Cofactors with PSII

Figure 23-34 Structure of PSII with assignment of protein subunits and cofactors. (A) Arrangement of transmembrane a-helices and cofactors in PSII. One monomer of the dimer is shown completely, with part of the second monomer related by the local-C2 axis (filled ellipse on the dotted interface). Chlorophyll a head groups and hemes are indicated by black wire drawings. The view direction is from the luminal side, perpendicular to the membrane plane. The a-helices of Dl, D2, and Cyt b-559 are labeled. D1/D2 are highlighted by an ellipse and antennae, and CP43 and CP47 by circles. Seven unassigned a-helices are shown in gray.

PSII has been extensively investigated and a great deal of information is now available regarding its reactivity,mechanism " and structure. " " Figure 5.3 depicts the structure and arrangement of the main cofactors of PSII in cyanobacteria Thermosynechococcus elongatus, where the sequence of electron transfer processes relevant for water oxidation is indicated with arrows. The overall redox process can be described as... 

In photosynthesis, water oxidation is accomplished by photosystem II (PSII), which is a large membrane-bound protein complex (158-161). To the central core proteins D1 and D2 are attached different cofactors, including a redox-active tyro-syl residue, tyrosine Z (Yz) (158-162), which is associated with a tetranuclear manganese complex (163). These components constitute the water oxidizing complex (WOC), the site in which the oxidation of water to molecular oxygen occurs (159, 160, 164). The organization is schematically shown in Fig. 18. 

Figure 16.6 Arrangement of cofactors in the dimeric (D1 and D2) subunits of PSII. The dashed line running through the non-haem Fe is the two-fold axis of pseudosymmetry. Thin lines show the centre-to-centre distances in A between cofactors. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

Figure 2. Arrangements of Chls and other PSII cofactors and their relative distances (A). A) View along the membrane plane. B) View from the lumenal side perpendicular to the membrane plane. The non-heme iron and two pheophytins are omitted for clarity. (Reproduced, with permission, from Kamiya and Shen, 2003. )...

Ca is an essential cofector in oxj en evolution. Depleting this cofactor suppresses OEC activity, which can be restored (up to 90%) by replenishing with Ca. Various cations compete vdth calcium for its binding site(s) in PSII. Sodium, potassium and cesium are weakly competitive with calcium, but they do not suppon oxygen evolution activity. Partial reactivation (up to 40%) results from addition of strontium to Ca-depleted PS II membranes and no other metal ions (excqjt VO, vanadyl ion) can restore activity. ... 

Photosystem II (PSII) is a large, heteromeric enzyme complex with more than twenty different protein subunits and an array of cofactors, that participate in the photosynthetic electron transport in the thylakoid membranes of chloroplasts and cyanobaaeria. PSII demonstrates the oxido-reductase aaivity and couples the oxidation of H2O with the reduction of plastoquinones through a series of intermediate redox reactions. The central core of the PSII reaction center is composed of D1 and D2 proteins associated >vith redox active components. These compounds include a tetra-manganese cluster, two redox-active residues, four to six chlorophyll a molecules, two pheophytins, and plastoquinones and The secondary plastoquinone Qg is a two-electron carrier which... 

By far, the dominant contributions to electronic absorption spectra of PSII arise from the pigment cofactors such as chi and carotenoids. Nonetheless, absorption features in the UV and NIR have been attributed to the Mu4 cluster. The period-four oscillations in the UV associated with the S-state transitions " " are shown in Figure 11 as difference spectra between successive... 

In contrast to the interaction of atrazine with Mn2+ described above, the interaction of atrazine with the two other inorganic cofactors (Ca and Cl") of PSII is different. It was observed that in NaCl-treated PSII preparations atrazine inhibition was higher compared to the native counterparts (Fig. 3A). However, readdition of the above two ions to the depleted preparations reverses the atrazine inhibition (Fig. 3B). These results indicate that Ca " " and/or Cl" display some parts of protective mechanism against atrazine inhibition. 

Ca " is an indispensable cofactor for O2 higher plant PSII Cl-33. The increased Ca has been attributed to release of Ca " ", although the role 5f still unclear Cl-4]. In this study, we report a unique property of the S2 state generated in spinach PSII stringently treated with NaCl under illumination followed by EDTA addition. 

Figure 5.3 Scheme of the structure of PSII with main cofactors and electron transfer processes involved in water oxidation. From the X-ray structure of cyanobacteria Thermosynechococcus vulcanus, with a resolution of 2.9A (X-ray structure PDB 3BZ1). ... 

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