Purification and properties of the photosystem I reaction center from chloroplasts

The effect on photosynthetic electron transport of temperature-dependent changes in the fluidity of the thylakoid membrane in a thermophilic blue-green alga

Various electron transport reactions in cell or isolated thylakoid membranes of the thermophilic blue-green alga, Synechococcus sp. were measured at different temperatures between 72 and 3 degrees C. They are classified into two groups with respect to their temperature dependency. The first group involves cytochrome 553 photooxidation, methyl viologen photoreduction with reduced 2,6-dichlorophenolindophenol as electron donor and 3-(3',4'-dichlorophenyl)-1,1-dimethylurea-resistant ferricyanide photoreduction determined in the presence or absence of silicomolybdate. The Arrhenius plot of these reactions showed a single straight line with the activation energy of about 10 kcal/mol throughout wide temperature ranges studied. Methyl viologen photoreduction with water as electron donor, reduction of flash-oxidized cytochrome 553, ferricyanide photoreduction and photosynthetic O2 evolution form the second group. Their arrhenius plots are characterized by discontinuities or breaks at about 30 and 10 degrees C, which respectively correspond to the upper and lower boundaries of the lateral phase separation of the membrane lipids. The first group reactions represent short spans of electron transport which are mediated either by Photosystem I or Photosystem II alone and not related to plastoquinone, whereas all the reactions of the second group involve plastoquinone. It is concluded therefore that the membrane fluidity affect electron transport specifically at the region of plastoquinone. It is proposed that the reaction center chlorophyll-protein complexes of both Photosystems I and II are closely associated with related electron carrier proteins to form functional supramolecular assemblies so that electron transfer within such a cluster of proteins proceeds independently of the phase changes in the membrane lipids. On the other hand, the role of plastoquinone as a mobile electron carrier mediating electron transfer from the protein assembly of Photosystem II to that of Photosystem I through the fluid hydrophobic matrix of the membranes is highly sensitive to the physical state of the membrane lipids.

Use of chemical modification to study the relationship between activity and net protein charge of the photosystem I core complex

The net charge on the photosystem I core complex of Shiozawa et al. (Shiozawa, J. A., Alberte, R. S., & Thornber, J. P. (1974) Arch. Biochem. Biophys. 165, 388-397) has been altered by using a water-soluble carbodiimide to form an amide bond between protein carboxyl groups and an amino group of ethylenediamine. This process replaces negatively charged carboxyl groups with positively charged free amino groups. Six hundred moles of ethylenediamine was incorporated per mole of P700 reaction center. The modification of the complex shifted the isoelectric pH of photosystem I from 5.0 to 9.5 without affecting the total amount of P700. The modified complex exhibited an increase in energy transfer from light-harvesting chlorophyll alpha molecules to the reaction center. This phenomenon has been previously observed upon addition of Mg2+ ions to the complex (Gross, E. L., & Grenier, J. (1978) Arch. Biochem. Biophys. 187, 387-398). The modification replaces the divalent cation requirement for electron donation to P700 by plastocyanin and lowered the Km for plastocyanin binding to 2.0 microM, compared to 32 microM for control photosystem I in the presence of Mg2+. In addition, the modification lowered the Km for the negatively charged electron donors dichlorophenolindophenol and ascorbate. These results suggest that changing the charge on the photosystem I complex from negative to positive stimulates both light utilization and electron transfer from electron donors to P700. We suggest that cation regulation of photosystem I activity occurs by a process in which cations alter the charge of the local environment around the complex.

