratensis was expressed in E. coli, a large amount of precursor plastocyanin accumulated in insoluble aggregates. Its accumulation level was increased by the addition of copper ions. About six percent of precursor plastocyanin molecules were transported into the periplasmic space and processed to the mature protein. On the other hand, expression of the intermediate size cDNA, which contains the hydrophobic domain and basic amino acid of C-terminal transit peptide, caused exclusive translocation to the periplasmic space and correct processing to the mature size. The addition of copper ions increased the holo-protein content, but did not change the polypeptide content of mature plastocyanin, indicating that translocation and processing are independent of the incorporation of copper ions. The mature plastocyanin content corresponds to 8% (w/w) of the total E. coli protein content (123 mg per liter of culture). The purified mature holo-protein showed almost the same spectroscopic and kinetic properties as those of purified spinach plastocyanin. Expression of the cDNA encoding the mature polypeptide and two preceeding amino acid residues caused the accumulation of only a small amount of plastocyanin.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron transfer from plastocyanin to photosystem I.

Mutant plastocyanins with Leu at position 10, 90 or 83 (Gly, Ala and Tyr respectively in wildtype) were constructed by site-specific mutagenesis of the spinach gene, and expressed in transgenic potato plants under the control of the authentic plastocyanin promoter, as well as in Escherichia coli as truncated precursor intermediates carrying the C-terminal 22 amino acid residues of the transit peptide, i.e. the thylakoid-targeting domain that acts as a bacterial export signal. The identity of the purified plastocyanins was verified by matrix-assisted laser desorption/ionization mass spectrometry. The formation of a complex between authentic or mutant spinach plastocyanin and isolated photosystem I and the electron transfer has been studied from the biphasic reduction kinetics of P700+ after excitation with laser flashes. The formation of the complex was abolished by the bulky hydrophobic group of Leu at the respective position of G10 or A90 which are part of the conserved flat hydrophobic surface around the copper ligand H87. The rate of electron transfer decreased by both mutations to < 20% of that found with wildtype plastocyanin. We conclude that the conserved flat surface of plastocyanin represents one of two crucial structural elements for both the docking at photosystem I and the efficient electron transfer via H87 to P700+. The Y83L mutant exhibited faster electron transfer to P700+ than did authentic plastocyanin. This proves that Y83 is not involved in electron transfer to P700 and suggests that electron transfer from cytochrome f and to P700 follows different routes in the plastocyanin molecule. Plastocyanin (Y83L) expressed in either E. coli or potato exhibited different isoelectric points and binding constants to photosystem I indicative of differences in the folding of the protein. The structure of the binding site at photosystem I and the mechanism of electron transfer are discussed.

Thermal denaturation of plastocyanin: The effect of oxidation state, reductants, and anaerobicity

The thermal stability of plastocyanin (PC) was determined as a function of oxidation state of the copper center and the presence of oxidants, reductants, oxygen, and EDTA. It was found that the copper center and its ligands play a crucial role in maintaining the stability of PC. Thermal denaturation was monitored by using far-uv circular dichroism (CD) spectra to monitor changes in secondary structure, the near-uv CD ellipticity at 280 nm to monitor changes in tertiary structure, and the absorbance at 597 nm and the 255-nm CD transition to monitor changes in the copper center. Reduced PC ( T m = 71 ° C) was found to be more stable than the oxidized form ( T m = 61 ° C). The T m was increased by addition of reductants, removal of oxygen, or addition of EDTA. Two distinct denatured forms (designated D 1 and D 2) were separated by anion exchange fast protein liquid chromatography. Neither form contained a native copper center. Form D 2 retained the characteristic 280-nm CD band but showed an altered far-uv CD spectrum. Its formation was inhibited by the addition of reductants or the removal of oxygen. It could be refolded to form native, Cu-PC upon incubation with copper plus a reductant such as dithionite. These results suggest that its formation involves the reversible oxidation of a group on the PC molecule, possibly a ligand to the copper such as Cys 84 or Met 92. Form D 1 occurred in the presence of ferricyanide or at high temperatures in the presence of oxygen. EDTA inhibited its formation. Form D 1 lost the 280-nm CD transition and its far-uv CD spectrum was altered. No renaturation was observed suggesting that Form D 1 is the product of an irreversible oxidation step possibly involving a histidine ligand to the copper. Forms D 1 and D 2 are not interconvertible and represent the endpoints of two different denaturation pathways.

