Characterization of a D1-D2-cyt b-559 complex containing 4 chlorophyll a/2 pheophytin a isolated with the use of MgSO4

An unusual role for the phytyl chains in the photoprotection of the chlorophylls bound to Water-Soluble Chlorophyll-binding Proteins

Water-Soluble Chlorophyll Proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins which tetramerize upon binding four chlorophyll (Chl) molecules. The bound Chls are highly photostable, despite the lack of bound carotenoids known, in Chl-containing photosynthetic proteins, to act as singlet oxygen and Chl triplet (3Chl) quenchers. Although the physiological function of WSCPs is still unclear, it is likely to be related to their biochemical stability and their resistance to photodegradation. To get insight into the origin of this photostability, the properties of the 3Chl generated in WSCPs upon illumination were investigated. We found that, unlike the excited singlet states, which are excitonic states, the triplet state is localized on a single Chl molecule. Moreover, the lifetime of the 3Chl generated in WSCPs is comparable to that observed in other Chl-containing systems and is reduced in presence of oxygen. In contrast to previous observations, we found that WSCP actually photosensitizes singlet oxygen with an efficiency comparable to that of Chl in organic solvent. We demonstrated that the observed resistance to photooxidation depends on the conformation of the phytyl moieties, which in WSCP are interposed between the rings of Chl dimers, hindering the access of singlet oxygen to the oxidizable sites of the pigments.

Low-temperature (77 K) phosphorescence of triplet chlorophyll in isolated reaction centers of photosystem II

Phosphorescence characterized by the main emission band at 952 ± 1 nm (1.30 eV), the lifetime of 1.5 ± 0.1 ms and the quantum yield nearly equal to that for monomeric chlorophyll a in aqueous detergent dispersions, has been detected in isolated reaction centers (RCs) of spinach photosystem II at 77 K. The excitation spectrum shows maxima corresponding to absorption bands of chlorophyll a, pheophytin a, and β-carotene. The phosphorescence intensity strongly depends upon the redox state of RCs. The data suggest that the phosphorescence signal originates from the chlorophyll triplet state populated via charge recombination in the radical pair [Formula: see text].

Isolation and spectral characterization of Photosystem II reaction center from Synechocystis sp. PCC 6803

We isolated highly-purified photochemically active photosystem (PS) II reaction center (RC) complexes from the cyanobacterium Synechocystis sp. PCC 6803 using a histidine-tag introduced to the 47 kDa chlorophyll protein, and characterized their spectroscopic properties. Purification was carried out in a one-step procedure after isolation of PS II core complex. The RC complexes consist of five polypeptides, the same as in spinach. The pigment contents per two molecules of pheophytin a were 5.8 +/- 0.3 chlorophyll (Chl) a and 1.8 +/- 0.1 beta-carotene; one cytochrome b(559) was found per 6.0 Chl a molecules. Overall absorption and fluorescence properties were very similar to those of spinach PS II RCs; our preparation retains the best properties so far isolated from cyanobacteria. However, a clear band-shift of pheophytin a and beta-carotene was observed. Reasons for these differences, and RC composition, are discussed on the basis of the three-dimensional structure of complexes.

HYSCORE Spectroscopy in the Cytochrome b559 of the Photosystem II Reaction Center

A HYSCORE investigation of the heme center in the cytochrome b(559) is presented. To assign the observed signals to specific nuclei, bis-imidazol coordinated heme compounds that model the iron environment in cytochrome b(559) are also studied. In the model compounds selective isotopic substitution of nitrogen atoms has been performed. The HYSCORE spectra allow us to obtain the hyperfine and quadrupolar coupling tensors of heme and imidazol bonding nitrogen atoms. The results can be interpreted in terms of the structure and the electronic distribution of the active center. The hyperfine tensors indicate that the unpaired electron is confined in a nonbonding iron orbital with a negligible nitrogen p orbital contribution. Quadrupolar coupling tensors suggest that the orientation of the semioccupied orbital is driven by the orientation of the two parallel imidazol rings of the axial histidine side chains. The results are discussed in terms of the structure-function relationship of cytochromes.