Evidence for complexed plastocyanin as the immediate electron donor of P-700

The reduction of P-700 by its electron donors shows two fast phases with half-times of 20 and 200 mus in isolated spinach chloroplasts. We have studied this electron transfer and the oxidation kinetics of cytochrome f. Incubation of chloroplasts with KCN or HgCl2 decreased the amplitude of the 20 mus phase. This provides evidence for a function of plastocyanin as the immediate electron donor of P-700. At low concentrations of salt and sugar the fast phases of P-700+ reduction were largely inhibited. Increasing concentrations of MgCl2, KCl and sorbitol (up to 5, 150 and 200 mM, respectively) were found to increase the relative amplitudes of the fast phases to about one-third of the total P-700 signal. Addition of both 3 mM MgCl2 and 200 mM sorbitol increased the relative amplitude of the 20 mus phase to 70%. The interaction between P-700 and plastocyanin is concluded to be favoured by a low internal volume of the thylakoids and compensation of surface charges of the membrane. The half-time of 20 mus was not changed when the amplitude of this phase was altered either by salt and sorbitol, or by inhibition of plastocyanin. This is evidence for the existence of a complex between plastocyanin and P-700 with a lifetime long compared to the measuring time. The 200 mus phase exhibited changes in its half-time that indicated the participation of a more mobile pool of plastocyanin. Cytochrome f was oxidized with a biphasic time course with half-times of 70--130 mus and 440--860 mus at different salt and sorbitol concentrations. The half-time of the faster phase and a short lag of 30--50 mus in the beginning of the kinetics indicate an oxidation of cytochrome f via the 20 mus electron transfer to P-700. An inhibition of this oxidation by MgCl2 suggests that the electron transfer from cytochrome f to complexed plastocyanin is not controlled by negative charges in contrast to that from plastocyanin to P-700.

Reconstitution of photosynthetic energy conservation. II. Photophosphorylation in liposomes containing photosystem-I reaction center and chloroplast coupling-factor complex

Photophosphorylation has been reconstituted in a liposomal system containing reaction centers of photosystem I and coupling-factor complex, both highly purified from spinach chloroplasts. This energy-converting model system was put together by diluting the preparation of the coupling-factor complex with an aqueous suspension of proteolipid vesicles, preformed from photosystem-I reaction centers and soybean phospholipids by sonication. In the presence of reduced N-methyl-phenazonium methosulfate the system catalyzed photophosphorylation with rates up to 50 mumol ATP formed x mg chlorophyll-1 x h-1, which was sensitive to uncouplers and to N,N'-dicyclohexyl-carbodiimide. The properties of the system in comparison to chloroplasts is discussed.

Reconstitution of photosynthetic energy conservation. I. Proton movements in liposomes containing reaction center of photosystem I from spinach chloroplasts

A preparation of reaction centers of photosystem I from spinach chloroplasts was incorporated into lipid vesicles by sonication. Incorporation was tested by chromatography on Sepharose-4B, by comparison of the elution profiles of photosystem-I reaction center liposomes and of free reaction centers. In the presence of reduced N-methylphenazonium methosulfate the reaction center liposomes catalyzed net proton extrusion in the light, but at the same time showed light-induced quenching of 9-aminoacridine fluorescence, with similar extent and kinetics as known for chloroplasts. We conclude that we are dealing with two vesicle populations, one right side-out and one inside-out with respect to the orientation of the incorporated reaction center complex. Net proton movements are influenced by the nature of the cations present in the suspending medium, and the possibilities for effects of surface charge on these movements are discussed.

Effect of detergents on the reliability of a chemical assay for P-700

A chemical assay for P-700 was developed using 0.36 mM potassium ferricyanide as oxidant and 1.6 mM sodium ascorbate as reductant. The major difference from other chemical assays for P-700 is procedural. The method is designed to take advantage of the availability of microprocessor-linked spectrophotometers to obtain greater accuracy by minimizing the spectral changes due to irreversible oxidized antenna chlorophyll molecules. The value measured for the P-700 concentration in a sample of chloroplasts was not changed by the presence of EDTA, Mg2+ or sucrose in the assayed solution. Similarly, half of the detergents tested (Triton X-100, Nonidet P-40, digitonin, Deriphat 160, Miranol S2M-SF and Miranol M2M) did not alter the value when added to the chloroplasts. The remainder of the detergents examined caused a significant decrease or increase in the value for P-700 content. Sodium dodecyl sufate, of particular interest due to its widespread use, caused a doubling in the amount of apparent P-700. This effect may be due to this detergent and some others enabling an additional long wavelength form of chlorophyll, possibly an intermediary electron acceptor in Photosystem I, to be chemically oxidized and reduced under the assay conditions.