Direct electrochemistry of spinach plastocyanin at a lipid bilayer-modified electrode: Cyclic voltammetry as a probe of membrane-protein interactions

The electron transfer reactions between a lipid bilayer-modified gold electrode and oxidized spinach plastocyanin have been studied by cyclic voltammetry, using either an electrically neutral phosphatidylcholine (PC) bilayer or a positively charged PC bilayer containing 40 mol% dimethyldioctadecylammonium chloride, at two ionic strengths of electrolyte (0.02 and 0.2 m NaClO 4). Plastocyanin was found to interact strongly enough with the lipid membrane to support an efficient electron transfer reaction with the electrode. The interaction forces, and therefore the mode of diffusion of plastocyanin molecules to the electrode, which limits the electron transfer rate, could be controlled by the PC concentration. At low lipid concentrations (0–5 mg/ml), electrostatically attractive interactions between specific microelectroactive sites on the surface of the lipid membrane and plastocyanin molecules predominate, producing a radial mode of diffusion of the protein molecules to the electrode surface. On the other hand, at high lipid concentrations (>5 mg/ml), interaction between plastocyanin and the lipid membrane occurs via hydrophobic forces, and a linear diffusion of protein molecules limits the electron transfer process. These observations support and extend other experimental and theoretical results which indicate two possible sites on the surface of the plastocyanin molecule, one hydrophobic and one negatively charged, which are able to participate in electron transfer reactions. We conclude that electrochemical measurements with the present system provide a new approach to the study of redox protein-membrane interactions.

Lateral distribution and diffusion of plastocyanin in chloroplast thylakoids.

The lateral distribution of plastocyanin in the thylakoid lumen of spinach and pea chloroplasts was studied by combining immunocytochemical localization and kinetic measurements of P700+ reduction at high time resolution. In dark-adapted chloroplasts, the concentration of plastocyanin in the photosystem I containing stroma membranes exceeds that in photosystem II containing grana membranes by a factor of about two. Under these conditions, the reduction of P700+ with a halftime of 12 microseconds after a laser flash of saturating intensity indicates that to greater than 95% of total photosystem I a plastocyanin molecule is bound. An analysis of the labeling densities, the length of the different lumenal regions, and the total amounts of plastocyanin and P700 shows that most of the remaining presumable mobile plastocyanin is found in the granal lumen. This distribution of plastocyanin is consistent with a more negative surface charge density in the stromal than in the granal lumen. During illumination the concentration of plastocyanin in grana increases at the expense of that in stroma lamellae, indicating a light-driven diffusion from stroma to grana regions. Our observations provide evidence that a high concentration of plastocyanin in grana in the light favors the lateral electron transport from cytochrome b6/f complexes in appressed grana across the long distance to photosystem I in nonappressed stroma membranes.

The effect of ethylenediamine chemical modification of plastocyanin on the rate of cytochrome f oxidation and P-700 + reduction

