Oxidized Amino Acid Residues in the Vicinity of QA and PheoD1 of the Photosystem II Reaction Center: Putative Generation Sites of Reducing-Side Reactive Oxygen Species

Under a variety of stress conditions, Photosystem II produces reactive oxygen species on both the reducing and oxidizing sides of the photosystem. A number of different sites including the Mn4O5Ca cluster, P-680, Pheo(D1), Q(A), Q(B) and cytochrome b(559) have been hypothesized to produce reactive oxygen species in the photosystem. In this communication using Fourier-transform ion cyclotron resonance mass spectrometry we have identified several residues on the D1 and D2 proteins from spinach which are oxidatively modified and in close proximity to Q(A) (D1 residues F-239, (241)Q, E-242 and the D2 residues P-238, T-239, E-242 and M-247) and Pheo(D1) (D1 residues E-130, L-133 and F-135). These residues may be associated with reactive oxygen species exit pathways located on the reducing side of the photosystem, and their modification may indicate that both Q(A) and Pheo(D1) are sources of reactive oxygen species on the reducing side of Photosystem II.