Oxygen evolving centre

Noguchi T. FTIR detection of water reactions in the oxygen-evolving centre of photosystem II. Phil Trans R Soc B. 2008 363(1494) 1189-95. 

Barber J, Ferreira K, Maghlaoui K, Iwata S. Structural model of the oxygen-evolving centre of photosystem II with mechanistic implications. Phys Chem Chem Phys. 2004 6 (20) 4737 42. 

The enzyme Photosystem II (PSII) is responsible for the conversimi of H2O to O2 during photosynthesis. Reaction 21.54 takes place in the oxygen evolving centre (OEC) in PSII, the active site consisting of a cubane-Uke Mn3Ca04-unit linked to an Mn spike (Fig. 21.19a). The structure was elucidated in 2004 through an X-ray diffraction study of PSII isolated from the cyanobacterium Thermosynecho-coccus elongatus. ... 

The structure of the reaction centre complexes appears to be well conserved in organisms carrying out oxygen-evolving photosynthesis. In contrast, the organization of antenna complexes seems to be more varied. 

In all oxygen-evolving organisms, the PS I reaction centres finally reduce a water-soluble ferredoxin. This small protein of around 10 kDa has a (2Fe-2S) cluster and a rather low midpoint reduction potential of -400 mV. Ferredoxin binds to the PS I centre after reduction it participates both in linear electron flow to NADP, via ferredoxin-NADP reductase, and in cyclic electron flow around the PS I centre. Two membrane-bound iron-sulfur centres, designated Centre A (or F, ) and Centre B (or Fg), appear to be the terminal acceptors in the reaction centre. Their mode of functioning is not clearly established and their structure is not well known, mainly because they cannot be extracted without their complete denaturation. F and Fq can be photoreduced at low temperature in cells or in purified PS I centres. Characteristic EPR spectra are thus obtained with g values of 1.86, 1.94, 2.05 for F, and 1.89, 1.92, 2.05 for F -. 

The TPD spectrum carried out on a fresh catalyst, is shown in Fig. 1. Four oxygen desorption peaks are observed a first small peak centred at Tm= 112°C, two distinct peaks at T ,= 478°C and Tm= 725°C, and a shoulder at T ,= 765°C. The temperatures T and the total amount of oxygen desorbed are indicated in Table 1 (column 2). The amounts of oxygen evolved in each peak (assumed symmetric) are respectively 0.07, 5, 7 and 12 pmoleO/g. The cerium content of this solid is 3.6 weight % (around 250 pmoles of Ce02 oxide/g of catalyst). 

The modification of these residues would be only on the peripheral proteins as we have used BBY particles. The exposed polypeptides are the oxygen evolving polypeptides (33, 24 and 18 kDa) and the chlorophyll antenna proteins, but not the reaction centre core (D-l/D-2 proteins). However, it may be possible that modification of peripheral polypeptides caused some structural alterations in... 

Oxygen evolving photosystem two (PS2) particles isolated using Triton X-100 consist of a number of polypeptides.These include the polypeptides termed D1 and D2 and the two apoproteins of cytochrome b559.The proteins D1 and D2 which bind the primary electron transport components show considerable sequence homology with the purple bacterial reaction centre proteins L and M. Computer models have indicated that D1 and D2 each have five transmembrane helices, and that the binding regions for electron acceptors Qa and Qb reside in the loops between helices IV and V on the stromal side of the thylakoid membrane [1]. 

Figure 1.1. Simplified scheme showing electron transport in a portion of a chloroplast thylakoid membrane. Electrons flow from water via an oxygen-evolving complex (OEC) to photosystem II (PS2), pheophytin (PHEO), plastquinone (PQ), plastocyanin (PC) to photosystem I (PSI). Aq, Chlorophyll FeS, iron-sulfur centres FD, ferredoxin. Phosphorylation is catalyzed by proton transport through a transmembrane proton channel (CFq) to the ATP-synthetase complex (CF,).

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