Chemical modification of plastocyanin was carried out using ethylenediamine plus a water-soluble carbodiimide, which has the effect of replacing a negatively charged carboxylate group with a positively charged amino group at pH 6–8. The conditions were adjusted to produce a series of singly and doubly modified forms of plastocyanin. Differences in charge configuration allowed separation of these forms on a Pharmacia fast protein liquid chromatograph using a Mono Q anion exchange column. These forms were used to study the interaction of plastocyanin with its reaction partner cytochrome f. The rate of cytochrome f oxidation was progressively inhibited upon incorporation of increasing numbers of ethylenediamine moieties indicating a positively charged binding site on cytochrome f. However, differential inhibition was obtained for the various singly modified forms allowing mapping of the binding site on plastocyanin. The greatest inhibition was found for forms modified at negatively charged residues Nos. 42–45 and Nos. 59–61 which comprise a negative patch surrounding Tyr-83. In contrast, the form modified at residue No. 68, on the opposite side of the globular plastocyanin molecule, showed the least inhibition. It can be concluded that the binding site for cytochrome f is located in the vicinity of residues Nos. 42–45 and Nos. 59–61. Modification of plastocyanin at residues Nos. 42–45 showed no effect on the rate of P-700 + reduction, suggesting that these residues are not involved in the binding of Photosystem I. However, an increase in the rate of P-700 + reduction was observed for plastocyanins modified at residue No. 68 or Nos. 59–61, which is consistent with the idea that the reaction domain of Photosystem I is negatively charged and Photosystem I binds at the top of the molecule and accepts electrons via His-87 in plastocyanin. These results raise the possibility that plastocyanin can bind both cytochrome f and Photosystem I simultaneously. The effect of ethylenediamine modification on the formal potential of plastocyanin was also examined. The formal potential of control plastocyanin was found to be +372 ± 5 mV vs. normal hydrogen electrode at pH 7. All modified forms showed a positive shift in formal potential. Singly modified forms showed increases in formal potentials between +8 and +18 mV with the largest increases being observed for plastocyanins modified at residues Nos. 42–45 or Nos. 59–61.

Turn-over of electron donors in Photosystem I: Double-flash experiments with pea chloroplasts and Photosystem I particles

Kinetic properties of the binding of plastocyanin to the Photosystem I (PS I) reaction center have been studied by measuring the reduction kinetics of photo-oxidized P-700 after two consecutive excitations, with flash absorption spectroscopy. In pea chloroplasts and in PS I particles, after two laser flash excitations separated by 50 μs or less, P-700 + is reduced with a t 1 2 = 110 μ s . No 12-μs reduction phase, due to ‘close’ bound plastocyanin electron donation to P-700 +, can be detected. By increasing the delay between the flashes, the 12-μs P-700 + reduction phase reappears and its half-time of reappearance is 110 μs. Plastocyanin is shown to be involved in P-700 + reduction after the first and also the second excitations. Results are interpreted in terms of two plastocyanin molecules bound to the reaction center, the ‘distant’ bound one replacing or re-reducing the ‘close’ bound one after it has been oxidized.

Molecular orientation of plastocyanin on spinach thylakoid membranes as determined by acetylation of lysine residues.

To reveal the molecular orientation of plastocyanin (PC) on spinach thylakoid membranes, the position of Lys residues modified by acetic anhydride was compared between thylakoid-bound PC and the isolated one. Digestion of the isolated PC by a trypsin yielded a peptide map with seven spots prior to the acetylation of the protein; none of the spots appeared after the isolated PC was acetylated. On the other hand, there were two spots on the peptide map of the PC acetylated when it was bound to the thylakoids. Those spots were revealed by their amino acid compositions to correspond to the peptide fragments Phe 82-Lys 95 and Val 96-Asn 99. Thus, the Lys residues 81 and 95 of the thylakoid-bound PC were not acetylated. These results suggest that the PC molecule binds to the thylakoids with a specific region including the Lys's 81 and 95 in contact with the membranes. The Lys's 81 and 95 are located near Tyr 83, which has been thought to be the delivery site of electrons from the Cu2+ center.

A non-fluorescent complex of chlorophyll [formula omitted] with plastocyanin

A complex between chlorophyll a and plastocyanin has been prepared by dialysis of mixtures of chlorophyll in Triton X-100 micellar solution with the protein. The complex appears to contain no more than one chlorophyll per plastocyanin molecule, and is non-fluorescent, whether the protein is in the oxidized or reduced state. The lack of fluorescence suggests that the chlorophyll is adsorbed very close to the Cu center.