Structural changes upon excitation of D1-D2-Cyt b559 photosystem II reaction centers depend on the beta-carotene content

Different preparations of D1-D2-Cyt b559 complexes from spinach with different beta-carotene (Car) content [on average from <0.5 to 2 per reaction center (RC)] were studied by means of laser-induced optoacoustic spectroscopy. phiP680(+)Pheo(-) does not depend on the preparation (or on the Car content) inasmuch as the magnitude of the prompt heat (produced within 20 ns) does not vary for the different samples upon excitation at 675 and 620 nm. The energy level of the primary charge-separated state, P680(+)Pheo(-), was determined as EP680(+)Pheo(-) = 1.55 eV. Thus, an enthalpy change accompanying charge separation from excited P680 of deltaH*P680Pheo-->P680(+)Pheo(-) = -0.27 eV is obtained. Calculations using the heat evolved during the time-resolved decay of P680(+)Pheo(-) (< or = 100 ns) affords a triplet (3[P680Pheo]) quantum yield phi3[P680Pheo] = 0.5 +/- 0.14. The structural volume change, deltaV1, corresponding to the formation of P680(+)Pheo(-), strongly depends on the Car content; it is ca. -2.5 A3 molecule(-1) for samples with <0.5 Car on average, decreases (in absolute value) to -0.5 +/- 0.2 A3 for samples with an average of 1 Car, and remains the same for samples with two Cars per RC. This suggests that the Car molecules induce changes in the ground-state RC conformation, an idea which was confirmed by preferential excitation of Car with blue light, which produced different carotene triplet lifetimes in samples with 2 Car compared to those containing less carotene. We conclude that the two beta-carotenes are not structurally equivalent. Upon blue-light excitation (480 nm, preferential carotene absorption) the fraction of energy stored is ca. 60% for the 9Chl-2Car sample, whereas it is 40% for the preparations with one or less Cars on average, indicating different paths of energy distribution after Car excitation in these RCs with remaining chlorophyll antennae.

Cytochrome b559 content in isolated photosystem II reaction center preparations

The cytochrome b559 content was examined in five types of isolated photosystem II D1-D2-cytochrome b559 reaction center preparations containing either five or six chlorophylls per reaction center. The reaction center complexes were obtained following isolation procedures that differed in chromatographic column material, washing buffer composition and detergent concentration. Two different types of cytochrome b559 assays were performed. The absolute heme content in each preparation was obtained using the oxidized-minus-reduced difference extinction coefficient of cytochrome b559 at 559 nm. The relative amount of D1 and cytochrome b559alpha-subunit polypeptide was also calculated for each preparation from immunoblots obtained using antibodies raised against the two polypeptides. The results indicate that the cytochrome b559 heme content in photosystem II reaction center complexes can vary with the isolation procedure, but the variation of the cytochrome b559alpha-subunit/D1 polypeptide ratio was even greater. This variation was not found in the PSII-enriched membrane fragments used as the RC-isolation starting material, as different batches of membranes obtained from spinach harvested at different seasons of the year or those from sugar beets grown in a chamber under controlled environmental conditions lack variation in their alpha-subunit/D1 polypeptide ratio. A precise determination of the ratio using an RC1-control sample calibration curve gave a ratio of 1.25 cytochrome b559alpha-subunit per 1.0 D1 polypeptide in photosystem II membranes. We conclude that the variations found in the reaction center preparations were due to the different procedures used to isolate and purify the different reaction center complexes.

Detergent effect on cytochrome b559 electron paramagnetic resonance signals in the photosystem II reaction centre

The detergent effect on Cytochrome b559 from spinach photosystem II was studied by electron paramagnetic resonance (EPR) spectroscopy in D1-D2-Cyt b559 complex preparations. Various n-dodecyl-beta-D-maltoside concentrations from 0 to 0.2% (w/v) were used to stabilise the D1-D2-Cyt b559 complexes. Low spin heme EPR spectra were obtained but the g(z) feature positions changed depending on the detergent conditions Redox potentiometric titrations showed a unique redox potential cytochrome b559 form (E'm = + 123-150 mV) in all the D1-D2-Cyt b559 complex preparations indicating that detergent does not affect this property of the protein in those conditions. A similar effect on Cytochrome b559 EPR spectrum was observed in more intact photosystem II preparations independently of their aggregation state. This finding indicates that changes due to detergent could be a common phenomenon in photosystem II complexes. Results are discussed in terms of the environment each detergent provides to the protein.

Modification of pigment composition in the isolated reaction center of photosystem II

The pigment content of isolated reaction centers of photosystem II was modified using an exchange protocol similar to that used for purple bacterial reaction centers. With this method, which is based on incubation of reaction centers at elevated temperature with an excess of chemically modified pigments, it was possible to incorporate [3-acetyl]-chlorophyll a and [Zn]-chlorophyll a into photosystem II reaction centers. Pigment exchange has been verified by absorption, circular dichroism and fluorescence spectroscopy, and quantitated by HPLC analysis of pigment extracts.

