Characterization of electron transfer from water to plastocyanin catalyzed by resolved electron transfer complexes from chloroplasts.

The photoreduction of soluble plastocyanin in a homologous reconstituted system requires a resolved Photosystem II preparation and the cytochrome b6-f complex from spinach chloroplasts. Oxygen evolution is linked quantitatively to plastocyanin reduction in the reconstituted system, indicating that water is the electron donor for the photoreduction. The photoreduction is sensitive to inhibitors which specifically interact with the Photosystem II preparation [DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea] as well as those which interact with the cytochrome complex (DNP-INT: 2-iodo-6-isopropyl-3-methyl-2',4,4'-trinitrodiphenyl ether; and DBMIB: 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone). The plastocyanin requirement in the reconstituted system indicated a Km of 4.6 microM and a Vmax of 4.2 mumols plastocyanin reduced per nmol cytochrome f per h. Inhibition curves for DNP-INT and DBMIB show that both are effective inhibitors (50% inhibition at a ratio of inhibitor to cytochrome f of 1.0). The rate of reduction of plastocyanin was found to be stimulated by cations. This dependency was localized in the transfer of electrons from the Photosystem II preparation to the cytochrome complex. These results are considered in terms of organizational aspects of integral protein complexes in thylakoid membranes.