Contribution to the structural characterization of eucaryotic PSI reaction centre-I. Critical analysis of the polypeptidic composition of different P700 enriched fractions

In thylakoid membranes, several peptides of high MW† are present which may interfere with the study of CP1's components. Modifying Cleveland's technique [7] for limited proteolysis, we have characterized the polypeptides found in the 60 kD region. Some may result from incomplete washing of the CF1 while others come from the CP1; indeed, this chlorophyll protein complex, which has a higher MW (above 100 kD), very often undergoes a dissociation into smaller components of about 60 KD MW.Analysis of the protein content of different preparations commonly used to obtain PSI reaction centre enriched fractions has been performed. The α and β subunits of CF1 are among the main contaminants of most of these preparations. A further purification step is described which can be applied to all these preparations, but numerous peptides are still present in the active fractions. It is most unlikely that all these polypeptides are required for the primary photochemical event, and this emphasizes the necessity to find a new simple method to purify PSI reaction centres.

Contribution to the structural characterization of eucaryotic PSI reaction centre - II. Characterization of a highly purified photoactive SDS-CP1 complex

Under precise conditions, SDS PAGE† allows purification of a photoactive P700-chla-protein complex from eucaryotic cells. The yield of P700 recovery is close to 100%. A total protein content equivalent to about 140 kD for one mole of P700 has been estimated by chemical analysis, and electrophoresis revealed the presence of two peptidic chains with MWs close to 65 kD. Photochemical and structural properties of this complex are given and compared with those of other complexes previously isolated.

The electron spin relaxation of the electron acceptors of photosystem I reaction centre studied by microwave power saturation

Photosystem I particles from spinach were reduced by illumination at 77 K. Under these conditions the one-electrom transfer from P-700 resulted in a reduction of only one acceptor molecule of the reaction centre. The EPR signals at g=2.05, 1.94 and 1.86 were attributed to reduced centre A and the smaller signals at g=2.07, 1.92 and 1.89 to reduced centre B. Reduction of both centres by dithionite in the dark lead to signals at g=2.05, 1.99, 1.96, 1.94, 1.92 and 1.89. Thus, the features at g=2.07 and 1.86 disappeared and new signals at g=1.99 and 1.96 were observed. From the spectral changes it followed that the iron-sulphur centres A and B interact magnetically. Temperature dependent EPR spectra demonstrated a faster electron spin relaxation of centre A than of centre B. These conclusions were corroborated using microwave power saturation of the respective EPR signals. The saturation data of the fully reduced centres A and B could not be fitted using the saturation equation for a one-electron spin system. The magnetic interaction between the (4Fe-4S) CENTRes of the electron acceptors A and B resulted in saturation properties which are simular to those of the 2(4Fe-4S) ferredoxin from Clostridium pasteurianum. For centre X a high proportion of homogeneous broadening of the EPR lines was inferred from the inhomogeneity parameter (b=1.83). It was, therefore, concluded that centre X is most probably an anion radical of chlorophyll. From the low temperature necessary for observing the EPR signal of centre X followed that the drastic relaxation enhancement has to be attributed to a magnetic interaction of the anion radical with iron.

The role of chloroplast-membrane-protein synthesis in the circadian clock. Purification and partial characterization of a polypeptide which is suggested to be involved in the clock

A polypeptide (polypeptide P39), which is presumed to involved in the photosynthetic circadian rhythm in the green alga Acetabularia, was purified from the EDTA-insoluble chloroplast membrane fraction by means of preparative dodecylsulfate gel electrophoresis and then partially characterized. The purity of the isolated polypeptide P39 was confirmed by a further electrophoresis on an analytical dodecylsulfate gel and further elucidated by amino-terminal analysis which shows that glycine is the only amino-terminal amino acid of the purified polypeptide material. The molecular weight of the polypeptide P39 was found to be about 39,000 on analytical gel electrophoresis and the value was further supported by those obtained from amino acid composition and peptide mapping. The amino acid composition of polypeptide P39 showed that the proportion of intermediate amino acid groups is high while the proportion of hydrophilic amino acid groups is well balanced by that of hydrophobic amino acid groups, a property characteristic of membrane proteins.

