Photosystems

An important part of the analysis of the structure of the WOC reviewed by Peloquin and Britt is a multi-author report by these and other workers who studied CW EPR (perpendicular and parallel mode) at X-band and Mn ESE-ENDOR (Davies) at ca. 10 GHz on the S2 EPR signal of PS II, out of which arose the trimer-monomer model. It is reported that four effective Mn hyperfine matrices are required to simulate the experimental data, by using a constrained approach to the simultaneous simulations of both types of experiment. Thus, for untreated and MeOH-treated PS II centres, the effective hyperfine components Ax, Ar, Az) in MHz are - 232, -232, - 270 200,200,250 - 311, - 311, - 270 180,180, 240, and for NHs-treated PS II the equivalent effective parameters are 208, 208, 158 -150, -150, -112 222, 222, 172 -295, -315, -390 MHz. Importantly, it was shown that the light-minus-dark S2-state spectrum measured with electron spin echo corresponded to that measured by CW EPR. 

The model compounds [(phen)2Mn 02Mn (phen)2](C104)3 and (Mn - 

An important addition to the literature of EPR spectrum simulation was presented as an Appendix in this work. A spin system consisting of one 5 = 3 and four / = I results in an energy matrix of order 2592. Even with the approximation of collinear matrices for electronic Zeeman and hyperfine interactions, matrix diagonalisation techniques are effectively unrealistic. The authors argue that starting from an uncoupled spin-Hamiltonian representation  

In another study involving this group, Hanley et al. have investigated the orientation dependence with respect to the thylakoid membrane of the Mn(II)-Mn(III) EPR spectrum at 11 K and X-band frequency, which results from the reduction of OEC by NO. The signal is centred on g k 2 with rich hyperfine structure and extending over 1600 G. The same set of spin-Hamil-tonian parameters were used to simulate the powder and partially oriented spectra fifii = 2.011,0 = 1.970, for Mn(II) A, = -62MHz, -549 MHz, 

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Kothe G, Weber S, BittI R, Ohmes E, Thurnauer M and Norris J 1991 Transient EPR of light-induced radical pairs in plant photosystem I observation of quantum beats Chem. Rhys. Lett. 186 474-80... 

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

Both PSI and PSII are necessary for photosynthesis, but the systems do not operate in the implied temporal sequence. There is also considerable pooling of electrons in intermediates between the two photosystems, and the indicated photoacts seldom occur in unison. The terms PSI and PSII have come to represent two distinct, but interacting reaction centers in photosynthetic membranes (36,37) the two centers are considered in combination with the proteins and electron-transfer processes specific to the separate centers. 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Photosystem II Inhibitors. The PSII complex usually is assumed to be that stmctural entity capable of light absorption, water oxidation, plastoquiaone reduction, and generation of transmembrane charge asymmetry and the chemical potential of hydrogen ions (41). The typical PSII complex... 

The work presented is part of a European project (Biosensors for Effective Environmental Protection BEEP) which is aimed at the assembly and application of Photosystem II (PS II)-based biosensors for large scale environmental screening of specific herbicides and heavy metals. 

A method of detecting herbicides is proposed the photosynthetic herbicides act by binding to Photosystem II (PS II), a multiunit chlorophyll-protein complex which plays a vital role in photosynthesis. The inhibition of PS II causes a reduced photoinduced production of hydrogen peroxide, which can be measured by a chemiluminescence reaction with luminol and the enzyme horseradish peroxidase (HRP). The sensing device proposed combines the production and detection of hydrogen peroxide in a single flow assay by combining all the individual steps in a compact, portable device that utilises micro-fluidic components. 

Michel, H., Deisenhofer, J. Relevance of the photosynthetic reaction center from purple bacteria to the structure of photosystem II. BicKhemistry 27 1-7, 1988. 

The existence of two separate but interacting photosystems in photosynthetic eukaryotes was demonstrated through analysis of the photochemical action spectrum of photosynthesis, in which the oxygen-evolving capacity as a function of light wavelength was determined (Figure 22.10). 

All photosynthetic cells contain some form of photosystem. Photosynthetic bacteria, unlike cyanobacteria and eukaryotic phototrophs, have only one photosystem. Interestingly, bacterial photosystems resemble eukaryotic PSII more than PSI, even though photosynthetic bacteria lack Og-evolving capacity. 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

What is the for the light-generated primary oxidant of photosystem II if the light-induced oxidation of water (which leads to Og evolution) proceeds with a AG° of —25 kj/mol ... 

Ghanotakis, D. F, and Yocum, C. F, 1990. Photosystem II and die oxygen-evolving complex. Annual Review of Plant Physiology and Plant Molecular 41 255-276. 

Hankamer, B., Barber,/, and Boekema, E. J., 1997. Structure and membrane organization of photosystem II in green plants. Annual Review of Plant Physiology and Plant Molecular Biology 48 641—671. 

Kranss, N., et al., 1996. Photosystem I at 4 A resolution represents the first structural model of a joint photosynthedc reaction centre and core antenna system. Nature Structural Biology 3 965-973. 

Rogner, M., Boekema, E. J., and Barber, J., 1996. How does photosystem 2 split water The structural basis of energy conversion. Trends in Biochemical Sciences 21 44—49. 

Photosynthetic(II) chloroplasts, 2,773 Photosystem II dioxygen evolving centre manganese, 6, 586 manganese protein, 6, 590 Photothermography, 6, 118 Phthalamic acid, /V-(2-phenanthrolyl)-hydrolysis... 

Schmitt-Jansen M, Altenburger R (2005) Predicting and observing responses of algal communities to photosystem Il-herbicide exposure using pollution-induced community tolerance and species-sensitivity distributions. Environ Toxicol Chem 24 304... 

B. The Fa/Fb-Binding Subunit of RCI-Type Photosystems 1. Global Structure and Differences between Species... 

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