Consequences of structural modifications in cytochrome b on the electron acceptor side of Photosystem II.

Cytb in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Mutants were previously designed and studied in Thermosynechococcus elongatus (Sugiura et al., Biochim Biophys Acta 1847:276-285, 2015) either in which an axial histidine ligand of the haem-iron was substituted for a methionine, the PsbE/H23M mutant in which the haem was lacking, or in which the haem environment was modified, the PsbE/Y19F and PsbE/T26P mutants. All these mutants remained active showing that the haem has no structural role provided that PsbE and PsbF subunits are present. Here, we have carried on the characterization of these mutants. The following results were obtained: (i) the Y19F mutation hardly affect the E of Cytb, whereas the T26P mutation converts the haem into a form with a E much below 0 mV (so low that it is likely not reducible by Q) even in an active enzyme; (ii) in the PsbE/H23M mutant, and to a less extent in PsbE/T26P mutant, the electron transfer efficiency from Q to Q is decreased; (iii) the lower E of the Q/Q couple in the PsbE/H23M mutant correlates with a higher production of singlet oxygen; (iv) the superoxide and/or hydroperoxide formation was not increased in the PsbE/H23M mutant lacking the haem, whereas it was significantly larger in the PsbE/T26P. These data are discussed in view of the literature to discriminate between structural and redox roles for the haem of Cytb in the production of reactive oxygen species.