Quinone, iron-complexed, donor

When the components of the PS II reaction centre are drawn on a redox scale and compared in this way to those of the purple bacterial reaction centre, a remarkable similarity can be seen between the electron acceptors in each system (Fig. 4). The chemical natures of these components are extremely similar, being made up of a complex of two quinones, an iron atom and a pheophytin (a bacteriopheo-phytin in bacteria). The donor side of PS II in the redox scheme is, however, not comparable to that in bacteria. P-680 may appear to be structurally similar to P-870 in bacteria in that it is made up of chlorophyll (bacteriochlorophyll in bacteria) and that is acts as the primary electron donor however, the P-680/P-680+ redox couple is approximately 600-800 mV more oxidizing than the equivalent bacterial redox couple P-870/P-870, = +450 mV). In addition, PS II has an array of high-potential components which make up the 02-evolving enzyme and which are clearly unique to that system. 

The first electron from the reduced quinone are transferred to a -type cytochrome and a second electron to the Rieske iron-sulfur protein and cytochrome c, in the cytochrome-6Cj complex. These electrons are subsequently transferred to cytochrome C2 on the periplasmic side ofthe plasma membrane and finally go to reduce the oxidized primary electron donor P870 to complete the cyclic reaction. 

The protein portion of the reaction center (RC) complex is composed of three subunits the intermembrane L and M chains, and the cytoplasmic H polypeptide. The cofactors are within the transmembrane region they are related by approximate twofold symmetry as are the homologous L and M chains [1-4]. The cofactors consist of a bacteriochlorophyll dimer that is the primary electron donor, two bacteriochlorophyll monomers, two bacteriopheophy-tins, a non-heme iron atom, and two quinones which serve as the final electron acceptors. 

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