Pigment quantitation and analysis by HPLC reverse phase chromatography: a characterization of antenna size in oxygen-evolving photosystem II preparations from cyanobacteria and plants

Photosystem II, the photosynthetic water-oxidizing complex, can be isolated from both plants and cyanobacteria. A variety of methods have been developed for purification of this enzyme, which can be isolated in several functional and structural forms. Knowledge of the pigment content of photosystem II preparations is important for precise spectroscopic, biochemical, and functional analysis. We have determined pigment stoichiometries in oxygen-evolving photosystem II preparations from plants and cyanobacteria. We have employed a solvent system for the isocratic elution of a reverse phase HPLC column in which we have determined the extinction coefficients of the relevant pigments. Pigments were extracted from four photosystem II preparations. These preparations included spinach photosystem II membranes [Berthold, D. A., Babcock, G. T., & Yocum, C. F. (1981) FEBS Lett. 134, 231-234], spinach photosystem II reaction center complexes [Ghanotakis, D. F., & Yocum, C. F. (1986) FEBS Lett. 197, 244-248], spinach photosystem II complexes [MacDonald, G. M., & Barry, B. A. (1992) Biochemistry 31, 9848-9856], and photosystem II particles isolated from the cyanobacterium, Synechocystis sp. PCC 6803 [Noren, G. H., Boerner, R. J., & Barry, B. A. (1991) Biochemistry 30, 3943-3950]. Pigment stoichiometries were determined using two different methods of data analysis and were based on the assumption that there are two pheophytin a molecules per photosystem II reaction center. The pigment stoichiometries obtained were comparable for the two methods of data analysis and agreed with previous biophysical and biochemical characterizations of the preparations. The average pigment stoichiometries (chlorophyll:plastoquinone-9 per 2 pheophytin a) determined using the two data analysis methods were as follows: photosystem II membranes, 274:3.2; photosystem II reaction center complexes, 78:2.5; Synechocystis PS II particles, 55:2.4; photosystem II complexes, 121:2.0.

Photosystem II of green plants: topology of core pigments and redox cofactors as inferred from electrochromic difference spectra

Three electrochromic difference spectra induced by the deposition of (1) a negative charge on the primary quinone acceptor, Q(A), (2) a positive charge on (or near) Tyr161 of the D1 subunit (Y(Z)), and (3) a positive charge on the manganese cluster were determined at room temperature in photosystem II (PSII) core particles from pea. They were deconvoluted into Gaussian components by Powell's numerical optimization procedure. All three spectra were fitted by four components, which we assigned to the Q(y) absorption bands of two chlorophyll a molecules of the primary donor P, the accessory chlorophyll a, and the pheophytin a molecules on the D1 subunit. On the basis of the electrochromic properties of chlorins and our data, we suggest an arrangement of pigments and redox cofactors in PSII that differs from current structural models, which have been shaped like the reaction centers (RC) of purple bacteria. Our model is compatible with sequence data, with the spectroscopic and electrochemical properties of chlorophyll a and pheophytin a, and with the extremely positive redox potential of water oxidation. We conclude the following: (1) P is formed from two orthogonally oriented chlorophyll a molecules that peak at 681 and 677 nm. (2) The accessory chlorophyll a on D1 is oriented perpendicular to the membrane, with ring V pointing to Q(A). It is presumably attached to His118 of D1. (3) The mutual arrangement of pheophytin a on the D1 subunit and Q(A) differs from that of their counterparts in bacterial RC. (4) The manganese cluster is located out of the axis that is formed by Y(Z) (Tyr161 of D1), P, and Y(D) (Tyr161 of D2).

Analysis of the absorption spectrum of photosystem II reaction centers: temperature dependence, pigment assignment, and inhomogeneous broadening

In this study a model for decomposition and pigment assignment of the low-temperature (10 K) absorption spectrum of the photosystem II reaction center (D1-D2-cytochrome b559 complex, PSII-RC) is developed. It is based on theoretical calculations of the line shapes of the inhomogeneously broadened pigment spectra, taking into account electron-phonon coupling. The analysis is performed under the hypothesis that exciton coupling is weak, except for the P680 special pair. In this way a detailed decomposition of the absorption spectrum is obtained. Within the model the temperature dependence of the spectrum can be well explained. It is mainly caused by the temperature-dependent changes of the homogeneous absorption spectra of the individual pigments in the PSII-RC. In addition, slight changes in the inhomogeneous distribution functions have to be taken into account. Two slightly different parameter sets are found. We prefer one of these parameter sets which indicates that an accessory chlorophyll (Chl) is the lowest energy pigment in the RC core and that the two antenna Chls have their spectral maxima at 667.7 and 677.9 nm, respectively. The relationship between the shape of the absorption spectrum and the pigment stoichiometry of the sample (ratio of chlorophyll a:pheophytin a), which was noticed by comparison of a variety of different independently prepared samples, can be explained by the presence of "additional" Chl molecules which are nonstoichiometrically bound to part of the PSII-RCs. These Chls can be grouped into three spectrally distinguishable pools. One of them has its absorption maximum at about 683 nm and is responsible for the prominent shoulder that is present in the 10 K absorption spectra of most PSII-RC preparations. Our results suggest that the Chl content of the samples has been underestimated in many spectroscopic studies on the PSII-RC.