Isolation and characterization of photosystem I and II membrane particles from the blue-green alga, Synechococcus cedrorum

Fractions enriched in either Photosystem I or Photosystem II activity have been isolated from the blue-green alga, Synechococcus cedrorum after digitonin treatment. Sedimentation of this homogenate on a 10--30% sucrose gradient yielded three green bands: the upper band was enriched in Photosystem II, the lowest band was enriched in Photosystem I, while the middle band contained both activities. Large quantities of both particles were isolated by zonal centrifugation, and the material was then further purified by chromatography on DEAE-cellulose. The resulting Photosystem II particles carried out light-induced electron transport from semicarbizide to ferricyanide of over 2000 mumol/mg Chlorophyll per h (which was sensitive to 3-(3,4-dichlorophenyl)-1, 1-dimethylurea), and was nearly devoid of Photosystem I activity. This particle contains beta-carotene, very little phycocyanin, has a chlorophyll absorption maximum at 675 nm, and a liquid N2 fluorescence maximum at 685 nm. The purest Photosystem II particles have a chlorophyll to cytochrome b-559 ratio of 50 : 1. The Photosystem I particle is highly enriched in P-700, with a chlorophyll to P-700 ratio of 40 : 1. The physical structure of the two Photosystem particles has also been studied by gel electrophoresis and electron microscopy. These results indicate that the size and protein composition of the two particles are distinctly different.

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

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

The field of possible structures for the chlorophyll a dimer in photosystem I of green plants delineated by polarized photochemistry

Photoselection experiments with immobilized photosystem I particles have been done to determine the mutual orientation of pigments in the reaction centre. When these particles are excited and interrogated with linearly polarized light, the flash-induced transient absorption changes (mainly from the chlorophyll a dimer) reveal linear dichroism, which yields information on the mutual orientation between the excited and the interrogated transition moments. The interpretation of the data, however, is ambiguous, (1) for reasons of principles inherent in the photoselection technique when applied to complex systems and (2) because of incomplete knowledge about the relative contribution of x- and of y-polarized transitions of chlorophyll a to absorption or to absorption changes at a given wavelength. We find it impossible to attribute any particular structure to the photooxidizable dimer based on photoselection data alone. Instead we present a field of possible structures, imposing constraints on proposed models for the dimer structure.

The light-harvesting chlorophylla a/b.protein complex of the green alga Acetabularia mediterranea. Isolation and characterization of two subunits

In the green alga Acetabularia mediterranea a light-harvesting chlorophyll a/b.protein complex of 67 000 daltons has been found which contains two polypeptide chains of 21 500 and 23 000 daltons. These two polypeptides were isolated on a preparative scale and were further characterized by several different methods. Both polypeptides proved to be very similar. While their amino acid and sugar compositions as well as their immunochemical properties were almost identical the tryptic peptides and the cyanogen bromide fragments of the two polypeptides revealed minor but significant differences. The 67 000-dalton chlorophyll a/b.protein complex and its two polypeptide components were compared to the light-harvesting chlorophyll a/b.protein of higher plants.

Electrophoretic and spectrophotometric studies of chlorophyll-protein complexes from tobacco chloroplasts. Isolation of a light harvesting pigment protein complex with a molecular weight of 70,000

Abstract— …After a short‐term solubilization with sodium dodecyl sulphate, chloroplast membranes of tobacco were separated by polyacrylamide gel electrophoresis into three chlorophyll‐protein complexes. In addition to the two major complexes termed I and IIc corresponding respectively to P700 chlorophyll a‐protein and light‐harvesting chlorophyll a/b‐protein described by Thornber (1975), a relatively stable complex termed IIa has been observed. This new complex has an apparent molecular weight of 70,000 daltons and possesses Chl a and b.

Complexes I, IIa and IIC have been isolated and precise spectroscopic analyses have been performed. Fourth derivative analyses of low temperature absorption spectra suggest that complex IIa seems more representative than IIC of chlorophyll a forms present in intact thylakoid membranes.

Moreover, the electrophoretic study reveals that CPI and CPII are composed of only one polypeptidic subunit with respective molecular weights of 68,000 and 24,000 daltons.

Composition of a photosystem I chlorophyll protein complex from Anabaena flos-aquae

