Degradation of OEC33 during strong illumination of the PS II: possible participation of reactive oxygen species generated at the lumenal side of PS II

Abstract

Three extrinsic proteins on the lumenal side of PS II (the OEC subunit proteins) are essential for efficient oxygen evolution in PS II. OEC33, one of the OEC subunits, either binds to the PS II core or is located in the thylakoid lumen as a free form. It is proposed that the OEC33 is released from and rebound to the PS II core during the turnover of the reaction center D1 protein. When we compared the amount of the OEC33 released from PS II and that retained in PS II after strong illumination, the former is always less than the value expected from the latter. It indicates that the OEC33 protein, which is stable and no degradation products have been detected under the illumination at physiological conditions, is partially degraded during strong illumination. When we examined the effects of reactive oxygen species on the OEC33, hydroxyl radicals degraded the free OEC33 whereas hydrogen peroxides had no effect. As the addition of scavengers of superoxide and chelating agents suppressed the degradation of OEC33 during strong illumination of PS II, it is suggested that reactive oxygen species generated in PS II during the illumination are responsible for the degradation of the OEC33. Interestingly, the addition of the scavengers of superoxide and EDTA did not suppress the damage of OEC33 during strong illumination of intact thylakoids. These results suggest that the reactive oxygen species responsible for the damage of OEC33 are generated at the lumenal side of PS II.