Copper effect on cytochrome b of photosystem II under photoinhibitory conditions

Toxic Cu (II) effect on cytochrome b(559) under aerobic photoinhibitory conditions was examined in two different photosystem II (PSII) membrane preparations active in oxygen evolution. The preparations differ in the content of cytochrome b(559) redox potential forms. Difference absorption spectra showed that the presence of Cu (II) induced the oxidation of the high-potential form of cytochrome b(559) in the dark. Addition of hydroquinone reduced the total oxidized high-potential form of cytochrome b(559) present in Cu (II)-treated PSII membranes indicating that no conversion to the low-potential form took place. Spectroscopic determinations of cytochrome b(559) during photoinhibitory treatment showed slower kinetics of Cu (II) effect on cytochrome b(559) in comparison with the rapid loss of oxygen evolution activity in the same conditions. This result indicates that cytochrome b(559) is affected after PSII centres are photoinhibited. The high-potential form was more sensitive to toxic Cu (II) action than the low-potential form under illumination at pH 6.0. The content of the high-potential form of cytochrome b(559) was completely lost; however, the low-potential content was unaffected in these conditions. This loss did not involve cytochrome protein degradation. The results are discussed in terms of different binding properties of the heme iron to the protonated or unprotonated histidine ligand in the high-potential and low-potential forms of cytochrome b(559), respectively.