Comparison between Electron Transfers through Plastocyanin in Spinach Chloroplasts and Cytochrome c_2 in Rhodopseudomonas sphaeroides

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Photosynthetic electron transfers through the water-soluble peripheral membrane proteins of plastocyanin and cytochrome c_2, were studied in spinach chloroplasts and the photosynthetic bacterium Rhodopseudomonas sphaeroides. In spinach chloroplasts, the rate of flash-induced oxidation of cytochrome f was highly dependent on the salt concentration in the suspending medium. The maximum rate with a halftime of 200μs was observed in the presence of 50mM KCl or 5mM MgCl_2. The salt effect was similar to that on the reaction rate between P700 in thylakoid fragments and externally added plastocyanin. On the other hand, in intact cells of R. sphaeroides, in which cytochrome c_2 is located in the periplasmic space exposed to the outer ionic environment, the rate of cytochrome c_1 oxidation via cytochrome c_2 was almost independent of salt concentration. This independence was a contrast to the strong dependence on salt concentration of reactions between isolated reaction centers and cytochrome c_2. These results suggest that plastocyanin reacts collisionally with the photosystem I reaction center and cytochrome b_6 f complex in a manner that is controlled by the surface electrostatic potential. Cytochrome c_2, on the other hand, reacts with the bacterial reaction center and cytochrome bc_1 complex probably by forming a complex prior to activation of the reaction center.
日本植物生理学会の論文