Differential mechanism of light-induced and oxygen-dependent restoration of the high-potential form of cytochrome b559 in Tris-treated Photosystem II membranes

The effect of illumination and molecular oxygen on the redox and the redox potential changes of cytochrome b₅₅₉ (cyt b₅₅₉) has been studied in Tris-treated spinach photosystem II (PSII) membranes. It has been demonstrated that the illumination of Tris-treated PSII membranes induced the conversion of the intermediate-potential (IP) to the reduced high-potential (HP^Fe2+) form of cyt b₅₅₉, whereas the removal of molecular oxygen resulted in the conversion of the IP form to the oxidized high-potential (HP^Fe3+) form of cyt b₅₅₉. Light-induced conversion of cyt b₅₅₉ from the IP to the HP form was completely inhibited above pH 8 or by the modification of histidine ligand that prevents its protonation. Interestingly, no effect of high pH or histidine modification was observed during the conversion of the IP to the HP form of cyt b₅₅₉ after the removal of molecular oxygen. These results indicate that conversion from the IP to the HP form of cyt b₅₅₉ proceeds via different mechanisms. Under illumination, conversion of the IP to the HP form of cyt b₅₅₉ depends primarily on the protonation of the histidine residue, whereas under anaerobic conditions, the conversion of the IP to the HP form of cyt b₅₅₉ is driven by higher hydrophobicity of the environment around the heme iron resulting from the absence of molecular oxygen.