Photosystem II

In PSII, water is oxidized by O2 in OEC. This type of photosynthesis is called oxygenic photosynthesis. OEC contains four manganese ions, in oxidation states varying between +3 and +4, and one calcium ion (Ca +). Oxygen may be regarded as waste product. In fact, if it happens to become oxidized to singlet oxygen, it threatens to destroy the system. The structure contains carotenes for protection. Nevertheless, the lifetime of PSII is fairly short ( 1 day). 

The excitation moves by EET in the antenna system until it reaches the SP, which acts as a very shallow trap. Charge separation takes place within the RC, probably due to the slightly smaller distance between the chromophores. Subsequently, the electron moves to Pheoi via the accessory Chip,. It seems that the ETs are only on one side (Dl) of the dimeric RC, just as in the bacterial RC. 

FIGURE 15.10 PSII reaction center. All protein structures and all hydrogen atoms are left out. One of the P-carotenes are shown. 

FIGURE 15.11 Oxygen-evolving complex (OEC). Wx are water molecnles. Large white is Ca. Dark is oxygen and grey manganese (Mnx). The picture is copied from the work of Umena et al. 

When the electron has left Tyr2, a neutral radical remains. It is believed that this radical picks up an electron and a proton from a water molecule and thus takes a direct part in the oxidation of water. 

The primary donor of photosystem II (P-680) is much more difficult to observe with EPR than that of PS I, because in normally functioning PS II the photooxi-dized donor is very rapidly (within at most a few hundred ns) rereduced by an electron donor called Z [21,22] (see for a review Refs. R3 and R4). This (re)reduction can be slowed down by various treatments and by cooling to low temperature. In intact chloroplasts in which P-700 is fully oxidized chemically or 

By far the most important role of manganese in nature is its direct involvement in the photocatalytic, four-electron oxidation of water to dioxygen in green plant photosynthesis, an essential process for the maintenance of life. Pirson, in 1937, first discovered the requirement of manganese in photosynthesis by showing that plants grown in a Mn-deficient medium lost their water oxidation capacity (184). During the next four decades, several researchers showed that two photosystems, photosystem I (PSI) and photosystem II (PSII), were involved in photosynthesis and that 02 evolution and Mn were localized at PSII (for a review, see Ref. 185). 

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). 

Amino acid analysis (196) and a complete amino acid sequence of the 33-kDa protein from spinach (201) indicated that no histidine was present. Thus, Mn ligation to this polypeptide must be to tyrosine phenoxide and/or to the carboxylate functionalities of aspartate and glutamate. 

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). 

However, Lavergne (240) and Renger and Weiss (241) presented alternative interpretations of the difference spectral data. To differentiate between the various proposals, Saygin and Witt (242) measured the absorbance difference spectra in the presence and absence of low concentrations of hydroxylamine, which shifts the S states by reduction backward two units. These experiments confirmed the +1, + 1, +1, — 3 redox change pattern. 

Another way of stating this relationship is that P680 has a more negative reduction potential than does P680. The following equation gives the free energy of the reduction reaction. 

From the quinones, the electron is transferred to plastocyanin and then to cytochrome bf. The two H+ ions (protons) left behind remain in the thylakoid lumen. As the electrons move down this electron transport chain, protons are pumped into the thylakoid lumen. Eventually the transported electron is given up to the oxidized P700 chlorophyll of Photosystem I. 

This movement of an electron to PS I from P680 leaves P680 in a non-excited, oxidized state. Oxidized P680 must be reduced to give up another electron. Hydrogen atoms derived from H2O reduce it according to the reaction  

This oxidation transfers four electrons to the Manganese Center, a complex metalloprotein, which then donates the electrons through an intermediate to oxidized P680. The protons derived from water are transported into the thylakoid lumen. The protons pumped into the thylakoid lumen by PSII are used to make ATP through the action of coupling factor, in a mechanism similar to that of mitochondrial ATP synthesis. 

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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. 

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. 

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... 

Weis, E. Berry, J. A. (1987). Quantum efficiency of photosystem II in relation to energy -dependent quenching of chlorophyll fluorescence. Biochimica Bio-physica Acta, 894, 198-208. 

Davis, M.S., Forman, A., and Fajer, J., Ligated chlorophyll cation radicals their function in photosystem II of plant photosynthesis, PNAS, 76, 4170, 1979. 

Langmuir and Langmuir-Blodgett Films of Chlorophyll a and Photosystem II Complex... 

El Bissati, K., E. Delphin, N. Murata, A. Etienne, and D. Kirilovsky (2000). Photosystem II fluorescence quenching in the cyanobacterium Synechocystis PCC 6803 Involvement of two different mechanisms. Biochim Biophys Acta 1457(3) 229-242. 

Rakhimberdieva, M. G., I. N. Stadnichuk, I. V. Elanskaya, and N. V. Karapetyan (2004). Carotenoid-induced quenching of the phycobilisome fluorescence in photosystem II-deficient mutant of Synechocystis sp. FEBS Lett 574(1-3) 85-88. 

Havaux, M. and F. Tardy. 1996. Temperature-dependent adjustment of the thermal stability of photosystem II in vivo Possible involvement of xanthophyll-cycle pigments. Planta 193 324—333. 

Structure of the Photosystem II Antenna Xanthophylls in LHCII Structure.117... 

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