The use of Triton X-100 to solubilize membrane fragments from Anabaena flos-aquae in conjunction with DEAE cellulose chromatography allows the separation of three green fractions. Fraction 1 is detergent-solubilized chlorophyll, and Fraction 2 contains one polypeptide in the 15 kdalton area. Fraction 3, which contains most of the chlorophyll and shows P-700 and photosystem I activity, shows by SDS gel electrophoresis varying polypeptide profiles which reflect the presence of four fundamental bands as well as varying amounts of other polypeptides which appear to be aggregates containing the 15 kdalton polypeptide. The four fundamental bands are designated Band I at 120, Band II at 52, Band III at 46, and Band IV at 15 kdaltons. Band I obtained using 0.1% SDS contains chlorophyll and P-700 associated with it. When this band is cut out and rerun, the 120 kdalton band is lost, but significant increases occur in the intensities of Bands II, III, and IV as well as other polypeptides in the 20-30 kdalton range. The use of 1% Triton X-100 coupled with sucrose density gradient centrifugation allows the separation of three green bands at 10, 25 and 40% sucrose. The 10% layer contains a major polypeptide which appears to be Band IV. The 25 and 40% layers show essentially similar polypeptide profiles, resembling Fraction 3 in this regard, except that the 40% layer shows a marked decrease in Band III. Treatment of the material layering at the 40% sucrose level with a higher (4%) concentration of Triton X-100 causes a loss (disaggregation) of the polypeptides occurring in the 60-80 kdalton region and in increase in the lower molecular weight polypeptides. Thus, aggregation of the lower molecular weight polypeptides accounts for the variability seen in the electrophoresis patterns. Possible relations of the principal polypeptides to the known photochemical functions in the original membrane are discussed.

Appearance and composition of chlorophyll-protein complexes I and II during chloroplast membrane biogenesis in Chlamydomonas reinhardi y-1

The chlorophyll-protein complexes I and II have been isolated and anlyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis during greening and degreening of Chlamydomonas reinhardi y-1. At all stages of membrane formation, the complexes, when present, have a constant composition. Chlorophyll-protein complex I consists of a major polypeptide(s) of molecular weight 64,000 synthesized in the chloroplast, to which about 29 chlorophyll a molecules are bound. The complex is not detected when other polypeptides of chloroplastic origin, related to both Photosystem I and Photosystem II activities, are not synthesized. However, Photosystem I activity can develop in membranes in which chlorophyll-protein complex I is not detectable. Chlorophyll-protein complex II consists of two polypeptides of cytoplasmic origin, molecular weights 24,000 and 22,000, which bind 12 chlorophylls (a and b). The chlorophyll-protein complex II can be detected in membranes in which the development of photosystem II activity is prevented. Clipping of a Mr = 2000 fragment(s) from the Mr = 22,000 polypeptide following trypsin digestion of membranes, does not affect the complex. The detection of the complexes is possible only in membranes in which the simultaneous synthesis of both the chlorophyll and the corresponding polypeptides occurs. The 28,000 dalton polypeptide, reported to be present in the chlorophyll-protein complex II, comigrates with the complex but apparently is not part of the complex itself. The apparent molecular weight of the chlorophyll-protein complexes I and II are 88,000 and 28,000, respectively. The minimal true value for complex I is 89,000 or 154,000 and for complex II is 56,000.

[Photosynthetic activity in the absence of CPl and CP2 pigmentary complexes (author's transl)]

Various photochemical activities were tested on chloroplasts of Zea mays that received 4 s of light every 4 h during the culture period. Photosystem I and Photosystem II were functioning, as well as the photosynthetic electron transport. These chloroplasts exhibited upon sodium dodecyl sulphate gel electrophoresis neither Complex 1 (Mr 70 000) generally associated with Photosystem I nor Complex 2 Mr 25 000) generally associated with Photosystem II. Chlorophyll is indeed attached to polypeptides of molecular weight 21 000 and 29 000. These results lead us to question the functional role of chloroplast protein-pigment complexes observed by sodium dodecyl sulphate gel electrophoresis.

PRoperties of the low-temperature photosystem I primary reaction in the P-700-chlorophyll alpha-protein

The Photosystem I primary reaction, as measured by electron paramagnetic resonance changes of P-700 and a bound iron-sulfur center, has been studied at 15 degrees K in P-700-chlorophyll alpha-protein complexes isolated from a blue-green alga. One complex, prepared with sodium dodecyl sulfate shows P-700 photooxidation only at 300 degrees K, whereas a second complex, prepared with Triton X-100, is photochemically active at 15 degrees K as well as at 300 degrees K. Analysis of these two preparations shows that the absence of low-temperature photoactivity in the sodium dodecyl sulfate complex reflects a lack of bound iron-sulfur centers in this preparation and supports the assignment of an iron-sulfur center as the primary electron acceptor of Photosystem I.