Destruction of a single chlorophyll is correlated with the photoinhibition of photosystem II with a transiently inactive donor side

Pigments destroyed during photoinhibition of water-splitting photosystem II core complexes from the green alga Chlamydomonas reinhardtii were studied. Under conditions of a transiently inactivated donor side, illumination leads to an irreversible inhibition of the electron transfer at the donor side that is paralleled by the destruction of chlorophylls a absorbing maximally around 674 and 682 nm. The observed stochiometry of 1 +/- 0.1 destroyed chlorophyll per inhibited photosystem II suggests that chlorophyll destruction could be the primary photodamage causing the inhibition of photosystem II under these conditions.

Photochemistry and spectroscopy of a five-chlorophyll reaction center of photosystem II isolated by using a Cu affinity column

A reaction center of photosystem II was isolated from Pisum sativum by using immobilized metal affinity chromatography. This reaction center is photochemically active and has a room temperature Qgamma chlorophyll (Chl) absorption band peaking at 677.5 nm. From HPLC analysis, the pigment stoichiometry was suggested to be 5 Chls per 1 beta-carotene per 2 pheophytins. Low-temperature absorption measurements at 77 K were consistent with the removal of one of the Chls associated with the usual form of the reaction center isolated by using ion-exchange chromatography. Transient absorption spectroscopy on the picosecond time scale indicated that the Chl removed belongs to a pool of Chl absorbing at approximately 670 nm (C670II) that transfers energy relatively slowly to the primary donor P680 in support of our recently proposed model. The results also support the previous conclusion that radical pair formation is largely associated with a 21-ps time constant when P680 is directly excited and that the identity of C670II is likely to be peripherally bound Chls possibly ligated to conserved His residues at positions 118 on the D1 and D2 proteins.

beta-Carotene quenches singlet oxygen formed by isolated photosystem II reaction centers

By measuring time-resolved luminescence emission at 1270 nm, we have detected singlet oxygen formation by illuminated, reaction centers of photosystem II isolated from Pisum sativum, which is in agreement with earlier work (Macpherson, A. N., Telfer, A., Barber, J., & Truscott, T. G. (1993) Biochim. Biophys. Acta 1143, 301-309). In this paper we show that the yield of singlet oxygen is significantly increased if the number of beta-carotene molecules bound per isolated complex is reduced from two to one. We conclude, therefore, that beta-carotene can act as an effective quencher of singlet oxygen in the photosystem II reaction center. This conclusion is supported by the finding that the rate of light-induced irreversible bleaching of chlorins in the reaction center is increased with decreasing beta-carotene levels. The results demonstrate the direct intermediacy of singlet oxygen in causing photooxidative damage within a biological environment and are discussed, specifically, in terms of the role of beta-carotene in protecting photosystem II against photoinhibition.

Core antenna complexes, CP43 and CP47, of higher plant photosystem II. Spectral properties, pigment stoichiometry, and amino acid composition

The core antenna complexes of photosystem II, CP43 and CP47, were purified from two higher plants by anion-exchange chromatography, using a combination of the chaotropic agent LiClO4 and the nonionic detergent beta-dodecyl maltoside. The Qy transition was resolved at 48 K into two main bands near 682.3 and 671.5 nm for CP43, while the CP47 spectrum showed a more complex structure with main bands at 688, 681.2, 676, 667, and 661 nm. Emission bands (77 K) were detected at 683 and 695 nm for CP43 and CP47, respectively. Fluorescence excitation spectra showed high efficiency of energy transfer between the different transitions of the chlorophylls and a somewhat lower efficiency from beta-carotene. The circular dichroism spectrum of CP47 indicated the presence of excitonic interactions between some chlorophylls. In contrast, CP43 showed a single negative circular dichroism band at 670 nm. The pigment content of the complexes was determined by both spectroscopic measurements and HPLC. Contents of 18 chlorophylls a and 5 beta-carotenes per CP43 polypeptide and 19 chlorophylls a and 3 beta-carotenes per CP47 polypeptide were found, using the methods of Lowry or Bradford for protein quantitation. When the protein concentration was determined from the amino acid analysis, 20 chlorophylls a and 5 beta-carotenes per CP43 and 21-22 chlorophylls a and 4 beta-carotenes per CP47 were obtained. Thus, a content of 46-48 chlorophylls a was obtained for the core complex, assuming 4-6 chlorophylls per reaction center, in agreement with the composition obtained experimentally using a highly purified oxygen-evolving core complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature-dependent triplet and fluorescence quantum yields of the photosystem II reaction center described in a thermodynamic model

A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 +/- 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at approximately 165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+l- is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.