Photosystem particle

The capacity of photosystem particles to bind to Ti02 particles and retain their photosynthetic activities was studied before immobilizing the particles on to the electrodes. Both PSI and PSII were able to bind Ti02 powder with retention of 80% PSI activity and 50% PSII activity. 

We acknowledge with thanks the co-operation of M.Brouers and D-J.Shi in the ammonium production studies, J.Hubbard and L.Tilling in the preparation of photosystem particles and N.Vlachopoulos and... 

If these two photosystems are located close to each other, the higher excitation energy of PS II (p680 versus p700) would pass a large fraction of its absorbed photons to PS I, via exciton transfer. These two particles are placed 10 nm away, thus eliminating this difficulty [37],... 

Photosystem II. Spinach and pea PSII particles coated on different Ti02 based electrodes were used for photocurrent measurements in the presence of PSII electron acceptor DMBQ. In all experiments, addition of DMBQ resulted in an increase in photocurrent which remained constant for long periods. In control experiments with no deposition of PSII on the electrodes, there was no change in the photocurrent pattern on addition of DMBQ. Addition of the PSII oxygen evolution inhibitor DCMU caused an immediate fall in photocurrent, suggesting that the electron transport to the Ti02 electrode is linked to water photolysis. 

Ortega, J.M., Hervas, M. and Losada, M. 1987. Redox and acid-base characterization of cytochrome b-559 in photosystem II particles. Eur. J. Biochem. (submitted). 

In contrast to LHCI, the light-harvesting chlorophyll a/b-antennae complex of photosystem II (LHCII) is the major component of the particles on the complementary protoplasmic fracture face of appressed membranes (PFs) (Simpson, 1979, Olive et al., 1979), and does not appear to be a significant component of the reaction centre EFs particles, although this is disputed. The LHCII in PFs particles is, nevertheless, in contact with the reaction centre particles and may provide a pathway for excitation energy transfer between several photosystem II reaction centres. 

FIGURE 4. Freeze-fracture electron micrographs of thylakoids from (a) wild type, (b) the photosystem II mutant viridis —115 and (c) the chlorophyll b-less mutant chlorina-f2. The EFs particles found in the wild type (a) are almost all missing from the photosystem II mutant (b), indicating that the photosystem II reaction centre is located in these particles. The PFs particles are missing from the chlorophyll b-less mutant (c), showing that LHCII, the major chlorophyll-containing protein is the major constituent of the PFs particles in wild type thylakoids. 

Miller, K.R. and Cushman R.A. 1979. A chloroplast membrane lacking photosystem II. Thylakoid stacking in the absence of the photosystem II particle. Biochim. Biophys. Acta 546.481 197. 

A relatively simple and quick procedure for the isolation of Photosystem I-enriched particles from the thermophilic cyanobacterium Phormidium laminosum, without the use of detergents for solubilization, is described. The procedure involves sonication of cells, centrifugation and DEAE-cellulose chromatography. The particles had an 02 uptake activity of up to 200 pmol 02. mg chlorophyll h 1 and appeared as vesicles of 200 100 nm diameter when observed under electron microscopy. The analysis of the chlorophyll-protein complexes by polyacrylamide gel electrophoresis showed that these particles are enriched in the complexes associated with Photosystem I and partially depleted in those associated with Photosystem II. The particles did not contain ferredoxin and were active in NADP-photoreduction only in the presence of added ferredox in. They were also able to photoreduce externally added electron mediators using ascorbate as electron donor, the reduced mediators can be coupled to hydrogenase for the production of H2 or for the activation of cyanobacterial phosphoribulokinase using a ferredoxin/thioredoxin system. 

Newman, PJ. and Sherman, L.A. 1978. Isolation and characterization of the photosystem I and II membrane particles from the blue-green alga Synechococcus cedrorum. Biochim. Biophys. Acta, 503. 343-361. 

Serra, J.L., Llama, M.J., Rao, K.K. and Hall, D.O. 1986. B-5 Hydrogen photoproduction using photosystem I-enriched particles from Phormidium laminosum. Book of Abstracts 6th International Conference on Photochemical Conversion and Storage of Solar Energy, Lab. Biophysique, INSERM, Paris. 

Advances in the study of photosynthetic manganese and the water oxidation complex have been accelerated by the development of techniques for the isolation of photosystem II particles by Triton-X and/or digitonin treatment of thylakoid membranes (188,189). Freeze-fracture electron microscopy indicates the particles are highly purified membrane fragments almost entirely devoid of photosystem I components (190). The lumenal side of the photosystem II membrane is exposed, allowing direct access to the water oxidation enzyme complex. These PSII preparations contain four atoms of manganese per PSII reaction center and possess large amounts of 02 activity (191, 192). 

Iodolabeling studies on photosystem II particles from higher plants and cyanobacteria (221) and on a PSII complex (227) specifically labeled the herbicide-binding protein. As 1 is believed to donate electrons to Z, the secondary electron donor which is believed to accept electrons from the photosynthetic manganese complex, these experiments indicate a role for this protein on the oxidizing side of PSII. Consequently, Z must at least be located near, if not in, the herbicidebinding polypeptide (222). 

The primary acceptor in PS II is a plastoquinone, PQ, as ascertained from optical absorbance difference spectroscopy [46], Until recently, the EPR spectrum of the semiquinone escaped observation, and only the advent of preparation methods for PS II subchloroplast particles made its recording possible. As surmised earlier, the spectrum of the intact acceptor [47] very much resembled the very broad qui-none-iron acceptor complex in purple bacteria, whereas in iron-depleted PS II particles the narrow spectrum typical of an immobilized semiquinone was found [48], As in the bacterial photosystem, flash-induced reduction of Q, of the second quinone, Qb, or of both resulted in somewhat different EPR spectra, indicative of structural changes that influence the magnetic interaction between the semiquinone and the non, and/or between the two semiquinones [49],... 

The four fracture faces in Pig. 18 (C) show imbedded particles of various sizes. In particular, the EFg face shows a profusion of 16 nm-diameter particles. The complementary fracture face PFs contains relatively tightly-packed, but rather indistinct and smaller particles. It is known that fractionation of a thylakoid membrane disrupted by detergent or by mechanical means yields separate granal and stromal fractions. The granal fraction has been found to be enriched in photosystem-II components and activity. 

Fig. 19. Chloroplast thylakoid-membrane structure revealed by freeze-fracture electron microscopy. The oxygen-evolving (BBY) PS-II particle its preparation (A) and electron micrographs (B). The inside-out and rightside-out vesicles preparation, structure, and properties (C) and electron micrographs (D). Figure source (A) and (B) Dunahay, Staehelin, Seibert, Ogilvie and Berg (1984) Structural, biochemical and biophysical characterization of four oxygen-evolving photosystem II preparations from spinach. Biochim Biophys Acta 764 190, 185 (C) and (D) from Andersson and Akerlund (1978) Inside-out membrane vesicles isolated from spinach thylakoids. Biochim Biophys Acta 503 465, 468. Figure (B) kindly furnished by Dr. Andrew Staehelin.

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