Amino acid oxidation of the D1 and D2 proteins by oxygen radicals during photoinhibition of Photosystem II
The Photosystem II reaction center is vulnerable to photoinhibition. The D1 and D2 proteins, lying at the core of the photosystem, are susceptible to oxidative modification by reactive oxygen species that are formed by the photosystem during illumination. Using spin probes and EPR spectroscopy, we have determined that both O 2 •– and HO • are involved in the photoinhibitory process. Using tandem mass spectroscopy, we have identified a number of oxidatively modified D1 and D2 residues. Our analysis indicates that these oxidative modifications are associated with formation of HO • at both the Mn 4 O 5 Ca cluster and the nonheme iron. Additionally, O 2 •– appears to be formed by the reduction of O 2 at either Pheo D1 or Q A . Early oxidation of D1: 332 H, which is coordinated with the Mn1 of the Mn 4 O 5 Ca cluster, appears to initiate a cascade of oxidative events that lead to the oxidative modification of numerous residues in the C termini of the D1 and D2 proteins on the donor side of the photosystem. Oxidation of D2: 244 Y, which is a bicarbonate ligand for the nonheme iron, induces the propagation of oxidative reactions in residues of the D- de loop of the D2 protein on the electron acceptor side of the photosystem. Finally, D1: 130 E and D2: 246 M are oxidatively modified by O 2 •– formed by the reduction of O 2 either by Pheo D1 •– or Q A •– . Furthermore, the identification of specific amino acid residues oxidized by reactive oxygen species provides insights into the mechanism of damage to the D1 and D2 proteins under light stress.